Melting mechanism in monolayers of flexible rod-shaped molecules

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.…”

“…We propose that these monolayers melt via the same "footprint reduction" mechanism found for shorter alkane monolayers [13] whereby molecular motion perpendicular to the adsorbing surface creates vacancies in the monolayer which allow translational and rotational disorder to develop. We find, though, a new feature in the monolayer melting of these longer alkanes.…”

“…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).…”

Different melting behavior in pentane and heptane monolayers on graphite: Molecular dynamics simulations

“…Most notably, the first study over hexane and butane proposes a "footprint reduction" mechanism 12 by which a phase transition is preempted by a space reduction in the monolayer that is a result of intermolecular scattering that supplies kinetic energy allowing the molecules to either tilt out of the surface plane, or else change conformation-in both cases decreasing their in-plane molecular footprint. This theory has been well adapted to these physisorbed systems, and most recent work over monolayer hexane on graphite 16 indicates that melting occurs primarily via the tilting mechanism, with only a very small contribution from gauche defects.…”