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 fil ms, as a result of increasing solid-methylene unit energetic affinity. The solid surfaces were deprived of any topographical features and we re modeled as atomically smooth 10-4 Lennard-Jones planes. We observed an abrupt transition in the structural features of the film at a crit ical value of the characteristic energy that quantified the affinity b etween 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 transi tion resulted in the freezing of molecular migration and rotation. The characteristics of the transition showed that it is a mild first orde r phase transition between a highly ordered liquid and a poorly organi zed solid. The solid constitutes a phase with order intermediate to th at of hydrocarbon ''rotator'' phases and two-dimensional smectics. The se findings demonstrate that solidification of nanoscopically thin fil ms of linear alkanes is a general, energetically driven phenomenon, wh ich does not require the aid of commensurate surface topography. Our s imulations provide a natural explanation for the solidlike features ex hibited by alkane films studied experimentally. (C) 1996 American Inst itute of Physics.