Diffusion Coefficients of Intrinsically Electroactive Polyelectrolytes and of Counterions Bound to Them

Abstract

An electroactive label was attached to poly(acrylic acid) (molecular weight ) 5000) by reaction with ferrocenylethanol. The resulting ferrocene label was stable when the polymer was dissolved in aqueous solutions, and the diffusion coefficient of the labeled polymer was estimated from the currents obtained as the ferrocene was oxidized at a rotating disk electrode. Although strong adsorption of the polymer on the electrode surfaces depressed the oxidation currents below the convection-diffusion controlled values at rotating disk electrodes, it was still possible to estimate diffusion coefficients by appropriate analysis of the rotation rate dependences of the measured currents. At pH values where the labeled polymer became a polyanion, its diffusion coefficient was also estimated from the reduction currents for Ru(NH 3 ) 6 3+ or Co(NH 3 ) 6 3+ counterions bound electrostatically to the polyelectrolyte. Good agreement was obtained between the diffusion coefficients estimated by the two independent routes.Measurements of diffusion coefficients of polyelectrolyte molecules in aqueous solutions can be useful in discerning the ways in which the structures of the molecules in solution are altered in response to changes in the ionic strength, pH or counterion composition of the supporting electrolytes. Relatively simple electrochemical techniques offer convenient means for estimating diffusion coefficients of electroactive molecules. 1 In previous reports from this laboratory the use of electroactive counterions to study the diffusion of oppositely charged, electroinactive polyelectrolytes has been described. 1-7 With concentrations of the polyelectrolyte sufficient to cause essentially all of the multiply charged electroactive counterions to bind to the polyelectrolyte, the electrochemically evaluated diffusion coefficients of the counterions were assumed to match that of the polyelectrolyte. Although considerable circumstantial evidence in support of this assumption has been obtained, it remained an assumption.In the present study, an intrinsically electroactive polyelectrolyte was synthesized and used in similar experiments with small, electroactive counterions. Electrochemical measurements were employed to evaluate two diffusion coefficients: one for the electroactive polyelectrolyte alone and a second for electroactive counterions bound to the polyelectrolyte. Good agreement between the two diffusion coefficients provided direct evidence in support of the previous assumption that the electrochemical responses obtained from counterions bound to polyelectrolytes offer a reliable means for measuring diffusion coefficients of the polyelectrolytes themselves. In addition, the labeling of poly(acrylic acid) with an electroactive ferrocene center allowed the diffusion of the polymer to be inspected at (low) pH values and (high) ionic strengths where the binding of electroactive counterions was absent or too weak to be useful for this purpose. Experimental SectionMaterials. Poly(acrylic acid) (PAA) having a specified molec...

Dissociation kinetics at polyacrylic latex-coated electrodes

“…Poly(acrylic acid) (PAA) has been used electrochemically for uptake of redox cations, e.g., bis-bipyridyl cobalt [1,2], cadmium in the presence of alumina [3], cupper [4], tris(2,2 0 -bipyridyl)ruthenium [5], nickel(II) tetraazamacrocyclic complex [6], bis-bipyridyl osmium [7], hexaammineruthenium and hexaamminecobalt [8], ferrocenylethanol [9], thallium [10], some alkali metal hydroxides without salts [11], and N,N-dimethylaminomethyl-ferrocene [12]. Negatively charged molecules such as hexacyanoferrate cannot be simply taken up to PAA [5].…”

“…The uptake of cations to PAA is caused by association of the carboxyl moiety in PAA, and hence a degree of the uptake should be determined by the pK a as well as pH in solution [5,9], as is predicted from the association in acetic acid. When PAA is in a solid state or a film, the association is complicated by block of a loss of permeation of solvent and ions.…”

“…When PAA is in a solid state or a film, the association is complicated by block of a loss of permeation of solvent and ions. It is also hindered by electrostatic interaction of the deprotonated carboxyl group [9], as has been found in carboxyl alkanethiol self-assembled monolayers [21][22][23]. In order to control quantitatively a degree of the association, PAA has to be dispersed in solution [10,11], or such PAA films have been employed that all the acrylic moiety might be exposed to solution [20,24].…”

Convenient synthesis of new water-soluble monosubstituted functional ferrocene derivatives

“…There is considerable interest in the synthesis of new polymers via introducing functional ferrocene derivative structure because of their potential applications in optical, electronic and magnetic materials [1]. However, the reports relating to conductive polymers with ferrocene moiety are scarce.As far as know, most of the known ferrocenyl compounds are only soluble in hydrophobic media that limited their application in aqueous systems [2,3]. Knox et al [4] had reported the preparation of water-soluble Ferrocenyl sulfonic acid (SAFc) by sulphonation of ferrocene at the presence of reagent of chlorosulphonic acid in acetic anhydride.…”

“…As far as know, most of the known ferrocenyl compounds are only soluble in hydrophobic media that limited their application in aqueous systems [2,3]. Knox et al [4] had reported the preparation of water-soluble Ferrocenyl sulfonic acid (SAFc) by sulphonation of ferrocene at the presence of reagent of chlorosulphonic acid in acetic anhydride.…”