THERMODYNAMICS OF (1,4-DIFLUOROBENZENE PLUS AN N-ALKANE) AND OF (HEXAFLUOROBENZENE PLUS AN N-ALKANE)

Excess molar enthalpies H-E and excess molar volumes V-E have been mea sured, as a function of mole fraction x(1), at 298.15 K and atmospheri c pressure for the five liquid mixtures (x(1)1,4-C6H4F2 + x(2)n-ClH2l2), l = 7, 8, 10, 12 and 16. In addition, H-E and excess molar heat ca pacities C-P(E) at constant pressure have been determined for the two liquid mixtures (x(1)C(6)F(6) + x(2)n-ClH2l+2), l = 7 and 14, at the s ame temperature and pressure. The instruments used were flow microcalo rimeters of the Picker design (the H-E version was equipped with separ ators) and a vibrating-tube densimeter, respectively. The excess entha lpies of the five difluorobenzene mixtures are all positive and quite large; they increase with increasing chain length l of the n-alkane fr om H-E(X(1) = 0.5)/(J mol(-1))= 1050 for l = 7 to 1359 for l = 16. The corresponding excess volumes V-E are all positive and also increase w ith increasing l: V-E(X(1) = 0.5)/(cm(3) mol(-1)) = 0.650 for l = 7 an d 1.080 for l = 16. Interestingly, the excess enthalpies of the corres ponding mixtures with hexafluorobenzene are only about 5% larger, wher eas the excess volumes of (x(1)C(6)F(6) + x(2)n-ClH2l+2) are roughly t wice as large as those of their counterparts in the series containing 1,4-C6H4F2. Specifically, at 298.15 K H-E(X(1) = 0.5)/(J mol(-1)) = 11 19 for (x(1)C(6)F(6) + x(2) n-C7H16) and 1324 for (x(1)C(6)F(6) + x(2) n-C14H30), and for the same mixtures V-E(x(1) = 0.5)/(cm(3) mol(-1)) = 1.882 and 2.093, respectively. The excess heat capacities for both sy stems are negative and of about the same magnitude as the excess heat capacities of mixtures of fluorobenzene with the same n-alkanes (Roux et al., 1984): C-P(E)(x(1) = 0.5)/(J K-1 mol(-1)) = -1.18 for (x(1)C(6 )F(6) + x(2)n-C7H16), and -2.25 for (x(1)C(6)F(6) + x(2)n-C14H30). The curve C-P(E) vs. (x(1) for x(1)C(6)F(6) + x(2)n-C14H30) shows a sort of ''hump'' for x(1) 0.5, which is presumed to indicate emerging W-sha pe composition dependence at lower temperatures.