Chapter 9 Is there hope for multi-site complexation (MUSIC) modeling?

Abstract

This chapter presents a discussion on multi-site complexation (MUSIC) modeling. The main difficulties with equilibrium modeling of solid-mineral interfaces have always been heterogeneity of surface sites and interfacial charging. Conventional surface complexation models (SCMs) provide corrections to apparent surface equilibrium constants, based on simple electrostatic models of the interface, but treat site types, site densities, and intrinsic surface equilibrium constants as adjustable parameters. This chapter discusses the current MUSIC model as a good attempt for bringing needed reform to the world of SCMs. It is also discussed that it is premature to dismiss all valence-based methods for p K a prediction, although they may not be suited to address certain problems on their own. There may yet be hope for the prospect of easy-to-implement SCMs, including some level of molecular-scale insight. In MUSIC, the basic idea for p K a prediction is to create an empirical correlation to predict the acidity of (hydr)oxy-acid solution monomers that is transferable to functional groups on oxide surfaces. To make this work, the following three main issues must be addressed: (1) electrostatic and intrinsic dissociation energy, (2) bond valence and intrinsic acidity, and (3) solvation. The chapter also analyzes whether the MUSIC method—(1) correctly partitions long-range electrostatic and intrinsic portions of the reaction energy, (2) correctly handles the effect of bond valence on intrinsic reaction energy, and (3) correctly describes the influence of solvation. It is possible to create a simple valence-based model of acidity that avoids many (but not all) of the problems inherent in the MUSIC formulation.