Coulombic hydration

As typical electrolytes we have taken LiCl, NaCl, Me4NBr, and Bu4NBr NaCl is the most studied alkali halide, Bu4NBr is a well-known hydrophobic electrolyte, Li+ is more solvated than Na+ (coulombic hydration) but its structural hydration is usually smaller, and Me4NBr is a weak structure breaker. A comparison of these electrolytes should therefore give us some idea of the various interactions between different electrolytes and mixed solvents. 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

CMC hydrates rapidly and forms clear solutions. Viscosity buUding is the single most important property of CMC. DUute solutions of CMC exhibit stable viscosity because each polymer chain is hydrated, extended, and independent. The sodium carboxylate groups are highly hydrated, and the ceUulose molecule itself is hydrated. The ceUulose molecule is linear, and conversion of it into a polyanion (polycarboxylate) tends to keep it in an extended form by reason of coulombic repulsion. This same coulombic repulsion between the carboxylate anions prevents aggregation of the polymer chains. Solutions of CMC are either pseudoplastic or thixotropic, depending on the type. 

Within the hydration process in Eqn. (8.8), a spherical ion becomes a (hydrated) octahedral ion, [M(H20)6]. Part of the Coulomb energy of the free ion concerns repulsion and exchange terms within the d" configuration. This is replaced by equivalent repulsion and exchange terms within the configuration. Let us estimate the trends in these quantities separately. 

Within the first-order estimations made here, it is apparent that no change in d-d repulsion energy accompanies the hydration process. Second-order adjustments would, of course, take account of the change in mean i/-orbital radius on complex formation. Let us agree to stop at the simple level of correction here. Overall, therefore, the significant Coulombic change on hydration concerns the loss of exchange stabilization. 

Since the hydration energy changes with p are driven by changes of Coulombic repulsion, a linear relationship between -A, and the reciprocal distance between the charged centers may be suspected, since the Coulombic energy is proportional to the reciprocal of the distance. The quantity, p + 2, may be assumed to be approximately proportional to the distance between the charged centers. We arrive... 

The basic formalism of the X-dynamics method has taken various forms in its application to problems of interest. In an early prototype calculation to assess umbrella sampling in chemical coordinates, the X-dynamics method was used to evaluate the relative free energy of hydration for a set of small molecules which included both nonpolar (C2H6,) and polar (CH3OH, CH3SH, and CH3CN) solutes.1 By assigning a separate X variable to the Lennard-Jones and Coulomb interactions, a linear partition of the potential part of the hybrid Hamiltonian was constructed... 

The ion-water interactions are very strong Coulomb forces. As the hydrated ion approaches the solution/metal interface, the ion could be adsorbed on the metal surface. This adsorption may be accompanied by a partial loss of coordination shell water molecules, or the ion could keep its coordination shell upon adsorption. The behavior will be determined by the competition between the ion-water interactions and the ion-metal interactions. In some cases, a partial eharge transfer between the ion and the metal results in a strong bond, and we term this process chemisorption, in contrast to physisorption, which is much weaker and does not result in substantial modification of the ion s electronic structure. In some cases, one of the coordination shell molecules may be an adsorbed water molecule. hi this case, the ion does not lose part of the coordination shell, but some reorganization of the coordination shell molecules may occur in order to satisfy the constraint imposed by the metal surface, especially when it is charged. 

The electrolyte concentration is very important when it comes to discussing mechanisms of ion transport. Molar conductivity-concentration data show conductivity behaviour characteristic of ion association, even at very low salt concentrations (0.01 mol dm ). Vibrational spectra show that by increasing the salt concentration, there is a change in the environment of the ions due to coulomb interactions. In fact, many polymer electrolyte systems are studied at concentrations greatly in excess of 1.0 mol dm (corresponding to ether oxygen to cation ratios of less than 20 1) and charge transport in such systems may have more in common with that of molten salt hydrates or coulomb fluids. However, it is unlikely that any of the models discussed here will offer a unique description of ion transport in a dynamic polymer electrolyte host. Models which have been used or developed to describe ion transport in polymer electrolytes are outlined below. 

Water can interact with ionic or polar substances and may destroy their crystal lattices. Since the resulting hydrated ions are more stable than the crystal lattice, solvation results. Water has a very high dielectric constant (80 Debye units [D] versus 21 D for acetone), which counteracts the electrostatic attraction of ions, thus favoring further hydration. The dielectric constant of a medium can be defined as a dimensionless ratio of forces the force acting between two charges in a vacuum and the force between the same two charges in the medium or solvent. According to Coulomb s law. 

The size assigned to the core and layer regions is an important consideration for such simulations. The size of the solute determines the minimum radius of the core region, but it is often necessary to include the first hydration layer as well, for instance when hydrated ions are studied. The distance of all particles included in the QM core to the QM/MM interface should exceed the non-Coulombic cutoff distance of all involved interactions. In the case of water the minimum size of the layer region is 2.5-3.0 A. 

From the examples given above, it is apparent that water solubility of a polysaccharide can be instilled or improved by placing substituents on the linear structure which reduce the fit of one polysaccharide molecule to another or by providing anionic groups which can improve hydration and present ionic charges which by coulombic repulsion aid in molecular separation. Cellulose is a good example for illustrating such effects it has been extensively studied because of its industrial availability at low cost (14). 

Cellulose Sulfates and Phosphates. Conversion of cellulose to sulfate or phosphate monoesters produces soluble derivatives. These ester groups are highly hydrated, offer steric interference to molecular fit, and are ionized at all PH levels so they continually produce coulombic repulsion. 

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