Water-in-Carbon Dioxide Microemulsions with Methylated Branched Hydrocarbon Surfactants

Nonionic methylated branched hydrocarbon surfactants, poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ethers, form water-in-carbon dioxide (W/C) microemulsions. A concentration of 1.0 wt % of the octa(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether (5b-C<INF>12</INF>E<INF>8</INF>) stabilized up to 1.1 wt % of water (corrected molar water/surfactant ratio, <EQUATION><EQNDATA>%@mt;sys@%%@ital@%W%@rsf@%%@mh;1q@%%@sx@%o%@be@%c%@sxx@%%@mx@%</EQNDATA> </EQUATION> ≈ 28) from 35 to 65 °C and above 240 bar. Methylation and branching of surfactant tails enhance the formation of stable W/C microemulsions as they (1) raise surfactant solubility in CO<INF>2</INF> by weakening interactions between tails and lower surfactant solubility in water by inhibiting micellization, (2) shift the curvature toward bending about water, and (3) reduce overlap between surfactant tails and weaken interdroplet interactions. The hydrodynamic radii of microemulsion droplets measured by dynamic light scattering (1.6−3.0 nm) increases with a decrease in temperature, at constant CO<INF>2</INF> density, due to stronger interactions between the headgroups and water. The W/C microemulsions solubilize lysozyme in CO<INF>2</INF> at a level of 0.28 mg/mL in CO<INF>2</INF> and 36 mg/mL in the water droplet.