Partial Molar Volumes of Ionic and Nonionic Organic Solutes in Water: A Simple Additivity Scheme Based on the Intrinsic Volume Approach

AbstractPartial molar volumes at infinite dilution, $$\overline {V^0 } $$ , in water at 25°C for organicelectrolytes and nonelectrolytes are described through a simple additivity schemebased on the intrinsic volume approach. The contributions to $$\overline {V^0 } $$ of hydrocarbongroups were determined from the “experimental” CH2 increment assuming thatall hydrocarbon portions have the same packing efficiency in aqueous solution.Twenty-two charged and 51 uncharged groups, are assigned a $$\overline {V^0 } $$ contribution and the partial molar volumes of nearly 400 monofunctional organic solutes arereproduced with a root mean square of deviation of 0.9 cm3-mol−1. The differentvolumetric behavior of linear, branched, and cyclic molecules is discussed. Acomparison is made of uncharged (hydrocarbon and polar) with charged groups,as well as of groups with small molecules and ions. A relative estimate is givenof the volume effects (cavity formation, hydrophobic hydration, solvent shrinkage,H bonding, and electrostriction) involved in the interaction of hydrophobic andhydrophilic groups with water. Deviations from additivity for mono- andpolyfunctional solutes, including polyelectrolytes, are analyzed in terms of (1) extensionof the hydration cosphere of different polar centers: (2) intramolecular interactionsand their dependence on nature, number, and mutual distance of interactinggroups. The van der Waals volume Vw is extensively used as molecular descriptor.Estimates of Vw for both charged and uncharged groups are presented.