Alkane fluids confined and compressed by two smooth gold crystalline surfaces: pure liquids and mixtures
Merchan Alvarez, Lina Paola
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With the use of grand canonical molecular dynamics, we studied the slow ompression(0.01m/s) of very thin liquid films made of equimolar mixtures of short and long alkane chains (hexane and hexadecane), and branched and unbranched alkanes (phytane and hexadecane). Besides comparing how these mixtures behave under constant speed compression, we will compare their properties with the behavior and structure of the pure systems undergoing the same type of slow compression. To understand the arrangement of the molecules inside the confinement, we present segmental and molecular density profiles, average length and orientation of the molecules inside well layered gaps. To observe the effects of the compression on the fluids, we present the number of confined molecules, the inlayer orientation, the solvation force and the inlayer diffusion coefficient, versus the thickness of the gap. We observe that pure hexadecane, although liquid at this temperature, starts presenting strong solid-like behavior when it is compressed to thicknesses under 3nm, while pure hexane and pure phytane continue to behave liquid-like except at 1.3nm when they show some weak solid-like features. When hexadecane is mixed with the short straight hexane, it remains liquid down to 2.8nm at which point this mixture behaves solid-like with an enhanced alignment of the long molecules not seen in its pure form; but when hexadecane is mixed with the branched phytane the system does not present the solid-like features seen when hexadecane is compressed pure.