Behavior Of Liquid Water Research Articles

Geometric relaxation in water. Its role in hydrophobic hydration

The thermodynamic quantities ΔG°, ΔH°, ΔS° and ΔC°p associated with the solvation of argon in water and aqueous mixtures are reinterpreted on the basis of two contributions. The first is related to the hydrogen-bonding connectivity of water and is assumed to be approximately represented by corresponding thermodynamic quantities in solvents such as hydrazine and ethylene glycol; following a proposal by Lumry and Frank [R. Lumry and H. S. Frank, Proc. 6th Int. Biophys. Congr.(1978), vol. 7, p. 554; R. Lumry, in Bioenergetics and Thermodynamics; Model Systems, ed. Y. Braibanti (Reidel, Dordrecht, 1980), p. 405] this contribution determines the free energy of hydrophobic hydration and is dominated by a positive enthalpy. The second contribution, which is responsible for marked enthalpy–entropy compensation and large heat-capacity effects in argon hydration, is assigned to the characteristic fluctuation behaviour of liquid water. This assignment is substantiated by comparisons of the bulk properties of water and various other liquids, and a model is suggested to rationalize the uncommon thermodynamic properties of water, aqueous mixtures and solutions of hydrophobic solutes. The phenomenological representation proposed is an overlay of a randomly connected H-bond network and a local fluctuation process defined in terms of a minimum cooperative unit. This process, labelled “geometric relaxation”, is pictured as a cooperative H-bond rearrangement involving a water molecule coordinated by four neighbours. The limiting microstates of the cooperative units are “short-bond” forms (with short, stiff, near-linear bonds) and “long-bond” forms (with long, weak, bent hydrogen bonds). The first is dominated by the low enthalpy of short hydrogen bonds and the second by the high entropy resulting from motions of the water molecules on flexible hydrogen bonds into the free volume not available to the “short-bond” form.

Study of liquid water by computer simulations. I. Static properties of a 2D model

A computer-simulation study of a water-like system is carried out by making use of a two-dimensional version of the Ben-Naim and Stillinger potential. The pair potential is set up such that at 0 K it yields a square net structure at low pressures and an interpretation of two square nets at high pressures. The liquid state is surveyed over a wide range of temperature and pressure. Various kinds of molecular distribution functions are derived to see how the hydrogen-bond network structure depends on temperature and density. The pressure and thermal equations of state are ’’experimentally’’ determined by a least square fitting to the pressures and energies calculated for about 200 different state points. The well-known anomalous behavior of liquid water is reproduced at least in a semiquantitative way. The singular properties of supercooled water also are reproduced and their origin is ascribed to the thermodynamical instability. New anomalies are predicted at high temperatures and pressures.

Interpretation of the unusual behavior of H2O and D2O at low temperatures: Tests of a percolation model

The unusual low-temperature behavior of liquid water is interpreted using a simple model based upon connectivity concepts from correlated-site percolation theory. Emphasis is placed on examining the physical implications of the continuous hydrogen-bonded network (or ’’gel’’) formed by water molecules. Each water molecule A is assigned to one of five species based on the number of ’’intact bonds’’ (the number of other molecules whose interaction energy with A is stronger than some cutoff VHB). It is demonstrated that in the present model the spatial positions of the various species are not randomly distributed but rather are correlated. In particular, it is seen that the infinite hydrogen-bonded network contains tiny ’’patches’’ of four-bonded molecules. Well-defined predictions based upon the putative presence of these tiny patches are developed. In particular, we predict the detailed dependence upon (a) temperature, (b) dilution with the isotope D2O, (c) hydrostatic pressure greater than atmospheric, and (d) ’’patch-breaking impurities’’—for four separate response functions, (i) the isothermal compressibility KT, (ii) the constant-pressure specific heat CP, (iii) the constant-volume specific heat CV, and (iv) the thermal expansivity αP, as well as for dynamic properties such as (v) the transport coefficients self-diffusivity Ds and shear viscosity η, (vi) the characteristic rotational relaxation time τch, and (vii) the Angell singularity temperature Ts. The experimentally observed dependence of these seven quantities upon the four parameters (a)–(d) is found in all cases to agree with the predicted behavior. The paradoxical behavior associated with the absence of a glass transition in pure liquid water is also resolved. Finally, we propose certain experiments and computer simulations—some of which are underway—designed to put the proposed percolation model to better tests than presently possible using available information.

Are concepts of percolation and gelation of possible relevance to the behaviour of water at very low temperatures?

Abstract It is proposed that the unusual low-temperature behavior of liquid water may be interpreted using a simple model based upon connectivity concepts from correlated-site percolation theory. Emphasis is placed on examining the physical implications of the continuous hydrogen-bonded network (or “gel”) formed by water molecules. Each water molecule A is assigned to one of five species based on the number of “intact bonds” (the number of other molecules whose interaction energy with A is stronger than some cutoff VHB). It is demonstrated that in the present model the spatial positions of the various species are not randomly distributed but rather are correlated. In particular it is seen that the infinite hydrogen-bonded network contains tiny “patches” of four-bonded molecules. Predictions based upon the putative presence of these tiny patches for KT, CP, and αP are developed as examples.

Adsorption on a microporous adsorbent along the liquid-vapor equilibrium curve (NaX zeolite-water)

1. Description is given of apparatus and experimental techniques which can be used for liquid adsorption on the liquid-vapor equilibrium curve over the interval from 230 to 480‡K and at pressures ranging up to 40 bar. 2. The measured adsorption of water on NaX zeolite was higher than the values obtained from the usual type of calculation. The maximum difference between the calculated and measured values was of the order of 15%. 3. Calculated by various methods, the thermal coefficient of adsorption was radically different from the measured value. 4. The mean density of water adsorbed on NaX zeolite at temperatures ranging from 273 to 473‡K was greater than the density of liquid water. 5. The curve showing the temperature variation of the density of adsorbed water is free of the maximum which appears on the corresponding curve for liquid water. The disappearance of the maximum is clearly the result of steric hindrances and is consistent with the behavior of liquid water at pressures in excess of 1 kbar.

Structure at the Free Surface of Water and Aqueous Electrolyte Solutions

Abstract During the past 20 years or so one of the regions of conspicuous growth in the field of physical chemistry has been the study of the structure and behaviour of water and aqueous solutions. There are practical reasons, for example technical and biological, for this interest, but it is also true that the complexity of water as a liquid provides its own motive to the rcsearch worker. It is unlikely that we would spend so much time in the study of water if it were as simple a liquid as Argon. However, strange though the behaviour of liquid water is, it is probably not as strange as it has sometimes been thought to be. The thermal “anomalies” of water and the abnormal “Poly-water” Seem rather likely to fade out of the scientific scene, as have other stimulating but nonviable scientific myths.

Effect of temperature on the hydration of hydrophobic lons. Apparent molal volumes and heat capacities of Bu4PBr, Ph4PBr, and NaBPh4 in aqueous solutions at various temperatures

The density and specific heat of aqueous solutions of Bu4PBr, Ph4PBr, and NaBPh4 have been measured at various temperatures between 10 and 45°C. The derived limiting apparent molal volumes (φvo) and heat capacities (φco) have been examined for specific effects in the hydration of alkyl- and aryl-substituted quaternary ions, using also similar results on the tetraalkylammonium series. While φvo and φco of Bu4PBr and Bu4NBr differ only in magnitude by a few percent, the properties of Ph4PBr are sharply distinct, especially φco. NaBPh4 also exhibits a distinct behavior in the temperature dependence of its φco; the effect, however, is much less pronounced than that previously reported by other investigators. Within the data presently available, φco of hydrophobic ions appears rather invariant with temperature; for the compounds studied here, the relative change in φco with temperature is significantly smaller than the relative change in φvo. These results are correlated with spectroscopic data reported earlier, using a phenomenological description of the two-state behavior of liquid water. The latter is used to calculate the solvent relaxational contribution to the heat capacity of water and to φco of the hydrophobic ions.

Statistical Mechanical Study of Hydrophobic Interaction. I. Interaction between Two Identical Nonpolar Solute Particles

The tendency of two nonpolar particles to adhere to each other in aqueous environment is examined within the framework of classical statistical mechanics. The system under investigation consists of N solvent molecules and two simple solute particles at fixed positions R1 and R2. The Helmholtz free energy for such a system is split into three terms: AN+2(R1, R2) = A0 + U12(R1, R2) + AHI(R1, R2). The main problem of the article is formulated in terms of the function AHI(R12). The hydrophobic interaction is defined as the indirect part of the work [AHI(R12 = ∞) − AHI(R12)] associated with the process of bringing the two solute particles from infinity to the distance R12. Three different estimates of the strength of the hydrophobic interaction for various solute–solute distances are discussed. A quantity, based on available experimental data, is suggested to serve as a simple and practical index for comparing the hydrophobic interaction in various media. Using this quantity, the unique behavior of liquid water with regard to hydrophobic interaction is established.

Analysis of the transient boiling of liquid metals

In the course of fast reactor safety studies various problems associated with the transient boiling of liquid metals have arisen. Some of these of general interest, associated with nucleation, bubble growth and channel voiding, are discussed here. The differences between the behaviour of liquid metals and water are emphasized, and in particular the effects of dissolved gas and ionizing radiation are dealt with. An approximate method of calculating the rate of growth of a vapour bubble is described, which includes the effects of inertia, heat transfer and surface tension. This is applied to the voiding by boiling of a blocked reactor coolant channel.

Liquid water in frozen tissue: study by nuclear magnetic resonance.

Nuclear magnetic resonance (NMR) spectroscopy was used to examine the behavior and extent of liquid water in postrigor-frozen tissue of cod at temperatures below 0 degrees C. A liquid-water phase persists in the tissue down to about -70 degrees C; the extent of the phase decreases rapidly between 0 degrees and -10 degrees C and slowly at lower temperatures. That the NMR absorption peak of the liquid water increases in width, with decreasing temperature, suggests loss of mobility or structuring of the phase. A technique for introducing geometrically uniform cores of muscle into the probe of the high-resolution spectrometer permits quantitative determinations of liquid water.