Effect of sodium polyacrylate molecular weight on the crystallogenesis of calcium carbonate

Aqueous solutions of sodium polyacrylates (NaPA) series having molecular weights (Mw) ranging from 2540 to 9890 g mol−1 are used as precipitation media to control the size and shape of calcium carbonate (CaCO3) particles. The retarding effect of polyacrylates on CaCO3 nucleation is evidenced by the increase of the induction time, τ, of the precipitated CaCO3, from τ=55 s in the absence of additives, to τ values in the range 100–2500 s in the presence of NaPA samples. The data also show the coexistence of two polymorphs, calcite and vaterite, for CaCO3 particles as prepared in the presence of NaPA samples. The vaterite fraction, fv, varies in all instances with the polymer concentration, Cpoly (g. L−1), and reaches its maximum value, fv,max at optimal ratio, R (mol. g−1), of Ca ion to polymer (NaPA), R=[Ca]/([NaPA]=Cpoly). No simple general trend is found to explain the influence of the molecular weight (Mw) of NaPA on the induction time, τ, and on the vaterite fraction, fv, since these two parameters are found to vary with Cpoly and Mw. However, under certain experimental conditions, an optimum polymer molecular weight (Mw=5530 g mol−1) of the NaPA series, gives the highest values of fv,max and τ. Such optimum indicates the influence of Mw of NaPA on CaCO3 nucleation and growth, and it is related to the surface density and the rate of adsorption of the polymer onto the growing crystal.The CaCO3 particle size is reduced from about 20 μm, as obtained in the control experiment, to sizes varying in the range 2–8 μm in the presence NaPA samples. Polymers having low Mw values (Mw<5000 g mol−1) are found to be more efficient in reducing the CaCO3 particle size.