Counterion association

Of the cations (counterions) associated with polar groups, sodium and potassium impart water solubiUty, whereas calcium, barium, and magnesium promote oil solubiUty. Ammonium and substituted ammonium ions provide both water and oil solubiUty. Triethanolammonium is a commercially important example. Salts (anionic surfactants) of these ions ate often used in emulsification. Higher ionic strength of the medium depresses surfactant solubihty. To compensate for the loss of solubiUty, shorter hydrophobes ate used for appHcation in high ionic-strength media. The U.S. shipment of anionic surfactants in 1993 amounted to 49% of total surfactant production. 

Some divalent cations such as Cu and Pb form very stable complexes with pectate, but are unlikely to be present at sufiScient concentration in the apoplast of plants to form a major fraction of the counterions associated with the pectic fraction in vivo. The Al ion may deserve closer examination, as it is certainly able to displace Ca from cell walls and reaches substantial concentrations in plant roots under some conditions [60,61]. aluminium is not usually considered to be freely translocated, however. Basic peptides with their negative charges spaced at a similar interval to galacturonans (0.43 nm or a small multiple thereof) can in principle have a very high afiBnity for pectate [62,63], but the extensins that are associated with the most insoluble pectic fractions [M] do not appear to have this type of structure. The possibility that the non-extractable pectic polymers in most cell walls are very strongly complexed with some cation other than Ca " cannot be ruled out, but there is little evidence to support it at present. 

The reaction is initiated by attack of a nucleophile (Ye), usually the counterion associated with the Cu ion, at one of the CN groups of the phthalonitrile, which is activated by its coordination with the Cu ion. Subsequently a cycliza-tion reaction to an isoindoline derivative takes place. These steps are three times repeated by a series of similar reactions finally resulting in a cyclization to a CuPc ring intermediate, whose formation is facilitated by the coordinating role of the copper ion. With a copper (II) salt as reactant it is suggested that Y is eliminated from the intermediate, e.g. the Cl ion in the case of CuCl2. The monochloro de-rivate of CuPc is than formed by an electrophilic attack of Cl on the CuPc initially formed. 

It was previously mentioned was that a large number of minor copolymers of PET have been developed over the past 50 years, with the intent of modifying textile fiber properties and processability [2, 3], Of broader interest is that some of these textile modifications, such as PET copolymers with metal salts of 5-sulfoisophthalic acid (SIPA), have their own rich chemistries when the extent of polymer modification is increased beyond textile levels. An example of such a modification is that changing the counterions associated with SIPA can significantly effect the kinetics of polyester transesterification reactions (the... 

The separation mechanism is based on stereoselective ion-pair formation of oppositely charged cationic selector and anionic solutes, which leads to a difference of net migration velocities of the both enantiomers in the electric field. Thus, the basic cinchona alkaloid derivative is added as chiral counterion to the BGE. Under the chosen acidic conditions of the BGE, the positively charged counterion associates with the acidic chiral analytes usually with 1 1 stoichiometry to form electrically neutral ion-pairs, which do not show self-electrophoretic mobility but... 

Tabled shows the results for the regression analysis of dodecylsul-fate surfactants with different alkali counterions. The degree of surfactant ion/counterion association in the adsorption layer is evidently high (from 89.9% to 92.6% counterion coverage). There is also a correlation between the hydrated radius (volume) of the counterions and Coo. The decrease in the hydrated volume of the coimterions results in the higher value of Coo, and increases the attractive force between the molecules. A pictorial presentation...

Consider the formation of a mixed micelle in aqueous solution from a binary surfactant solution consisting of a nonionic and an anionic surfactant. The process is depicted as the aggregation of ng molecules of nonionic surfactant B, of n molecules of anionic surfactant A", and in addition there will be counterions, C" ", of the anionic surfactant in the amount of an where a is the fraction of the counterions associated or bound with the surfactant anions in the micelle. The process as depicted is... 

To this point we have used no specific mixing rule to describe the interactions of monomers of surfactants 1 and 2 in the micellar pseudophase. We have assumed, however, that only one micellar pseudo-phase exists. For our calculations we have used the Redlich-Kister expansion for w(x) with up to two parameters (10,12). Moreover, we have not yet specified the form of the function y9(x), which can be varied for modeling specific counterion association behavior. For our calculations we have used the following linear function for /3(x) ... 

In Chapter 1, we arbitrarily defined Lewis superacids as those that are stronger than anhydrous aluminum chloride in their reactivity, the most commonly used Friedel-Crafts catalyst. Of course, Lewis acidity is only a relative term concerning specific bases and involved counterions (association, steric hindrance, etc.). The physical properties of some of the Lewis superacids are given in Table 2.3. 

For counterions, Eq. (5.7) predicts a larger rate constant for the association process than Eq. (5.6) since the ions are attracted by the micelle. An estimate using Eq. (5.7) and a solution of the Poisson-Boltzmann equation (cf. Sect. 6) gives k2 - lO M s-1 for the counterion association. 

Overall, this chapter shows how ASAXS data can be acquired and carefully interpreted to yield information related to the spatial distribution and number of counterions associated to DNA. Due to the small size of the signal, ASAXS requires careful attention to experimental detail. Background scattering must be minimized to the greatest possible extent. 

The counterion associated with the sorbent when it is manufactured is replaced by another ion of like charge existing on the analyte to achieve retention. However, analyte retention is affected by the ionic strength of the sample matrix because other ions present will compete with the analyte of interest for retention by ion-exchange mechanisms [75],... 

D. Effect of Buffer Cation and Buffer Anion The electroosmotic flow is proportional to the potential drop across the diffuse layer of counterions associated with the capillary wall. Because the potential drop is formed by counterions in the buffer attracted to the charged silica surface, the nature of the counterions will affect the zeta potential and therefore the EOF. Figure 4.5 shows the effect of the buffer cation on the mobility of both the EOF (using mesityl oxide as the marker) and a solute (dansylalanine).16 The highest mobility is obtained with the smallest cations however, high mobility may decrease solute resolution, so care must be taken in choosing the cation. The buffer anion also affects the mobility of the EOF, although trends are less apparent. Therefore, the effect of the EOF on a separation can be altered by careful selection of both the buffer anion and cation. 

Here, c3 and cd are the concentrations of salt and polyion, respectively. The difference in association of counterions between the products and the reactants is given by (1 + 2T3d) [65-73]. Consequently, counterions associated with the polyion will be released into the bulk solution if the (1 + 2T3D) is negative. The release of counterions leads to density changes in the condensed layer and provides an additional driving force, except that it is entropic in nature [65-73]. 

It is insightful to view counterion association through the Donnan equilibrium experiment. Let cq be the concentration of polyion, and let c3 be the salt concentration in the sample chamber [65-70, 74], In the presence of... 

The density of phosphate charges [74-83] in tertiary structures, such as pseudoknots, RNA, inter-helix junctions, Holliday and branched recombination junctions, exceeds that in linear DNA molecules [74—93], In fact, the degree of counterion association has been linked with the overall stability of these types of structures [46, 82-84, 95, 96]. 

The number of counterions associated with the tetrahedral junction was less than sensitive not only to parameters that define the junction region, but also to the interbranch angle between 90° and 109.5° [74] The number of associated counterions is substantially larger in the four-way junction than other junction geometries and constructs studied [74]. As salt concentration is increased, the stability of the junction is enhanced over a linear polyelectrolyte molecule of identical length as the junction [74], For junctions with symmetrical branches, the counterions associated with the junction in excess of that of a linear construct increases with the length of the branches and then saturates [74],... 

In an attempt to rationalize the large variety of buffers used to date in protein HPLC separations, it is necessary to briefly return to the separation mechanism based on the hydrophobic effect. This mechanism related the retention of a sample on a reversed-phase column to the nonpolar surface area of a sample molecule. A possible explanation of the dramatic effect of ion-pairing reagents on the retention of proteins on reversed-phase columns can then be based on the modification of surface polarity of the protein molecule on association with suitable counterions. In the absence of salts dissolved in the mobile phase, the peptide or protein sample probably has some counterions associated with the sample. Alternatively, the basic side chains of the protein may be neutralized by a salt bridge with an acidic residue which is adjacent in the three-dimensional structure. In either case, the contribution of the ammonium group to the surface polarity is relatively small. Figure 13 shows the result of association of the amine cation with a highly polar anion such as phosphate, which has a substantial sphere of hydration. In this case, the nonpolar area of the... 

This expression for differs by a factor of 2 from the length below which ion association is assumed to take place in electrolyte solutions, according to Bjerrum s theory, see (1.5.2.30a). The reason for this factor of 2 is that for counterion association on a polyelectrolyte only the former loses its kinetic energy, whereas for association of two small ions this occurs for both. At low polylon charge, is of course simply given by... 

Whether such systems may or may not exhibit shell-core behavior depends on the relative difiusivities and on whether the entering B counterion forms the stronger or weaker complex. In both cases the presence of co-ions in the exchange can be neglected because the Donnan effect (normally excluding co-ions) is greatly reduced by counterion association with the fbced charges. 

The counterion associated with the charged group of ionic surfactants has a significant effect on the micellar properties. There is an increase in micellar size for a particular cationic surfactant as the counterion is changed according to the series Ch < Br < T, and for a particular anionic surfactant according to Na < < Cs. ... 

The conductance of the bromide counterion associated with the emulsifier micelles is 4.212x10% divided by the conductance readout (8). The concentration of hexadecyltrimethylammonium bromide in the micelles at a 1 3 molar ratio is (53.1-41.0)/4.212xl03 or 2.87x10"%. The total concentration of hexadecyltrimethylannnaniun bromide in the system is (0.15/364.6) (1000/25) or 1.65x10"%. The solute and micellar hexadecyltrimethylammonium bromide concentrations are subtracted from the total concentration to give the concentration in the crystalline rodlike particles. Table III gives these concentrations for the hexadecyltrimethylammonium bromide-cetyl alcohol ratios of 1 6 to 1 0.33. 

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