Candidate of Chemical Sciences, Associate Professor

Topic 1. Surface phenomena and adsorption

Lesson 1. Introduction. Classification and methods of production

dispersed systems

Lectures

Saratov – 2010

Introduction

1. Introduction. Subject and tasks of colloid chemistry.

2. Classification of disperse systems.

3. Methods for obtaining dispersed systems

3. Free surface energy and surface tension.

Conclusion

LITERATURE

1. Frolov colloid chemistry. – M.: Chemistry, 1989. – P. 10-20, 115-127.

2. Gelfman M., Kovalevich O., Yustratov V. Colloid chemistry. – St. Petersburg: “Lan”, 2003. – P. 6-15.

VISUAL AIDS AND APPLICATIONS

1. Slides No. 1,2,3,4:

Classification of disperse systems

Methods for obtaining dispersed systems

Specific surface

Surface tension coefficients

INTRODUCTION

The discipline “Surface phenomena and adsorption” was previously called “Colloidal Chemistry”. Colloidal chemistry is studied after completing other chemical sciences (inorganic, analytical, physical, organic chemistry), and this is no coincidence.

Having mainly real substances and materials as objects of research, colloidal chemistry completes general chemical education. At the same time, it is a frontier field of knowledge that combines physical chemistry and the physics of surface phenomena and dispersed systems and considers many natural processes that have not previously received attention. Therefore, colloidal chemistry plays an important role in scientific and technological progress. It is almost impossible to name an industry in which there would be no colloidal chemical processes (food industry, artificial silk production, textile dyeing, leather industry, agriculture, soil science, medicine, military chemistry, etc.).

1. INTRODUCTION. SUBJECT AND TASKS OF COLLOID CHEMISTRY

The task of colloidal chemistry is the study of heterogeneous systems with a highly developed phase interface. Such systems are called dispersed .

One of the phases of the dispersed system is usually highly crushed and is called dispersed phase . The dispersed phase in a disperse system is distributed in the volume of a continuous phase called dispersion medium . The number of dispersed phases in a dispersed system can generally be unlimited.

The founder of colloidal chemistry is rightfully considered the English chemist Thomas Graham (G.G.), who first gave general ideas about disperse systems and developed some methods for their study (1861). While studying the diffusion of substances in solutions, Graham noted the slow diffusion of particles of colloidal solutions and their inability to penetrate membranes, unlike molecules of ordinary solutions. Comparing ordinary solutions with colloidal (sols), Graham came to the conclusion that it was necessary to separate substances into “crystalloids” and “colloids”.

At the beginning of the 20th century, a professor at the St. Petersburg Mining Institute showed that there is no “special type of colloid” and that the same substance, depending on the conditions of dissolution, can be both a “crystalloid” and a “colloid”. Thus, the idea of ​​the colloidal state of matter was established, which Weimarn considered to be the universal state of matter.

Dispersed systems are the most typical and at the same time complex objects of colloidal chemistry, because they exhibit the whole variety of surface phenomena that form the special volumetric properties of these systems.

The majority of real bodies around us are dispersed systems, so there is reason to call the science of surface phenomena and dispersed systems the physics and chemistry of real bodies. Almost all bodies in the world around us are dispersed. These are polycrystalline, fibrous, layered, porous, granular and other substances consisting of filler and binder, as well as substances in the state of suspensions, pastes, emulsions, foams, dust, etc. Soil, bodies of flora and fauna, clouds and fogs, many industrial products, building materials, metals, polymers, paper, leather, fabrics, food - all these are dispersed systems whose properties are studied by colloidal chemistry.

The universality of the dispersed state, the presence of external and internal surfaces in most real bodies determine the fundamental and general scientific nature of colloidal chemistry.

Let's get acquainted with the basic concepts of colloidal chemistry.

Colloid chemistry is the science of surface phenomena and dispersed systems, their physical, chemical and mechanical properties. Another name for colloidal chemistry is used - Surface phenomena and disperse systems, which more accurately reflects the subject of study of this science.

Thus, subject The study of colloidal chemistry is dispersed systems and surface phenomena. Let's consider the relationship between these concepts.

TO superficial phenomena These include processes occurring at the phase boundary, in the interphase surface layer of conjugate phases.

Dispersed system is a two- or multiphase, i.e., heterogeneous system in which one of the phases is represented by very small particles, the sizes of which, however, significantly exceed molecular ones. The disperse system consists of dispersed phase And dispersion medium.

Dispersed phase – this is the crushed phase of the dispersed system. Particles of the dispersed phase can have a spherical or cubic shape, as well as the shape of long thin filaments (fibrillar systems), very thin films, and capillaries.

Dispersive medium – continuous medium in which the dispersed phase is distributed.

The measure of fragmentation of the dispersed phase is dispersion .

Dispersity D is the reciprocal of the particle size. For spherical particles this is the diameter d, for cubic particles this is the edge of the cube l . Hence

(1)

The finer the particles are crushed (i.e., the higher the dispersion), the larger the total surface of the particles of the dispersed phase, i.e., the larger the phase interface. Therefore, an important characteristic of disperse systems is specific surface .

Specific surface – interfacial surface per unit volume or per unit mass of the dispersed phase

; , (2)

where Ssp. – specific surface, m2;

Vd. f. – volume of dispersed phase, m3;

m d.f. – mass of the dispersed phase, g or kg.

Formulas (2) are also valid for one particle of the dispersed phase. A simple calculation shows that as the particle size decreases, the specific surface area increases. For a cubic particle with an edge  , volume V = 3, and surface area S = 62 (6 sides of a cube with area 2).

(3)

From formula 3 it follows that the smaller , the greater Ssp (see Table 1).

To make sure that the specific surface area increases with increasing degree of dispersion, consider a cube with an edge length of 1 cm (Fig. 1). The volume of a cube is 1 cm3, the surface area of ​​six squares with a side of 1 cm is 6 cm2. Specific surface Ssp = 6 cm2 / 1 cm3 = 6 cm2 / cm3. Let's break this cube into smaller cubes with an edge size of 1 mm and calculate the specific surface area. 10*10*10 = 1000 cubes were formed. The total volume of all cubes remained equal to 1 cm3. The surface area of ​​each cube is 6 mm2. The total surface area of ​​a thousand cubes is 1000 * 6 mm2 = 6000 mm2 = 60 cm2. We obtain the specific surface area by dividing the surface area by the volume Ssp = 60 cm2 /1 cm3 = 60 cm2 / cm3. Please note that you cannot abbreviate units (cm) in this expression, since these units refer to different phases - cm2 refers to the interphase area, and cm3 refers to the volume of the dispersed phase. Comparing the results of calculating the specific surface area of ​​an uncrushed cube and a crushed one, we come to the conclusion that the phase interface has increased 10 times.

Fig.1. Dependence of specific surface area on particle size

If the crushing process is continued further, then by making the necessary calculations, we can be convinced that as the particle size decreases, the specific surface area increases. The data in Table 1 confirms this. Thus, for particles with an edge size of 1 nm, the specific surface area increases to 6000 m2/cm3.

Table 1

Specific surface area of ​​cubic bodies depending on

depending on the degree of grinding

Similar calculations can be performed for particles of other shapes; they will give similar results. Thus, disperse systems have a large phase interface. It can reach several thousand m2 per 1 g of dispersed phase.

The above examples show that dispersed systems and surface phenomena are inseparable: in dispersed systems with their highly developed surface, it is surface phenomena that determine the specific properties of these systems and the ways to control these properties.

Unlike other areas of chemistry, which are primarily interested in the bulk properties of phases, colloidal chemistry focuses on surface phenomena.

General signs of objects colloid chemistry are as follows:

– heterogeneity (particles of the dispersed phase, despite their small sizes, represent an independent phase);

– large specific surface area (therefore, surface phenomena have a great influence on the properties);

– high dispersion (small particle sizes affect the optical, kinetic and other properties of systems).

From all of the above it follows tasks colloid chemistry:

– study of surface phenomena and properties of surface layers;

– study of the conditions for the production and existence of dispersed systems and factors affecting their stability;

– study of molecular-kinetic, optical, electrical, mechanical and other properties of disperse systems.

2. CLASSIFICATION OF DISPERSE SYSTEMS

Classification of dispersed systems is carried out according to various criteria.

Classification according to the degree of connectivity of dispersed phase particles

Freely dispersed systems – dispersed systems in which particles of the dispersed phase are mobile. In such systems, small particles of the dispersed phase move freely in liquid or gaseous dispersion medium. These are emulsions, aerosols, suspensions, etc.

Cohesively dispersed systems – dispersed systems in which particles of the dispersed phase or dispersion medium are interconnected and cannot move freely. This class includes dispersed systems with solid dispersion environment, namely all capillary-porous bodies (soils, soils, rocks, adsorbents, activated carbons), as well as gels and jellies, in which a continuous spatial network (matrix) includes very small cells filled with liquid or gas (jelly, frozen glue, marmalade).

Classification by degree of dispersion

Let's consider this classification for freely dispersed systems.

1.Coarsely dispersed (microheterogeneous) systems – systems with particle sizes from 100 donm (10-5 – 10-3 cm). Dispersed phase particles contain more than 109 atoms.

Coarse systems include: powders, suspensions, emulsions, foams, fumes. These systems are unstable, stratify when standing, their particles are visible under a microscope, and they are retained by a paper filter.

2. Colloid-dispersed (ultramicroheterogeneous) systems – systems with particle sizes from 1 to 100 nm (10-7 – 10-5 cm). Dispersed particles contain from 103 to 109 atoms.

Such systems are called colloidal (colloidal solutions) or sols . There are solid sols ( solidozols ) with a solid dispersion medium, lyosols with a liquid dispersion medium and aerosols with a gaseous medium.

Particles of colloidal systems are invisible in a regular microscope, pass through a paper filter, and are stable for a long time.

3. Molecular dispersed systems – these are true solutions, with a particle size of ~10-8 cm (less than 103 atoms). True solutions are homogeneous systems; they are not the subject of study of colloidal chemistry; their properties differ sharply from the properties of heterogeneous colloidal solutions.

For cohesively dispersed systems which include porous bodies, another classification is applicable: microporous (pore sizes up to 2 nm), transition-porous (2-200 nm) and macroporous (above 200 nm). It is more convenient to classify other dispersed systems with a solid dispersion medium by dispersion in the same way as freely dispersed ones.

In general, the above classification can be presented in the form of a diagram.

This classification is the most common. It is based on the state of aggregation of particles of the dispersed phase and the dispersion medium. The combination of three states of aggregation (solid, liquid, gaseous) allows us to distinguish nine types of dispersed systems - for brevity, they are conventionally designated as a fraction, the numerator of which indicates the aggregative state of the dispersed phase, and the denominator - the dispersion medium. For example, the designation t/f shows that the system consists of a solid dispersed phase and a liquid dispersion medium (solid in liquid). Table 2 shows possible options for dispersed systems and examples of different types of dispersed systems.

Classification according to the state of aggregation of phases

Mixtures of gases are, generally speaking, homogeneous systems. However, in this case, one should take into account the microheterogeneity of this system, caused by fluctuations (oscillations) of density. It is the presence of density fluctuations and the scattering of light on them that explains the blue color of the sky: if the atmosphere were completely homogeneous, then the sky would be black.

Table 2

Classification of disperse systems according to the state of aggregation of phases

Dispersive	Dispersed phase
	Solid	Liquid
	Suspensions and sols: industrial suspensions, suspensions, pastes, sludges, medicines, natural waters	Emulsions : natural oil, milk, creams, medications	Foam : flotation, fire, soap
	Solid heterogeneous systems: minerals, alloys, concrete, composite materials, plastics	Capillary systems: gels, liquid in porous bodies, in adsorbents, soils, soils, tissues of living organisms, pearls	Porous bodies: adsorbents and catalysts in gases, activated carbons, foam concrete, polyurethane foam, pumice, aerated chocolate
Gaseous	Aerosols: dust, fumes, powders, cirrus clouds, bacteria in the air	Aerosols: fogs, including industrial fogs, cumulus clouds, Earth's atmosphere	mixture of gases

3. METHODS FOR OBTAINING DISPERSE SYSTEMS

Let us briefly discuss the methods for obtaining dispersed systems. As is known, sols, in terms of the particle size of the dispersed phase, occupy an intermediate position between true solutions and suspensions, therefore, naturally, they can be obtained either by combining individual molecules or ions of a dissolved substance into aggregates, or as a result of the dispersion of relatively large particles. In accordance with this, Svedberg divides methods for the synthesis of colloidal systems into condensation and dispersion . The method stands apart from these methods peptization , which consists of transferring sediments, the primary particles of which already have colloidal sizes, into a colloidal solution. Finally, in some cases, colloidal systems can be formed by spontaneous dispersion of the dispersed phase in a dispersion medium.

The main two conditions for obtaining colloidal systems, regardless of the synthesis methods used, are: the insolubility of the dispersed phase in the dispersion medium and the presence in the system in which the particles are formed of substances capable of stabilizing these particles. Such substances can be either foreign substances specially introduced into the system, or compounds formed during the interaction of the dispersed phase with the dispersion medium.

Dispersion methods for obtaining disperse systems

Dispersing is the grinding of solids and liquids in an inert (not interacting with the substance being ground) environment, in which the dispersion sharply increases and a dispersed system is formed with a significant specific interfacial surface area. In contrast to dissolution, dispersion, as a rule, does not occur spontaneously, but with the expenditure of external work spent on overcoming intermolecular forces when crushing the substance.

The dispersion process is of great practical importance in a number of industries and technological processes: in the production of highly dispersed powders, pigments for paints, in the grinding of mineral ores, in the production of flour and other food products, etc.

Various dispersion methods are known.

To obtain coarse systems, ball mills are used, which are hollow, rotating cylinders containing a certain amount of steel or ceramic balls. When the cylinder rotates, these balls roll, crushing and abrading the crushed material. Ball mills produce powders, cement, thickly ground paints, etc.; the particle size of the dispersed phase in them can be increased only to 1000 nm. For finer grinding - up to 100 nm and less - colloidal mills are used, in which the crushed material (coarse suspension), passing through the gap between the rotating rotor and the mill body, is subjected to further grinding. Watercolor paints, powder, medicines, etc. are produced in colloid mills.

Condensation methods for obtaining dispersed systems

Condensation methods, compared to dispersion methods, make it possible to obtain colloidal systems of higher dispersion.

Condensation methods for producing dispersed systems are based on creating conditions under which the future dispersion medium is supersaturated with the substance of the future dispersed phase. Depending on the methods for creating these conditions, the condensation method is divided into physical And chemical .

Physical methods include:

A) Vapor condensation by passing them through a cold liquid, resulting in the formation of lyosols. Thus, when vapors of boiling mercury, sulfur, and selenium are passed into cold water, their colloidal solutions are formed.

b) Solvent replacement . The method is based on the fact that the substance from which one wants to obtain a sol is dissolved in a suitable solvent, then a second liquid is added, which is a poor solvent for the substance, but mixes well with the original solvent. The initially dissolved substance is released from the solution in a highly dispersed state. For example, in this way you can obtain hydrosols of sulfur, phosphorus, rosin, and paraffin by pouring their alcohol solution into water.

Chemical condensation differs from all the methods discussed above in that the dispersible substance is not taken in finished form, but is obtained directly in solution by a chemical reaction, as a result of which the desired compound, insoluble in the given medium, is formed. The task boils down to obtaining the precipitate that falls out in a finely dispersed state. In chemical condensation methods, any reactions leading to the formation of a new phase are used: double exchange reactions, decomposition, oxidation-reduction, etc. The colloidal solution is usually stabilized by one of the reaction participants or a by-product, from which the particles are formed at the particle-medium interface adsorption layers of ionic or molecular type that prevent particles from sticking together and settling.

4. FREE SURFACE ENERGY AND SURFACE TENSION

Surface phenomena are of particular importance for the properties of dispersed systems with a developed phase interface. Surface phenomena are associated with processes such as wetting and spreading of liquids over the surface, adhesion, washing, surface adsorption, capillary phenomena, and flotation. Various technological processes are based on these phenomena: dyeing and printing, heterogeneous catalysis, the use of binding materials and adhesives, the manufacture of gas masks, and wastewater treatment. Knowledge of the nature of surface phenomena is necessary for a military chemist, since it is these processes that are associated with the contamination of the surfaces of military equipment and their degassing, special treatment of uniforms, and the operation of gas masks.

Any phase interface is very different in physical and chemical properties from both contacting phases. Let's take two phases in contact: gas and liquid, consider the behavior of liquid molecules inside the volume and on the surface (Fig. 1)

Fig.2. Action of intermolecular forces in volume and on the surface

There is intermolecular interaction between molecules. If a molecule is inside, it experiences attraction from all neighboring molecules. The resultant of all these forces is equal to 0. A molecule located on the surface experiences attraction only from internal molecules (the gas, due to its rarefied state, interacts weakly), the resultant of these forces is directed into the body, i.e., the tendency to draw surface molecules into the body is clearly expressed, the surface of the body seems to be in a tense state and tends to contract. Since the action of forces on surface molecules is not compensated, such molecules have free surface energy. Let's give a definition.

Free surface energy – this is the excess energy of the molecules of the surface layer compared to the molecules located inside DE = E* – Eavg.

This energy depends on the nature of the substance of the contacting phases, on the temperature and area of ​​phase separation.

where Fs is free surface energy, J;

s – phase interface area, m2;

s – proportionality coefficient, called the surface tension coefficient (or simply surface tension), J/m2.

As you know, any system strives for a minimum of energy. To reduce the free surface energy (Fs = ss), the system has two ways: reduce the surface tension s or the interfacial area s.

A decrease in s occurs during the adsorption of substances on solid and liquid surfaces (this is the driving force of adsorption), when one liquid spreads over another.

The desire to reduce the surface area S leads to the merging of particles of the dispersed phase, to their enlargement (at the same time, the specific surface area is reduced), i.e., this process is the cause of the thermodynamic instability of dispersed systems.

The tendency of a liquid to reduce its surface area leads to the fact that it tends to take the shape of a sphere. Mathematical calculations show that a sphere has the smallest area at a constant volume, so liquid particles take on a spherical shape, unless these drops are flattened by gravity. Drops of mercury on the surface take the form of balls. In zero gravity, all liquids take the shape of a ball; the spherical shape of the planets is also attributed to the action of surface forces.

Surface tension

The physical meaning of the surface tension coefficient can be interpreted from different points of view.

1.Free surface energy (specific surface

energy)

From expression (3) it follows

https://pandia.ru/text/77/498/images/image009_29.gif" width="57" height="48"> [J/m2], (6)

where W is the work to create a new phase interface, J;

S – interface area, m2.

From expression (5) it follows that s is the work that must be done in order to increase the interface area of ​​the phases by one unit under isothermal conditions with a constant volume of liquid (i.e., transfer the appropriate number of liquid molecules from the volume to the surface layer).

For example, when a liquid is splashed, work is done, which turns into free surface energy (when splashed, the interface between the phases increases many times over). The same work is expended when crushing solids.

Since surface tension is associated with the work expended on breaking intermolecular bonds when transferring molecules from the bulk to the surface layer, it is obvious that surface tension is a measure of the forces of intermolecular interaction inside a liquid. The more polar the liquid, the stronger the interaction between molecules, the stronger the surface molecules are drawn inward, the higher the value of s.

Among liquids, water has the largest s value. This is no coincidence, since fairly strong hydrogen bonds are formed between water molecules. In nonpolar hydrocarbons, only weak dispersion interactions exist between molecules, so their surface tension is low. The value of s is even greater for liquid mercury. This indicates significant interatomic interaction (and a large amount of free surface energy).

Solids are characterized by a high s value.

3.Surface force

There is also a force interpretation of surface tension. Based on the dimension of the surface tension coefficient J/m2, we can write

Thus, surface tension is the surface force applied per unit length of the contour delimiting the surface and aimed at reducing the interface phases .

The existence of this force is clearly illustrated by Dupre's experience. A movable jumper is fixed to a rigid wire frame (Fig. 2). A soap film is stretched in the frame (position 1). To stretch this film to position 2, it is necessary to apply a force F, which is counteracted by the surface tension force F2. This force is directed along the surface (tangentially), perpendicular to the contour limiting the surface. For the film in Fig. 2, the role of part of the circuit is played by a movable jumper.

Rice. 3. Dupre's experience

Hence,

where F is the force tightening the surface contour, N;

 – contour length, m.

The action of surface tension can be visualized as a set of forces that pull the edges of a surface towards the center (therefore this force is called surface tension). These forces are shown in Fig. 3 arrows – vectors; the length of the arrows reflects the magnitude of surface tension, and the distance between them corresponds to the unit length of the contour.

Rice. 4. Action of surface tension forces

Thus, surface tension forces have the following properties:

1) evenly distributed along the phase separation line;

Surface tension occurs at all phase interfaces; in accordance with the state of aggregation of these phases, the following designations have been introduced:

sJ-G (at the liquid-gas boundary)

sZh1-Zh2 (at the boundary of two immiscible liquids)

sТ-Г (at the solid-gas boundary)

sТ-Л (at the solid-liquid boundary)

Surface tension at the liquid–gas and liquid–liquid interface can be determined directly experimentally. Methods for determining surface tension at the interface with a solid are based on indirect measurements.

CONCLUSION

Today we got acquainted with the basic concepts of colloidal chemistry, and moved on to consider surface phenomena that have a large role in nature and technology. In the next lecture we will continue our acquaintance with such surface phenomena as adhesion and cohesion, wetting and spreading, adsorption.

Associate Professor of the Department of Physical Education

The production of dispersed systems is primarily associated with the production of dispersed particles. The following tasks need to be solved:

• 1) distribute dispersed particles in a dispersion medium to the required concentration;
• 2) stabilize the dispersed system in order to preserve its structure and properties for a sufficiently long time;
• 3) clean the dispersed system from various impurities.

These problems are solved depending on the specifics (type) of a particular disperse system.

Preparation of dispersed systems

Emulsions. Since emulsions are coarse systems, they are usually prepared using the dispersion method. Liquids that are to form an emulsion are vigorously mixed or subjected to mechanical vibrations or ultrasound. To obtain droplets of the same size (i.e., a monodisperse system), homogenization is performed. This process consists of forcing a dispersed phase liquid into a dispersion medium through small holes of the required diameter under high pressure. This technique is used, for example, when processing milk. As a result of homogenization, the average size of fat droplets decreases from approximately 1 -3 to 0.1 -0.2 microns.

Emulsions are also obtained by condensation methods (usually by replacing the solvent).

An independent task is the production of highly concentrated emulsions. These include emulsions with a dispersed phase concentration of more than 74 vol. %, up to 99 vol. %. Droplets of the dispersed phase in such emulsions, having the shape of multifaceted prisms, are separated by thin films of a liquid dispersion medium.

Concentrated emulsins can have the mechanical properties of solids - strength and elasticity.

The specificity of preparing concentrated emulsions is that the dispersed phase is introduced into the dispersion liquid medium in small portions with vigorous stirring.

Foam. Like emulsions, foams are coarse systems. Therefore, in many technological processes, foams are obtained by the same dispersion methods that are used to obtain gas bubbles.

Condensation methods for producing foams are based on the supersaturation of a gas solution in a given liquid with a corresponding change in temperature or pressure. Chemical reactions that release gas are also used. As an example, we give the reaction underlying the preparation of foam in fire extinguishers:

NaHCO 3 + HCl > NaCl + H 2 O+ CO 2 ^

Another condensation method for producing foams is based on the use of microbiological processes.

Colloidal solutions. Colloidal solutions (sols) are obtained by various condensation methods. To obtain highly dispersed sols, it is necessary to ensure that the following condition is met: the rate of formation of solid particles must be many times higher than the rate of their growth. To fulfill this condition, when producing dispersed particles using chemical reactions, the following method is often used: a concentrated solution of one component is poured in a small amount into a highly diluted solution of another component with very vigorous stirring.

Gels. The above systems are freely dispersed. The production of coherently dispersed systems has certain specifics. Let us consider the preparation of gels as an example. They are usually obtained from colloidal solutions (sols). Under certain conditions, dispersed particles stick together and a coagulation process occurs.

If the particles have an anisodiametric shape (rods, ellipsoids), then they are connected predominantly by their ends and form a spatial structure (network), in the cells of which there is a liquid dispersion medium. The process of converting sols into gels is called sol-to-gel transition. It is important in nanotechnology. Thus, gels, like concentrated emulsions, can sometimes be bicontinuous disperse systems.

The properties of gels are very effectively controlled by changing the concentration of the dispersed phase and the shape of dispersed particles. Another important factor is temperature: its increase makes it difficult to form contacts between dispersed particles and therefore the strength of the gels decreases.

Methods for obtaining dispersed systems

Lecture 20. Electrokinetic phenomena

Self-test questions

1. What is the difference between adsorption on a solid surface and adsorption on a liquid surface?

2. What is physical and chemical adsorption, what is their essence?

4. On what principles is Langmuir’s theory of monomolecular adsorption based?

5. Give the equation for the Langmuir adsorption isotherm. What is limiting adsorption?

6. Consider the Freundlich equation. Under what conditions and for what systems is it applicable?

7. Explain the principle of graphically determining adsorption constants using the Freundlich equation?

20.1 Methods for obtaining dispersed systems

20.2 Electrophoresis, electroosmosis, sedimentation and percolation potentials

20.3 Electrokinetic potential and its definition

A chemical substance can be obtained in a colloidal state under the following conditions:

1) the particle size of a given substance must be brought to colloidal sizes (10−5–10−7 cm), which can be done by two methods: a) crushing the particles of the substance to the size of a colloidal degree of dispersion (dispersion methods); b) enlargement of molecules, atoms, ions to particles of colloidal size (condensation methods);

2) the presence of a stabilizer, for example, electrolyte ions, which form an ionic hydrate shell on the surface of colloidal particles and create a charge that prevents particles from sticking together when they collide in a solution;

3) colloidal particles (dispersed phase) must have poor solubility in a dispersion medium, at least at the time of their preparation.

If these conditions are met, colloidal particles acquire an electrical charge and a hydration shell, which prevents them from precipitating.

Dispersion methods for producing colloidal systems are based on grinding relatively large particles of the dispersed phase substance to colloidal sizes by mechanical, electrical, chemical, and ultrasonic dispersion. Chemical methods of dispersion also include the so-called. spontaneous dispersion method. For example, by dissolving in water, colloidal solutions of starch, gelatin, agar-agar, etc. can be obtained. Spontaneous dispersion occurs without external mechanical influences. This method is widely used to obtain solutions of high molecular weight substances from solid polymers.

Condensation methods are based on the transition of molecular or ionic solutions into colloidal solutions due to the enlargement of particles of the dispersed phase substance. Condensation methods include the solvent replacement method, chemical methods for producing colloidal solutions using reactions of oxidation, reduction, exchange decomposition, hydrolysis, etc., as well as the peptization method. As a result of all chemical reactions, molecular or ionic solutions become colloidal by converting dissolved substances into an insoluble state. Condensation methods, in addition to chemical processes, can also be based on physical processes, mainly the phenomenon of vapor condensation. In chemical methods for producing dispersed systems, one of the starting substances acts as a stabilizer, and is taken in excess.

Oxidation method. It is based on oxidation reactions, as a result of which one of the substances can be obtained in a colloidal state. For example, by oxidizing hydrogen sulfide with atmospheric oxygen or sulfur dioxide, a sulfur sol can be obtained:

2H 2 S + O 2 → 2H 2 O + 2S

2H 2 S + SO 2 → 2H 2 O + 3S

Recovery Method. As an example, we give the reaction of obtaining a gold sol by reducing its salt with hydrogen peroxide or formaldehyde:

2HAuCI 4 + 3H 2 O 2 → 2Au + 8HCI + 3O 2

2HAuCI 4 + 3HCHO + 11KOH → 2Au + 3HCOOK + 8KCI + 8H 2 O

The reduction reaction produced many metals in a colloidal state, for example, Au, Ag, Pt, Pd, Os, Hg, etc.

Exchange decomposition method. An example is the reaction for producing barium sulfate sol:

BaCI 2 + K 2 SO 4 → BaSO 4 + 2KCI

or silver chloride

AgNO 3 + KCI → AgCI + KNO 3.

Hydrolysis method. Slightly soluble Fe(III) hydroxide is formed during the hydrolysis of iron(III) chloride:

FeCI 3 + 3HOH → Fe(OH) 3 + 3HCI,

Fe(OH) 3 + HCI → FeOCI + 2H 2 O

The ferric oxychloride formed as a result of these reactions dissociates partially into ions:

FeOCI ↔ FeO + + CI −

These ions provide an ionic layer around the Fe(OH) 3 particles, keeping them suspended.

Peptization method. Peptization is the transition of precipitates formed during coagulation into a colloidal solution. It can occur when washing sediments under the influence of peptizers, which use electrolytes. There is no change in the degree of dispersion of sediment particles, but only their disconnection.

For this reason, the peptization method, condensation in the initial stages, and dispersion in the final stages, occupies an intermediate position between condensation and dispersion. An example of a sol obtained by peptization is the synthesis of Prussian blue sol.

Two methods for producing disperse systems - dispersion and condensation

Dispersion and condensation are methods for producing freely dispersed systems: powders, suspensions, sols, emulsions, etc. Under dispersion understand the crushing and grinding of a substance; condensation is the formation of a heterogeneous dispersed system from a homogeneous one as a result of the association of molecules, atoms or ions into aggregates.

In the global production of various substances and materials, the processes of dispersion and condensation occupy one of the leading places. Billions of tons of raw materials and products are obtained in a freely dispersed state. This ensures ease of transportation and dosage, and also makes it possible to obtain homogeneous materials when preparing mixtures.

Examples include crushing and grinding of ores, coal, and cement production. Dispersion occurs during the combustion of liquid fuel.

Condensation occurs during the formation of fog, during crystallization.

It should be noted that during dispersion and condensation, the formation of dispersed systems is accompanied by the appearance of a new surface, i.e., an increase in the specific surface area of ​​substances and materials, sometimes thousands or more times. Therefore, the production of dispersed systems, with some exceptions, requires energy expenditure.

When crushing and grinding, materials are destroyed primarily in places of strength defects (macro- and microcracks). Therefore, as grinding progresses, the strength of the particles increases, which leads to an increase in energy consumption for their further dispersion.

The destruction of materials can be facilitated by use Rehbinder effect – adsorption reduction in the deterioration of solids. This effect is to reduce the surface energy with the help of surfactants, resulting in easier deformation and destruction of the solid. As such surfactants, here called hardness reducers, For example, liquid metals can be used to destroy solid metals or typical surfactants.

Hardness reducers are characterized by small quantities causing the Rebinder effect and specificity of action. Additives that wet the material help the medium to penetrate into defects and, with the help of capillary forces, also facilitate the destruction of the solid. Surfactants not only contribute to the destruction of the material, but also stabilize the dispersed state, preventing the particles from sticking together.

Systems with the maximum degree of dispersion can only be obtained using condensation methods.

Colloidal solutions can also be prepared by chemical condensation method, based on chemical reactions accompanied by the formation of insoluble or slightly soluble substances. For this purpose, various types of reactions are used - decomposition, hydrolysis, redox, etc.

Cleaning of dispersed systems.

Sols and solutions of high molecular weight compounds (HMCs) contain low molecular weight compounds as undesirable impurities. They are removed using the following methods.

Dialysis. Dialysis was historically the first method of purification. It was proposed by T. Graham (1861). The diagram of the simplest dialyzer is shown in Fig. 3 (see appendix). The sol to be purified, or IUD solution, is poured into a vessel, the bottom of which is a membrane that retains colloidal particles or macromolecules and allows solvent molecules and low-molecular impurities to pass through. The external medium in contact with the membrane is a solvent. Low molecular weight impurities, the concentration of which is higher in the ash or macromolecular solution, pass through the membrane into the external environment (dialysate). In the figure, the direction of flow of low molecular weight impurities is shown by arrows. Purification continues until the concentrations of impurities in the ash and dialysate become close in value (more precisely, until the chemical potentials in the ash and dialysate are equalized). If you update the solvent, you can almost completely get rid of impurities. This use of dialysis is appropriate when the purpose of purification is to remove all low molecular weight substances passing through the membrane. However, in some cases the task may turn out to be more difficult - it is necessary to get rid of only a certain part of low molecular weight compounds in the system. Then, a solution of those substances that need to be preserved in the system is used as the external environment. This is precisely the task that is set when purifying the blood from low molecular weight wastes and toxins (salts, urea, etc.).

Ultrafiltration. Ultrafiltration is a purification method by forcing a dispersion medium along with low molecular weight impurities through ultrafilters. Ultrafilters are membranes of the same type as those used for dialysis.

The simplest installation for purification by ultrafiltration is shown in Fig. 4 (see appendix). The purified sol or IUD solution is poured into the bag from the ultrafilter. Excessive pressure compared to atmospheric pressure is applied to the sol. It can be created either by an external source (compressed air tank, compressor, etc.) or by a large column of liquid. The dispersion medium is renewed by adding a pure solvent to the sol. To ensure that the cleaning speed is high enough, the update is carried out as quickly as possible. This is achieved by using significant excess pressure. In order for the membrane to withstand such loads, it is applied to a mechanical support. Such support is provided by meshes and plates with holes, glass and ceramic filters.

Microfiltration . Microfiltration is the separation of microparticles ranging in size from 0.1 to 10 microns using filters. The performance of the microfiltrate is determined by the porosity and thickness of the membrane. To assess porosity, i.e., the ratio of pore area to the total area of ​​the filter, various methods are used: squeezing liquids and gases, measuring the electrical conductivity of membranes, squeezing systems containing calibrated particles of the dispersion phase, etc.

Microporous filters are made from inorganic substances and polymers. By sintering powders, membranes from porcelain, metals and alloys can be obtained. Polymer membranes for microfiltration are most often made from cellulose and its derivatives.

Electrodialysis. The removal of electrolytes can be accelerated by applying an externally imposed potential difference. This purification method is called electrodialysis. Its use for the purification of various systems with biological objects (protein solutions, blood serum, etc.) began as a result of the successful work of Dore (1910). The device of the simplest electrodialyzer is shown in Fig. 5 (see appendix). The object to be cleaned (sol, IUD solution) is placed in the middle chamber 1, and the medium is poured into the two side chambers. In the cathode 3 and anode 5 chambers, ions pass through the pores in the membranes under the influence of an applied electrical voltage.

Electrodialysis is most suitable for purification when high electrical voltages can be applied. In most cases, at the initial stage of purification, systems contain a lot of dissolved salts and their electrical conductivity is high. Therefore, at high voltages, significant amounts of heat can be generated, and irreversible changes can occur in systems containing proteins or other biological components. Therefore, it is rational to use electrodialysis as a final cleaning method, using dialysis first.

Combined cleaning methods. In addition to individual purification methods - ultrafiltration and electrodialysis - their combination is known: electroultrafiltration, used for the purification and separation of proteins.

You can purify and simultaneously increase the concentration of the IUD sol or solution using a method called electrodecantation. The method was proposed by W. Pauli. Electrodecantation occurs when the electrodialyzer operates without stirring. Sol particles or macromolecules have their own charge and, under the influence of an electric field, move in the direction of one of the electrodes. Since they cannot pass through the membrane, their concentration at one of the membranes increases. As a rule, the density of particles differs from the density of the medium. Therefore, at the place where the sol is concentrated, the density of the system differs from the average value (usually the density increases with increasing concentration). The concentrated sol flows to the bottom of the electrodialyzer, and circulation occurs in the chamber, continuing until the particles are almost completely removed.

Colloidal solutions and, in particular, solutions of lyophobic colloids, purified and stabilized, can, despite thermodynamic instability, exist for an indefinitely long time. The red gold sol solutions prepared by Faraday have not yet undergone any visible changes. These data suggest that colloidal systems may be in metastable equilibrium.

Methods for producing dispersed systems are divided into two fundamentally different groups: dispersion and condensation.

Dispersing

The production of dispersed systems by the dispersion method involves crushing and grinding of substances. Dispersion can be carried out by mechanical, electrical, chemical (peptization) and ultrasonic methods.

Mechanical dispersion of substances constantly occurs in nature - weathering of rocks, formation of glaciers and other processes. Mechanical dispersion is of great importance in industrial processes - ore dressing, metallurgical production during the formation of slag, in oil refining, construction, medicine, pharmaceuticals. In this case, various types and designs of mills are used to ensure the desired degree of grinding. Thus, ball mills provide the production of coarse particles (~ 10 4 m); in colloidal mills, finer particles are obtained, for example, when crushing sugar, coffee, starch, graphite, and chemical reagents, colloidal mills are used to obtain a high degree of dispersion of the substance.

Dispersing begins with crushing, grinding the substance is the next stage. Job W, spent on dispersing the substance, according to Rehbinder’s equation, consists of two terms:

Where W^- work spent on crushing; - work spent on grinding a substance; A K and As- change in the volume of the system and the surface of dispersed particles in it; and - proportionality coefficients.

If the volume of a body is proportional to a cube of linear size, and the area is proportional to its square, then Rehbinder’s equation can be rewritten as the relation

where /Г and are proportionality coefficients.

For the first stage of dispersion, the first term is important K.a*,

since the work spent on deformation and crushing is related to the size of the original pieces of the substance (usually large and with a small surface) and their mechanical strength. At the second stage of dispersion, the work is proportional to the size of the resulting surface. For large particle sizes, the work of surface formation can be neglected and, conversely, for small sizes, the work of volumetric deformation can be neglected.

If in general the proportionality coefficients K^ And TO 2 depend

from the nature of the substance, the medium, the crushing method, then in the second term the coefficient /C takes on the function of the energy of formation of a unit surface, that is, surface tension: k^ = K^ c5.

During crushing and grinding, the destruction of bodies occurs in places of strength defects - microcracks, which are present in weak points of the crystal lattice, while the strength of the particles increases, which is used to obtain more durable materials.

To facilitate the dispersion of materials and reduce energy costs, special additives called strength reducers are usually used. Typically, the addition of strength reducers in an amount of -0.1% by weight of the crushed substances reduces the energy costs for obtaining dispersed systems by approximately half. The effect of reducing the strength of solids in the presence of strength reducers is called the effect

Rebindera. It is based on the fact that the development of microcracks under the influence of force occurs more easily with the adsorption of various substances from the environment, that is, the environment itself does not destroy the surface of bodies, but only facilitates the destruction. The effect of additives, which are most often surfactants, is primarily to reduce surface tension and reduce grinding work. In addition, additives, by wetting the material, help the medium penetrate into defects in the solid and, using capillary forces, facilitate its destruction. The Rehbinder effect is widely used in industry. For example, ore grinding is always carried out in an aqueous environment in the presence of a surfactant; the quality of processing parts on machines in the presence of a surfactant emulsion increases sharply, the service life of metal-cutting tools increases and energy costs for the process are reduced.

Dispersion is widely used in the production of emulsions - dispersed systems in which one liquid is dispersed in another liquid, that is, both phases are liquid (L/L). A necessary condition for the formation of emulsions is the complete or partial insolubility of the dispersed phase in the dispersion medium. Therefore, the liquid substances forming the emulsion must differ in polarity. Usually water (polar phase) is a component of emulsions. The second phase should be a non-polar or slightly soluble liquid, called oil regardless of its composition (benzene, toluene, vegetable and mineral oils).

Emulsions are divided into two types: direct O/W emulsions are called (dispersed phase - oil, dispersion medium - water); reverse (invert) - W/O emulsions (dispersions of water in oil). Examples of type I emulsions include emulsions formed during condensation of exhaust steam in an engine, food emulsions (milk, cream); A typical type II emulsion is crude oil containing up to 50% brine. Crude oil is a W/O emulsion stabilized by oil-soluble surfactants (paraffins, asphaltenes). Examples of food reverse emulsions include margarines or butter. The type of emulsion is determined by the volumetric ratio of the phases: the dispersed phase is the liquid that is in smaller quantities. The type can be determined by its ability to mix with polar and non-polar solvents or to dissolve polar or non-polar dyes, as well as electrical conductivity (for an aqueous dispersion medium, the electrical conductivity is several orders of magnitude higher than for a non-aqueous one).

Emulsions are widely used in nature and various technological processes. Emulsions play a major role in human life, for example, blood is an emulsion in which erythrocytes are the dispersed phase.

The uniformity of the state of aggregation of two adjacent phases determines the characteristics of the stability of emulsions. The sedimentation stability of emulsions is quite high and the greater, the smaller the difference in the densities of the dispersed phase and the dispersion medium. The process of sedimentation in emulsions can be superimposed by the process of flocculation (aggregation), leading to the enlargement of particles and, consequently, to an increase in the rate of their settling (or floating).

The aggregative stability of emulsions, like all dispersed systems, is determined by their lyophilicity or lyophobicity. Most emulsions are lyophobic systems. They are thermodynamically unstable and cannot form spontaneously due to the presence of excess free energy at the interphase surface. This instability manifests itself in the spontaneous merging of liquid droplets with each other (coalescence), which can lead to the complete destruction of the emulsion and its separation into two layers. The aggregative stability of such emulsions is possible only in the presence of a stabilizer that prevents the merging of particles. The stabilizer can be a component of the system that is in excess, or a substance specially introduced into the system; in this case, the stabilizer is called an emulsifier. Surfactants or high molecular weight substances are usually used as emulsifiers. Emulsifiers can be hydrophilic or hydrophobic. The most common hydrophilic emulsifiers are sodium (potassium) salts of fatty acids, which are more soluble in water than in hydrocarbons. They are capable of stabilizing direct O/W emulsion. The orientation of the surfactant adsorption layer occurs in accordance with Rehbinder's rule: the nonpolar radical faces the nonpolar liquid, and the polar group faces the polar one. In direct emulsions, the polar parts of the emulsifier are located on the outside of the oil droplets and prevent them from approaching each other. The same substances in reverse emulsions are adsorbed by polar groups on the inner surface of water droplets and do not interfere with their merging (Fig. 1.3).

Rice. 1.3. The location of the hydrophilic emulsifier in straight lines (A) and inverse ( 6 ) emulsions

Under certain conditions, a phenomenon called inversion is possible - reversal of the phases of an emulsion (or simply reversal of the emulsion), when, when conditions change or the introduction of any reagents, an emulsion of a given type turns into an emulsion of the opposite type.