Effect of nonionic surfactant upon surface tension of anionic surfactant

The surface tension of sodium dodecyl sulfate was determined in the presence of nonionic surfactant. The nonionic surfactant used was homogeneous pentaethyleneglycol-n-dodecylether which has no poisson distribution of added mole numbers of ethyleneoxide. The concentration of sodium dodecyl sulfate was changed at various fixed concentrations of homogeneous pentaethyleneglycol-n-dodecylether. Two inflection points were observed on each surface tension curve, and the surface tension maintained a constant value between the inflections, regardless of the concentrations of added homogeneous pentaethyleneglycol-n-dodecylether. However, the concentrations of sodium dodecyl sulfate at the inflections were affected at the concentrations of homogeneous pentaethyleneglycol-n-dodecylether, respectively. When the homogeneous pentaethyleneglycol-n-dodecylether concentrations were fixed below the critical micelle concentration, the concentration of sodium dodecyl sulfate at the lower inflection point decreased with increasing concentration of homogeneous pentaethyleneglycol-n-dodecylether, while that of the higher was hardly influenced. If the homogeneous pentaethyleneglycol-n-dodecylether concentrations were fixed above the critical micelle concentration, the sodium dodecyl sulfate concentrations at the two inflections both increased with an increment of homogeneous pentaethyleneglycol-n-dodecylether concentration. These results were interpreted in terms of mixed surface layer and mixed micelles consisting of sodium dodecyl sulfate and homogeneous pentaethyleneglycol-n-dodecylether. Also, the surface tension curves of homogeneous pentaethyleneglycol-n-dodecylether in the presence of sodium dodecyl sulfate, as well as those of sodium dodecyl sulfate with given concentrations of homogeneous pentaethyleneglycol-n-dodecylether, were studied.