Active control of interfacial properties: A comparison of dimeric and monomeric ferrocenyl surfactants at the surface of aqueous solutions

The surface activity and micellization of the dimeric, ferrocenyl surfactant Fc(CH2N+(CH3)(2)(CH2)(3)-(CH3N+(CH2)(8)Fc 2Br(-) (DI2+), where Fc = ferrocene = [eta(5)-C5H5]Fe[eta(5)-C5H5], are compared and contrasted to those of the monomeric, ferrocenyl surfactant Fc(CH2N+(CH3Br- (I+) in aqueous solution. Dimeric DI2+, when compared to I+, was measured to be surface active in aqueous solutions of 0.1 M Li2SO4 at lower concentrations (0.001 mM of DI2+ versus 0.03 mM of I+) and found to possess a lower critical micelle concentration (cmc) (0.02 mM for DI2+ as compared to 2.0 mM for I+). Linear sweep cyclic voltammograms show that the (two) ferrocenes of DI2+ oxidize independently to form ferrocenium cations. Light-scattering measurements confirm that DI2+ forms micelles in solution and that oxidation of DI2+ to DI4+ is accompanied by the disassembly of micelles. Reduction of DI4+ back to DI2+ reforms the micelles. Measurements of surface tension demonstrate that oxidation of DI2+ to DI4+ leads to desorption of surfactant from the surface of aqueous solutions. Because DI4+ was not measurably surface active at concentrations less than 0.4 mM, oxidation of DI2+ to DI4+ made possible changes in surface tension of 21 mN/m over a range of concentrations that exceeded an order of magnitude (0.02-0.4 mM). Control of the oxidation state of I+, in contrast, permits changes in surface tension of 20 mN/m at one concentration (2.0 mM) only. Dimeric redox-active surfactants in combination with electrochemical methods do, therefore, permit active control of interfacial properties of aqueous solutions over a wider range of surfactant concentrations than is possible with the corresponding monomeric redox-active surfactant. Simple models for the standard free energies of micellization and adsorption can describe the lower cmc of DI2+ as compared to If and the wider range of concentrations over which oxidation of DI2+ leads to changes in surface tension as compared to I+.