LAUSR

Abstract This study presents zeta potential (ZP) measurements of synthetic calcite in glycine solutions using the electrophoretic method. We tested samples with glycine concentrations of 1, 3, and 5 wt% at 25°C, while the ZP of 5 wt% glycine solutions was also measured at 50 and 75°C. The first set of measurements consisted of single-component brines prepared with CaCl2, MgCl2, Na2SO4, and NaCl salts with an ionic strength of 0.05 M. The second set of measurements consisted of formation brine (FB) with an ionic strength of 6.37 M and FB diluted 2-, 10-, and 100-times (2dFB, 10dFB, 100dFB, respectively). Results show that the effect of glycine on calcite’s ZP depended on its initial ZP value. Moreover, the increase in temperature significantly decreased calcite’s ZP in most samples. The ZP values in CaCl2 and MgCl2 brines were initially positive and decreased with the addition of glycine. Conversely, the ZP values of calcite in NaCl and Na2SO4 brines were initially negative and increased with the addition of glycine. The trends of ZP versus glycine concentration for 2dFB and 10dFB were analogous to the ones with CaCl2 and MgCl2 brines. Similar trends were observed between 100dFB, NaCl, and Na2SO4 brines. Finally, the increase of temperature from 25°C to 50°C and 70°C significantly reduced the ZP of calcite’s particles in CaCl2, Na2SO4, and NaCl brines, while the ZP of FB and its dilutions slightly decreased at 50°C. This is the first time the ZP measurements of calcite in glycine solutions were reported, to the best of our knowledge. Results are mainly explained by (1) the adsorption of glycine anion onto the calcite’s surface, (2) the decrease of the brine’s pH from the addition of glycine, and (3) the complex formation of glycine with divalent ions. Given the ubiquity and abundance of calcite in nature, we expect this study to increase the understanding of the electrostatic interactions between amino acids and calcite in phenomena such as wettability alteration and biomineralization.