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The modification of the clays (montmorillonite) by surfactants finds several applications in various fields, such as cosmetics, drugs, nanomaterials, etc. The experimental and the simulation of the adsorption of the cetylpyridinium surfactant on a Moroccan   Na-Montmorillonite-type clay surface have been investigated. The adsorption rate of cetylpyridinium reached 2 CEC (Cation Exchange Capacity) and the maximum amount of adsorption increases for co-ion Br^- more than Cl^-. These results have been proven and we can conclude that the amount of adsorption varies according to the surfactant critical micelle concentration (CMC). The simulation shows the arrangement of the cationic surfactant’s ions adsorbed on the basal surface. Where the cetylpyridinium are organized in two layers in a bimolecular and paraffinic manner. The binding coefficients for the formation of neutral complexes of either cationic surfactant’s ions and the clay are calculated by a theoretical model and the binding force of the cationic surfactant ion on the surface of the clay depends on the co-ion. The FT-IR and XRD analyses showed that the surfactant is in strong interaction with montmorillonite for a surfactant adsorption rate lower than the CEC, and that the orientation of the surfactant molecules on the surface of the clay changes with the amount adsorbed to CP-Cl, especially for an adsorbed amount greater than the CEC.