Structural change and moisture sorption behavior of DL-valine induced by grinding

Abstract

Structural change of DL-valine crystals induced by grinding and the moisture sorption behavior were investigated. DL-valine crystals were ground using a vibrational mill under liquid nitrogen flow. Decrease in crystallinity of DL-valine, concurrently with shift of the X-ray powder diffraction peaks of lattice planes (020) and (040) to the lower diffraction angles were observed with increasing grinding time. The intact DL-valine crystals took up less than O.1% of moisture content, while ground samples showed characteristic water vapor sorption isotherms. After humidifying the 30-min ground sample with 82% RH at 40°C, the crystallinity of the sample was increased and the diffraction peaks due to planes (020) and (040) were shifted back to the angles observed in unmilled crystals. It was concluded that DL-valine crystal lattice was extended along the b-axis by grinding, via disordering of the hydrogen bond layers. Moisture-induced crystallization should occur by water absorption inside the ground sample.