Structural properties of magnesium stearate pseudopolymorphs: effect of temperature

A thorough review of the relevant literature reveals that the interaction between water vapour and magnesium stearate, in contrast to many other metal soaps, is not properly understood. The structural modifications associated with the up-take or loss of water of vegetable-derived commercial magnesium stearate powders exposed to humid air or vacuum at room temperature are investigated using standard powder X-ray diffractometry. It is found that in such conditions magnesium stearate reacts reversibly with the vapour phase with structural consequences very similar to the high temperature transition between the crystalline and rotator phases of other anhydrous metal soaps. When temperature is increased under dry nitrogen the diffraction band characteristic of the rotator phase shifts towards higher angle values and the corresponding lattice spacing increases at the rate of 6.9x10(-4)C(-1). Melting takes place gradually above 100 degrees C as revealed by the collapse of the diffraction band and the growth of the broader diffusion band characteristic of the liquid state. Full clarification of the structure of the hydrated and dried phases proves impossible based on powder diffraction spectra obtained with conventional high resolution X-ray diffraction equipment.