Quasielastic neutron scattering and molecular dynamics simulation studies of the melting transition in butane and hexane monolayers adsorbed on graphite

Abstract

Quasielastic neutron scattering experiments and molecular dynamics ͑MD͒ simulations have been used to investigate molecular diffusive motion near the melting transition of monolayers of flexible rod-shaped molecules. The experiments were conducted on butane and hexane monolayers adsorbed on an exfoliated graphite substrate. For butane, quasielastic scattering broader than the experimental energy resolution width of 70 eV appears abruptly at the monolayer melting point of T m ϭ116 K, whereas, for the hexane monolayer, it appears 20 K below the melting transition (T m ϭ170 K). To facilitate comparison with experiment, quasielastic spectra calculated from the MD simulations were analyzed using the same models and fitting algorithms as for the neutron spectra. This combination of techniques gives a microscopic picture of the melting process in these two monolayers which is consistent with earlier neutron diffraction experiments. Butane melts abruptly to a liquid phase where the molecules in the trans conformation translationally diffuse while rotating about their center of mass. In the case of the hexane monolayer, the MD simulations show that the appearance of quasielastic scattering below T m coincides with transformation of some molecules from trans to gauche conformations. Furthermore, if gauche molecules are prevented from forming in the simulation, the calculated incoherent scattering function contains no quasielastic component below T m . Modeling of both the neutron and simulated hexane monolayer spectra below T m favors a plastic phase in which there is nearly isotropic rotational diffusion of the gauche molecules about their center of mass, but no translational diffusion. The elastic scattering observed above T m is consistent with the coexistence of solid monolayer clusters with a fluid phase, as predicted by the simulations. For T/T m у1.3, the elastic scattering vanishes from the neutron spectra where the simulation indicates the presence of a fluid phase alone. The qualitative similarities between the observed and simulated quasielastic spectra lend support to a previously proposed ''footprint reduction'' mechanism of melting in monolayers of flexible, rod-shaped molecules. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50237-6͔

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

“…The alkane molecules are represented by a unified atom model in which the methyl and methylene groups are replaced by pseudoatoms of their respective masses at the carbon atom position, and the pseudoatom separation is constrained to the C-C bond length of 1.53 Å. Molecular flexibility is retained by allowing dihedral torsional motion about the C-C bond as well as the C-C-C angle-bending motion. We used the same intermolecular and intramolecular potentials as previously [13,20] except for the atom-substrate potential where we have substituted the parameters of Velasco and Peters [21]. Their simulations gave better agreement with the melting temperatures observed for butane and hexane monolayers [22].…”

“…Simulations of a hexane monolayer (n-C 6 H 14 ) also showed gauche defects appearing in the molecules below the melting transition at 175 K [13,20]. Furthermore, we could demonstrate their role in driving the melting transition by tripling the height of intramolecular trans-gauche energy barrier so that no gauche defects could be formed.…”

“…In order to interpret microscopically the temperature dependence of the diffraction patterns, we have performed extensive MD simulations of both the low-and highdensity phases of monolayer C32 by a method similar to that used in our previous simulations of n-butane ͑n 4͒ and n-hexane ͑n 6͒ monolayers on graphite [13,20]. This method employs a canonical sampling scheme (constant volume, temperature, and a fixed number of molecules).…”

Molecular diffusion in monolayer and submonolayer nitrogen

“…Although most of the data for monolayers of molecular nitrogen [1][2][3][4][5] are for solids, the recent extensions of quasielastic neutron [6][7][8] and helium atom 9 scattering experiments raise the prospect that data on the dynamics of the fluid phases may become available. Here, we characterize the diffusive motions in monolayer N 2 fluids using molecular dynamics simulations and explore how these motions are reflected in the correlation functions for inelastic scattering.…”

Effects on the structure of monolayer and submonolayer fluid nitrogen films by the corrugation in the holding potential of nitrogen molecules

“…Subsequent experiments with electron diffraction 6,7 and helium atom 8,9 scattering for nitrogen physisorbed on metal surfaces have shown similarities with the graphite results and differences in the occurrence of commensurate structures. Most of these studies are for solid films of nitrogen, but the recent extensions of quasielastic neutron 10 and helium atom 11 scattering experiments may make it possible to obtain data for the dynamics of the fluid phases as well.…”