Molecular modeling of adsorption of simple and complex fluids on nanoporous materials for characterization

This is the authors' abstract. We don't add key points for this paper.

The process of adsorption – in particular physisorption – occurs when the molecules of a fluid accumulate upon a solid surface. Adsorption is influenced by temperature, pressure, and adsorbate composition – as well as the adsorbate-adsorbent interactions, which are dependent on the chemistry and topology of the adsorbent. The influence of these factors is reflected in the adsorption behavior – the quantitative and qualitative interactions between the adsorbed species and the adsorbent. The unique features of adsorption have led to its widespread use for the characterization of porous materials. Of particular interest is the class of materials with pores which have a characteristic dimension on the order of nanometers, termed nanoporous materials. This dissertation focuses on the accurate modeling of adsorption through analytical, computational and simulation techniques and the application of adsorption modeling to characterization of nanoporous materials. Analytical and computational models were developed to predict the specific behavior of simple fluids (N2, Ar, Kr, CO2) and complex fluids (polymers) interacting with nanoporous materials and transferrable tools were created to facilitate adsorbent characterization based upon adsorption behavior. The tools and models presented in this dissertation aid one to understand the peculiarities of adsorption behavior, in particular the phenomena of capillary hysteresis, scanning hysteresis, and the so-called “critical conditions” of adsorption and to exploit these peculiarities to derive useful information about nanoporous materials. In particular: a suite of analytical models was developed for the analysis of scanning isotherms, suitable for any simple nonpolar fluid, from which pore size distributions and pore domain correlation/connectivity may be derived. A set of adsorption isotherm kernels was developed using Quenched Solid density functional theory to analyze adsorption isotherms of Ar and CO2 on micro-mesoporous carbons, capable of distinguishing slit, cylindrical, and spherical pore geometries. A new criterion for the critical conditions of polymer adsorption on surfaces – the incremental chemical potential – has been demonstrated and applied to the case of liquid polymer chromatography. An expression for the overall partition coefficient was developed which takes into account adsorbent geometry and column porosity. It was shown that the adsorption and elution behavior of polymers can be predicted for nonporous and porous column substrates using this partition coefficient, with minimal parameterization.