Counter-ion conductivity

Polypyrrole Counter-ion Conductivity (S cm-1) Carrier Density (cm-3) Carrier Mobility, 2., -1 -U (cm Vs)... 

Newkome and co-workers have demonstrated the operation of a template effect in the formation of a pyrido-ester-crown. In the reaction shown in Eq. (2.8), they treated 2-clTloronicotinoyl cliloride with either the disodium or dipotassium salt of pentaethylene glycol. TJie two reactions were conducted under identical conditions except for the presence of sodium vs. potassium cations. Since the product is a six-oxygen macrocycle, its formation would be expected to be favored by K" rather than Na" counter ions for the glycolate. In fact, the yields of crown-lactone were 30% and 48% respectively when Na" and K" were the templating cations. 

Room temperature ionic liquids arc currently receiving considerable attention as environmentally friendly alternatives to conventional organic solvents in a variety of contexts.144 The ionic liquids have this reputation because of their high stability, inertness and, most importantly, extremely low vapor pressures. Because they are ionic and non-conducting they also possess other unique properties that can influence the yield and outcome of organic transformations. Polymerization in ionic liquids has been reviewed by Kubisa.145 Commonly used ionic liquids are tetra-alkylammonium, tetra-alkylphosphonium, 3-alkyl-l-methylimidazolium (16) or alkyl pyridinium salts (17). Counter-ions are typically PF6 and BF4 though many others are known. 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

Figure 20. Artificial muscle under work. In reduction (A) electrons are injected into the polymer chains. Positive charges are annihilated. Counter-ions and water molecules are expelled. The polymer shrinks and compaction stress gradients appear at each point of the interface of the two polymers. The free end of the bilayer describes an angular movement toward the left side. (B) Opposite processes and movements occur under oxidation. (Reprinted from T. F. Otero and J. Rodriguez, in Intrinsically Conducting Polymers An Emerging Technology, M. Aldissi, ed., pp. 179-190, Figs. 1,2. Copyright 1993. Reprinted with kind permission of Kluwer Academic Publishers.)...

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

These equations describe the full oxidation of a conducting polymer Submitted to a potential step under electrochemically stimulated confer-mational relaxation control as a function of electrochemical and structural variables. The initial term of /(f) includes the evolution of the current consumed to relax the structure. The second term indicates an interdependence between counter-ion diffusion and conformational changes, which are responsible for the overall oxidation and swelling of the polymer under diffusion control. 

Otero and co-workers208,212 have visually observed nuclei of oxidized polymer in thin polypyrrole films on electrodes. They attribute these to sites of counter-ion and solvent ingress. A nucleation model based on the growth of ionically conductive zones provides good agreement with experimental chronoamperometric responses. 

There has been much interest in modifying the ion-exchange properties of conducting polymers with substituents or polymeric counter-ions. Cationic substituents, such as ammonium234 and pyridinium235 groups, increase the polymer s anion-exchange capacity and increase anion trans-... 

The involvement of halogen bonding in conducting molecular materials is essentially based on the use of halogenated TTFs in electrocrystallization experiments with counter ions of Lewis base character prone to act as halogen bond acceptors. This concept was first successfully introduced by Imakubo... 

NB. Where only a current density was given in the original Ref. this is quoted in place of the polymerisation potential. c Counter ions are as incorporated during the electrochemical polymerisation process or by subsequent electrochemical doping unless suffixed with (chem.) which indicates the use of chemical doping. d Conductivities f-film, p-pressed pellet. 

The maximum in optical absorption for anodic oxides on Au as determined by McIntyre and Kolb [66] at a photon energy of 4.0 eV may be attributed on the basis of the UPS data to a transition from the maximum of the valence band to the conduction band. The differences between the spectra of the anodized and rinsed sample in Fig. 17 is due to counter ion removal after rinsing the sample with H20 [67]. 

We have described above the evolution of the magnetic properties of the [Cp2M (dmit)]AsFg salts upon isomorphous Mo/W substitution. Another possibility offered by this attractive series is the isomorphous substitution of the counter ion, that is PFg- vs AsF6 vs Sbl- fi. Electrocrystallization experiments conducted with [Cp2Mo(dmit)] and the three different electrolytes afforded an isomorphous series, with a smooth evolution of the unit cell parameters with the anion size [32], This cell expansion with the anion size leads to decreased intermolecular interactions between the [Cp2Mo(dmit)]+ radical cation, as clearly seen in Table 2 from the decreased Curie-Weiss temperatures and Neel temperatures (associated with the transition they all exhibit to an AF ground state). 

Y Cao, P Smith, and AJ Heeger, Counter-ion induced processability of conducting polyaniline and of conducting polyblends of polyaniline in bulk polymers, Synth. Met., 48 91-97, 1992. 

In many cases, it will be impossible to prevent unwanted reactions at the counter electrode. Then a separation of the anolyte and catholyte is needed. An optimal compromise has to be found for the separator between separation effectiveness and ion conductivity, that is, minimized electrical resistance and low energy consumption. Moreover, chemical, thermal, and mechanical stability and price of the separator have to be considered. Naturally, a complete separation is impossible, because a slight diffusion rate is inevitable. In laboratory scale experiments, probably a high cell voltage is acceptable in order to realize a maximal separation. 

In the case of PEMs, the situation is more complicated because the sulfonate counter-ions (in the case of a PEM such as Nafion ) are bound to the polymer chain and are thus relatively immobile, in contrast to the free counter-ion in a small molecule acid such as sulfuric or acetic acid. Tethering of the sulfonate group can be considered to be an impediment to the mobility of the proton as it traverses the membrane. Proton mobility is also affected by the effective mean-free path of connectivity of the conduction pathway as shown in Figure 3.2. In situation (a), the increased number of dead ends and tortuosity of the aqueous domains through which proton transport occurs over the situation in (b) leads to lower overall mobility. This has been demonstrated by Kreuer and will be discussed later in this section. 

Miyamoto and Shibayama (1973) proposed a model which is essentially an extension to free volume theory, allowing explicitly for the energy requirements of ion motion relative to counter ions and polymer host. This has been elaborated (Cheradame and Le Nest, 1987) to describe ionic conductivity in cross-linked polyether based networks. The conductivity was expressed in the form... 

Williams, R. C., Bocuher, R., Brown, J., Scull, J. R., Walker, J., and Paolini, D. (1997). Analysis of acetate counter ion and inorganic impurities in pharmaceutical substances by capillary ion electrophoresis with conductivity detection. /. Pharm. Biomed. Anal. 16, 469—479. 

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