Understanding the Kinetics of Barium Sulfate Precipitation from Water and Water-Methanol Solutions
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It is well understood that the solvent can affect precipitation kinetics, and all chemists are very familiar with the notion that adding an organic solvent to an aqueous solution induces the rapid precipitation of dissolved inorganic salts. The general explanation for this observation is that the solubility of the salt in the organic/water mixture is lower than in pure water and therefore precipitation occurs. Here we study the kinetics and thermodynamics of precipitation of barium sulfate from water/methanol mixtures.It is shown that addition of an organic solvent to a water solution affects not only the thermodynamics but also the kinetics of the precipitation. The kinetics of precipitation in water/methanol mixtures is faster than in pure water. This is the opposite of what would be expected from classical nucleation theory. Molecular dynamics simulations show that nucleation proceeds through the formation of amorphous aggregates that are thermodynamically more stable than the crystalline phase. The formation of a crystalline nucleus involves the restructuring of the amorphous aggregates to form ordered particles. We suggest that this step can be ratelimiting for nucleation. The kinetics of crystallization for divalent salts in general are usually very similar; therefore, this result may be also valid for other salts of industrial or biological relevance.
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