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

Growth kinetics of the two common polymorphs of dl-methionine (dl-met), α-dl-met and γ-dl-met, and dissolution kinetics of γ-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 α-dl-met and γ-dl-met were found to be linearly dependent on the relative supersaturation of dl-met in the system. The dissolution rate of γ-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 α-dl-met and the dissolution rate of γ-dl-met are faster than the growth rate of γ-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 γ-dl-met.

► Growth and dissolution kinetics measured for dl-methionine polymorphs.
► Kinetic dependence on super/undersaturation and temperature were determined.
► Dissolution has larger rate constant than growth for the γ-form.
► Polymorph transformation kinetics are likely controlled by growth of the γ-form.