Neutron diffraction and quasielastic neutron scattering studies of films of intermediate-length alkanes adsorbed on a graphite surface.

摘要

Over the past several years, we have conducted a variety of elastic neutron diffraction and quasielastic neutron scattering experiments to study the structure and the dynamics of films of two intermediate-length alkane molecules (C nH2n+2), adsorbed on a graphite basal-plane surface. The two molecules are the normal alkane n-tetracosane [n-CH 3(CH2)22CH3] and the branched alkane squalane (C30H62 or 2, 6, 10, 15, 19, 23-hexamethyltetracosane) whose carbon backbone is the same length as teteracosane.; The temperature dependence of the monolayer structure of tetracosane and squalane was investigated using elastic neutron diffraction and evidence of two phase transitions was observed. Both the low-coverage tetracosane (C 24H50) and squalane (C30H62) monolayers have crystalline-to-"smectic" and "smectic"-to-isotropic fluid phase transitions upon heating.; The diffusive motion in the tetracosane and squalane monolayers has been investigated by quasielastic neutron scattering. Two different quasielastic neutron scattering spectrometers at the Center for Neutron Research, National Institute of Standards and Technology (NIST) have been used. The spectrometers differ in both their dynamic range and energy resolution allowing molecular motions to be investigated on time scales in the range 10-13--10 -9 s. On these time scales, we observe evidence of translational, rotational, and intermolecular diffusive motions in the tetracosane and squalane monolayers. We conclude that the molecular diffusive motion in the two monolayers is qualitatively similar. Thus, despite the three methyl sidegroups at each end of the squalane molecule, its monolayer structure, phase transitions, and dynamics are qualitatively similar to that of a monolayer of the unbranched tetracosane molecules.; With the higher resolution spectrometer at NIST, we have also investigated the molecular diffusive motion in multilayer tetracosane films. The analysis of our measurements indicates slower diffusive motion in the bottom layer of a bilayer tetracosane film compared to that in a monolayer, while the rate of diffusive motion in the bottom two layers of the trilayer film is comparable to that in a monolayer.