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.

Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of organic species

The effects of urea on protein stability have been studied using a model system in which we have determined the energetics of dissolution of a homologous series of cyclic dipeptides into aqueous urea solutions of varying concentration at 25 degrees C using calorimetry. The data support a model in which urea denatures proteins by decreasing the hydrophobic effect and by directly binding to the amide units via hydrogen bonds. The data indicate also that the enthalpy of amide hydrogen bond formation in water is considerably higher than previously estimated. Previous estimates included the contribution of hydrophobic transfer of the alpha-carbon resulting in an overestimate of the binding between urea and the amide unit of the backbone and an underestimate of the binding enthalpy.

Urea effects on protein stability: hydrogen bonding and the hydrophobic effect.

In this critical review, the significance of the term 'activity' is examined in the context of the properties of aqueous solutions. The dependence of the activity of water(l) at ambient pressure and 298.15 K on solute molality is examined for aqueous solutions containing neutral solutes, mixtures of neutral solutes and salts. Addition of a solute to water(l) always lowers its thermodynamic activity. For some solutes the stabilisation of water(l) is less than and for others more than in the case where the thermodynamic properties of the aqueous solution are ideal. In one approach this pattern is accounted for in terms of hydrate formation. Alternatively the pattern is analysed in terms of the dependence of practical osmotic coefficients on the composition of the aqueous solution and then in terms of solute-solute interactions. For salt solutions the dependence of the activity of water on salt molalities is compared with that predicted by the Debye-Huckel limiting law. The analysis is extended to consideration of the activities of water in binary aqueous mixtures. The dependence on mole fraction composition of the activity of water in binary aqueous mixtures is examined. Different experimental methods for determining the activity of water in aqueous solutions are critically reviewed. The role of water activity is noted in a biochemical context, with reference to the quality, stability and safety of food and finally with regard to health science.

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Activity of water in aqueous systems; A frequently neglected property

Das Bestreben relativ unpolarer Verbindungen, sich in wasriger Losung aneinander zu lagern, wird mit dem Begriff hydrophobe Wechselwirkungen beschrieben. Diese Wechselwirkungen sind in vielen Gebieten der Chemie, einschlieslich der Chemie in vivo, von entscheidender Bedeutung. Enzym-Substrat-Wechselwirkungen, die Anordnung von Lipiden in biologischen Membranen, die Aggregation oberflachenaktiver Stoffe und kinetische Solvenseffekte in wasserreichen Losungsmittelsystemen – sie alle werden in erster Linie durch hydrophobe Wechselwirkungen bestimmt. Trotz enormer Anstrengungen weis man noch wenig uber die Hydratation unpolarer Molekule und uber die nichtkovalenten Wechselwirkungen zwischen diesen Molekulen in Wasser. Wenn man sich nun die Frage stellt, was denn die treibende Kraft hinter hydrophoben Wechselwirkungen ist, so fuhrt dies zu der Forderung, die herausragenden Eigenschaften flussigen Wassers genau zu verstehen. Diese ubersicht beschreibt einige neue Erkenntnisse auf diesem Gebiet aus den letzten zehn Jahren. Schwerpunkte sind sowohl die hydrophobe Hydratation als auch hydrophobe Wechselwirkungen, denn beide Phanomene hangen ganz eng zusammen. Es hat sich gezeigt, das viele der bisherigen Sichtweisen zutiefst unbefriedigend sind, und man hat mutige Versuche unternommen, die treibende Kraft fur hydrophobe Wechselwirkungen zwischen zwei oder vielen Molekulen zu verstehen. Hier wird eine zugegebenerweise personliche Auswahl experimenteller und theoretischer Entwicklungen der letzten Zeit vorgestellt und, wenn notig, durch Hinweise auf relevante fruhere Studien erganzt.

Hydrophobe Effekte – Ansichten und Tatsachen

The enthalpy of dilution of all one-and two-solute aqueous mixtures of a series of compounds were measured from about 0.2 to 2.0 mole-kg−1 at 25°C. The compounds included in the study wereN-methylformamide,N-methylacetamide,N-methylpropionamide,N-butylacetamide, urea, ethylene glycol, pentaerythritol, glucose, and sucrose. The results of the enthalpy measurements were used to calculate the pairwise enthalpy of interaction for each compound with all the other compounds. A simple additivity principle is used to correlate the data. The principle assumes that each functional group on one molecule interacts with every functional group on the other molecule and that each of these interactions has a characteristic effect on the enthalpy that is independent of the positions of the functional groups in the two molecules. The resulting equation gives a rough but useful correlation of the results. Of the six interactions between the CH2, CONH, and CHOH functional groups, the CONH−CONH interaction is the strongest, the CHOH−CHOH interaction is the weakest, and the CH2−CH2 interaction is about equal in magnitude to the rest of the interactions. Thus, the CH2−CH2 and CONH−CONH are not the only interactions making important contributions to the enthalpy of a wide variety of systems.

Enthalpy of dilution of aqueous mixtures of amides, sugars, urea, ethylene glycol, and pentaerythritol at 25°C: Enthalpy of interaction of the hydrocarbon, amide, and hydroxyl functional groups in dilute aqueous solutions

James J. Savage

Papers

Enthalpy of dilution of aqueous mixtures of amides, sugars, urea, ethylene glycol, and pentaerythritol at 25°C: Enthalpy of interaction of the hydrocarbon, amide, and hydroxyl functional groups in dilute aqueous solutions