Abstract
Pulsed-field-gradient NMR methods are used for determining the partitioning of surfactants between unimeric and micellar forms in mixed surfactant systems. The method allows determination of the composition dependence of activity coefficients in binary surfactant mixtures. Application of the regular solution approach to NMR data for several mixtures gives β parameters which are dependent on concentration and which differ systematically among the surfactants within the same binary mixtures. The β parameter summarizes interactions among different pairs of surfactants and, in principle, should not be composition dependent. The observed behavior is therefore not consistent with expectations from the regular solution model. Use of the van Laar expressions, on the other hand, accounts well for the composition dependence of the activity coefficients. The van Laar expressions also account for the often-observed composition dependence of regular solution β parameters determined from CMC measurements. Though simila...
Published Version
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