The majority of chemical processes takes place in fluid systems, hence properties of gases, vapours and liquids, pure and mixed, are of prime scientific as well as engineering importance. In fact, many of the significant developments in physical chemistry, biophysical chemistry, geochemistry and chemical engineering have originating from chemical thermodynamics as applied to fluid systems.^1–3  However, the most profitable approach for both applications and theoretical advances is based on the combination of chemical thermodynamics with molecular theory and statistical mechanics^4,5  as a means for or as an aid to the calculation of thermodynamic properties⁶  (the term molecular thermodynamics was coined by Prausnitz more than forty years ago). During the last decades, the impressive growth of this field has been stimulated by the steadily increasing need for thermodynamic property data and phase equilibrium data in the applied sciences, and has profited greatly by advances in experimental techniques, by advances in the theory of fluids in general, and by advances in computer simulations of reasonably realistic model systems.

Internal energy and enthalpy belong to the most important thermodynamic/thermophysical properties, and have played a central role in the development of formal thermodynamics, i.e. in the formulation of the first law of thermodynamics,⁷  introduced as a generalisation and abstraction of experimental results concerning energy conservation. Measurements of changes of these properties using calorimeters established one of the oldest fields of physical chemistry – calorimetry.^8,9  In fact, calorimetric determinations of molar enthalpies of mixing/molar excess enthalpies H^E belong to the earliest methods yielding quantitative measures of deviations from ideal-solution behaviour, and H^E data play a pivotal role in equation-of-state research, which is intimately connected with applied statistical mechanics. Excess enthalpies provide information complementary to that associated with the excess molar Gibbs energy G^E which is obtained from measurements of vapour–liquid equilibria,¹⁰  that is to say,

Here, the temperature is denoted by T, P is the pressure, and {x_i} is the set of compositional variables (mole fractions) characterising the (multicomponent) mixture. Modern flow calorimeters allow reliable measurements of excess enthalpies over extended temperature ranges and at elevated pressures,¹¹  and the results have to be consistent with heat capacity measurements,¹²  i.e.,and with volumetric data,¹³  i.e.,Here, C$PE$ denotes the excess molar heat capacity at constant pressure, and V^E is the excess molar volume. Vibrating-tube densimetry,¹⁴  in particular, is well suited for this purpose.

This book, entitled Enthalpies and Internal Energies: Liquids, Solutions and Vapours, is published under the auspices of the International Association of Chemical Thermodynamics (IACT) and is the third volume in our series published by the Royal Society of Chemistry, Cambridge, UK; the previous two, published, respectively, in 2010 and 2015 are:

and

Our three books follow a long tradition of publishing reviews of important areas/topics of current interest in chemical thermodynamics, that started in 1956 with Experimental Thermochemistry, Volume 1 (Interscience Publishers, Inc., New York, USA), edited by F. D. Rossini (see details in the preface to Volume Properties¹³ ). As done previously, we have looked to the world-wide international field of thermodynamics for our authors, and in true IACT spirit our new book has contributing authors from 14 countries: Austria, Belgium, China, France, Germany, Greece, Israel, Japan, Mexico, South Africa, Spain, Switzerland, United Kingdom and the United States of America.

The main objectives of our book are as follows. First and foremost, the book presents reviews and surveys which are of particular value for researchers wanting to stay informed on recent developments, for researchers expanding their area of interest and/or are entering a new field, and for those who desire to understand the origins and the background of a scientific area and endeavour to see where it is leading to. Many of the topics discussed in this new volume have not been reviewed before as they have only recently been researched, or have only recently attracted renewed interest. The reviews published here include copious references to the literature including the latest relevant publications (up to mid 2016). Another objective is to bring together research from disparate disciplines, either from academia or from the applied sciences, which has a bearing on enthalpy, internal energy and related properties. Of particular note are the expanding activities in biophysical chemistry aimed at broadening our understanding of the thermodynamic basis of physicochemical phenomena associated with biological processes in living organisms. This book thus provides an overview and various highlights of a huge field, and we strongly believe that cross-linking chapters will yield synergistic effects, leading to new ways of looking at problems in physics, chemistry and chemical engineering, and expanding the horizons to which solutions can be applied. The book's success ultimately rests with the authors and we, the editors, would like to thank all of them for their cooperation and enthusiastic contributions which are highly valued. Finally, we reiterate (with passion) our philosophy that a book, be it in electronic or printed form, is the most important vehicle for disseminating knowledge.

Where possible, the units, symbols and names of thermodynamic quantities have been checked against the rules laid down in the Green Book of the International Union of Pure and Applied Chemistry.¹⁵  In almost all cases the authors have adhered to the suggestions of IUPAC, and deviations are solely due to the desire to present a concise, unequivocal and logically consistent notation in compliance with usage preferred by the scientific community interested in this book's topics, i.e. by physical chemists, physicists, geophysicists, biophysicists and chemical engineers. Such an approach is in accord with the spirit of the Green Book expressed in the Introduction on p. XII, i.e., with the principle of “good practice of scientific language”. Finally, we wish to thank the Royal Society of Chemistry, whose representatives were most helpful and patient in producing Enthalpy and Internal Energy: Liquids, Solutions and Vapours.

Emmerich Wilhelm

University of Wien, Austria

Trevor M. Letcher

University of KwaZulu-Natal, South Africa