The effect of surfactants on the crystallization and polymorphic transformation of glutamic acid

Glutamic acid can crystallize in two polymorphic structures depending on the crystallization regime. The study demonstrates an efficient method to preferentially crystallize the non-stable polymorphic structure (the α-form) in the presence of surface active agents. The rate of transformation was found to depend on the rate of growth of β and not on the rate of dissolution of α. The growth rate of β was a function of the supersaturation of the solute in solution. It was shown that the transformation could be inhibited by the addition of surfactants. The surfactants are capable of adsorbing preferentially to the α-growing crystals and solution mediating (retarding) the transformation of the α-to the β-form. It was suggested that the surfactant nature and steric considerations were important for the inhibition of both nucleation and growth of the β-polymorph. A Langmuir approach indicated that the kinetic parameter was related to the volume of surfactant adsorbed at the crystal surface. No changes in crystal morphology were observed, indicating that adsorption was not specific to any crystal face. Different mechanisms of surfactant adsorption were suggested: adsorption of single molecules at low concentrations of surfactant and formation of hemimicelles at higher concentrations.