Ferrocene and its derivatives are excellent electroactive probes. Cyclic voltammetry measurements are employed to yield diffusion coefficients of surfactant aggregates assuming the electroactive probe strongly binds to the micelles using a standard two site model. Ferrocene and hexylferrocenyl alcohol were investigated in the presence of two surfactants, dodecyltrimethylammonium bromide (DTAB) and, propandiyl-α-ω-bis(dodecyldimethylammonium bromide, a cationic gemini surfactant (labelled 12-3-12). Preliminary data shows the observed ferrocene diffusion coefficients of 2.0 x 10 – 10 m2/s and 7.1 x 10 – 11 m2/s in the presence of 40mM DTAB and 40 mM gemini surfactants. Similarly for the hexylferrocenyl alcohol probe, the diffusion coefficients are 8.4 x 10 – 11 m2/sec and 1.8 x 10 – 11 m2/sec, in the presence of 40 mM DTAB, and 40 mM 12-3-12 bromide, respectively. The hexylferrocenyl alcohol is more hydrophobic than the ferrocene so the predicted diffusion coefficients for the former are predicted to be smaller in magnitude. In recognition of the two site model, the hexylferrocenyl alcohol diffusion coefficients more adequately show the diffusion character of the micellized surfactant than ferrocene. Using DOSY NMR methods, the surfactant self-diffusion coefficient of 12-3-12 at 15 mM is 5.6 x 10 – 11 m2/sec which compares well to the electrochemical diffusion coefficient of hexylferrocenyl alcohol in 15 mM 12-3-12 surfactant which is 7.6 x 10-11 m2/sec. In conclusion, the hexylferrocenyl alcohol probe must partition into the micellar aggregate more strongly than ferrocene and yields diffusion coefficients more closely paralleling the NMR self-diffusion coefficients of the aggregated surfactants. The cyclic voltammetry methods are generally applicable using simple inexpensive instrumentation and could be useful compliment to NMR methods.

Electrochemical diffusion coefficients for cationic surfactants using ferrocene probes

255th National Meeting (2018)

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