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.