Structure-breaking effects

Schwenk CF, Hofer TS et al (2004a) The structure breaking effect of hydrated Cs(I). I Phys Chem A 108 1509... 

Hofer TS, Randolf BR et al (2005c) Structure-breaking effects of solvated Rb(I) in dilute aqueous solution - an ab initio QM/MM MD approach. J Comput Chem 26 949... 

Not only Diels-Alder cycloadditions but also 1,3-dipolar cycloaddition reactions can be subject to hydrophobic rate enhancements. For example, the reaction of C,N-diphenylnitrone with di-n-butyl fumarate at 65 °C to yield an isoxazolidine is about 126 times faster in water than in ethanol, while in nonaqueous solvents there is a small 10-fold rate decrease on going from n-hexane to ethanol as solvent - in agreement with an isopolar transition-state reaction [cf. Eq. (5-44) in Section 5.3.3] [858]. Because water and ethanol have comparable polarities, the rate increase in water cannot be due to a change in solvent polarity. During the activation process, the unfavourable water contacts with the two apolar reactants are reduced, resulting in the observed rate enhancement in aqueous media. Upon addition of LiCl, NaCl, and KCl (5 m) to the aqueous reaction mixture the reaction rate increases further, whereas addition of urea (2 m) leads to a rate decrease, as expected for the structure-making and structure-breaking effects of these additives on water [858]. 

The fluidity of the solvent is changed because of the structure-breaking effects caused by the presence of macroions. Self-diffusion data on water in the presence of proteins allows one to measure their hydration. This is usually measured in water per gram of anhydrous protein. 

In methyl alcohol-water mixtures there is a structure-making effect, maximizing the correlation factor at 40 molar per cent of alcohol. By contrast, both n-propyl and isopropyl alcohols, at ca. 50 molar perceat, have a marked structure-breaking effect, the correlation factors of these... 

We have already discussed in detail the appropriate functional forms of the position-dependent "microscopic viscosity" in our previous paper Here we employ the following two functions First, we assume that the magnitude of the viscosity reduction around an ion is proportional to the strength of the structure breaking effect arising from the ion-dipole interaction This leads to the following function ... 

Gurney considered that the negative B coefficient is an indication of the local loosening of the solvent structure in the vicinity of the solute molecule and can be used as a measure for the structure breaking effect. The present quantitative analysis of the effect of the local viscosity change supports this idea. 

The very much greater dielectric constant of N-methylformamide (e = 185) vs. DMF (c = 36) is partly attributed by Dawson (1963) to the H-bonded structme of the former solvent. Polar solutes are more soluble in DMF than in N-methylformamide. Addition of a polar solute, which is itself not a hydrogen-bond donor or acceptor, to a protic solvent creates some disturbance of the solvent structure (Arnett et ah, 1965). Thus the standard chemical potential of polar solutes in protic solvents is somewhat different from that which might have been expected from purely electrostatic considerations. It may be that polar solutes of this type exert a net structure-breaking effect on protic solvents. 

Density measurements on solutions of alkali-metal salts in methanol from 0 to 60 °C over a wide concentration range show that the structure-breaking effect of the salts decreases in the order NaC104 > Nal, NaBr and KI> Nal > Lil.94... 

One type of Raman study of solutions concentrates on water-water bonding as it is affected by the presence of ions. Hydrogen bonds give Raman intensities, and the variation of these with ionic concentration can be interpreted in terms of the degree and type of structure of water molecules around ions. The CIO4 ion has often been used in Raman studies to illustrate structure-breaking effects because it is a relatively large ion. 

As mentioned above, however, a decrease in the LCST was observed when urea was added to an aqueous PNIPA solution. We thus tried to determine the LCSTs of both the PNIPA solution and the PNIPA-PAAc mixture in the presence of 4 M urea (Table 2). It is generally believed that urea breaks up the hydrogen bonds between solute molecules and also disrupts the cluster structure of water molecules ( structure breaking effect ). The latter brings about a weakening of the hydrophobic interaction between solute molecules (e.g., see Ref. 76). In the case of an aqueous PNIPA system, however, the addition of urea shifted the LCST to a low-temperature range. Therefore we cannot simply state that hydrophobic interaction between NIPA residues is weakened by the addition of urea. 

Figure 8. Rivalry of ion and ether hydrates of PlOP-9 at fugh ion contents. Above 2m Kl, the tubidity point, depends on the PIOP concentration (ng/L), arut above 4m, the structure-breaking effect of K disappears.

Dack MR (1976) Solvent structure. II. a study of the structure-making and structure-breaking effects of dissolved species in water by internal pressure measurements. J Aust J Chem 29 771-778 Dack MR (1976a) Solvent structure. III. the dependence of partial moM volumes on internal pressure and solvent compressibility. J Aust J Chem 29 779-786 Davies J, Ormondroyd S, Symons MCR (1971) Solvation spectra. 41. Absolute proton magnetic resonance shifts for water protons induced by cations and anions in aqueous solutions. Trans Faraday Soc 67 3465-3473... 

Proton resonance measurements can also give information on the structurebuilding or structure-breaking effects of various ions on the solvent [Fr 65, Ha 67a, Kr 67a]. 

Central to such a study is a need for basic information to deepen our understanding of hydrophobic hydration and all other specific intermolecular interactions that take place in mixed systems, which are the driving forces of structure-making and structure-breaking effects in these solutions. Obviously, these opposite effects enhance or depress rheological and viscokinetic properties of the systems, and all... 

Also, in further developments of this kind of treatment, specific structureforming and structure-breaking effects of the organic adsorbate must be considered in relation to the orientation of solvent molecules, e.g., as indi-cated by entropies and heats of adsorption of pyridine at Hg electrodes. Analysis of various types of isotherms which represent organic molecule adsorption has been made by Kovac and Bockris and by Wroblowa and Mueller in relation to the theory presented above. 

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