Probing molecular-scale adsorption and dissolution-growth processes using nonlinear optical and scanning probe methods suitable for hydrothermal applications

Experimental hydrothermal geochemistry has made great strides on the basis of stoichiometric equilibrium studies, measured kinetic rates, and bulk-sensitive spectroscopic data, but there are current problems in hydrothermal geochemistry ranging from deep aquifer chemistry to nuclear waste tank chemistry that can benefit from new techniques able to provide direct spectroscopic and microscopic information about processes operative at mineral surfaces. We present and discuss applications of two inherently interface-sensitive techniques that are amenable to application in hydrothermal experimentation: optical second harmonic generation (SHG) and hydrothermal atomic force microscopy (HAFM). Results of experimental SHG from a corundum-water interface are presented, along with a model relating SHG signal in part to the structure of water in the electric double layer. Although the SHG signal is closely related to surface potential, direct extraction of surface potential over a wide range of pH and ionic strength will depend upon better understanding of water structure near oxide-water interfaces. We present applications of HAFM to dissolution and growth of barite, and in particular show that the velocity and spacing of kink sites along [010] and [120] steps can be extracted from the HAFM imagery using a simple step model. The model enables us to conclude that although steps may appear straight in AFM images, the atomic scale roughness can be large, giving rise to kinks at a large percentage of step sites. Both techniques are promising ways to extend our ability to probe and better understand the nature of mineral-water interface chemistry.