Thermodynamics of liquid binary (alkanenitrile–alkane) mixtures. Part 2.—Experimental excess molar heat capacity at 298.15 K and structure in solution

Abstract

Excess molar heat capacities at constant pressure (C^E_p) have been determined at 298.15 K, as a function of mole fraction, for 17 binary liquid mixtures of propanenitrile, butanenitrile, pentanenitrile and hexanenitrile mixed individually with some of the following alkanes: hexane, heptane, octane, decane, dodecane and tetradecane. C^E_p is positive and extremely large for propanenitrile with hexane and heptane, increasing in magnitude with the alkane chain length, 6 and 8 J mol^–1 K^–1 equimolar composition, respectively, whereas for the systems with butanenitrile, pentanenitrile and hexanenitrile as the common component, C^E_p, varies from small positive values for the systems with lower alkanes to negative values for the systems with the larger alkanes. For some of these systems the curve C^E_p(x) presents a ‘W shape’, i.e. it exhibits two minima separated by a maximum, that is, two changes of sign of the curvature. The observed behaviour is discussed in terms of structure in the solutions due to non-randomness or local concentration fluctuations and randomness balance. In this way, the concentration–concentration correlation function, S_cc, is calculated using a molecular groups contribution model for the excess Gibbs energy of the systems studied in order to correlate these effects.