Starting from known properties of non-specific salt effects on the surface tension at an air–water interface, we propose the first general, detailed qualitative molecular mechanism for the origins of ion-specific (Hofmeister) effects on the surface potential difference at an air–water interface; this mechanism suggests a simple model for the behaviour of water at all interfaces (including water–solute interfaces), regardless of whether the non-aqueous component is neutral or charged, polar or non-polar Specifically, water near an isolated interface is conceptually divided into three layers, each layer being 1 water-molecule thick We propose that the solute determines the behaviour of the adjacent first interfacial water layer ( I 1 ); that the bulk solution determines the behaviour of the third interfacial water layer ( I 3 ), and that both I 1 and I 3 compete for hydrogen-bonding interactions with the intervening water layer ( I 2 ), which can be thought of as a transition layer The model requires that a polar kosmotrope (polar water-structure maker) interact with I 1 more strongly than would bulk water in its place; that a chaotrope (water-structure breaker) interact with I 1 somewhat less strongly than would bulk water in its place; and that a non-polar kosmotrope (non-polar water-structure maker) interact with I 1 much less strongly than would bulk water in its place We introduce two simple new postulates to describe the behaviour of I 1 water molecules in aqueous solution The first, the ‘relative competition’ postulate, states that an I 1 water molecule, in maximizing its free energy (—δG), will favour those of its highly directional polar (hydrogen-bonding) interactions with its immediate neighbours for which the maximum pairwise enthalpy of interaction (—δ H ) is greatest; that is, it will favour the strongest interactions We describe such behaviour as ‘compliant’, since an I 1 water molecule will continually adjust its position to maximize these strong interactions Its behaviour towards its remaining immediate neighbours, with whom it interacts relatively weakly (but still favourably), we describe as ‘recalcitrant’, since it will be unable to adjust its position to maximize simultaneously these interactions The second, the ‘charge transfer’ postulate, states that the strong polar kosmotrope–water interaction has at least a small amount of covalent character, resulting in significant transfer of charge from polar kosmotropes to water–especially of negative charge from Lewis bases (both neutral and anionic); and that the water-structuring effect of polar kosmotropes is caused not only by the tight binding (partial immobilization) of the immediately adjacent ( I 1 ) water molecules, but also by an attempt to distribute among several water molecules the charge transferred from the solute When extensive, cumulative charge transfer to solvent occurs, as with macromolecular polyphosphates, the solvation layer (the layer of solvent whose behaviour is determined by the solute) can become up to 5- or 6-water-molecules thick We then use the ‘relative competition’ postulate, which lends itself to simple diagramming, in conjunction with the ‘charge transfer’ postulate to provide a new, startlingly simple and direct qualitative explanation for the heat of dilution of neutral polar solutes and the temperature dependence of relative viscosity of neutral polar solutes in aqueous solution This explanation also requires the new and intriguing general conclusion that as the temperature of aqueous solutions is lowered towards o °C, solutes tend to acquire a non-uniform distribution in the solution, becoming increasingly likely to cluster 2 water molecules away from other solutes and surfaces (the driving force for this process being the conversion of transition layer water to bulk water) The implications of these conclusions for understanding the mechanism of action of general (gaseous) anaesthetics and other important interfacial phenomena are then addressed

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0033583500005369

The Hofmeister effect and the behaviour of water at interfaces.

The solution structures of organic carbonate solvents (ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)) as electrolyte solutions of LiPF6 were investigated with FTIR and NMR spectroscopy and DFT computational methods. Both coordinated and uncoordinated solvents are observed by IR spectroscopy, allowing the determination of solvent coordination numbers, which a range from 2 to 5. The predominant species in solution changes as a function of LiPF6 concentration. At low salt concentrations ( 2.0 M) the predominant species in solution is the contact ion pair. In mixed solvent systems (PC–DMC, PC–DEC, EC–DMC, or EC–DEC), the mixed solvated cations are observed in the presence of high concentrations of uncoordinated cyclic carbonate despite the much larger dielectric constant of the cyclic carbonates than dielectric constant of linear carbonate.

Role of Mixed Solvation and Ion Pairing in the Solution Structure of Lithium Ion Battery Electrolytes

The heat capacities per unit volume and the densities of aqueous solutions of 2-propanol, neopentanol, tert-amylalcohol, 2-amino-2-methylpropanol, triethylamine and diethylmethylamine were measured, in many cases as a function of temperature, over the whole mole fraction or solubility range. Apparent and partial molal heat capacities, volumes and expansibilities were derived. The concentration dependence of these functions suggest the existence of transitions in some of these systems, in the water-rich region, qualitatively similar to micellization. The large relaxation contribution observed with some of the thermodynamic functions of hydrophobic alcohols and amines suggests a reinforcement of hydrophobic hydration due to strong hydrogen-bonding interactions of the polar groups with water.

Microheterogeneity in aqueous-organic solutions: Heat capacities, volumes and expansibilities of some alcohols, aminoalcohol and tertiary amines in water

THE solvation of sugars by water has not been extensively studied in spite of its biochemical importance. Relaxation rates for 17O in monosaccharides have been used to study solvation1,2, the revised2 estimate of the solvation number being 5±1 water molecules for D-glucose and 2.5±1 for D-ribose at 5 °C, in agreement with 6±1 and 2.5±1 (respectively) derived from dielectric relaxation amplitudes. Again, compressibility data for aqueous sugars have been used3 to derive solvation numbers for monosaccharides, with results ranging from 2.5 to 4.0 at 25 °C. One limiting factor in nuclear magnetic resonance (NMR) studies of aqueous sugars is that the rate of proton exchange is fast and no separate resonances from the hydroxyl protons appear. We now report that at low temperatures the exchange rate becomes sufficiently slow in the pH range 4 to 8 to enable us to detect resolved resonances from the individual hydroxyl protons. The results are interpreted in terms of strong hydrogen bonding between water and glucose, the effective solvation number being at least 10.

Proton magnetic resonance study of the hydration of glucose.

Thermo‐Solvatochromism of Merocyanine Polarity Probes – What Are the Consequences of Increasing Probe Lipophilicity through Annelation?

The hydroxyl proton resonance frequency for various binary solvent mixtures, including water with methanol, t-butyl alcohol, triethylamine and acetone, has been measured over the complete mole fraction range. Results of similar experiments for alcohol + alcohol and alcohol + amine mixtures are used for comparative purposes. At low concentrations water behaves normally in these solvents, but in the water-rich region shifts to low-field were generally observed, especially for aqueous alcohols and amines. These are interpreted in terms of an acid-base effect, and an effect involving local changes in water structure, equivalent to a temperature reduction. Various methods for differentiating between these two factors are outlined. It is concluded that for aqueous methanol, the acid-base effect dominates, whilst for aqueous t-butyl alcohol the water-structure effect is of major significance. Other solvents fit between these extremes.

Barbara Kingston

Papers

Solvation spectra. Part 44.—Nuclear magnetic resonance study of binary solvent mixtures: water structural effects

Solvation spectra part 44, nuclear magnetic resonance study of binary solvent mixtures, water structural effects