Amphiphilic counterions

Figure 7.7 Schematic representation of the temperature-induced change in the MSP composed of terpy end-capped ditopic monomers and Fe dihexadecyl phosphate. The temperature-induced transition resuits in a distortion of the metal ion coordination geometry, which occurs because of the melting of the amphiphilic counterions, giving rise to a reversible transition from a diamagnetic low spin state to a paramagnetic high spin state (Bodenthin et al. 2005).

Fig. 30 Top view of a linear array of six devices and a section of a single junction containing a monolayer of catenanes anchored with amphiphilic counterions.

Phenomenologically, two different mechanisms could be envisioned (a) the formation of the ion pair between the analyte and amphiphilic counterion with subsequent adsorption of this complex on the stationary phase and (b) adsorption of the amphiphilic counterion itself on the stationary phase surface and subsequent retention of charged analyte in essentially an ion-exchange mode. Melander and Horvath [117] concluded that, in reality, probably both mechanisms coexist in the chromatographic system. 

Electron microscopy provides perfect pictures of vacuum collapsed vesicle membranes after negative staining with heavy metal salts. BLMs appear usually as collapsed balls, MLMs often as flat disks (see Figure 4.29). There is no requirement for double-chain amphiphiles in order to form vesicles. The same single-chain amphiphiles which form micelles also form vesicles if their charge is neutralized. This was practised, for example, via the protonation of soaps or through addition of an amphiphilic counterion. In both cases, fatty acids function perfectly well in the form of vesicles. 

Micellization depends upon a balance of forces and the cmc decreases with increasing hydrophobicity of the apolar groups, and for ionic amphiphiles also depends on the nature and concentration of counterions in solution. Added electrolytes decrease the cmc, and the effect increases with decreasing charge density of the counterion. Divalent counterions, however, lead to... 

Figure 5.45 Cationic gemini amphiphiles having chiral counterions. TEM images of representative twisted ribbons formed by 16-2-16 tartrate (49 + 50) at 0.1% in water for various values of enantiomeric excess (a) 0 (racemate) (b) 0.5 (c) 1 (pure L) (d) 1 (pure L) in presence of 1 eq sodium L-tartrate. Bar = 100 nm. Reprinted with permission from Ref. 165. Copyright 1999 by Macmillan Magazines.

For therapeutical purposes, a likewise frequently used group of drug compounds are the nonsteroidal anti-inflammatory drugs (NSAID). Among the best known representatives of the aryl acetic acid derivatives is diclofenac as well as ibuprofen, an aryl propionic acid derivative. As both have acidic properties, they dissociate while being dissolved and may form salts with amphiphilic properties. Together with appropriate counterions these amphiphilic organic acids may form lyotropic mesophases with water even at room or body temperature, for example, diclofenac diethylamine... 

While the ion pair combination 1 is found to be a successful example to afford artificial ion channels, it may not be difficult to find analogous solutions. Illustrated in Figure 7 is a small expanded scheme for constructing artificial supramolecular ion channels from synthetic amphiphilic pairs of hydrophilic and hydrophobic counterions (note that the combination 2a-2e corresponds to 1). All of these compounds gave stable single-channel currents when incorporated into a bilayer lipid membrane. 

Ref. 13 contains a more complete discussion of static quadrupolar effects for amphiphilic mesophases. That work also includes a treatment of counterion quadrupole relaxation for liquid crystalline systems a brief outline of this discussion is given in the next section. 

In the previous discussion, activity coefficients have been totally neglected, for simplicity, but they should be included in a proper treatment. However, for ionic amphiphiles, where activity corrections are expected to be most important, additional complications arise. If the counterions are explicitly incorporated in the equilibria the number of possible chemical species is greatly enhanced making a detailed analysis even more complex. Furthermore a description of a process in terms of an equilibrium constant is only really suitable when the forces involved are of a short range... 

If one considers solely the consecutive equilibria, the concentration of monomer can only increase with increasing total amphiphile concentration even above the CMC. (Apart from the trivial decrease in the monomer concentration calculated on the total volume which may arise when the micelles occupy a substantial volume fraction). However, if one realizes that micelles are not only composed of amphiphile, the result may be different. Thus counterion binding helps to stabilize the micelles and for ionic surfactants it can be predicted that the monomer activity may decrease with increasing surfactant concentration above the CMC. Good evidence for a decreasing monomer concentration above the CMC has been provided in the kinetic investigations of Aniansson et al.104), and recently Cutler et al.46) demonstrated, from amphiphile specific electrode studies, that the activity of dodecylsulfate ions decreases quite appreciably above the CMC for sodium dodecylsulfate solutions (Fig. 2.14). 

The somewhat unexpected fact that the different types of methods give approximately the same values of 0 is discussed in Sect. 6. Quantitatively, there are significant differences though, so in comparisons between different amphiphile ions, counterions etc., the adherence to a single method is necessary. Having outlined the generalities of counterion binding, we will with selected examples discuss some of its features on a molecular level as revealed mainly by spectroscopic studies. 

Counterion specificity has been observed to be markedly more pronounced for cationic surfactants than for anionic ones. This can certainly be mainly referred to a weaker hydration of typical counter-anions. From the variation of CMC with counterion and from ion activity measurements it can be inferred that the binding to -N(CH3)3 and -NH3 headgroups follows the sequence P>NOj >Br > CP. (As an example a recent study223-1 of decylammonium salts shows the CMC to decrease from 0.064 M for the chloride to 0.038 M for the iodide). The counterion specific effects on micellar shape for -N(CH3)3 surfactants were discussed above. For cationic (as well as some anionic) amphiphiles, a marked counterion specificity is also indicated in the phase diagrams8 but systematic studies of the counterion dependence have not yet been reported. 

The catalytic effect for reactions involving an ionic reactant usually shows a strong dependence on the total amphiphile concentration. The maximal effective rate constant is attained at concentrations just over the CMC. Romsted284 showed that this occurs due to the competition between the ion binding of the reactive ions (OH- in the example above) and the counterions of the amphiphile. Recently, Diekman and Frahm285 286 showed that it is possible to rationalize the kinetic data by describing the ion distribution through a solution of the Poisson-Boltzman equation. (See Fig. 5.1). 

A system of an ionic amphiphile contains in its simplest form three entities the solvent water, an amphiphilic ion, and a hydrophilic counterion. The properties of the total system can then be understood as the effects of the mutual interactions between these three species. A comprehensive treatment of all the interactions on a molecular level is not at present feasible. However, the development of methods for determining intermolecular potentials and for making statistical mechanical simulations292-294 should change this in the not too distant future. 

Micellar solutions of anionic amphiphiles are usually not stable with respect to the addition of di- or multivalent cations since a precipitation occurs (hard water). In exceptional cases, where precipitation does not occur, the question arises as to how the uni-, di- and multivalent ions compete for binding to the micelles. Due to the high value of I l close to the micellar surface a counterion of high charge will be strongly favored and there is a discrimination between the different types of ions299. For example for SDS close to the CMC, -e(r[)/kT — 7 and if the presence of small amounts of calcium ions does not affect [Pg.74]

The case of Srcl8C6(PFO)2 complexes differs, as the PFO" counterions are amphiphilic in nature and their carboxylate head coordinates to strontium, while their perfluoroalkyl chain cannot and do not attract each other, thus preventing selfaggregation of the complexes. The affinity of these fluorinated chains for C02 contributes to solubilize the strontium complexes. As a result, the Srcl8C6(PFO)2 complexes sitting at the interface are "diluted", while the others migrated from the interface to SC-C02 40 % are found at more than 8 A from the interface, and can thus be considered as "extracted" at 305 K. 

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