Counterions unbound

If the mixed micelle model already presented is used to predict the ionic surfactant monomer concentration, and a simple concentration—based solubility product is assumed to hold between the unbound counterion and monomer, the salinity tolerance of an anionic/nonionic surfactant mixture can be accurately predicted (91). supporting this view of the mechanism of tolerance enhancement by nonionic surfactant. 

Figure 2 Conventional representation of micelles formed by an ionic surfactant, such as sodium dodecyl sulfate. The inner core region consists of the methylene tails of the surfactants. The Stem layer consists of surfactant headgroups and bound counterion species. The diffuse double layer consists of unbound counterions and coions which preserve the electrical neutrality of the overall solution. Also pictured are the transition moment vectors for the S-O stretching modes of sodium dodecyl sulfate.

Figure 5. Four-state model of hydration-mediated equilibrium between unbound and sidechain-associated counterions in ionomeric membranes (9,10, 13).

The fraction of delocalized or free charge is approximately a constant with a logarithmic increase as the volume fraction of spheres is decreased. This occurs because the entropy of the counterions becomes more important for small , thus stabilizing the free charge. The fraction of charge that is free or unbound is much larger than that of the one-dimensional case where the entropy is much more restricted and either another variational calculation... 

Six structures are shown in Fig. 6. Unbound 12-crown-4 is shown in panel (a). It is shown complexed in panel (b), along with 18-crown-6, in an unsymmetrical sandwich. Two additional molecules of 18-crown-6 are present in the unit cell but do not bind the cation. Note that because 10 oxygen donors are present. is not in the 18-crown-6 macroring s plane. Potassium cation is in the plane in (c), (d). and (e). The C104 ion is shown in contact with the cation in (d). The counterions are not shown for (c) and (e) (A-heptylaza-15-crown-5). The molecular complex between 18-crown-6 and acetonitrile is shown in panel (f). 

It was taken into account in the models developed in Refs 75, 78 and 80 that some portion of the counter-ions is bound to surface-active ions within the Stern-Helmholtz (S-H) layer, while another (unbound) portion is located within the diffuse region of the DEL. The equivalent relations of Eqs (56)-(58) in this case contain the difference F — y"X instead of Fj, where F x is the adsorption of counterions localized within the monolayer. It follows from the model described by Eqs (56)-(58) that if all counterions are lo-... 

Fig. 1 ORTEP representation of the X-ray crystal structure of [(NN)PtPh2]2Cu (CF3S03). Ellipsoids are drawn at 50% probability. Hydrogen atoms and the unbound CF3S03 counterion are omitted for clarity. Selected distances [A] Cul-Ptl 2.7254(3), Cul-Cl 2.138(5), Cul-C2 2.336(5), Ptl-Cl 2.033(5), and Ptl-C2 2.022(5). Reprinted with permission from ref [24]. Copyright 2009 American Chemical Society...

Neutral carrier-based membranes require the addition of a lipophilic ion-exchanger for proper functioning. This ion-exchanger forms the counterion of the complexed analyte ion in the membrane. Its concentration should not be too large in order to allow for a substantial concentration of unbound ionophore in the membrane. Eor instance, a cation-selective membrane may contain the ionophore and the lipophilic tetraphe-nylborate derivative cation-exchanger potassium tetrakis(4-chlorophenyl)borate, while anion-selective membranes may be doped with tridodecylmethylammonium chloride as anion-exchanger in addition to the ionophore. 

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