Formation of thin calcium carbonate films on chitosan biopolymer substrates

Crystallization of two-dimensional spherulitic calcium carbonate films was performed on planar films of chitosan in the presence of poly(acrylic acid) (PAA). PAA, electrostatically bound to the chitosan substrate, increased the local concentration of calcium at the surface, thereby promoting CaCO3 crystallization there. Simultaneously, interaction between PAA and calcium ions in the surrounding solution suppressed bulk crystallization. Increasing the PAA concentration at fixed pH increased the surface supersaturation of calcium ions leading to larger spherulites. Saturation of the surface with PAA at 0.004% led to a maximum in the nucleation rate at this concentration. Above a certain PAA concentration, a regime existed at which the surface was completely covered. Above even higher PAA concentrations no crystallization occurred, since the bulk PAA sequestered all of the calcium ions in solution previously available for surface crystallization. As the pH increased, PAA became more charged and therefore interacted with more calcium ions in solution. Increasing sequestration of bulk calcium ions with increasing pH countered the increase in surface calcium supersaturation and carbonate ion formation, leading to a maximum in surface coverage at an intermediate pH value, at fixed PAA concentration. In situ crystallization experiments demonstrated that both vaterite and calcite spherulites grew simultaneously in separate parts of the film, albeit at different rates. The coexistence of polymorphs may therefore indicate the importance of local supersaturation conditions in determining the film morphology.