Hydration shell thermodynamic properties

The thermodynamic properties of the hydradon shell (Table VIII) show it to be slightly, but not strongly, different from the bulk water. The free-energy difference is only 0.5 kcal/mol of water, slighdy less than the ambient thermal energy. The heat capacity, enthalpy, and volume changes associated with hydration are 10—15% of the bulk water values. 

The most expensive part of a simulation of a system with explicit solvent is the computation of the long-range interactions because this scales as Consequently, a model that represents the solvent properties implicitly will considerably reduce the number of degrees of freedom of the system and thus also the computational cost. A variety of implicit water models has been developed for molecular simulations [56-60]. Explicit solvent can be replaced by a dipole-lattice model representation [60] or a continuum Poisson-Boltzmann approach [61], or less accurately, by a generalised Bom (GB) method [62] or semi-empirical model based on solvent accessible surface area [59]. Thermodynamic properties can often be well represented by such models, but dynamic properties suffer from the implicit representation. The molecular nature of the first hydration shell is important for some systems, and consequently, mixed models have been proposed, in which the solute is immersed in an explicit solvent sphere or shell surrounded by an implicit solvent continuum. A boundary potential is added that takes into account the influence of the van der Waals and the electrostatic interactions [63-67]. 

In this part of the paper we examine the thermodynamic properties of hydrated ionomers. By strongly hydrated we mean that we are beyond the state of solvation shells, where V, the number of water molecules per cation, is a small number ("v A to 6). In a strongly hydrated sample, the water molecules are considered to be free, and make a concentrated solution with the cations (the counter ions) and eventually with some mobile anions (the coions). This subject has already been extensively studied because of its practical importance (1-4). From the following discussion, we shall see that some of the usual classical laws are no longer valid. For instance, the variation of the chemical potential of water with the concentration of cations may no longer hold. 

The interplay between ion adsorption and ion hydration, and the relation between thermodynamic quantities like the free energy of adsorption and microscopic structure, characterized, e.g., by hydration shell properties and hole formation, has been elucidated. 

Aqueous Solvation.—A review, covering the 1968—1972 publications, deals with physical properties, thermodynamics, and structures of non-aqueous and aqueous-non-aqueous solutions of electrolytes, and complete hydration limits. Thermodynamic aspects of ionic hydration also reviewed include the thermodynamic theory of solvation the molecular interpretation of ionic hydration hydration of gaseous ions (AG s, H s, and AA s) thermodynamic properties of ions at infinite dilution in water, solvent isotope effect in hydration reference solvents and ionic hydration and excess properties. A third review on the hydration of ions emphasizes the structure of water in the gaseous, liquid, and solid states the size of ions and the hydration numbers of ions and the structure of the hydrated shell from measurements of mobility, compressibility, activity, and from n.m.r. spectra. Pure water and aqueous LiCl at concentrations up to saturation have been examined by neutron and X-ray diffraction. For the neutron studies LiCl and D2O are employed. The data are consistent with a simple model involving only... 

In several systems, interfacial water, which is associated with the hydrophilic surfaces (polar groups and counterions) of surfactant microstructures, is present. This kind of water is also called bound water, hydration shell, hydration water, solvent shell [182], or vicinal water [171]. This water can be operationally defined as water detected by a certain technique as it had been influenced by the surface of the substrate in contact with the water [177]. The presence of the microstructure surface may alter the thermodynamic properties (such as melting point, melting enthalpy and entropy, and heat capacity) and the spectroscopic properties (such as IR absorption frequencies and band shapes) of water [61,214]. The chemical potential of bound water is different from that of bulk water [216]. Properties of bound water (viscosity, density, fl-eezing point, etc.) adsorbed on different surfaces of adsorbents differ from those of bulk water [216-223]. 

Another aspect of these problems is the effect of ions existing in the electrolytic solutions that constitute the fluid component of the cytoplasm. These ions can affect the conformation, interactions and biochemical functions of molecules in the cell. The Hofmeister series, which was first noted in 1888 [95], is invoked in this more modem context. It ranks the relative influence of ions on the physical behaviour of a wide variety of aqueous processes ranging from colloidal assembly to protein folding. The influence of an ion on the properties of macromolecules was initially thought to arise, at least in part, from its capacity of modifying bulk water stmcture. However, recent time-resolved and thermodynamic studies of water molecules in salt solutions show that bulk water structure is not central to the Hofmeister effect. Models are now being developed that take into account direct interactions between ion and macromolecule, and the interactions with water molecules that are operative in the first hydration shell of the macromolecule. 

So far we have not precisely defined the term protein . For the statistical thermodynamic treatment we differentiate between protein and bulk buffer. In a thermodynamic sense the term protein or protein system is meant to refer to the polypeptide chain plus the hydration shell. The hydration shell is considered to have physical properties different from those of the bulk buffer . The term bulk buffer refers to those parts of the protein solution that have the same physical properties as the pure buffer without protein chains. The dimensions of the hydration shell are generally assumed to be in the order of one monolayer [50]. Recently the hydration shell of RNase was estimated by dielectric relaxation studies. It was found that the amount of water accounting for hydration effects was smaller than the number of water molecules calculated for full monolayer coverage of the surface [51]. 

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