AGGREGATION OF POLYETHYLENE-GLYCOL DODECYL ETHERS IN SUPERCRITICAL CARBON-DIOXIDE AND ETHANE

Fourier transform infrared spectroscopy has been used to measure the degree of intermolecular hydrogen bonding between solute molecules of dodecanol-d, triethylene glycol dodecyl ether (C12EO3-d), or octaethylene glycol dodecyl ether (C12EO8-d) in supercritical carbon dioxide, supercritical ethane, and liquid heptane. Both supercritical and subcritical binary solutions containing up to 0.04 mole fraction amphiphile were examined under conditions ranging from 200 to 400 bar and 40-degrees-C. An equilibrium is established in these fluids between the "free" non-hydrogen-bonded monomer and various hydrogen-bonded species. The amphiphile concentration significantly affects the equilibrium distribution of the monomer and oligomeric species. Aggregation numbers of these three different amphiphiles in carbon dioxide are in the range of 2.5-5.0 molecules per aggregate. The aggregate size in CO2 is, within experimental error, the same as found for supercritical ethane and liquid n-heptane. These solvation studies provide a basis for more complete thermodynamic treatments of these systems.