At (25±0.2) ℃, the cmc values of nonionic-anionic surfactant sodium octylphenol polyoxyethylenated ethylsulfonate (OPS) in the aqueous solution in the presence of inorganic salt, LiCl, NaCl, KCl, MgCl₂, CaCl₂, BaCl₂, AlCl₃, FeCl₃ and SmCl₃, were measured by the method of surface tension. An influence on the cmc of nonionic-anionic surfactant OPS with inorganic salt counter ion was generalized from the variation of the cmc. It is indicated from the mathematic analysis of experimental data that although an additional hydrophilic group of nonionic oxyethylene unit was introduced in the molecular chain of nonionic-anionic surfactant OPS, the relationship between its cmc and the concentration of inorganic salt counter ion [M] is similar to that of ionic surfactant as the followed linear formula: lg cmc＝－4.242－(0.2980－0.0102 Z/r)lg[M] ([M]＜1 mol/L), where Z and r are the valence number and the average Van der Waals radius of atom of counter ion, respectively, then is characteristic of ionic surfactants and not of nonionic or amphoteric surfactants. It is obviously showed from the above formula that the cmc of nonionic-anionic surfactant OPS is related to the concentration, the valence number and the average Van der Waals radius of atom of counter ion, and the degree of binding to micelle of counter ion depends on the ratio of the valence number to the average Van der Waals radius of atom of counter ion. And the mechanism of the inorganic salt counter ion effect on the cmc of OPS is briefly analyzed.

Key words: nonionic-anionic surfactant, surface tension, critical micelle concentration, inorganic salt, counter ion, radius of atom, mechanism