Energetically driven liquid-solid transitions in molecularly thin n-octane films

In this paper we present findings from molecular dynamics simulations that investigated the changes induced in molecularly thin n-octane films, as a result of increasing solid-methylene unit energetic affinity. The solid surfaces were deprived of any topographical features and were modeled as atomically smooth 10-4 Lennard-Jones planes. We observed an abrupt transition in the structural features of the film at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. The transition was signaled by a discontinuous increase in the degree of intermolecular order and facilitated by a precipitous extension of the octane molecules, which adopted almost fully extended configurations. Furthermore, the transition resulted in the freezing of molecular migration and rotation. The characteristics of the transition showed that it is a mild first order phase transition between a highly ordered liquid and a poorly organized solid. The solid constitutes a phase with order intermediate to that of hydrocarbon ''rotator'' phases and two-dimensional smectics. These findings demonstrate that solidification of nanoscopically thin films of linear alkanes is a general, energetically driven phenomenon, which does not require the aid of commensurate surface topography. Our simulations provide a natural explanation for the solidlike features exhibited by alkane films studied experimentally. (C) 1996 American Institute of Physics.