EFFECT OF STRUCTURE ON THE ELECTROCHEMISTRY OF LANGMUIR-BLODGETT MONOLAYERS OF 16-FERROCENYLHEXADECANOIC ACID ON A SELF-ASSEMBLED ALKANETHIOL MONOLAYER

The voltammetry of bilayers of a Langmuir-Blodgett monolayer of 16-ferrocenylhexadecanoic acid (FCAC) transferred onto a decanethiol self-assembled monolayer on Au is presented. Monolayers of FCAC and mixed monolayers containing hexadecanoic acid (HDAC) are transferred onto decanethiol-modified Au electrodes by vertical dipping and horizontal touching techniques, resulting in the ferrocene groups residing at the hydrophobic monolayer-monolayer interface. In-trough voltammetric measurements are carried out immediately after monolayer transfer without recrossing the air/water interface. Surface pressure-area isotherms of monolayers of FCAC and its mixtures with HDAC are collected at the air/water and air/aqueous electrolyte interfaces in order to investigate the monolayer and mixed monolayer structures at these air/liquid interfaces. The structures of the pure and mixed FCAC monolayers at the air/liquid interface depend greatly on the monolayer composition and the nature and concentration of the subphase electrolyte and pH. The monolayer structure in turn has a dramatic impact on the electrochemical response of the ferrocene groups in the bilayer. While voltammetric waves are observed in loosely packed LB monolayers of pure FCAC in a 1 M NaClO4 subphase (pH 6.8) and in immiscible LB films of 1:4 FCAC/HDAC in 1 M HClO4, no faradaic response was obtained with 1:4 FCAC/HDAC monolayers in 1 M NaClO4. Well-defined voltammetry was best observed in expanded monolayers obtained by horizontal touch transfer. Isotherm results suggest a more rigid and better molecular packing of 1:4 FCAC/HDAC on the NaClO4 subphase than on the HClO4 subphase. The electrochemical behavior is interpreted on the basis of control of the counterion motion by the structure of the hydrophobic tail region of the outer LB monolayer. Complementary experiments using AU electrodes modified with 12-ferrocenylalkanethiol as LB transfer substrates support this interpretation.