Exploration of molecular dynamics during transient sorption of fluids in mesoporous materials

Abstract

In recent years, considerable progress has been made in the development of novel porous materials with controlled architectures and pore sizes in the mesoporous range. An important feature of these materials is the phenomenon of adsorption hysteresis: for certain ranges of applied pressure, the amount of a molecular species adsorbed by the mesoporous host is higher on desorption than on adsorption, indicating a failure of the system to equilibrate. Although this phenomenon has been known for over a century, the underlying internal dynamics responsible for the hysteresis remain poorly understood. Here we present a combined experimental and theoretical study in which microscopic and macroscopic aspects of the relaxation dynamics associated with hysteresis are quantified by direct measurement and computer simulations of molecular models. Using nuclear magnetic resonance techniques and Vycor porous glass as a model mesoporous system, we have explored the relationship between molecular self-diffusion and global uptake dynamics. For states outside the hysteresis region, the relaxation process is found to be essentially diffusive in character; within the hysteresis region, the dynamics slow down dramatically and, at long times, are dominated by activated rearrangement of the adsorbate density within the host material.

Mean field kinetic theory for a lattice gas model of fluids confined in porous materials

“…Examples include phase separation dynamics for confined liquid mixtures, 10,11 the kinetics of nonwetting liquids in pores, 16,17 and the relaxation dynamics during gas adsorption in porous glasses. 15,18 These approaches are an efficient alternative to very time consuming nonequilibrium molecular dynamics calculations. 19,20 However, even with the lattice model simplifications, Kawasaki dynamics simulations can be computationally intensive, especially if one wants to consider many trajectories of the dynamics to determine an ensemble average.…”

Physical Adsorption Characterization of Nanoporous Materials

“…Generally, hysteresis is being considered: (i) on the level of a single pore of a given shape, (ii) cooperative effects due to the specifics of connectivity of the pore network, and (iii) in highly disordered, and for inhomogenous porous materials a combination of kinetic and thermodynamic effects spanning the complete disordered pore system has to be taken into account. Progress has been achieved in understanding the underlying internal dynamics of hysteresis in disordered pore systems [71], however a discussion of this topic is beyond the scope of this Section.…”

A new view of diffusion in nanoporous materials

“…Fig. 17a illustrates the novel options of PFG NMR when applied to a sample of nanoporous material in contact with a guest atmosphere of variable pressure: Simultaneously with the variation of the external pressure, the relative amount adsorbed (lower part, right ordinate) and the self-diffusivities (upper part, left ordinate) can be determined [93]. The data have been obtained with cyclohexane in Vycor porous glass.…”