Collapse of preformed cobalt stearate film on water surface

Abstract

Preformed cobalt stearate (CoSt) molecules form a film on the water surface, which with barrier compression shows multilayers of different heights that are evidenced from the structures of the films deposited on hydrophilic silicon (0 0 1) substrates by using a horizontal deposition technique at different positions of the surface pressure ( π)–specific molecular area ( A) isotherm. In-plane morphology and out-of-plane structures are obtained from the atomic force microscopy (AFM) and X-ray reflectivity studies. Electron density profiles (EDPs), extracted from the reflectivity data, show that the monolayer coverage is maximum when π is far before the collapse point ( π c) but with barrier compression domains of multilayers start to form even before π c. After π c, two different bilayer repeat distances have been observed from the two different series of the Bragg peaks implying the formation of domains by both the tilted and untilted CoSt molecules. Far after π c, reflectivity decreases rapidly and morphology of the deposited films changes totally. Structures before and after π c of the CoSt film have also been obtained by changing the pH of the subphase water. From all the structural information it is clear that the preformed CoSt film collapses in a different way in comparison with the collapse of the standard cobalt stearate monolayer where cobalt stearate molecules were formed at the air–water interface. Reasons for obtaining different structures on the water surface with barrier compression have been proposed.