Monolayer formation of synthetic ammonium amphiphiles carrying cyanobiphenyloxy mesogens

Abstract

New amphotropic molecules 4–6 have been prepared and the monolayer properties have been investigated on a Langmuir trough. The results are compared with the monolayer properties of 1–3. At high area per molecule the amphotropic molecules lie flat on the aqueous surface and both the ammonium head group and the cyanobiphenyloxy mesogens interact with water. Upon compression the cyanobiphenyloxy alkyl moieties are expelled from the surface. The presence of an extra n-dodecyl chain in 4 and 5 does not affect the lift-off area, compared with 1 and 2. The higher π(coex) observed for 4 and 5, compared with 1–3 and 6, is interpreted as stabilization of the monolayer in which the cyanobiphenyl mesogens still interact with water. The presence of a third mesogenic moiety in 6 results in a large increase in lift-off area. Transfer experiments of monolayers of 3 onto quartz substrates show that only one monolayer can be deposited during the upstroke movement. If the transfer is carried out from the solid monolayer phase, the presence of stacked H aggregates could be detected from the optical absorption spectra. In contact with water, this monolayer is not stable and reorganization is observed.