A Langmuir–Schaefer approach for the synthesis of highly ordered organoclay thin films

Abstract

The Langmuir-Schaefer (LS) method has been investigated as a means to control the structure of hybrid organoclay thin films consisting of montmorillonite and dimethyldioctadecylammonium (DODA) cations. We observed a significant modification of the compression isotherms as a function of clay mineral concentration in the subphase, implying clay interaction with the alkylammonium monolayer. For a particular range of clay concentrations, LS hybrid monolayers could be readily prepared on a hydrophobic substrate. The structure of hybrid multilayers of DODA and clay platelets, prepared by repeated LS deposition, was found to be governed by the synthetic route: when the multilayer is fabricated by transferring the hybrid Langmuir films from the surface of the clay dispersion, the DODA-clay particles "flip over" while passing through the meniscus during the even cycles of the deposition, as demonstrated from the elemental analysis of the surface by X-ray photoelectron spectroscopy. In our new model for these multilayers, the structural building block consists therefore of two interdigited DODA layers and two clay layers held together by Na(+). Additionally, a minority phase forms, probably differing from the majority one in the conformation of the alkylammonium cations, and can be eliminated by annealing. This deposition procedure leads to a less ordered structure than an alternative route which combines LS deposition and self-assembly to produce a multilayer consisting of two interdigited DODA layers and one clay layer: here the hydrophilic surface of the transferred hybrid Langmuir film is converted to a hydrophobic surface by dipping into a solution of DODA cations before proceeding with the LS deposition of the next layer.