Water structural temperature

Walrafen G E 1967 Raman spectral studies of the effects of temperature on water structure J. Chem. Phys. 47 114-26... 

It will be recalled that in Fig. 28 we found that for the most mobile ions the mobility has the smallest temperature coefficient. If any species of ion in aqueous solution at room temperature causes a local loosening of the water structure, the solvent in the co-sphere of each ion will have a viscosity smaller than that of the normal solvent. A solute in which both anions and cations are of this type will have in (160) a negative viscosity //-coefficient. At the same time the local loosening of the water structure will permit a more lively Brownian motion than the ion would otherwise have at this temperature. Normally a certain rise of temperature would be needed to produce an equal loosening of the water structure. If, in the co-sphere of any species of ion, there exists already at a low temperature a certain loosening of the water structure, the mobility of this ion is likely to have an abnormally small temperature coefficient, as pointed out in Sec. 34. 

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. 

For most organic chemicals the solubility is reported at a defined temperature in distilled water. For substances which dissociate (e.g., phenols, carboxylic acids and amines) it is essential to report the pH of the determination because the extent of dissociation affects the solubility. It is common to maintain the desired pH by buffering with an appropriate electrolyte mixture. This raises the complication that the presence of electrolytes modifies the water structure and changes the solubility. The effect is usually salting-out. For example, many hydrocarbons have solubilities in seawater about 75% of their solubilities in distilled water. Care must thus be taken to interpret and use reported data properly when electrolytes are present. 

Usually, dissolution of a small amount of one compound in a pure liquid is enthalpically unfavourable and driven by an increase in (mixing) entropy. At room temperature, the opposite is true for the dissolution of a small apolar compound in water. This unexpected behaviour is referred to as the hydrophobic effect [4]. Classically, this effect has been rationalised by ordered water structures around apolar compounds (entropy reduction) and the increase in number... 

Free energy variations with temperature can also be used to estimate reaction enthalpies. However, few studies devoted to the temperature dependence of adsorption phenomena have been published. In one such study of potassium octyl hydroxamate adsorption on barite, calcite and bastnaesite, it was observed that adsorption increased markedly with temperature, which suggested the enthalpies were endothermic (26). The resulting large positive entropies were attributed to loosening of ordered water structure, both at the mineral surface and in the solvent surrounding octyl hydroxamate ions during the adsorption process, as well as hydrophobic chain association effects. 

VPIE s of H20/D20 and LV fractionation factors for H20/H0D and H20/H2180 have been carefully measured and thoroughly interpreted over the complete coexistence range. Data for H20/T20 and intermediate isotopomer pairs are limited to lower temperatures. Liquid molar density IE data are complete for H20/D20. Departures from the law of geometric mean are small and the liquid molar density IE for H20/H0D is available to good precision. At low temperature, ln(p7p) for H20/D20 (and presumably for the other water isotopomer pairs) shows a minimum which has been ascribed to H-bonding ( water-structure effects ). 

Miki, H., Hayashi, S., Kikura, H., and Hamaguchi, H., Raman spectra indicative of unusual water structure in crystals formed from a room-temperature ionic liquid, J. Raman Spectres., 37,1242-1243,2006. 

This estimate of the lifetime of the excited state resulting from the charge transfer described here results from seeing aprincipal process in the deexcitation as the rotation of a water molecule (originally attached to the proton) away from the position in the first layer next to the electrode from which the proton transfer from H30+ occurred. The rotating rate ofa free molecule is 109 s-1,but in solution there will be a hindrance to such a motion by the tendency to re-form H bonds and become part of the water structure. There is some evidence that the potential energy barrier for hindered rotation in this situation is quite low, about 6 kJ mol-1. Accepting this reduces the rotation rate to 10 e =10 at room temperature = 10 s, i.e., 10 s. 

As further evidence of the importance of hydrophobic interactions in these systems, we examined the partition coefficient of methyl orange in the presence of water structure-forming and water structure-breaking salts above and below the transition temperature [70], Methyl orange is an easily detected, hydro-phobic dye which has been sulfonated to improve water solubility. Water structure-breaking salts like tetraethylammonium chloride (TEAC) are known to minimize hydrophobic interactions while water structure-forming salts like ammonium sulfate are known to increase hydrophobic interactions [165, 166]. 

Selected compound-specific functions, property-temperature-property relationships, or structure-temperature-property relationships are supplied and discussed in this book for density (Section 3.5), refractive index (Section 4.5), surface tension (Section 5.4), viscosity (Section 6.4), vapor pressure (Section 7.4), enthalpy of vaporization (Section 8.5), aqueous solubility (Section 11.8), and air-water partition coefficients (Section 12.5). 

Frontas ev (59) observed an anomalous temperature dependence in the thermal conductivity of water around 30°-40°C. (Figure 2). (In this illustration the data points are those given by Frontasev, but I believe the curve shown gives a reasonable fit to the experimental data.) He stated specifically that an anomaly existed near 30°-40°C. and that it implies a fundamental modification in water structure in this temperature range. 

It is appropriate at this point to make a few comments about the importance of the observed thermal anomalies in connection with the theories of water structure mentioned above. If the reality of the thermal anomalies is accepted, the ultimate theory of water structure must be able to allow for the existence of these anomalies and, hopefully, eventually predict their existence. If the thermal anomalies do indeed manifest higher-order phase transitions, structured elements of a certain size must be present in water. In other words, the uniformists , average structural models must definitely be ruled out. Furthermore, noting that the anomalies tend to center around discrete temperatures and apparently are completed over a few degrees, we concluded that if they do manifest... 

Safford and Naumann (128) have shown that the time-of-flight spectra for 4.6M solutions of KF, KC1, CsCl, NaCl, and LiCl show peaks in the inelastic scattering region which coincide both in frequency and shape with the ice-like (structured) frequencies of pure water. Also, solutions of KSCN, KI, KBr, and NaC104 have lattice frequencies where they are found for water although in these cases apparently with less resolution and less intensity. Even an 18.5-M solution of KSCN showed a similar behavior. We take this to suggest that elements of water structure remain in these solutions (as discussed elsewhere in this paper, where we noted that the thermal anomalies occur at approximately the same temperatures, even for relatively concentrated solutions, as where they occur in pure water see also Ref. 103). 

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