The influence of pH on barite nucleation and growth
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Nanoscale Atomic Force Microscopy (AFM) experiments show that barite (BaSO4) growth is influenced by the pH of the growth solution. AFM observations provide evidence that growth and nucleation rates measured along the  crystallographic direction on the initial layer grown on barite (001) natural surfaces increase at both high and low pH of the growth solutions. At alkaline pH, growth is arrested in the second and successive layers, possibly as a result of the structure distortion resulting from incorporation of foreign ions (OH- and/or CO32-). Macroscopic nucleation experiments also show that with increasing pH, the induction times, the precipitation rate and the interfacial tension are all reduced, consistent with nanoscale observations. Smaller particle size at high pH provides further evidence for enhanced barium sulfate nucleation in alkali solutions. This enhancement in growth as well as in nucleation of barite at high pH could be explained by taking into account the effect of hydroxyl ions on hydration shells of aqueous Ba2+ and SO42- in solution and on the barite surface. The energetic interaction between water molecules and the barite building units is affected by the presence of OH- ions in solution. The frequency of water exchange around Ba2+ and SO42- could increase due to the effect of OH- ions on the structure of water and consequently promote nucleation and growth. Increased growth at low pH can be attributed to increased Ba2+ activity with respect to SO42-.
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