A disperse system consisting of a solid dispersed phase. Dispersed systems material in chemistry (grade 11) on the topic

Dispersed systems. Definition. Classification.

Solutions

In the previous paragraph we talked about solutions. Let us briefly recall this concept here.

Solutions are called homogeneous (homogeneous) systems consisting of two or more components.

Homogeneous system- This homogeneous system, chemical composition And physical properties in which all parts are the same or change continuously, without jumps (there are no interfaces between parts of the system).

This definition of a solution is not entirely correct. It rather refers to true solutions.

At the same time, there are also colloidal solutions, which are not homogeneous, but heterogeneous, i.e. consist of different phases separated by an interface.

In order to achieve greater clarity in definitions, another term is used - dispersed systems.

Before considering dispersed systems, let’s talk a little about the history of their study and the appearance of such a term as colloidal solutions.

Background

Back in 1845, the chemist Francesco Selmi, while studying the properties of various solutions, noticed that biological fluids - serum and blood plasma, lymph and others - differ sharply in their properties from ordinary true solutions, and therefore such liquids were called pseudo-solutions.

Colloids and crystalloids

Further research in this direction, carried out since 1861 by the English scientist Thomas Graham, showed that some substances that quickly diffuse and pass through plant and animal membranes easily crystallize, while others have a low ability to diffusion, do not pass through membranes and do not crystallize , but form amorphous precipitates.

Graham named the first crystalloids, and the second – colloids(from the Greek word kolla - glue and eidos - kind) or glue-like substances.

In particular, it was found that substances capable of forming amorphous sediments, such as albumin, gelatin, gum arabic, iron and aluminum hydroxides and some other substances, diffuse in water slowly compared to the diffusion rate of crystalline substances such as table salt , magnesium sulfate, cane sugar, etc.

The table below shows the diffusion coefficients D for some crystalloids and colloids at 18°C.

The table shows that there is an inverse relationship between molecular weight and diffusion coefficient.

In addition, crystalloids were found to have the ability not only to diffuse quickly, but also dialyze, i.e. pass through membranes, as opposed to colloids, which have larger molecular sizes and therefore diffuse slowly and do not penetrate membranes.

The walls of a bull's bladder, cellophane, films of ferrous-cyanide copper, etc. are used as membranes.

Based on his observations, Graham established that all substances can be divided into crystalloids and colloids.

Russians disagree

Against such strict division chemical substances a professor at Kyiv University objected I.G. Borschev(1869). Borshchev's opinion was later confirmed by the research of another Russian scientist Weimarn, who proved that the same substance, depending on conditions, can exhibit the properties of colloids or crystalloids.

For example, a solution of soap in water has the properties colloid, and soap dissolved in alcohol exhibits properties true solutions.

In the same way, crystalline salts, for example, table salt, dissolved in water, give true solution, and in benzene – colloidal solution and so on.

Hemoglobin or egg albumin, which has the properties of colloids, can be obtained in a crystalline state.

DI. Mendeleev believed that any substance, depending on the conditions and nature of the environment, can exhibit properties colloid. Currently, any substance can be obtained in a colloidal state.

Thus, there is no reason to divide substances into two separate classes - crystalloids and colloids, but we can talk about the colloidal and crystalloid states of the substance.

The colloidal state of a substance means a certain degree of its fragmentation or dispersion and the presence of colloidal particles in suspension in a solvent.

The science that studies the physicochemical properties of heterogeneous highly dispersed and high-molecular systems is called colloid chemistry.

Dispersed systems

If one substance, which is in a crushed (dispersed) state, is evenly distributed in the mass of another substance, then such a system is called dispersed.

In such systems, the fragmented substance is usually called dispersed phase, and the environment in which it is distributed is dispersion medium.

So, for example, a system representing agitated clay in water consists of suspended small particles of clay - the dispersed phase and water - the dispersion medium.

Dispersed(fragmented) systems are heterogeneous.

Dispersed systems, in contrast to heterogeneous ones with relatively large, continuous phases, are called microheterogeneous, and colloidal dispersed systems are called ultramicroheterogeneous.

Classification of disperse systems

Classification of dispersed systems is most often made based on degree of dispersion or state of aggregation dispersed phase and dispersion medium.

Classification by degree of dispersion

All dispersed systems Based on the size of dispersed phase particles, they can be divided into the following groups:

For reference, here are the units of size in the SI system:
1 m (meter) = 102 cm (centimeter) = 103 mm (millimeters) = 106 microns (micrometers) = 109 nm (nanometers).

Sometimes other units are used - mk (micron) or mmk (millimicron), and:
1 nm = 10 -9 m = 10 -7 cm = 1 mmk;
1 µm = 10 -6 m = 10 -4 cm = 1 µm.

Coarse dispersed systems.

These systems contain as a dispersed phase the largest particles with a diameter of 0.1 microns and above. These systems include suspensions And emulsions.

Suspensions are systems in which a solid substance is in a liquid dispersion medium, for example, a suspension of starch, clay, etc. in water.

Emulsions are called dispersion systems of two immiscible liquids, where droplets of one liquid are suspended in the volume of another liquid. For example, oil, benzene, toluene in water or droplets of fat (diameter from 0.1 to 22 microns) in milk, etc.

Colloidal systems.

They have the particle size of the dispersed phase from 0.1 µm to 1 µm(or from 10 -5 to 10 -7 cm). Such particles can pass through the pores of filter paper, but do not penetrate the pores of animal and plant membranes.

Colloidal particles if they have electric charge and solvation-ion shells remain in suspension and, without changing conditions, may not precipitate for a very long time.

Examples of colloidal systems include solutions of albumin, gelatin, gum arabic, colloidal solutions of gold, silver, arsenic sulfide, etc.

Molecular dispersed systems.

Such systems have particle sizes not exceeding 1 mm. Molecular dispersed systems include true solutions of non-electrolytes.

Ion-dispersed systems.

These are solutions of various electrolytes, such as salts, bases, etc., which disintegrate into corresponding ions, the sizes of which are very small and go far beyond
10 -8 cm.

Clarification on the representation of true solutions as dispersed systems.

From the classification given here it is clear that any solution (both true and colloidal) can be represented as a dispersed medium. True and colloidal solutions will differ in the particle sizes of the dispersed phases. But above we wrote about the homogeneity of true solutions, and dispersion systems are heterogeneous. How to resolve this contradiction?

If speak about structure true solutions, then their homogeneity will be relative. The structural units of true solutions (molecules or ions) are much smaller than the particles of colloidal solutions. Therefore, we can say that compared to colloidal solutions and suspensions, true solutions are homogeneous.

If we talk about properties true solutions, then they cannot be fully called dispersed systems, since the mandatory existence of dispersed systems is the mutual insolubility of the dispersed substance and the dispersion medium.

In colloidal solutions and coarse suspensions, the dispersed phase and the dispersion medium practically do not mix and do not react chemically with each other. This cannot be said at all about true solutions. In them, when dissolved, substances mix and even interact with each other. For this reason, colloidal solutions differ sharply in properties from true solutions.

The sizes of some molecules, particles, cells.

As the particle sizes change from the largest to the smallest and back, the properties of dispersed systems will change accordingly. Wherein colloidal systems occupy as it were intermediate position between coarse suspensions and molecular disperse systems.

Classification according to the state of aggregation of the dispersed phase and dispersion medium.

Foam is a dispersion of gas in a liquid, and in foams the liquid degenerates into thin films separating individual gas bubbles.

Emulsions are dispersed systems in which one liquid is crushed by another liquid that does not dissolve it (for example, water in fat).

Suspensions are called low-disperse systems of solid particles in liquids.

Combinations of three types of aggregative states make it possible to distinguish nine types of dispersed systems:

Dispersed phase	Dispersive medium	Title and example
Gaseous	Gaseous	No disperse system is formed
Gaseous		Gas emulsions and foams
Gaseous		Porous bodies: foam pumice
	Gaseous	Aerosols: fogs, clouds
		Emulsions: oil, cream, milk, margarine, butter
		Capillary systems: Liquid in porous bodies, soil, soil
	Gaseous	Aerosols (dusts, fumes), powders
		Suspensions: pulp, sludge, suspension, paste
		Solid systems: alloys, concrete

Sols are another name for colloidal solutions.

Colloidal solutions are also called sols(from Latin solutus - dissolved).

Dispersed systems with a gaseous dispersion medium are called aerosols. Fogs are aerosols with a liquid dispersed phase, and dust and smoke are aerosols with a solid dispersed phase. Smoke is a more highly dispersed system than dust.

Dispersed systems with a liquid dispersion medium are called lysols(from the Greek “lios” - liquid).

Depending on the solvent (dispersion medium), i.e. water, benzene alcohol or ether, etc., there are hydrosols, alcosols, benzols, etherosols, etc.

Cohesively dispersed systems. Gels.

Dispersed systems can be freely dispersed And cohesively dispersed depending on the absence or presence of interaction between particles of the dispersed phase.

TO freely dispersed systems include aerosols, lysols, diluted suspensions and emulsions. They are fluid. In these systems, particles of the dispersed phase have no contacts, participate in random thermal motion, and move freely under the influence of gravity.

The pictures above show free-dispersed systems:
In the pictures a B C depicted corpuscular-dispersed systems:
a, b- monodisperse systems,
V- polydisperse system,
On the image G depicted fiber-dispersed system
On the image d depicted film-dispersed system

- solid. They arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network.

This structure limits the fluidity of the dispersed system and gives it the ability to retain its shape. Such structured colloidal systems are called gels.

The transition of a sol to a gel, which occurs as a result of a decrease in the stability of the sol, is called gelation(or gelatinization).

In the pictures a B C depicted cohesive dispersed systems:
A- gel,
b- coagulum with a dense structure,
V- coagulum with a loose “arched” structure
In the pictures g, d depicted capillary-dispersed systems

Powders (pastes), foams– examples of cohesively dispersed systems.

The soil, formed as a result of contact and compaction of dispersed particles of soil minerals and humus (organic) substances, is also a coherently dispersed system.

A continuous mass of substance can be penetrated by pores and capillaries, forming capillary-dispersed systems. These include, for example, wood, leather, paper, cardboard, fabrics.

Lyophilicity and lyophobicity

A general characteristic of colloidal solutions is the property of their dispersed phase to interact with the dispersion medium. In this regard, two types of sols are distinguished:

1. Lyophobic(from Greek phobia - hatred) And

2.Lyophilic(from Greek philia – love).

U lyophobic In sols, the particles have no affinity for the solvent, interact weakly with it, and form around themselves a thin shell of solvent molecules.

In particular, if the dispersion medium is water, then such systems are called hydrophobic, for example, sols of metals iron, gold, arsenic sulfide, silver chloride, etc.

IN lyophilic systems there is an affinity between the dispersed substance and the solvent. The particles of the dispersed phase, in this case, acquire a more voluminous shell of solvent molecules.

In the case of an aqueous dispersion medium, such systems are called hydrophilic, such as solutions of protein, starch, agar-agar, gum arabic, etc.

Coagulation of colloids. Stabilizers.

Substance at the interface.

All liquids and solids are limited by an outer surface at which they come into contact with phases of a different composition and structure, for example, vapor, another liquid or a solid.

Properties of matter in this interfacial surface, with a thickness of several diameters of atoms or molecules, differ from the properties inside the volume of the phase.

Inside the volume of a pure substance in a solid, liquid or gaseous state, any molecule is surrounded by similar molecules.

In the boundary layer, molecules are in interaction with another number of molecules (different in comparison with the interaction inside the volume of the substance).

This occurs, for example, at the interface of a liquid or solid with its vapor. Either in the boundary layer molecules of a substance interact with molecules of another chemical nature, for example, at the boundary of two mutually poorly soluble liquids.

As a result, differences in the nature of the interaction inside the bulk of the phases and at the phase boundary arise force fields associated with this unevenness. (More on this in the section Surface tension of a liquid.)

The greater the difference in the intensity of intermolecular forces acting in each of the phases, the greater the potential energy of the interphase surface, briefly called surface energy.

Surface tension
To estimate surface energy, a quantity such as specific free surface energy is used. It is equal to the work spent on the formation of a unit area of ​​a new phase interface (assuming a constant temperature).
In the case of a boundary between two condensed phases, this quantity is called boundary tension.
When talking about the boundary of a liquid with its vapors, this quantity is called surface tension.

Coagulation of colloids

All spontaneous processes occur in the direction of decreasing the energy of the system (isobaric potential).

Similarly, processes spontaneously occur at the phase interface in the direction of decreasing free surface energy.

The smaller the interphase surface, the smaller the free energy.

And the phase interface, in turn, is related to the degree of dispersion of the dissolved substance. The higher the dispersion ( smaller particles dispersed phase), the larger the interface between the phases.

Thus, in dispersed systems there are always forces leading to a decrease in the total interphase surface, i.e. to particle enlargement. Therefore, the merging of small droplets in fogs, rain clouds and emulsions occurs - the aggregation of highly dispersed particles into larger formations.

All this leads to the destruction of dispersed systems: fogs and rain clouds rain, emulsions separate, colloidal solutions coagulate, i.e. are separated into a sediment of the dispersed phase (coagulate) and a dispersion medium or, in the case of elongated particles of the dispersed phase, turn into a gel.

The ability of fragmented systems to maintain their inherent degree of dispersion is called aggregative stability.

Stabilizers for dispersed systems

As stated earlier, dispersed systems are fundamentally thermodynamically unstable. The higher the dispersion, the greater the free surface energy, the greater the tendency to spontaneously reduce dispersion.

Therefore, to obtain stable, i.e. long-lasting suspensions, emulsions, colloidal solutions, it is necessary not only to achieve the desired dispersion, but also to create conditions for its stabilization.

In view of this, stable disperse systems consist of at least three components: a dispersed phase, a dispersion medium and a third component - disperse system stabilizer.

The stabilizer can be either ionic or molecular, often high-molecular, in nature.

Ionic stabilization of sols of lyophobic colloids is associated with the presence of low concentrations of electrolytes, creating ionic boundary layers between the dispersed phase and the dispersion medium.

High-molecular compounds (proteins, polypeptides, polyvinyl alcohol and others) added to stabilize dispersed systems are called protective colloids.

Adsorbed at the phase interface, they form mesh and gel-like structures in the surface layer, creating a structural-mechanical barrier that prevents the integration of particles of the dispersed phase.

Structural-mechanical stabilization is crucial for the stabilization of suspensions, pastes, foams, and concentrated emulsions.

General ideas about dispersed systems

Chemical interaction in homogeneous reactions occurs during effective collisions of active particles, and in heterogeneous reactions - at the interface of phases upon contact of reacting substances, and the speed and mechanism of the reaction depend on the surface area, which is larger, the more developed the surface is. From this point of view, dispersed systems with a high specific surface area are of particular interest.

A disperse system is a mixture consisting of at least two substances that do not react chemically with each other and have almost complete mutual insolubility. Dispersed system - This is a system in which very crushed particles of one substance are evenly distributed within the volume of another.

When considering dispersed systems, two concepts are distinguished: dispersed phase and dispersion medium (Fig. 10.1).

Dispersed phase – This is a collection of particles of a substance dispersed to small sizes, evenly distributed in the volume of another substance. Signs of the dispersed phase are fragmentation and discontinuity.

Dispersive mediumis a substance in which particles of the dispersed phase are evenly distributed. A sign of a dispersion medium is its continuity.

The dispersed phase can be separated from the dispersion medium by physical means (centrifugation, separation, settling, etc.).

Figure 10.1 – Dispersed system: particles of the dispersed phase s (in the form of small solid particles, crystals, liquid drops, gas bubbles, associates of molecules or ions), having an adsorption layer d, are distributed in a homogeneous continuous dispersion medium f.

Dispersed systems are classified according to various distinctive features: dispersity, the state of aggregation of the dispersed phase and the dispersion medium, the intensity of interaction between them, the absence or formation of structures in dispersed systems.

Classification by degree of dispersion

Depending on the particle size of the dispersed phase, all dispersed systems are conventionally divided into three groups (Fig. 10.2).

Figure 10.2 – Classification of disperse systems by particle size (for comparison, the particle sizes in true solutions are given)

1. Coarse systems , in which the particle size is more than 1 µm (10 –5 m). This group of dispersed systems is characterized by the following characteristics: particles of the dispersed phase settle (or float up) in the field of gravitational forces and do not pass through paper filters; they can be viewed under a regular microscope. Coarse systems include suspensions, emulsions, dust, foam, aerosols, etc.

Suspension – is a dispersed system in which dispersedthe phase is a solid, and the dispersion medium is a liquid.

An example of a suspension can be a system formed by shaking clay or chalk in water, paint, or paste.

Emulsion – This is a dispersed system in which the liquid dispersed phase is uniformly distributed throughout the volume of the liquid dispersion medium, i.e. an emulsion consists of two mutually insoluble liquids.

Examples of emulsions include milk (in which the dispersed phase is drops of liquid fat, and the dispersion medium is water), cream, mayonnaise, margarine, and ice cream.

When settling, suspensions and emulsions are separated (stratified) into their component parts: dispersed phase and dispersion medium. So, if you vigorously shake benzene with water, an emulsion is formed, which after some time separates into two layers: the upper benzene and the lower aqueous. To prevent emulsions from separating, add to them emulsifiers– substances that impart aggregate stability to emulsions.

Foam – a cellular coarsely dispersed system in which the dispersed phase is a set of gas (or vapor) bubbles, and the dispersion medium is liquid.

In foams, the total volume of gas contained in the bubbles can be hundreds of times greater than the volume of the liquid dispersion medium contained in the layers between the gas bubbles.

2. Microheterogeneous (orfinely dispersed ) intermediate systems in which the particle size varies within 10 – 5 –10 –7 m. These include thin suspensions, fumes, and porous solids.

3. Ultramicroheterogeneous (orcolloidal dispersed ) systems in which particles with a size of 1–100 nm (10–9 –10 –7 m) consist of 10 3_ 10 9 atoms and are separated from the solvent by an interface. Colloidal solutions are characterized by an extremely highly dispersed state; they are usually called sol, or often lyosolsto emphasize that the dispersion medium is a liquid. If water is taken as the dispersion medium, then such sols are calledhydrosols, and if the organic liquid -organosols.

Most finely dispersed systems have certain features:

low diffusion rate;

particles of the dispersed phase (i.e. colloidal particles) can only be examined using an ultramicroscope or electron microscope;

scattering of light by colloidal particles, as a result of which in an ultramicroscope they take on the appearance of light spots - the Tyndall effect (Fig. 10.3);

Figure 10.3 – Ultramicroheterogeneous (finely dispersed) system: a) colloidal solution; b) diagram of the deflection of a narrow beam of light when passing through a colloidal solution; c) scattering of light by a colloidal solution (Tyndall effect)

• on the phase interface in the presence of stabilizers (electrolyte ions), an ionic layer or solvation shell is formed, which promotes the existence of suspended particles;
• the dispersed phase is either completely insoluble or slightly soluble in the dispersion medium.

Examples of colloidal particles include starch, proteins, polymers, rubber, soaps, aluminum and ferum (III) hydroxides.

Classification of dispersed systems based on the relationship between the states of aggregation of the dispersed phase and the dispersion medium

This classification was proposed by Ostavld (Table 10.1). When schematically recording the state of aggregation of dispersed systems, first indicate with the letters G (gas), L (liquid) or T (solid) the state of aggregation of the dispersed phase, and then put a dash (or fraction sign) and write down the state of aggregation of the dispersion medium.

Table 10.1 – Classification of disperse systems

Classification of dispersed systems according to the intensity of molecular interaction

This classification was proposed by G. Freundlich and is used exclusively for systems with a liquid dispersion medium.

1. Lyophilic systems , in which the dispersed phase interacts with the dispersion medium and, under certain conditions, is capable of dissolving in it - these are solutions of colloidal surfactants (surfactants), solutions of high molecular weight compounds (HMW). Among the various lyophilic systems, the most important in practical terms are surfactants, which can be found both in a molecularly dissolved state and in the form of aggregates (micelles) consisting of tens, hundreds or more molecules.
2. Lyophobic systems , in which the dispersed phase is not able to interact with the dispersion medium and dissolve in it. In lyophobic systems, the interaction between molecules of different phases is much weaker than in the case of lyophilic systems; the interfacial surface tension is high, as a result of which the system tends to spontaneously enlarge the particles of the dispersed phase.

Classification of dispersed systems by physical state

The author of the classification is P. Rebinder. According to this classification, a dispersed system is designated by a fraction in which the dispersed phase is placed in the numerator and the dispersion medium in the denominator. For example: T 1 / L 2 denotes a dispersed system with a solid phase (index 1) and a liquid dispersion medium (index 2). The Rebinder classification divides disperse systems into two classes:

1. Freely dispersed systems – sols in which the dispersed phase does not form continuous rigid structures (grids, trusses or frames), has fluidity, and the particles of the dispersed phase do not contact each other, participating in random thermal movement and moving freely under the influence of gravity. These include aerosols, lyosols, diluted suspensions and emulsions.

Examples of freely dispersed systems:

• Dispersed systems in gases with colloidal dispersion (T 1 / G 2 - dust in the upper layers of the atmosphere, aerosols), with coarse dispersion (T 1 / G 2 - fumes and G 1 / G 2 - fogs);
• Dispersed systems in liquids with colloidal dispersion (T 1 / G 2 - lyosols, dispersed dyes in water, latexes of synthetic polymers), with coarse dispersion (T 1 / G 2 - suspensions; G 1 / G 2 - liquid emulsions; G 1 / Zh 2 – gas emulsions);
• Dispersed systems in solids ah: T 1 / T 2 - solid sols, for example, yellow metal sol in glass, pigmented fibers, filled polymers.

2. Cohesively dispersed (or continuous) systems . In continuous (cohesively dispersed) systems, particles of the dispersed phase form hard spatial structures. Such systems resist shear deformation. Cohesively dispersed systems are solid; they arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network, limiting the fluidity of the dispersed system and giving it the ability to retain its shape. Such structured colloidal systems are called gels.

Examples of cohesively dispersed systems:

• Dispersed systems with a liquid interface (G 1 / Zh 2 - foams; Zh1 / Zh 2 - foam emulsions);
• Dispersed systems with a solid interface between phases (G 1 / T 2 - porous bodies, natural fibers, pumice, sponge, charcoal; G 1 / T 2 - moisture in granite; T 1 / T 2 - interpenetrating networks of polymers).

Preparation and purification of colloidal solutions

Preparation of colloidal solutions

Colloidal solutions can be prepareddispersive or to condensation methods.

1. Dispersion methods- these are methods for producing lyophobic sols by crushing large pieces into aggregates of colloidal sizes.

Mechanical Crushing of coarse systems is carried out by: crushing, impact, abrasion, splitting. Particles are crushed to sizes of several tens of microns using ball millsVery fine crushing (up to 0.1-1 microns) is achieved using specialcolloid millswith a narrow gap between a rapidly rotating rotor (10-20 thousand rpm) and a stationary housing, and the particles are torn or abraded in the gap.The work of P. A. Rebinder established the phenomenon of a decrease in the resistance of solids to elastic and plastic deformations, as well as mechanical destruction under the influence of adsorption of surfactants. Surfactants facilitate dispersion and contribute to a significant increase in the degree of dispersion.

2. Condensation methods- these are methods for producing colloidal solutions by combining (condensing) molecules and ions into aggregates of colloidal sizes. The system changes from homogeneous to heterogeneous, i.e., a new phase (dispersed phase) appears. Required condition is oversaturation original system.

Condensation methods are classified according to the nature of the forces causing condensation into physical condensation and chemical condensation.

Physical condensation can be done from vapor or by replacing the solvent.

Condensation from vapors. The starting material is in vapor. As the temperature decreases, the steam becomes supersaturated and partially condenses, forming a dispersed phase. In this way, hydrosols of mercury and some other metals are obtained.

Solvent replacement method. The method is based on changing the composition and properties of the dispersion medium. For example, pour an alcohol solution of sulfur, phosphorus or rosin into water; due to a decrease in the solubility of the substance in the new solvent, the solution becomes supersaturated and part of the substance condenses, forming particles of the dispersed phase.

Chemical condensation is that the substance forming the dispersed phase is obtained as a result chemical reaction. In order for a reaction to form a colloidal solution rather than a true solution or precipitate, at least three conditions must be met:

1. the substance of the dispersed phase is insoluble in the dispersion medium;
2. the rate of formation of dispersed phase crystal nuclei is much greater than the rate of crystal growth; this condition is usually met when a concentrated solution of one component is poured into a highly dilute solution of another component with vigorous stirring;
3. one of the starting substances is taken in excess; it is this that is the stabilizer.

Methods for purifying colloidal solutions.

Colloidal solutions obtained in one way or another are usually purified from low molecular weight impurities (molecules and ions). The removal of these impurities is carried out by dialysis, (electrodialysis), and ultrafiltration.

Dialysis– a purification method using a semi-permeable membrane that separates the colloidal solution from the clean dispersion medium. Parchment, cellophane, collodion, ceramic filters and other fine-porous materials are used as semi-permeable (i.e. permeable to molecules and ions, but impermeable to dispersed phase particles) membranes. As a result of diffusion, low molecular weight impurities pass into the external solution.

Ultrafiltration called dialysis, carried out under pressure in an internal chamber. Essentially, ultrafiltration is not a method for purifying sols, but only a method for concentrating them.

Optical properties of colloidal solutions

When light falls on a dispersed system, the following phenomena can be observed:

• passage of light through the system;
• refraction of light by dispersed phase particles (if these particles are transparent);
• reflection of light by particles of the dispersed phase (if the particles are opaque);
• light scattering;
• absorption ( absorption) of light by the dispersed phase.

Light Scattering observed for systems in which the dispersed phase particles are smaller or comparable to the wavelength of the incident light. Let us recall that the particle size of the dispersed phase in colloidal solutions is 10 -7 -10 -9 m. Consequently, light scattering is a characteristic phenomenon for the colloidal systems we study.

Rayleigh created the theory of light scattering. He derived an equation that relates the intensity of scattered light I to the intensity of incident light I 0 . fair provided that:

• the particles have a spherical shape;
• particles do not conduct electricity(i.e. they are non-metallic);
• the particles do not absorb light, i.e. they are colorless;
• a colloidal solution is dilute to such an extent that the distance between particles is greater than the wavelength of the incident light.

Rayleigh equation:

• Where V - volume of one particle,
• λ - wavelength;
• n 1 - refractive index of the particle;
• n o - refractive index of the medium.

The following conclusions follow from the Rayleigh equation:

1. The more the refractive indices of the particle and the medium differ, the greater the intensity of scattered light. (n 1 - P 0 ).
2. If the refractive index P 1 And n 0 are the same, then light scattering will be absent in an inhomogeneous medium.
3. The greater the partial concentration v, the greater the intensity of the scattered light. Mass concentration c, g/dm 3, which is usually used when preparing solutions, is related to the partial concentration by the expression:

where ρ is the particle density.

It should be noted that this dependence is preserved only in the region of small particle sizes. For the visible part of the spectrum, this condition corresponds to values ​​of 2 10 -6 cm< r < 4 10 -6 см. С увеличением r рост I slows down, and for r > λ, scattering is replaced by reflection. The intensity of the scattered light is directly proportional to the concentration.

4. The intensity of the scattered light is inversely proportional to the wavelength to the fourth power.

This means that when a beam of white light passes through a colloidal solution, short waves - the blue and violet parts of the spectrum - are predominantly scattered. Therefore, colorless sol has a bluish color in diffuse light, and a reddish color in transmitted light. The blue color of the sky is also due to the scattering of light by tiny droplets of water in the atmosphere. The orange or red color of the sky at sunrise or sunset is due to the fact that in the morning or evening there is mainly light passing through the atmosphere.

light absorption. Rayleigh's equation was derived for uncolored sols, i.e., those that do not absorb light. However, many colloidal solutions have a certain color, i.e. absorb light in the corresponding region of the spectrum - the sol is always colored in a color complementary to that absorbed. Thus, absorbing the blue part of the spectrum (435-480 nm), the sol turns out to be yellow; when absorbing the bluish-green part (490-500 nm), it takes on a red color.If rays from the entire visible spectrum pass through a transparent body or are reflected from an opaque body, then the transparent body appears colorless, and the opaque body appears white. If a body absorbs radiation from the entire visible spectrum, it appears black.The optical properties of colloidal solutions capable of absorbing light can be characterized by changes in light intensity as it passes through the system. To do this, use the Bouguer-Lambert-Beer law:

where I 0 - intensity of incident light ; I etc- intensity of light passed through the sol; k - absorption coefficient; l- thickness of the sol layer; With- sol concentration.

If we take the logarithm of the expression, we get:

The quantity is called optical density solution . When working with monochromatic light, always indicate at what wavelength the optical density was determined, designating it D λ .

Mycellar theory of the structure of colloidal systems

Let us consider the structure of a hydrophobic colloidal particle using the example of the formation of an AgI sol by an exchange reaction

AgNO 3 + KI → AgI + KNO 3.

If the substances are taken in equivalent quantities, then a crystalline precipitate of AgI precipitates. But, if one of the starting substances is in excess, for example KI, the AgI crystallization process leads to the formation of a colloidal solution - AgI micelles.

The structure of an AgI hydrosol micelle is shown in Fig. 10.4.

Figure 10.4 – Scheme of an AgI hydrosol micelle formed with an excess of KI

An aggregate of 100-1000 [mAgI] molecules (microcrystals)—the core—is the nucleus of a new phase, on the surface of which the adsorption of electrolyte ions occurring in a dispersion medium occurs. According to the Paneth-Faience rule, ions that are the same as ions that enter the crystal lattice of the core and complete this lattice are better adsorbed. Ions that are adsorbed directly on the nucleus are called potential-determining, since they determine the magnitude of the potential and the sign of the surface charge, as well as the sign of the charge of the entire particle. The potential-determining ions in this system are I - ions, which are in excess, are part of the crystal lattice of the AgI core, act as stabilizers and form the inner shell in the rigid part of the electrical double layer (DEL) of the micelle. The aggregate with I - ions adsorbed on it forms the core of the micelle.

To the negatively charged surface of AgI particles at a distance close to the radius of the hydrated ion, ions of the opposite sign (counterions) - positively charged K + ions - are attracted from the solution. The counterion layer is the outer shell of the electrical double layer (DEL), held together by both electrostatic forces and adsorption attractive forces. An aggregate of molecules together with a solid double layer is called a colloidal particle - granule.

Due to thermal movement, some of the counterions are located diffusely around the granule and are associated with it only due to electrostatic forces. The colloidal particles, together with the diffuse layer surrounding it, are called a micelle. The micelle is electrically neutral, since the charge of the nucleus equal to charge all counterions, and the granule usually has a charge, which is called electrokinetic or ξ - zeta - potential. In abbreviated form, the micelle structure diagram for this example can be written as follows:

One of the main provisions of the theory of the structure of colloidal particles is the concept of the structure of a double electric layer (EDL). According to modern ideas, electric double layer DESconsists of adsorption and diffusion layers. The adsorption layer consists of:

• the charged surface of the micelle core as a result of the adsorption of potential-determining ions on it, which determine the magnitude of the surface potential and its sign;
• a layer of ions of the opposite sign - counterions, which are attracted from the solution to the charged surface. Counter ion adsorption layer is located at a distance of molecular radius from the charged surface. Both electrostatic and adsorption forces exist between this surface and the counterions of the adsorption layer, and therefore these counterions are especially strongly bound to the core. The adsorption layer is very dense, its thickness is constant and does not depend on changes in external conditions (electrolyte concentration, temperature).

Due to thermal movement, some of the counterions penetrate deep into the dispersion medium, and their attraction to the charged surface of the granule is carried out only due to electrostatic forces. These counterions constitute a diffuse layer, which is less tightly bound to the surface. The diffuse layer has a variable thickness, which depends on the concentration of electrolytes in the dispersion medium.

When solid and liquid phases move relative to each other, a rupture of the EDL occurs in the diffuse part and a potential jump occurs at the interface, which is called electrokinetic ξ - potential(zeta potential). Its value is determined by the difference between the total number of charges (φ) of potential-determining ions and the number of counterion charges (ε) contained in the adsorption layer, i.e. ξ = φ - ε. The drop in interfacial potential with distance from the solid phase deeper into the solution is shown in Fig. 10.5.

Figure 10.5 Structure of diesel power plant

The presence of a potential difference around the particles of a hydrophobic sol prevents them from sticking together upon collision, that is, they are a factor in the aggregate stability of the sol. If the number of diffuse ions decreases or tends to zero, then the granule becomes electrically neutral (isoelectric state) and has the lowest stability.

Thus, the magnitude of the electrokinetic potential determines the repulsive forces and, consequently, the aggregate stability of the colloidal solution. Sufficient stability of the colloidal solution is ensured at an electrokinetic potential value of ξ = 0.07 V; at values ​​lower than ξ = 0.03 V, the repulsive forces are too weak to resist aggregation, and therefore coagulation occurs, which inevitably ends in sedimentation.

The value of the electrokinetic potential can be determined using an electrophoresis device according to the formula (10.5):

where η is viscosity; ϑ - particle movement speed; l is the distance between the electrodes along the solution; E - electromotive force, D - dielectric constant.

Factors influencing ξ - potential:

1. The presence in the solution of an indifferent electrolyte - an electrolyte that does not contain a potential-determining ion.

• An indifferent electrolyte contains a counterion. In this case, compression of the diffusion layer occurs and ξ drops and, as a consequence, coagulation.
• An indifferent electrolyte contains an ion of the same sign as the counterion, but not the counterion itself. In this case, ion exchange occurs: the counterion is replaced by ions of the indifferent electrolyte. A drop in ξ is observed, but the degree of drop will depend on the nature of the substituent ion, its valence, and the degree of hydration. Lyotropic rows of cations and anions are rows in which ions are arranged according to an increase in their ability to compress the diffuse layer and cause a drop in ξ - potential.

Li + - Na + - NH 4 + - K + - Rb + - Cs + - Mg 2+ - Ca 2+ - Ba 2+ ...

CH 3 COO – - F – - NO 3 – - Cl – - I – - Br – - SCN – - OH – - SO 4 2–

2. Adding solution electrolyte stabilizer– an electrolyte containing a potential-determined ion causes an increase in ξ - potential, which means it contributes to the stability of the colloidal system, but up to a certain limit.

Stability and coagulation of colloidal systems

The modern theory of stability and coagulation of colloidal systems was created by several famous scientists: Deryagina, Landau, Verwey, Overbeck and therefore it is abbreviated as DLFO theory . According to this theory, the stability of a dispersed system is determined by the balance of attractive and repulsive forces that arise between particles when they approach each other as a result of Brownian motion. A distinction is made between kinetic and aggregate stability of colloidal systems.

1. Kinetic (sedimentation) stability- the ability of dispersed particles to be in suspension and not settle (not sediment). In disperse systems, as in natural solutions, Brownian motion exists. Brownian motion depends on the particle size, viscosity of the dispersed medium, temperature, etc. Finely dispersed systems (sols), the particles of which practically do not settle under the influence of gravity, are classified as kinetically (sedimentation) stable. These also include hydrophilic sols - solutions of polymers, proteins, etc. Hydrophobic sols and coarse systems (suspensions, emulsions) are kinetically unstable. In them, the separation of phase and medium occurs quite quickly.
2. Aggregate stability- the ability of dispersed phase particles to maintain a certain degree of dispersion unchanged. In aggregation-stable systems, particles of the dispersed phase do not stick together during collisions and do not form aggregates. But when aggregate stability is violated, colloidal particles form large aggregates with subsequent precipitation of the dispersed phase. This process is called coagulation, and it proceeds spontaneously, since in this case the free energy of the system decreases (Δ G<0) .

Factors that affect the stability of colloidal systems include:

1. The presence of an electrical charge of dispersed particles. Dispersed particles of lyophobic sols have the same charge, and therefore, when they collide, they will repel each other the more strongly, the higher the zeta potential. However, the electrical factor is not always decisive.
2. The ability to solvation (hydration) of stabilizing ions. The more hydrated (solvated) counterions are in the diffuse layer, the larger the total hydration (solvate) shell around the granules and the more stable the dispersed system.

According to the theory, during Brownian motion, colloidal particles freely approach each other at a distance of up to 10 -5 see. The nature of the change in the van der Waals forces of attraction (1) and electrostatic forces of repulsion (2) between colloidal particles is shown in Fig. 10.6. The resulting curve (3) is obtained by geometrically adding the corresponding ordinates. At minimal and large distances, the energy of attraction prevails between particles (I and II energy minima). At energy minimum II, the cohesion energy of particles is insufficient to keep them in an aggregated state. At average distances corresponding to the thickness of the electric double layer, the repulsion energy predominates with the potential barrier AB, which prevents particles from sticking together. Practice shows that at a zeta potential ξ = 70 mV, colloidal systems are characterized by a high potential barrier and high aggregation stability. To destabilize the colloidal system, i.e. implementation of the coagulation process, it is necessary to reduce - potential up to values ​​0 - 3 mV.

Figure 10.6. Potential interaction curves of colloidal particles

Coagulation of dispersed systems

Coagulation is the process of colloidal particles sticking together. This process proceeds relatively easily under the influence of a variety of factors: the introduction of electrolytes, non-electrolytes, freezing, boiling, stirring, exposure to sunlight, etc. In the process electrolytic coagulation (under the influence of electrolytes) ion-exchange adsorption is often observed: coagulant ions with higher valence or higher adsorption potential displace counterions, first of the diffuse layer, and then of the adsorption layer. The exchange takes place in an equivalent amount, but the replacement of counterions leads to the fact that, with a sufficient concentration of electrolytes in the dispersed medium, the particles lose stability and stick together upon collision.

A number of experimental general rules have been established for electrolytic coagulation:

1. Coagulation of lyophobic sols is caused by any electrolytes, but at a noticeable rate it is observed when a certain electrolyte concentration is reached. Coagulation threshold(C to) is the minimum electrolyte concentration required to begin coagulation of the sol. In this case, external changes are observed, such as turbidity of the solution, change in its color, etc.

• where Sel is the molar concentration of the electrolyte, mmol/l;
• Vel - volume of electrolyte solution, l;
• Vz - volume of sol, l.

The reciprocal of the coagulation threshold is called the coagulating ability () of the electrolyte:

where Sk is the coagulation threshold.

2. Schultz–Hardy rule:

• the coagulating effect is exhibited by the ion whose charge is opposite in sign to the charge of the surface of colloidal particles (charge of the granule), and this effect increases with increasing valency of the ion;
• The coagulating effect of ions increases many times with increasing valency of the ions. For one - two and trivalent ions, the coagulating effect is approximately 1: 50: 500.

This is explained by the fact that multivalent highly charged coagulant ions are much more strongly attracted by the charged surface of a colloidal particle than monovalent ones, and displace counterions from the diffuse and even adsorption layer much more easily.

3. The coagulating effect of organic ions is much higher than that of inorganic ions. This is due to their high adsorption capacity, the ability to be adsorbed in over-equivalent amounts, and also to cause recharging of the surface of colloidal particles.

4. In a number of inorganic ions with the same charges, the coagulating ability depends on the radius of the coagulant ion: the larger the radius, the greater the coagulating ability (see. lyotropic series). This is explained by the fact that the degree of ion hydration decreases, for example, from L + to Cs +, and this facilitates its incorporation into the ionic double layer.

5. Electrically neutral particles of lyophobic colloidal sols coagulate at the highest speed.

6. The phenomenon of sol addiction. If a coagulant is quickly added to the sol, then coagulation occurs, but if it is added slowly, there is no coagulation. This can be explained by the fact that a reaction occurs between the electrolyte and the sol, as a result of which peptizers are formed, which stabilize the dispersed system:

Fe (OH) 3 + HCl →FeOCl + 2H 2 O,

FeOCl → FeO + + Cl - ,

where FeO + is a peptizing agent for Fe (OH) 3 sol.

The coagulating effect of a mixture of electrolytes manifests itself differently depending on the nature of the ion - the coagulator. In a mixture of electrolytes, the effect can be summed up with the coagulating effect of each electrolyte. This phenomenon is called additivity ions (NaCl, KCl). If the coagulating effect of electrolyte ions decreases with the introduction of ions of another electrolyte, antagonism of ions (LiCl, MgCl 2 ). In the case when the coagulating effect of electrolyte ions increases with the introduction of ions of another electrolyte, this phenomenon is called synergy ions.

The introduction of, for example, 10 ml of a 10% NaCl solution into 10 ml of Fe (OH) 3 sol leads to coagulation of this sol. But this can be avoided if one of the protective substances is additionally added to the sol solution: 5 ml of gelatin, 15 ml of egg albumin, 20 ml of dextrin.

Protection of colloidal particles

Colloidal protection- increasing the aggregate stability of the sol by introducing a high molecular weight compound (HMC) into it. For hydrophobic sols, proteins, carbohydrates, and pectins are usually used as BMCs; for non-aqueous sols - rubbers.

The protective effect of the IUD is associated with the formation of a certain adsorption layer on the surface of colloidal particles (Figure 10.7). The reverse phenomenon of coagulation is called peptization.

Figure 10.7 Peptization mechanism

To characterize the protective effect of various IUDs, Zsigmondy proposed using the golden number.Golden number- this is the number of milligrams of the IUD that must be added to 10 cm 3 0.0006% red gold sol to prevent it from turning blue (coagulation) when adding 1cm to it 3 10% NaCl solution. Sometimes, to characterize the protective effect of IUDs, colloidal solutions of silver (silver number), iron hydroxide (iron number), etc. are used instead of gold sol.Table 10.2 shows the values ​​of these numbers for some IUDs.

Table 10.2 Protective effect of the IUD

I allocate 2 hours to study this topic. I consider it advisable to study dispersed systems in the form of a separate block, since they are widespread in everyday life, nature, and play a large role in various industrial and natural processes (geological, soil). It is necessary to know the types of dispersed systems and their properties in order to learn to understand the manifestations of undesirable processes in the environment and correctly solve many scientific, technical and environmental problems.

If at the previous stages of studying chemistry, students became familiar with the variety of substances and the establishment of relationships between the structure, composition and properties of a substance, then when studying dispersed systems they will learn about a new dependence - the dependence of the properties of a substance on the state of their fragmentation.

When studying dispersed systems, many new terms are encountered, so it is necessary to compile a list of them with appropriate explanations and, as you become more familiar with dispersed systems, refer to this list.

I plan lessons on this topic as follows:

1. Dispersed systems, their types.
2. Conference “Properties of disperse systems. The role of dispersed systems in everyday life, nature and production processes.”

Objective of the lessons: Summarize, systematize knowledge on the topic; create an atmosphere of search and cooperation in the classroom, giving each student the opportunity to achieve success.

Educational objectives:

1. Check the degree of mastery of basic knowledge on the topic:
   - Formulate the concept of a disperse system.
   - Introduce the classification of dispersed systems according to various criteria.
   - To attract students’ attention to dispersed systems of great practical significance:
   suspensions, emulsions, colloidal solutions, true solutions, aerosols, foams.;
2. Continue developing general academic skills (exercise self-control; collaborate; use a computer, laptop, interactive whiteboard).
3. Continue to develop the skills of students to work independently with a textbook, additional literature, and Internet sites.

Educational tasks:

1. Continue to develop students’ cognitive interests;
2. To cultivate a culture of speech, hard work, perseverance;
3. Continue to develop a responsible, creative attitude to work;

Developmental tasks:

1. Develop the ability to use chemical terminology
2. Develop mental operations (analysis, synthesis, establishing cause-and-effect relationships, putting forward hypotheses, classification, drawing analogies, generalization, ability to prove, highlighting the main thing);
3. Develop interests and abilities of the individual;
4. Develop the ability to conduct, observe and describe a chemical experiment;
5. To improve the communication skills of students in joint activities (the ability to conduct a dialogue, listen to an opponent, substantiate their point of view) and the information and cognitive competence of students.

Preliminary preparation:

1. Formulation of the problem;
2. Forecasting practical results of work;
3. Organization of independent (individual, pair, group) activities of students in class and outside of class hours;
4. Structuring the content of the research work (indicating stage-by-stage results and indicating roles);
5. Research work in small groups (discussion, searching for sources of information);
6. Creating a slide presentation;
7. Defense of research work at a conference.

Equipment:

• List: “Terms and their explanations.”
• Table No. 6 “Dispersed systems” is displayed on the board and given to each table.
• On the demonstration table: samples of various disperse systems and a device for demonstrating the Tyndall effect.
• Computers, media projector.

Lesson #1. Dispersed systems, their types.

During the classes.

The introductory speech substantiates the need to study dispersed systems, emphasizing that dispersed systems are not a separate class of substances, as was previously thought when faced with colloidal systems (egg white, soy protein, etc.), but a state of substances, but not an aggregate state, but a state fragmentation of a substance, which determines its properties.

The meaning of the term “dispersed” is explained, definitions of a dispersed system, dispersed phase, and dispersed medium are given.

It is noted that dispersed systems surround us everywhere. These include air, water, food products, cosmetics, medicines, natural bodies (rocks, plant and animal organisms), as well as a variety of building and structural materials.

Samples of dispersed systems are demonstrated: tap water, solutions of various salts, egg white solution, alcohol extract of chlorophyll, office glue, milk, clay in water, the drug “Almagel”, nourishing cream, toothpaste, a piece of pumice, a piece of polystyrene foam, a mixture of vegetable oil and water, mayonnaise, aerosol cans.

It is noted once again that dispersed systems are understood as formations of two or more phases with a highly developed surface between them, and that the main feature of a dispersed system is a highly developed surface of the dispersed phase.

The classification of dispersed systems according to particle size (see diagram No. 1) and the state of aggregation of the dispersed phase and dispersed medium (see table No. 6) is considered.

Scheme No. 1.

Dispersed systems:

1. Coarsely dispersed (suspensions, emulsions, aerosols)
2. Finely dispersed (colloidal and true solutions)

Types of disperse systems. Table No. 6.

Dispersed systems		Type of dispersed system, its designation.	Examples of dispersed systems
Dispersed phase	Dispersive medium
Solid		Aerosol (t/g)	Dust, smoke, snow flakes
	Liquid (l)	Suspensions (t/l) Colloidal solutions (t/l) True solutions	Clay, toothpaste, lipstick. Egg white solution, blood plasma, alcohol extract of chlorophyll, silicic acid. Solutions of salts, alkalis, sugar.
	Solid (t)	Solid solutions (t/t)	Alloys, minerals, colored glasses.
Liquid		Aerosol (l/g)	Fog, clouds, drizzling rain, spray from an aerosol can.
	Liquid(l)	Emulsion (w/w) True solutions (l/l)	Milk, butter, mayonnaise, cream, ointments, emulsion paints. Lower alcohols + water, acetone + water.
	Solid (t)	Solid emulsion (w/t)	Pearl, opal.
Gas		No disperse system is formed
	Liquid (l)	Foam (g/l)	Soda foam, soap foam, whipped cream, whipped cream, marshmallow.
	Solid (t)	Solid foam (g/t)	Polystyrene foam, foam concrete, foam glass, pumice, lava.

Based on the data in Scheme No. 1 and Table No. 6, each type of dispersed system is characterized, and natural objects are classified on the demonstration table according to the most important types of dispersed systems.

The class is divided into 5 groups. Each group is asked to characterize a particular disperse system according to the plan below.

Plan.

1. Characteristics of a disperse system, examples where it occurs.
2. Properties (appearance, visibility of particles, ability to settle, ability to be retained by the filter, presence of charge).
3. Obtaining and destroying a dispersed system.
4. The importance of dispersed systems in everyday life and production processes, in environmental protection.

In accordance with the plan, participants in each group select material for the following types of dispersed systems: aerosols, emulsions, suspensions, foams, colloidal solutions or true solutions. Electronic textbooks and Internet materials are required. The material is downloaded to its own folder on the computer and used to create a presentation for a speech at a conference on the topic “Dispersed systems around us.”

In addition, each group receives a practical problem that was faced by chemists and was solved by specialists. The task is written on a card and given to the group leader.

Task No. 1.

The following method is known to reduce air dust: polluted air is passed through chambers in which ordinary water is sprayed. Droplets of water absorb dust particles and settle to the bottom of the chamber.

It is proposed to find a way to increase the degree of purification of dusty air using sprayed water.

(One of the answers can be found in the book by G.V. Lisichkin and V.I. Betaneli “Chemists Invent.” M., Prosveshchenie, 1990, p. 85).

Task No. 2.

Small droplets of fat are emulsified in the milk infusion medium. They gradually rise to the surface because their density is less than that of water. A layer of cream forms in the milk within a few hours. Milk is not a stable emulsion.

Milk sold from the dairy industry must be more resistant to separation. How can the stability of this emulsion be increased?

Task No. 3.

Suspensions are dispersed systems in which small solid particles are distributed in a liquid. Suspensions are unstable and gradually solid particles precipitate under the influence of gravity. The main method of separating solids from liquids in suspensions is filtration. At a pharmaceutical factory, the problem arose of quickly separating a suspension by filtration, and it was necessary to separate both the liquid and the solid phase suspended in it for further processing. To do this, the suspension was passed through a fine-mesh metal mesh filter. As sediment accumulated, the filtration rate decreased and, finally, the process practically stopped.

It is necessary to find a schematic diagram of a device that would allow the process of filtering the suspension to be carried out in a continuous mode.

(One of the answers can be found in the book by G.V. Lisichkin and V.I. Betaneli “Chemists Invent.” M., Prosveshchenie, 1990, p. 76).

Task No. 4.

To obtain heat and sound insulating polymer materials, they must be foamed ("expanded"), i.e. receive foam plastics. These are materials in which the solid polymer mass contains a large number of gas bubbles. One of the methods for producing foam plastics is the use of gas-forming substances. These substances decompose during polymerization, releasing gas.

It is necessary to propose substances that can be used as gas-forming agents and to compose equations for the reactions of their decomposition.

Task No. 5.

Find out what a hemostatic pencil is. Explain what its action is based on.

For the conference lesson, students in each group determine what visual aids they will use, i.e. what natural objects they will use during their group’s performance, what experiments they can demonstrate, what diagrams they can show, etc. In computer science class they are finalizing their presentations. Teachers can consult with any questions. The performance time of each group is limited: no more than 6-7 minutes.

To prepare for the conference, you can use the chemistry classroom library:

• Encyclopedic dictionary of a young chemist. M., Pedagogy, 1990.
• Petryanov I.V., Sutugin A.G. Ubiquitous aerosols. M., Pedagogy, 1989.
• Yudin A.M., Suchkov V.N. Chemistry in everyday life. M., Chemistry, 1982.
• Reference materials. M., Education, 1984.
• Davydova S.L. chemistry in cosmetics. M., Knowledge, 1990.
• G.V. Lisichkina and V.I. Betaneli “Chemists invent.” M., Education, 1990.

Lesson #2. Conference “Properties of disperse systems. The role of dispersed systems in everyday life, nature and production processes.”

Conference Lesson Plan:

1. Teacher's opening speech.
2. Messages from student groups (aerosols, emulsions, suspensions, foams, colloidal solutions, true solutions) - students use prepared presentations and demonstration material. Application .
3. Summing up the conference.

In the introductory speech, it is recalled what types of dispersed systems students have become acquainted with, where dispersed systems are found in life, and how they are classified.

Students defend their work in the form of a presentation and make notes by filling out pre-prepared reference tables.

Information about the studied disperse systems.

Characteristics of disperse systems.	Types of disperse systems.
	aerosols	emulsions	suspensions	Colloidal solutions	True solutions
Particle sizes
Appearance
Ability to settle
Receipt
Destruction
Meaning

In the final speech, the teacher once again notes the great practical importance of dispersed systems. They are used in the food industry, artificial silk production, textile dyeing, leather industry, agricultural production, soil science, medicine, construction and other sectors of the national economy. Knowledge about disperse systems, methods of formation and destruction, patterns of their behavior in natural processes allows us to solve scientific, technical and environmental problems.

Used Books:

1. Gabrielyan O.S. Chemistry 11th grade. – M. Bustard 2005.
2. Lagunova L.I. Teaching a general chemistry course in high school. – Tver, 1992
3. Politova S.I. General chemistry. Supporting notes. Grade 11. – Tver, 2006

The classification of dispersed systems can be carried out on the basis of various properties: by dispersity, by the aggregative state of phases, by the interaction of the dispersed phase and the dispersed medium, by interparticle interaction.

Classification by dispersion

The dependence of the specific surface area on the dispersion Ssp = f(d) is graphically expressed by an equilateral hyperbola (Fig.).

The graph shows that with a decrease in the transverse dimensions of the particles, the specific surface area increases significantly. If a cube with an edge size of 1 cm is crushed to cubic particles with dimensions d = 10 -6 cm, the value of the total interfacial surface will increase from 6 cm 2 to 600 m 2.

At d ≤ 10 -7 cm, the hyperbola breaks off, since the particles are reduced to the size of individual molecules, and the heterogeneous system becomes homogeneous, in which there is no interfacial surface. According to the degree of dispersion, dispersed systems are divided into:

• coarse systems, d ≥ 10 -3 cm;
• microheterogeneous systems, 10 -5 ≤ d ≤ 10 -3 cm;
• colloidal disperse systems or colloidal solutions, 10 -7 ≤ d ≤ 10 -5 cm;
• true solutions, d ≤ 10 -7 cm.

It must be emphasized that the particles of the dispersed phase in colloidal solutions have the largest specific surface area.

Classification according to the state of aggregation of phases

Classification according to the state of aggregation of phases was proposed by Wolfgang Ostwald. In principle, 9 combinations are possible. Let's present them in the form of a table.

Aggregate state of the dispersed phase	Aggregate state of the dispersed medium	Convention	System name	Examples
G	G	y/y	aerosols	Earth's atmosphere
and	G	w/g		fog, stratus clouds
TV	G	tv/g		smoke, dust, cirrus clouds
G	and	g/f	gas emulsions, foams	sparkling water, soap foam, therapeutic oxygen cocktail, beer foam
and	and	w/w	emulsions	milk, butter, margarine, creams, etc.
TV	and	TV/W	lyosols, suspensions	lyophobic colloidal solutions, suspensions, pastes, paints, etc. d.
G	TV	g/tv	solid foams	pumice, hard foams, polystyrene foam, foam concrete, bread, porous bodies in gas, etc. d.
and	TV	g TV	solid emulsions	water in paraffin, natural minerals with liquid inclusions, porous bodies in liquid
TV	TV	tv/tv	solid sols	steel, cast iron, colored glass, precious stones: Au sol in glass - ruby ​​glass (0.0001%) (1t glass - 1g Au)

Classification according to the interaction of the dispersed phase and the dispersed medium (by interphase interaction).

This classification is only suitable for systems with liquid dispersion media. G. Freundlich proposed dividing disperse systems into two types:

1. lyophobic, in which the dispersed phase is not able to interact with the dispersion medium and, therefore, dissolve in it; these include colloidal solutions and microheterogeneous systems;
2. lyophilic, in which the dispersed phase interacts with the dispersion medium and, under certain conditions, is capable of dissolving in it; these include solutions of colloidal surfactants and solutions of IUDs.

Classification by interparticle interaction

According to this classification, dispersed systems are divided into:

• freely dispersed (structureless);
• cohesively dispersed (structured).

In freely dispersed systems, particles of the dispersed phase are not associated with each other and are able to move independently in a dispersion medium.

In cohesively dispersed systems, particles of the dispersed phase are connected to each other due to intermolecular forces, forming unique spatial networks or frameworks (structures) in the dispersion medium. The particles that form the structure are not capable of mutual movement and can only perform oscillatory movements.

List of used literature

1. Gelfman M. I., Kovalevich O. V., Yustratov V. P. Colloidal chemistry. 2nd ed., erased. - St. Petersburg: Lan Publishing House, 2004. - 336 pp.: ill. ISBN 5-8114-0478-6 [p. 8-10]

No. 6. For classification of disperse systems, see table. 3.

CLASSIFICATION OF DISPERSE SYSTEMS Table ACCORDING TO STATE OF AGREGATION
Dispersive medium	Dispersed	Examples of some natural and household disperse systems
	Liquid	Fog, associated gas with oil droplets, carburetor mixture in car engines (gasoline droplets in the air), aerosols
	Solid	Dust in the air, fumes, smog, simooms (dust and sand storms), solid aerosols
Liquid		Effervescent drinks, foams
	Liquid	Emulsions. Liquid media of the body (blood plasma, lymph, digestive juices), liquid contents of cells (cytoplasm, karyoplasm)
	Solid	Sols, gels, pastes (jelly, jellies, glues). River and sea silt suspended in water; mortars
solid,		Snow crust with air bubbles in it, soil, textile fabrics, brick and ceramics, foam rubber, aerated chocolate, powders
	Liquid	Moist soil, medical and cosmetic products (ointments, mascara, lipstick, etc.)
	Solid	Rocks, colored glasses, some alloys