An innovative approach is presented to interpret the refractive index of binary liquid mixtures. The concept of refractive index "before mixing" is introduced and shown to be given by the volume-fraction mixing rule of the pure-component refractive indices (Arago-Biot formula). The refractive index of thermodynamically ideal liquid mixtures is demonstrated to be given by the volume-fraction mixing rule of the pure-component squared refractive indices (Newton formula). This theoretical formulation entails a positive change of refractive index upon ideal mixing, which is interpreted in terms of dissimilar London dispersion forces centred in the dissimilar molecules making up the mixture. For real liquid mixtures, the refractive index of mixing and the excess refractive index are introduced in a thermodynamic manner. Examples of mixtures are cited for which excess refractive indices and excess molar volumes show all of the four possible sign combinations, a fact that jeopardises the finding of a general equation linking these two excess properties. Refractive indices of 69 mixtures of water with the amphiphile (R,S)-1-propoxypropan-2-ol are reported at five temperatures in the range 283-303 K. The ideal and real refractive properties of this binary system are discussed. Pear-shaped plots of excess refractive indices against excess molar volumes show that extreme positive values of excess refractive index occur at a substantially lower mole fraction of the amphiphile than extreme negative values of excess molar volume. Analysis of these plots provides insights into the mixing schemes that occur in different composition segments. A nearly linear variation is found when Balankina's ratios between excess and ideal values of refractive indices are plotted against ratios between excess and ideal values of molar volumes. It is concluded that, when coupled with volumetric properties, the new thermodynamic functions defined for the analysis of refractive indices of liquid mixtures give important complementary information on the mixing process over the whole composition range.

Apparent molal volumes, [Formula: see text], of secondary and tertiary amines and linear hydrocarbons were determined in benzene at 25 °C, using a vibrating tube densimeter. These quantities have been extrapolated to infinite dilution to obtain partial molal volumes. The contribution to partial molal volume of the amine groups, calculated using a simple additive scheme, [Formula: see text], were interpreted in terms of conformational effects present in these molecules. A first attempt to find a measure of the contribution to the partial molal volume of the specific interaction amine–benzene in tertiary and secondary amines was made. The results agree well in the two different approaches used.

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Experimental densities were obtained in the binary mixtures water + 3-ethoxypropan-1-amine, for the whole composition range and at temperatures between (283.15 and 303.15) K. Derived thermodynamic properties such as apparent molar, excess molar, and excess partial molar volumes have been calculated. Limiting partial molar volumes and isobaric expansions were obtained for the two components. Different patterns of molecular aggregation and hydration schemes were detected.