Chiral sulphur-containing molecules in Langmuir–Blodgett films

Abstract

Long, straight alkane chains have been functionalised with sulphur-containing groups at the chain-end, near the chain-end and at the mid-chain. The behaviour of such compounds in Langmuir films has been investigated. Several gave stable monolayers from which molecular areas have been determined. Surface potential (ΔV) measurements carried out simultaneously with the pressure–area (π–A) isotherms provided an important insight into the surface behaviour of some of the compounds. On compression, changes in the ΔV–A characteristics were clearly visible, well before any significant changes occurred in the π–A curves. New evidence presented here suggests also that the previous interpretation of the π–A behaviour of C₁₀H₂₁SO(CH₂)₁₀CO₂H in terms of molecular orientation was incorrect, dissolution of the monolayer in the water subphase at higher pressure being the most likely explanation. Attempts at depositing chiral L.B. multilayers of (+)-C₁₈H₃₇SO-p-tolyl were not successful because of crystallite formation on the quartz support. Multilayers of (+)-S-octadecyl cysteine were deposited successfully, albeit with a monolayer of stearic acid interposed every seven monolayers of the chiral compound. No optical rotation was observed in the films.