Counterions escape

The theory accounts for the fact that some fraction of counterions escapes from the interior of the macromolecular coil. The macromolecular coil on the whole possesses by the electrostatic charge and interacts via the screened electrostatic potential with low-molecular ions escaped from the interior of the macroion. 

In the proposed inert molecule-separated ion pair, the distance carbocat-ion-N2 -counterion is not changed as illustrated by 7.33, 7.34 and 7.35 in Scheme 7-17, but structures 7.34 and 7.35 are characterized by rotation of the car-bocation relative to the counterion. Escape of the inert molecule leads to the intimate ion pairs 7.36 and 7.37. The formation of solvent-derived products becomes dominant only if the carbocation and the solvent have a relatively high reactivity. The distribution of in the products is reasonably close to 50 50 in order to conclude that rotation of the carboxylate is almost free in the inert molecular-separate ion pair stage. 

We can conclude that that the charged osmotic brush regime, in which counterions escape into bulk solvent, can be observed when the experiments are performed in solutions with pH close to p A at very low ionic strength. Because our study provided the very first indirect evidence of behavior of PE micelles that, at that time, was only hypothetical and highly doubted by recognized theoreticians, we were looking for independent support and performed an MC study (see Sect. 4.2). 

One attraction of MD simulation is the possibility of computer animation. The mobility of ions inside a charged cylindrical pore can be visualized. Some movie clips of EMD and NEMD are downloadable at http //chem.hku.hk/ kyc/movies/. mpg. Some features that escape statistical averages can be learned in watching the animation. While the coions are present mainly in the center of the pore, occasional collisions with the wall do occur, as observed in the movie. The time scale of a coion staying near the wall is of the order of 1 ps, compared to 10 ps for the counterion. While the averaged equilibrium distributions indicate an infinitesimal concentration of coion at the wall, reaction of coion with the wall can occur within a time scale of 1 ps. From the video, it can also be observed that the radial mobility of the counterion is more significant compared to the coion s and compared to the axial mobility. It is consistent with the statistical results. 

Thus, in general for any counterion species the free energy of adhesion will probably always be higher and the contact angle lower when more counterions are able to escape the local fields of the individual charges on the polyelectrolyte. Figure 8 depicts schematically this behavior for the different counterions. Therefore, for grafted polyquaternary cations on a nylon surface, the counterions F", CT, and I03 are... 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

Effects Due to Polarization of Counterion Distribution.—We consider first spherical particles (of radius a) with a layer of counterions which are freely mobile on the surface, but which cannot escape into the solvent because of the strong attractive forces of the highly charged partide. Without external manipulations, a symmetrical distribution of counterions can be assumed. After application of an external field, a displacement of counterions will occur which leads to some asymmetry of the distribution. The final equilibrium is determined by the opposing effects of the field and the diffusion of counterions, which tends to restore a random distribution. It can be shown that the phenomenon may be quantitatively described by a frequency-dependent complex surface conductivity... 

The polyion domain volume can be computed by use of the acid-dissociation equilibria of weak-acid polyelectrolyte and the multivalent metal ion binding equilibria of strong-acid polyelectrolyte, both in the presence of an excess of Na salt. The volume computed is primarily related to the solvent uptake of tighdy cross-linked polyion gel. In contrast to the polyion gel systems, the boundary between the polyion domain and bulk solution is not directly accessible in the case of water-soluble linear polyelectrolyte systems. Electroneutrality is not achieved in the linear polyion systems. A fraction of the counterions trapped by the electrostatic potential formed in the vicinity of the polymer skeleton escapes at the interface due to thermal motion. The fraction of the counterion release to the bulk solution is equatable to the practical osmotic coefficient, and has been used to account for such loss in the evaluation of the Donnan phase volume in the case of linear polyion systems. 

Arj Po] have a better chance to escape decomposition in the case of o-and p-tolyl derivatives and to produce Arj °PoX by associating with the counterion X in the solution in solvents of low polarity, than the m-tolyl derivatives. In the case of (m-Tol)3 °Bi, the excited molecular cations have higher probability to decompose... 

FIG. 4 Schematic presentation of macroion. Counterions inside the inner dashed sphere are those kept by the macroion others are escaped. External dashed sphere shows the external volume per one chain. 

It was taken into account that counterions could leave the interior of the microgel for the external solution and that both contributions of electrostatic interaction of a macroion with escaped counterions and of the osmotic pressure of counterions contribute to the swelling law of a microgel. The swelling behavior of a microgel was analyzed as a function of the number v of polymer chains in the microgel. 

Vasilevskaya VV, Khokhlov AR, Yoshikawa K. Single polyelectrolyte macromolecule in the salt solution effect of escaped counterions. Macromolecular Theory and Simulations 1999, submitted for publication. 

Benzene formed from photolysis of the 1 1 complex is a cage-escape product from 3(Jul-CHO+ /Ph ). Benzene formed from the photolysis of the 2 1 complex is an in-cage product from 3((Jul-CHO)2 /Ph ). The formation of 2 1 complexes of amino-substituted ketones and iodonium salts has been suggested to account for the high photosensitivity of polymeric Mannich bases with iodonium salts [102]. Formation of 2 1 donor iodonium cation complexes has been rationalized by consideration of the crystal structures of diphenyliodonium halides, which crystallize as dimers with square planar iodine atoms with two bridging halide counterions [102,108]. 

This very simplified model of micellization is illustrated in scheme 4 for a cationic surfactant. At concentrations below the cmc only monomeric surfactant is present, but at higher concentration the solution contains micelle, free surfactant and counterions which escape from the micelle. It is assumed that submicellar aggregates are relatively unimportant for normal micelles in water, although, as we shall see, this assumption fails in some systems. However it is probably reasonable for relatively dilute surfactant, although at high surfactant concentration, and especially in the presence of added salt, the micelle may grow, and eventually, new organized assemblies form, for example, liquid crystals are often detected in relatively concentrated surfactant [1]. However, this discussion will focus on the relatively dUute surfactant solutions in which normal micelles are present. 

Figure 6.3 Stimuli-responsive polymers and their intracellular mechanisms leading to transfection. (A) Cationic polymers containing bioreducible disulfide bonds (S-S) can form tight complexes with DNA, releasing it in the cytosol, due to reduction of disulfide bonds to two thiol groups. (B) Associated polyplexes can he dissociated in the cytosol by temperature decrease, leading to DNA release. (C) The endosomal escape can be promoted by pH-responsive polymers such as PEI that act as proton sponges, preventing the normal acidification of the endosomes the continuous influx of protons, counterions and water eventually leads to endosomal disruption and DNA release.

The strong osmotic pressure is also the decisive factor for the understanding of the interaaion of the SPEB in solution. A simple model was developed that is based on the compression of the surface layer upon mutual interaction of two SPEBs. Since the counterions cannot escape, the... 

In conclusion, the solution behavior of the SPEB is fairly well understood. There are only a small fraction of counterions that may escape the bmsh layer. Hence, the entire system is neatly elertrically neutral and the decisive factor is the osmotic pressure of the counterions confined in the bmsh layer. We explore the consequences of this fact on practical applications related to catalysis in Section 6.07.5.1. 

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