Water Structure in Aqueous Solutions

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. 

---

An understanding of equilibrium phenomena in naturally occurring aqueous systems must, in the final analysis, involve understanding the interaction between solutes and water, both in bulk and in interfacial systems. To achieve this goal, it is reasonable to attempt to describe the structure of water, and when and if this can be achieved, to proceed to the problems of water structure in aqueous solutions and solvent-solute interactions for both electrolytes and nonelectrolytes. This paper is particularly concerned with two aspects of these problems—current views of the structure of water and solute-solvent interactions (primarily ion hydration). It is not possible here to give an exhaustive account of all the current structural models of water instead, we shall describe only those which may concern the nature of some reported thermal anomalies in the properties of water and aqueous solutions. Hence, the discussion begins with a brief presentation of these anomalies, followed by a review of current water structure models, and a discussion of some properties of aqueous electrolyte solutions. Finally, solute-solvent interactions in such solutions are discussed in terms of our present understanding of the structural properties of water. 

Molecular modeling using either Monte-Carlo simulations or molecular dynamics is used to apply molecular mechanics energy minimizations to very complex systems [348]. In complex flexible molecules such as proteins or nucleic adds, the number of variable parameters, i.e., bond torsion angles, is such that the global search for energy minima becomes impossible The same problem occurs with theoretical calculations of water structure in aqueous solutions or in heavily hydrated crystals. 

My considerations do not exclude inductive and other contributions. Rather it is the case that the effects of hydrophobically enforced water-structuring in aqueous solution override the other contributions. I did not say that proline is the most flexible amino acid residue in proteins. 

Nickolov, Z. S., and Miller, J. D. 2005. Water structure in aqueous solutions of alkali halide salts FTIR spectroscopy of the OD stretching band. J. Colloid Interface Sci. 287 572. 

Most of the molecules introduced in this chapter are hydrophobic. Even those molecules that have been functionalized to improve water-solubility (for example, CCVJ and CCVJ triethyleneglycol ester 43, Fig. 14) contain large hydrophobic structures. In aqueous solutions that contain proteins or other macromolecules with hydrophobic regions, molecular rotors are attracted to these pockets and bind to the proteins. Noncovalent attraction to hydrophobic pockets is associated with restricted intramolecular rotation and consequently increased quantum yield. In this respect, molecular rotors are superior protein probes, because they do not only indicate the presence of proteins (similar to antibody-conjugated fluorescent markers), but they also report a constricted environment and can therefore be used to probe protein structure and assembly. 

Fig. 30 Types of nanocarriers for drug delivery, (a) Polymeric nanoparticles polymeric nanoparticles in which drugs are conjugated to or encapsulated in polymers, (b) Polymeric micelles amphiphilic block copolymers that form nanosized core-shell structures in aqueous solution. The hydrophobic core region serves as a reservoir for hydrophobic drugs, whereas hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble.

Marinov VS, Nickolov ZS, Matsuura H. 2001. Raman spectroscopic study of water structure in aqueous nicotinic surfactant solutions. J Phys Chem B 150 9953-9959. 

Studies of ionic solutions have been overwhelmingly aqueous in the hundred years or so in which they have been pursued. This has been a blessing, for water has a dielectric constant, s, of 80, about ten times larger than the range for most nonaqueous solvents. Hence, because the force between ions is proportional to 1/e, the tendency of ions in aqueous solutions to attract each other and form groups is relatively small, and structure in aqueous solutions is therefore on the simple side. This enabled a start to be made on the theory of ion-ion attraction in solutions. 

Jiang, X. Zhang, G. Narain, R. Liu, S.H. Fabrication of two types of shell-cross-linked micelles with Inverted structures in aqueous solution from schizophrenic water-soluble ABC triblock copolymer via click chemistry. Langmuir 2009, 25 (4), 2046-2054. 

Besides PEG, other water soluble polymers, such as poly(vinyl alcohol) (PVA) [41] and polyacrylamide (PAAm) [29], can also form water-mediated supramolecular structures in aqueous solution. SMFS results indicated that the force curves of PVA and PAAm measured in water could not be fitted well to the WLC model (for PVA) or the M-FJC model (for PAAm). However, the force curves in aqueous 8 M urea solution (urea is generally considered to break H-bonds) were remarkably distinct from those measured in water and fitted well to the WLC model and M-FJC model for PVA and PAAm, respectively. This fact demonstrated the existence of H-bonded supramolecular structures in PVA and PAAM in water, which were destroyed in 8 M urea. 

Dutkiewicz E, Jakubowska A (2002) Water activity in aqueous solutions of homogeneous electrolytes the effects of ions on the structure of water. Chem Phys Chem 3 221-224 Endon L, Hertz HG, Thuel B, Zeidler MD (1967) Microdyntimic model of electrolyte solutions as derived from nuclear magnetic relaxation and self diffusion. Ber Bunsenges Phys Chem 71 1008-1031... 

The most common and important complex ions are hydrated metal ions. The coordination numbers and structures of some of these simple complexes have been determined. Isotope dilution techniques were used to show that Cr and Al are bonded rather firmly to six water molecules in aqueous solutions. The interpretation of the visible spectra of solutions of transition metal ions using CFT indicates that ions such as Mn, Fe, Co, Ni, Cr, and Fe are octahedral [M(H20)6] species. For non-transition metal ions it has been more difficult to obtain structural information. Flowever, nuclear magnetic resonance spectroscopy demonstrates that Be in aqueous solution is surrounded by four water molecules. These data support the importance of six coordination. The only exception cited here is Be, an element which obeys the octet rule. 

While not a completed model of a single-strand sheet, Tew s extended sheet-like structures [75,76] lack only connections between individual strands to fit the definition. In this case, structures were assembled at an air-water interface, and controlled by amphiphilic patterning as well as TT-stacking. X-ray studies confirmed an organized sheet-like structure in aqueous solution indicating that the patterning of polar and non-polar functionality is a path to sheet formation [76b]. These strand-Uke stractures were shown to have biological activity similar to many sheet folded peptides [26]. 

Under the influence of water, cationic manganese compounds decompose immediately, as in organic donor solvents such as acetone and tetrahydrofuran. In contrast to isoelectronic compounds of the chromium group, diolefin technetium and rhenium complexes have cis structures. In aqueous solution, the cation [Re(CO)4 ( 2114)2] is stable the Re —C2H4 bond in [Re(CO)5 ( 2114)] is also stable. There is no exchange between free ethylene and the ethylene in the complex. 

Apart from the crystalline solids of clathrate hydrates, some cage structures in aqueous solutions around hydrophobic solutes have been observed by computer simulation [80-84], Therefore, it is important to investigate how the water-water interaction, and the tetrahedral hydrogen-bonded structure, is connected with the formation of clathrate hydrate structure when discussing the thermodynamic and structural properties of these aqueous solutions. 

---