The Hydrogen-Bonding Structure of Water

The molal lowering of nouelectiolyles is illustrated by sucrose and H202. These enter so easily into the hydrogen bonded structure of water that they give the theoretical lowering, 1.86° up to 0.1 M in the case of sucrose and to 10 M by H2O2. 

P. Jedlovsky, J.P. Brodholt, F. Bruni, M.A. Ricci, A.K. Soper and R. Vallauri, An ysis of the hydrogen-bonded structure of water from ambient to supercritical conditions, J. Chem. Phys., 108 (1998) 8528-8540. 

Early studies were carried out at the liquid gas interface [22, 23]. Castro et al. [24] studied the adsorption of / -propyl-phenol from aqueous solutions at the air interface as a function of phenol concentration in the bulk. They showed that the square root of the second-harmonic intensity plotted against bulk phenol concentration followed a Langmuir isotherm with a standard Gibbs energy of adsorption equal to -24.3 kJmol Similar results were obtained for other alkylphenols and alkylanilines. In other work with phenols, the orientation of phenol at the water air interface was determined by studying the phase of the xfl component of the susceptibility. As expected, the OH was oriented toward the water phase [25] so that it could participate in the hydrogen-bonded structure of water. The same conclusion was reached for / -bromophenol and -nitrophenol. 

By the 1970s, larger computers permitted the statistical mechanical treatment of molecules with complicated (other than spherical) potentials. By using potentials similar to MM2, molecular dynamics and Monte Carlo methods were developed, and calculations could be carried out on whole assemblies of molecules. A successful simulation of the molecular dynamics of water by Rahman and Stillinger allowed the calculation of properties such as dielectric constants. The hydrogen bonding structure of water was finally revealed. Thus, some early approximate developments had begun to pay off. 

The hydrophobic effect is an aggregate phenomenon, distinguished from all other molecular forces (e.g., covalent, ionic, dipolar, hydrogen bonding, 7i interactions, van der Waals, and London forces) in that it arises from the collective behavior of many molecules by disruption of the hydrogen-bonded structure of water. 

In aqueous systems there is also an indirect type of interaction between nonpolar groups, which is related to the hydrogen-bonded structure of water itself. 

Experimentally measurable quantities, such as solubilities, EMF data, etc., yield A/us° of Eq. (3.23) and the sublimation enthalpies of the D2O and H2O ices yield A °6hb values. Hence, AGhb, the effect of the solute S on the (hydrogen bonded) structure of water, can be determined. Non-ionic solutes, such as argon or methane, have positive values of AGhb (Ben-Naim 1975) and are known from several approaches to enhance the structure of water, diminishing with increasing temperatures. This is expected from the structure of water being diminished in this direction (Table 1.6). 

Chaotropic The property of being able to disrupt the hydrogen bonding structure of water. Substances that are good hydrogen bonders, such as urea or guanidine hydrochloride, are chaotropic. Concentrated solutions of these substances tend to denature proteins because they reduce the hydrophobic effect. 

Jedlovszky, P. Brodholt, J. P. Bruni, F. Ricci, M. A. Soper, A. K. Vallauri, R. (1998) Analysis of the Hydrogen-bonded Structure of Water from Ambient to Supercritical Conditions, Journal of Chemical Physics 108, 8528-8540... 

Stellwagen E, Dong Q, Stellwagen NC (2005) Monovalent cations affect the free solution mobility of DNA by perturbing the hydrogen-bonded structure of water. Biopolymers 78 62-68... 

Tlie carboxylate salts of fatty acids have long, nonpolar, hydrocarbon chains. Therefore, they do not form solutions of individual ions, but are dispersed as weakly associated structures called micelles, which are spherical aggregations of molecules or ions. In a micelle of carboxylate salts, the nonpolar hydrocarbon chains occupy the interior of the sphere, and the polar carboxylate heads lie on the surface of the sphere. This spherical arrangement encloses the maximum amount of hydrocarbon material in the smallest surface area. Therefore, a micelle disrupts the hydrogen-bonded structure of water to the smallest extent possible. 

Fig. 4. A sketch of the hydrogen-bonded structure of water in a slit-shaped pore. The circles represent the positions of the oxygen atoms.

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