Hydrogen bonding structure of water

The presence of alcohols in the aqueous medium generally decreases grafting. This is expected since the addition of alcohol breaks the tetrahedral hydrogen bonded structure of water and thus disturbs the association of active sites with water. This will lead to a decrease in grafting. In the presence of alcohols, chain... 

If the principal cohesive forces between solute molecules are London forces, then the best solvent is likely to be one that can mimic those forces. For example, a good solvent for nonpolar substances is the nonpolar liquid carbon disulfide, CS2-It is a far better solvent than water for sulfur because solid sulfur is a molecular solid of S8 molecules held together by London forces (Fig. 8.19). The sulfur molecules cannot penetrate into the strongly hydrogen-bonded structure of water, because they cannot replace those bonds with interactions of similar strength. 

The second approach that has been rather popular with mixed aqueous solvents is to assume that the mixture is more or less structured than that of pure water. There is much evidence to show that the particular hydrogen-bonded structure of water influences many of the properties of electrolytes in water (15). If nonelectrolytes can modify the structure of water (15), they can have an indirect effect on the properties of electrolytes. This explanation has been particularly successful in the case of U + W mixtures (1,2). Such a simple approach is not as successful with hydrophobic cosolvents. For example, AHe°(W — W + TBA) are positive for both alkali halides (16) and tetraalkylammonium bro-... 

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. 

Figure 9.27. Hydrogen-bonded structure of water in a slit-shaped pore (Carrott etai, 1991).

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... 

Suresh SJ (2(X)7) Disruption of hydrogen bond structure of water near charged electrode surfaces. J Chem Phys 126(20) 204705... 

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