Model for microemulsions: I. Effect of sulfonate surfactant cation and chain size and concentration on phase behavior

Abstract

One/one aqueous NaCl/decane systems containing C 12 o-oxylene sulfonates neutralized with mono-, di-, and triethanol amines were equilibrated and phase volumes determined. The systems phase split into 1 to 3 surfactant-rich phases in equilibrium with excess brine and/or decane. Both birefringent and optically homogeneous surfactant phases showed trends in phase volumes with varying salinity, head group size, chain length, and surfactant concentration. Monoethanol ammonium (MEA) and triethanol ammonium (TEA) sulfonates showed opposite trends with increasing concentration; optimal salinity rose with increasing TEA sulfonate and dropped with MEA sulfonate concentration. The diethanol ammonium (DEA) sulfonate showed an optimal salinity essentially independent of surfactant concentration as did equal weight mixtures of MEA and TEA sulfonates. Optimal salinity increased in the order MEA < DEA = MEA/TEA ( 1 1 ) < TEA indicating that surfacant hydrophilicity increases with increasing head group size. These trends and those with varying chain length are consistent with a proposed model mutually relating water and oil uptakes, interfacial curvature, and head and chain size. The model focuses on the structure of the surfactant at the brine/oil interface and relates structure to surfactant capacity to hold brine and oil together in the microemulsion. It examines the influence of surfactant head and chain volumes and water oil interfacial “solubility” on the direction and extent of interfacial curvature. The effect of these variables on interfacial curvature is in turn related to water and oil uptake. Water uptake increases and oil uptake decreases with increasing head/chain volume ratio, oil alkane carbon number and temperature (sulfonates), and decreasing salinity and aromaticity. The model postulates the coexistence of domains of water and oil continuity and applies spherical geometry to oil and water droplets in these domains. The resulting equations relate water and oil uptakes to the relative thicknesses and volumes of surfactant heads and chains. These structural parameters at balance are evaluated from surfactant heads and chains molecular olumes and interfacial areas. The model provides a physical explanation for the well-known empirical hydrophile/lipophile balance (HLB) system. It extends the HLB concept to include the influence of salinity, temperature, and oil composition.