Molecular hydration

Molecular as well as ionic substances can form hydrates, but of an entirely different nature. In these crystals, sometimes referred to as clathrates, a molecule (such as CFI4, CHCI3) is quite literally trapped in an ice-like cage of water molecules. Perhaps the best-known molecular hydrate is that of chlorine, which has the approximate composition Cl2- 7.3H20. This compound was discovered by the great... 

Molecular hydration in solution is described not only by the inner-sphere water molecules (first and second coordination spheres, see Section II.A.l) but also by solvent water molecules freely diffusing up to a distance of closest approach to the metal ion, d. The latter molecules are responsible for the so-called outer-sphere relaxation (83,84), which must be added to the paramagnetic enhancement of the solvent relaxation rates due to inner-sphere protons to obtain the total relaxation rate enhancement,... 

S. R. Gadre, M. M. Deshmukh, and R. P. Kalagi, Quantum chemical investigations on explicit molecular hydration, Proc. Indian Natl. Sci. Acad. 70, 709-724 (2004). 

Of particular interest in connexion with our subject is the case of compound formation by a macromolecular solid e. g., the hydrate formation by cellulose and gelatin or the formation of an addition compound between nitrocellulose and acetone. Whereas in ordinary low molecular hydrates the composition of the successive compounds X, X. HgO, X. 2HaO etc. differs considerably as regards the percentage of water and, hence, the Gibbs potentials of these compounds differ by considerable jumps, the situation is different in macromolecular substances. If e.g., each monomeric residue R of a molecule consisting of a chain of n residues can bind one water molecule, the following hydrates are possible ... 

When all of the variables that contribute to the diffusion coefficient are examined in unison, it is seen that temperature, molecular shape, solute concentration, molecular weight, molecular hydration, intrinsic viscosity of the sample, and mobile phase viscosity all play a role in the ability of a solute to diffuse readily into the pores of a support. 

Hydrates are solid structures composed of water molecules joined as crystals that have a system of cavities. The structure is stable only if at least one part of the cavities contains molecules of small molecular size. These molecules interact weakly with water molecules. Hydrates are not chemical compounds rather, they are clathrates . 

The SPC/E model approximates many-body effects m liquid water and corresponds to a molecular dipole moment of 2.35 Debye (D) compared to the actual dipole moment of 1.85 D for an isolated water molecule. The model reproduces the diflfiision coefficient and themiodynamics properties at ambient temperatures to within a few per cent, and the critical parameters (see below) are predicted to within 15%. The same model potential has been extended to include the interactions between ions and water by fitting the parameters to the hydration energies of small ion-water clusters. The parameters for the ion-water and water-water interactions in the SPC/E model are given in table A2.3.2. 

The relative acidities in the gas phase can be detennined from ab initio or molecular orbital calculations while differences in the free energies of hydration of the acids and the cations are obtained from FEP sunulations in which FIA and A are mutated into FIB and B A respectively. 

Israelachvili J N and Pashley R M 1983 Molecular layering of water at surfaces and origin of repulsive hydration forces Nature 306 249-50... 

Straatsma, T.P, Berendsen, H.J.C. Free energy of ionic hydration Analysis of a thermodynamic integration technique to evaluate free energy differences by molecular dynamics simulations. J. Chem. Phys. 89 (1988) 5876-5886. 

Tieleman, D.P., Berendsen, H.J.C. A molecular dynamics study of the pores formed by E. coli OmpF porin in a fully hydrated POPE bilayer. Biophys. J., in print (1998). 

A possible explanation of the hysteresis could be the non-equilibrium of the DNA hydration. In that case the value of hysteresis has to depend on the size of the experimental sample. However, such a dependence is not observed in the wide range of DNA film thicknesses (0.05-0.2 fmi) [14], [12]. Thus, hysteresis cannot be a macroscopic phenomenon and does reflect the molecular interaction of water and the biopolymer. 

D. Beglov and B. Roux. Dominant solvations effects from the primary shell of hydration Approximation for molecular dynamics simulations. Biopolymers, 35 171-178, 1994. 

Essex J W, M M Harm and W G Richards 1994. Molecular Dynamics of a Hydrated Phospholipi Bilayer. Philosophical Transactions of the Royal Society of London 8344 239-260. 

In the hope of having done away with these misunderstandings, we now address the molecular origin of the hydrophobic hydration as well as the hydrophobic interaction. Note that comprehension of hydrophobic hydration is a prerequisite for understanding hydrophobic interactions, since hydrophobic interactions always involve a (partial) reversal of the hydrophobic hydration. 

Page 396 of the March 2000 issue of the Journal of Chem ical Education outlines some molecular modeling exercises concerning the regioselectiv ity of alkene hydration... 

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