The Hydrogen Bonded Structure of Water

Contrary to expectations from the open tetrahedral hydrogen-bonded structure of ice, liquid water is a close-packed rather than an open liquid. It shares this property with 1,2-ethanediol, glycerol, and formamide, among common solvents, see Table 1.5. These four liquids have (1 — Vx/V) 0.1, whCTeas most otho common solvents for ions have larger values of this quantity. Liquids that have large fractions of free volume, i.e., are open , are quite compressible, and there exists a moderate positive correlation of (1 — Vx /1 ) with the isothermal compressibility. Water shares a low isothermal compressibility, kj = 0.457 GPa at 25 °C, with the above named low-openness solvents for which kj 0.5 GPa , whereas for other common solvents the values range from 0.524 (dimethylsulfoxide) to 1.706 (n-hexane) (Marcus 1998). 

Over the temperature range of the existence of water as a liquid at the standard pressure P° = 0.1 MPa, i.e., at 0 °C t 100°C, the structuredness of water diminishes with rising temperatures, as is shown in Table 1.6 (Marcus 2009). This feature, common to other solvents as well, is due to the increased order-destroying 

With rising pressures at a given temperature water compresses and its density increases, so that its openness diminishes. At 25 °C but under 100 bar (10 MPa) pressure (1 - Vx/V) = 0.0743 compared with 0.0757 at ambient pressure. It decreases to 0.0578 at 500 bar and to 0.0402 at 1000 bar (0.1 GPa). The free volume practically vanishes at very high pressures ( 2.4 GPa at 25 °C), water being then completely compressed. As will be discussed in Chap. 2, such pressures are prevailing in the vicinity of ions, due to their extremely large electric fields. 

According to this two-state model, at the ambient pressure of 1.01325 bar ( 0.1 MPa) the fraction of the bulky state decreases from 0.4746 at 0 °C via 0.3855 at 25 °C to 0.2705 at 70 °C, its corresponding molar volumes being 20.017, 20.242, and 20.645 cm mol The fraction of the dense state makes up the rest to unity and the molar volumes of the dense state are 16.112,16.705, and 17.594 cm mol at the respective temperatures (Cho et al. 2002). It was expected that other forms of broken down hydrogen bonded structures are present above 70 °C. At higher pressures the fraction of the bulky state is reduced, of course, the more the lower the temperature. The molar volumes of the two states also diminish with higher pressures, the more so for the bulky state than for the dense state, as expected, and for the latter being more sensitive to the temperature. The temperature and pressure variation of other bulk properties of water, such as the viscosity and the refractive index, could also be interpreted in terms of the two-state model and the respective fractions of these two states. 

Rull (2002) recently provided Raman spectroscopic evidence supporting the mixture model, a major fraction consisting of domains with linear HBs in a tetrahedral like configuration, the other of interstitial molecules, with either bifurcated or else weak or no HBs. Soper (2010) commented on the two-state model that the different domains must be very short lived, in view of the rapid diffusion of the water molecules, one of them moving over 150 molecular diameters away in 1 ms. 

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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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