Molecular dynamics under confinement to one dimension: options of measurement and accessible information

Two types of host systems for one-dimensional molecular arrangements are considered, namely zeolites containing one- and twodimensional arrays of channels of sub-nanometre dimension and porous silicon with channel diameters in the range of a few nanometres. After a discussion of the potential of zeolites as host systems, in particular for molecular arrangements under the conditions of single-file diffusion and of molecular traffic control, actual diffusion measurements by means of pulsed-field gradient NMR and interference/IR microscopy are shown to reveal substantial differences between the real and ideal zeolite structure. In contrast, porous silicon with onedimensional channel arrays is successfully exploited as a host system allowing the experimental observation of such most important features of molecular confinement like hysteresis in mesoscalic systems and surface diffusion. Thus, the attainable experimental insight offers promising conditions for a comparison of the results with those of the theoretical treatment of the observed phenomena

Structural characterization of porous solids by simultaneously monitoring the low-temperature phase equilibria and diffusion of intrapore fluids using nuclear magnetic resonance

Nuclear magnetic resonance (NMR) provides a variety of tools for the structural characterization of porous solids. In this paper, we discuss a relatively novel approach called NMR cryodiffusometry, which is based on a simultaneous assessment of both the phase state of intraporous liquids at low temperatures, using NMR cryoporometry, and their transport properties, using NMR diffusometry. Choosing two model porous materials with ordered and disordered pore structures as the host systems, we discuss the methodological and fundamental aspects of the method. Thus, with the use of an intentionally micro-structured mesoporous silicon, we demonstrate how its structural features give rise to specific patterns in the effective molecular diffusivities measured upon progressive melting of a frozen liquid in the mesopores. We then present the results of a detailed study of the transport properties of the same liquid during both melting and freezing processes in Vycor porous glass, a material with a random pore structure.

On the Comparative Analysis of Different Phase Coexistences in Mesoporous Materials

Alterations of fluid phase transitions in porous materials are conventionally employed for the characterization of mesoporous solids. In the first approximation, this may be based on the application of the Kelvin equation for gas–liquid and the Gibbs–Thomson equation for solid–liquid phase equilibria for obtaining pore size distributions. Herein, we provide a comparative analysis of different phase coexistences measured in mesoporous silica solids with different pore sizes and morphology. Instead of comparing the resulting pore size distributions, we rather compare the transitions directly by using a common coordinate for varying the experiment’s thermodynamic parameters based on the two equations mentioned. Both phase transitions in these coordinates produce comparable results for mesoporous solids of relatively large pore sizes. In contrast, marked differences are found for materials with smaller pore sizes. This illuminates the fact that, with reducing confinement sizes, thermodynamic fluctuations become increasingly important and different for different equilibria considered. In addition, we show that in the coordinate used for analysis, mercury intrusion matches perfectly with desorption and freezing transitions

History-dependent molecular dynamics in nanoporous host matrices

Ensembles of molecules confined to pore spaces with diameters of the order of a couple of molecular diameters reveal features which may notably deviate from their behaviour both in the bulk phase and under dominating host-guest interaction. We are going to demonstrate that under identical external conditions (temperature, pressure), different ensemble \"histories\" may give rise to dramatic differences in the states of these ensembles. PFG NMR diffusion measurements are introduced as a most sensitive tool for probing these differences