Characterization of Langmuir-Blodgett films of 3,4-didecyloxy-2,5-di(4-nitrophenylazomethine) thiophene in a stearic acid matrix

The surface pressure π of two component monolayers of stearic acid (SA) and 3,4-didecyloxy-2,5-di(4-nitrophenylazomethine) thiophene (DNAT) has been measured at the air-water interface by the Wilhelmy method as a function of mean molecular area at various compositions. The surface properties of these mixed films were interpreted in terms of both the additivity rule of molecular areas and the two-dimensional phase rule applied to collapse pressures. Results indicate that SA is miscible with DNAT to up to χ_SA = 0.5. At χ_SA > 0.5 phase separation occurs and at π > 14 mN m^-1 DNAT is ejected from the highly ordered SA film matrix. The index of refraction and thickness of these films dielectrics were determined through surface plasmon (SP) measurements on approximately 500 Å thick silver substrates. Horizontal lifting was used to transfer the Langmuir-Blodgett monolayers to glass slides coated with silver. Mixed solid films of DNAT up to 10% (by weight) in polystyrene matrices with thicknesses ranging from 4 to 12 μm were also prepared, indices and thicknesses being measured using the m-line technique. This allowed us to estimate the index of pure DNAT as a cross-check on the SP results. Since index determinations from these techniques are in essential agreement in terms of linear refractivity, we are able to discuss the structural implications of the SP-measured monolayer film thicknesses.