The degree of dispersion. Dispersed phase. Dispersion medium

Most of the substances around us are mixtures of various substances, therefore, the study of their properties plays an important role in the development of chemistry, medicine, food industry and other sectors of the economy. The article discusses what is the degree of dispersion, and how it affects the characteristics of the system.

What are dispersed systems?

Before discussing the degree of dispersion, it is necessary to explain to which systems this concept can be applied.

Imagine that we have two different substances that can differ from each other in chemical composition, for example, salt 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 it comes to dispersed systems, it is believed that no reaction occurs during their formation, that is, the starting materials retain their structure at the micro level and their physical properties, for example, 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. In its formation, the concepts of “dispersion medium” and “phase” are used. The first has the property of continuity within the system and, as a rule, is in it in a large relative amount. The second (dispersed phase) is characterized by the discontinuity property, 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 the above 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 sand grains existing in the water, whose chemical formula is SiO ₂ , will not differ from the state when they were not in the water. In such cases, they speak of heterogeneity. In other words, a heterogeneous system is a mixture of several (two or more) phases. The latter is understood as a certain 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 dissolve in any medium can become so small that they cease to show their individual properties. In this case, they speak of homogeneous or homogeneous substances. Although they contain several components, they all form one phase throughout the entire system. An example of a homogeneous system is a solution of NaCl in water. When it is dissolved due to interaction with polar H ₂ O molecules, the NaCl crystal breaks up into individual cations (Na ⁺ ) and anions (Cl ^- ). They are uniformly mixed with water, and it is no longer possible to find the interface between a soluble substance and a solvent in such a system.

Particle size

What is the degree of dispersion? This value must be considered in more detail. What is she like? It is inversely proportional to the particle size of the dispersed phase. It is this characteristic that underlies the classification of all considered substances.

In the study of dispersed systems, students are often confused in their names, because they believe that the aggregate state is also the basis of their classification. This is not true. Mixtures of different states of aggregation really have different names, for example, emulsions are water substances, and aerosols already assume 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

The classification of dispersed systems by 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 and 100 nm, then the substance under consideration will be called a colloidal solution.
• If the particles are larger than 100 nm, then we are talking about suspensions or suspensions.

Regarding the above classification, we will clarify two points: first, the figures given are indicative, that is, a system in which the particle size will be 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 can see that the phrase "conditional size" is used in the list. 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 talk about a certain average (conditional) size.

Further in the article we give a brief description of the noted types of disperse systems.

True solutions

As mentioned above, the degree of dispersion of particles in these solutions is so great (their size is very small, <1 nm) that there is no interface between them and the solvent (medium), that is, a single-phase homogeneous system takes place. For completeness, recall that the size of an atom is about one angstrom (0.1 nm). The last figure indicates that the particles in these solutions have atomic sizes.

The main properties of true solutions that distinguish them from colloids and suspensions are as follows:

• The state of the solution exists indefinitely unchanged, that is, no precipitate of the dispersed phase forms.
• The dissolved substance cannot be separated from the solvent by filtration through plain paper.
• The substance also does not separate as a result of the transition through a porous membrane, which is called dialysis in chemistry.
• The solute can be separated from the 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 do not scatter light.

An example of true solutions is the mixing of various salts with water, for example, NaCl (sodium chloride), NaHCO ₃ (baking soda), KNO ₃ (potassium nitrate) and others.

Colloidal solutions

These are intermediate systems between real solutions and suspensions. However, they have a number of unique characteristics. We list them:

• They are mechanically stable indefinitely 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 by filter paper, however, the dialysis process results in the separation of the dispersed phase and the medium.
• As for true solutions, electrolysis can be carried out for them.
• Transparent colloidal systems are characterized by the so-called Tyndall effect: passing a ray of light through this system, you can see it. This is due to the dispersion of electromagnetic waves in the visible part of the spectrum in all directions.
• The ability to adsorb other substances.

Colloidal systems, due to these properties, are widely used by man 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, we will explain in more detail some of these properties for colloids.

What are colloidal solutions?

The classification can be given for this type of dispersed systems, taking into account the different aggregate states of the medium and the phase dissolved in it. Below is the corresponding table /

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, recalling that the difference with colloids in them is only in the size of the dispersed phase. However, suspensions are mechanically unstable, therefore, are of less interest for practice than colloidal systems.

The reason for the mechanical stability of colloids

Why can mayonnaise lie for a long time in the refrigerator, and suspended particles in it do not precipitate? Why do particles of paints dissolved in water do not “fall” over time to the bottom of the vessel? The answer to these questions is the Brownian movement.

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 particles of pollen 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 you increase the temperature of the liquid. It is this type of movement that makes small particles of colloidal solutions be in suspension.

Adsorption property

Dispersion 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 S / m ratio is a large value, here S is the total interface between two phases (dispersion medium and particles), m - total mass of particles in solution.

The 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 other molecules. 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 m ² / g.

The degree of dispersion and specific surface

The calculation of S / m is not an easy task. The fact is that particles in a colloidal solution have different sizes, shapes, and also the surface of each particle has a unique relief. Therefore, theoretical methods for solving this problem lead to qualitative results, not quantitative ones. Nevertheless, it is useful to give the specific surface area formula 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 simple calculations we get the expression: S _ud = 6 / (d * ρ), where S _ud is the surface area (specific), d is the particle diameter, ρ - the density of the substance of which it consists. It can be seen from the formula that the smallest and heaviest particles will make the largest contribution to the quantity under consideration.

The experimental method for determining S _ud consists in calculating the volume of gas that is adsorbed by the test substance, as well as in measuring the pore size (dispersed phase) in it.

Lyophilic and lyophobic systems

Lyophilicity and lyophobicity are those characteristics that, in essence, determine the existence of a classification of dispersed systems in the form in which it is given above. Both concepts characterize the force bond between the solvent molecules and the solute. If this connection is great, then they talk about lyophilism. So, all true solutions of salts in water are lyophilic, because their particles (ions) are electrically connected with polar H ₂ O molecules. If we consider such systems as butter or mayonnaise, then these are representatives of typical hydrophobic colloids, as long as they contain fat molecules (lipids) are repelled by polar molecules of H ₂ O.

It is important to note that lyophobic (hydrophobic, if the solvent is water) systems are thermodynamically unstable, which distinguishes them from lyophilic ones.

Suspension Properties

Now we consider the last class of dispersed systems - suspensions. Recall that they are characterized in that the smallest particle in them is greater than or of the order of 100 nm. What properties do they have? Below is the corresponding list:

• They are mechanically unstable, therefore, in a short period of time, a precipitate forms in them.
• They are cloudy and opaque to sunlight.
• The phase from the medium can be separated using filter paper.

Examples of suspensions in nature include cloudy water in rivers or volcanic ash. The use of suspensions by a person is usually associated with medicine (drug solutions).

Coagulation

What can be said of mixtures of substances with varying degrees of dispersion? Partially this question has already been covered in the article, since in any dispersed 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 disrupted, and coagulation will occur. Its reason is the influence of the electric fields of the ions of the true solution on the surface charge of colloidal particles.