We should distinguish two kinds of stability of dispersion systems: sedimentation (kinetic) and aggregative. Sedimentation stability is the ability of particles of a dispersed phase to remain in the suspension state.

Sedimentation stability depends on the:

size of the particles of dispersed phase (It is less than 10−6 m a system has a high sedimentation stability. When the particles are larger, the system is not stable),

temperature,

viscosity of dispersion medium,

density of the dispersed phase and dispersion medium.

Aggregative stability characterizes the ability of particles from dispersed phase to show resistances to their adhesion and in this way to keep a definite degree of dispersion.

The main factors of aggregative stability of dispersion systems are the following: particles have an ionic shell, i.e. double electric layer, the diffuse layer of counter ions and also their solvation (hydrated) sphere. The loss of aggregative stability leads to coagulation.

The colloidal of disperse is determined by their ability to preserve their appearance – color, transparency, and «homogeneity». We conditionally distinguish sedimentation and aggregative stability. A disperse system is considered to how sedimentation stability if its dispersed particles do not settle and the system does not stratity, i.e. is in stable sedimentation equilibrium.

Coagulation is the process of joining of colloid particles and forming of greater aggregates which leads to their precipitation under the influence of the forces of gravity followed by further phase division.

Coagulation can be caused by different factors: change in temperature, action of light, mechanical influence, irradiation, the increase in sol concentration, adding of electrolytes.

Factors govering coagulation:

A little amount of suitable electrolyte may bring coagulation.

Coagulation is brought about by oppositely charged ions of the electrolyte.

Coagulation of a sol is more pronounced at high temperatures.

Сritical coagulation concentration:

C.C.C.=𝑪𝒆𝒍∙𝑽𝒆𝒍

𝑽𝒔𝒐𝒍+𝑽𝒆𝒍

Сel – concentration of an electrolyte

Vel – volume of an electrolyte

Vsol – volume of a sol.

The valence of the effective ion. The coagulating power increases with the increase of the valence of the active cation i.e. Al3+ > Ba2+ > Na+ and active anion i.e. [Fe(CN)6]4- > PO4

3- > SO42- > Cl- (Schulze-Hardy rule).

P=𝟏

𝑪.𝑪.𝑪.

Where P – coagulation ability.

The type of the colloidal solution. The lyophobic colloids are easily coagulated while lyophilic colloids require more amount of electrolyte.

Coagulation is caused by the ion which charge is opposite in sign to the surface

charge of colloid particles. The coagulation of positively charged sols is caused by

anions of the added electrolyte, of negatively charges sols – by cations of electrolyte.

Coagulation action of electrolytes is determined by Shulze-Gardi rule that reads:

“Coagulation action is caused by a counter ion and the coagulating ability increases progressively to some high degree of its charge.”

According to the Schulze-Hardy rule the coagulating ability of ion increases with increasing of its charge, therefore the lowest critical concentration of coagulation have electrolytes with multi-charged coagulating ion.

For inorganic ions with the same charges their coagulating ability increases with decreasing their degree of hydratation and increasing of the ion radius.

The phenomenon of coagulation by electrolytes plays a great role in the living organism as colloid solutions of cells and biological fluids contact with electrolytes.

That’s why at the introduction of some electrolyte in the organism we should take into account not only its concentration but the ion charge. For example, saline of sodium chloride can’t be substituted by isotonic solution of magnesium chloride because this salt contains a divalent ion of magnesium exhibiting a higher coagulating property.

Coagulation of any colloid solution doesn’t take place immediately, it takes some time.

We should distinguish two stages of coagulation: latent and explicit.

During the first stage we can see the enlargement of particles without any vivid changes in the optical properties of the solution (latent coagulation). During the second stage we can observe the further enlargement of particles accompanied by explicit changes in sol (explicit coagulation).

At fig. we can see the curve (OSKN) which reflects the dependence of the sol coagulation rate on the concentration of the added electrolyte.

The segment OS corresponds to latent coagulation and point A is the electrolyte concentration at coagulation threshold which can be fixed. The characteristic of explicit coagulation is the change in its colour.

At the beginning of explicit coagulation (segment SKN) its rate is small. But with the increase in electrolyte concentration the rate is increased too. That’s why we should distinguish slow (SK) and quick (KN) coagulation. Point B corresponds the electrolyte concentration at some residual value of ξ -potential (in scientific literature it is called the critical potential).

Oppositely charged sols when mixed in almost equal proportions, neutralise their charges and get partially or completely precipitated. This type of coagulation is called mutual coagulation.

1. The coagulating action of electrolyte mixture is summed up, i.e. the mixture of electrolytes acts in the same way as one of them taken in the same amount. It’s an additive action.

2. The coagulating action of electrolyte mixture is less than each of them taken separately. It’s antagonism. It’s characteristic for ion mixtures having different valence.

3.The coagulating action of electrolyte mixture is greater than each of them taken separately. It’s synergism.

The stability of colloid solutions can be increased using not only small amount of electrolyte but adding to it high molecular compounds (HMC). The increase in the stability of a colloid solution when adding to it HMC is called colloid protection.

For example, if we add a small amount of gelatin solution into sol of iron (III) hydroxide, it will require much more electrolyte for the sol coagulation than for the coagulation of an unprotected sol.

The mechanism of protective action is based on the formation of adsorption shell from HMC molecules around the colloid particle while these molecules form a structural and mechanical barrier preventing the particles from adhesion. At the same time not only aggregative but sedimentation sol stability is increased as a result of the increasing viscosity of dispersion medium.

Peptization. If a freshly precipitated Fe(OH)3 is treated with a small amount of FeCl3 solution, a reddish brown coloured sol of Fe(OH)3 is obtained. Thus process of transferring precipitate back into colloidal form is called peptization. The FeCl3 which has caused this dispersion, is called peptizing agent.

It is evident that peptization is just reverse to coagulation.

Colloidal solution are formed by aggregation of atoms or molecules to give particles of colloidal size. Yet there are substances which are themselves composed of giant molecules and dissolve in a solvent to yield colloidal solution directly. These giant molecules are formed macromolecules. Proteins (gelatin),synthetic polymers (plastics), synthetic rubber, cellulose and starch all possess macromolecules.

The molecular weight is an important property of HMS such as proteins, polymers and other macromolecules. Generally, molecules of protein or a polymer may not be of the same size. Therefore all the experiment al methods of molecular weight determination will give some kind of an average value. Two types of average molecular weight have been defined.

1. Number average molecular weight. It is defined as:

ni ∙ Mi stands for the weight of macromolecules numbering ni and having molecular weight Mi. The experimental methods based on properties present e.g., osmotic pressure, yield number average molecular weight.

2. Weight average molecular weight. It is defined as:

The mechanism of polymer dissolution differs from that of monomer dissolution. When monomers dissolve, their particles are «carried away» (diffuse) into the volume of the solvent, whereas when polymers dissolve, their enormous molecules are firmly retained by one another and instead to the polymer molecules diffusing into the volume of the solvent, molecules of the latter diffuse into the volume of the polymer.

The intermolecular bonds in the polymer weaken, and the volume of the polymer grows – the polymer swells. During the initial period from to to t1 the solvent molecules. Contraction of the total volume of the system occurs attended by lowering of the liquid level by ∆h.

During the period from t1 to t2 the intramolecular bonds in the polymer weaken, and the solvent molecules penetrate deeper and deeper into the polymer.

The volume of the latter grows, while the total volume of the system remains constant; the level of the liquid on the vessel not change. In this stage of dissolution, the pylymer molecules apparently straighten and stretch out into filaments.

If the intramolecular bonds in the polymer are strong enough and the solvent cannot separate the molecules, then swelling stops. In this case it is called limited. Limited swelling is generally due to the presence of stable chemical bonds between the polymer molecules.

A quantitative characteristic of the limited swelling of polymers is the degree of swelling L, determing as the ratio of the growth in the mass (m – mo) or volume (V – Vo) of a polymer specimen occurring as a result of the swelling to its initial mass mo or volume Vo:

If there are no strong bonds between the molecules of a polymer, it continues

to swell until it fills the entire volume of the solvent taken, i.e. until a homogeneous system forms. When the amount of solvent is large enough, the polymer molecules are finally detached from one another, and the system acquires fluidity – a liquid solution forms. Such swelling is called unlimited.

The swell ratio is influenced with various factors: the nature of the polymer and the solvent, temperature, addition of electrolytes and pH. The latter variable is of the greatest importance. Electrolytes affecting is based on the anions hydratation ability. Some anions have the low ability for hydratation and can enhance the polymers swelling while other anions being well-hydrated would reduce the swelling.

Proteins are polyelectrolytes whose macromolecules contain acid and basic functional groups in the amino acids side chains. Amino acids in aqueous solutions are in the form of bipolar ions:

H2N–R–COOH ⇄ +H3N–R–COO – .

In acidic medium, when the ionization of carboxylic groups decreases, the protein molecule shows basic properties and gains positive charge:

+H3N–R–COO – + H+ ⇄ +H3N–R–COOH.

In basic medium vice versa the ionization of amino groups decreases, the protein molecule shows acidic properties and gains negative charge:

+H3N–R–COO – + OH– ⇄ H2N–R–COO – + H2O.

So proteins are amphoteric molecules while changes in pH value of the medium influence whether protein will be positively or negatively charged. Proteins net charge is the sum of all positive and all negative charges of amino acid side chains and of amino and carboxyl groups. Isoelectric point (pI) is characteristic pH value at which proteins net charge is zero. Proteins are positively charged at pH values of solution below their pI value and negatively charged when pH value of solution is higher then their pI value.

1) The relative viscosity of a solution of a HMS, denoted by ηr, is given by the

expression.

η=η

η0

=η·ρ

η0·ρ

0

where η is viscosity of HMS solution and ηo that of the solvent at the same

temperature.

2) The sperific viscosity, denoted by ηsp, is given by

ηsp = ηr-1

3) The intrinsic viscosity id defined as

[η] = lim(η𝑠𝑝

𝑐)

It was shown by Staudinger that an empirical relationship exists between intrinsic viscosity [η] and the molecular weight M of the higt polymer [η] = k∙M2,

where k and α are constants for a specific polymer in a specific solvent.