Surfactant-encapsulated polyoxoanion: Structural characterization of its Langmuir films and Langmuir - Blodgett films

We synthesized a surfactant-encapsulated polyoxoanion, (DODA)₁₆As₄W₃₀Cu₄O₁₁₂ ·¹¹⁴H₂O (denoted SEC-1), which possesses a hydrophobic dimethyl dioctadecylammonium (DODA) shell and an encapsulated hydrophilic polyoxoanion core in organic phases. The novel organic/inorganic composite can form stable Langmuir monolayers, but it is significantly distorted when spread onto an air/water interface although the general core-shell structure seems to remain. Langmuir-Blodgett (LB) films of SEC-1 are readily prepared by transferring the monolayers onto substrates with a transfer ratio near 1. It has been proposed that there exist well-orientated alkyl chains and highly defined layer structure in LB films. Fourier transformation infrared spectroscopy at different temperatures indicates that two obvious phase transitions appear near 34 and 47 °C for multilayer LB films of SEC-1. The polyoxoanion can be organized to some extent within LB films with its long axis parallel to the substrate by comparing the IR transmission spectrum with the reflection-absorption spectrum. The one-monolayer LB film of SEC-1 possesses a homogeneous and flat surface morphology, and a water contact angle of 104 ± 3° for odd-layer LB films shows a highly hydrophobic surface.