Hydration electrostricted

Initial screening conditions are suggested in Table 6.1. Multiple pH values are included because mobile-phase pH can significantly affect retention. Major selectivity shifts such as transpositions in elution order are fairly common changes in resolution are much more so.2,14-16 Changes in retention due to pH variation relate to protein hydration. Proteins are minimally charged at their isoelectric points (pis). This means that they carry the minimum of electrostricted hydration water. Both protein surface hydrophobicity and HIC retention should therefore reach their maximum at a protein s pi.6 As pH is either increased or... 

Average number of water molecules in the electrostricted hydration shell, estimated from standard molar Gibbs energies of hydration [121]. 

A subsequent development by Marcus (1994) of this concept for Astmc>S is based on a model common for various thermodynamic functions of ion hydration (Marcus 1987) (Sect. 2.3.4). The width An of the electrostricted hydration shell is the key quantity of this model. The water molecules in this shell have a volume nd l6 each rather than Vw/Aa, that is, they are fully compacted, and t(w = 0.276 nm is the diameter of a water molecule. The value of Ar is obtained from the volume of the hydration shell with hi water molecules ... 

The entropic effects of the creation of a cavity in the water to accommodate the ion (similar to ASn in (Abraham et al. 1982)) and of the compression from the gas to the solution (similar to Acomp in (Marcus 1986)) are taken care of together in a neutral term, A5nt. It is calculated from the entropies of hydration of small neutral molecules or rare gas atoms, interpolated for a radius n the same as that of the ion A5 t = -3 - 600(n/nm) J K mol In the electrostricted hydration shell the permittivity and its temperature derivative are assumed to have the infinitely large field values. Using for this purpose d, the relfactive index of water at the sodium D line, e = = 1-776 and (de /dT)p = 2(dni)/dT)p = -1 x 10-"K (at... 

Subsequently, A j 5 was based by Marcus [51] on a model common for various thermodynamic functions of ion hydration used by him [52], based in turn on the width AZj of the electrostricted hydration shell. The volume of the fully compacted water molecules in this shell was itd V6 each (where water molecule), rather than VJN pertaining to bulk water. The hydration shell with Aj compacted water molecules had a volume... 

Another, related effect leading to non-bulk response in the first hydration shell is electrostriction[131], which is the change in solvent density due to the high electric fields in the first solvation shell of an ion. 

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

Fig. 2.3 A schematic representation of the hydration layer near a small ion (left) and a large ion (right), showing the region where the water is dielectrically saturated (with a low relative permittivity e ), hence electrostricted (squeezed) and immobilized. The thickness of this layer, Ar, depends reciprocally on the size of the ion.

Negative values ofN —N0, the electrolyte effect on the association numbers of water, are called the structure-breaker effect. One can speak of negative hydration31. The estimation of the hydration numbers by spectroscopic or solubility methods gives only an approximation of the sum effect. The spectra of the H-bond bands show in second approximation distinct differences between the ion effects on the H-bonds7 ). — The partial molar volume Vx of water in electrolyte solutions is negative in all solutions but the series of -values corresponds to the Hofmeister ion series too. The negative V1 volume indicates an electrostriction effect around the ions. 

Taking into account a finite compressibility of the hydration waters led Onori to suggest solvation numbers that differed from those of Passynski with his assumption of zero compressibility of the inner region of the solvation shell. For example, for a 1.5 M solution, Onori has the value 19 for the sum of the solvation numbers of Na and cr, whereas the Passynski at 0.05 M solution is 6 However, later on (Section 2.22), when electrostriction is discussed in detail, Onori s estimate will be shown to be unlikely. 

Fig. 2.75. (a) Local compressibility fi of water as afunctbn of log(field) near an bn. (b) Electrostriction, AW, in water as a function of bg(fielcl) near an bn. (Reprinted from B. E. Conway, Ionic Hydration in Chemistry and Biophysics, Elsevier, New Yoik, 1981.)... 

Maximum Electrostrictive Decrease in the Volume of Water in the First Hydration Shell... 

R448 L. A. Marky and D. W. Kupke, Enthalpy - Entropy Compensations in Nucleic Acids Contribution of Electrostriction and Structm-al Hydration , Methods Enzymol, 2000,323,419 R449 A. F. Martins, Measurement of Viscoelastic Coefficients for Nematic Mesophases Using Magnetic Resonance , EMIS Datarev. Ser., 2000, 25,405... 

Frank, his theory of electrostriction, 190 Free eneigies of hydration dependence on radius. 54 how to get them, described, 53 Free eneigies of solvation, described, 53 Friedman... 

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