文档介绍：Adsorption in Porous Materials at High Pressure: Theory and
Experiment
A. L. Myers
Department of Chemical and Biomolecular Engineering
University of Pennsylvania
Philadelphia, PA, 19104
P. A. Monson
Department of Chemical Engineering
University of Massachusetts
Amherst, MA, 01003
October 9, 2002
Abstract
We consider the thermodynamics of adsorption of gases in porous solids from both the per-
spective of absolute properties which appear naturally in theoretical studies and that of excess
properties which are measured in experiments. Our thermodynamic description starts by treat-
ing the gas (or gas mixture) plus porous solid system as a mixture to which we can apply
solution thermodynamics. We show that equations for the absolute thermodynamic properties
for adsorption in rigid porous materials do not require an explicit reference to the pressure of
the fluid confined in the porous material. We discuss how to relate absolute properties to excess
properties by using an estimate of the helium void volume. We illustrate the thermodynamic
formalism with calculations for a simple thermodynamic model in which the Langmuir equation
is used to describe the absolute adsorption isotherm and the ideal gas equation of state is used
for the bulk properties. The simplified model explains the apparently anomalous behavior of
the thermodynamic functions for adsorption at high pressure up to 1,000 bar.
1
1 Introduction
Statistical thermodynamic theories and molecular simulations of adsorption of gases on porous
solids are formulated in the language of absolute thermodynamic variables [1]. On the other hand,
experimental measurements are reported as excess thermodynamic variables [2]. At low pressure
(sub-atmospheric), the difference between absolute and excess adsorption is negligible. However,
for higher pressures the difference will be more substantial. In particular, for supercritical fluids
adsorbed at high pressure in the range f