Counterion activity, additivity

Fig. 16. Influence of the ionic strength on the counterion activity of PDADMAC with different molar masses. Variation of the ionic strength by addition of NaCl. (X=Cp / cs cp=10-3 mono-mol L-1 T=20 °C PDADMAC Mn 12,000 g mol-1 22,000 g mol-1 72,000 g mol-1 A 170,000 g mol-1 - - - Gueron -Manning .Iwasa) (Data taken from[38])...

It should be stressed that such condensation of counterion by polyion is determined just by the structural parameter that defines charge density along the length of the macromolecule. It is not influenced by external condition, such as Cp or the addition of salt. The fact that the colligative properties of salt and polyelectrolyte are found to be additive [32,33] when salt is added to the polyelectrolyte provides insight with respect to the uniqueness of ( )p and y,. Such behavior is attributable to the inaccessibility of the polyion, the condensed Na" " ions and the solvent associated with the polyion domain, to the measurements being carried out. Their presence as a separate phase, however, is not detectable by the counterion activity measurement in the absence of simple salt. 

Fig. 24. 32 Left Structure of guanidinium calix[6]arene 7. Right. Vesicle experiments (i) Addition of polyions (e.g., ctDNA) and counterion activators (e.g., DG, guanidiniocalixarenes) to vesicles with internal reporter ions (e.g., DPX, HPTS) and counterion inactivators (e.g., pK) triggers (ii) the formation of membrane-active polyion-counterion complexes (e.g., ctDNA-DG), (ill) DPX export, (iv) the formation of internal polyion-counterion complexes (e.g., ctDNA-DG-pK), etc M = Na [100]. (Reprinted from Ref. [100])...

Influence of the counterion stmcture Addition versus hydride transfer Friedei-Cratts-type initiators Direct initiation versus coinitiation Propagation Stmcture of active species Reactivity of active species Transfer and Termination Chain transfer to poiymer - transacetaiization Termination... 

Fig. 4. Activity coefficients of univalent small ions in the presence of added simple salt. The solid lines are the limiting laws for (top to bottom) the co-ion activity coefficient 72, the counterion activity coefficient yi, and the square of the mean activity coefficient y y2. The curves are drawn for = 1.85. The dashed line is the additivity rule for yi and 7172 the additivity rule for 72 coincides with the top baseline 72 = 1. The data points are measurements of 7ci (Reference [44]) for a polyvinylalcohol sulfate sample of degree of substitution corresponding to = 1.85 and NaCl concentration 1 x lO" M.

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

The initial rates in the 2" mns decreased in the following order Co-OTs > Co-OAc > Co-Cl > Co-1 (see Table 43.2), which is consistent with the idea of deactivation by counterion addition to epoxide. As reported by Nielsen et al. (7), the rate of counterion addition to epoxide determines the kinetics of HKR. We suspect that the iodide and chloride counterions undergo such rapid addition to epoxide that most of the HKR reaction in the 2" mn occurred with the much less active Co-OH catalyst formed in situ. In contrast, the weakly nucleophilic tosylate counterion reacted more slowly with the epoxide and was therefore more stable. 

Certain surface-active compounds [499], when dissolved in water under conditions of saturation, form self-associated aggregates [39,486-488] or micelles [39,485], which can interfere with the determination of the true aqueous solubility and the pKa of the compound. When the compounds are very sparingly soluble in water, additives can be used to enhance the rate of dissolution [494,495], One can consider DMSO used in this sense. However, the presence of these solvents can in some cases interfere with the determination of the true aqueous solubility. If measurements are done in the presence of simple surfactants [500], bile salts [501], complexing agents such as cyclodextrins [489 191,493], or ion-pair-forming counterions [492], extensive considerations need to be applied in attempting to extract the true aqueous solubility from the data. Such corrective measures are described below. 

Although the exact nature of the active center in polymerizations of butadiene with these Ba-Mg-Al catalysts is not known, we believe that the preference for trans-1,4 addition is a direct consequence of two aspects of this polymerization system, namely (1) the formation of a specific organobarium structure in a highly complexed state with Mg and A1 species, and (2) the association of the polybutadiene chain end with a dipositive barium counterion which is highly electropositive. 

These reactions comprise nucleophilic SN2 substitutions, -eliminations, and nucleophilic additions to carbonyl compounds or activated double bonds, etc. They involve the reactivity of anionic species Nu associated with counterions M+ to form ion-pairs with several possible structures [52] (Scheme 3.4). 

Sulfur has four unique characteristics related to its occurrence and chemistry in soil. As sulfate, it is one of the principle counterions that keep the soil electrically neutral. Soil receives constant additions of sulfur through volcanic activity around the world and industrial pollution, usually in the form of acid rain. This means that soils usually have sufficient sulfur for plant growth. Lastly, plants can take and use sulfur dioxide from the air as a source of sulfur for growth [22,38],... 

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

The choice of the counterion has a significant influence on the activity of the catalyst, no methylester is produced by complex 19. Similar results were obtained for compounds 20 (980%) and 21 (0%), where the steric demand of the ligands R (= methyl) is significantly lower. Additional reactions under different conditions show that the yields can be improved. After 14 hours at 90 °C catalyst 20 yielded 3000% relative to palladiiun (TON 30) [55]. 

---