(256d) Structure and Transport in Monolayers of Charged Colloid Particles at Oil-Water Interfaces: The Role of Oil Soluble Surfactant

The behavior of colloidal particles in liquid-liquid interfaces is relevant to numerous commercial products and industrial processes, such as food processing, pharmaceutical formulation, flotation, oil extraction and materials fabrication. Over the past century, colloid and surface forces have been well understood in bulk phase, but the presence of an ?oil-water interface? introduces significant complexity to the colloidal interactions. We have studied the absorption, structure and interaction of charged polystyrene particles at the oil (decane) - water interface by optical microscopy. The electrostatic interaction between charged particles at a water-low dielectric medium interface leads to the formation of a typical crystalline 2-D monolayer at the interface. Such particle monolayers are remarkably insensitive to the addition of salt in the aqueous phase, but small amounts of surfactant in either liquid phase disturb the crystalline order and thus promote liquid-like structures. Two-dimensional aggregation of particles was observed when increasing the surfactant concentration in the oil phase. We discuss the role of interfacial and electrostatic energy for the observed behavior and propose a multi-faceted effect of the added surfactant.