Hydration Numbers from Bulk Properties

Solvation numbers obtained from bulk properties refer to those solvent molecules that are bound strongly to the ion and move along with it in the solution but may pertain to both the first and the second solvation shells (for multivalent ions). They are operationally defined according to the methods employed to obtain them, but when the numbers obtained from different methods agree with one another, they can be taken to have a more universal meaning. The bulk properties are measured at finite concentrations but are generally extrapolated to infinite dilution, and special considerations are required to obtain the values for individual ions. On the other hand, such measurements are also made in more concentrated solutions, and the concentration dependence of the solvation numbers can then be elucidated. 

One method to employ bulk properties of electrolytes to obtain solvation numbers is to consider the electrostriction caused by the electric field of the ion, that is, the compression of the water in its hydration shell. The compression of electrostricted wsiier per mole of water is Ay j=-3.5cm mol at 25°C, independently of the nature of the ion [83]. This value revised an earlier estimate of -2.9cm mol by Marcus [84] based on less accurately established second pressure derivatives of the density 

Cation IW(first) IW(second) Reference Anion Reference 

Another way to use bulk properties of the solutions is to consider the ion and the water in its first hydration shell to be non-compressible by an external pressure, the huge electric field of the ion having already produced the maximal possible compression. The hydration number is then defined by using the standard molar ionic compression, (dVf/dP), that is a negative quantity, as  

Experimental (SV values for electrolytes are split into individual ionic values on the assumption by Matheison and Conway [86], that is, (0V (Cl , aq) = 

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There are a number of laboratories around the world where hydrates are produced artificially for study. The bulk of our knowledge of the structures and properties of gas hydrates emerged from such synthetic materials. Additionally, many hydrates with unusual guest species... 

As with most hydrogels, PHEMA possesses a network of pores, the size and connectivity of which determine the transport properties of molecules so important for many applications of this polymer. It is well known that the structure of PHEMA hydrogels is sensitive to the polymerization conditions and in particular the presence of additives or co-solvents during reaction of the monomer. While numerous studies of the pore structure of PHEMA polymerized from HEMA in the bulk, or from HEMA in aqueous solution have been conducted, a number of questions remain. As will be apparent from the brief review below, there remains uncertainty of the details of the pore stucture of PHEMA. Also it is unclear whether within the so-called homogeneous PHEMA hydrogels there exist relatively hydrophobic domains separated from the hydrated porous network. The experiments reported here address these issues. 

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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