Divalent Surfactants—The Influence of Headgroup Charge on the Phase Behavior of an Alkyldiamine Surfactant/Water System

This paper reports the first phase diagram study of a three-component surfactant/neutral cosurfactant/water system which includes a divalent surfactant. The system is composed of dodecyl-1,3-propylene-bis(ammonium chloride) [CH3-(CH2)11-N+H2-(CH2)3-N+H3 + 2Cl-], dodecyl-1,3-propylene-bisamine [CH3-(CH2)11-NH-(CH2)3-NH2], and water. The average surfactant charge (z) varies between 0 and 2.0 in this system. Partial phase diagrams have been obtained at four different temperatures (25, 37, 50, and 60°C). Small-angle X-ray scattering, 2 H NMR, and optical polarizing microscopy are the main techniques used in order to determine the phase diagrams and the aggregate structures. A micellar solution phase and a lamellar phase are obtained at low and high surfactant concentrations, respectively. A hexagonal phase forms at intermediate compositions when z ⩾ 0.8. A centered rectangular and/or a bicontinuous cubic phase is obtained between the hexagonal and the lamellar phases, but two-phase regions with hexagonal and lamellar phases are also obtained. A discontinuous cubic phase, with discrete surfactant aggregates forms at surfactant concentrations intermediate between those of the micellar solution phase and the hexagonal phase for z > 1.75. In addition, the part of the lamellar phase region which borders to both the micellar solution phase and the hexagonal phase shows some properties that differ from the ones that are otherwise obtained for the lamellar phase. The general trend for z ⩾ 0.8 is a shift of the individual phase transitions towards higher surfactant concentrations when z increases, but the succession of phases with increasing surfactant concentration is not affected to any considerable extent by the average surfactant charge for these cases. However, the formation of the discontinuous cubic phase is strongly favoured by a high-average surfactant charge. Furthermore, the intermediate phase with centered rectangular symmetry is substituted for the bicontinuous cubic phase when the temperature increases.