Electrostatic Stabilization of Colloids in Carbon Dioxide:  Electrophoresis and Dielectrophoresis

Abstract

Over the past decade, steric stabilization has been achieved for a variety of inorganic and organic colloids in supercritical fluid carbon dioxide (scCO2). Herein we demonstrate that colloids may also be stabilized in CO2 by electrostatic forces, despite the ultralow dielectric constant of 1.5. Zeta potentials of micrometer-sized water droplets, measured in a microelectrophoresis cell, reached -70 mV corresponding to a few elementary charges per square micrometer of droplet surface. This degree of charge was sufficient to stabilize water/CO2 emulsions for an hour, even with water volume fractions of 5%. Hydrogen ions partition preferentially, relative to bicarbonate ions, from the emulsion droplets to the cores of surfactant micelles in the diffuse double layer surrounding the droplets. The micelles, formed with a low molecular weight branched hydrocarbon surfactant, prevent ion pairing of the hydrogen counterions to the negatively charged emulsion droplets. Dielectrophoresis of the water droplets at a frequency of 60 Hz leads to chains containing a dozen droplets with lengths of 50 mum. The ability to form electrostatically stabilized colloids in carbon dioxide is particularly useful in practical applications, because steric stabilization in CO2 is often limited by the poor solvation of the stabilizers.

High internal phase CO2-in-water emulsions stabilized with a branched nonionic hydrocarbon surfactant

“…The excess of negatively charged bicarbonate anions on these droplets was balanced by excess hydronium ions in the cores of the microemulsion droplets in the double layer surrounding the macroemulsion droplets. The preferential adsorption of hydronium ions to EO groups [53] of 5b-C 12 E 8 in W/C microemulsions was consistent with wellknown adsorption of cations such as Li to EO [62]. It should be noted that the concentrations of hydronium and bicarbonate ions in the bulk water are on the order of 10 −3 M (pH 3) [63].…”

“…At a surfactant concentration of 1 mM, γ was 3.1 mN/m in both cases. At the same density and 55 • C, γ measured in a recent study [53] was also about the same value. For this CO 2 density, the change in temperature from 25 to 55 • C was not large enough to produce a significant change in γ .…”

“…The lack of a cµc for 5b-C 12 E 8 in Figs. 7 and 8, and at 55 • C and 300 bar in a recent study of the same surfactant [53], is likely the result of a continual increase in surfactant aggregation as the surfactant concentration increased. The lack of a distinct cµc for nonionic surfactants in CO 2 is not surprising since the short hydrophilic ethylene oxide chains are also somewhat CO 2 -philic [54].…”

“…(3). At higher pressures and temperatures studied recently for this surfactant, it appeared that aggregates were not present, and it was possible to determine the surfactant adsorption quantitatively [53]. The higher pressure favors monomer solvation over aggregates, as both the hydrophobe and the ethylene oxide tail become more soluble in CO 2 .…”

“…In the present study, the accumulation of charge at the W/C interface is very likely based on a recent study. Ryoo et al [53] investigated micron-sized W/C macroemulsion droplets surrounded by W/C microemulsion droplets in the CO 2 continuous phase, with the same surfactant as in this study. On the basis of microelectrophoretic measurements, zeta potentials of the macroemulsion droplets reached −70 mV, corresponding to a few elementary charges per square micrometer of droplet surface.…”