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The adsorption isotherms of cetyltrimethylammonium ion (CTA+) together with that of the Br counterion on silica gel, and the effects of pH and added salts (NaF, NaCl and NaBr) have been systematically determined at 25°C. Electrophoretic mobilities of the silica gel particles have also been measured in the same conditions. The adsorption isotherm of CTA+ consists of four regions. Region I, at low concentrations of surfactant, the adsorption results primarily from electrostatic force between CTA+ and the negatively charged silica surface. Region II (first plateau), at medium concentrations, the adsorption is due to both the electrostatic force and the specific attraction (vdW forces) between CTA+ and the surface. Region III, characterized by an abrupt increase in the slope of the isotherm when the concentration reaches a particular point known as hemimicelle concentration (HMC). The abrupt increase in the adsorption is due to the hydrophobic interaction between hydrocarbon chains. Region IV (second plateau), at or above CMC, the limiting adsorption is reached as the micelle is not adsorbed. Based on this model, the experimental results can be explained reasonably. The results show that the HMC is about half of the CMC. According to the assumption that each adsorbed CTA+ ion in the first plateau is an active center for surface aggregation, the average aggregation number of hemimicelle have been calculated.