Knudsen thermogravimetry approach to the thermodynamics of aqueous solutions

The use of isothermal TGA with Knudsen-like cells allows determination ofthe thermodynamic activity of water, aW. The typical experiment implies aslow dehydration ofthe aqueous solution atconstant temperature in dynamic vacuum conditions.The method isalternative to the classical isopiestic approach and offers the advantage ofacontinuous record onincreasing the solute concentration.These data can be directly treated according tothe classical thermodynamic relationships drawn from the Gibbs–Duhem expression to evaluate the activity and osmotic coefficientofthe aqueous solutions ofelectrolytes and non-electrolytes, and,inthe case ofelectrolytes,allow determination ofsolubility ofthe solute.Discrepancies with respect to the literature data are observed when the viscosity ofthe systems becomes too high, asinthe case ofsugars with avery large solubility.Such amismatch may however beaccounted for either slowing the dehydration rate with use ofanarrower Knudsen orifice,orcorrecting the experimental aW with a calibration curve. The same approach can beapp lied tonon-aqueous solutions.