To clarify how a fluorocarbon (FC)-hydrocarbon (HC) tail of a hybrid surfactant acts for aggregation and solubilization in supercritical CO2, this study examined the phase behavior and aggregate nanostructures of both hybrid and non-hybrid surfactants in a water/supercritical CO2 mixture. By high-pressure small-angle neutron scattering (HP-SANS) technique, symmetric double-FC-tail surfactants (non-hybrid, FC length = 4, 6 and 8) with an Aerosol-OT-like headgroup and backbone were found to form spherical reversed micelles in CO2 at 45 �C, 350 bar and water-to-surfactant molar ratios (W) of 0-80. These reversed miceller systems were identified as a Winsor-II or IV W/CO2 microemulsion. On the other hand, phase behavior and solubilizing capacity of hybrid surfactants (FC length m = 4 and 6, HC length n = 2, 4, 5, 6 and 8) were strongly affected by HC length as well as FC one, and the short FC (m = 4) and/or HC (n=2) remained surfactant precipitates in water/CO2 mixture at W=20, 350 bar and 45 �C. The hybrid surfactants with m = 6 and n = 4 were able to yield transparent single-phases at the same P and T conditions, and the solubilizing capacity became the maximum value of W =~60 at hydrocarbon chain length close to that of the hybridized FC (m = 6). The HP-SANS experiments for the hybrid surfactants interestingly found not only spherical reversed micelles but also non-spherical (rod-like) one in the transparent single-phases. The length and aspect ratio of the rod-like reversed micelles depended on W and surfactant concentration, and these values at W=10 and 50 mM reached 360 � and 10, respectively.

Effect of fluorocarbon-hydrocarbon hybrid tail on phase behavior and aggregate nanostructure of surfactant in supercritical carbon dioxide

247th National Meeting (2014)

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