Dissolution enthalpies of anhydrous sodium sulfate in water were measured up to saturation atT = (287.8, 317.5, and 352.1) K with a SETARAM C80 mixing calorimeter. Experimental isotherms, and previous ones at T = 297.6 K and T = 317.5 K (Hubert, N. et al., Thermochim. Acta1995, 259, 41–48), all intersect around a molality of 2 mol · kg − 1. These five isotherms have been fitted with the electrolyte-NRTL model according to a methodology presented in this paper. This methodology uses enthalpy values along with values atT = 298.15 K from the literature and leads to the proposal of a unique set of parameters that allows a simultaneous representation of dissolution enthalpies, apparent relative molar enthalpies, and osmotic coefficients of sodium sulfate in water. Very good agreement was obtained between experimental and calculated values of dissolution enthalpies and osmotic coefficients in (water + sodium sulfate).

Two quaternary equimolar alloys (C 22/C26/C30/C34 and C24/C28/C32/C36) and three commercial products, which respectively consist of 23 and 33 consecutiven-alkanes with chain lengths between 20 and 52 carbon atoms, are studied by X-ray diffraction analyses. A single orthorhombic solid solution, identical to one of the two intermediate phases seen in binary n-alkane systems, is observed in two commercial products and in their 50–50 wt% mixture. The n-alkane molar concentrations and the long crystallographic c parameters have been determined and allow us to demonstrate that the molecule layer stacking periodicity is quasi-equal to the average length of n-alkane chains of these complicated mixtures: this periodicity corresponds to that of a hypothetical orthorhombic pure n-alkane whose equivalent carbon atom number is quasi-equal to the average carbon atom number of commercial multin-alkane mixtures. Discontinuous distributions of chain lengths lead to the observation of two or three orthorhombic phases in the quaternary alloys and the presence of an orthorhombic solid solution and of an amorphous solid in the third commercial multin-alkane product. q 1999 Elsevier Science Ltd. All rights reserved.

X-ray diffraction patterns of binary mixtures of normal tricosane and tetracosane at 20°C (powder method) show the presence of two new phases, here denoted β′ and β′1. The crystal structure of these phases is orthorhombic; the structure of the β′ phase is isomorphous with the tricosane β′ phase observed after the δ transition. With increasing temperature and concentration, the experimental observations, both by X-ray diffraction and differential thermal analysis, made it possible to modify the binary phase diagram of n-C23–n-C24.