Hydrophobic interaction connection with experimental

In our discussion of halide ion quadrupole relaxation we will follow a path of increasing complexity of the systems. First, we consider in the following subsection the relaxation rates observed for the ions at infinite dilution in water and in connection with this the theoretical treatment of relaxation due to ion-solvent interactions. In Subsection 5.1.3, experimental data for aqueous solutions of alkali halides are considered and in connection with this we outline theoretical attempts to account for effects of ion-ion interactions on the relaxation rates. Apart from alkali halide solutions few inorganic systems, mainly earth alkali halide solutions, have been studied and these are treated in Subsection 5.1.4. Hydrophobic solutes have particularly strong effects on chloride, bromide and iodide relaxation and the explanation to this is considered in Subsection 5.1.5. Long-chain hydrophobic solutes in aqueous systems form various types... 

The studies dealing with the effects of hydrophobic species on halide ion relaxation are of great significance in connection with attempts to use halide ion quadrupole relaxation to elucidate anion binding sites in proteins (cf. Ref. [304] and Chapter 8) and also in relation to theories of ion quadrupole relaxation in solution. As has been described above, the experimental observations are explainable in terms of electrostatic ion-water interactions. On the other hand. 

A word of cantion is in order. A better connectivity in SSC cannot be instantly eqnated to higher condnctivity. In the simulations of Nafion and SSC, the same amount of water is distributed in the same volume. A change in connectivity is almost certainly accompanied by a change in the characteristic dimension and geometry of the cluster channel. In other words, if one stretches out clusters in SSC in order to better connect them, under the constraint that the volume of the aqueous domain is the same as that in Nafion, then one must accept that the dimension of a channel in a clusters in SSC is smaller. (That the characteristic channel width in SSC is smaller than in Nafion has also been observed experimentally at least for the medium and high water contents.) This change in dimension can affect the environment of the water and hydronium ions. If the channel is smaller and more spread out in SSC PFSA membrane than in Nafion, then it has more surface area with the hydrophobic phase per unit volume. This additional interaction with the hydrophobic phase can be characterized as additional confinement. The effects of confinement on both the diffusion of water and the vehicular and structural components of diffusion of the proton are not fully understood. Thus it is important to corroborate the suggestions of this water cluster distribution analysis with other measures of structure and transport. 

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