Powders of nano-particles used in high performance ceramic production often require a protection both against chemical reactions and aggregation in aqueous environment. As well known, surfactant films adsorbed on the particle interfaces can provide a protective coating. Using Monte-Carlo-simulations (µ-V-T ensemble) for a coarse grained model the adsorption equilibrium of amphiphilic molecules in polar solvents (e.g. water) is investigated. In aqueous solution adsorbed surfactant molecules aggregate to bilayers on hydrophilic solid interfaces to reduce the hydrophobic interaction between the hydrocarbon chains and water. For moderate adsorption energies adsorbed bilayers are formed at surfactant concentrations lower than the threshold for aggregate formation in the bulk phase. It is investigated how the forces between powder particles are influenced by the adsorbed surfactant film. In most cases the adsorbed bilayers are useful as a barrier against particle aggregation in water. The repulsion force between the nanoparticles is strongly modified by the phase equilibrium between adsorbed and dissolved surfactant molecules.