IN-SITU AND REVERSIBLE CONTROL OF THE SURFACE-ACTIVITY OF FERROCENYL SURFACTANTS IN AQUEOUS-SOLUTIONS

The redox-active surfactant (11-ferrocenylundecyl)trimethylammonium br omide (Fc(CH2N+(CH3Br-; Fc = [eta(5)-C5H5]Fe[eta(5)-C5H5]), in combina tion with electrochemical methods, permits reversible control over the surface tension of an aqueous solution. The change in surface activit y of the surfactant was caused by oxidation of the electrically neutra l ferrocenyl moiety to the polar ferrocenium cation. Oxidation of 0.1 mM Fc(CH2N+(CH3)(3) dissolved in aqueous 0.1 M Li2SO4 caused the surfa ce tension of the solution to increase from 49 to 72 mN/m. The change in surface tension was reversible; reduction of the dicationic Fc(+)(C H2N+(CH3)(3) to Fc(CH2N+(CH3)(3) recovered the initial surface tension of the solution (49 mN/m). Surface tensions of aqueous solutions were cycled repeatedly (> 14 cycles) between these limits. Surprisingly, o xidation of Fc(CH3)(3) at concentrations greater that 10 mM caused a d ecrease in the equilibrium surface tension. The limiting area per mole cule at the surface of the aqueous solution (0.1 M Li2SO4, pH 2) was u nexpectedly similar for both Fc(CH2N+(CH3)(3) (85 +/- 4 Angstrom(2)) a nd Fc(+)(CH2N+(CH3)(3) (75 +/- 4 Angstrom(2)).