Developing a Theory of Relaxation Dynamics for Fluids Confined in Porous Materials

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

0853068 Monson "This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)." Intellectual Merit. This project supports for a research program on development and application of modeling techniques that can simultaneously describe both thermodynamic and dynamic transport phenomena for fluids confined in porous materials. The scientific motivation for this work is two fold. One motivation is the need to understand adsorption desorption hysteresis in mesoporous materials. The absence of true equilibrium in these systems makes it of particular interest to understand the dynamics. Another motivation is to develop a more sophisticated understanding of the transport resistances to equilibration in adsorption isotherms and how these depend on the structure of the porous materials. These are both problems with long histories and are even in some sense classical. However, the project proposes modeling research on developing a consistent description of both the thermodynamics and transport phenomena. This has the potential for a truly transformative contribution in this field. The approach we take here is to develop a theory of the time dependent molecular density distribution in the system. We then consider the dynamic evolution of this distribution in terms of the probabilities of transitions between states of the system. We use a lattice gas model to describe the interactions in the system and the PIs theory, dynamic mean field theory (DMFT), yields a mean field (classical density functional) description of the equilibrium and metastable states of the system. Ther proposed research has following components: i) Application of the DMFT to a variety of model pore geometries. They will choose geometries that illustrate the impact of pore structure on the relaxation dynamics. ii) Application to pore geometries in real systems. They will study systems of independent pores, including MCM 41 and porous silicon and ordered pore network systems such as KIT 6 and SBA 16, as well as disordered pore network systems exemplified by porous glasses. iii) Developing a theory of transport and self-diffusivities. The DMFT can also be analyzed from the point point of view of diffusion and provides a theory of transport diffusivity as well as self diffusivity. iv) Application to mercury porosimetry. We will use the DMFT to understanding the nature of mercury entrapment in mercury porosimetry, building on their recent work showing how gas adsorption and mercury porosimetry can be modeled in a single framework. v) Accuracy assessment and additional developments. They will make a detailed assessment of the impact of the approximations built into the approach using Kawasaki dynamics and molecular dynamics simulations. They also investigate including thermal fluctuations in the theory and plan to investigate an extension of the approach to off lattice molecular models. Broader Impacts. The research proposed here is fundamental in nature and addresses the nanoscale modeling of dynamic relaxation for fluids confined in porous materials. This is potentially transformative research since it will provide a bridge between two research communities in the area of confined fluid properties: one that focuses primarily on adsorption measurements and thermodynamics and the other that focuses on transport phenomena. The immediate impact is in the development of characterization methods for porous materials. However, given the very extensive world-wide effort in developing new porous materials for applications ranging from separations to catalysis to microelectronics the ultimate impact could be very broad including energy-related applications. The project has a number of educational components including research experience for under- graduates, outreach to community colleges and development of course materials for undergraduate courses in thermodynamics, reaction engineering and transport phenomena based on the research.