Temperature-dependence of the Heat of Mixing of Two n-Alkanes

Abstract

FOR any of a wide variety of pairs of n-alkanes it is known^1,2 that at 20° C the heat of mixing is positive. Until about two years ago it was assumed on the basis of the measurements by van der Waals and Hermans¹ that as the temperature was increased the heat of mixing of any pair decreased, markedly at first, but remained positive. Then we published² some measurements of the heat of mixing of n-hexane + n-hexadecane at 20°, 30°, 40°, and 50° C. At 20° C our measurements were in fair agreement with those of van der Waals and Hermans. The temperature-dependence of our measurements was, however, strikingly different from that found by them for other pairs. By extrapolation of our results³ and on theoretical grounds⁴ we predicted that for n-hexane + n-hexadecane the heat of mixing should change sign at about 64° C and become negative at higher temperatures. That prediction became the subject of a wager (of a good dinner) between members of the ‘Liquids Club’ at the Shell Laboratory in Amsterdam and one of us (M.L.M.) in Reading, and has recently been publicly challenged on theoretical grounds by Holleman and Hijmans⁵. We have now used a modified version of our calorimeter⁶ to make a few measurements on equimolar mixtures of n-hexane + n-hexadecane at 40°, 75°, and 100° C. The results of these new measurements are shown plotted against temperature in Fig. 1 together with the results of all the previous measurements made at more than one temperatur.