Spontaneous multilayer formation of an amphiphilic alkylammonium cation with a long hydrocarbon chain at an air-clay dispersion interface

Abstract

Langmuir film formation behavior of a trimethylammonium cation with a long hydrocarbon chain (CH3(CH2)29N+(CH3)3, C30TMA+) was investigated. To weaken the hydrophobic interactions between the hydrocarbon chains, methanol was added to the subphases. A solution of C30TMA+·I– was spread at air-water and air-clay dispersion interfaces. The surface pressure-molecular area curves of the floating C30TMA+ films showed that the surface pressure at the collapse point decreased with increasing methanol concentration in water and in the dispersion, indicating the weakening of the hydrophobic interaction. For the floating films on clay dispersions, the molecular area at the collapse point decreased with increasing methanol concentration. The decrease in the molecular area indicates multilayer formation of C30TMA+ at the air-dispersion interface at a low surface pressure, which was confirmed by the atomic force microscope images and X-ray diffraction (XRD) patterns of the deposited films. The out-of-plane and in-plane XRD results of the multilayers showed not only layer-by-layer ordering, but also lateral ordering of the closely packed C30TMA+ cations in each layer.