Interaction of copper ions with stearic acid Langmuir monolayer and formation of cluster structures in monolayers and Langmuir-Blodgett films

The interaction of copper ions with the stearic acid Lagmuir monolayer resulting in an extremely high level of copper binding to monolayer in quantities that significantly exceeded the amount of stearic acid molecules in the monolayer was studied. The shape of the pressure-area isotherm changed drastically upon the pH changes from 4 to 6 in the presence of copper ions in aqueous phase (at concentrations from 10(-5) to 10(-3) M) or upon the addition of copper ions to the aqueous phase under different monolayer compression. The bulk phase copper ion concentration changes, caused by binding to the monolayer, were studied with the use of EPR at the equilibrium after intensive mixing of the bulk phase and were found to depend on the aqueous phase pH and monolayer compression extent. The highest level of binding (up to 100 copper ions per one stearic acid molecule, pH 5.6, initial copper concentration 5 . 10(-4) M) was observed at the surface pressure of about 20 mN/m; further monolayer compression and the corresponding increase in the surface pressure caused the reverse growth of the aqueos phase copper ion concentration. At collapse (monolayer fracture and destruction) the copper ion concentration in the bulk phase was similar to that with absence of monolayer. The EPR spectra and SAXS difractograms of copper-containing stearic acid multilayers confirmed the high copper content in the obtained LB films. STM study of pure stearic acid and the copper-containing monolayer LB films, transferred to graphite wafers from the water subphase surface (pH 5.4) with various copper concentration, discovered nanosized (about 5 nm) cluster formation an the monolayer surface. The obtained data point out that interaction of the charged Langmuir monolayer with copper ions and formation of copper containing nanostructures depended on the monolayer compression and was determined by arrangement, order, mobility of the monolayer stearic acid molecules and by electrostatics at the interface.