Hydration shell binding

At the limiting concentration the enhancer will bind all available water in hydration shells, leaving none to participate in the solution of the drug substance. 

The right-most term is similar to the familiar PDT formula except that the indicator function combinations forbid binding of solution molecules to the defined inner shell. That last factor is recognized as the Boltzmann factor of the hydration free energy that would result if inner-shell binding were prohibited. The Km are recognizable ratios of equilibrium concentrations - equilibrium constants - that are discussed... 

It is interesting to note that smaller ions (e.g., Na, Mg, Ca, Cl") form hydration shells larger than bigger ions, which tend to bind water molecules only very weakly. In a simple way, the salting out of nonpolar and weakly polar compounds was explained by Schwarzenbach et al. (2003) by imagining that the dissolved ions compete successfully with the organic compound for solvent molecules. The freedom of some water molecules to solvate an organic molecule depends on the type and concentration of salts. 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

Anions bind also to other metals, like gold, platinum, or silver [74,81], Why do anions adsorb specifically to metals, while cations do not The explanation is a strong hydration of cations. A cation would have to give up its hydration shell for an adsorption. This is energetically disadvantageous. Anions are barely hydrated and can therefore bind more easily to metals [82], Another possible explanation is the stronger van der Waals force between anions and metals. The binding of ions to metallic surfaces is not yet understood and even the idea that cations are not directly bound to the metal, was questioned based on molecular-dynamics simulations [83],... 

This seems quite satisfying, but interesting (and apparently anomalous) results have been observed by Marinelli and Squire and others concerning the energy of interaction of successive molecules as the hydration shell is built up in the gas phase. Thus, it would be expected that the first hydrating water would have the greatest heat of binding, because there are no other molecules present in the hydration shell with... 

The most inclnsive definition of hydration shell describes it as consisting of all thermodynamically altered water molecnles in the vicinity of a solnte. From a thermodynamic standpoint, hydration can be viewed as binding of water molecnles to the hydration sites of a solnte. The energetics of this association is modulated by the type of solute-solvent interactions (electrostatic, hydrogen bonding, van der Waals) and by solnte-indnced solvent reorganization. The latter occnrs even in the absence of appreciable solute-solvent interactions becanse the eqnUib-rium distribution of hydrogen-bonded water networks of the bulk becomes disrupted at the solute surface. 

---