Surfactant adsorption polarity equalization rule

One of the essential features of the solid-liquid interface is that the adsorbing substance may not only be bound to the surface by relatively weak physical forces, but also may form true chemical bonding with molecules or ions located at the surface of the solid phase. This phenomenon, referred to as the chemisorption, may seem to invalidate the polarity equalization rule at the interface between a polar crystal (e.g. silicate or sulfide) and a polar medium (water) the adsorption due to chemical bond formation may occur in such a way that the hydrocarbon chains are facing the water phase (Fig. III-9, a). At sufficiently high concentrations of chemisorbing surfactant, when the entire solid surface is covered with a monolayer, the formation of a second, oppositely oriented, surfactant layer starts, i.e., regular surfactant adsorption... 

The adsorption from solutions on finely dispersed powders and porous adsorbents is used for the removal of dissolved toxic components, as well as for concentrating and entrapping valuable substances from dilute solutions. In agreement with the polarity equalization rule, surface active substances dissolved in aqueous medium can be removed by adsorption on non-polar adsorbents (such as activated carbon), or on adsorbents that are capable of chemisorbing the surfactant polar heads. In order to increase the effectiveness... 

Surfactants may not only stabilize system against coagulation, but may have an opposite effect, i.e. cause destabilization in cases when the surfactant adsorption proceeds against the polarity equalization rule (Chapter III,2), e.g., during chemisorption of surfactants from aqueous medium on a hydrophilic surface. For example, small additives of cationic surfactants cause coagulation of aqueous dispersions of clays and other silicates due to hydrophobization at T< rmax. Further increase in surfactant concentration results in the formation of a second (hydrophilizing) adsorption layer and leads to an increased... 

In froth flotation, the adsorption takes place from the polar aqueous phase, but the hydrophobic portions of the surfactant molecules in the adsorbed layer must be facing the polar phase, which appears to contradict the thermodynamically based polarity equalization rule. This is possible only if surfactant molecules are chemisorbed at the mineral surface. Chemisorption involves the formation of a very strong chemisorption bond between the polar groups and the particle surface. The energy of such a bond needs to well exceed 50 nJ/m in order to compensate for the free energy increase associated with the formation of the hydrocarbon/water interface (Figure 2.11c). 

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