Kinetic effect of L-phenylalanine on growth process of L-glutamic acid polymorph

The mechanism of additive effect of L-phenylalanine (L-Phe) on the growth kinetics of L-glutamic acid (L-Glu) polymorphs was investigated. With L-Phe concentration the growth rate of the (1 0 1) face of the beta crystal decreased and finally stopped at a critical concentration C-p*. As for alpha crystal the growth rate in the [1 1 1] direction was suppressed and the new (1 1 0) face appeared, while the growth rate of the (0 0 1) face was not influenced by L-Phe. The fluctuation of the growth rates was observed for both polymorphs at the higher concentration of L-Phe. The critical concentration of L-Phe for the (1 1 0) face of alpha is about twice of that for the (1 0 1) face of beta. At the constant L-Phe concentration each polymorph grew at higher supersaturations than the critical relative supersaturation sigma*. The value of sigma* for the beta crystal was a few times larger than that for the alpha crystal. The relationship between the growth rates and the concentration of the additive was expressed using the growth model of a curved step in connection with the adsorption density of the additive. The dependence of the growth rate of L-Glu on the L-Phe concentration and the relative supersaturation could be represented by Freundlich-type equations for both polymorphs. The calculated values of the adsorption density and the distance between the adsorbed neighboring L-Phe molecules also showed the preferential adsorption of L-Phe on beta crystals. The method to determine the optional supersaturation at the certain concentration of additives was proposed. (C) 1998 Elsevier Science B.V. All rights reserved.