Colloidal systems are extremely important in the life of any person. This is due not only to the fact that almost all biological fluids in a living organism form colloids. But many natural phenomena (fog, smog), soil, minerals, food, medicines are also colloidal systems.
The unit of such formations, reflecting their composition and specific properties, is considered to be a macromolecule, or micelle. The structure of the latter depends on a number of factors, but it is always a multilayer particle. Modern molecular kinetic theory considers colloidal solutions as a special case of true solutions, with larger particles of solute.
Methods for producing colloidal solutions
The structure of the micelle formed during the appearance of the colloidal system, partly depends on the mechanism of this process. Methods for producing colloids are divided into two fundamentally different groups.
Dispersion methods are associated with the grinding of fairly large particles. Depending on the mechanism of this process, the following methods are distinguished.
- Grinding It can be carried out dry or wet. In the first case, the solid is first crushed, and then liquid is added. In the second case, the substance is mixed with a liquid, and only after that is it turned into a homogeneous mixture. Grinding is carried out in special mills.
- Swelling. Grinding is achieved due to the fact that the particles of the solvent penetrate into the dispersed phase, which is accompanied by the expansion of its particles until separation.
- Dispersion by ultrasound. Material subject to grinding is placed in a liquid and is acted upon by ultrasound.
- Dispersion by electric current. Demanded on receipt of metal sols. It is carried out by placing electrodes of dispersible metal in a liquid with subsequent supply of high voltage to them. As a result, a voltaic arc is formed in which the metal is sprayed and then condensed into the solution.
These methods are suitable for producing both lyophilic and lyophobic colloidal particles. The structure of the micelle is carried out simultaneously with the destruction of the original structure of a solid substance.
Condensation methods
The second group of methods based on particle enlargement is called condensation. This process may be based on physical or chemical phenomena. The methods of physical condensation include the following.
- Solvent replacement. It boils down to the transfer of a substance from one solvent in which it dissolves very well, into another, in which the solubility is much lower. As a result of this, small particles will merge into larger aggregates and a colloidal solution will appear.
- Condensation from vapors. As an example, we can mention mists, the particles of which are able to settle on cold surfaces and gradually coarsen.
Chemical condensation methods include some chemical reactions accompanied by precipitation of a complex structure:
- Ion exchange: NaCl + AgNO 3 = AgCl ↓ + NaNO 3 .
- Redox processes: 2H 2 S + O 2 = 2S ↓ + 2H 2 O.
- Hydrolysis: Al 2 S 3 + 6H 2 O = 2Al (OH) 3 ↓ + 3H 2 S.
Chemical Condensation Conditions
The structure of micelles formed during these chemical reactions depends on the excess or lack of substances involved in them. Also, for the appearance of colloidal solutions, it is necessary to observe a number of conditions that prevent the precipitation of an insoluble compound:
- the content of substances in mixed solutions should be low;
- their mixing speed should be low;
- one of the solutions should be taken in excess.
Micelle structure
The main part of the micelle is the core. It is formed by a large number of atoms, ions and molecules of an insoluble compound. Usually the core is characterized by a crystalline structure. The surface of the nucleus has a supply of free energy, which allows selective adsorption of ions from the environment. This process obeys the Peskov rule, which states: on the surface of a solid substance, those ions that are capable of completing its crystal lattice are predominantly adsorbed. This is possible if these ions are related or similar in nature and shape (size).
During adsorption, a layer of positively or negatively charged ions, called potential-determining, forms on the micelle core. Thanks to electrostatic forces, the resulting charged unit attracts counterions from the solution (ions with the opposite charge). Thus, the colloidal particle has a multilayer structure. Micella acquires a dielectric layer built of two types of oppositely charged ions.
Hydrosol BaSO4
As an example, it is convenient to consider the structure of the barium sulfate micelle in a colloidal solution prepared in excess of barium chloride. The reaction equation corresponds to this process:
BaCl 2 (p) + Na 2 SO 4 (p) = BaSO 4 ( t ) + 2NaCl (p) .
Barium sulfate, sparingly soluble in water, forms a microcrystalline aggregate constructed from the mth number of BaSO 4 molecules. The surface of this aggregate adsorbs the nth amount of Ba 2+ ions . 2 (n - x) Cl - ions are associated with a layer of potential-determining ions. And the rest of the counterions (2x) are located in the diffuse layer. That is, the granule of this micelle will be positively charged.
If sodium sulfate is taken in excess, then the SO 4 2– ions will be the potential-determining ions, and Na + will be the counter ions. In this case, the charge of the granule will be negative.
This example clearly demonstrates that the sign of the charge of a micelle granule directly depends on the conditions for its preparation.
Micelle record
The previous example showed that the chemical structure of micelles and the formula reflecting it is determined by the substance, which is taken in excess. Let us consider the ways of recording the names of individual parts of a colloidal particle using the example of a copper sulfide hydrosol. To prepare it, an excess of a solution of copper chloride is slowly poured into a solution of sodium sulfide:
CuCl 2 + Na 2 S = CuS ↓ + 2NaCl.
The structure of the CuS micelle obtained in excess CuCl 2 is written as follows:
{[mCuS] · nCu 2+ · xCl - } + (2n-x) · (2n-x) Cl - .
Structural parts of a colloidal particle
In square brackets write the formula of a sparingly soluble compound, which is the basis of the whole particle. It is commonly called an aggregate. Usually, the number of molecules making up an aggregate is written in Latin letter m.
Potentially determining ions are contained in excess in solution. They are located on the surface of the unit, and in the formula they are written immediately after square brackets. The number of these ions is denoted by n. The name of these ions indicates that their charge determines the charge of the micelle granule.
The granule is formed by the core and part of the counterions in the adsorption layer. The granule charge is equal to the sum of the charges of the potential-determining and adsorbed counterions: + (2n - x). The rest of the counterions are in the diffuse layer and compensates for the charge of the granule.
If Na 2 S were taken in excess, then for the resulting colloidal micelle the structure scheme would have the form:
{[m (CuS)] ∙ nS 2– ∙ xNa + } - (2n - x) ∙ (2n - x) Na + .
Surfactant micelles
In the event that the concentration of surfactants in the water is too high, aggregates from their molecules (or ions) may begin to form. These enlarged particles have the shape of a sphere and are called Gartley-Rebinder micelles. It should be noted that not all surfactants possess this ability, but only those in which the ratio of the hydrophobic and hydrophilic parts is optimal. This ratio is called the hydrophilic-lipophilic balance. The ability of their polar groups to protect the hydrocarbon core from water also plays a significant role.
Aggregates of surfactant molecules are formed according to certain laws:
- unlike low molecular weight substances, the aggregates of which may include a different number of molecules m, the existence of surfactant micelles is possible with a strictly defined number of molecules;
- if for inorganic substances the start of micelle formation is determined by the solubility limit, then for organic surfactants it is determined by the achievement of critical micelle formation concentrations;
- First, the number of micelles in the solution increases, and then their size increases.
Effect of concentration on micelle shape
The structure of surfactant micelles is influenced by their concentration in solution. Upon reaching some of its values, colloidal particles begin to interact with each other. This leads to a change in their shape as follows:
- the sphere turns into an ellipsoid, and then into a cylinder;
- a high concentration of cylinders leads to the formation of a hexagonal phase;
- in some cases, a lamellar phase and a solid crystal (soap particles) arise.
Types of micelles
According to the peculiarities of the organization of the internal structure, three types of colloidal systems are distinguished: suspensionoids, micellar colloids, and molecular colloids.
Suspension can be irreversible colloids, as well as lyophobic colloids. This structure is typical for metal solutions, as well as their compounds (various oxides and salts). The structure of the dispersed phase formed by suspensoids does not differ from the structure of a compact substance. It has a molecular or ionic crystal lattice. The difference from suspensions is a higher dispersion. Irreversibility is manifested in the ability of their solutions to form a dry precipitate after evaporation, which cannot be turned into sol by simple dissolution. They are called lyophobic due to the weak interaction between the dispersed phase and the dispersion medium.
Micellar colloids are solutions whose colloidal particles arise upon the adhesion of diphilic molecules containing polar groups of atoms and nonpolar radicals. Examples are soaps and surfactants. Molecules in such micelles are retained by dispersion forces. The shape of these colloids can be not only spherical, but also lamellar.
Molecular colloids are completely stable without stabilizers. Their structural units are individual macromolecules. The particle shape of the colloid may vary depending on the properties of the molecule and intramolecular interactions. So a linear molecule can form a rod or a ball.