The precise molecular structure of organically modified mineral surfaces is Still not well understood. To establish a relation between experimental observations and underlying molecular structure, we performed Monte Carlo simulations of the aggregation behavior of alkyltrimethylammonium surfactants (C(n)TMA(+)) at the interface between C(n)TMACl solution and cleaved K+-muscovite. The structures were examined with regard to the in fluence of varying alkyl chain length n (n = 8, 12, 16) and surface coverage of C(n)TMA(+) ions. The simulation results indicate that the water film structure at the muscovite surface is considerably influenced by the adsorption of C(n)TMA(+). A fraction of the C(n)TMA(+) ions forms inner-sphere and outer-sphere adsorption complexes with nitrogen-surface distances of 3.3-3.8 and 5.5-8.4 angstrom, respectively. The simulated monolayer aggregates exhibit thicknesses of 31-35, 22-27, and similar to 18 angstrom for C(16)TMA(+), C(12)TMA(+), and C(8)TMA(+), respectively. C(16)TMA(+) and C(12)TMA(+) ions form bilayer aggregates, which show a strong interdigitation of the two opposing layers composing them. The aggregate thicknesses equal 35-39 and 30-35 angstrom, respectively, and are in agreement with available experimental data. In contrast, the short-chained C(8)TMA(+) ions do not form bilayer aggregates In agreement with previous experimental studies, the alkyl chains of the aggregated ions show high conformational order markedly decreasing with decreasing chain length. We suggest that the stimulated structures represent C(n)TMA(+) aggregates, which are formed on Muscovite during the experimentally observed initial equilibration phase characterized by the presence of inorganic ions within the aggregates.