Hygroscopic growth of self-assembled layered surfactant molecules at the interface between air and organic salts

We report here the self-assembly of surfactant molecules at the interface of air and the hygroscopic quaternary ammonium salt tetrabutylammonium acetate (TBAAc). Homogeneously dissolved surfactant molecules at 100 °C self-assemble upon contacting air due to high moisture adsorption by the organic salt when cooling down. Highly ordered lamellar phases with different lattice spacings have been observed when surfactants with various lengths of alkyl chains were used. CnTMAB/TBAAc systems showed all-trans conformation of interior methylene carbons and interdigited bilayers with an average CH2 increment of 0.119 nm, while CnNH2/TBAAc systems showed trans/gauche mixed conformations of interior methylene carbons and bilayers with an average CH2 increment of 0.247 nm. CnNH2s in CnNH2/TBAAc formed bilayers through water-mediated intermolecular hydrogen bonds with a water layer thickness of 0.51-0.61 nm. In CnTAB/TBAAc, as the head group of CnTAB is bigger, the interdigited bilayer thickness (d-spacing) is smaller, because the bigger head groups accommodate enough space for alkyl tails to come in between them.