Structure breakers

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. 

The largest solubility isotope effects are found for sparingly soluble salts. For example, lead chloride and potassium bichromate are 36% and 33.5% more soluble in H20 than D20 at 298.15 and 278.15 K, respectively. For the more soluble salts, NaCl and KC1, the values are 6.4% and 9.0%. Interestingly LiF and LiCl.aq have inverse effects of 13% and 2%, respectively. Recall that lithium salts are commonly designated as structure makers . Almost all other electrolytes are structure breakers . 

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. 

As typical electrolytes we have taken LiCl, NaCl, Me4NBr, and Bu4NBr NaCl is the most studied alkali halide, Bu4NBr is a well-known hydrophobic electrolyte, Li+ is more solvated than Na+ (coulombic hydration) but its structural hydration is usually smaller, and Me4NBr is a weak structure breaker. A comparison of these electrolytes should therefore give us some idea of the various interactions between different electrolytes and mixed solvents. 

Similarly, AHu°(W — W + E) is of opposite sign to the expected structural hydration contribution of E (28). There does not seem to be too much specificity in the interactions involving U which probably acts as a statistical structure breaker the local U-W interactions are probably not too different from the W-W ones, but the long-range ordering is destroyed. Schrier et al. (28) have interpreted the Bue parameters in terms of a destructure overlap cosphere model. Since... 

The transfer functions of Me4NBr (Figure 3) also show relatively little specificity if we assume that the high concentration data is tending to AY°Me4NBr(W — N). The deviations from this ideal behavior are in the direction we would expect if Me4NBr is a structure breaker. 

These and similar findings have led to the concept of structure makers and structure breakers among the ions (6). Thus, the alkali ions are all considered structure makers. Of the halide ions, Cl", Br", and I" are all structure breakers, whereas F" is an exception. 

Hard ions These have high charge and/or a small radius. They usually have a low polarizability, participate principally in electrostatic bonding, and are structure breakers or formers in a polar solvent. 

There are some ions which effect the water spectra like a -increase. Ions with the largest structure-breaker effect can have salt-in effects on organic molecules. This can be understood as follows the water becomes more hydrophilic because the content of orientation defects of OH groups increases. Structure breakers are mainly large mono-valent anions. 

Negative values ofN —N0, the electrolyte effect on the association numbers of water, are called the structure-breaker effect. One can speak of negative hydration31. The estimation of the hydration numbers by spectroscopic or solubility methods gives only an approximation of the sum effect. The spectra of the H-bond bands show in second approximation distinct differences between the ion effects on the H-bonds7 ). — The partial molar volume Vx of water in electrolyte solutions is negative in all solutions but the series of -values corresponds to the Hofmeister ion series too. The negative V1 volume indicates an electrostriction effect around the ions. 

The unfolded state U is modeled as a single conformation even though this is by no means obvious and usually cannot be verified. The unfolding equilibrium between N and U can be shifted to the unfolded form by measures such as an increase in temperature or an increase in the concentration of structure breakers , termed chaotropes, such as urea or guanidinium hydrochloride. When either temperature or the concentration of chaotropes is decreased (or the concentration of structure formers , kosmotropes, is increased), the folding equilibrium reverts to the native state N. The two-state model is an approximation very often, analytically verified folding intermediates render the two-state model incomplete. 

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. 

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. 

There are several advantages, particularly in the context of aqueous solutions, in representing water using eqn (16). Thus, to a first approximation, a solute which increases (H20)b at the expense of (H20)d is a structure former a structure breaker has the opposite effect. The large heat capacity for water can be attributed to the need to melt part of (H20)b. In these terms, the partial molar heat capacity of solutes in water often indicates their effect on water structure. 

An experimental basis has been suggested by Hepler (1969) for the classification of solutes for structure formers, (32F /9r2)>0 and for structure breakers <0. More recently, Ben-Naim (1975) has demonstrated how the difference between solubilities of a solute in D20 and H20 can also be used in a similar fashion. Ben-Naim (1972a, 1973a) has also shown from theoretical arguments how it is possible for a solute to stimulate H-bond formation, i.e. structure-formation, between water molecules. However, the effect of a structure-former on water is not straightforward, the induced structure being apparently different from that of pure water at a lower temperature (Hertz, 1970). 

In contrast to urea itself, its N-methylated derivatives enhance water-water interactions, i.e. lower the structural temperature hexa-methylene tetramine produces similar marked effects (Barone et al., 1968). Glycine and /3-alanine appear to be structure breakers (Devine and Lowe, 1971) according to their effect on the viscosity of water (Herskovitz and Kelly, 1973). The viscosities and diffusion properties of urea solutions show striking changes as the concentration increases (MacDonald and Guerrera, 1970). 

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