The simplified form of an integral adsorption isotherm based on Butter's equation was applied to describe surface behavior of a series of anionic (sodium alkylsulfonates) and cationic (alkylpyridinium halides) surfactants. This theory allows for the calculation of the free energy of adsorption (DeltaG(jk)) value corresponding to the ability of a particular surfactant to undergo adsorption. The obtained results indicate that the value of DeltaG(jk) depends linearly on the length of the hydrocarbon chain as well as on the kind and concentration of the added inorganic electrolyte. Moreover, it has been found that in the case of surfactants, which have the same length of the alkyl chain and adsorb from solutions containing the same inorganic electrolyte, the charge of hydrophilic group has insignificant influence on the value of DeltaG(jk) (C) 2004 Elsevier Inc. All rights reserved.