Impact of Solution {Ba²⁺}:{SO₄^2-} on Charge Evolution of Forming and Growing Barite (BaSO₄) Crystals: A ζ-Potential...

The impact of solution stoichiometry on formation of BaSO₄ (barite) crystals and the development of surface charge was investigated at various predefined stoichiometries (r_aq = 0.01, 0.1, 1, 10, and 100, where r_aq = {Ba²⁺}:{SO₄^2-}). Synthesis experiments and zeta potential (ζ-potential) measurements were conducted at a fixed initial degree of supersaturation (Ω_barite = 1000, where Ω_barite = {Ba²⁺}{SO₄^2-}/K_sp), at circumneutral pH of ∼6, 0.02 M NaCl, and ambient temperature and pressure. Mixed-mode measurement-phase analysis light scattering (M3-PALS) showed that the particles stayed negative for r_aq < 1 during barite crystal formation and positive for r_aq > 1. At r_aq = 1, two populations with a positive or negative ζ-potential prevailed for ∼2.5 h before a population with a circumneutral ζ-potential (-10 to +10 mV) remained. We relate the observations of particle charge evolution to particle size and morphology evolution under the experimental conditions. Furthermore, we showed that the ζ-potential became more negative when the pH was increased for every r_aq. In addition, our results demonstrated that the type of monovalent background electrolyte did not influence the ζ-potential of barite crystals significantly, although NaCl showed slightly different behavior compared to KCl and NaNO₃. Our results show the important role of surface charge (evolution) during ionic crystal formation under nonstoichiometric conditions. Moreover, our combined scanning electron microscopy and ζ-potential results imply that the surface charge during particle formation can be influenced by solution stoichiometry, besides the pH and ionic strength, and may aid in predicting the fate of barite in environmental settings and in understanding and improving industrial barite (surface chemistry) processes.