Physicochemical fundamentals of purification activity and characteristic of detergents

Purification property is removing liquid and solid pollutions from surface and transforming them into purification mixture as solvent, emulsion or dispersion. Purification activity is well seen in complex processes of interaction between pollution, detergents and surfaces.

Basic physicochemical phenomena, which determine purification property are processes: wetting, emulsification, dispersion, foaming and stabilizing. These phenomena are closely connected with boundary stresses and boundary activity of detergents.

Surfactants. Usually each molecular A inside the liquid is surrounded by other, molecules and forces of interaction between them are distributed symmetrically. Forces of interaction of molecular B of boundary layer of liquid aren’t balanced with molecules of lover layers which contact with liquid. That’s why molecular B is taken into liquid by force P, according to this surface of liquid is reduced by area. So we can assume that along liquid surface are forces of stress try to reduce the area. The forces are called surface stresses.

Surface stress is measured by work which we need to apply for increasing surface area for 1 sm². Purification substances have different surface stress.

Product of surface stress on surface area is called free surface energy. Ability of liquids to reduce free surface energy is characterized by surface activity. Substances which reduce surface stress are called surfactants.

Reduction of surface stress causes better damping by liquid. Damping is characterized by boundary angle. If edge angle is <90°, so the surface is well damped by liquid, if >90° so the surface is not damped.

Fig. 1.8. Scheme of spreading drops of liquid along the rigid surface: a – liquid damps the surface well, angle <90°; b – liquid doesn’t damp the surface well, angle >90°.

Usually surfactants are polar organic mixtures. Polarity of surfactants is caused by molecular structure which consist of 2 different parts differed by properties. One part of molecules is hydrophobias and causes dissolving surfactant in oil and other part is hydrophilic. It causes dissolving of surfactant in hydro.

Emulsion is called system of insoluble in each other liquids, one of which is distributed as small drops in other. Emulsification of liquid phase happens in hydro solvents of surfactants.

Dispersion of rigid phase of pollution occurs due to adsorption of surfactants on particles of pollution. Small surface stress of solvent allows penetrating in smallest cracks of particles of pollution and adsorbing on the particles surface. Adsorbed molecules of surfactants create discharging pressure on particles, destroying and crashing them.

Fig 1.9. Scheme of purification process: A – emulsion particle; B – rigid particle of surface pollution: 1- hydrophobias molecular particle (radical); 2 – hydrophilic molecular particle of surfactant.

Important step in purification process is stabilization of cleaned pollutions in solvent and predicting its oscillation on cleaned surface. Stabilization of pollutions basically depends on content of detergent and technological conditions of its application (concentration, temperature, pollution).

Alkalinity of detergents is the most important factor that determines efficiency of purification. Alkalinity determines ability of solvents to neutralize acid components of pollution, soap the oil, reduce hydro rigidity. Alkaline indicator as well as acidity one is hydro indicator pH.

Synthetic detergents. Recently synthetic detergents (SD) are widely spread. The base for SD is surfactants, activity of which is increased by application of alkaline. Solvents of SD by purification property are much better than solutions of sodium hydroxide and different alkaline mixtures. SD can include up to 10 different active components taking into account even components which protect surface of detail (the cleaned one) from corrosion.

Corrosion inhibitors and sequestering. Detergents which include alkaline, acids or their salts have correlation influence on metals. For preventing of corrosion special additives are included into detergents, so called inhibitors (decelerators) of corrosion.

Inhibitors are substances small additives of which have selective deceleration influence on the process of any chemical reaction without preventing the process of other reactions which exist simultaneously.

Solvents. Solvents which are used in process of purification the details must satisfy some requirements. Basically they are:

· High solvent property as for pollution which are removed;

· Allowable toxicity according to standard and fire safety;

· Exclusion of destructive activity on surface area which is being cleaned;

· Physicochemical stability under process condition.

Chemically active substances. Chemically active substances are used for removing oxides or residues pollution. For this purpose hydro solvents of alkaline or acid of different concentration are prepared. Process of purification the surface in case of using chemically active substances is transformed to chemical or electrochemical etching – destruction of boundary layers of metal for removing pollution.

Дата добавления: 2014-01-05