Formation of hybrid monolayers of alkylammonium cations and clay particles at the air-water interface has been studied by surface pressure-molecular area (pi -A) isotherm measurements, atomic force microscopy (AFM), and infrared (IR) spectroscopy. The pi -A isotherm of a monolayer of octadecylammonium chloride (ODAH(+)Cl(-): C-18) on a clay suspension in a Langmuir-Blodgett trough approaches that of ODAH(+)Cl(-) on pure water when the amount of clay in the suspension decreases from 100 to 1 ppm. The AFM images of the films reveal. hybrid monolayers with clay particles. At 0 mN m(-1) the film is only partially covered with clay particles. As the molecular area decreases, the clay particles come closer and closer to each other in the film until they touch each other. Then the surface pressure increases. The IR spectra of multilayer films of ODAH(+) and clay (3-10 layers) reveal a linear increase of the amount of the alkylammonium cations with the number of deposited layers. The area per ODAH(+) cation in the film estimated from the spectral 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 interestingly, even alkylammonium cations with short chains (C-4-C-12) that are soluble in water form hybrid monolayers with clay particles at the air-water interface. This means that clay particles in the subphase play a role as stabilizers for monolayer formation of water-soluble ammonium cations. On the basis of these data, a plausible mechanism of formation of the hybrid monolayers was proposed.