SOLVENT EFFECT OF ETHANOL ON CRYSTALLIZATION AND GROWTH-PROCESS OF L-HISTIDINE POLYMORPHS

The solvent effect of ethanol on precipitation behavior and crystal gr owth rates of the A and B polymorphs of L-histidine were investigated in aqueous solutions. The fraction of A in precipitates decreases quic kly when the volume fraction of ethanol in solvent (VOL) increases abo ve 0.2, and finally pure B is obtained at 0.4 VOL. The transformation rate from B to A decreases with ethanol concentration. The growth rate of B was larger than that of A at the same supersaturation degree in both aqueous and ethanol solutions. However, the growth rates of both polymorphs are almost the same at the same concentration in aqueous so lution, corresponding to the precipitation behaviors. In ethanol solut ions, the growth rate of B is slightly larger than that of A at the sa me concentration of 0.2 VOL. However, the growth rate of B is much lar ger than A at 0.4 VOL. The results indicate that the precipitation beh avior is controlled mainly by the relative growth rates of the polymor phs. The change of the growth mechanism with increasing VOL is suggest ed. The concentration decrease of the conformer which corresponds to A is suggested as responsible for the solvent effect.