THERMODYNAMICS OF LIQUID BINARY (ALKANENITRILE-ALKANE) MIXTURES .2. EXPERIMENTAL EXCESS MOLAR HEAT-CAPACITY AT 298.15 K AND STRUCTURE IN SOLUTION

Excess molar heat capacities at constant pressure (C-p(E)) have been d etermined at 298.15 K, as a function of mole fraction, for 17 binary l iquid mixtures of propanenitrile, butanenitrile, pentanenitrile and he xanenitrile mixed individually with some of the following alkanes: hex ane, heptane, octane, decane, dodecane and tetradecane. C-p(E) is posi tive and extremely large for propanenitrile with hexane and heptane, i ncreasing in magnitude with the alkane chain length, 6 and 8 J mol(-1) K-1 at equimolar composition, respectively, whereas for the systems w ith butanenitrile, pentanenitrile and hexanenitrile as the common comp onent, C-p(E) varies from small positive values for the systems with l ower alkanes to negative values for the systems with the larger alkane s. For some of these systems the curve C-p(E)(X) presents a 'W shape', i.e. it exhibits two minima separated by a maximum, that is, two chan ges of sign of the curvature. The observed behaviour is discussed in t erms of structure in the solutions due to non-randomness or local conc entration fluctuations and randomness balance. In this way, the concen tration-concentration correlation function, S-cc, is calculated using a molecular groups contribution model for the excess Gibbs energy of t he systems studied in order to correlate these effects.