AmericanChemicalSociety.com

Organic molecules are ubiquitous in sedimentary basins and other natural environments, where they exert a strong influence on the stability and growth/dissolution kinetics of geochemical significant "sparingly soluble" minerals such as calcite (CaCO₃), barite (BaSO₄) and gypsum (CaSO₄.2H₂O) that comprise petroleum reservoirs, evaporites, and pore-filling precipitates, thus affecting petroleum, natural gas and groundwater formation, migration and flow, and contributing to the global carbon cycle (in the case of calcite).

We are investigating the fundamental molecular-level mechanisms by which organic acids influence calcite growth/dissolution rates. We hypothesize that organic acids adsorb at steps on growth spirals/etch pits, thus either stabilizing the steps thermodynamically, or hindering step movement kinetically by blocking sites.

Our approach is to use Molecular Mechanics/Molecular Dynamics (MM/MD) simulations of organic acids at calcite surfaces and compare our results to previously published experimental Atomic Force Microscopy (AFM) studies of the same systems. We have examined adsorption of succinate, a dianion at neutral pH, along with two Na⁺ counter-ions at steps in various directions on the calcite {104} cleavage face, in a fully hydrated system. Adsorption strength is measured in terms of the reaction energy (-DE_r, _{suc + 2Na+, hyd}) for succinate adsorption at each step, and stability of steps with succinate adsorbed is represented by surface energy of the step (G _{suc+2Na+, hyd}). Our results indicate that succinate adsorbs least strongly (least negative value of -DE_r, _{suc + 2Na+, hyd}) at etc pit step directions that are observed experimentally to open up the earliest in a time-sequence, and most strongly on step directions that appear the last in time. Stronger adsorption slows down step velocity, suggesting that the mechanism is under kinetic control. Stereochemical matching between organic functional groups and calcite surface sites may allow preferred adsorption in certain crystallographic directions, affecting crystal morphology. Furthermore, if the mechanism were under thermodynamic control, in the presence of adsorbed succinate, we would expect to see greatest step stability (least positive value of G _{suc+2Na+, hyd}) for the slowest moving step and lowest step stability at the fastest moving step. We do not observe this sequence, confirming a lack of thermodynamic control. Our study provides a close comparison of MD simulations and experimental AFM studies, and establishes the mechanism for succinate-mediated etch pit dissolution/growth spiral morphology. Our results on calcite should have implications for other geochemically significant sparingly soluble minerals. Future work includes analogous MD simulations for citrate, a trianion, and for chiral amino-acids.