New Concepts on Surface Chemistry From Ab Initio and Monte Carlo Methods

Antonio C. Lasaga Department of Geology and Geophysics, Yale University,

210 Whitney Ave., New Haven, CT, 06511, USA

The integration of the dependence of mineral surface rates on temperature, pH, solution composition and deviation from equilibrium into a unified overall rate law can be significantly enhanced by a combination of first-principles atomic studies and new Monte Carlo simulations.

The ab initio results have led to new conceptual understanding of the rate laws and mechanisms of geochemical reactions. An important example is the pH effect on the dissolution rate of many mineral-fluid reactions. The interactions of a proton with the bridging oxygen of an Si-O-Si or Si-O-Al group in an aluminosilicate (e.g. feldspars, clays, zeolites) have been shown to strongly lengthen and weaken the Si-O and Al-O bonds, thereby catalyzing the dissolution process. This result is in agreement with experimental work on mineral-fluid reactions. But, in addition, the ab initio work has also shown that the chemical effect of protonating the bridging oxygen diminishes greatly with distance, so that the neighboring Si-O or Al-O bonds are not much changed.

This latter result will be shown to explain the pH contribution to the overall rate law obtained in recent experimental work on albite, forsterite, and kaolinite. From a kinetic mechanism point of view, the only way that a dissolution rate law can depend on a higher power of the total adsorption hydrogen ion concentration (e.g. n = 2 or 3) involves the necessity of having two or more adsorbed protons next to each other before any substantial bond breaking or hydrolysis can occur. In a reaction mechanism that sequentially clips the Si-O-Si or Si-O-Al bonds, catalysis by H+ adsorption would lead to a linear rate law. A non-linear dependence requires a multi-proton adsorption precursor. Energetically, the strong coulombic repulsion of the two protons would make the number of adsorption sites of this type very very small. Nonetheless, in kinetics very small concentrations of very reactive sites (e.g. H+ catalysis of feldspars) can dominate the rate. Thus, a non-linear rate law would be reasonable if the diminished population of the multi-proton adsorption complex is compensated by a huge increase in reactivity of this complex. However, the very short range nature of the chemical effect of the adsorbed proton based on the high-level ab initio studies, strongly indicates that the chemical destabilization gain of having two protons near each other will be close to the sum of the individual effects from each proton. This bonding picture from the ab initio calculations, therefore, leads to an important generalization of the few experimental studies done so far.

One of the most important "functions" in the area of water-rock kinetics is the variation of the growth/dissolution of minerals as a function of the deviation from equilibrium, f(Gr). Without knowledge of f(Gr), one cannot develop models for fluid movement in the earth's crust that incorporate a realistic treatment of the chemical interaction with minerals. Experimental work has found rather interesting non-linear f(Gr)) functions. Theoretical understanding of f(Gr) is very important at this stage of the development of the kinetic theory of water-rock interactions. The new Monte Carlo methods presented here directly involve the crystal structure, as opposed to earlier Ising-like box Monte Carlo models, and can provide much needed insight into our concepts of the f(Gr) function. For example, the new Monte Carlo model of the dissolution of kaolinite would track the hydrolysis reaction of all the possible surface complexes. The actual rates of these hydrolysis reactions may depend on neighbor interactions.

The difference between this new Monte Carlo approach to mineral dissolution and growth and earlier Monte Carlo models is the shift in focus from the molecular units or "blocks" that attach or detach to the surface to a focus on the bonds themselves. Thus the breaking of a particular bond depends on the nature of the neighboring bonds. By shifting to this "bond-centered" scheme}, the Monte Carlo simulations can deal with actual mineral structures, which is very compatible with the type of fundamental results coming from the ab initio work. The integration of the elementary dissolution/growth processes into an overall mechanism, leads to a non-trivial interpretation of the temperature dependence of the kinetics as well as to possible explanations of the experimental f(Gr) functions.