Dispersed systems
Pure substances are very rare in nature. Mixtures of different substances in different states of aggregation can form heterogeneous and homogeneous systems - dispersed systems and solutions.
Dispersed
are called heterogeneous systems in which one substance in the form of very small particles is evenly distributed in the volume of another.
The substance that is present in smaller quantities and distributed in the volume of another is called dispersed phase
. It may consist of several substances.
The substance present in larger quantities, in the volume of which the dispersed phase is distributed, is called dispersion medium
. There is an interface between it and the particles of the dispersed phase; therefore, dispersed systems are called heterogeneous (inhomogeneous).
Both the dispersion medium and the dispersed phase can be represented by substances in different states of aggregation - solid, liquid and gaseous.
Depending on the combination of the aggregate state of the dispersion medium and the dispersed phase, 9 types of such systems can be distinguished.
Based on the particle size of the substances that make up the dispersed phase, dispersed systems are divided into coarsely dispersed (suspensions) with particle sizes of more than 100 nm and finely dispersed (colloidal solutions or colloidal systems) with particle sizes from 100 to 1 nm. If the substance is fragmented into molecules or ions less than 1 nm in size, a homogeneous system is formed - a solution. It is uniform (homogeneous), there is no interface between the particles and the medium.
Already a quick acquaintance with dispersed systems and solutions shows how important they are in everyday life and in nature.
Judge for yourself: without the Nile silt the great civilization of Ancient Egypt would not have taken place; without water, air, rocks and minerals, the living planet would not exist at all - our common home - the Earth; without cells there would be no living organisms, etc.
Classification of disperse systems and solutions
Suspend
Suspend
- these are dispersed systems in which the phase particle size is more than 100 nm. These are opaque systems, individual particles of which can be seen with the naked eye. The dispersed phase and the dispersion medium are easily separated by settling. Such systems are divided into:
1) emulsions
(both the medium and the phase are liquids insoluble in each other). These are well-known milk, lymph, water-based paints, etc.;
2) suspensions
(the medium is a liquid, and the phase is a solid insoluble in it). These are construction solutions (for example, “lime milk” for whitewashing), river and sea silt suspended in water, a living suspension of microscopic living organisms in sea water - plankton, which giant whales feed on, etc.;
3) aerosols
- suspensions in gas (for example, in air) of small particles of liquids or solids. Distinguish between dust, smoke, and fog. The first two types of aerosols are suspensions of solid particles in gas (larger particles in dust), the latter is a suspension of small droplets of liquid in gas. For example, natural aerosols: fog, thunderclouds - a suspension of water droplets in the air, smoke - small solid particles. And the smog hanging over the world's largest cities is also an aerosol with a solid and liquid dispersed phase. Residents of settlements near cement factories suffer from the finest cement dust always hanging in the air, which is formed during the grinding of cement raw materials and the product of its firing - clinker. Similar harmful aerosols - dust - are also present in cities with metallurgical production. Smoke from factory chimneys, smog, tiny droplets of saliva flying out of the mouth of a flu patient, and also harmful aerosols.
Aerosols play an important role in nature, everyday life and human production activities. Cloud accumulations, chemical treatment of fields, spray paint application, fuel atomization, production of milk powder, and respiratory tract treatment (inhalation) are examples of phenomena and processes where aerosols provide benefits. Aerosols are fogs over the sea surf, near waterfalls and fountains; the rainbow that appears in them gives a person joy and aesthetic pleasure.
For chemistry, dispersed systems in which the medium is water and liquid solutions are of greatest importance.
Natural water always contains dissolved substances. Natural aqueous solutions participate in soil formation processes and supply plants with nutrients. Complex life processes occurring in human and animal bodies also occur in solutions. Many technological processes in the chemical and other industries, for example, the production of acids, metals, paper, soda, fertilizers, take place in solutions.
Colloidal systems
Colloidal systems
- these are dispersed systems in which the phase particle size is from 100 to 1 nm. These particles are not visible to the naked eye, and the dispersed phase and the dispersion medium in such systems are difficult to separate by settling.
They are divided into sols (colloidal solutions) and gels (jelly).
1.
Colloidal solutions, or sols.
This is the majority of the fluids of a living cell (cytoplasm, nuclear juice - karyoplasm, contents of organelles and vacuoles) and the living organism as a whole (blood, lymph, tissue fluid, digestive juices, humoral fluids, etc.). Such systems form adhesives, starch, proteins, and some polymers.
Colloidal solutions can be obtained as a result of chemical reactions; for example, when solutions of potassium or sodium silicates (“soluble glass”) react with acid solutions, a colloidal solution of silicic acid is formed. A sol is also formed during the hydrolysis of iron chloride (III) in hot water. Colloidal solutions are similar in appearance to true solutions. They are distinguished from the latter by the “luminous path” that is formed - a cone when a beam of light is passed through them.
Particles of the dispersed phase of colloidal solutions often do not settle even during long-term storage due to continuous collisions with solvent molecules due to thermal movement. They do not stick together when approaching each other due to the presence of electric charges of the same name on their surface. But under certain conditions, a coagulation process can occur.
Coagulation - the phenomenon of colloidal particles sticking together and precipitating - is observed when the charges of these particles are neutralized when an electrolyte is added to the colloidal solution. In this case, the solution turns into a suspension or gel. Some organic colloids coagulate when heated (glue, egg white) or when the acid-base environment of the solution changes.
2.
Gels
, or jellies, which are gelatinous sediments formed during the coagulation of sols. These include a large number of polymer gels, so well known to you confectionery, cosmetic and medical gels (gelatin, jellied meat, jelly, marmalade, Bird's Milk cake) and of course an endless variety of natural gels: minerals (opal), jellyfish bodies, cartilage , tendons, hair, muscle and nervous tissue, etc. The history of the development of life on Earth can simultaneously be considered the history of the evolution of the colloidal state of matter. Over time, the structure of the gels is disrupted and water is released from them. This phenomenon is called syneresis
.
Solutions
A solution is called
homogeneous system consisting of two or more substances.
Solutions are always single-phase, that is, they are a homogeneous gas, liquid or solid. This is due to the fact that one of the substances is distributed in the mass of the other in the form of molecules, atoms or ions (particle size less than 1 nm).
Solutions are called true
, if you want to emphasize their difference from colloidal solutions.
A solvent is considered to be a substance whose state of aggregation does not change during the formation of a solution. For example, water in aqueous solutions of table salt, sugar, carbon dioxide. If a solution was formed by mixing gas with gas, liquid with liquid, and solid with solid, the solvent is considered to be the component that is more abundant in the solution. So, air is a solution of oxygen, noble gases, carbon dioxide in nitrogen (solvent). Table vinegar, which contains from 5 to 9% acetic acid, is a solution of this acid in water (the solvent is water). But in acetic essence, acetic acid plays the role of solvent, since its mass fraction is 70-80%, therefore, it is a solution of water in acetic acid.
When crystallizing a liquid alloy of silver and gold, solid solutions of different compositions can be obtained.
Solutions are divided into:
molecular - these are aqueous solutions of non-electrolytes - organic substances (alcohol, glucose, sucrose, etc.);
molecular ion- these are solutions of weak electrolytes (nitrous, hydrosulfide acids, etc.);
ionic - these are solutions of strong electrolytes (alkalies, salts, acids - NaOH, K 2 S0 4, HN0 3, HC1O 4).
Previously, there were two points of view on the nature of dissolution and solutions: physical and chemical. According to the first, solutions were considered as mechanical mixtures, according to the second - as unstable chemical compounds of particles of a dissolved substance with water or another solvent. The last theory was expressed in 1887 by D.I. Mendeleev, who devoted more than 40 years to the study of solutions. Modern chemistry considers dissolution as a physicochemical process, and solutions as physicochemical systems.
A more precise definition of a solution is:
Solution
- a homogeneous (homogeneous) system consisting of particles of a dissolved substance, a solvent and the products of their interaction.
The behavior and properties of electrolyte solutions, as you well know, are explained by another important theory of chemistry - the theory of electrolytic dissociation, developed by S. Arrhenius, developed and supplemented by the students of D. I. Mendeleev, and primarily by I. A. Kablukov.
Questions to consolidate:
1. What are disperse systems?
2. When the skin is damaged (wound), blood clotting is observed - coagulation of the sol. What is the essence of this process? Why does this phenomenon perform a protective function for the body? What is the name of a disease in which blood clotting is difficult or not observed?
3. Tell us about the importance of various disperse systems in everyday life.
4. Trace the evolution of colloidal systems during the development of life on Earth.
Dispersed systems.
Dispersed systems are widespread in nature and have been used by humans in their life activities for a long time. Almost any living organism either represents a dispersed system or contains them in various forms.
Example: freely dispersed systems(there are no solid rigid structures - sols): blood, lymph, gastric and intestinal juices, cerebrospinal fluid, etc.
cohesive dispersed systems(there are rigid spatial structures - gels): protoplasm, cell membranes, muscle fiber, eye lens, etc.
Dispersed systems are actively used in medicine, primarily colloidal solutions, aerosols, creams, and ointments. Biochemical processes in the body occur in dispersed systems. The absorption of food is associated with the transition of nutrients into a dissolved state. Biofluids (dispersed systems) are involved in the transport of nutrients (fats, amino acids, oxygen), drugs to organs and tissues, as well as in the excretion of metabolites (urea, bilirubin, carbon dioxide) from the body.
Knowledge of the patterns of physical and chemical processes in dispersed systems is important for future doctors both for studying biomedical and clinical disciplines, and for a deeper understanding of the processes occurring in the body and consciously changing them in the desired direction.
Dispersed systems- these are multicomponent systems in which some substances in the form of small particles are distributed in another substance. The substance that is distributed is called the dispersed phase. The substance in which the dispersed phase is distributed is called a dispersion medium.
Example: aqueous glucose solution
glucose molecules – dispersed phase
water – dispersion medium
Dispersity is a value characterizing the size of suspended particles in dispersed systems. It is the inverse of the particle diameter of the dispersed phase. The smaller the particle size, the greater the dispersion.
Classification of disperse systems.
Dispersed systems are classified according to five criteria.
1. By degree of dispersion:
· coarse
D = 10 4 – 10 6 m –1 , are characterized by instability and opacity.
Example: suspensions, emulsions, foams, suspensions.
· colloidal dispersed
D = 10 7 – 10 9 m –1 , can be transparent and cloudy, stable and unstable.
Example: colloidal solutions, solutions of high molecular weight compounds.
molecular-disperse and ion-disperse
D = 10 10 – 10 11 m –1 , are characterized by transparency and stability.
Example: solutions of low molecular weight compounds.
2. By the presence of a physical interface between the dispersed phase and the dispersion medium:
· homogeneous (single-phase systems, no interface.
Example: solutions of low molecular weight and high molecular weight compounds.
· heterogeneous
there is an interface between the dispersed phase and the dispersion medium.
Example: colloidal solutions and coarse systems.
3. According to the nature of the interaction between the dispersed phase and the dispersion medium:
· lyophilic
There is an affinity between the dispersed phase and the dispersion medium.
Example: all homogeneous systems.
· lyophobic
There is little or no interaction between the dispersed phase and the dispersion medium.
Example: all heterogeneous systems.
4. According to the state of aggregation of the dispersed phase and dispersion medium:
| control phase control medium | gaseous | hard | liquid |
| gaseous | mixture of gases (air) | tobacco smoke flour dust, cosmic aerosols | fog steam clouds |
| liquid | dissolved in blood CO 2 , O 2 , N 2 , foam mineral water fruit carbonated drinks | colloidal solutions suspensions IUD solutions NMS solutions | emulsions: milk butter margarine creams ointments oil |
| hard | solid foams (foam plastic, activated carbon) ion exchange resins molecular sieves | metal alloys colored glass, crystal precious stones (ruby, amethyst) suppositories (medicinal suppositories) | crystal hydrates minerals with liquid inclusions (pearls, opal) wet soils |
5. By the nature of the dispersion medium:
True solutions.
The true solution is a homogeneous lyophilic disperse system with particle sizes of 10 –10 – 10 –11 m.
True solutions are single-phase disperse systems; they are characterized by a high bond strength between the dispersed phase and the dispersion medium. A true solution remains homogeneous indefinitely. True solutions are always transparent. Particles of the true solution are not visible even with an electron microscope. True solutions diffuse well.
A component, the state of aggregation of which does not change during the formation of a solution, is called a solvent (dispersion medium), and the other component is called a solute (disperse phase).
If the components have the same state of aggregation, the solvent is the component whose amount in the solution predominates.
In electrolyte solutions, regardless of the ratio of components, electrolytes are considered as dissolved substances.
True solutions are divided:
· by type of solvent: aqueous and non-aqueous
· by type of dissolved substance: solutions of salts, acids, alkalis, gases, etc.
· in relation to electric current: electrolytes and non-electrolytes
by concentration: concentrated and diluted
· according to the degree of reaching the solubility limit: saturated and unsaturated
· from a thermodynamic point of view: ideal and real
· by state of aggregation: gaseous, liquid, solid
True solutions are:
· ion-dispersed (dispersed phase – hydrated ions): aqueous solution of NaCl
· molecularly dispersed (dispersed phase – molecules): aqueous solution of glucose
Each ion, individually or together, performs certain functions in the body. The decisive role in the transfer of water in the body belongs to the Na + and Cl – ions, i.e. they participate in water-salt metabolism. Electrolyte ions are involved in the processes of maintaining a constant osmotic pressure, establishing acid-base balance, in the processes of transmitting nerve impulses, and in the processes of enzyme activation.
From the perspective of living systems, solutions in which water is the solvent are of greatest interest.
A huge number of substances dissolve in it. It is not only a solvent that ensures the molecular dispersion of substances throughout the body. It is also a participant in many chemical and biochemical processes in the body. For example, hydrolysis, hydration, swelling, transport of nutrients and drugs, gases, antibodies, etc.
There is a continuous exchange of water and substances dissolved in it in the body. Water makes up the bulk of any living creature. Its content in the human body changes with age: in a human embryo - 97%, in a newborn - 77%, in adult men - 61%, in adult women - 54%, in old people over 81 years old - 49.8%. Most of the water in the body is inside the cells (70%), about 23% is intercellular water, and the rest (7%) is inside the blood vessels and as part of the blood plasma.
In total there are 42 liters of water in the body. 1.5 - 3 liters of water enters and leaves the body per day. This is the normal water balance of the body.
The main route for removing water from the body is the kidneys. A loss of 10–15% of water is dangerous, and 20–25% is fatal to the body.
The most important characteristic of a solution is its concentration.
Ways to express the concentration of solutions:
1. Mass fraction w(x)– a value equal to the ratio of the mass of the dissolved substance m(x) to the mass of the solution m(p-p)
w(x) = × 100%
2. Molar concentration of solution with(X)– a value equal to the ratio of the amount of substance n(x) contained in a solution to the volume of this solution V(solution).
With(x) = [mol/l], where n(x) = [mol]
Millimolar solution - a solution with a molar concentration equal to 0.001 mol/l
Centimolar solution - a solution with a molar concentration equal to 0.01 mol/l
Decimolar solution - a solution with a molar concentration equal to 0.1 mol/l
3. Molar concentration equivalent With ( x) – a value equal to the ratio of the amount of substance equivalent n ( x) in solution to the volume of this solution.
c ( x) = [mol/l], where n ( x) = [mol], and M( x) = × M(x)
Equivalent – is a real or conditional particle of matter X, which in a given acid-base reaction is equivalent to one hydrogen ion or in a given ORR - one electron.
Equivalence number z And equivalence factor f= . The equivalence factor shows what fraction of a real particle of matter X equivalent to one hydrogen ion or one electron. Equivalence number z equals for:
a) acids - acid basicity H 2 SO 4 z = 2.
b) bases – acidity of the base Aℓ(OH) 3 z = 3.
c) salts - the product of the oxidation state (s.o.) of the metal by the number of its atoms in the Fe 2 (SO 4) 3 molecule z= 2 × 3 = 6.
d) oxidizing agents - the number of attached electrons
Mn +7 + 5ē → Mn +2 z = 5
e) reducing agents - the number of electrons given up
Fe +2 – 1ē → Fe +3 z = 1
4. Molal concentration b(x)– a value equal to the ratio of the amount of substance to the mass of solvent (kg)
b(x) = = [mol/kg]
5. Mole fraction c (x i) equal to the ratio of the amount of substance of a given component to the total amount of all components of the solution
Formulas for the relationship between concentrations:
With(x)= c(x)×z
Solutions have a number of properties that do not depend on the nature of the solute, but depend only on its concentration. The most important is osmosis.
Thanks to osmosis, the complex process of metabolism of the body with the external environment occurs through the membranes of cells of organs and tissues.
Diffusion is the process of spontaneous equalization of concentration per unit volume.
Osmosis is the one-way diffusion of solvent molecules through a semi-permeable membrane from a solvent to a solution or from a solution with a lower concentration to a solution with a higher concentration.
solvent solution
Solvent transfer through the membrane is due to osmotic pressure . It is equal to the excess external pressure that should be applied from the solution to stop the process, that is, to create conditions of osmotic equilibrium. Exceeding excess pressure over osmotic pressure can lead to reversal of osmosis - reverse diffusion of the solvent. Reverse osmosis occurs when blood plasma is filtered in the arterial part of the capillary and in the renal glomeruli.
Osmotic pressure is the pressure that must be applied to a solution for osmosis to stop.
Van't Hoff equation: P osm = c RT×10 3
Blood osmotic pressure: 780 – 820 kPa
All solutions, from the point of view of osmotic phenomena, can be divided into 3 groups:
· Isotonic solutions are solutions that have the same osmotic pressure and osmolar concentration. Examples: bile, NaCl solution (w=0.9%, c=0.15 mol/l), glucose solution (w=7%, c=0.3 mol/l)
Osmolar concentration (osmolarity) is the total amount of substance of all kinetically active particles contained in 1 liter of solution. with osm, osmol/l
Osmolality concentration (osmolality) is the total amount of substance of all kinetically active particles contained in 1 kg of solvent. b osm, osmol/kg
For dilute solutions, the osmolar concentration is the same as the osmolal concentration. c osm ≈ b osm
· Hypertonic solution - a solution with a higher concentration of dissolved substances, therefore, with a higher osmotic pressure compared to another solution and, in the presence of permeable membranes, capable of drawing water out of it. Examples: intestinal juice, urine.
· Hypotonic solution - a solution with a lower concentration of dissolved substances, therefore, with a lower osmotic pressure compared to another solution and capable of losing water in the presence of permeable membranes. Examples: saliva, sweat.
Animal and plant cells are separated from the environment by a membrane. When a cell is placed in solutions of different osmolar concentrations or pressures, the following phenomena will be observed:
Plasmolysis – reduction of cell volume. In this case, the cell is placed in a hypertonic solution. The difference in osmotic pressure causes the solvent to move from the cell into the hypertonic solution.
· lysis – increase in cell volume. In this case, the cell is placed in a hypotonic solution. The difference in osmotic pressure causes the solvent to move into the cell. In the case of rupture of erythrocyte membranes and the transfer of hemoglobin into plasma, the phenomenon is called hemolysis.
Isoosmia – cell volume does not change. In this case, the cell is placed in an isotonic solution.
With the help of osmotic phenomena, water-salt metabolism is maintained in the human body. Osmosis is the basis of the mechanism of kidney function. Isotonic (physiological) NaCl solution (0.9%) is used for large blood losses. Hypertonic NaCl solution (10%) is used when applying gauze bandages to purulent wounds.
Oncotic pressure- This is part of the osmotic pressure created by proteins.
In human blood plasma it makes up only about 0.5% of the osmotic pressure (0.03-0.04 atm or 2.5 - 4.0 kPa). However, oncotic pressure plays a crucial role in the formation of intercellular fluid, primary urine, etc. The capillary wall is freely permeable to water and low molecular weight substances, but not to proteins. The rate of fluid filtration through the capillary wall is determined by the difference between the oncotic pressure of plasma proteins and the hydrostatic pressure of the blood created by the work of the heart. At the arterial end of the capillary, the saline solution along with nutrients passes into the intercellular space. At the venous end of the capillary, the process proceeds in the opposite direction, since the venous pressure is lower than the oncotic pressure. As a result, substances released by the cells pass into the blood. In diseases accompanied by a decrease in the concentration of proteins (especially albumin) in the blood, oncotic pressure decreases, and this may be one of the reasons for the accumulation of fluid in the intercellular space, resulting in the development of edema.
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Disperse system- formations of two or more phases (bodies) that practically do not mix and do not react chemically with each other. In a typical case of a two-phase system, the first of the substances ( dispersed phase) finely distributed in the second ( dispersion medium). If there are several phases, they can be separated from each other physically (centrifuge, separate, etc.).
Typically dispersed systems are colloidal solutions, sols. Dispersed systems also include the case of a solid dispersed medium in which the dispersed phase is located. Solutions of high molecular weight compounds by you
Classification of disperse systems
The most general classification of disperse systems is based on the difference in the state of aggregation of the dispersion medium and the dispersed phase (phases). Combinations of three types of state of aggregation make it possible to distinguish nine types of two-phase disperse systems. For brevity, they are usually denoted by a fraction, the numerator of which indicates the dispersed phase, and the denominator indicates the dispersion medium; for example, for the gas-in-liquid system the designation G/L is accepted.
| Designation | Dispersed phase | Dispersive medium | Title and example |
|---|---|---|---|
| Y/Y | Gaseous | Gaseous | Always homogeneous mixture (air, natural gas) |
| F/G | Liquid | Gaseous | Aerosols: fogs, clouds |
| T/G | Hard | Gaseous | Aerosols (dusts, fumes), powdery substances |
| G/F | Gaseous | Liquid | Gas emulsions and foams |
| F/F | Liquid | Liquid | Emulsions: oil, cream, milk |
| T/F | Hard | Liquid | Suspensions and sols: pulp, sludge, suspension, paste |
| H/T | Gaseous | Hard | Porous bodies: foam polymers, pumice |
| W/T | Liquid | Hard | Capillary systems (fluid-filled porous bodies): soil, soil |
| T/T | Hard | Hard | Solid heterogeneous systems: alloys, concrete, glass ceramics, composite materials |
Based on the kinetic properties of the dispersed phase, two-phase disperse systems can be divided into two classes:
- Freely dispersed systems, in which the dispersed phase is mobile;
- Cohesively dispersed systems, in which the dispersion medium is solid, and the particles of their dispersed phase are interconnected and cannot move freely.
In turn, these systems are classified according to the degree of dispersion.
Systems with dispersed phase particles of equal size are called monodisperse, and systems with particles of unequal size are called polydisperse. As a rule, the real systems around us are polydisperse.
There are also dispersed systems with a larger number of phases - complex dispersed systems. For example, when a liquid dispersion medium boils with a solid dispersed phase, a three-phase system “vapor - drops - solid particles” is obtained.
Another example of a complex disperse system is milk, the main components of which (not counting water) are fat, casein and milk sugar. The fat is in the form of an emulsion and when the milk stands, it gradually rises to the top (cream). Casein is contained in the form of a colloidal solution and is not released spontaneously, but can easily be precipitated (in the form of cottage cheese) when milk is acidified, for example, with vinegar. Under natural conditions, casein is released when milk sours. Finally, milk sugar is in the form of a molecular solution and is released only when water evaporates.
Freely dispersed systems
Based on particle size, freely dispersed systems are divided into:
Ultramicroheterogeneous systems are also called colloidal or sols. Depending on the nature of the dispersion medium, sols are divided into solid sols, aerosols (sols with a gaseous dispersion medium) and lyosols (sols with a liquid dispersion medium). Microheterogeneous systems include suspensions, emulsions, foams and powders. The most common coarse systems are solid-gas systems (for example, sand).
Colloidal systems play a huge role in biology and human life. In biological fluids of the body, a number of substances are in a colloidal state. Biological objects (muscle and nerve cells, blood and other biological fluids) can be considered as colloidal solutions. The dispersion medium of blood is plasma - an aqueous solution of inorganic salts and proteins.
Cohesively dispersed systems
Porous materials
Porous materials are divided according to pore size, according to the classification of M. M. Dubinin, into:
Based on geometric characteristics, porous structures are divided into regular(in which in the body volume there is a correct alternation of individual pores or cavities and channels connecting them) and stochastic(in which the orientation, shape, size, relative position and relationships of the pores are random). Most porous materials are characterized by a stochastic structure. The nature of the pores also matters: open the pores communicate with the surface of the body so that liquid or gas can be filtered through them; dead-end pores also communicate with the surface of the body, but their presence does not affect the permeability of the material; closed pores .
Solid heterogeneous systems
A typical example of solid heterogeneous systems are the recently widely used composite materials (composites) - artificially created solid, but heterogeneous, materials that consist of two or more components with clear interface boundaries between them. In most of these materials (with the exception of layered ones), the components can be divided into matrix and included in it reinforcing elements; in this case, the reinforcing elements are usually responsible for the mechanical characteristics of the material, and the matrix ensures the joint operation of the reinforcing elements. The oldest composite materials include adobe, reinforced concrete, damask steel, and papier-mâché. Nowadays, fiber-reinforced plastics, fiberglass, and metal-ceramics are widely used and have found application in a wide variety of fields of technology.
Movement of dispersed systems
The mechanics of multiphase media deals with the study of the movement of dispersed systems. In particular, the problems of optimizing various heat and power devices (steam turbine units, heat exchangers, etc.), as well as the development of technologies for applying various coatings, make the problem of mathematical modeling of near-wall flows of a gas-liquid droplet mixture relevant. In turn, the significant diversity of the structure of near-wall flows of multiphase media, the need to take into account various factors (inertia of droplets, formation of a liquid film, phase transitions, etc.) require the construction of special mathematical models of multiphase media, which are currently being actively developed
In the world around us, pure substances are extremely rare; basically, most substances on earth and in the atmosphere are various mixtures containing more than two components. Particles ranging in size from approximately 1 nm (several molecular sizes) to 10 µm are called dispersed(Latin dispergo – scatter, spray). Various systems (inorganic, organic, polymer, protein), in which at least one of the substances is in the form of such particles, are called dispersed. Dispersed - these are heterogeneous systems consisting of two or more phases with a highly developed interface between them or a mixture consisting of at least two substances that are completely or practically immiscible with each other and do not react with each other chemically. One of the phases - the dispersed phase - consists of very small particles distributed in another phase - the dispersion medium.
Dispersed system
According to their state of aggregation, dispersed particles can be solid, liquid, gaseous, and in many cases have a complex structure. Dispersion media are also gaseous, liquid and solid. Most of the real bodies of the world around us exist in the form of dispersed systems: sea water, soils and soils, tissues of living organisms, many technical materials, food products, etc.
Classification of disperse systems
Despite numerous attempts to propose a unified classification of these systems, it is still missing. The reason is that in any classification, not all properties of disperse systems are taken as a criterion, but only one of them. Let us consider the most common classifications of colloidal and microheterogeneous systems.
In any field of knowledge, when one has to deal with complex objects and phenomena, in order to facilitate and establish certain patterns, it is advisable to classify them according to certain criteria. This also applies to the field of dispersed systems; At different times, different classification principles were proposed for them. Based on the intensity of interaction between the substances of the dispersion medium and the dispersed phase, lyophilic and lyophobic colloids are distinguished. Other techniques for classifying disperse systems are briefly outlined below.
Classification by presence or absence of interactionbetween particles of the dispersed phase. According to this classification, dispersed systems are divided into freely dispersed and coherently dispersed; the classification is applicable to colloidal solutions and solutions of high molecular weight compounds.
Freely dispersed systems include typical colloidal solutions, suspensions, suspensions, and various solutions of high-molecular compounds that have fluidity, like ordinary liquids and solutions.
Cohesively dispersed systems include the so-called structured systems, in which, as a result of the interaction between particles, a spatial openwork mesh-framework arises, and the system as a whole acquires the property of a semi-solid body. For example, sols of some substances and solutions of high-molecular compounds, when the temperature decreases or with an increase in concentration above a certain limit, without undergoing any external changes, they lose fluidity - they gelatinize (gelatinize), and pass into a gel (jelly) state. This also includes concentrated pastes and amorphous precipitates.
Classification by dispersion. The physical properties of a substance do not depend on the size of the body, but at a high degree of grinding they become a function of dispersion. For example, metal sols have different colors depending on the degree of grinding. Thus, colloidal solutions of gold of extremely high dispersion have a purple color, less dispersed ones have a blue color, and even less dispersed ones have a green color. There is reason to believe that other properties of sols of the same substance change as they are ground: A natural criterion for classifying colloidal systems by dispersity suggests itself, i.e., division of the region of the colloidal state (10 -5 -10 -7 cm) into a number of narrower intervals. Such a classification was proposed at one time, but it turned out to be useless, since colloidal systems are almost always polydisperse; monodisperse are very rare. In addition, the degree of dispersion can change over time, i.e., it depends on the age of the system.
There are no elements in nature that are pure. Basically, they are all different mixtures. They, in turn, can be heterogeneous or homogeneous. They are formed from substances in an aggregate state, creating a specific dispersion system in which various phases are present. In addition, mixtures usually contain a dispersion medium. Its essence lies in the fact that it is considered an element with a large volume in which a substance is distributed. In a disperse system, the phase and medium are located in such a way that there are particles at the interface between them. Therefore, it is called heterogeneous or heterogeneous. In view of this, the action of the surface, and not the particles as a whole, is of great importance.
Classification of dispersed system
A phase, as is known, is represented by substances having different states. And these elements are divided into several types. The aggregate state of the dispersed phase depends on the combination of the medium in it, resulting in 9 types of systems:
- Gas. Liquid, solid and element in question. Homogeneous mixture, fog, dust, aerosols.
- Liquid dispersed phase. Gas, solid, water. Foams, emulsions, sols.
- Solid dispersed phase. Liquid, gas and the substance considered in this case. Soil, medicine or cosmetics, rocks.
As a rule, the dimensions of a disperse system are determined by the size of the phase particles. There is the following classification:
- coarse (suspensions);
- subtle and true).
Dispersion system particles
By examining coarse mixtures, one can observe that the particles of these compounds in the structure can be visible to the naked eye, due to the fact that their size is more than 100 nm. Suspensions generally refer to a system in which the dispersed phase is separable from the medium. This is because they are considered opaque. Suspensions are divided into emulsions (insoluble liquids), aerosols (small particles and solids), and suspensions (solids in water).
A colloidal substance is any substance that has the quality of having another element dispersed evenly throughout it. That is, it is present, or rather, it is part of the dispersed phase. This is a state when one material is completely distributed in another, or rather in its volume. In the milk example, liquid fat disperses into an aqueous solution. In this case, the smaller molecule is within 1 nanometer and 1 micrometer, making it invisible to the optical microscope once the mixture becomes homogeneous.
That is, no part of the solution has a higher or lower concentration of the dispersed phase than any other. It can be said to be colloidal in nature. The larger one is called a continuous phase or dispersion medium. Because its size and distribution do not change, and the element in question spreads across it. Types of colloids include aerosols, emulsions, foams, dispersions, and mixtures called hydrosols. Each such system has two phases: dispersed and continuous phase.
Colloids in history
Intense interest in such substances was present throughout the sciences at the beginning of the 20th century. Einstein and other scientists carefully studied their characteristics and applications. At the time, this new field of science was a leading area of research for theorists, researchers and manufacturers. After a peak of interest before 1950, research on colloids declined significantly. It is interesting to note that with the recent advent of higher power microscopes and "nanotechnology" (the study of objects on a specific tiny scale), there is a renewed scientific interest in the study of new materials.
Read more about these substances
There are elements observed both in nature and in artificial solutions that have colloidal properties. For example, mayonnaise, cosmetic lotion and lubricants are types of artificial emulsions, while milk is a similar mixture that occurs naturally. Colloidal foams include whipped cream and shaving foam, while edibles include butter, marshmallows and jelly. In addition to food, these substances exist in the form of some alloys, paints, inks, detergents, insecticides, aerosols, polystyrene foam and rubber. Even beautiful natural objects such as clouds, pearls and opals have colloidal properties because they have other matter distributed evenly through them.
Preparation of colloidal mixtures
By enlarging small molecules to the 1 to 1 micrometer range, or by reducing large particles to the same size. Colloidal substances can be obtained. Further production depends on the type of elements used in dispersed and continuous phases. Colloids behave differently than ordinary liquids. And this is observed in transport and physicochemical properties. For example, a membrane may allow a true solution with solid molecules attached to liquid molecules to pass through it. While a colloidal substance, which has a solid dispersed through a liquid, will be stretched by the membrane. The parity of the distribution is uniform to the point of microscopic equality in the gap throughout the second element.
True solutions
A colloidal dispersion is presented in the form of a homogeneous mixture. The element consists of two systems: continuous and dispersed phase. This indicates that this case is related to for they are directly related to the above mixture consisting of several substances. In a colloid, the second has a structure of tiny particles or droplets that are evenly distributed in the first. From 1 nm to 100 nm is the size constituting the dispersed phase, or more precisely particles, in at least one dimension. In this range, the dispersed phase with the indicated dimensions can be called approximate elements that fit the description: colloidal aerosols, emulsions, foams, hydrosols. The particles or droplets present in the compositions under consideration are largely exposed to the chemical composition of the surface.
Colloidal solutions and systems
One should take into account the fact that the size of the dispersed phase is a difficult to measure variable in the system. Solutions are sometimes characterized by their own properties. To make it easier to perceive the indicators of the compositions, colloids resemble them and look almost the same. For example, if it has a solid form dispersed in a liquid. As a result, particles will not pass through the membrane. While other components such as dissolved ions or molecules are able to pass through it. If we analyze it more simply, it turns out that the dissolved components pass through the membrane, but colloidal particles cannot with the phase under consideration.
Appearance and disappearance of color characteristics
Due to the Tyndall effect, some such substances are translucent. In the structure of the element it is the scattering of light. Other systems and compositions come with some kind of tint or are completely opaque, with a certain color, even if some are dim. Many familiar substances, including butter, milk, cream, aerosols (fog, smog, smoke), asphalt, paints, paints, glue, and sea foam, are colloids. This field of study was introduced in 1861 by Scottish scientist Thomas Graham. In some cases, a colloid can be considered a homogeneous (not heterogeneous) mixture. This is because the distinction between "dissolved" and "granular" matter can sometimes be a matter of approach.
Hydrocolloid types of substances
This component is defined as a colloidal system in which particles are dispersed in water. Hydrocolloid elements, depending on the amount of liquid, can take on different states, for example, gel or sol. They can be irreversible (one-part) or reversible. For example, agar, the second type of hydrocolloid. Can exist in gel and sol states, and alternate between states with the addition or removal of heat.
Many hydrocolloids are obtained from natural sources. For example, carrageen is extracted from algae, gelatin is derived from bovine fat, and pectin is derived from citrus peels and apple pomace. Hydrocolloids are used in foods primarily to affect texture or viscosity (sauce). Also used for skin care or as a healing agent after injury.
Essential characteristics of colloidal systems
From this information it is clear that colloidal systems are a subsection of the dispersed sphere. They, in turn, can be solutions (sols) or gels (jelly). The former, in most cases, are created on the basis of living chemistry. The latter are formed under sediments that arise during the coagulation of sols. Solutions can be aqueous with organic substances, with weak or strong electrolytes. The particle sizes of the dispersed phase of colloids range from 100 to 1 nm. They cannot be seen with the naked eye. As a result of settling, the phase and medium are difficult to separate.
Classification by types of dispersed phase particles
Multimolecular colloids. When, upon dissolution, atoms or smaller molecules of substances (having a diameter less than 1 nm) combine together to form particles of similar sizes. In these sols, the dispersed phase is a structure that consists of aggregates of atoms or molecules with a molecular size of less than 1 nm. For example, gold and sulfur. These are held together by van der Waals forces. They are usually lyophilic in nature. This means significant particle interaction.
High molecular weight colloids. These are substances that have large molecules (so-called macromolecules), which, when dissolved, form a certain diameter. Such substances are called macromolecular colloids. These elements forming the dispersed phase are usually polymers having very high molecular weights. Natural macromolecules are starch, cellulose, proteins, enzymes, gelatin, etc. Artificial ones include synthetic polymers such as nylon, polyethylene, plastics, polystyrene, etc. They are usually lyophobic, which means in this case weak interaction particles.
Bound colloids. These are substances that, when dissolved in a medium, behave like normal electrolytes at low concentrations. But they are colloidal particles with a larger enzymatic component of components due to the formation of aggregated elements. The aggregate particles thus formed are called micelles. Their molecules contain both lyophilic and lyophobic groups.
Micelles. They are clustered or aggregated particles formed by the association of a colloid in solution. Common examples are soaps and detergents. Formation occurs above a certain Kraft temperature, and above a certain critical micellization concentration. They are capable of forming ions. Micelles can contain up to 100 molecules or more, with sodium stearate being a typical example. When it dissolves in water, it produces ions.