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College Park, Maryland      June 6 - 10 , 2004

W2-B1 (1:30 PM): Surface constraints and membrane phase miscibility gaps (Invited)

W. A. Hamilton (Condensed Matter Sciences Division, Oak Ridge National Laboratory), L. Porcar (NIST Center for Neutron Research), G.S. Smith (Condensed Matter Sciences Division, Oak Ridge National Laboratory)

In many surfactant membrane systems the competition between intrinsic curvature and membrane composition leads to large miscibility gaps in which membrane phases of very different geometries coexist in the bulk. Depending on the phase geometries we would expect the bulk equilibrium balance to be affected by the ordering potential exerted by a proximate surface. In this work we consider a situation in which the effect should be relatively strong: when one of the coexisting phases, the so-called L3 "sponge", is a manifestly isotropic convolution of membranes for which any accommodation to a flat surface will represent a significant distortion, whereas for the other, a regularly stacked Lα lamellar phase, it should not. Using neutron reflectivity and "Near-Surface" SANS (NS-SANS) we have tracked a temperature driven L3 to biphasic L3+Lα transition in an AOT membrane solution near the quartz surface of a reflection geometry sample cell. The NR signal allows us to probe the membrane conformation to a depth of order 1000, a relatively few membrane separations from the solid-solution interface (the expected range of any ordering potential), while the NS-SANS signal allows simultaneous monitoring of "bulk" phase behavior corresponding to the surface signal to depths of tens of μm. While these are static measurements, we note that these phases are to some extent stabilized by hydrodynamic fluctuations. These have well known structural consequences in the bulk, such as logarithmic correction of the scaling of characteristic lengths with respect to dilution. In the vicinity of the surface these fluctuation modes will inhibited, narrowing their spectrum, so the surface may be expected to frustrate these membrane phases dynamically as well as geometrically.

* Research sponsored by the U.S. Department of Energy under contract DE-AC05-00OR22725 with ORNL managed by UT-Battelle, LLC.

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