Laboratoire de Physique des Solides, Orsay
Organic monolayer of amphiphilic molecules, like stearic acid in presence of different divalent ions in subphase water, behaves differently under pressure. For certain cations (Cd2+, Pb2+) it collapses at constant area, while for other cations (Mg2+, Mn2+, Co2+, Zn2+) collapses nearly at constant pressure. Two different kinds of out-of-plane structures of these collapsed films are obtained from x-ray reflectivity analysis. The information obtained from x-ray reflectivity and AFM analysis show that there is a striking similarity with the Stranski-Krastanow (SK) growth mode in constant pressure collapse. Trivalent ions like Fe3+ ions when incorporated chemically into the stearic acid molecules, the preformed three-tailed fatty acid salt ferric stearate (FeSt) form a bi-molecular layer structure of these molecules on water surface and this drastically enhances capillary wave fluctuations on water surface due to a reduction in surface tension of water to 1mN/m. The growth nature of such bi-molecular layer on water surface with increasing surface pressure and it’s membrane like behavior has also been obtained. In presence of Cd2+ ions but at high pH (≈ 9.5) of the water subphase, a bidentate bridging type headgroup structure is formed at the water surface that changes the morphology of the deposited Langmuir-Blodgett (LB) films.