Spectroscopic investigations and light scattering experiments with saturated, liquid hydrocarbons and their mixtures indicate a specific and distinct influence of the constitution, conformation, and flexibility of the molecule on the structure and macroscopic behavior of such liquids. Orientational order present in pure liquid n-alkanes, for example, characteristically affects the thermodynamic mixing properties, such as the enthalpy of mixing AHM and the entropy of mixing ASM, when these liquids are mixed with each other, or with other liquids. Nowadays it is possible to determine thermodynamic mixing properties experimentally with such precision that systematic investigations of these properties allow the behavior of liquids to be studied qualitatively and-with molecular theories of liquidsto some extent also quantitatively. The latest results in this respect, exemplified by mixtures of alkanes, are discussed. These results not only demonstrate the progress made in understanding the relations between molecular (microscopic) and macroscopic properties, but are also of importance for industrial applications (e. g. separation processes) in which mixtures of hydrocarbons are involved.[**I Present address: cal properties of the substances to be processed are known. It is, therefore, not surprising that the properties of hydrocarbons have been, and still are collected in comprehensive tabular compilations. Furthermore, many attempts have been made to correlate these data using semiempirical or theoretical approaches. In order to find out, for example, whether a liquid mixture with known composition can be separated into its components by distillation, it is necessary to know the composition of the gaseous phase with which it is in equilibrium. Of course an experiment will reveal the data, but it might be quite expensive and very time consuming. Thermodynamics does not give an answer to this question directly, but defines the conditions of equilibrium. To calculate the compositions of the different phases in equilibrium with one another, however, the chemical constitution of the molecules forming the mixture and the intermolecular interactions between them must be known. Only then does statistical thermodynamics, which connects molecular and macroscopic properties, in principle allow physical properties such as e.g. densities, vapor pressures, and phase compositions to be calculated.
