After studying the topic of the lesson, you will learn:
- What are disperse systems?
- what are disperse systems?
- What properties do disperse systems have?
- the importance of dispersed systems.
Pure substances are very rare in nature. Crystals of pure substances - sugar or table salt, for example, can be obtained in different sizes - large and small. Whatever the size of the crystals, they all have the same internal structure for a given substance - a molecular or ionic crystal lattice.
In nature, mixtures of various substances are most often found. Mixtures of different substances in different states of aggregation can form heterogeneous and homogeneous systems. We will call such systems dispersed.
A dispersed system is a system consisting of two or more substances, one of them in the form of very small particles evenly distributed in the volume of the other.
A substance breaks down into ions, molecules, atoms, which means it “splits” into tiny particles. “Crushing” > dispersing, i.e. substances are dispersed to different particle sizes, visible and invisible.
A substance that is present in a smaller quantity, dispersed 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 dispersed medium. There is an interface between it and the particles of the dispersed phase; therefore, dispersed systems are called heterogeneous (inhomogeneous).
Both the dispersed medium and the dispersed phase can be represented by substances in various states of aggregation - solid, liquid and gaseous.
Depending on the combination of the aggregate state of the dispersed medium and the dispersed phase, 9 types of such systems can be distinguished.
Table
Examples of dispersed systems
| Dispersive medium | Dispersed phase | Examples of some natural and household disperse systems |
| Gas | Gas | Always homogeneous mixture (air, natural gas) |
| Liquid | Fog, associated gas with oil droplets, carburetor mixture in car engines (gasoline droplets in the air), aerosols | |
| Solid | Dust in the air, smoke, smog, simooms (dust and sand storms), aerosols | |
| Liquid | Gas | Effervescent drinks, foams |
| Liquid | Emulsions. Liquid media of the body (blood plasma, lymph, digestive juices), liquid contents of cells (cytoplasm, karyoplasm) | |
| Solid | Sols, gels, pastes (jelly, jellies, glues). River and sea silt suspended in water; mortars | |
| Solid | Gas | Snow crust with air bubbles in it, soil, textile fabrics, brick and ceramics, foam rubber, aerated chocolate, powders |
| Liquid | Moist soil, medical and cosmetic products (ointments, mascara, lipstick, etc.) | |
| Solid | Rocks, colored glasses, some alloys |
Based on the size of the particles of substances that make up the dispersed phase, dispersed systems are divided into coarse (suspensions) with particle sizes greater 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 - solution. It is homogeneous, there is no interface between the particles and the medium.
Dispersed systems and solutions are very important 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.
SUSPENSION
Suspensions 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 dispersed medium are easily separated by settling and filtration. Such systems are divided into:
- Emulsions ( both the medium and the phase are liquids insoluble in each other). An emulsion can be prepared from water and oil by shaking the mixture for a long time. These are well-known milk, lymph, water-based paints, etc.
- Suspensions(medium is a liquid, phase is a solid insoluble in it). To prepare a suspension, you need to grind the substance to a fine powder, pour it into the liquid and shake well. Over time, the particle will fall to the bottom of the vessel. Obviously, the smaller the particles, the longer the suspension will persist. These are construction solutions, river and sea silt suspended in water, a living suspension of microscopic living organisms in sea water - plankton, which feeds giants - whales, etc.
- Aerosols suspensions in a gas (for example, in air) of small particles of liquids or solids. There are dusts, smokes, and fogs. The first two types of aerosols are suspensions of solid particles in gas (larger particles in dust), the latter is a suspension of liquid droplets in gas. For example: 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. Smoke from factory chimneys, smog, tiny droplets of saliva flying out of the mouth of a flu patient are also harmful aerosols. Aerosols play an important role in nature, everyday life and human production activities. The accumulation of clouds, the treatment of fields with chemicals, the application of paint and varnish coatings using a spray gun, the treatment of the respiratory tract (inhalation) are examples of those phenomena and processes where aerosols are beneficial. 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 (translated from the Greek “colla” - glue, “eidos” - glue-like type) – 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 dispersed medium in such systems are difficult to separate by settling.
You know from your general biology course that particles of this size can be detected using an ultramicroscope, which uses the principle of light scattering. Thanks to this, the colloidal particle in it appears as a bright dot against a dark background.
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, 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 (III) chloride in hot water.
A characteristic property of colloidal solutions is their transparency. 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. This phenomenon is called the Tyndall effect. The particles of the dispersed phase of the sol, larger than in the true solution, reflect light from their surface, and the observer sees a luminous cone in the vessel with the colloidal solution. It is not formed in a true solution. You can observe a similar effect, but only for an aerosol and not a liquid colloid, in the forest and in cinemas when a beam of light from a movie camera passes through the air of the cinema hall.
Passing a beam of light through solutions;
a – true sodium chloride solution;
b – colloidal solution of iron (III) hydroxide.
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. This is explained by the fact that substances in a colloidal, i.e., finely divided, state have a large surface area. Either positively or negatively charged ions are adsorbed on this surface. For example, silicic acid adsorbs negative ions SiO 3 2-, of which there are many in solution due to the dissociation of sodium silicate:
Particles with like charges repel each other and therefore do not stick together.
But under certain conditions, a coagulation process can occur. When some colloidal solutions are boiled, desorption of charged ions occurs, i.e. colloidal particles lose their charge. They begin to enlarge and settle. The same thing is observed when adding any electrolyte. In this case, the colloidal particle attracts an oppositely charged ion and its charge is neutralized.
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 are gelatinous precipitates 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, marmalade, Bird's Milk cake) and of course an endless variety of natural gels: minerals (opal), jellyfish bodies, cartilage, tendons , hair, muscle and nerve tissue, etc. The history of development 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 (flakes off) - water is released from them. This phenomenon is called syneresis.
Perform laboratory experiments on the topic (group work, in a group of 4 people).
You have been given a sample of the dispersed system. Your task: to determine which disperse system was given to you.
Given to students: sugar solution, iron (III) chloride solution, a mixture of water and river sand, gelatin, aluminum chloride solution, table salt solution, a mixture of water and vegetable oil.
Instructions for performing laboratory experiments
- Carefully examine the sample given to you (external description). Fill out column No. 1 of the table.
- Stir the disperse system. Observe the ability to settle.
It settles or stratifies within a few minutes, or with difficulty over a long period of time, or does not settle. Fill out column No. 2 of the table.
If you do not observe particle settling, examine it for the coagulation process. Pour a little solution into two test tubes and add 2-3 drops of yellow blood salt to one and 3-5 drops of alkali to the other, what do you observe?
- Pass the dispersed system through the filter. What are you observing? Fill out column No. 3 of the table. (Filter some into a test tube).
- Shine a flashlight beam through the solution against a background of dark paper. What are you observing? (Tyndall effect can be observed)
- Draw a conclusion: what kind of dispersed system is this? What is a dispersed medium? What is the dispersed phase? What are the particle sizes in it? (column No. 5).
Writing rules syncwine:
- The first line uses one word (usually a noun) to name the topic.
- The second line is a description of this topic with two adjectives.
- The third line is three verbs (or verb forms) naming the most characteristic actions of the subject.
- The fourth line is a four-word phrase that shows a personal attitude towards the topic.
- The last line is a synonym for the topic, emphasizing its essence.
Summer 2008 Vienna. Schönbrunn.
Summer 2008, Nizhny Novgorod region.
Clouds and their role in human life All the nature that surrounds us - animal and plant organisms, the hydrosphere and atmosphere, the earth's crust and subsoil are a complex collection of many different and different types of coarse and colloidal systems. Krasnova Maria, |
P.S.
Many thanks to O.G. Pershina, chemistry teacher at the Dmitrov Gymnasium, during the lesson we worked with the presentation we found, and it was supplemented with our examples.
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.
The substance that is present in smaller quantities and distributed in the volume of another is called the 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 a 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, 8 types of such systems can be distinguished (Table 11).
Table 11
Examples of dispersed systems
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 of the dispersed phase and the medium.
Even a quick acquaintance with dispersed systems and solutions shows how important they are in everyday life and in nature (see Table 11).
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.
The classification of disperse systems and solutions is presented in Scheme 2.
Scheme 2
Classification of disperse systems and solutions
Suspend
Suspensions 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 three groups:
- emulsions (both the medium and the phase are liquids insoluble in each other). These are well-known milk, lymph, water-based paints, etc.;
- 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.;
- aerosols are suspensions in a gas (for example, in the 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, milk powder production, and respiratory 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 water is the medium are of greatest importance.
Colloidal systems
Colloidal systems 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 (III) chloride 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. This phenomenon is called the Tyndall effect. The particles of the dispersed phase of the sol, larger than in the true solution, reflect light from their surface, and the observer sees a luminous cone in the vessel with the colloidal solution. It is not formed in a true solution. You can observe a similar effect, but only for an aerosol rather than a liquid colloid, in cinemas when a beam of light from a movie camera passes through the air of the cinema hall.
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. The second subgroup of colloidal systems is gels, or jellies y representing 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, aspic, jelly, marmalade, Bird's Milk soufflé 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.
Dispersion systems can be divided according to the particle size of the dispersion phase. If the particle size is less than one nm, these are molecular ionic systems, from one to one hundred nm are colloidal, and more than one hundred nm are coarse. The group of molecularly dispersed systems is represented by solutions. These are homogeneous systems that consist of two or more substances and are single-phase. These include gas, solid or solutions. In turn, these systems can be divided into subgroups:
- Molecular. When organic substances such as glucose combine with non-electrolytes. Such solutions were called true so that they could be distinguished from colloidal ones. These include solutions of glucose, sucrose, alcohol and others.
- Molecular-ionic. In case of interaction between weak electrolytes. This group includes acidic solutions, nitrogenous, hydrogen sulfide and others.
- Ionic. Compound of strong electrolytes. Prominent representatives are solutions of alkalis, salts and some acids.
Colloidal systems
Colloidal systems are microheterogeneous systems in which the sizes of colloidal particles vary from 100 to 1 nm. They may not precipitate for a long time due to the solvation ionic shell and electric charge. When distributed in a medium, colloidal solutions uniformly fill the entire volume and are divided into sols and gels, which in turn are precipitates in the form of jelly. These include albumin solution, gelatin, colloidal silver solutions. Jellied meat, soufflé, puddings are bright colloidal systems found in everyday life.
Coarse systems
Opaque systems or suspensions in which fine particle ingredients are visible to the naked eye. During the settling process, the dispersed phase is easily separated from the dispersed medium. They are divided into suspensions, emulsions, and aerosols. Systems in which a solid with larger particles are placed in a liquid dispersion medium are called suspensions. These include aqueous solutions of starch and clay. Unlike suspensions, emulsions are obtained by mixing two liquids, in which one is distributed in droplets into the other. An example of an emulsion is a mixture of oil and water, droplets of fat in milk. If small solid or liquid particles are distributed in a gas, these are aerosols. Essentially, an aerosol is a suspension in gas. One of the representatives of a liquid-based aerosol is fog - this is a large number of small water droplets suspended in the air. Solid aerosol - smoke or dust - a multiple accumulation of small solid particles also suspended in the air.
Dispersed systems. Definition. Classification.
Solutions
In the previous paragraph we talked about solutions. Let us briefly recall this concept here.
Solutions are called homogeneous (homogeneous) systems consisting of two or more components.
Homogeneous system is a homogeneous system, the chemical composition and physical properties of which are the same in all parts or change continuously, without jumps (there are no interfaces between parts of the system).
This definition of a solution is not entirely correct. It rather refers to true solutions.
At the same time, there are also colloidal solutions, which are not homogeneous, but heterogeneous, i.e. consist of different phases separated by an interface.
In order to achieve greater clarity in definitions, another term is used - dispersed systems.
Before considering dispersed systems, let’s talk a little about the history of their study and the appearance of such a term as colloidal solutions.
Background
Back in 1845, the chemist Francesco Selmi, while studying the properties of various solutions, noticed that biological fluids - serum and blood plasma, lymph and others - differ sharply in their properties from ordinary true solutions, and therefore such liquids were called pseudo-solutions.
Colloids and crystalloids
Further research in this direction, carried out since 1861 by the English scientist Thomas Graham, showed that some substances that quickly diffuse and pass through plant and animal membranes easily crystallize, while others have a low ability to diffusion, do not pass through membranes and do not crystallize , but form amorphous precipitates.
Graham named the first crystalloids, and the second – colloids(from the Greek words kolla - glue and eidos - kind) or glue-like substances.
In particular, it was found that substances capable of forming amorphous sediments, such as albumin, gelatin, gum arabic, iron and aluminum hydroxides and some other substances, diffuse in water slowly compared to the diffusion rate of crystalline substances such as table salt , magnesium sulfate, cane sugar, etc.
The table below shows the diffusion coefficients D for some crystalloids and colloids at 18°C.
The table shows that there is an inverse relationship between molecular weight and diffusion coefficient.
In addition, crystalloids were found to have the ability not only to diffuse quickly, but also dialyze, i.e. pass through membranes, as opposed to colloids, which have larger molecular sizes and therefore diffuse slowly and do not penetrate membranes.
The walls of a bull's bladder, cellophane, films of ferrous-cyanide copper, etc. are used as membranes.
Based on his observations, Graham established that all substances can be divided into crystalloids and colloids.
Russians disagree
A professor at Kyiv University objected to such a strict separation of chemicals I.G. Borschev(1869). Borshchev's opinion was later confirmed by the research of another Russian scientist Weimarn, who proved that the same substance, depending on conditions, can exhibit the properties of colloids or crystalloids.
For example, a solution of soap in water has the properties colloid, and soap dissolved in alcohol exhibits properties true solutions.
In the same way, crystalline salts, for example, table salt, dissolved in water, give true solution, and in benzene – colloidal solution and so on.
Hemoglobin or egg albumin, which has the properties of colloids, can be obtained in a crystalline state.
DI. Mendeleev believed that any substance, depending on the conditions and nature of the environment, can exhibit properties colloid. Currently, any substance can be obtained in a colloidal state.
Thus, there is no reason to divide substances into two separate classes - crystalloids and colloids, but we can talk about the colloidal and crystalloid states of the substance.
The colloidal state of a substance means a certain degree of its fragmentation or dispersion and the presence of colloidal particles in suspension in a solvent.
The science that studies the physicochemical properties of heterogeneous highly dispersed and high-molecular systems is called colloid chemistry.
Dispersed systems
If one substance, which is in a crushed (dispersed) state, is evenly distributed in the mass of another substance, then such a system is called dispersed.
In such systems, the fragmented substance is usually called dispersed phase, and the environment in which it is distributed is dispersion medium.
So, for example, a system representing agitated clay in water consists of suspended small particles of clay - the dispersed phase and water - the dispersion medium.
Dispersed(fragmented) systems are heterogeneous.
Dispersed systems, in contrast to heterogeneous ones with relatively large, continuous phases, are called microheterogeneous, and colloidal dispersed systems are called ultramicroheterogeneous.
Classification of disperse systems
Classification of dispersed systems is most often made based on degree of dispersion or state of aggregation dispersed phase and dispersion medium.
Classification by degree of dispersion
All dispersed systems Based on the size of dispersed phase particles, they can be divided into the following groups:
For reference, here are the units of size in the SI system:
1 m (meter) = 102 cm (centimeter) = 103 mm (millimeters) = 106 microns (micrometers) = 109 nm (nanometers).
Sometimes other units are used - mk (micron) or mmk (millimicron), and:
1 nm = 10 -9 m = 10 -7 cm = 1 mmk;
1 µm = 10 -6 m = 10 -4 cm = 1 µm.
Coarse dispersed systems.
These systems contain as a dispersed phase the largest particles with a diameter of 0.1 microns and above. These systems include suspensions And emulsions.
Suspensions are systems in which a solid substance is in a liquid dispersion medium, for example, a suspension of starch, clay, etc. in water.
Emulsions are called dispersion systems of two immiscible liquids, where droplets of one liquid are suspended in the volume of another liquid. For example, oil, benzene, toluene in water or droplets of fat (diameter from 0.1 to 22 microns) in milk, etc.
Colloidal systems.
They have the particle size of the dispersed phase from 0.1 µm to 1 µm(or from 10 -5 to 10 -7 cm). Such particles can pass through the pores of filter paper, but do not penetrate the pores of animal and plant membranes.
Colloidal particles if they have an electric charge and solvation-ion shells, they remain in a suspended state and, without changing conditions, may not precipitate for a very long time.
Examples of colloidal systems include solutions of albumin, gelatin, gum arabic, colloidal solutions of gold, silver, arsenic sulfide, etc.
Molecular dispersed systems.
Such systems have particle sizes not exceeding 1 mm. Molecular dispersed systems include true solutions of non-electrolytes.
Ion-dispersed systems.
These are solutions of various electrolytes, such as salts, bases, etc., which disintegrate into corresponding ions, the sizes of which are very small and go far beyond
10 -8 cm.
Clarification on the representation of true solutions as dispersed systems.
From the classification given here it is clear that any solution (both true and colloidal) can be represented as a dispersed medium. True and colloidal solutions will differ in the particle sizes of the dispersed phases. But above we wrote about the homogeneity of true solutions, and dispersion systems are heterogeneous. How to resolve this contradiction?
If speak about structure true solutions, then their homogeneity will be relative. The structural units of true solutions (molecules or ions) are much smaller than the particles of colloidal solutions. Therefore, we can say that compared to colloidal solutions and suspensions, true solutions are homogeneous.
If we talk about properties true solutions, then they cannot be fully called dispersed systems, since the mandatory existence of dispersed systems is the mutual insolubility of the dispersed substance and the dispersion medium.
In colloidal solutions and coarse suspensions, the dispersed phase and the dispersion medium practically do not mix and do not react chemically with each other. This cannot be said at all about true solutions. In them, when dissolved, substances mix and even interact with each other. For this reason, colloidal solutions differ sharply in properties from true solutions.
The sizes of some molecules, particles, cells.
As the particle sizes change from the largest to the smallest and back, the properties of dispersed systems will change accordingly. Wherein colloidal systems occupy as it were intermediate position between coarse suspensions and molecular disperse systems.
Classification according to the state of aggregation of the dispersed phase and dispersion medium.
Foam is a dispersion of gas in a liquid, and in foams the liquid degenerates into thin films separating individual gas bubbles.
Emulsions are dispersed systems in which one liquid is crushed by another liquid that does not dissolve it (for example, water in fat).
Suspensions are called low-disperse systems of solid particles in liquids.
Combinations of three types of aggregative states make it possible to distinguish nine types of dispersed systems:
Dispersed phase | Dispersive medium | Title and example |
|---|---|---|
Gaseous | Gaseous | No disperse system is formed |
Gaseous | Gas emulsions and foams |
|
Gaseous | Porous bodies: foam pumice |
|
Gaseous | Aerosols: fogs, clouds |
|
Emulsions: oil, cream, milk, margarine, butter |
||
Capillary systems: Liquid in porous bodies, soil, soil |
||
Gaseous | Aerosols (dusts, fumes), powders |
|
Suspensions: pulp, sludge, suspension, paste |
||
Solid systems: alloys, concrete |
Sols are another name for colloidal solutions.
Colloidal solutions are also called sols(from Latin solutus - dissolved).
Dispersed systems with a gaseous dispersion medium are called aerosols. Fogs are aerosols with a liquid dispersed phase, and dust and smoke are aerosols with a solid dispersed phase. Smoke is a more highly dispersed system than dust.
Dispersed systems with a liquid dispersion medium are called lysols(from the Greek “lios” - liquid).
Depending on the solvent (dispersion medium), i.e. water, benzene alcohol or ether, etc., there are hydrosols, alcosols, benzols, etherosols, etc.
Cohesively dispersed systems. Gels.
Dispersed systems can be freely dispersed And cohesively dispersed depending on the absence or presence of interaction between particles of the dispersed phase.
TO freely dispersed systems include aerosols, lysols, diluted suspensions and emulsions. They are fluid. In these systems, particles of the dispersed phase have no contacts, participate in random thermal motion, and move freely under the influence of gravity.
The pictures above show free-dispersed systems:
In the pictures a B C depicted corpuscular-dispersed systems:
a, b- monodisperse systems,
V- polydisperse system,
On the image G depicted fiber-dispersed system
On the image d depicted film-dispersed system
- solid. They arise when particles of the dispersed phase come into contact, leading to the formation of a structure in the form of a framework or network.
This structure limits the fluidity of the dispersed system and gives it the ability to retain its shape. Such structured colloidal systems are called gels.
The transition of a sol to a gel, which occurs as a result of a decrease in the stability of the sol, is called gelation(or gelatinization).
In the pictures a B C depicted cohesive dispersed systems:
A- gel,
b- coagulum with a dense structure,
V- coagulum with a loose “arched” structure
In the pictures g, d depicted capillary-dispersed systems
Powders (pastes), foams– examples of cohesively dispersed systems.
The soil, formed as a result of contact and compaction of dispersed particles of soil minerals and humus (organic) substances, is also a coherently dispersed system.
A continuous mass of substance can be penetrated by pores and capillaries, forming capillary-dispersed systems. These include, for example, wood, leather, paper, cardboard, fabrics.
Lyophilicity and lyophobicity
A general characteristic of colloidal solutions is the property of their dispersed phase to interact with the dispersion medium. In this regard, two types of sols are distinguished:
1. Lyophobic(from Greek phobia - hatred) And
2.Lyophilic(from Greek philia – love).
U lyophobic In sols, the particles have no affinity for the solvent, interact weakly with it, and form around themselves a thin shell of solvent molecules.
In particular, if the dispersion medium is water, then such systems are called hydrophobic, for example, sols of metals iron, gold, arsenic sulfide, silver chloride, etc.
IN lyophilic systems there is an affinity between the dispersed substance and the solvent. The particles of the dispersed phase, in this case, acquire a more voluminous shell of solvent molecules.
In the case of an aqueous dispersion medium, such systems are called hydrophilic, such as solutions of protein, starch, agar-agar, gum arabic, etc.
Coagulation of colloids. Stabilizers.
Substance at the interface.
All liquids and solids are limited by an outer surface at which they come into contact with phases of a different composition and structure, for example, vapor, another liquid or a solid.
Properties of matter in this interfacial surface, with a thickness of several diameters of atoms or molecules, differ from the properties inside the volume of the phase.
Inside the volume of a pure substance in a solid, liquid or gaseous state, any molecule is surrounded by similar molecules.
In the boundary layer, molecules are in interaction with another number of molecules (different in comparison with the interaction inside the volume of the substance).
This occurs, for example, at the interface of a liquid or solid with its vapor. Or, in the boundary layer, molecules of a substance interact with molecules of a different chemical nature, for example, at the boundary of two mutually poorly soluble liquids.
As a result, differences in the nature of the interaction inside the bulk of the phases and at the phase boundary arise force fields associated with this unevenness. (More on this in the section Surface tension of a liquid.)
The greater the difference in the intensity of intermolecular forces acting in each of the phases, the greater the potential energy of the interphase surface, briefly called surface energy.
Surface tension
To estimate surface energy, a quantity such as specific free surface energy is used. It is equal to the work spent on the formation of a unit area of a new phase interface (assuming a constant temperature).
In the case of a boundary between two condensed phases, this quantity is called boundary tension.
When talking about the boundary of a liquid with its vapors, this quantity is called surface tension.
Coagulation of colloids
All spontaneous processes occur in the direction of decreasing the energy of the system (isobaric potential).
Similarly, processes spontaneously occur at the phase interface in the direction of decreasing free surface energy.
The smaller the interphase surface, the smaller the free energy.
And the phase interface, in turn, is related to the degree of dispersion of the dissolved substance. The higher the dispersion (smaller particles of the dispersed phase), the larger the interface between the phases.
Thus, in dispersed systems there are always forces leading to a decrease in the total interphase surface, i.e. to particle enlargement. Therefore, the merging of small droplets in fogs, rain clouds and emulsions occurs - the aggregation of highly dispersed particles into larger formations.
All this leads to the destruction of dispersed systems: fogs and rain clouds rain, emulsions separate, colloidal solutions coagulate, i.e. are separated into a sediment of the dispersed phase (coagulate) and a dispersion medium or, in the case of elongated particles of the dispersed phase, turn into a gel.
The ability of fragmented systems to maintain their inherent degree of dispersion is called aggregative stability.
Stabilizers for dispersed systems
As stated earlier, dispersed systems are fundamentally thermodynamically unstable. The higher the dispersion, the greater the free surface energy, the greater the tendency to spontaneously reduce dispersion.
Therefore, to obtain stable, i.e. long-lasting suspensions, emulsions, colloidal solutions, it is necessary not only to achieve the desired dispersion, but also to create conditions for its stabilization.
In view of this, stable disperse systems consist of at least three components: a dispersed phase, a dispersion medium and a third component - disperse system stabilizer.
The stabilizer can be either ionic or molecular, often high-molecular, in nature.
Ionic stabilization of sols of lyophobic colloids is associated with the presence of low concentrations of electrolytes, creating ionic boundary layers between the dispersed phase and the dispersion medium.
High-molecular compounds (proteins, polypeptides, polyvinyl alcohol and others) added to stabilize dispersed systems are called protective colloids.
Adsorbed at the phase interface, they form mesh and gel-like structures in the surface layer, creating a structural-mechanical barrier that prevents the integration of particles of the dispersed phase.
Structural-mechanical stabilization is crucial for the stabilization of suspensions, pastes, foams, and concentrated emulsions.
It is quite difficult to find a pure substance in nature. In different states they can form mixtures, homogeneous and heterogeneous - dispersed systems and solutions. What are these connections? What types are they? Let's look at these questions in more detail.
Terminology
First you need to understand what disperse systems are. This definition refers to heterogeneous structures, where one substance, as tiny particles, is distributed evenly in the volume of another. The component that is present in smaller quantities is called the dispersed phase. It may contain more than one substance. The component present in larger volume is called the medium. There is an interface between the particles of the phase and it. In this regard, dispersed systems are called heterogeneous - heterogeneous. Both the medium and the phase can be represented by substances in various states of aggregation: liquid, gaseous or solid.
Dispersed systems and their classification
In accordance with the size of the particles included in the phase of substances, suspensions and colloidal structures are distinguished. The former have element sizes of more than 100 nm, and the latter - from 100 to 1 nm. When a substance is fragmented into ions or molecules whose size is less than 1 nm, a solution is formed - a homogeneous system. It differs from others in its homogeneity and the absence of an interface between the medium and particles. Colloidal disperse systems are presented in the form of gels and sols. In turn, suspensions are divided into suspensions, emulsions, and aerosols. Solutions can be ionic, molecular-ionic and molecular.
Suspend
These disperse systems include substances with particle sizes greater than 100 nm. These structures are opaque: their individual components can be seen with the naked eye. The medium and phase are easily separated upon settling. What are suspensions? They can be liquid or gaseous. The former are divided into suspensions and emulsions. The latter are structures in which the medium and phase are liquids that are insoluble in each other. These include, for example, lymph, milk, water-based paint and others. A suspension is a structure where the medium is a liquid and the phase is a solid, insoluble substance. Such dispersed systems are well known to many. These include, in particular, “milk of lime,” sea or river silt suspended in water, microscopic living organisms common in the ocean (plankton), and others.
Aerosols
These suspensions are distributed small particles of liquid or solid in a gas. There are fogs, smoke, dust. The first type is the distribution of small liquid droplets in a gas. Dusts and fumes are suspensions of solid components. Moreover, in the former the particles are somewhat larger. Natural aerosols include thunderclouds and fog itself. Smog, consisting of solid and liquid components distributed in gas, hangs over large industrial cities. It should be noted that aerosols as dispersed systems are of great practical importance and perform important tasks in industrial and domestic activities. Examples of positive results from their use include treatment of the respiratory system (inhalation), treatment of fields with chemicals, and spraying paint with a spray bottle.
Colloidal structures
These are dispersed systems in which the phase consists of particles ranging in size from 100 to 1 nm. Such components are not visible to the naked eye. The phase and medium in these structures are separated with difficulty by settling. Sols (colloidal solutions) are found in living cells and in the body as a whole. These fluids include nuclear juice, cytoplasm, lymph, blood and others. These dispersed systems form starch, adhesives, some polymers, and proteins. These structures can be obtained through chemical reactions. For example, during the interaction of solutions of sodium or potassium silicates with acidic compounds, a silicic acid compound is formed. Externally, the colloidal structure is similar to the true one. However, the former differ from the latter by the presence of a “luminous path” - a cone when a beam of light is passed through them. Sols contain larger phase particles than true solutions. Their surface reflects light - and the observer can see a luminous cone in the vessel. There is no such phenomenon in a true solution. A similar effect can also be observed in a movie theater. In this case, the light beam passes not through a liquid, but an aerosol colloid - the air of the hall.
Precipitation of particles
In colloidal solutions, phase particles often do not settle even during long-term storage, which is associated with continuous collisions with solvent molecules under the influence of thermal motion. When approaching each other, they do not stick together, since electric charges of the same name are present on their surfaces. However, under certain circumstances, a coagulation process can occur. It represents the effect of colloidal particles sticking together and precipitating. This process is observed when charges are neutralized on the surface of microscopic elements when an electrolyte is added. In this case, the solution turns into a gel or suspension. In some cases, the coagulation process is observed when heated or in case of changes in the acid-base balance.
Gels
These colloidal disperse systems are gelatinous sediments. They are formed during the coagulation of sols. These structures include numerous polymer gels, cosmetics, confectionery, and medical substances (Bird's Milk cake, marmalade, jelly, jellied meat, gelatin). These also include natural structures: opal, jellyfish bodies, hair, tendons, nervous and muscle tissue, cartilage. The process of development of life on planet Earth can, in fact, be considered the history of the evolution of the colloidal system. Over time, the gel structure is disrupted, and water begins to be released from it. This phenomenon is called syneresis.
Homogeneous systems
Solutions include two or more substances. They are always single-phase, that is, they are a solid, gaseous substance or liquid. But in any case, their structure is homogeneous. This effect is explained by the fact that in one substance another is distributed in the form of ions, atoms or molecules, the size of which is less than 1 nm. In the case when it is necessary to emphasize the difference between a solution and a colloidal structure, it is called true. In the process of crystallization of a liquid alloy of gold and silver, solid structures of different compositions are obtained.
Classification
Ionic mixtures are structures with strong electrolytes (acids, salts, alkalis - NaOH, HC104 and others). Another type is molecular-ion disperse systems. They contain a strong electrolyte (hydrogen sulfide, nitrous acid and others). The last type is molecular solutions. These structures include non-electrolytes - organic substances (sucrose, glucose, alcohol and others). A solvent is a component whose state of aggregation does not change during the formation of a solution. Such an element may, for example, be water. In a solution of table salt, carbon dioxide, sugar, it acts as a solvent. In the case of mixing gases, liquids or solids, the solvent will be the component of which there is more in the compound.
