Ion-pair networks

Fig. 5. Schematic representation of the 12-residue ion pair network at the dimer interface. Ion pairs were identified with a distance less than or equal to 4 A.

The continuing development of such systems exhibits great potential in the areas of sensing, molecular switches, and nanoscale machines capable of doing work. Realization of these ambitious goals will likely require new synthetic methods to construct more complex hierarchical ion pair networks containing multiple aligned mechanically interlocked assemblies for amplified functionality and macroscopic device incorporation. 

K.S.P. Yip, T.L Stillman, K.L. Britton, PL Artymiuk, PL Baker, S.E. Sedelnikova, PC. Engel, A. Pasquo, R. Chiaraluce, V. Consalvi, R. Scandurra, D.W. Rice, The structure of pyrococcus-furiosus glutamate-dehydrogenase reveals a key role for ion-pair networks in maintaining enzyme stability at extreme temperatures. Structure 3 (11) (1995) 1147-1158. 

When the network chains contain ionic groups, there will be additional forces that affect their swelling properties. Translational entropy of counterions, Coulomb interactions, and ion pair multiplets are forces that lead to interesting phenomena in ion-containing gels. These phenomena were studied in detail by Khokhlov and collaborators [74-77]. The free energy of the networks used by this group is... 

Figure 13. a-Cyclodextrin-based bimotecular ion channels, (a) Hexatcar-boxymethyh-substituted a-cydodextrin combined with dioctadecyldimethylam-monium cation to afford hexa ion pairs 9, hexa(stearoyl)substituted a-cyclodextrin tO, and hexa trimethylcholyl)-substituted a-cydodextrin 11. (b) A hypothetical side view for the passage of metal ion through a hydrophilic channel wall comprised of ether network convergently extended from steroidal backbone. 

It is well known that the association-dissociation reaction goes in both directions and that there is always some finite fraction of ion pairs [28]. Counter ions that form ion pairs do not participate in creating osmotic pressure accounting for such counter ions is thus important for the correct determination of the dimensions of the network. 

The simplest way to consider the formation of ion pairs was discussed in Ref. [29]. It was shown that the concentration of ion pairs exponentially increases with the decrease of the dielectric constant of the solvent . This effect should be taken into account in the theory of collapse of polyelectrolyte networks, because effective values of in the collapsed state are usually much less than in the swollen state (e depends mainly on the water content which is much larger in swollen networks). This effect has not been taken into account in the theories developed so far. 

The experimental result obtained was explained by the formation of ion pairs between the charges of a network and counter ions. The theoretical analysis of this problem has shown that the degree of ion pairs formation very strongly (exponentially) depends on e (cf. Sect. 2.2). Thus, if the precipitant has a small dielectric constant e (e.g. dioxane) the degree of dissociation of ion pairs is sufficiently small and this fact leads to the decrease of the osmotic pressure of counter ions which defines the swelling of the gel and the point of the transition in the collapsed state. As a result, in this case the degree of swelling of the gel near the transition point is less pronounced than for other solvents and only a relatively small amount of the precipitant is required to reach this point. In... 

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