SELF-ASSEMBLED AND LANGMUIR-BLODGETT ARACHIDIC ACID MONOLAYERS ON SILVER - A COMPARATIVE REFLECTION-ABSORPTION FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY

Self-assembled (SA) and Langmuir-Blodgett (LB) monolayers of arachidic acid on silver surfaces have been investigated by a reflection-absorp tion Fourier transform infrared spectroscopy. Arachidic acid was adsor bed on silver as carboxylate with its two oxygen atoms bound symmetric ally to the surface. Although both the SA and LB monolayers consisted of fully extended trans zigzag carbon chains, a closer examination ind icated that the SA monolayers should possess a more ordered crystallin e structure than the LB monolayers. The infrared spectral data dictate d that the extent of methyl group exposure at the air-film interface w as greater in the SA monolayers than the LB monolayers, in agreement w ith the contact angle measurement. From a theoretical analysis, the al kyl chains in each monolayers seemed to be tilted away from the surfac e normal by less than 3.5-degrees, but in opposite directions. Arachid ic acid monolayers were concluded to have same structure as stearic ac id monolayers.