Investigation of interaction, hysteresis and reactions of mixed langmuir monolayers over air/aqueous interface

Abstract

Mixed Langmuir films of N-octadecyl-N’-phenylthiourea/stearic acid (OPT/SA), [(octadecylimino)methyl]phenol/octadecylamine (ODIMP/ODA) and 2-methoxy-6- [(octadecylimino)methyl]phenol/stearic acid (MODIMP/SA) over air/water interface have been investigated for their pressure-area isotherms and the hysteresis behaviour of the isotherms. Likewise films of OPT over copper ions containing aqueous subphase were investigated. Calculations of the areas/molecules of films formed by spreading pre-mixed solutions of various ratios reveals that a 1:1 complex is formed between OPT and SA. When re-mixed solutions of ODIMP and ODA are spread homogeneous monolayers result, but when the components are spread individually, the monolayers formed are non-homogeneous. For homogeneous MODIMP/SA film, the calculated excess areas exhibited negative deviation from the ideal values at all compositions for lower surface pressures. Gibbs free energies of mixing had been calculated and the most stable state of the mixed monolayer was found to be one having the mole fraction of SA between 0.5 and 0.6. The study of OPT over copper ions reveals that the interaction between thiourea and copper ions could be controlled by the addition of protons. The mechanism of this reaction is based on the intermolecular shift of two electrons from the thiolates to Cu (II) atoms to form the disulfides, and the corresponding reduction of the Cu2+ to Cu+ and the formation of Cu (I)-disulfide complexes.