CHAPTER 7. Excess Volumes of Liquid Nonelectrolyte Mixtures

Abstract

Chapter 7 of this monograph is devoted to excess volumes and related properties of liquid mixtures. The importance of this topic is firmly grounded in the use of excess volumes for improving liquid-state theory in general, as well as in the role of excess volumes in chemical engineering and biophysical chemistry (for instance, in discussions of hydrophobic effects). Real mixtures are non-ideal, and the extent to which they deviate from ideal behaviour is most conveniently expressed through use of thermodynamic excess quantities. Note that ∂lnγi/∂P=ViE/RT, where γi is the activity coefficient of mixture component i, and ViE is its excess partial molar volume which is thus seen to play a central role in describing the pressure dependence of the excess molar Gibbs energy GE/RT=∑xilnγi. The excess molar volumes VE=∑xiViE are obtained either indirectly via density measurements or directly via dilatometry. A few important types of instruments belonging to each group, such as the vibrating-tube densimeter (originally designed by Stabinger, Kratky and Leopold in 1967) and the tilting dilution dilatometer of Bottomley and Scott (1974), are presented. Density determinations at elevated pressures, which are based on measuring the speed of ultrasound as a function of pressure, are also discussed. In the section on experimental results, the discussion of excess molar volumes focuses on binary systems belonging to one of the following three groups: (I) mixtures of organic, aprotic liquids, (II) mixtures containing alcohols or alkanoic acids, and (III) aqueous solutions of nonelectrolytes. In general, emphasis is placed on mixtures which, we believe, will stimulate theoretical advances and on mixtures of importance for practical, chemical engineering applications. Various aspects of empirically describing the composition dependence of experimentally obtained excess molar volumes of binary as well as ternary systems are discussed in some detail.