Water-structure breaker

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. 

Raman spectra of uncoupled OH and OD stretch bands show a pronounced asymmetry on the high energy side (Walrafen, 1973). Analysis of the band shape reveals contributions from two bands assigned by Walrafen to non-hydrogen bonded and hydrogen bonded oscillators. Addition of sodium perchlorate, a water structure breaker, results in an increase in intensity of the high energy component, consistent with an increase in proportion of free OH. 

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

The addition of inert electrolyte containing a common, nonsurfactant ion to a solution of ionic surfactant in an aqueous medium causes a large increase in the efficiency of its adsorption at the liquids-air interface (Boucher, 1968). In sharp contrast to their lack of significant effect on the effectiveness of adsorption, the addition of water structure breakers, such as urea and /V-methylacetamide, to aqueous solutions of nonionic surfactants results in a decrease in efficiency of adsorption at the interface (Schwuger, 1969), whereas the addition of structure formers, such as fructose and xylose, increases the efficiency of adsorption (Schwuger, 1971b). 

Investigation of the effect of electrolyte on the CMC of a high-molecular-weight nonionic of the POE polyoxypropylene type (Pandit, 2000) found that the CMC decreased in the order Na3P04 > Na2S04 > NaCl. The addition of NaSCN increased the CMC, consistent with its action as a water structure breaker. 

Urea, formamide, and guanidinium salts are believed to increase the CMC of surfactants in aqueous solution, especially polyoxyethy-lenated nonionics because of their disruption of the water structure (Schick, 1965). This may increase the degree of hydration of the hydrophilic group, and since hydration of the hydrophilic group opposes micellization, may cause an increase in the CMC. These water structure breakers may also increase the CMC by decreasing the entropy effect... 

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. 

The addition of water structure promoters (fructose, xylose) or a water structure breaker GV-methylacetamide) to an aqueous solution of a POE nonionic has been shown to affect markedly the efficiency of the surfactant in reducing surface tension (Schwuger, 1971). Water structure promoters appear to increase the efficiency of the surfactant, whereas structure breakers decrease it. The reasons for these changes are probably the same as those that account for the effect of these additives on the critical micelle concentrations of nonionics (Chapter 3). 

Urea, being a water structure breaker, would weaken the hydrophobic interaction between solute molecules as reported in several previous studies (see Ref. 76). It is thus necessary for us to examine the effects of urea on the hydrophobic interaction in the present gel system. However, this is a considerably difficult problem which, to our knowledge, has not yet been dealt with by any researchers in the field of polyelectrolyte gels. The main reason is the lack of information about whether hydrophobic interaction plays a role in the volume collapse of usual polyelectrolyte gels with a lot of hydrophilic ionizable groups, such as the LPEI gels in question. As a novel approach in order to overcome this difficulty, we examined the swelling curves of the LPEI gel as a function of the concentration of anionic surfactants in the presence and absence of urea. 

On the other hand, cesium, rubidium, and, to a lesser extent, potassium are known as water structure breakers they cause the water molecules to be very disordered around the central cation. These larger cations have a small tendency to coordinate water molecules in a hydration shell, so lithium cations interact strongly with the (inner)... 

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. 

Structure breaker. A similar problem is encountered with the study of the hydration of Au+ ions (Armunanto et al. 2004), where absolute Af/ r values for t of 2 and 0.5 ps are reported (unusually shorter for the first than the second hydration sphere), but not the corresponding value for bulk water. The relative MRT values for the first and second shells were reversed in a more recent study (Lichtenberger et al. 2011). Still, also Au+ was considered to be a structure-breaking ion. The hydration dynamics of T1+ (Vchirawongkwin et al. 2007) share with those of Au" the phenomenon that the RMRT of the first hydration sphere (76 %) is shorter than that of the second sphere (88 %) for t of 0.5 ps, but the list of RMRT values given in this paper for Rb+, Cs+, Ag+, and Au+ are at variance with their being classified as water structure breakers in the papers quoted above. This point has not been explained so far. 

The second hypothesis claims that the denaturants preferentially bind to the snr-face of the proteins (Timasheff 2002a) the larger the snrface area, the more denatur-ant molecules are bound to each protein the denatured state therefore becomes more stable than the native state. Both of these proposals have been founded upon primitive and antiquated models of solutions the lattice theory of solution is the foundation of the water structure breaker hypothesis (Frank and Franks 1968), whereas the stoichiometric binding model of solvation is the basis of the preferential binding hypothesis (Scheltman 1987 Timasheff 2002a). Consequently, the weak theoretical foundation had prompted much debate, not only over the validity of these hypotheses, but also over the true meaning of these hypotheses at a molecular level. 

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. 

Breslow, R. and Guo, T. (1990) Surface tension measurements show that chaotropic salting-in denaturants are not just water-structure breakers, Proc. Natl. Acad. Sci. USA 87, 167-169. 

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 ... 

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