Environmental Effects on Adsorption

An increase in electrolyte content will generally cause a decrease in adsorption of surfactants onto surfaces of opposite charge and an increase in adsorption of like charged molecules (Fig. 9.13a). An important exception to that rule is the case of ionic surfactants in which the added cation has a specific interaction with the adsorbing solute (i.e., surfactant) that reduces its solubility in the solution leading to enhanced adsorption. A typical example is the addition of polyvalent cations to carboxylic acid soaps. 

An increase in temperature usually results in a decrease in the adsorption of ionic surfactants, although the change may be small when compared to those due to pH and electrolyte changes. Nonionic surfactants solubilized by hydrogen bonding, which usually have an inverse temperature-solubility relationship in aqueous solution, generally exhibit the opposite effect. In other words, adsorption will increase as the temperature increases, often having a maximum near the Krafft point of the particular surfactant. 

FIGURE 9.13. In systems of negatively charged surfaces and anionic snrfactants, the presence of polyvalent cations snch as Ca+ and AF, can prodnce bridging phenomena that result in strong snrface-snrfactant interactions that would not occur in the presence of monovalent ions such as Na+ or K+. 

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