Cd. Eads et Lc. Robosky, NMR studies of binary surfactant mixture thermodynamics: Molecular size model for asymmetric activity coefficients, LANGMUIR, 15(8), 1999, pp. 2661-2668
Pulsed-field-gradient NMR methods are used for determining the partitioning
of surfactants between unimeric and micellar forms in mixed surfactant sys
tems. The method allows determination of the composition dependence of acti
vity coefficients in binary surfactant mixtures. Application of the regular
solution approach to NMR data for several mixtures gives beta parameters w
hich are dependent on concentration and which differ systematically among t
he surfactants within the same binary mixtures. The beta parameter summariz
es interactions among different pairs of surfactants and, in principle, sho
uld not be composition dependent. The observed behavior is therefore not co
nsistent with expectations from the regular solution model. Use of the van
Laar expressions, on the other hand, accounts well for the composition depe
ndence of the activity coefficients. The van Laar expressions also account
for the often-observed composition dependence of regular solution beta para
meters determined from CMC measurements. Though similar to the symmetric re
gular solution model, the van Laar expressions contain an additional parame
ter which reflects differences in the sizes of the mixture components. The
results therefore suggest that headgroup size and headgroup packing are imp
ortant contributors to nonideal surfactant behavior. Computer algorithms ar
e described for extracting the van Laar interaction energy- and size-relate
d parameters from NMR-derived results, from mixed critical micelle concentr
ations, or from heats of mixing. Data for several binary surfactant mixture
s are presented and discussed. The results emphasize that when accurate dat
a are available, the single-parameter regular solution model will not alway
s fully account for nonideal surfactant mixing.