X-ray diffuse scattering was measured from oriented stacks and unilamellar vesicles of dioleoylphosphatidylcholine lipid bilayers to obtain the temperature dependence of the structure and of the material properties. The area/molecule, A, was 75.5Å2 at 45°C, 72.4Å2 at 30°C, and 69.1Å2 at 15°C, which gives the area expansivity αA=0.0029/deg at 30°C, and we show that this value is in excellent agreement with the polymer brush theory. The bilayer becomes thinner with increasing temperature; the contractivity of the hydrocarbon portion was αDc=0.0019/deg; the difference between αA and αDc is consistent with the previously measured volume expansivity αVc=0.0010/deg. The bending modulus KC decreased as exp(455/T) with increasing T (K). Our area compressibility modulus KA decreased with increasing temperature by 5%, the same as the surface tension of dodecane/water, in agreement again with the polymer brush theory. Regarding interactions between bilayers, the compression modulus B as a function of interbilayer water spacing D′W was found to be nearly independent of temperature. The repulsive fluctuation pressure calculated from B and KC increased with temperature, and the Hamaker parameter for the van der Waals interaction was nearly independent of temperature; this explains why the fully hydrated water spacing, D′W, that we obtain from our structural results increases with temperature.
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