Gas adsorption on commercial magnesium stearate: Effects of degassing conditions on nitrogen BET surface area and isotherm characteristics.

Commercial grades of magnesium stearate have been analysed by nitrogen adsorption having been pre-treated at temperatures between 30°C and 110°C and in the as-received state. Characteristics of nitrogen adsorption/desorption isotherms are assessed through the linearity of low relative pressure isotherm data and the BET transform plot together with the extent of isotherm hysteresis. Comparison is made between thermal gravimetric analysis and mass loss on drying. Features of gas adsorption isotherms considered atypical are identified and possible causes presented. It is shown that atypical isotherm features and issues of applying BET theory to the calculation of S are linked to the presence of hydrated water and that these depend on the hydration state: being more pronounced for the di-hydrate than the mono-hydrate. Dehydration reduces the extent of atypical features. S of a mono-hydrate sample is 5.6mg and 3.2mg at 40°C and 100°C degassing respectively but 23.9mg and 5.9mg for di-hydrate containing samples under comparable degassing. Di-hydrated samples also show S >15mg, BET C-values <7 and BET correlation coefficients <0.98 before dehydration. Possible mechanisms for atypical isotherms are critically discussed together with the suitability of applying BET theory to nitrogen adsorption data.