Preparation and characterisation of Langmuir-Blodgett films of 4-(4-(N-octadecyl-N-methylamino) phenylazo)benzoic acid

Langmuir films of the title compound have been spread on an aqueous subphase at various values of pH at 10, 15 and 20 °C. Increasing pH and increasing temperature favour stable films, but at the higher subphase pH and temperature values the pressure-area isotherms exhibit a transition to aggregated forms. Tilt angles between the alkyl chains and the normal to the subphase increase with increasing subphase pH and temperature. The area per molecule for films deposited at 15 °C decreased steadily with time at a fixed surface pressure, except at the highest pH, indicating poor stability. Langmuir-Blodgett films deposited at a surface pressure of 30mN m^-1 and a subphase temperature of 15 °C were of Y-type and showed transfer ratios above unity for the lower subphase pH values. UV/visible spectra of the LB films showed features characteristic of the formation of H aggregates for deposition at the higher subphase pH values. Over long periods of time the spectra for high pH showed evidence of increasing aggregation. Small-angle X-ray diffraction confirmed molecular tilts larger than those deduced in the floating monolayer. Reflection-absorption infrared spectroscopy of the LB films showed differences from the bulk also indicative of significant tilt, as did surface-enhanced Raman spectroscopy. The LB films showed weak second-harmonic generation from 1064 nm radiation consistent with a polar film structure parallel to the substrate.