DIVALENT SURFACTANTS - EXPERIMENTAL RESULTS AND THEORETICAL MODELING OF SURFACTANT WATER PHASE-EQUILIBRIA

The influence of the headgroup charge on the phase behavior of surfactant/water systems has been investigated. Two binary phase diagrams with divalent surfactant and water are presented together with a ternary phase diagram for a system comprising a divalent (anionic) surfactant, a monovalent (anionic) surfactant and water (at 50-degrees-C). The studied surfactants are dodecylpentamethyl-1,3-propylenebis-(ammonium chloride) [DoPPDAC, CH3(CH2)11N+(CH3)2(CH2)3N+(CH3)3 + 2Cl-], dipotassium dodecylmalonate [K2DoM, CH3(CH2)11CH(COO-K+)2] and potassium tetradecanoate [KTD, CH3(CH2)IICH2COO-K+]. The absolute value of the average surfactant charge (z) varies between 1 and 2 for these systems. H-2 NMR, crossed polarizing microscopy, and small angle X-ray scattering are the main techniques used in order to determine the phase diagrams. The isotropic phase region with micellar solution and discontinuous cubic phases is followed by a hexagonal phase. A lamellar phase is obtained only for low values of z, on account of the fact that the solubility temperature increases when z increases. Two intermediate phases are obtained for z almost-equal-to 1.5, or less. One of these is a ''ribbon phase,'' presumably a centered rectangular phase, while the other may be an orthorhombic phase. The formation of the discontinuous cubic phase, which most likely is constituted by slightly prolate-shaped micelles, is strongly promoted by high z-values. Moreover, the presence of two different discontinuous cubic phases in the K2DoM/water system is indicated. Hence, the studied systems show the same succession of phases when the total surfactant concentration increases (although the number of phases varies between the systems), but they differ in that the phase transitions are shifted towards higher surfactant concentrations when z increases. These observations are in accord with theoretical predictions, as demonstrated by a phase diagram, which has been calculated by use of the Poisson-Boltzmann cell model. In addition, there is a marked decrease in the free amphiphile concentration when the total surfactant concentration increases above the critical micelle concentration for the case of the investigated divalent surfactants. This observation is also accounted for by Poisson-Boltzmann cell-model calculations.