Growth and dissolution kinetics of α and γ polymorphs of DL-methionine

Growth kinetics of the two common polymorphs of DL-methionine (DL-met), alpha-DL-met and gamma-DL-met, and dissolution kinetics of gamma-DL-met, were studied in aqueous solution as part of an attempt to complete an a-priori model of the solution-mediated transformation (SMT) of polymorphs in this system, which will then be compared to measured rates of polymorph transformation. The growth rates of alpha-DL-met and gamma-DL-met were found to be linearly dependent on the relative supersaturation of DL-met in the system. The dissolution rate of gamma-DL-met was found to linearly depend on the relative undersaturation of DL-met in the system. Both the growth and dissolution rate constants are temperature dependent and follow an Arrhenius relationship. At all temperatures studied, both the growth rate of alpha-DL-met and the dissolution rate of gamma-DL-met are faster than the growth rate of gamma-DL-met, indicating that if the dissolution is a diffusion controlled process, then the SMT of the polymorphs of DL-met is likely to be controlled by the growth rate of gamma-DL-met. (C) 2011 Elsevier B.V. All rights reserved.