The complications near the critical micellar concentration (CMC) of a sodium dodecyl sulfate (SDS) solutions have been existing over half of a century. Early interpretation involves the presence of an impurity. The complicated situation renders the CMC determination difficult. Meanwhile, impure nonionic surfactant systems do not always show complicated phenomena. In this work, turbidity, surface tension, zeta potential and solubilization measurements were applied to SDS solutions containing 0 - 0.15M NaCl. All solution preparation follows the normal dilution procedure. Detailed interpretation of data follows the same order: from high concentration to low concentration. In addition to the turbidity maximum and surface tension minimum, a zeta potential minimum was also found. The presence of some highly charged, large, emulsified particles may have contributed significantly to the complications. Even minute amount (< 0.006 wt%) of added organic impurity can introduce all these complications. In such impure surfactant systems, the micellar dissociation concentration (MDC) can be measured easily whereas the CMC value can only be estimated. The knowledge of the MDC and CMC is the extremely useful in date interpretation.
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