The Second CMC of the Aqueous Solution of Sodium Dodecyl Sulfate. I. Conductivity

The conductivity studies of the aqueous solution of sodium dodecyl sulfate (SDS) over the concentration range of 0–0.12 m revealed that there exists a so-called 2nd critical micelle concentration (CMC) at about 65 mm in addition to the 1st CMC at 8.3 mm. The 2nd CMC was greatly influenced by additives; sodium chloride lowered the 2nd CMC, whereas organic substances, such as dodecyl alcohol and n-hexane, raised it. Moreover, the specific conductivity-concentration curve of the pure SDS solution broke upward at the 2nd CMC, and its slope increased in the presence of sodium chloride, while it bent toward the concentration axis in the presence of a certain amount of dodecyl alcohol or n-hexane. These facts may afford a clue for explaining the micelle structure above the 2nd CMC.