Experimental Determination of Third Derivative of the Gibbs Free Energy, G II: Differential Pressure Perturbation Calorimetry.

We have been evaluating third deriv. quantities of the Gibbs free energy, G, by graphically differentiating the second derivs. that are accessible exptl., and demonstrated their power in elucidating the mixing schemes in aq. solns. Here we det. directly one of the third derivs. of G, the partial molar entropy-vol. cross fluctuation d. of 2-butoxyethanol (BE) in the BE-H2O system, SV δ BE . The difference of the heats of compression were directly detd. using two identical cells and applying the same pressure change to both cells concurrently. Both cells are filled with sample solns. having a small appropriate difference in mole fraction. The results indicated that this method is feasible with the prior knowledge of the thermal expansivity of the soln. to within a few per cent accuracy. If the vols. of the two cells are identical within the order of 0.01%, the method provides the required results to within 0.1% without the thermal expansivity data. This success opens a possibility of evaluating the fourth deriv. graphically, which is expected to provide much more detailed information about the mol. processes in aq. solns. [on SciFinder(R)]