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Original Articles

Mapping the Stability and Curvature of Emulsions of H2O and Supercritical CO2 with Interfacial Tension Measurements

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Pages 65-80 | Received 28 Jun 2001, Accepted 30 Sep 2001, Published online: 12 Feb 2010
 

ABSTRACT

The stability and curvature of emulsions of H2O and CO2 are reported and analyzed in terms of measurements of interfacial tension versus formulation variables, including salinity, CO2 density, temperature and pH. Among the surfactants studied are, quaternary ammonium cationic ones with perfluoropolyether tails, block copolymer ionomers and a poly (hydroxyethyl methacrylate) with polydimethylsiloxane tails, and a nonionic ethylene oxide surfactant with a fluoroalkane tail. The interfacial tension measurements were made at surfactant concentrations from 0.05 to 1.0 wt% with a variable-volume pendant drop tensiometer up to 345 bar and 363°K. As a formulation variable was varied, the system reached a balanced state characterized by a minimum in interfacial tension, a loss in emulsion stability and in some cases an inversion from a W/C to C/W emulsion. Here the Marangoni-Gibbs stabilization weakens, and also it becomes easy to bend and rupture the surfactant monolayer, causing coalescence. Except in the case of the nonionic fluorinated surfactant C8F17—SO2NEt-(CH2CH2O)12–14CH3, the crossover from the CO2-continuous (W/C) to the H2O-continuous (C/W) emulsion occurred abruptly due to clouding of the surfactant out of the CO2 phase. For PFPE-TMAA, the plot of γ versus surfactant concentration revealed both pre-micellar aggregates and a critical micro emulsion, each of which was dependent on salinity.

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