Excess heat capacities and excess volumes of n-alkane mixtures

Abstract

Using a Picker flow microcalorimeter, excess heat capacities (C_p^E) have been measured through the composition range at 25, 40, 55 and 70 °C for the following mixtures of normal alkanes: C₇+ C₉, C₇+ C₁₁ and C₇+ C₁₃. The mixture C₇+ C₁₅ was studied at 25 °C and the mixture C₇+ C₂₀ at 40, 55 and 70 °C. Using a Sodev vibrating-cell densimeter, the excess volume (V^E) was measured for the same mixtures and in the same temperature range as above. The C_p^E values are all negative, decreasing in magnitude with increasing T, although for some mixtures the magnitude of C_p^E increases at high values of T. This is consistent with the decrease of short-range orientational order as T increases and with an increase of free volume at high T. The V^E values also are all negative, increasing in magnitude with increasing T for a given mixture and with the difference in chain length for a given T. Both C_p^E and V^E obey the Brönsted principle of congruence. An extended corresponding-states principle for chain molecules gives excellent prediction of the temperature and composition dependences of C_p^E and V^E.