Excess enthalpy, excess heat capacity and excess volume of 1,2,4-trimethylbenzene +, and 1-methylnaphthalene + an n-alkane

Abstract

A flow microcalorimeter of the Picker design was used to measure excess molar enthalpies H E at 298.15 K as a function of mole fraction χ 1 for several mixtures belonging to series I: {χ 11,2,4-C 6H 3(CH 3) 3 + χ 2n-C ℓH 2ℓ+2}, and series II: {χ 11-C 10H 7CH 3 + χ 2n-C ℓH 2ℓ+2}. The chain length ℓ of the n-alkanes ranged between 7 and 16. 1,2,4-trimethylbenzene and 1-methylnaphthalene have about the same size and shape as the previously investigated chloro derivatives 1,2,4-C 6H 3Cl 3 and 1-C 10H 7Cl but a much smaller reduced dipole moment. The calorimeter was used in the discontinuous mode. A plot of H E max (i.e., the maximum value of H E with respect to composition) against ℓ for series I shows a shallow minimum around ℓ = 11 with H E max (ℓ = 11) ≈ 250 J mol −1, whereas H E max for series II decreases over the whole range 7 ⩽ ℓ ⩽ 16: H E max (ℓ = 7) ≈ 760 J mol −1, and H E max (ℓ = 16) ≈ 595 J mol −1. The corresponding enthalpic interaction parameters h 12, calculated from zeroth-order KGB (Kehiaian-Guggenheim-Barker) theory, decrease with increasing ℓ, and the rate of decrease, d h 12/dℓ, diminishes for larger chain lengths. For three mixtures belonging to series I (ℓ = 7, 10, 14), excess molar volumes V E and excess molar heat capacities C E P at constant pressure were mesured at the same temperature. V E was determined with a vibrating-tube densimeter (flow conditions), and C E P was obtained with another type of flow calorimeter (stepwise procedure). V E(χ 1 = 0.5)/(cm 3 mol −1) = −0.207 for ℓ = 7, 0.060 for ℓ = 10, and 0.145 for ℓ = 14. The corresponding values for C E P x 1 = 0.5)/(J K −1 mol −2) are 0.32, 0.66 and −0.09. Thus the chain length dependence of the excess molar heat capacity is qualitatively similar to that observed for the series with the homomorphic chloro derivative, (1,2,4-C 6H 3Cl 3 + n-C ℓH 2ℓ+2), and to that of (1-C 10H 7Cl+n-C ℓH 2ℓ+2).