Strength of Alkane-Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms
YQ Qiu and V Molinero, CRYSTALS, 7, 86 (2017).
Alkanes are important building blocks of organics, polymers and biomolecules. The conditions that lead to ordering of alkanes at interfaces, and whether interfacial ordering of the molecules leads to heterogeneous crystal nucleation of alkanes or surface freezing, have not yet been elucidated. Here we use molecular simulations with the united-atom OPLS and PYS alkane models and the mW water model to determine what properties of the surface control the interfacial orientation of alkane molecules, and under which conditions interfacial ordering results in homogeneous or heterogeneous nucleation of alkane crystals, or surface freezing above the melting point. We find that liquid alkanes present a preference towards being perpendicular to the alkane-vapor interface and more parallel to the alkane-water interface. The orientational order in the liquid is short-ranged, decaying over -1 nm of the surface, and can be reversed by tuning the strength of the attractions between alkane and the molecules in the other fluid. We show that the strength of the alkane-fluid interaction also controls the mechanism of crystallization and the face of the alkane crystal exposed to the fluid: fluids that interact weakly with alkanes promote heterogeneous crystallization and result in crystals in which the alkane molecules orient perpendicular to the interface, while crystallization of alkanes in the presence of fluids, such as water, that interact more strongly with alkanes is homogeneous and results in crystals with the molecules oriented parallel to the interface. We conclude that the orientation of the alkanes at the crystal interfaces mirrors that in the liquid, albeit more pronounced and long-ranged. We show that the sign of the binding free energy of the alkane crystal to the surface, triangle G(bind), determines whether the crystal nucleation is homogeneous (triangle G(bind) >= 0) or heterogeneous (triangle G(bind) < 0). Our analysis indicates that water does not promote heterogeneous crystallization of the alkanes because water stabilizes more the liquid than the crystal phase of the alkane, resulting in triangle G(bind) > 0. While triangle G(bind) < 0 suffices to produce heterogeneous nucleation, the condition for surface freezing is more stringent, triangle G(bind) < -2 gamma(xl), where gamma(xl) is the surface tension of the liquid- crystal interface of alkanes. Surface freezing of alkanes is favored by their small value of gamma(xl). Our findings are of relevance to understanding surface freezing in alkanes and to develop strategies for controlling the assembly of chain-like molecules at fluid interfaces.
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