Development of High-Quality Models for Anhydrous and Aqueous Hydrogen Fluoride

Suny At Buffalo, Amherst NY

Investigators

Abstract

ABSTRACT CTS-0076515 Kofke, D. A. SUNY @ Buffalo This project is concerned with the development of models that can predict and characterize a broad range of properties of hydrogen fluoride (HF) and its mixtures with water. The specific aim of the project is to develop and test molecular models appropriate for these systems. The larger aim in all of this work is the advancement of modeling techniques that bridge ab initio quantum chemistry and bulk-phase modeling via molecular simulation. Thus the specific focus of the project provides a vehicle for treating a problem of much broader impact. The modeling incorporates as much as possible the fundamental quantum chemistry that governs the molecular interactions. Several approaches are examined. The first approach adopts methods that are currently in wide used for quantitative modeling. Dispersion and repulsive interaction are treated with 12-6 exp-6 models, and hydrogen-bonding is modeled via a simple electrostatic treatment (e.g., point charges). It is anticipated that this methodology will fail because it does not capture important features of HF interactions. Other approaches are considered to include the quantum-mechanical effects present in HF that are difficult to capture using an analytic potential. These include a combined quantum- and molecular-mechanics treatment that ahs been applied to water, and a treatment that considers three-body effects explicitly coupled with a high-quality model for the dimer. Advances in modeling of HF are then introduced to molecular models of water to examine the behavior of HF/water mixtures. It is of interest to test the validity of any HF/water model by examining its ability to explain the thermodynamic inconsistency obtained in the application of simple models to published experimental HF/water vapor-liquid equilibrium data. All modeling efforts here are concerned with the volumetric properties, phase equilibria and heat effects. The behavior of HF in all of these directions is highly anomalous. Additional components of this study are concerned with the effect of intermolecular association on surface tension, and on the development of improved molecular simulation techniques of associating fluids. The interest in the former is driven by the anomalously small surface tension of HF, which contributes to its ability to form aerosols (a significant safety problem), while the motivation for the latter is related to the natural inefficiency of simulation when applied to associating systems, combined with our interest in studying such systems with computationally expensive molecular models.