The relative partial specific enthalpy of water in montmorillonite-water systems and its relation to the swelling of these systems

Abstract

A Calvet calorimeter was used to determine the heats of immersion of montmorillonite-water mixtures at different initial values of m w m m , the mass ratio of water to montmorillonite. The heats of immersion were plotted against the respective values of m w m m and values of h ̄ w − h w 0 , the relative partial specific enthalpy of the water, were determined from the slopes of the resulting curve at selected values of m w m m . These values of h ̄ w − h w 0 were combined with values of the relative partial specific free energy of the water, determined from previously measured values of Π, the swelling pressure, to calculate the corresponding values of s ̄ w − s w 0 , the relative partial specific entropy of the water. Then the values of h ̄ w − h w 0 and s ̄ w − s w 0 were used with other thermodynamic data for the same systems to calculate the enthalpic and entropic contributions to Π and values of ( ∂Π ∂T ) m , the partial derivative of Π with respect to temperature at constant composition, for different values of m w m m . The results showed that: (1) h ̄ w − h w 0 becomes increasingly negative with decreasing m w m m , (2) s ̄ w − s w 0 becomes increasingly positive with decreasing m w m m until a maximum is reached at m w m m ⋍ 1.0 g/g and then it becomes increasingly negative thereafter, (3) except at very small values of m w m m , the entropic contribution to Π predominates, and (4) the value of ( ∂Π ∂T ) m is very small and its change with m w m m is similar to that of s ̄ w − s w 0 .