Stability and occurrence of polymorphic modifications of the stearic acid in polar and nonpolar solutions

Abstract

The crystallization of A, B and C polymorphs of stearic acid was examined in polar (methanol) and nonpolar (decane and benzene) solutions. First, the stability of the three modifications was determined by comparing the relative degree of solubility of each polymorph; B and C are the most stable forms below and above about 30°C respectively, whereas the A form is unstable at any temperature. Thereafter, the occurrence of the three forms was investigated by controlling the effects of temperature, supersaturation and solvent independently. The general trend is that: in nonpolar solvents, the A and C forms prevail at higher supersaturation, while the B form preferably occurs at lower supersaturation; in polar solvents, the occurrence domain of B is more extended towards the higher supersaturation range. In all cases, particularly in nonpolar solvents, the relative importance increases for A and C but decreases for B with increasing temperature. This trend was semi-quantitatively interpreted on the basis of a homogeneous nucleation rate function. Further, it was shown that a variety of previous studies, which have been so far rather contradictory to each other, could consistently be explained by the present study taking the thermodynamic and kinetic parameters in the crystallization process into account.