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

Abstract

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 12 E 8 ) stabilized up to 1.1 wt % of water (corrected molar water/surfactant ratio, W o c ≈ 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 2 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 2 density, due to stronger interactions between the headgroups and water. The W/C microemulsions solubilize lysozyme in CO 2 at a level of 0.28 mg/mL in CO 2 and 36 mg/mL in the water droplet.

Characterization of Water/Supercritical CO2 Microemulsion by UV-visible Spectroscopy and Dynamic Light Scattering

“…The same phenomena were found in water/AOT/supercritical alkane (ethane and propane) systems, and were ascribed to the clustering of the droplets 40,41) . Johnston et al 11) also reported that D h values for TMN-6/W/scCO 2 mE with W 0 c = 20-25 were ca. 5-6 nm at CO 2 densities of 0.80-0.95 g/cm 3 and temperature of 35 or 55 .…”

“…The differences in the diameters of the two systems can be attributed to the following two reasons: (1) solvent density : low-density scCO 2 promotes the aggregation of mE droplets, and (2) Measurement Method: DLS using a long wavelength of 532 nm easy-monitored the aggregates of mE droplets rather than mono-dispersed mE droplets, when they were compared with a high density liquid CO 2 and SANS using short wavelength of several angstroms. Therefore, the drastic increase in D h with W 0 c for the W/scCO 2 mE system reflected two phenomena: the swelling of mE droplets and the promotion of their aggregation and fusion 11,40,41) . shows the effect of CO 2 density on the D h value of W/scCO 2 mE.…”

“…A number of studies have been directed toward the development of a surfactant system for stable formation of a W/scCO 2 mE, and more than 150 surfactants have been examined in these studies [7][8][9][10][11][12][13][14][15][16][17][18][19] . However, only a few have succeeded in developing a W/scCO 2 mE with W 0 c > 0 (W 0 c :water-to-surfactant molar ratio corrected by subtracting the moles of water soluble in CO 2 ).…”

“…Oleo Sci. 58, (2) 75-83 (2009) 11) . Similar to our conclusions, they 11) also concluded that the relationship was driven by interdroplet interactions that resulted in the aggregation of droplets as surfactant tails become poorly solvated due to less-dense CO 2 .…”

“…58, (2) 75-83 (2009) 11) . Similar to our conclusions, they 11) also concluded that the relationship was driven by interdroplet interactions that resulted in the aggregation of droplets as surfactant tails become poorly solvated due to less-dense CO 2 . The same phenomena were found in water/AOT/supercritical alkane (ethane and propane) systems, and were ascribed to the clustering of the droplets 40,41) .…”