Structure Changes in Model Membranes Monitored by Variable Period X-ray Standing Waves: Effect of Langmuir-Blodgett Film Thickness on Thermal Behavior

Variable period X-ray standing wave (XSW) measurements have been used previously to monitor and to interpret structure rearrangements undergone during a phase transition in a model membrane with angstrom resolution. In the present study, an extended length scale ranging from 40 up to ca. 1000 8, has been used without sacrificing resolution to address the question of whether the phase transition characteristics of a lipid multilayer are sensitive to the number and identity of the lamellae in the membrane stack. The system chosen for examination consisted of an octadecanethiol (ODT) coated gold mirror on top of which a variable number (0, 2, 8, and 16) of w-tricosenoic acid (C23, UTA) bilayers followed by a single upper inverted bilayer of zinc arachidate (C20, ZnA) was deposited by the Langmuir-Blodgett (LB) technique. Variable period XSW measurements provided precise information on the zinc layer mean position and width and were used to track the collapse of the heavy atom layer during the thermotropic phase transition. Samples with and without the UTA layers showed disparate pretransitional rearrangements, transition temperatures, and apparent cooperativity as well as final high-temperature zinc distribution. At sufficiently high temperatures, the ODT monolayer appears to desorb from the gold mirror surface to be replaced by a layer of zinc. In all samples containing wTA, the cooperative transition occurred at ca. 72 "C, which has been shown by simultaneous calorimetry and lowand wide-angle time-resolved X-ray diffraction to correspond to the onset of melting of bulk UTA. Thus, the remarkable result that bulk behavior is displayed by as few as two LB bilayers of UTA. With all thin film samples, the temperature-induced structure change was not reversed upon cooling to and subsequent storage at room temperature.