Fabrication of hybrid films of alkylammonium cations (CnH2n+1NH3+; n=4-18)and a smectite clay by the Langmuir-Blodgett method

Formation of hybrid monolayers of alkylammonium cations and clay particles at the air-water interface has been studied by surface pressure-molecular a rea (pi -A) isotherm measurements, atomic force microscopy (AFM), and infra red (IR) spectroscopy. The pi -A isotherm of a monolayer of octadecylammoni um chloride (ODAH(+)Cl(-): C-18) on a clay suspension in a Langmuir-Blodget t trough approaches that of ODAH(+)Cl(-) on pure water when the amount of c lay in the suspension decreases from 100 to 1 ppm. The AFM images of the fi lms reveal. hybrid monolayers with clay particles. At 0 mN m(-1) the film i s only partially covered with clay particles. As the molecular area decreas es, the clay particles come closer and closer to each other in the film unt il they touch each other. Then the surface pressure increases. The IR spect ra of multilayer films of ODAH(+) and clay (3-10 layers) reveal a linear in crease of the amount of the alkylammonium cations with the number of deposi ted layers. The area per ODAH(+) cation in the film estimated from the spec tral data (0.14 nm(2) molecule(-1)) is consistent with the molecular area ( 0.12 nm(2) molecule(-1)) from the pi -A isotherm at 20 mN m(-1). Most inter estingly, even alkylammonium cations with short chains (C-4-C-12) that are soluble in water form hybrid monolayers with clay particles at the air-wate r interface. This means that clay particles in the subphase play a role as stabilizers for monolayer formation of water-soluble ammonium cations. On t he basis of these data, a plausible mechanism of formation of the hybrid mo nolayers was proposed.