Ordered Structures of Dichain Cationic Surfactants at Interfaces

• Published in: Langmuir 2003-09, Vol.19 (19), p.7719-7726
• Main Authors: McGillivray, Duncan J, Thomas, Robert K, Rennie, Adrian R, Penfold, Jeffrey, Sivia, Devinder S
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827; 1520-5827
• DOI: 10.1021/la026745v

We have observed stable layered structures adsorbed at the silicon−water and air−water interfaces from N,N-didodecyl-N,N-dimethylammonium bromide surfactant (DDAB) and the corresponding diundecyl compound (DUDAB). The structures were studied by neutron reflection at the wet hydrophilic silicon interface, and at the air−water interface, at concentrations of 0.2−2% w/w. The structures formed are found to have a high sensitivity to temperature, with the repeat spacing decreasing approximately reversibly by about 50% with an increase of temperature of about 40 °C. Associated with the specular scattering there is pronounced off-specular scattering, indicative of a degree of lateral structure or dynamic fluctuation. The observations are consistent with two to four correlated layers which could arise from adsorbed monodisperse unilamellar vesicles, a lamellar phase, or a structured bicontinuous phase. Studies of the bulk solutions with X-ray scattering and cryo transmission electron microscopy show only coexisting unilamellar vesicles and lamellar phase.