Gas adsorption on commercial magnesium stearate: The origin of atypical isotherms and BET transform data
The causes of atypical nitrogen adsorption isotherm features of commercial magnesium stearate grades have been investigated through a combination of alternative adsorbates and variation of analytical conditions. Atypical features have previously been identified as having poor linearity of adsorption isotherms at relative pressures applied to the calculation of BET surface area and low pressure hysteresis between adsorption and desorption branches. These features are most pronounced for di-hydrated MgSt and are absent for the anhydrous form: the presence of hydrated water causing atypical features. The possible interaction between hydrated water molecules and nitrogen via the nitrogen quadrupole is investigated through application of non-polar adsorbates; argon and krypton. Low pressure hysteresis is investigated through alteration of equilibration conditions and through truncating adsorption isotherms to successively lower maximum relative pressures before commencing desorption. It is found that atypical gas sorption isotherms are not limited to nitrogen. The effects of these conditions on adsorption and desorption volumes are discussed and a model for the origin of atypical isotherm features is presented. The difference in gas sorption behaviour of anhydrous, mono-hydrated and di-hydrated magnesium stearate is discussed and ultimately the consequences on the calculation of BET surface area is considered.