Colloid 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 the solute.
Methods for obtaining colloidal solutions
The structure of a micelle formed when a colloidal system appears, partly depends on the mechanism of this process. Methods for obtaining colloids are divided into two fundamentally different groups.
Dispersion methods are associated with the grinding of rather large particles. Depending on the mechanism of this process, the following methods are distinguished.
- Refining. Can be done dry orwet way. In the first case, the solid is first crushed, and only then the liquid is added. In the second case, the substance is mixed with a liquid, and only after that it is turned into a homogeneous mixture. Grinding is carried out in special mills.
- Swelling. Grinding is achieved due to the fact that the solvent particles penetrate into the dispersed phase, which is accompanied by the expansion of its particles up to separation.
- Dispersion by ultrasound. The material to be ground is placed in a liquid and sonicated.
- Electric shock dispersion. Demanded in the production of metal sols. It is carried out by placing electrodes made of a dispersible metal into a liquid, followed by applying high voltage to them. As a result, a voltaic arc is formed in which the metal is sprayed and then condenses into a solution.
These methods are suitable for obtaining both lyophilic and lyophobic colloidal particles. The micelle structure is carried out simultaneously with the destruction of the original structure of the solid.
Condensation methods
The second group of methods based on particle enlargement is called condensation. This process can be based on physical or chemical phenomena. Physical condensation methods include the following.
- Replacement of the solvent. It comes 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, small particleswill combine into larger aggregates and a colloidal solution will appear.
- Vapour condensation. An example is fogs, whose particles are able to settle on cold surfaces and gradually grow larger.
Chemical condensation methods include some chemical reactions accompanied by precipitation of a complex structure:
- Ion exchange: NaCl + AgNO3=AgCl↓ + NaNO3.
- Redox processes: 2H2S + O2=2S↓ + 2H2 O.
- Hydrolysis: Al2S3 + 6H2O=2Al(OH) 3↓ + 3H2S.
Conditions for chemical condensation
The structure of micelles formed during these chemical reactions depends on the excess or deficiency of the substances involved. Also, for the appearance of colloidal solutions, it is necessary to observe a number of conditions that prevent the precipitation of a sparingly soluble compound:
- 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 a 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 reserve of free energy, which makes it possible to selectively adsorb ions from the environment. This processobeys the Peskov rule, which says: on the surface of a solid, those ions are predominantly adsorbed that are capable of completing its own crystal lattice. 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 ions, is formed on the micelle core. Due to electrostatic forces, the resulting charged aggregate attracts counterions (ions with the opposite charge) from the solution. Thus, a colloidal particle has a multilayer structure. The micelle acquires a dielectric layer built from two types of oppositely charged ions.
Hydrosol BaSO4
As an example, it is convenient to consider the structure of a barium sulfate micelle in a colloidal solution prepared in an excess of barium chloride. This process corresponds to the reaction equation:
BaCl2(p) + Na2SO4(p)=BaSO 4(t) + 2NaCl(p).
Slightly soluble in water, barium sulfate forms a microcrystalline aggregate built from the m-th number of BaSO molecules4. The surface of this aggregate adsorbs the n-th amount of Ba2+ ions. 2(n - x) Cl- ions are connected to the layer of potential-determining ions. And the rest of the counterions (2x) is located in the diffuse layer. That is, the granule of this micelle will be positively charged.
If sodium sulfate is taken in excess, thenthe potential-determining ions will be SO42- ions, and the counterions will be Na+. In this case, the charge of the granule will be negative.
This example clearly demonstrates that the charge sign of a micelle granule directly depends on the conditions of its preparation.
Recording micelles
The previous example showed that the chemical structure of micelles and the formula that reflects it is determined by the substance that is taken in excess. Let us consider ways of writing the names of individual parts of a colloidal particle using the copper sulfide hydrosol as an example. To prepare it, sodium sulfide solution is slowly poured into an excess amount of copper chloride solution:
CuCl2 + Na2S=CuS↓ + 2NaCl.
The structure of a CuS micelle obtained in excess of CuCl2 is written as follows:
{[mCuS]·nCu2+·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 entire particle. It is commonly called an aggregate. Usually, the number of molecules that make up the aggregate is written with the Latin letter m.
Potential-determining ions are contained in excess in solution. They are located on the surface of the aggregate, and in the formula they are written immediately after square brackets. The number of these ions is denoted by the symbol n. The name of these ions indicates that their charge determines the charge of the micelle granule.
A granule is formed by a core and a partcounterions in the adsorption layer. The value of the granule charge is equal to the sum of the charges of the potential-determining and adsorbed counterions: +(2n – x). The remaining part of the counterions is in the diffuse layer and compensates for the charge of the granule.
If Na2S was taken in excess, then for the formed colloidal micelle the structure scheme would look like:
{[m(CuS)]∙nS2–∙xNa+}–(2n – x) ∙(2n – x)Na+.
Micelles of surfactants
In the event that the concentration of surface-active substances (surfactants) in water is too high, aggregates of 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 have this ability, but only those in which the ratio of 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 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 micellization is determined by the solubility limit, then for organic surfactants it is determined by the achievement of critical concentrations of micellization;
- 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 affected by their concentration in solution. Upon reaching some of its values, colloidal particles begin to interact with each other. This causes their shape to change as follows:
- sphere turns into an ellipsoid and then into a cylinder;
- high concentration of cylinders leads to the formation of a hexagonal phase;
- in some cases, a lamellar phase and a solid crystal (soap particles) appear.
Types of micelles
Three types of colloidal systems are distinguished according to the peculiarities of the organization of the internal structure: suspensoids, micellar colloids, molecular colloids.
Suspensoids can be irreversible colloids, as well as lyophobic colloids. This structure is typical for solutions of metals, as well as their compounds (various oxides and s alts). 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 after evaporation to form a dry precipitate, which cannot be converted into a sol by simple dissolution. They are called lyophobic because of the weak interaction between the dispersed phase and the dispersion medium.
Micellar colloids are solutions whose colloidal particles are formedwhen sticking diphilic molecules containing polar groups of atoms and non-polar radicals. Examples are soaps and surfactants. Molecules in such micelles are held by dispersion forces. The shape of these colloids can be not only spherical, but also lamellar.
Molecular colloids are quite stable without stabilizers. Their structural units are individual macromolecules. The shape of a colloid particle can vary depending on the properties of the molecule and intramolecular interactions. So a linear molecule can form a rod or a coil.
