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

Abstract

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.

Molecular view of polymer flow into a strongly attractive slit

“…glassy dynamics. 36 Our observations of the flow of a polymer melt into a narrow slit with attracting walls are consistent with the dynamics of other strongly driven fluid systems.…”

Simulation of polymer melt intercalation in layered nanocomposites

“…The influence of surface-molecule interactions on equilibrium and dynamical properties of confined molecules has been the subject of several computational investigations. [26][27][28][33][34][35] In these studies, an increased surface-polymer affinity is found to decrease molecular mobilities. Manias et al 28 have performed a moleculardynamics study of the motion of chain molecules in a thin film, which employed the same potentials that are used in the present work.…”

“…8,9 Understanding the formation kinetics and physical properties of these nanocomposites requires a molecular picture of the structure and dynamics of confined polymers. Laboratory studies employing the surface forces apparatus [20][21][22][23][24] and computational studies [25][26][27][28][29][30][31][32][33][34][35][36] have greatly increased our understanding of confined fluids. Confinement of a fluid on length scales comparable to the molecular size has been demonstrated to dramatically alter its structural and dynamical a͒ Author to whom correspondence should be addressed.…”

“…For example, confined fluids have been shown to solidify or vitrify at temperatures well above the bulk transition temperatures. 24,34 The mobility of the molecules is strongly reduced compared to the bulk because of interactions between the confined molecules and the atoms or molecules of the confining medium. [16][17][18][19]28 Molecules in films of nanometer thickness organize in layers parallel to the surfaces.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [711]. Exponentially decaying oscillatory solvation forces across confined liquids were first predicted by computer simulations and theory [711][712][713][714][715][716][717][718][719][720][721][722][723]. Experimental proof came a few years afterwards [724][725][726][727].…”