A method has been purposed to calculate some of the thermodynamic quantities for the thermal deformation of a smectite without
using any basic thermodynamic data. The Hançılı (Keskin, Ankara, Turkey) bentonite containing a smectite of 88% by volume
was taken as material. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of the sample were obtained.
Bentonite samples were heated at various temperatures between 25–900°C for the sufficient time (2 h) until to establish the
thermal deformation equilibrium.
Cation-exchange capacity (CEC) of heated samples was determined by using the methylene blue standard method. The CEC was used
as a variable of the equilibrium. An arbitrary equilibrium constant (Ka) was defined similar to chemical equilibrium constant and calculated for each temperature by using the corresponding CEC-value.
The arbitrary changes in Gibbs energy (ΔGa0) were calculated from Ka-values. The real change in enthalpy (ΔH0) and entropy (ΔS0) was calculated from the slopes of the lnK vs. 1/T and ΔG vs. T plots, respectively. The real changes in Gibbs energy (ΔG0) and real equilibrium constant (K) were calculated by using the ΔH0 and ΔS0 values. The results at the two different temperature intervals are summarized as below: ΔG10=ΔH10−ΔS10T=−RTlnK1=47000−53t, (200–450°C), and ΔG20=ΔH20-ΔS20T=−RTlnK2=132000−164T, (500–800°C).