Water near ions

F igure 14.1. A schematic illustration of the arrangement of water moleeules around a small ion. The figure is reprodueed from ht //www.porous-35.com/ electrochemistry-semieonductors-4.html 

Depending on their relative abilities to induce stmctural changes in water, ions have often been classified as structure-makers, termed kosmotropes, and structure-breakers, termed chaotropes. These are of course qualitative or pictorial terms and need to be quantified. As in other non-ideal solutions, this is done through the concentration dependence of the viscosity of the aqueous solution, which in the case of electrolytes can be described by the following expansion in concentration c, 

In some sense, therefore, small monovalent and divalent ions can he regarded as hydrophilic witile large monovalent ions are hydrophobic. For large monovalent ions, the enthalpic gain is less than the entropic loss of free energy, as discussed below. 

The concentration dependence of ionic conductivity needs to be understood from a molecular viewpoint [4]. In this approach, a time correlation function representation of viscosity derived long ago by Green and Kubo was used along with a molecular-level description of the equilibrium correlation between the positions of ions and water 

As indicated earlier, when the size of the monovalent ion becomes much larger than the size of a water molecule, then the ion starts to interfere with the HB network of liquid water. In addition, the solvation energy due to ion-solvent interaction decreases. According to Bom s expression, this energy decreases with size rio as 1/rion- Thus, beyond a certain size, the entropic loss to the system due to the size of a large ion becomes greater than the enthalpic stabilization due to the ion-solvent interaction. Thus, the ion can behave as a hydrophobic solute. 

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Spectroscopic methods, molten salts, 702 Spectroscopy detection of stmctnral nnits in liquid silicates, 747 and structure near an ion, 72 Standard partial gram ionic entropies, absolute, II Thermodynamics, applied to heats of solvation, 51 of ions in solution, 55 Time average positions of water near ions. 163 Tools, for investigating solvation, 50 Transformation, chemical, involving electrons, 8 Transition metals... 

In these early reactions the reactivities of the individual phases are important in determining the overall reaction rate. However, as the cement particles become more densely coated with reaction products, diffusion of water and ions in solution becomes increasingly impeded. The reactions then become diffusion-controUed at some time depending on various factors such as temperature and water—cement ratio. After about 1 or 2 days, ie, at ca 40% of complete reaction, the remaining unhydrated cement phases react more nearly uniformly. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

The impedance factor is strictly empirical, accounting primarily for the geometry of the soil pore network bnt also for ion exclusion by negative adsorption from narrow pores, and for the increased viscosity of water near charged surfaces. It is similar for all simple ions and molecules. It can be measured by following the self diffusion of a nonadsorbed ion, such as Cl , for which C = 0lCl and hence D =... 

Ultrafast motion of water molecules near ions... 

McKim and Erickson (1991) proposed three explanations to explain this phenomena. First, it is possible that, when the non-ionic chemical is absorbed at the gill surface, rapid acid-base equilibration results in the transformation of ionized compound into non-ion-ized chemical which can be further absorbed. Second, respiration causes the pH in the gill water near the gills to drop, causing more of the ionic chemical to be converted to the nonionic form. Third, the chemical in the ionic form may be taken up through the gills but at a much lower rate than the non-ionic form. McKim and Erickson (1991) showed that incorporating any of these three scenarios in Equation (4) improves estimates of the chemical bioavailability of weak acids in fish. 

The xanthate method has been used in the U.S. and Europe for a number of larger scale pilot plant studies. In the case of rayon, the technique has been explored for flameproofing, high water sorbency, ion exchange characteristics and bacteriostatic and fungistatic properties. Also nonwoven grafted fibers with excellent dispersibility for wet processing and improved binder affinities have been produced. It does not appear, however, that industrial exploitation of these technical successes will take place in the near future. 

The other procedure used in the present work for estimating absolute enthalpies of hydration of ions is empirical, although the approach may have some theoretical justification on the grounds of the Born—Bjernim equation (46, 47) if appropriate allowance is made for the fact that the effective dielectric constant of water near an ion is less than that in bulk water (47—52). 

Metal Ion Effects. The metal ion effects on the acid-catalyzed hydrolysis of PPS also were examined by Benkovic and Hevey (5). However, they observed that in water near pH 3, the rate enhancement in the presence of an excess of metal ion was at most only threefold (Mg2+, Ca2+, Al3+) and in some cases (Zn2+, Co2+, Cu2+) the rate was actually retarded. We thought that the substrate PPS and Mg2+ ion should be hydrated heavily in water so that their complexa-tion for rate enhancement is weak. If, however, the hydrolysis is carried out in a solvent of low water content, such complexation would not occur, and therefore, the rate enhancement might be more pronounced. This possibility appears to be supported by the fact that the active sites of many enzymes are hydrophobic. Of course, there is a possibility that the S—O fission may not require metal ion activation. In this connection, it is interesting to note that in biological phosphoryl-transfer reactions the enzymes generally require divalent metal ions for activity (7, 8, 9), but such metal ion dependency appears to be less important for sulfate-transfer enzymes. For example, many phosphatases require metal ions, but no sulfatase is known to be metal... 

There are some qualitative difficulties when the specific ion effects are explained via the dispersion forces of the ions. Particularly the anions, for which the dispersion coefficients / , are large, affect the double layer interactions. However, experiments on colloid stability [6] or colloidal forces [11] revealed strong specific ion effects especially for cations. Furthermore, the ions which affect most strongly the solvating properties of the proteins are those from their vicinity, since they perturb mostly the structure of water near the proteins. However, the van der Waals interactions of ions predict that the cations remain in the vicinity of an interface, and the anions are strongly repelled, while Hofmeister concluded that anions are mainly responsible for the salting out of proteins. 

A simple model that illustrated the behavior of the polarization when the water molecules are organized in water layers between perfectly flat surfaces was previously suggested.13 That model took into account the nearest-neighbor dipole interactions, but ignored the surface charges and the electrolyte ions. The model is extended here to cases in which an electrolyte as well as surface charges are also present. It will be shown that a treatment of all electrostatic interactions, in the assumption of an icelike structuring of water near interfaces, can predict an oscillatory behavior for both the polarization and the electric potential as well as a nonproportionality between the polarization and the electric fields. 

The limitations of this simplified model have been immediately recognized, and the first criticism [3] even preceded the full development of the DLVO theory. Since then many improvements of the theory have been proposed, to account for the finite size of the ions [4], image forces [5], dielectric corrections [6], ion correlations [7], ion-dispersion [8] and ion-hydration forces [9], to name only a few. Despite the many corrections brought to the traditional DLVO theory, there are some experiments, such as those regarding the stability of neutral lipid multilayers, which could still not be explained within this framework. It is therefore commonly accepted that an additional repulsion occurs when two surfaces approach each other at a distance shorter than a few nanometers. Because this force was initially related to the structuring of water near surfaces, it is commonly named hydration force [10]. 

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