Dp. Bossev et al., Pressure-induced freezing of the hydrophobic core leads to a L-1 -> H-1 phase transition for C12E5 micelles in D2O, LANGMUIR, 17(25), 2001, pp. 7728-7731
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.