Thermogravimetric determination of specific surface area for soil colloids

Abstract

A simple and easy method based on differential drying of samples at different temperatures has been proposed for quantitative determination of the specific surface areas of colloidal disperse systems. The following fundamental dependence of moisture thermodynamic potential [ψ] on absolute drying temperature T is used in the method: ψ = Q–aT, where Q is the specific heat of evaporation and a is a parameter related to the initial temperature and relative air humidity in an external thermodynamic reservoir (laboratory apartment). Gravimetric data on moisture mass fraction W and the value of ψ have been used to plot Polanyi potential curves W(ψ) for the studied samples. The curves have been employed to calculate the isotherms of moisture sorption. The capacity of a monolayer and the effective specific surface area have been determined from these isotherms in terms of the BET theory. The surface area estimated from the published results of classical experiments is about 1000 m²/g. The problem of the “absolute zero of humidity,” which is of great importance for determining the properties of colloidal disperse bodies normalized with respect to the solid phase mass, has been discussed.