Pressure-induced freezing of the hydrophobic core leads to a L-1 -> H-1 phase transition for C12E5 micelles in D2O

We apply small-angle neutron scattering (SANS) to study the effect of press ure on micelle structure in a solution of 1% by weight pentaethylene glycol mono-n-dodecyl ether (C12E5) in D2O at 20 degreesC and pressures up to sim ilar to 3000 bar. At ambient pressure, the structure is a network of branch ed, semiflexible, cylindrical micelles with the branch points comprised of three-armed junctions. Our SANS results reveal that pressure induces a phas e transition from this network of threadlike micelles to hexagonally ordere d bundles of cylindrical micelles. Using geometric packing constraints for three-arm junctions and cylinders, we show that the formation of three-arm junctions becomes increasingly unfavorable with increasing pressure due to the compression of the micelle hydrophobic core, and as such, the network b ecomes unstable at pressures close to those observed in our SANS experiment s. We also measured the temperature dependence of the transition pressure a nd find that it follows the pressure-temperature freezing curves for liquid n-alkanes of comparable hydrocarbon chain length. These observations lead us to propose that the phase transition is related to a loss of flexibility or conformational entropy of the C12E5 micelles upon the pressure-induced freezing of the micelle hydrophobic core to form an amorphous solid. The fo rmation of hexagonally ordered bundles of cylindrical micelles follows as a ttractive van der Waals forces between the micelles are not offset by the l oss of repulsive undulation forces arising from the fluidity of the hydroph obic core.