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.

Publisher Summary This chapter discusses the development of microscopic models of the electronic structure of polymers in general and of conjugated organic polymers in particular The underlying physical model—namely, the one-particle picture, is being widely used in this field and is considered basically correct On a practical level, this means that periodic models are usually constructed, which justify using the terminology of energy–band theory With refinements of calculation tools, self-consistent calculations at even the ab initio level have become widespread The chapter focuses on methodological problems related to the treatment of the electronic structure of polymers, such as the boundary conditions and problems related to the large size of these systems, ab initio techniques, and a symmetry dilemma connected with the quasidegeneracy of partially filled bands The results of the application of these and semi-empirical techniques to the carbon atomic chain to (CH) x and a series of related polymers, polydiacetylene (PDA), and some further conjugated polymers, are compared with experimental results Most of the discussion is based on one-dimensional (1D) model

Electronic Structure of Polymers

CIRCULAR hydrogen bonds present a new, experimentally demonstrated principle showing how hydration water molecules and hydroxyl groups of macromolecules can cooperate to form a network-like pattern. Quantum chemical calculations show that chain-like H-bonds in the crystal lattice1 are energetically favoured above individual ones2,3. This is due to the cooperative effect, which leads to increased H-bonding activity of an OH-group if it is already accepting or donating an H-bond. These linear structures can close up to form circular arrangements comprising four and more OH-groups4–6. Such circles have actually been described for crystal structures of ice7 and of ice clathrates8, but in these cases the water molecules within the circles are related by crystallographic symmetry elements and are therefore not independent of each other. One should expect to find lattice-independent circular H-bonds in crystal structures of large O—H-rich molecules which co-crystallise with water of hydration, conditions which are satisfied by the cyclodextrin family. The smallest member, α-cyclodextrin (α-CD; cyclohexaamylose), consists of six α(1, 4)-linked glucose molecules and contains six primary and 12 secondary hydroxyl groups ((C6H10O5)6, molecular weight 973). From aqueous solution, α-CD crystallises as hexahydrate, α-CD·6H2O, and this complex, with a total of 120 hydroxyl groups (4 × 18 from α-CD and 4 × 12 from the six H2O) in one unit cell has been studied by X-ray and neutron diffraction methods (ref. 9, and Klar, Hingerty and W.S., unpublished). Two other complexes, a second modification of the hexahydrate (K. Lindner and W.S., unpublished) and α-CD·methanol·4H2O (ref. 10) have been investigated by X rays. As refinement in all cases is around R = 4% for data extending to 0.89 A resolution, hydrogen atom positions could be assigned. I discuss here results obtained from the X-ray/neutron study of α-CD·6H2O.

Circular hydrogen bonds

The interaction between F− or the Cl− anion and water has been computed in the Hartree‐Fock approximation with a large basis set of Gaussian functions: The obtained energies and wavefunctions are therefore close to the Hartree‐Fock limit. We find that the geometrical configuration of maximal stability corresponds to a hydrogen‐bonded configuration between the water and the anion; however, the hydrogen bond is somewhat bent with a deviation from linearity of about 4.5° for F−–H2O and about 14.6° for Cl−–H2O. At the most stable configuration the oxygen‐anion distance is 4.75 a.u. for F− and 6.26 a.u. for Cl−. At the equilibrium configuration the Hartree‐Fock binding energy is 23.54 and 11.86 kcal/mole for the F−–H2O and the Cl−–H2O complex, respectively. The energy surfaces are analyzed in terms of an energy partitioning consisting of the energy of the water molecule in the field of the anion, the anion in the field of the molecule of water, and of the interaction energy between the water molecule and the a...

Study of the structure of molecular complexes. III. Energy surface of a water molecule in the field of a fluorine or chlorine anion

The electron projection function P(x, z) = ∫ ρ(x, y, z) dy is used to evaluate charge transfer and covalency in two series of molecules, LiX and CH3X (X = Li, BeH, BH2, CH3, NH2, OH, and F), with wavefunctions derived from STO-3G, 4-31G, and, in some cases, 6-31* ab initio calculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH3Li which by our criteria is wholly ionic.

Integrated spatial electron populations in molecules: Application to simple molecules

Hydrate der Ionen Li+, Na+, Be2+, F− und Cl− mit den Koordinationszahlen 1, 2, 4, 6 und 8 sowie bis zu drei Hydrathullen wurden mit Hilfe des CNDO/2-Verfahren berechnet. Die Starke der Wasserstoffbrucken in den auseren Spharen wird durch die Ladungsubertragung vom Zentralion zu den Liganden stark beeinflust. Bei F− unterscheidet sich die mittlere Bindungsenergie in der dritten Sphare kaum mehr vom reinen Wasser; bei Li+ reicht der Einflus um eine Wasserhulle weiter. Fur beide Ionen wird 6 als gunstigste Koordinationszahl erhalten.

Modellrechnungen zur Solvatation einatomiger Ionen (LCAO-MO-Untersuchungen von Molekülstrukturen VIII)

Ab initio calculations on the interaction of oxygen containing ligands with alkali cations. The system H2CO/Li+☆

Energy band structures of one-dimensional (HF)n- and (H2O)n-chains have been calculated (1) by extrapolation of CNDO/2-MO levels to infinite chain length and (2) by the CNDO/2 crystal orbital (CO) method. In the CO-calculations interactions up to fifth neighbours have been taken into account. Both types of calculations were performed using experimental geometries and CNDO/2 minimum geometries of the corresponding dimers (HF)2 and (H2O)2. With the same geometries CO calculations on two-dimensional sheets of hydrogen bonded chains were performed too.

Peter Russegger

Papers

Modellrechnungen zur Solvatation einatomiger Ionen (LCAO-MO-Untersuchungen von Molekülstrukturen VIII)

Ab initio calculations on the interaction of oxygen containing ligands with alkali cations. The system H2CO/Li+☆

Hydrogen bonding in long chains of hydrogen fluoride and long chains and large clusters of water molecules: X. LCAO-MO Studies on molecular structure

Ab initio calculations of molecules with efficient GTO basis sets