1. Surface charge development at the Barite-Water interface in NaCl media, from 15 to 50˚C Heather Williams and Moira K. Ridley Department of Geosciences, Texas Tech University INTRODUCTION Barite is found in a variety of geologic environments, including marine deposits from evaporation and biogenic processes, as well as hydrothermally mineralized veins. Barite is also of industrial importance. For example, it is found as scale in pipes which causes a decrease in the effective radii of pipes and limits fluid flow. Barite is an additive to drilling fluids used for oil and gas exploration. Consequently, a number of studies have been performed analyzing crystal growth and the dissolution behavior of barite. Similarly, numerous studies have been performed to evaluate the reactive processes occurring between the surface of calcite and aqueous solutions. Barite has a crystal structure similar to calcite; additionally, the solubility of barite has been shown to increase when calcite or gypsum are present. Therefore, studies to evaluate the surface reactivity of barite in aqueous solution may assist in elucidating calcite–water interactions. In this study, potentiometric titrations were performed to understand the surface charging behavior of barite in NaCl media at a variety of ionic strengths (0.03, 0.06, and 0.1 m) and temperatures (15, 25, 35, 50, and 65 ˚C). All experiments were performed in NaCl media. Sodium chloride media was selected as it most closely matches the natural environmental conditions were barite is found. CHARACTERIZATION The commercial barite sample used in this study was characterized extensively. Characterization included SEM and TEM imaging, XRD analysis, and BET surface area measurements. Ionic Strength (m)0.03, 0.06, 0.1 Temperature (⁰C)15, 25, 35, 50, 65 MATERIALS & METHODS At each experimental condition base and acid calibrations solutions, and a base titrant were prepared. The experimental solutions were prepared from concentrate HCl, NaOH, and NaCl stock solution, and 18 MΩ water. The potentiometric titrations were performed using a single titration cell. The titration procedure included a two point calibration of the electrode, first at base then acid pH. The synthetic barite powder was added to the acid calibration solution, allowed to equilibrate then the solid–solution mixture was titrated with the base titrant. For each titration, a constant mass of ~ 0.65 g of barite was used. The maximum variation in the mass of solid added was 5 %. In addition, background titrations were performed at each experimental condition. These titrations were conducted in the same manner as the barite titration, but omitted the solid (i.e., only the prepared solutions were used). The background titrations accounted for any pH-dependent solution effects. RESULTS ACKNOWLEDGEMENTS Thanks to Dr. A. Stack for providing the barite samples, and Ms. J. Riedel for help with sample characterization. STEM/TEM (upper) and SEM (lower) images of the barite used in the titrations. The particles are euhedral and approximate 50 nm in diameter. Results from XRD analysis of the barite sample. XRD confirmed that the sample was crystalline synthetic barite. The effective radii of industrial pipes are reduced by barite; thus, decreasing necessary fluid flow. Barite is commonly used in drilling fluids for oil and gas exploration. Barium Oxygen Sulfur Orthorhombic crystal structure of barite Barite surface charge is strongly dependent on ionic strength. Surface charge decreases as ionic strength decreases. The titration curves are shallow from pH 3 to 8 then steepen significantly. The barite surface is positively charged over the entire experimental pH range, and at all ionic strengths and temperatures studied. The results suggest that the zero net proton charge (pH znpc ) value is at high pH, likely above a pH of 9. Experimental Conditions: at each ionic strength, titrations were performed at all 5 temperatures. The proton induced surface charge of barite shows a strong temperature dependence, as the surface becomes progressively more positive (deficit of H + in solution) with increasing temperature. Background titrations show pH-dependent solution effects. The titrations were fitted to polynomial functions and used to correct the concentration of calculated H + in the solid-solution titrations.