Most of the substances around us are mixtures of various substances, so the study of their properties plays an important role in the development of chemistry, medicine, the food industry and other sectors of the economy. The article discusses the issues of what is the degree of dispersion, and how it affects the characteristics of the system.
What are disperse systems?
Before discussing the degree of dispersion, it is necessary to clarify to which systems this concept can be applied.
Let's imagine that we have two different substances that may differ from each other in chemical composition, for example, table s alt and pure water, or in the state of aggregation, for example, the same water in liquid and solid (ice) states. Now you need to take and mix these two substances and mix them intensively. What will be the result? It depends on whether the chemical reaction took place during mixing or not. When talking about dispersed systems, it is believed that when theyno reaction occurs in the formation, that is, the initial substances retain their structure at the micro level and their inherent physical properties, such as density, color, electrical conductivity, and others.
Thus, a dispersed system is a mechanical mixture, as a result of which two or more substances are mixed with each other. When it is formed, the concepts of "dispersion medium" and "phase" are used. The first has the property of continuity within the system and, as a rule, is found in it in a large relative amount. The second (dispersed phase) is characterized by the property of discontinuity, that is, in the system it is in the form of small particles, which are limited by the surface that separates them from the medium.
Homogeneous and heterogeneous systems
It is clear that these two components of the dispersed system will differ in their physical properties. For example, if you throw sand into the water and stir it, it is clear that the grains of sand that exist in the water, the chemical formula of which is SiO2, will not differ in any way from the state when they were not in the water. In such cases, one speaks of heterogeneity. In other words, a heterogeneous system is a mixture of several (two or more) phases. The latter is understood as some finite volume of the system, which is characterized by certain properties. In the example above, we have two phases: sand and water.
However, the size of the particles of the dispersed phase when they are dissolved in any medium can become so small that they cease to show their individual properties. In this case, one speaks ofhomogeneous or homogeneous substances. Although they contain several components, they all form one phase throughout the entire volume of the system. An example of a homogeneous system is a solution of NaCl in water. When it dissolves, due to the interaction with polar molecules H2O, the NaCl crystal decomposes into separate cations (Na+) and anions (Cl -). They are homogeneously mixed with water, and it is no longer possible to find the interface between the solute and the solvent in such a system.
Particle size
What is the degree of dispersion? This value needs to be considered in more detail. What does she represent? It is inversely proportional to the particle size of the dispersed phase. It is this characteristic that underlies the classification of all substances under consideration.
When studying disperse systems, students often get confused in their names, because they believe that their classification is also based on the state of aggregation. This is not true. Mixtures of different states of aggregation really have different names, for example, emulsions are water substances, and aerosols already suggest the existence of a gas phase. However, the properties of disperse systems depend mainly on the particle size of the phase dissolved in them.
Generally accepted classification
Classification of disperse systems according to the degree of dispersion is given below:
- If the conditional particle size is less than 1 nm, then such systems are called real, or true solutions.
- If the conditional particle size lies between 1 nm and100 nm, then the substance in question will be called a colloidal solution.
- If the particles are larger than 100 nm, then we are talking about suspensions or suspensions.
With regard to the above classification, let's clarify two points: firstly, the given figures are indicative, that is, a system in which the particle size is 3 nm is not necessarily a colloid, it can also be a true solution. This can be established by studying its physical properties. Secondly, you may notice that the list uses the phrase "conditional size". This is due to the fact that the shape of the particles in the system can be completely arbitrary, and in the general case has a complex geometry. Therefore, they speak of some average (conditional) size of them.
Later in the article we will give a brief description of the noted types of disperse systems.
True solutions
As mentioned above, the degree of dispersion of particles in real solutions is so high (their size is very small, < 1 nm) that there is no interface between them and the solvent (medium), that is, there is a single-phase homogeneous system. For completeness of information, we recall that the size of an atom is on the order of one angstrom (0.1 nm). The last number indicates that the particles in real solutions are atomic in size.
The main properties of true solutions that distinguish them from colloids and suspensions are as follows:
- The state of the solution exists for an arbitrarily long time unchanged, that is, no precipitate of the dispersed phase is formed.
- Dissolvedthe substance cannot be separated from the solvent by filtration through plain paper.
- The substance is also not separated as a result of the process of passage through the porous membrane, which is called dialysis in chemistry.
- It is possible to separate a solute from a solvent only by changing the state of aggregation of the latter, for example, by evaporation.
- For ideal solutions, electrolysis can be carried out, that is, an electric current can be passed if a potential difference (two electrodes) is applied to the system.
- They don't scatter light.
An example of true solutions is the mixing of various s alts with water, for example, NaCl (table s alt), NaHCO3 (baking soda), KNO3(potassium nitrate) and others.
Colloid solutions
These are intermediate systems between real solutions and suspensions. However, they have a number of unique characteristics. Let's list them:
- They are mechanically stable for an arbitrarily long time if the environmental conditions do not change. It is enough to heat the system or change its acidity (pH value), as the colloid coagulates (precipitates).
- They are not separated using filter paper, however, the dialysis process leads to separation of the dispersed phase and the medium.
- As with true solutions, they can be electrolyzed.
- For transparent colloidal systems, the so-called Tyndall effect is characteristic: passing a beam of light through this system, you can see it. It is connected withscattering of electromagnetic waves in the visible part of the spectrum in all directions.
- Ability to adsorb other substances.
Colloidal systems, due to the listed properties, are widely used by humans in various fields of activity (food industry, chemistry), and are also often found in nature. An example of a colloid is butter, mayonnaise. In nature, these are fogs, clouds.
Before proceeding to the description of the last (third) class of disperse systems, let us explain in more detail some of the named properties for colloids.
What are colloidal solutions?
For this type of disperse systems, the classification can be given, taking into account the different aggregate states of the medium and the phase dissolved in it. Below is the corresponding table/
| Wednesday/Phase | Gas | Liquid | Rigid body |
| gas | all gases are infinitely soluble in each other, so they always form true solutions | aerosol (fog, clouds) | aerosol (smoke) |
| liquid | foam (shaving, whipped cream) | emulsion (milk, mayonnaise, sauce) | sol (watercolors) |
| solid body | foam (pumice, aerated chocolate) | gel (gelatin, cheese) | sol (ruby crystal, granite) |
The table shows that colloidal substances are present everywhere, both in everyday life and in nature. Note that a similar table can also be given for suspensions, remembering that the difference withcolloids in them is only in the size of the dispersed phase. However, suspensions are mechanically unstable and therefore are of less practical interest than colloidal systems.
The reason for the mechanical stability of colloids
Why mayonnaise can lie in the refrigerator for a long time, and suspended particles in it do not precipitate? Why don't paint particles dissolved in water eventually "fall" to the bottom of the vessel? The answer to these questions will be Brownian motion.
This type of movement was discovered in the first half of the 19th century by the English botanist Robert Brown, who observed under a microscope how small pollen particles move in water. From a physical point of view, Brownian motion is a manifestation of the chaotic movement of liquid molecules. Its intensity increases if the temperature of the liquid is raised. It is this type of movement that causes small particles of colloidal solutions to be in suspension.
Adsorption property
Dispersity is the reciprocal of the average particle size. Since this size in colloids ranges from 1 nm to 100 nm, they have a very developed surface, that is, the ratio S / m is a large value, here S is the total interface area between the two phases (dispersion medium and particles), m - total mass of particles in solution.
Atoms that are on the surface of the particles of the dispersed phase have unsaturated chemical bonds. This means that they can form compounds with othermolecules. As a rule, these compounds arise due to van der Waals forces or hydrogen bonds. They are able to hold several layers of molecules on the surface of colloidal particles.
A classic example of an adsorbent is activated carbon. It is a colloid, where the dispersion medium is a solid, and the phase is a gas. The specific surface area for it can reach 2500 m2/g.
Degree of fineness and specific surface area
Calculating S/m is not an easy task. The fact is that the particles in a colloidal solution have different sizes, shapes, and the surface of each particle has a unique relief. Therefore, theoretical methods for solving this problem lead to qualitative results, and not to quantitative ones. Nevertheless, it is useful to give the formula for the specific surface area from the degree of dispersion.
If we assume that all particles of the system have a spherical shape and the same size, then as a result of straightforward calculations, the following expression is obtained: Sud=6/(dρ), where Sud - surface area (specific), d - particle diameter, ρ - density of the substance of which it consists. It can be seen from the formula that the smallest and heaviest particles will contribute the most to the quantity under consideration.
The experimental way to determine Sud is to calculate the volume of gas that is adsorbed by the substance under study, as well as to measure the pore size (dispersed phase) in it.
Freeze-drying andlyophobic
Lyophilicity and lyophobicity - these are the characteristics that, in fact, determine the existence of the classification of disperse systems in the form in which it is given above. Both concepts characterize the force bond between the molecules of the solvent and the solute. If this relationship is large, then they speak of lyophilicity. So, all true solutions of s alts in water are lyophilic, since their particles (ions) are electrically connected with polar molecules H2O. If we consider such systems as butter or mayonnaise, then these are representatives of typical hydrophobic colloids, since fat (lipid) molecules in them repel polar molecules H2O.
It is important to note that lyophobic (hydrophobic if the solvent is water) systems are thermodynamically unstable, which distinguishes them from lyophilic ones.
Properties of suspensions
Now consider the last class of disperse systems - suspensions. Recall that they are characterized by the fact that the smallest particle in them is larger than or of the order of 100 nm. What properties do they have? The corresponding list is given below:
- They are mechanically unstable, so they form sediment in a short period of time.
- They are cloudy and opaque to sunlight.
- Phase can be separated from medium with filter paper.
Examples of suspensions in nature include muddy water in rivers or volcanic ash. Human use of suspensions is associated asusually with medicine (drug solutions).
Coagulation
What can be said about mixtures of substances with different degrees of dispersion? Partially, this issue has already been covered in the article, since in any disperse system the particles have a size that lies within certain limits. Here we only consider one curious case. What happens if you mix a colloid and a true electrolyte solution? The weighted system will be broken, and its coagulation will occur. Its reason lies in the influence of the electric fields of the true solution ions on the surface charge of colloidal particles.
