Excess heat capacities of n-alkane mixtures

Abstract

Using the Picker flow microcalorimeter, excess heat capacities have been measured through the composition range at 20, 40 and 55 °C for the following mixtures of normal alkanes: C₆+ C₁₆, C₈+ C₁₄, C₈+ C₁₆ and C₉+ C₁₆. The following mixtures were studied at two of the above temperatures: C₆+ C₈, + C₉, + C₁₂ and + C₁₄, C₈+ C₁₂, C₉+ C₁₂ and C₁₂+ C₁₆. The C^E_p values are negative, usually decreasing in magnitude with increasing T. Unlike regular solution theory, C^E_p per unit volume is not symmetrical in volume fraction, the curves being skewed toward the higher alkane side of the composition range. On raising T, C^E_p per unit volume tends to symmetry in volume fraction. The magnitudes and temperature and composition dependences of C^E_p are not consistent with current theory, but are predicted by a corresponding states approach which implies the existence of short-range orientational order in the higher alkane liquids. At all three temperatures, C^E_p values for the mixtures are mutually consistent within the framework of the Brönsted Principle of Congruence. This implies that orientational order between n-alkane chains does not depend directly on chain length, but on the density of packing of segments. However, C^E_p values at 20 °C do not coincide with values calculated from data of the pure n-alkanes using the Principle of Congruence. This failure is related to an anomalous value for the internal C_p of pure n-C₁₆ at 20 °C close to the freezing point.