Entrainment of aqueous subphase in Langmuir-Blodgett films

Abstract

An on-line capacitance technique has been employed for the detection of entrainment of the aqueous subphase during Langmuir-Blodgett deposition. This technique exploits the high dielectric constant of the subphase relative to air and to deposited monolayers. An interdigitated electrode structure was patterned on a substrate, and the capacitance was recorded throughout a dipping cycle. Electrodes were oriented parallel and perpendicular to the air-water interface during dipping. Subphase entrainment was detected by means of two indicators, drying time and the slope of the capacitance vs. time trace. Entrainment occurred during the deposition of all the monolayer materials employed, namely arachidic acid and n-docosyl homologues of merocyanine and hemicyanine. Comparable levels of entrainment observed for both orientations of the electrode indicate that entrainment is independent of topography. The dyes had characteristics different from those of arachidic acid. Poor deposition was obtained on the downstroke with partially dry substrates, indicating the strong influence of entrainment on the quality of deposition. This emphasizes the need for entrainment studies, and also the applicability of the capacitance technique in determining the necessary dwell times.