UNDERSTANDING BEHAVIOR OF PARTICLE STABILIZED EMULSIONS AND TRANFORMATIONS IN COLLOIDAL SYSTEMS

Abstract

Practically irreversible attachment of partially wettable particles to liquid-liquid interfaces presents an effective strategy for dispersing oil as stable drops in water column in the event of an oil spill. We investigate the potentials of using carboxyl-terminated carbon black particles for stabilizing oil-in-water emulsions motivated by the need for developing alternative dispersants. These particles on exposure to either acid or salt, become partially hydrophobic and stabilize emulsions which remain stable for months. Once at the interface, these particles can adsorb naphthalene, a potentially toxic polycyclic aromatic hydrocarbon and reduce its transport into the aqueous phase. Stable crude oil-in-sea water emulsions are formed using these CB particles (with no added acid or salt). These attributes make these particles a viable alternative or supplement to conventional dispersants for oil spill remediation. While partially wettable particles are known to stabilize emulsions, we report a new strategy for stabilizing emulsions using very hydrophilic and hydrophobic particles which are otherwise incapable of forming emulsions alone. When such a hydrophilic and a hydrophobic particle suspension are mixed, attractive van der Waals interactions between the particles cause them to assemble at the oil-water interfaces into supraparticle aggregates that are partially wettable in both phases and stabilize emulsions. Van der Waals interaction energy between two particle types across an aqueous-organic interface provide a systematic guide to particle and liquid combinations that can be used for stabilizing emulsions using our strategy. A combination of optical microscopy, cryogenic electron microscopy and zeta potential measurements are used for these studies. Time-resolved cryogenic transmission electron microscopy (TR-cryo-TEM) is used to investigate early stages of gypsum formation from a supersaturated solution of calcium sulfate hemihydrate. Our results indicate that a multi-step particle formation model, where an amorphous phase forms first, followed by transformation into a crystalline product, is applicable even at time scales of the order of tens of seconds for this system. Addition of a small amount of citric acid significantly delays reorganization to gypsum crystals due to complexation of available calcium ions with carboxylic groups. This induces disorder, and extends the time over which amorphous phase exists. Information about phase and morphology is obtained by energy dispersive X-ray spectroscopy (EDX) and selected area electron diffraction (SAED). Complementary X-ray diffraction experiments confirm our observations. Direct imaging of transient nanoscale samples by TR-cryo-TEM is a powerful technique for fundamental understanding of crystallization, and manyother evolving systems. We have also used cryo-TEM to understand