Neutron diffraction studies of the structure, growth, and melting of intermediate-length n-alkane films adsorbed on graphite

Over the past several years, we have conducted a variety of elastic neutron diffraction experiments to study the structure of films of two intermediate-length deuterated n-alkanes [n-CD ₃(CD ₂) _{n - 2}CD ₃] where n = 24 (n-tetracosane) and n = 32 (n-dotriacontane). The issues of interest in our studies include: the structure of monolayers deposited from an n-heptane (n = 7) solution as well as from the vapor phase, the melting behavior of these monolayers, the multilayer film structure, and the film growth mode. At room temperature, neutron diffraction patterns reveal striking differences between the molecular orientations, lattice constant, and coherence length of vapor-deposited and solution-deposited monolayers. Vapor-deposited monolayers of both molecules show qualitatively similar behavior upon heating. There is little change in their structure until reaching a temperature of about 340 and 350 K for n-tetracosane and n-dotriacontane, respectively, where a large thermal expansion begins, accompanied by a rapid decrease in the monolayer coherence length. Molecular dynamics simulations indicate that a melting transition is occurring in which there is simultaneous loss of both intrachain and interchain order as a result of the introduction of gauche defects along the chains. At higher coverages, diffraction patterns of vapor-deposited films of both molecules are consistent with crystallization of bilayer but not thicker films. Bulk Bragg peaks appear in the case of n-tetracosane at coverages between two and three layers, demonstrating incomplete wetting of the solid film as is found for films of shorter n-alkanes.