Critical composition of ionic–nonionic mixed micelles for counterion condensation

In the mixed micelles of an ionic surfactant (sodium dodecyl sulfate) with a nonionic surfactant (N-decanoyl-N-methylglucamide, hexaoxyethylene glycol-mono-n-decylether, and hexaoxyethylene glycol-mono-n-dodecylether), the critical mole fraction, X_ic, of the ionic surfactant has been determined, below which the counterion is completely released from the micelles. The values of X_ic are 0.074, 0.11, and 0.11, for the respective nonionic surfactants. The valences, i.e., the aggregation numbers of the ionic surfactant, of the mixed micelles at X_ic are almost close to each other, around 6. At X_ic, the critical surface charge density (about 0.03 Cm^-2) for counterion condensation was tentatively calculated. In the present study, a differential conductivity method was applied.