Structure 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. 

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. 

The criteria (9.23), (9.24), and (9.26) are all rather obvious properties of Euclidean geometry. All of these properties can be traced back to mathematical properties of the scalar product (R Ry), the key structure-maker of a metric space. We therefore wish to determine whether a proposed definition of scalar product satisfies these criteria, and thus guarantees that M is a Euclidean space. 

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. 

If we calculated with the idealized co-operative model by the content of spectroscopic determined Op values the number Nei of H-bonded water molecules we would get — with different 1 molar salt solutions — the result of Fig. 11. The values Nei with salt additions depend strongly on the salt concentrations because of the disturbance of the big H-bonded system3At small concentrations the Nel-N0 numbers (7V0 association number in pure water) of structure-makers are in size of the order of Debye-Sack s or Azzam s calculations. They are of the same size of order as the secondary hydration numbers calculated by solubility measurements of organic substances in water (Chapter b) or as the hydration numbers of hydrophilic organic molecules (Chapter lld-e) or biopolymers (Chapter III). 

Structure maker ions increase the concentration of micelles and reduce the concentration of monomers. In a very rough model one can assume the fixed hydration sphere around the ions cannot solve the ethylenoxide products and increase its concentrations in the rest bulk water phase (salt-out effect). In this model one can estimate the size of the hydration sphere of the ions. The hydration numbers gained by this method are surprisingly large3 They start at 200 water molecules per ion pair at 0.1 mole solutions of ions and decrease about 20 at 1 mole solutions31,72,130). 

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. 

Protein stabilization by trehalose in aqueous solution is also an example of the biological functions of trehalose [84-86]. According to reports by Timasheff and coworkers, preferential hydration should occur when the interaction of a cosolvent with water is stronger than its interaction with a protein [85]. In other words, a water structure maker like trehalose is a good cosolvent causing... 

An ion with a positive VBC is a structure maker and tends to enhance the hydrophobic interaction, while one with a negative VBC has the opposite effect. 

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 stmctme breaker, while the other is a structure maker the dissolution occurs because the ions are mismatched... 

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

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