Characterization of porous glasses: Simulation models, adsorption isotherms, and the Brunauer-Emmett-Teller analysis method

We have developed a realistic model for studying adsorption in porous glasses which reproduces the complex structure of these materials. The model porous material is generated by a quench molecular dynamics procedure which mimics the processes by which Vycor glass and controlled-pore glasses are produced. We examine this procedure and the resulting model materials by a variety of methods and find that they have porosities, pore sizes, and surface areas very similar to the real glasses. These simulated glasses have precisely known properties (surface area, pore size distribution, etc.), in contrast to experimental glasses; computer experiments on such model glasses can therefore be used to test new and existing experimental methods of characterization. We calculate the adsorption isotherms for a model of nitrogen adsorbing onto these materials and analyze these data using the BET isotherm. The BET monolayer density exhibits systematic variations with both the average pore size and the porosity of the model glasses, which can be partially explained by studying the variations of adsorption energies in the different systems. We have done similar calculations in a series of ideal planar and cylindrical pore systems with comparable surface micro-structure. We find that there is a systematic error involved in comparing the networked glasses to the ideal cylinders as is commonly done in the analysis of these materials. For glasses with large pores this error is negligible, but for very small pores it can be significant. We discuss the implications of these findings for other standard techniques of isotherm analysis.