Hydrophilic and Hydrophobic Capacity

concepts that characterize the affinity for water of substances or the bodies formed by them. This affinity arises from forces of molecular interaction. The words “hydrophilic” and “hydrophobic” can relate equally to a substance, to the surface of a body, and to a thin layer (at least one molecule thick) at the boundary separating phases (bodies). Hydrophilic and hydrophobic capacities are special cases of lyophilicity and lyophobicity, which are characteristics of the molecular interaction of substances with various liquids.

The binding energy of water molecules to the surface of a body is a general measure of hydrophilic capacity; it can be determined from the heat of immersion if the substance of which a particular body is made is insoluble. Hydrophobic capacity should be regarded as a low degree of hydrophilic capacity, because to a greater or lesser extent forces of molecular attraction will always exist between water molecules and any body. Hydrophilic and hydrophobic capacity may be estimated from the spreading of a drop of water on the smooth surface of a body. A drop spreads completely on a hydrophilic surface, whereas on a hydrophobic surface it spreads only partly; the angle between the surface of the drop and the surface being wetted depends on how hydrophobic the particular body is. All bodies with large enough molecular (atomic or ionic) interaction are hydrophilic. The hydrophilic capacity of minerals with ion crystal lattices (such as carbonates, silicates, sulfates, and clays), as well as of silicate glasses, is particularly well defined. Metals without oxide films, organic compounds in which hydrocarbon groups predominate in the molecule (for example, paraffins, fats, waxes, and some plastics), graphite, sulfur, and other substances with weak molecular interaction are hydrophobic.

The concepts of hydrophilic and hydrophobic capacity apply not only to bodies and their surfaces but also to single molecules and the separate parts of molecules. Thus, in molecules of surface-active substances, a distinction is made between hydrophilic (polar) and hydrophobic (hydrocarbon)groups. The hydrophilic capacity of the surface of a body can change markedly through the absorption of such substances.An increase in the hydrophilic capacity is called hydrophili-zation, and a decrease is called hydrophobization. Both phenomena play an important part in the beneficiation of oresby flotation. In textile technology the hydrophilization of cloth (fibers) is necessary for successful dyeing, bleaching,and laundering, and hydrophobization is necessary to impart water and moisture resistance to cloth.