Langmuir–Blodgett multilayers of imidazoles

Langmuir–Blodgett (LB) multilayers have been formed from a wide variety of compounds but the basic studies from which others have developed have involved a long hydrocarbon chain terminated at one end by a hydrophilic group. The quaternary ammonium ion has a greater attraction for water than do the other groups which have been used and is so hydrophilic as to preclude the formation of LB layers of straight chain molecules terminated by this group. Certainly all such molecules up to and including octadecyl trimethyl ammonium bromide form micelles in preference to monolayers at the air/water interface though metastable monolayers of this compound have been reported. Quaternized imidazoles have, like quaternary ammonium ions, a positive charge but seem more likely to form LB films. We have thus explored the properties of several of these compounds with a view to producing one which has the desired properties. In order to combat the highly hydrophilic nature of the imidazole group it is necessary to employ a highly hydrophobic ‘tail group’. Thus our most successful compound contains two octadecyl alkyl chains. We have studied the behaviour of this compound at the air/water interface and have found that the nature of the subphase has a very marked influence on the shape of the isotherms. With nitric acid added to the subphase to bring about a pH of 3.5 it is possible to obtain good Y deposition with deposition ratios equal to one in both the upward and downward directions. However we have not been able to achieve LB deposition using any other acid. On heating, the compound in question it changes from a crystalline structure to a smectic structure at 55°C.