Simulation Study of Confinement Effects on the Melting Transition in Hexane and Decane Monolayers Between Two Graphite S
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0899-N07-21.1
Simulation Study of Confinement Effects on the Melting Transition in Hexane and Decane Monolayers Between Two Graphite Slabs Cary L. Pint1 and Michael W. Roth Department of Physics, University of Northern Iowa Cedar Falls, IA 50614, U.S.A. 1 electronic mail: [email protected] ABSTRACT The melting transition in hexane and decane monolayers are studied through use of a canonical ensemble molecular dynamics (MD) simulation method, where the monolayer is physisorbed onto a single, infinite graphite slab, with another confining graphite slab some varying vertical distance from the monolayer. Using a simple united-atom model to depict the alkanes, simulations suggest that the monolayer melting transition is largely dependent upon the average density of the physisorbed monolayer in addition to the spacing between the two graphite slabs. In particular, hexane is studied at two distinct coverages: (i) a fully commensurate (FC) monolayer, and (ii) a uniaxially incommensurate (UI) monolayer with an average density 0.9 times that of the FC monolayer. In the case of no confinement, both the FC hexane and decane monolayers exhibit a power law relation between the melting temperature of the monolayer and the graphite slab separation. In contrast, the UI hexane monolayer deviates from this behavior, suggesting that the power law behavior is an effect of the in-plane space, and how the monolayer melting transition depends upon this in-plane space. Finally, the role of the top (confining) layer corrugation in the power-law relationship, and the importance of this study toward future applications are briefly discussed. INTRODUCTION Interfacial behavior of atoms and molecules physisorbed onto solid surfaces is a topic that has received a great deal of attention in the past several decades. From simple atoms to the most complex molecules, a thin layer of adsorbate on a solid surface induces competing interactions between the molecules in the absorbed layers and the solid surface that often results in new phases and interesting phenomena that is not observed in the bulk. This study deals with the specific case of hexane (C6H14) and decane (C10H22) as the adsorbate, as these are very simple molecules whose behavior on a graphite surface has been studied very well through both experiment [1-3] and simulations [4-7]. The n-alkanes are well known for their properties as lubricants and their presence as major constituents in many complex biological molecules (such as lipids, which have hydrophobic alkane tails). Thus, a study of these simple chain molecules provides great insight into several important applications in a broad classification of topics ranging from lubrication to coating technology. Whereas the phase transitions in a physisorbed monolayer of hexane or decane on a single graphite slab have been studied through simulations in previous work [4-6], this study proposes to better understand these phase transitions by imposing a graphite slab above the monolayer that serves to both (i) eliminate some of the molecular motion
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