SURFACTANT MONOLAYERS ON ELECTRODE SURFACES - SELF-ASSEMBLY OF A SERIES OF AMPHIPHILIC VIOLOGENS ON GOLD AND TIN OXIDE

The aggregation of a series of N-alkyl-N'-ethyl viologens, with alkyl chains from 10 to 18 carbons long, N-allyl-N'-hexadecyl viologen, N-allyl-N'-octadecyl viologen, and N,N'-diheptyl viologen at the electrode-solution interface was surveyed on Au in order to establish correlations between the structural features of the viologen amphiphiles and the electrochemical properties of the interfacial assemblies. The octadecyl viologens 1 and 7 were found to self-assemble on gold from aqueous solutions, forming compact monolayers with a limiting coverage of 4.1 x 10(-10) mol/cm2. The hexadecyl derivatives 2 and 6 also self-assembled on gold but the measured limiting coverages were lower. The symmetric viologen 8 was not found to self-assemble appreciably under identical experimental conditions. Aggregation was also observed on SnO2 surfaces. Compact interfacial monolayer assemblies formed by compounds 1 and 7 mediate the heterogeneous electron transfer reaction between the water-soluble complex Ru(NH3)6(3+) and the underlying electrode surface. In contrast, no mediation was observed for the hexadecyl analogues. A zwitterionic derivative of ferrocene was also used to probe the degree of packing and determine the barrier properties of the viologen monolayers. The results indicate that the blocking effects arise mainly from the lipophilic nature of the monomers' alkyl chains.