Uncovering the Role of Surfactants in Controlling the Crystal Growth of Pyridoxine Hydrochloride

Pyridoxine hydrochloride, commonly known as vitamin B-6 (VB6) is considered to be one of the most essential vitamin enhancers. Understanding its crystal morphology is of paramount interest in the pharmaceutical industry. Herein, we systematically investigate the crystallization of VB6 in the absence and presence of 0.5-2 mM nonionic and ionic surfactants by single-crystal growth experiments and theoretical calculations. VB6 crystals exhibit the most common blocklike morphology in aqueous and nonionic surfactant solution, while needlelike crystals are observed, for the first time, in ionic surfactant solutions. Two surfaces, (100) and (010), play a decisive role in modulating the crystal morphology. The preferential adsorption of anionic sodium dodecyl sulfate surfactant on the radial (010) surface inhibiting its growth over the axial (100) surface results in a high aspect ratio of the crystals. On the other hand, the cationic hexadecyl trimethylammonium bromide surfactant enhances the growth of both surfaces by promoting Cl- ion integration to the surfaces. The zwitterionic dodecyl dimethyl betaine surfactant hinders (010) surface growth by surface adsorption while it promotes (100) surface growth by accelerating Cl- integration. In addition, a slight growth inhibition of both surfaces by the nonionic surfactant Tween 80 is due to surfactant crowding on the growing surfaces.