Water structure breakers/makers

More complicated and less known than the structure of pure water is the structure of aqueous solutions. In all cases, the structure of water is changed, more or less, by dissolved substances. A quantitative measure for the influence of solutes on the structure of water was given in 1933 by Bernal and Fowler 23), introducing the terminus structure temperature, Tsl . This is the temperature at which any property of pure water has the same value as the solution at 20 °C. If a solute increases Tst, the number of hydrogen bonded water molecules is decreased and therefore it is called a water structure breaker . Vice versa, a Tsl decreasing solute is called a water structure maker . Concomitantly the mobility of water molecules becomes higher or lower, respectively. 

It is possible to indicate by thermodynamic considerations 24,25,27>, by spectroscopic methods (IR28), Raman29 , NMR30,31 ), by dielectric 32> and viscosimetric measurements 26), that the mobility of water molecules in the hydration shell differs from the mobility in pure water, so justifying the classification of solutes in the water structure breaker and maker, as mentioned above. 

Hydration of ionic species water structure breakers and structure makers... 

Ions that are water structure formers, lower the cloud point of POE nonionics, OH > F > Cl > Br, by decreasing the availability of nonassociated water molecules to hydrate the ether oxygens of the POE chain. Ions that are water structure breakers (large, polarizable anions soft bases SCN I-) increase the cloud point by making more water molecules available to interact with the POE chain (Schott, 1984). Thus, chloride ions, which are water structure makers, lower the cloud point iodide ions, which are structure breakers, raise it bromide ions have no pronounced effect. 

These algebraic signs have led to the classification of ions into water-structure-makers (Bt,i>0) and water-structure-breakers (Bt I<0) (Gurney 1953), and such effects are fully discussed in Sect. 3.1. 

Probably the first scientific study on specific salt effects was performed by Jean Luis PoiseuUle in 1847 [1]. He discovered that some salts increase the viscosity of water, whereas others decrease it. In the first half of the twentieth century, the investigations on specific ion effects on viscosity were further refined by Jones and Dole in 1929 [2] and Cox and Wolfenden in 1934 [3], Based on these studies, Frank and Evans [4] proposed the expressions water structure-maker and water-structure breaker, a concept that recently turned out to be slightly misleading, at least for simple 1-1 electrolytes in water. 

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 ... 

However, it should be mentioned that specific ion effects were found even several decades before Hofineister. In 1847, Poiseuille was probably the first who noted that some salts increase the viscosity of water, whereas others decrease it. Jones and Dole in 1929, Cox and Wolfenden in 1934, and several other groups further refined the specific ion effect on water viscosity. From these viscosity smdies and in particular the Jones-Dole viscosity B coefficients, the expressions for water-structure maker and water-structure breaker were finally derived. They were first introduced in 1945 by Frank and Evans, who showed the relationship between viscosity and entropy of dilution. There is a third concept introduced by A. Voet [see also Eckfeldt ] the ordering ofions according to their lyotropic numbers. It nicely correlates with ion effects on the swelling... 

Fig. 16. Division of the group lA cations and the VIIA halide anions into [strongly hydrated] kosmotropes (water structure makers) and [weakly hydrated] chaotropes (water structure breakers). The ions are drawn approximately to scale. A virtual water molecule is represented by a zwitterion of radius 1.78 A for the anionic portion and 1.06 A for the cationic portion. In aqueous solution, Li+ has 0.6 tightly attached water molecules, Na+ has 0.25 tightly attached water molecules, F has 5.0 tightly attached water molecules, and the remaining ions have no tightly attached water molecules. (After ColKns. °)...

O Neil and Truesdell (1991) have introduced the concept of structure-making and structure-breaking solutes structure makers yield more positive isotope fractionations relative to pure water whereas structure breakers produce negative isotope fractionations. Any solute that results in a positive isotope fractionation is one that causes the solution to be more structured as is the case for ice structure, when compared to solutes that lead to less structured forms, in which cation - H2O bonds are weaker than H2O - H2O bonds. 

Organic molecules may influence c.m.c. s at higher additive concentrations by virtue of their influence on water structuring. Sugars are structure-makers and as such cause a lowering of c.m.c., whereas urea and formamide are structure-breakers and their addition causes an increase in c.m.c. 

All the non-freezing water values in Figure 7 were determined in 1 M salt solutions. It was therefore of Interest to explore the effect of salt concentration. Figure 8 shows the effect of salt concentration for one structure breaker (KNO ) and one structure maker (Ll-SO ). The results show that Ll2S0 is strongly concentration dependent while KNO is concentration independent. These results cast some doubt that the results in Figure 7 are due solely to the cation of the salt solution. Further work is needed for clarifying these results. 

The structure breaker salts decrease the bound water whereas the structure maker salts Increase the bound water. To explain the observed bound water trend. It Is hypothesized that structure breakers decrease the critical pore size and structure makers Increase this value. The viscosities of aqueous salt solutions are used as criteria to separate structure breaker from structure maker salts. 

The precipitation of salts according to Rule I is accompanied by a large favorable entropy change, as the strongly hydrated cations and anions release numerous waters of hydration. In contrast, the dissolution of salts according to Rule II is accompanied by very little entropy change, since one ion is a structure breaker, while the other is a structure maker the dissolution occurs because the ions are mismatched... 

LiCl from water to several organic solvents. The large negative values of A5 suggests that an increase in the structure of the organic solvent accompanies the transfer. The evidence suggests that most salts which are classified as structure breakers in water are structure-makers in the organic solvent. 

Depending on their relative abilities to induce stmctural changes in water, ions have often been classified as structure-makers, termed kosmotropes, and structure-breakers, termed chaotropes. These are of course qualitative or pictorial terms and need to be quantified. As in other non-ideal solutions, this is done through the concentration dependence of the viscosity of the aqueous solution, which in the case of electrolytes can be described by the following expansion in concentration c,... 

---