The adsorption films of silica nanoparticles modified by a cationic surfactant (cetyltrimethylammonium bromide, CTAB) at the air–water interface were investigated by the dilational surface rheology and optical methods. Special attention was paid to the slow changes of surface properties with the surface age. The surface tension and dynamic dilational surface elasticity were measured as a function of CTAB concentration. The whole surfactant concentration range can be divided into four regions characterized by different surface rheological behavior. Depending on the CTAB concentration, the surface elasticity close to the equilibrium may reach extremely high values (∼1000 mN m⁻¹) and strongly depends on the applied surface strain. This occurs at surfactant concentrations below the region where the dispersion becomes turbid, indicating particle aggregation in the bulk. The Brewster angle microscopy and ellipsometry show that the adsorption layer becomes fragile and inhomogeneous in this region.