Hybrid Langmuir–Blodgett monolayers containing clay minerals: effect of clay concentration and surface charge density on the film formation

Abstract

To control the properties of hybrid organo-clay films prepared by the Langmuir–Blodgett (LB) method, the film formation mechanism should be understood. This work aimed to understand what occurs at the air–water interface after the spreading of cationic surfactants (octadecyl rhodamine B, 3,3′-dioctadecyl oxacarbocyanine) on aqueous dispersions of smectite clay minerals (saponite, montmorillonite, hectorite, laponite), resulting in hybrid organo-clay films. Information on the amount of surfactant molecules and clay particles in the hybrid films was obtained with surface pressure versus molecular area isotherms, attenuated total reflection infrared spectroscopy, ultraviolet-visible spectroscopy and atomic force microscopy. X-ray reflectivity measurements indicated that the surfactant molecules had adsorbed on only one side of the clay mineral lamella. With increasing clay concentration of the dispersion, the isotherms shifted to a lower lift-off area, a minimum lift-off area (MLA) was reached and then the lift-off area increased again. Films made at lower than the MLA clay concentration consisted of two phases: an organic phase and a hybrid organo-clay phase. Films made at the MLA clay concentration consisted of dense monolayers of the surfactant molecules and single clay mineral lamellae. The density of the surfactant molecules was highly correlated with the surface charge density of the clay minerals. These films had low water content. Films made at higher than the MLA clay concentration contained less surfactant, aggregates of the clay mineral particles, residual Na⁺ ions and water. With the clay concentration of the dispersion and the surface charge density of the clay mineral, the properties of hybrid organo-clay LB films can be adjusted.