The effect of dopant pKa and the solubility of corresponding acid on the electropolymerisation of pyrrole

Abstract

a b s t r a c tIn this paper, attempts to dope polypyrrole (PPy) with two small sized anionic drugs, diclofenac sodium salt (NaDF) and valproic acid sodium salt (NaVPA), are described. For PPy doped with DF − , unusual patterns in growth and morphology were observed. During the deposition of the polymer, the rate of electropolymerisation decreased with increasing time and higher applied potentials. The polymer had features of an insulating film, while SEM confirmed the presence of crystal-like shards on the surface of the polymer. Analyses of these crystals indicate them to be drug that may have precipitated out of solution. These findings suggest that insoluble drug crystals are formed during electropolymerisation. The formation of PPy doped with a small soluble anti-epilepsy anionic drug, VPA − , was also studied; however it was not possible to incorporate this drug during electrochemical polymerisation. Again, this was explained in terms of the equilibrium between the anion and the acid forms of VPA. At pH values below 5.6, the equilibrium of the VPA − is shifted towards the insoluble HVPA. As the monomer is oxidised, there is a decrease in the local pH in the vicinity of the electrode and this causes the HVPA to precipitate from solution. This, in turn, prevents any PPy from being deposited at the electrode.

Electropolymerized Molecularly Imprinted Polypyrrole Film for Sensing of Clofibric Acid

“…, 2.7 mM, at 25 °C, estimated with EPI Suite v4.11) at the polymer surface, because due to clofibric acid’s pK a of 2.8–3.2, at pH 3.0 only about 50% of the molecules are dissociated [ 42 , 43 ]. Anionic drugs with low solubility such as diclofenac (4.5 mg·L −1 ) and valproic acid (895 mg·L −1 at 25 °C) were also found to inhibit the polymerization of polypyrrole [ 44 ]. Therefore, the pH of the solution was increased to pH 7.0 by the use of phosphate buffer solution instead of KNO 3 .…”

The formation and properties of polypyrrole doped with an immobile antibiotic

“…If the neutral molecule has a low solubility, which is generally the case with large organic-like molecules, then precipitation o f t h e m o l e c u l e o c c u r s t o l i m i t o r p r e v e n t electropolymerisation. This was shown by Ryan et al [24] for the sodium salts of diclofenac and valproic acid, where the formation of the insoluble neutral molecule prevented the formation of polypyrrole doped with valproate and gave rise to the deposition of crystals of diclofenac, limiting the formation of the polymer doped with diclofenac.…”