STRUCTURE AND INTERACTIONS OF SURFACTANT-COVERED SURFACES IN NONAQUEOUS (OIL SURFACTANT WATER) MEDIA

We have measured the forces between various surfactant-coated mica surfaces immersed in various hydrocarbon liquids (oils). The aim was to clarify the colloidal interaction occurring in nonaqueous systems of practical interest (e.g., lube oils, paints, toners, emulsions) where the literature is complex and often confusing. We find that due to solvent penetration-either of hydrocarbon molecules into the hydrophobic chain regions or of water (when present) into the hydrophilic headgroup regions-the surface monolayers swell. This is similar to the well-known swelling of micelles or microemulsion droplets except that it is now occurring at the solid-solution interface. When surfactant is also present in the oil phase, the surface layers may incorporate oil, surfactant, and water from the three-component bulk solution. Even though the bulk concentration of surfactant and water may be very low, the adsorbed layer may grow into a thick, concentrated mesophase structure, e.g., of ordered lamellae or disordered cylindrical micelles. These structural changes alter the range, magnitude, and sometimes even the sign of the forces between the surfaces across the oil. We describe our results on the changes of and correlations between the surface structures and the interaction forces of different surfactant systems adsorbed on solid surfaces in various nonpolar liquids. We also describe the effects of head group type, headgroup counterion, monolayer coverage, temperature, surfactant and water concentration in the oil, and time (aging effects) on these interactions.