Adsorption phase change and wetting condition at solid-vapor interface

Abstract

Adsorption is the process in which atoms or molecules from a substance adhere to a surface of a adsorbent. Adsorption is a typical surface-based process, which has been a hot research topic in interfacial science and attracts great attention in recent years. Adsorption at a solid-vapor interface is not only important in many industry technologies, but also a phenomenon commonly encountered in nature. The first isotherms model namely Langmuir assumes that one gas molecule could be adsorbed on one adsorption site. Later, the well known BET isotherm formulation was introduced, the concept is that adsorption takes place in layer. However, these models of adsorption isotherms all indicate infinity in the limit of pressure approaching the saturation vapor pressure. Recently, the ζ adsorption isotherm was proposed by approximating the adsorbated as molecular clusters with a maximum of one cluster adsorbed at an adsorption site. This model has the essential characteristic of predicting a finite amount adsorbed at the saturation-vapor pressure. When the Gibbsian thermodynamics is combined with the equilibrium adsorption isotherm, the expression for the solid-vapor surface tension can be established. The ζ adsorption isotherm has been tested successfully for some materials in the limit of pressure approaching the saturation pressure. When a smooth, rigid, homogeneous solid surface is exposed to a vapor at pressure equal or greater than the saturation pressure, the ζ isotherm indicates that an adsorbed vapor film forms. However, no experimental investigation has been reported to examine the validity. To investigated the adsorption behaviors at the solid-vapor interface and to examine the ζ isotherm model, the amount of adsorption are obtained by means of two sets of experiments. First, when the pressure is less than the saturation pressure, the gravimetric measurements of vapor adsorption on silicon powder are conducted. Then the interferometer spectra analysis is used to measure the adsorbed thickness when the pressure is greater than the saturation pressure. Based on the ζ adsorption isotherm, the phase change and wetting condition are discussed. Results show that the isotherms predicted by ζ adsorption equation agree well the experimental measurements. When the pressure ratio is greater than a certain value, the amount of adsorption undergoes a sharp increase and then tends to a constant. The adsorbed vapor is approximated as a collection of molecular clusters. When the pressure is near zero, the adsorbate consists primarily of single molecules, and the number of occupied sites is small. As the pressure ratio progressively increases, the clusters with a larger number of molecules appear in the adsorbate and the number of occupied sites decreases. When the pressure ratio reaches 1.14, the number of unoccupied adsorption sites goes to zero, all the adsorption sites are occupied and the clusters merge to initiate the liquid phase. When the pressure ratio exceeds 1.14, only the large liquid-like clusters are present, which suggests a phase change takes place in the adsorbate and wetting occurs. The surface tension of the solid-vapor are found to be decreased as the adsorption increases, and the predicted wetting condition is in good agreement with the experimental measurements.