Andreas A. Zavitsas

The current belief that water solutions are not "ideal" is due to the assumption that the mass action law is not obeyed and activity coefficients are used to avoid violating an immutable law. The law is not violated when expressed by including waters of hydration as part of the reactants and as products in the formation of ion pairs by electrolytes. Mole fractions calculated by adhering to the law and recognizing effects of incomplete dissociation and amounts of water interacting strongly with solutes obey Raoult's law to high concentrations. Another belief is that, for a solution to be "ideal", there must be no volume or heat change on mixing the components. We demonstrate "ideality" despite volume changes and nonzero enthalpies of solution. Results with a variety of electrolytes correlate with residence times of waters near ions, viscosities, entropies of hydration, enthalpies of solution and dilution, ion ionization energies, Hofmeister series, and so forth. Older reports of "negative hydration", that had been considered unreliable, are now understood; the effect is due to formation of mobile, or "fast", water by strong hydrophobic associations. Our results are supported by findings from techniques perfected relatively recently: dielectric relaxation, mid-IR pump-probe spectroscopy, 2D-IR, atomic force microscopy, and so forth.

Despite their abundance in nature and their importance in biology, medicine, nutrition, and in industry, gas phase enthalpies of formation of many long chain saturated and unsaturated fatty acids and of dicarboxylic acids are either unavailable or have been estimated with large uncertainties. Available experimental values for stearic acid show a spread of 68 kJ mol^–1. This work fills the knowledge gap by obtaining reliable values by quantum theoretical calculations using G4 model chemistry. Compounds with up to 20 carbon atoms are treated. The theoretical results are in excellent agreement with well established experimental values when such values exist, and they provide a large number of previously unavailable values.

Common misconceptions about the properties of water solutions appear in the work of Yang (RSC Adv., 2015, 5, 1213). The issues addressed pertain to all aspects of the properties and behaviors of aqueous solutions of strong electrolytes: proper hydration numbers and their dependence on temperature, the extent of ion pair formation and its effect on hydration numbers, and the correct form of the equilibrium constant for ion pair formation. These aspects are fundamental for understanding the behavior of water solutions in chemistry and in biology. The present work points out the misconceptions, the faults resulting from them, and the ensuing confusion.