Polyelectrolyte solutions, reaction rates

Studies of vinyl polyelectrolyte solutions connected with problems of this type have dealt with specific effects of neighboring groups in intramolecular reactions and rates of reaction between polyelectrolytes and simple substrates. Comprehensive reviews of this subject have been published (27, 22, 23). 

The addition of PVMI was expected to enhance the hydroxide-ion catalyzed hydrolysis of the anionic ester, since both the hydroxide ions and the negatively charged ester are attracted to the polycation so that the rate of their mutual collision is increased. For any given polyelectrolyte concentration the increase in the rate of hydrolysis should be independent of pH. And this is what has actually been found at pH values greater than 9. At lower pH values, however, a completely unexpected behavior resulted, i. e. the polycation was found to increase the rate of hydrolysis of NABS by the largest factor in that pH range (pH s 6) in which direct water attack on the ester makes the dominant contribution to the overall reaction rate (Fig. 8). The influence of hydrophobic forces appears to be ruled out in this case since PVMI has no effect on the solvolytic rate of the neutral ester p-nitrophenyl acetate and p-nitrophenyl hexanoate. Thus, the causes of the above-mentioned phenomenon are obscure this very fact adds, in the author s opinion, further interest to the study of reactions in polyelectrolyte solutions. The examination of such factors as the enthalpy and entropy of activation may be of particular relevance for a deeper insight into these complex reaction systems. 

This effect suggests an interesting possibility for the utilization of kinetic data of ionic reactions in polyelectrolyte solutions to characterize the distribution of the electrostatic potential in such systems. Consider a system in which the local concentrations of two reagents, Ca and Cb, vary widely as a function of the spatial coordinates. If we assume that in any volume element 3F the process takes place at a rate k CACB V,... 

This effect has been known for quite some time [76-81] and used to influence the reaction rate between the charged particles. Examples include some hydrolysis reactions [80] where a small addition of polyelectrolyte causes a dramatic acceleration of the chemical reaction between equally charged divalent counterions in solution. The effect of a polyelectrolyte on ion-ion collision frequencies has also been used to probe the distribution of ions around the polyion. For example, Meares and coworkers [82] probed the electrosta-... 

This step is diffusion-controlled it depends on the rate of collisions between the oppositely charged species. Factors influencing this step are the polyelectrolyte concentration and the temperature. The second step is rearrangement of these initial complexes towards their equilibrium state. These rearrangements occur via polyelectrolyte exchange reactions between the polymer chains within the complex or with free polymer chains in solution [3, 4]. 

Morawetz and coworkers have also studied the catalysis of ionic reactions by polyelectrolytes [26—28]. These authors argue that the fluctuations of the electrostatic potential in a polyelectrolyte solution may in principle be studied by monitoring the effect a polyelectrolyte has upon the rate of reaction involving two small species. Qualitatively, if the charge of these two species is opposite to that of the polyion, the reagents... 

On the other hand t and A r contain the rate constants of the equilibrium under study and, for bimolecular reactions, the concentrations of the species involved in the reaction [4b, 8]. Had counter-ion site binding been a one step process the determination of t would have provided a direct estimation of the lifetime of the bound counter-ions and, therefore, of the exchange rate between bound and free counterions. On the other hand the study of as a function of concentration would have permitted to obtain informations on the distribution of bound counterions between those bound with and without dehydration. Unfortunately, as will be shown in Section 4, site binding is a multistep process involving at least two equilibria. All of the unknown quantities involved in such a process (four rate constants, two volume changes and the concentrations of the species) cannot be obtained from ultrasonic absorption data alone. Independent measurements become necessary. For this purpose we have measured the density d of the polyelectrolyte solutions from which can be obtained the apparent molal volume FcP of the polyelectrolyte CP (C counterion, P polyion) according to ... 

Spectacular increases in spontaneous and base-catalyzed or metal-assisted aquation rate constants of particularly pentaammineco-balt(III) complexes have been observed in the presence of polyelectrolytes and micelles. Polyelectrolytes are known to cause marked acceleration of reactions, though decelerations can be observed, and the effects are very large compared with those produced by equivalent amounts of corresponding low molecular weight substances similar effects are observed in solutions containing charged micelles. This area of research has been extensively reviewed (63,101,133,139,195, 206, 207), so discussion will be selective. 

Table 1. Rate data at 25°C for the reaction CV - -OH carbinol in buffer soltUion and in polyelectrolyte buffered solutions (spectrophotometric measurements at 590 mp)...

We have initiated a series of investigations to study the catalytic effects of a class of cationic polyelectrolytes ranging in solution behavior from "normal" polyions to polysoaps upon the alkaline hydrolysis of neutral and anionic phenyl esters of varying chain lengths. Employing these catalysts of varying hydrophylic-hydrophobic character in reactions of neutral and anionic substrates of varied hydrophilic-hydrophobic character, it should be possible to elucidate the contributions of both the hydrophobic interactions and electrostatic interactions on the rate of reaction. 

A very important monograph outlined the state of knowledge up to 1974, and also noted other associated species which could influence the rates of thermal, photochemical and radiation induced reactions [1]. The initial studies of micellar effects were made in water, but subsequently micelle-like aggregates were observed in non-aqueous solution. These aggregates can also influence chemical reactivity. In some respects the effects of micelles on reactivity are similar to those of cyclo-dextrins or synthetic polyelectrolytes. 

Product solutions were passed over a size exclusion diromatograpl column to remove 1-ArOH formed from reaction of l-ArN2 with water. Hiese solutions containing tagged polyelectrolyte were hydrolyzed in 1 M NaOH at ambient temperature for 30 hours to ensure complete hydrolysis of the ester linkages. The half-life for the hydrolysis of the model compound 1-ArOAc is only about 5 minutes under these conditions based on its measured second order rate constant of 2.8 X 10 M sec After the hydrolyses were complete, the pH of each solution was adjusted to 7.0 by titrating wiA 6 M HCl and monitoring with a pH meter. The yields of 1-ArOH were analyzed by HPLC by fluoresence detection in triplicate 100 pi injections. 

Reactions in the polyelectrolyte medium of the coating are usually slower than in solution. Collisional restrictions may be responsible for the lower rates when the diffusion-controlled rate constant is much smaller within the polymeric phase. Mediated reactions were studied under conditions where the collisional restrictions on the rates were minimal. The IrCl incorporated into a cationic polyelectrolyte exhibited high diffusional rates. Oxidation of catechol or L-dopa (3-(3,4-dihydroxy-... 

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