Stabilization of dispersions in carbon dioxide and in other low-permittivity media
Abstract
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
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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.
Department
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