Irrotationally bound water

Hydration of biopolymers is a mechanism for stabilizing these materials (Fig. 2.78). When proteins are conpletely dry, they tend to decompose. One way of evaluating hydration in polyions is to measure the dielectric constant of a solution containing a dissolved protein as a function of concentration at radio frequency. The dielectric constant falls with increase in concentration and the water per polyion can be calculated by assuming that water bound to the protein no longer makes any contribution to the dielectric constant. Thus, Buchanan calculated the irrotationally bound water from such expaiments. Some of this water is hidden in cavities within the structure of the protein molecule. 

The linear relation found between dielectric constant and concentration can be interpreted in a first approximation as the result of a number of irrotationally bound waters. Such waters would constitute the primary hydration water referred to in Section 2.4. 

If the structure of water depends on distance from a surface, so must its physical properties, including its dielectric function. We noted in Section 9.5 that at microwave frequencies the dielectric function of water changes markedly when the molecules are immobilized upon freezing as a consequence, the relaxation frequency of ice is much less than that of liquid water. Water irrotationally bound to surfaces is therefore expected to have a relaxation frequency between that of water and ice. 

Those water molecules that are prevented from orienting ( irrotationally bound ) to oppose the field will be withdrawn from those contributing to the counter field and hence the dielectric constant of the ionic solution will be reduced from what the solvent would have without the ions. 

Some of the facts that Hasted et al. established are shown in Table 2.11, They found that the lowering of the dieleetrie eonstant of 1 M solutions is in the range of 10-20%. This ean be nieely explained by taking the water in eontaet with the ion as dielectrically saturated (unable to orient on the demand of the external field), but still having a dieleetrie eonstant of only 6, eompared with the value of 80 for bulk water unaffected by ions. The table shows the number of water moleeules pa ion pair that one has to assume are saturated (i.e., irrotationally bound in the vicinity of each ion) to make the above model come out right (i.e., reproduce the measured dieleetrie constants of solutions). This model leads to a very simple equation for the dielectric constant of a solution ... 

We shall now discuss the depression of the static permittivity of water by the addition of eiectrolyte solutes, which is a phenomenon of some importance in the understanding of the hydration sheath of the ions. It is essentially a dielectric saturation phenomenon the strong electric fields in the neighbourhood of the ions produce a non-linear polarization, which renders the local water moleodes ineffective as regards orientation in the applied field. It is possible to make estimates of the extent of hydration, or hydration number , of water molecules considered to be bound irrotationally to the average ion these estimates are in reasonable agreement with hydration numbers estimated on the basis of activity coefficients, entropies, mobilities, and viscosities. The hydration number must be distinguished from the number of water molecules actually adjacent to the ion in the first or second layers of hydration (the hydration sheath) it does not follow that all of these molecules can be considered to be attached to the ion as it moves in the solution. 

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