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-fer rocenylhexadecanoic acid (FCAC) transferred onto a decanethiol self-as sembled monolayer on Au is presented. Monolayers of FCAC and mixed mon olayers containing hexadecanoic acid (HDAC) are transferred onto decan ethiol-modified Au electrodes by vertical dipping and horizontal touch ing techniques, resulting in the ferrocene groups residing at the hydr ophobic monolayer-monolayer interface. In-trough voltammetric measurem ents are carried out immediately after monolayer transfer without recr ossing the air/water interface. Surface pressure-area isotherms of mon olayers of FCAC and its mixtures with HDAC are collected at the air/wa ter and air/aqueous electrolyte interfaces in order to investigate the monolayer and mixed monolayer structures at these air/liquid interfac es. The structures of the pure and mixed FCAC monolayers at the air/li quid interface depend greatly on the monolayer composition and the nat ure and concentration of the subphase electrolyte and pH. The monolaye r structure in turn has a dramatic impact on the electrochemical respo nse of the ferrocene groups in the bilayer. While voltammetric waves a re observed in loosely packed LB monolayers of pure FCAC in a 1 M NaCl O4 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 monolaye rs in 1 M NaClO4. Well-defined voltammetry was best observed in expand ed 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 be havior is interpreted on the basis of control of the counterion motion by the structure of the hydrophobic tail region of the outer LB monol ayer. Complementary experiments using AU electrodes modified with 12-f errocenylalkanethiol as LB transfer substrates support this interpreta tion.