Structure of perfluoroalkane fluids at the liquid-vapor interface: a molecular dynamics study

D. A. Hernandez and H. Domínguez

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, México, D.F. 04510, e-mail: hectordc@unam.mx

Received 17 February 2014. ]]> accepted 17 June 2014.

Abstract

Structure of perfluoroalkane fluids at the liquid-vapor interface was studied by molecular dynamics simulations at different temperatures. It was observed that fluids formed a freezing layer at the liquid-vapor interface with a strong structure which it was more pronounced at low temperatures, close to the melting point. Whereas molecules did not have any preference orientation in the bulk fluid they developed a monolayer of molecules aligned perpendicular at the interface. Similar results have been observed for alkanes fluids in the same interface, however, the structure of those molecules is weaker than that reported for perfluoroalkanes. As the temperature increased the layer melted until it vanished. Finally, thermodynamic studies were also conducted in terms of surface tensions and good agreement with experimental data was found.

Keywords: Structure; freezing layer; surface tension; computer simulation.

Resumen

Se estudia la estructura de los fluidos perfluoroalcanos en la interfase líquido-vapor mediante simulaciones por computadora. Se observó que los fluidos forman una monocapa congelada en la interfase a temperature bajas, cercanas al punto de congelamiento. Las moléculas que se encuentran en la interface se orientan perpendicularmente a la superficie mientras que las moléculas en el centro no lo hacen. Este fenómeno se ha observado en fluidos tipo alcanos, sin embargo, el orden que tienen esas moléculas es menor a la que presentan los perfluoroalcanos. Cuando la temperatura se aumenta la estructura de la capa desaparece. Finalmente, se realizan cálculos de propiedades termodinámicas, como la tensión superficial obteniéndose un buen acuerdo con datos experimentales.

Descriptores: Estructura; capa congelada; tensión superficial; simulaciones por computadora.

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Acknowledgments

We acknowledge support from grants DGAPA-UNAM-Mexico, IN102812 and Conacyt-Mexico, 154899. We also thank DGTIC-UNAM for supercomputer facilities.

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