The Second CMC of the Aqueous Solution of Sodium Dodecyl Sulfate. II. Viscosity and Density

Our viscosity and density studies of an aqueous solution of sodium dodecyl sulfate have revealed that the 2nd critical micelle concentration (CMC) exists in addition to the 1st CMC and that the 2nd CMC is greatly influenced by such additives as sodium chloride, dodecyl alcohol, and benzene; these results are in good agreement with those obtained from the conductivity measurements described in the previous paper. The reduced viscosity of the micelle (ηsp.m/Cm) vs. Cm curve showed a knick at the concentration corresponding to the 2nd CMC as obtained by Tyuzyo and his co-workers. The molar volume of the hydrated micelle was calculated by using the equation: ηr.m=1+2.5φm+14.1φm2; it has been found to decrease at about the 2nd CMC, similarly to the partial specific volume (\barv) of SDS obtained from the density measurement. This may be attributed to a change in the micelle structure at the 2nd CMC, caused by a variation in the type of aggregation and by a decrease in the counter-ion binding by the micelle.