Controlled variation of calcite morphology using simple carboxylic acids

Abstract

Water-soluble straight-chain monocarboxylic acids have been used to manipulate the processes of crystal nucleation and growth of calcium carbonate using the vapour diffusion method. The effect of solution pH was also investigated. Low initial solution pH (pH<2.5) led to >95% calcite nucleation, as compared with approximately 30% in control experiments with no added acid (pH∼5.3), irrespective of whether a mineral or organic acid was used to control the pH (note that the final solution pH was approximately 9 for reaction media that produced crystals). The presence of the mineral acid did not alter the expressed morphology of the calcite crystals, with the thermodynamically stable rhombohedral crystal habit dominating. In the presence of carboxylic acids, calcium carbonate crystal habit was considerably altered. The calcite habit expressed could be manipulated by varying the hydrophilic/hydrophobic balance within the carboxylic acid additive, by changing the hydrocarbon chain length, for a given calcium ion and additive concentration and partial pressure of carbon dioxide. The exact habit expressed could be further dictated by independently varying the partial pressure of carbon dioxide for a given concentration of calcium and carboxylic acid additive.