THE STABILITY OF LAMELLAR PHASES IN WATER, PROPYLENE-GLYCOL, AND SURFACTANT MIXTURES

The stability of the lamellar phases formed with the anionic surfactant sodium 4-( 1'-heptylnonyl)benzenesulfonate, the cationic surfactant didodecyldimethylammonium bromide, and the nonionic surfactant pentaethylene glycol mono-n-dodecyl ether (C(12)E(5)) is reported as water is gradually replaced with propylene glycol. In all cases, ternary phase diagrams at 25 degrees C show that the lamellar phase disappears as the concentration of propylene glycol increases. The stability of the lamellar phase is a function of the periodic repeat distances and surfactant head group areas as measured by small angle X-ray scattering, and the results are discussed in terms of the inter- and intra-bilayer interactions. Lamellar phases of nonionic surfactant apparently result as a balance between van der Waals attractions and steric repulsions, and become unstable because propylene glycol decreases the value of the Hamaker constant. For ionic surfactants, electrostatic interactions between the lamellae and intra-aggregate interactions are important. Lamellar stability is probably limited by the increase in surfactant head group area.