From Big Medical Encyclopedia

WETTING — the phenomenon arising at contact of liquid or gas with a surface of a solid or other liquid and caused by molecular interaction between them. The page plays an essential role in many biol. processes: blood and a lymph moisten a surface of circulatory and absorbent vessels, the same occurs at current of solutions of the nutrients rising from the soil to leaves of plants, etc. S. is of great importance at digestion, extraction of medicinal substances from vegetable and animal fabrics during the processing of skin liquid pharmaceuticals, at disinfection, deodorization, etc. Besides, S. is used in many tekhnol. processes — flotation, amalgamation, dyeing of skin and fabrics, washing.

If forces of interaction of liquid with a surface of a solid considerably surpass forces of interaction between molecules of the liquid, then the drop of liquid applied on a surface of a solid will spread on this surface until covers it all or so far the nappe will not become monomolecular. So the drop of water on a clean surface of glass behaves, to-ruyu it almost completely moistens. If forces of interaction between molecules of liquid considerably exceed forces of interaction between liquid and a surface of a solid, then the liquid applied on a surface of a solid does not moisten it and gathers in the drops having almost spherical shape. So mercury on the surface of glass or water on the surface of paraffin behaves.

S.'s phenomenon is shown also in a curvature of a surface of liquid (or interfaces between two immiscible liquids) about walls of capillaries, i.e. in education convex (liquid does not moisten walls of a capillary) and bent (liquid moistens walls of a capillary) meniscuses.

the Diagrammatic representation of situation on a surface of a solid of a drop of the nonwetting and moistening liquid: θ — a regional corner.

The drop of liquid forms the certain corner 0 measured from liquid and called by a regional corner (fig.) with a surface of a solid. The size of a regional corner θ can be ranging from 0 ° (full wetting) to 180 ° (full not wetting). Measurement of a regional corner is performed as follows: the drop of liquid applied on a surface of a solid by means of a parallel beam of light is projected on the screen, the contour of a drop is outlined, build a tangent to its surface and measure a corner θ. Take a cosine (cos) of a corner for a quantitative measure of S. θ. If 0 less than 90 ° and, therefore, cosθ is more than 0, liquid moistens the solid surface called in this case lyophilic (at S. with water — hydrophilic) if 0 more than 90 ° and cosθ is less than 0, the surface of a solid is not moistened with liquid and is called lyophobic (at not wetting by water — hydrophobic).

The nature of a solid surface is more accurately shown at contact with two immiscible liquids with extremely differing polyarnost, napr, with water (liquid with the expressed polar properties) and an octane (unpolar liquid).

The liquid moistening a surface of a solid it is fuller in the presence of other liquid, finds so-called selective Page. According to selective S. solid surfaces can be divided more precisely on hydrophilic and hydrophobic. At the same time the regional corner 0 is measured from water. The surfaces of quartz, glass, sulfates, carbonates, metal oxides, etc. belong to hydrophilic. The surfaces of metals, sulfides, sulfurs, talc, paraffin, etc. belong to hydrophobic pure (not covered with oxide films).

The nature of a solid surface can be changed processing by solutions of surfactants — surfactant (see. Detergents ). At the same time molecules surfactants, being adsorbed on hydrophilic surfaces, are guided the polar groups to a surface, and the last becomes hydrophobic (process of a gidrofobization). At adsorption on hydrophobic surfaces of molecule surfactant are guided to a surface unpolar sites, and the surface becomes hydrophilic (process of a gidrofilization).

S. are influenced even by insignificant surface contaminations of a solid, especially pollution by organic matters (e.g., fatty and oil films), and also geometrical properties of a solid surface — a roughness, a form and particle sizes of a body. The roughness reduces 0 hydrophilic surfaces, i.e. improves their wettability water and increases 0 hydrophobic surfaces.

In case of powdery solids, for to-rykh regional the corner cannot be measured, as S.'s measure serves the coefficient of hydrophily (K) representing the size of the relation of the warmth which is marked out during the wetting of the studied powder with water Q H 2 O = (J/g), to the warmth which is marked out during the wetting of this powder with benzene Q With 6 R 6 . For hydrophilic powders size K more than 1, for hydrophobic (oleophilic) — less than 1.

See also Hydrophily , Water repellency .

Bibliography: Adamson A. U. Physical chemistry of surfaces, the lane with English, M., 1979; Voyutsky S. S. Course of colloid chemistry, M., 1975.

V. P. Mishin.