Intramolecular and Lattice Melting inn-Alkane Monolayers: An Analog of Melting in Lipid Bilayers

Molecular dynamics (MD) simulations and neutron diffraction experiments have been performed on $n$-dotriacontane ( $n-{\mathrm{C}}_{32}{\mathrm{D}}_{66}$) monolayers adsorbed on a graphite basal-plane surface. The diffraction experiments show little change in the crystalline monolayer structure up to a temperature of $\sim 350\mathrm{K}$ above which a large thermal expansion and decrease in coherence length occurs. The MD simulations provide evidence that this behavior is due to a phase transition in the monolayer in which intramolecular and translational order are lost simultaneously. This melting transition is qualitatively similar to the gel-to-fluid transition found in bilayer lipid membranes.