On the morse potential in liquid phase and at liquid–vapor interface
U.F. Galicia–Pimentel, D. Osorio–González, and J. López–Lemusa
Facultad de Ciencias, Universidad Autónoma del Estado de México, Av. Instituto Literario 100, Toluca 50000, México, ae–mail: jllemus@uaemex.mx
Recibido el 11 de mayo de 2006
Aceptado el 27 de septiembre de 2006
Abstract
Canonical Molecular Dynamics simulations have been performed to calculate thermodynamic properties in the liquid phase and at the liquid–vapor interface for fluids interacting by Morse potential. Transport properties such as self diffusion and shear viscosity have been calculated in one phase. Self diffusion shows an important dependence on particle number whereas shear viscosity does not show such dependence. At the liquid–vapor interface, properties such as orthobaric densities, vapor pressure, and surface tension were calculated. Equilibrium densities were compared with results obtained by NpT plus test particle method, and an excellent agreement was found. The surface tension and the vapor pressure are computed for the first time in this work. We also analyzed the cut–off distance dependence in both bulk and interfacial properties. No significant difference was found in the data obtained when two different cut–off distances were used, Rc = 2.5σ y Rc = 4.0σ. This is a consequence of the short–range nature of the potential.
Keywords: Surface tension; Morse potential; molecular dynamics.
Resumen
Se realizaron simulaciones canónicas de Dinámica Molecular para obtener propiedades termodinámicas en la fase líquida y en la interfase líquido–vapor para fluidos que interactuan mediante el potencial de Morse. Se calcularon en una fase las propiedades de transporte tales como la autodifusión y la viscosidad de corte. La autodifusión muestra una dependencia en el número de partículas mientras que la viscosidad de corte no mostró tal comportamiento. Las propiedades que se calcularon en la interfase líquido–vapor fueron las densidades ortobaricas, la presión de vapor, y la tensión superficial. Las densidades de equilibrio fueron comparadas con aquellas densidades que se obtuvieron mediante la metodología NpT mas partícula de prueba, y de esto se halló un acuerdo excelente. La tensión superficial y la presión de vapor se calculan aquí por primera vez. También hemos analizado la dependencia en el radio de corte por parte de las propiedades de bulto e interfaciales. Los datos obtenidos con dos distintos radios de corte, Rc = 2.5σ y Rc = 4.0σ, fueron comparados entre sí, y como resultado de ello, no hallamos diferencias significativas. Esto es una consecuencia de la naturaleza de corto alcance del potencial.
Descriptores: tensión superficial; potencial de Morse; dinámica molecular.
PACS:31.15.Qg
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Acknowledgments
The authors acknowledge CONACyT–México for partial support Grand P/49607. JLL wishes to thank UAEM–México Grands 2152/2005 and 2263/2006, for partial support. JLL would also like to thank Jorge Orozco–Velazco for his useful comments.
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