Models for Interactions between Ionic Surfactants and Nonsurface-Active Ions in Foam Fractionation Processes

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Abstract
Interactions are analyzed between an ionic surfactant and nonsurface-active ions (colligends) of opposite charge being separated in foam fractionations. Surfactant selectivity for competing colligends is determined in terms of models based on surfactant—colligend ion pair formation in the feed solution to a foam fractionation unit, based on colligend-surfactant counterion exchange at the gas-solution, bubble interfaces, and based on surface exchange coupled with ion pair formation in the bulk solution. Accurate, continuous-flow, single-equilibrium-stage foam fractionation data for NO− 3, BrO− 3, CIO− 3, and I−, each versus Br−, the counterion of the ethylhexadecyldimethylammonium cation, are used to discriminate among the models. Based on a detailed statistical analysis of the selectivity coefficients determined by two interaction models for each of the four colligends, the hypothesis of colligend-counterion exchange at the gas-solution interface is shown to be valid and that of solution ion pair formation is not substantiated. The surface exchange model provides selectivity coefficients which are quite constant over a tenfold concentration range and yet which are very sensitive to data inaccuracies.