Abstract: The cmc's of C_8H_(17) (OC_2H_4)_nSO_4Na (C_8E_nS; n=0, 1, 3) under varied conditions have been measured by the drop-volume method. The free energy (ΔG°) and entropy (ΔS°) of the micellization of surfactants, and the counter-ion binding constant (K) of micelle have been calculated. The results show that the cmc of C_8E_nS decreases with increasing n, but the rate of declination gradually slows down; K decreases as n increased; ΔS°increases with n, but the rate of increase slows down untill ΔS°being constant, which is the general trend (for ionic or nonionic surfactants, the conclusion is correct). This may be interpreted as that the oxyethylene group (EO) decrease with the increasing of EO number. The feature of the EO decides its special function in surfactant molecules.

Key words: Oxyethylenated surfactant, Thermodynamic function of micellization