Ions, dehydration

The polarity of the interior of the channel, usually lower than in the case of ionophores, often prevents complete ion dehydration which results in a decrease in the ion selectivity of the channel and also in a more difficult permeation of strongly hydrated ions as a result of their large radii (for example Li+). 

Summarizing, it appears that D-mannitol with boiling hydrochloric acid first suffers dehydration between Cl and C4 (because when the treatment is interrupted after only a few hours duration, 1,4-anhydro-D-mannitol is obtained), then between C3 and C6. The dehydration is probably achieved by the catalytic influence of the chloride ion. Dehydration is accompanied by the formation of smaller amounts of other ring compounds and by substitution of secondary hydroxyl groups by chlorine. With fuming hydrochloric acid, D-mannitol gives mainly 1,6-dichloro-... 

In the second item above, the presence of bound and free water molecules was noted. Both bound ions and ionic surfactant groups are hydrated to about the same extent in the micelle as would be observed for the independent ions. The dehydration of these ionic species is an endothermic process, and this would contribute significantly to the AH of micellization if ion dehydration occurred. In the next section we discuss the thermodynamics of micellization, but it can be noted for now that there is no evidence of a dehydration contribution to the AH of micelle formation. The extent of micellar hydration can be estimated from viscosity... 

Ion dehydration effects can be responsible for a variety of ion-dependent RNA folding properties, especially for the tertiary structure folding (Draper, 2008). Refinement of the TBI model should include a more accurate treatment for the possible ion dehydration effect. Further development of the model should also consider the all-atom details of the RNA structure and the improvements of computational efficiency. 

An X-ray structure analysis was carried out 179) on a single crystal of natural faujasite which had been exchanged with Cu2+ ions, dehydrated, and then exposed to butadiene. The major effect of adsorbing butadiene was to induce a migration of copper cations to site III, located at the pore entrances to the supercages (see Fig. 17). The unsaturated coordination of... 

The classical picture of the role of ions in aqueous polymer solutions is related to the notion that hydrophilic colloids are heavily hydrated in solution. The ions are considered to exert their influence by competing successfully with the polymer for the available molecules of water. Because of the charge on the ions, it might be expected that the ion-dipole interactions would be stronger than the dipole-dipole interactions between the water and the polymer. The classical viewpoint would therefore claim that the ions dehydrate the polymer and so induce flocculation. 

It should be pointed out that the BCF theory was derived for crystal growth from the vapour and while it should also apply to growth from solutions (and melts), it is difficult to quantify the relationships because of the more complex nature of these systems. Viscosities, for example, are higher and diffusivities lower in solutions ( 10 Nsm (1 cP) and 10 m s ) than in vapours ( 10 Nsm and lO m s" ). In addition, the dependence of diffusivity on solute concentration can be complex (section 2.4). Transport phenomena in ionic solutions can be complicated, especially if the different ions exhibit complex hydration characteristics. Furthermore, little is known about surface diffusion in adsorbed layers, and ion dehydration in or near these layers must present additional complicating factors. 

The potential importance of the ion dehydration step in the crystallization of electrolytes from aqueous solution has been discussed by several authors (Reich and Kahlweit, 1968 Nielsen, 1984), and there is evidence that an allowance for these effects could account substantially for discrepancies between theoretical... 

What is noteworthy about the series is that for the monatomic alkali metal cations their order does not agree with their size or charge density or their lyotropic series (Voet 1937b). This apparent disorder (note the position of Cs+) is not universal, however, since cases where the lyotropic series is followed are also known. An instance is the rate of the penetration of the alkali metal cations through leaf cuticles that decreases in the order Cs+ >Rb+ > K+ > Na+ > Li+, i.e., in the expected order according to their surface charge densities. The cuticular pores were supposed by McFarlane and Berry to be lined with a protein that has exposed positive sites (McFarlane and Berry 1974). The critical micelle concentration (cmc) of sodium dodecylsulphate increases in the reverse order by these cations (Maiti et al. 2009), where Cs+ is at the expected position. The transition of a mixed surfactant (sodium dodecylsulfate + dodecyltrimethylammonium bromide with an excess of the former) from micelles to vesicles (Sect. 4.5) is also promoted in this sequence, explained by counter-ion association depending on relative ease of ion dehydration (Renoncourt et al. 2007). 

Figure 13 Determination of ion selectivity (a) from cis-trans ion gradients in planar bilayer conductance experiments and (b, c) by external ion exchange in the HPTS assay Description of the resnlts in (d) anion and (e) cation selectivity sequences or topologies, the latter showing selectivities as a function of reciprocal ion radii or ion dehydration enwgies.

---