A Molecular Dynamics Study of the Effect of Steric Properties on the Melting of Quasi Two-Dimensional Systems
The effect of molecular steric properties on the melting of quasi two-dimensional solids is investigated by comparing molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the graphite (002) surface. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)nCH3], where n=2 for butane and n=4 for hexane and have similar solid monolayer structures on graphite. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x ray scattering experiments. The melting of the low-temperature herringbone (hb) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid (hb) phase transforms to a rectangular centered (rc) structure with a short coherence length in coexistence with a fluid phase. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. A mechanism for the melting of the (hb) phase is proposed. It is based on vacancy creation within the monolayer by molecules reducing their “footprint” on the surface either by a conformation change, tilting with respect to the surface, or promotion to a second layer.
KeywordsMelting Behavior Orientational Order Vacancy Formation Bend Angle Mono Layer
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