Surfactant induced orientation of non-centrosymmetric polyoxometalate clusters in Langmuir–Blodgett films

The trivacant Keggin-type polyoxometalate Na10-α-SiW9O34 (SiW9) was encapsulated by the cationic surfactant dimethyl dioctadecylammonium (DODA) and forms an organic–inorganic complex. Stable Langmuir monolayers of this surfactant-encapsulated cluster complex form at the air–water interface and can be transferred onto solid substrates with a transfer ratio of 1.0 ± 0.02. Fourier transform infrared spectroscopy at grazing incidence reflection condition reveals that the non-centrosymmetric SiW9 clusters are highly oriented in the films with their symmetry axis aligned preferably perpendicular to the layer plane. We propose a model in which the orientation state of SiW9 is induced by the different electrostatic interactions between DODA molecules and different positions at the SiW9 surface.