Subject and tasks colloid chemistry. Concept of the colloidal state of matter. Stages of development of colloidal chemistry. Classification of disperse systems.

Previously, colloid chemistry was considered a branch physical chemistry, and is now an independent discipline.

The subject of study of colloidal chemistry is heterogeneous mixtures of substances (dispersed systems), their properties, and processes occurring in these systems.

The tasks of colloidal chemistry are predicting the direction and studying the features of the occurrence of physicochemical processes in disperse systems.

Colloidal chemistry uses special methods research how electron microscopy, ultramicroscopy, ultracentrifugation, electrophoresis, nephelometry, etc.

To better understand the role of colloidal chemistry, let us briefly look at the history of the development of this science.

Colloidal systems began to be studied in the mid-19th century. In 1845, the Italian scientist Francesco Selmi found that some water-insoluble substances (for example, AgCl, S, Prussian blue) dissolve under certain conditions, form homogeneous solutions, and the precipitation is not accompanied by a change in temperature, i.e. . abnormal behavior of a substance. He called them pseudosolutions. Later, at the suggestion of K. Negeli, they received the name “sol”. In 1857 M. Faraday discovered hallmark pseudosolutions – light scattering.

The English scientist Thomas Graham is considered the founder of colloid chemistry. He studied Selmi's solutions and found (1861) that they differed from compounds that were highly soluble in water. These compounds in solution form not crystalline, but loose amorphous sediments, diffuse slowly, and do not pass through semi-permeable membranes with molecular-sized holes. This indicated big size particles of such compounds. Graham called the solutions and substances that form them colloids (from the gr. kolla - glue + eidos appearance), because He conducted experiments with gelatin, solutions of which are used as wood glue, and believed that glue was one of the representatives of these compounds. The main distinctive provisions of T. Graham’s “Colloid Chemistry” are as follows:

1) the properties of colloidal systems largely depend on the particle size of the dispersed phase;

2) all colloidal systems are capable of intense light scattering;

3) diffusion of dispersed particles in colloidal systems is expressed to a minimum extent;

4) colloidal systems are capable of dialysis;

5) colloidal systems are thermodynamically unstable.

One of the shortcomings of the concepts expressed by T. Graham was his division of all substances into two worlds. Graham believed that colloids by their nature differed from ordinary substances and divided all substances into two groups - crystalloids (ordinary substances that crystallize when the solution is saturated) and colloids (glue-like substances).

Later, the Russian botanist I.G. Borschov (1869) established the dependence of the diffusion rate of colloidal particles on their size and came to the conclusion that colloids have a crystalline structure.

At the beginning of the 20th century, P.P. Weimarn (1907–1912) studied about 200 substances and showed that the same substance can have the properties of a crystalloid under some conditions, and a colloid under others. Thus, rosin in alcohol forms a true solution, and in water - a colloidal solution, or when NaCl is dissolved in water, a true solution is formed, and in benzene - a colloidal one. Thus, it has been established that it is more correct to speak not about a colloidal substance, but about the colloidal state of a substance.

In 1903, the Czech scientist R. Zsigmondy and the German scientist G. Siedentopf designed an ultramicroscope, which can be used to make direct observations of particles of a colloidal solution.

Later (1907), F.F. Rayleigh, M. Smoluchovsky, A. Einstein established that the substance of colloidal solutions is not in the form of individual molecules or ions, but in the form of clusters - aggregates of molecules called micelles (from the Latin micella crumb, grain). A. Einstein and M. Smoluchowski developed the molecular statistical theory Brownian motion colloidal particles and the theory of fluctuations. J.B.Perrin, T.Svedberg tested this theory by determining Avogadro's number in independent ways. At the beginning of the 20th century, W. Ostwald quite fully studied the influence of the state of aggregation and dispersion on the properties of colloidal objects.

In 1920, N.P. Peskov introduced 2 concepts (types) of stability of dispersed systems: aggregative and sedimentation stability. The theory of the structure of the double electric layer was developed in the works of H. Helmholtz and J. Perrin (80s of the twentieth century), G. Gouy and D. Chapman (1910 and 1913), O. Stern (1924) and later in the middle of the twentieth century in the works of A.N. Frumkin.

P.P. Weymarn studied in detail condensation methods for the formation of lyosols. The theory of the formation of amorphous and crystalline particles during the synthesis of colloidal systems was studied by V.A. Kargin. F.F. Rayleigh, and later L.I. Mandelstam, P. Debye developed the fundamentals of the theory of light scattering by inhomogeneities in the medium and successfully applied these concepts to the analysis of colloidal systems. In 1908, G. Freundlich formulated the main principles of the adsorption theory of coagulation. B.V. Deryagin, A.D. Landau, E. Verwey, T. Overbeck developed (1939-1943) and developed physical theory coagulation. G. Kroyt proposed the theory of coagulation of the IUD.

Currently, dispersed systems in which the particle size is 1–100 nm (or 1.10–7–1.10–9 m) are considered colloidal. These boundaries are conditional, because There are systems with more or less large particles that have the properties of colloidal solutions and those, having the same sizes, do not exhibit the properties of colloidal solutions. Therefore, it can be noted that a colloidal system is a dispersion of one body in another, and colloidal chemistry studies physical laws surface phenomena and the resulting properties of colloidal solutions. It follows that colloidal chemistry is the science of the properties of heterogeneous highly dispersed systems and the processes occurring in them.

It should be noted that there are substances with very large molecules - high molecular weight compounds (proteins, cellulose, rubber and other polymers). The molecules of such compounds can exceed the size of colloidal particles; their solutions can have many of the properties of colloidal solutions, but are not clusters of molecules. They cannot be classified as typical colloidal systems. To differentiate, they are called IUD solutions. IUD solutions are also objects of study in colloidal chemistry.

Colloidal systems and solutions of IUDs are widespread in nature. Proteins, blood, lymph, carbohydrates, pectins are in a colloidal state. Many industries (food, textile, rubber, leather, paint and varnish, ceramics, artificial fiber technology, plastics, lubricants) are associated with colloidal systems. Production building materials(cement, concrete, cementitious mortars) is based on knowledge of the properties of colloids. Coal, peat, mining and oil industry deal with dispersed materials (dust, suspensions, foams). Special meaning colloidal chemistry acquires in the processes of mineral processing, crushing, flotation and wet concentration of ores. Photographic and cinematographic processes are also associated with the use of colloidal disperse systems.

The objects of colloidal chemistry include all the diversity of forms of plant and animal life, in particular, typical colloidal formations are muscle and nerve cells, cell membranes, fibers, genes, viruses, protoplasm, blood. Therefore, colloid scientist I.I. Zhukov stated that “man is essentially a walking colloid.” In light of this, technology medicines(ointments, emulsions, suspensions, aerosols, powders), the effect of various drugs on the body cannot be imagined without knowledge of colloid chemistry.

Dispersed system. Dispersion measure.

Disperse systems are called heterogeneous (heterogeneous) mixtures of substances in which one finely divided substance is evenly distributed in a homogeneous medium (mass) of another substance.

Dispersed systems consist of a dispersed phase and a dispersion medium. Dispersed phase (DP) is a collection of small particles of a substance distributed (dispersed) in a homogeneous medium of another substance.

A dispersion medium is a homogeneous medium in the form of molecules or ions in which small particles of another substance are evenly distributed.

A disperse system, in contrast to homogeneous (true) solutions, is characterized by heterogeneity and dispersity.

Heterogeneity is the multiphase nature of the system, i.e. the presence of phase boundaries, which is due to the insolubility of the substance of one phase in another. Since only between such substances can physical interfaces exist.

Dispersity is a measure of the fragmentation of one substance in a dispersed system. According to A.V. Dumansky (1913), a measure of the fragmentation of a disperse system can be the transverse particle size (R) or the degree of dispersion (D): D = 1/R (m ─1). How smaller size particles, the greater the degree of dispersion. Systems with different sizes particles are called polydisperse, and with particles of the same size - monodisperse. Since the particle sizes in real systems are different, the degree of dispersion does not very accurately characterize the system. Therefore, in 1909, W. Ostwald proposed using specific surface area (S sp.) as a measure of fragmentation: , where S d.f. and V d.f. – surface area and volume of the dispersed phase. The specific surface area can be calculated if the size and shape of the particles are known: in the case of cubic particles, and in the case of spherical particles: . Where l– length of the edge of the cube, r and d – radius and diameter of the sphere. All indicators are interconnected by the equation S beat. = k. D = k/R. As can be seen from the equation, the specific surface area is related to dispersion. With increasing dispersion, the specific surface area increases sharply, for example, if R = 0.1 cm, then Ssp. = 30 cm - 1, and when R = 10 - 7 cm, then S beat. will be 30 cm +7 cm - 1, i.e. 1 cm 3 of these particles have an interphase surface equal to 3000 m 2. An increase in the specific surface area determines the specific properties of dispersed systems associated with surface phenomena.

Classification of disperse systems.

Dispersed systems are classified according to particle size, the state of aggregation of substances, and the intensity of interaction between the phases of the system. They also differ in the rate of diffusion of particles, in their ability to pass through membranes and filters, and in light scattering.

By particle size distinguish molecularly dispersed (r< 1 . 10 –9 м), коллоидно-дисперсные (1 . 10 –7 –1 . 10 –9 м), микрогетерогенные (1 . 10 –4 –1 . 10 –7 м) и грубодисперсные системы (r >1 . 10 –4 m).

Molecular disperse systems or true solutions. In these systems, molecules or ions do not have a surface in the usual sense and therefore are not considered a disperse system. They are isolated only to compare the properties of colloidal solutions and microheterogeneous systems. The particle size is less than 1 nm or 1. 10 –9 m. The substance is crushed to the limit and therefore such systems are completely homogeneous. These systems are thermodynamically stable: due to their small size, the particles have a high diffusion rate, they pass through semi-permeable membranes and filters, and are not visible in an optical microscope. True solutions are transparent and do not scatter light. Examples of true solutions are aqueous solutions of highly soluble salts, organic compounds, fats in organic solvents, mixtures of gases, etc.

Colloidal disperse systems. The particle sizes of the dispersed phase in such systems range from 1–100 nm (or 1.10–7–1.10–9 m). These particles, although not too large, have an interface, which is why colloidal systems are sometimes called ultramicroheterogeneous. Colloidal systems are thermodynamically unstable; colloidal particles are capable of diffusion, pass through paper filters, but do not pass through semi-permeable membranes, are retained on ultrafilters, are not visible in an optical microscope, but are observed in ultramicroscopes, have electric charge(electric double layer), move in electric field. Colloidal solutions are transparent, but scatter light (exhibit the Faraday-Tyndall effect). Examples of colloidal systems are smoke, fog, and liquid colloidal solutions of compounds that are difficult to dissolve in water.

Microheterogeneous systems(suspensions, powders, emulsions, foams). Particle size 1. 10 –4 –1. 10 –7 m. These systems are thermodynamically unstable: they are destroyed quite quickly due to the settling of particles. The particles are not capable of diffusion, do not even pass through paper filters, and are visible in an optical microscope. Solutions are cloudy due to the absorption of light, reflection and refraction of its particles. Examples: suspensions of clay, silt, sand in water, clouds of dust, powders, etc.

Classification according to the state of aggregation of the dispersed phase and dispersion medium (according to W. Ostwald)

Considering that a substance can be found in three states of aggregation, 8 combinations of dispersion medium and DF are possible:

T–G T–F T–T

Classification according to the interaction of the dispersion medium and the dispersed phase (according to G. Freundlich)

This classification is only suitable for systems with a liquid dispersion medium.

If the particle surface and the solvent molecule have the same polarity (i.e. affinity), then they will interact with each other. Therefore, thick multilayer solvation shells are formed around colloidal particles. Freundlich called such systems lyophilic (from the gr. lyo liquid + phileo love). Examples of such systems are solutions of protein, starch, agar-agar, gum arabic, highly concentrated emulsions, emulsols. In the case when the particles and molecules of the solvent are oppositely polar, then there is no interaction between the colloidal particles and the dispersion medium, which means that there are no solvation shells, or thin solvation shells are formed. Such systems were called lyophobic colloidal solutions (from the gr. lyo liquid + phobos fear). In the case when the dispersion medium is water, these systems are called hydrophilic and hydrophobic, respectively.

Lyophobic systems include typical colloidal systems formed by substances that are difficult to dissolve in a dispersion medium (weak bases, some salts, metals, aerosols, foams).

Lyophilic systems do not have all the typical colligative properties; they dissolve spontaneously, are thermodynamically stable, and form homogeneous solutions. Therefore, lyophilic systems are currently distinguished as special groups dispersed systems – solutions of high-molecular substances (proteins, polysaccharides, nucleic acids) and micellar surfactant solutions.