Surfactant monolayers on electrode surfaces: Self-assembly of a viologen derivative having a cholesteryl hydrophobic residue

The self-assembly properties of N-[(cholesteryloxy)carbonyl]-N′-ethylviologen (1²⁺) and N-ethyl-N′-octadecylviologen (2²⁺) at the electrode surface were compared on Au in order to assess the effects of the two different hydrophobic tails. Voltammetric data indicated that both compounds self-assemble from aqueous solution forming compact monolayers with the following saturation surface coverages: 3.7 × 10^-10 mol/cm² for 1²⁺ and 4.2 × 10^-10 mol/cm² for 2²⁺. The formal potential for viologen reduction is more positive in 1²⁺ monolayers (-0.345 V vs SSCE) than in 2²⁺ monolayers (-0.458 V vs SSCE). The voltammetric reduction wave for interfacial 1²⁺ is much broader than that for reduction of 2²⁺. However, the monolayers of the cholesteryl derivative seem to be less organized than those made from the octadecyl analog, as judged from the blocking properties of the two monolayers for the reduction of the water-soluble complex Ru-(NH₃)₆³⁺. These results suggest that the cholesteryl-based monolayers are less organized due to the difficulties associated with the packing of the rigid and sterically bulky cholesteryl subunits.