On the difficulty of assessing the specific surface area of magnesium stearate

The water content of as-received commercial magnesium stearate batches from animal and vegetable sources have been modified by ageing in humid air at room temperature or by vacuum treatment. The complete adsorption-desorption isotherms of nitrogen and krypton vapours by samples of these as received and modified materials have been measured at liquid nitrogen temperature after standardised vacuum degassing. They are greatly affected by the initial water content of the material. In particular: (a) the BET surface area values computed from the adsorption branch vary widely and is increasing with increasing water content; (b) anomalous hysteresis of varying amplitude is observed in all cases except adsorption of krypton on the material with the lowest water content; (c) the hysteresis loops extend down to very low desorption pressure values and cannot be accounted for by capillary condensation. Lastly, the surface area value of a given material computed from nitrogen and krypton adsorption may differ by a factor as high as six. Accordingly, the very significance of BET surface area values obtained from routine adsorption experiments should be regarded as questionable, at least until the mechanisms of adsorption are fully clarified.