Water Structure Effects of Ions

Water structural effects of ions may be assessed by a variety of experimental methods as well as by computer simulations. These methods lead to the recognition that some ions enhance the native structure of the water whereas other ions destroy it, up to some distance in the water away from the ion. The former ions are called structure-makers, or cosmotropic, a term favoured in the biophysical literature, and the latter kind of ions are structure-breakers or chaotropic. Naturally, there are also ions that are borderline between these groups, neither structure-makers nor -breakers to a significant extent. 

Eq. (3.23) the geometrical hydrogen bonding parameters for the ions that describe their effects on the structure of the water. These may be taken for correlations with other quantities that describe the water structural effects of ions that are better established, such as Bi, and Astruc-5. The resulting expressions, calculated with = — 929 J mor are ... 

The effects of the ions on the structure of the water were then described by Marcus [51, 53] as the ratios AG bj, = A i" °according to Equation 5.13. The water stracture effects of ions according to this approach are shown in Table 5.2 — structure makers having positive values and structure-breakers negative values. These results are unsatisfactory, due to the inaccuracy of the AjU,°° ° data, making the divalent cations Ba and Cd appear as strong water-structure breakers and LP as a mild structure breaker, contrary to aU other information concerning these ions. The available data for the nine alkah metal and hahde ions appear to be the most accurate, and their correlations with other quantities that describe the water structural effects of ions are ... 

The standard molar entropy of hydration of an ion, Ahydr5° , should contain contributions from the formation of the ionic hydration shell and also from the limitation of the ionic rotation of a multiatomic ion in the solution compared with the gas. Hence, such contributions should be deducted, according to Krestov (1962, 1962a), from Ahydr5° in order to obtain the water structural effects of the ion beyond the hydration shell, Astmc (AS ii in the notation of Krestov) ... 

The question now arises, whether the biophysical phenomena ascribed to the ions being classified as chaotropic or kosmotropic indeed result from the effects the ions have on the structure of water or from other causes. This problem should be judged on the premise that water stmcture effects of ions are manifested in dilute homogeneous solutions. In fact, few biophysical phenomena take place in such solutions, since biomolecules such as proteins and nucleic acids tend to be large and colloidal, and when dispersed in water may form micro-heterogeneous domains. 

The most extensive treatment of the structural effects of ions on the solvent surrounding them has been made by quantum-chemical treatment (charge field modified, in more recent years) of their first (or first and second) solvation shell combined with molecular-mechanical computer simulations of the solvent beyond the(se) solvation shell(s), the interface between these two regions being also carefully treated. Only small solvent molecules with few atoms, namely water (and for very few ions also ammonia), could be treated in this manner, because of the large expenditure of computer time required for the quantum chemical simulation. 

Feng et al. [3] have studied the structural effect of acetanilide on the AAM polymerization either in water-for-mamide [3], water-acetonitrile [4], and water-DMF [26] mixed solution using Ce(IV) ion-acetanilide and its substituted derivatives as the initiator. The results showed that an electron donating substituent on the phenyl group would enhance the Rp, while an electron withdrawing group would decrease it, as shown in Table 1 [26]. 

EFFECT OF IONS ON STRUCTURE AND DIELECTRIC CONSTANT OF WATER... 

Conductometric and spectrophotometric behavior of several electrolytes in binary mixtures of sulfolane with water, methanol, ethanol, and tert-butanol was studied. In water-sulfolane, ionic Walden products are discussed in terms of solvent structural effects and ion-solvent interactions. In these mixtures alkali chlorides and hydrochloric acid show ionic association despite the high value of dielectric constants. Association of LiCl, very high in sulfolane, decreases when methanol is added although the dielectric constant decreases. Picric acid in ethanol-sulfolane and tert-butanol-sulfolane behaves similarly. These findings were interpreted by assuming that ionic association is mainly affected by solute-solvent interactions rather than by electrostatics. Hydrochloric and picric acids in sulfolane form complex species HCl and Pi(HPi). ... 

However, as the shell builds up and breaks more and more H bonds in the surrounding solvent, the effect of ions on liquid water begins to become more structure-breaking (Fig. 2.31). 

One of the most exciting questions in solution chemistry is concerned with the effect of the pressure on the solvation shell of ions. From conductance data it was suggested that the pressure breaks up the structure in the bulk water and also in the local water structure near the ions. Opposite opinions also exist according to which the increase in pressure leads to an enhancement of the close hydration. MD simulation studies on a concentrated aqueous NaCl solution [23] showed almost no changes in the hydrate structures of the ions even at 10 kbar pressure. On the other hand, the H-bonded network of the water molecules is distorted in a similar way as it was found in pure water [40]. Because of extreme technical difficulties, only a few attempts were to determine the structure of pure solvents at elevated pressures and only one ND experiment is reported on aqueous solutions of a LiCl and a NiClj aqueous solution [24], leading to contradictory results. No XD diffraction studies were reported. 

Table 3.1. I Effects of ions on the density and structure of liquid water...

Another problem with the MSA is that it does not distinguish between the solvation of cations and anions of the same size. Thus, although the K and F ions have approximately the same radius, the F anion is more strongly solvated than the K cation (table 3.4). This can be understood in terms of the effect that each ion has on local water structure. The K ion disrupts this structure more so that the stabilizing effect of the local ion-dipole interactions is offset by the work done to break up the water structure, that is, to disrupt attractive dipole-dipole interactions and hydrogen bonding between local water molecules. This means that the parameter 8s should be different for cations and anions in the same solvent (table 3.5). 

The electrolyte can also have another effect, not commonly recognized by many researchers, the chaotropic effect.40 This effect, more commonly discussed in biochemical circles, is related to variations in the water-structuring ability of different salts. This ability is used, for example, to dehydrate and salt out macromolecular proteins. The same effect can be expected with conducting polymers. As well as ion-size effects, this may be used to explain the large shifts in switching potentials observed in different electrolytes41 when the cation was varied and the difference in overoxidation potentials observed in different electrolytes.42... 

Note the distmction in the effect of ions and apolar solutes on the structure of water only becomes apparent in terms of A5 values when thermodynamic transfer A5 values are used. 

Structural Significance of ion Distribution Curves. The effect of solutes, ionic and not ionic, on the structure of water has been the object of much research, especially since the fundamental investigations of H. S. Frank and co-workers (52, 53, 54, 88, 132, 133, 135). By contrast, relatively little is known about the way solutes fit into the ice structure. Subsequent sections of this paper review some pertinent evidence. 

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