The two‐phase, mixed indicator method for anionic surfactants has been modified and applied to zwitterionic surfactants. The modifications entail: (i) lowering the pH; (ii) adding 95% ethanol, and (iii) increasing the concentration of mixed indicator solution and adjusting the ratio of the indicators. By adjusting one parameter (the amount of 95% ethanol added), the method is applicable to different types of zwitterionic surfactants. Six different surfactants were examined; three were pure materials synthesized and purified in our laboratory, and the remaining three were commercial materials.
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Rosen et al. (1987) studied this question.
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