Stabilization of dispersions in carbon dioxide and in other low-permittivity media
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Electrostatic stabilization of emulsions and particle dispersions in liquid and supercritical carbon dioxide (CO2) reduces the limitations on steric stabilization imposed by the weak solvent strength of CO2. This research focuses on charging and electrostatic stabilization of particles dispersed in CO2 and other low-permittivity media. Despite ultralow dielectric constant (1.5–2.5), dispersal of counterions in reverse micelles can prevent pairing with particle surfaces, enabling the required particle charging. TiO2 particles were electrostatically stabilized in CO2 at spacing of several diameters. Dispersions in low-permittivity solvents require sensitive instrumentation to measure electrophoretic mobilities (zeta potentials), which are low relative to those in aqueous solvents, motivating electrophoresis with velocimetry by differential-phase optical coherence tomography (DP-OCT). DP-OCT with transparent electrodes enables close electrode spacing and thus high electric fields despite low applied electric potential, to avoid electrohydrodynamic instability and electrochemical interference. Further advantages include ability to analyze small volumes and turbid dispersions, avoidance of electro-osmosis, and potential for single-particle measurement. The effect of surface viii hydrophilicity on the charging of TiO2 particles in low-permittivity toluene was studied. Partitioning of ions between particle surfaces and micelle cores was analyzed according to differences in their hydrophilicities and extents. The strategic design of particle surfaces and counterion stabilizers and the continued use of phase-sensitive velocimetry for electrophoresis in low-permittivity solvents should further clarify these complex charging mechanisms. Another important stabilization mechanism in CO2 is steric stabilization with surfactants active at the CO2–water interface. The fractional free volume (FFV) available to CO2 was demonstrated to be important in the design of surfactants with stubby tails. An understanding of the role of FFV and the use of a weakly hydrophilic headgroup enabled design of tertiary amine esters, a new class of nonfluorinated surfactants with extremely low aspect ratio. The structures adapt easily for study of surfactant architecture. These low-molecular weight surfactants are attractive replacements for more widely used fluorinated surfactants for tunable CO2-inwater emulsions. A fundamental understanding of electrostatic stabilization in lowpermittivity media and further investigation of structure–property relationships for interfacial activity of surfactants will aid development of new classes of colloids in the unusual medium of CO2.