Quantitative treatment of indicator equilibria in micellar solutions of sodium decyl phosphate and sodium lauryl sulfate

Laurence S. Romsted, Dino Zanette

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The pseudophase ion-exchange (PIE) model provides both a qualitative and quantitative interpretation of micellar effects on reaction rates and equilibria for a number of reactions between organic molecules and ions in aqueous solution in the presence and absence of buffer. Our results show that this model is also applicable to micellar solutions of sodium decyl phosphate monoanion, NaDP. We measured the ratio of the acid and base forms of the indicator, pyridine-2-azo-p-dimethylaniline (PADA), in the pH range of 4-6, succinate buffer, at a variety of surfactant and counterion concentrations. Measurements were carried out at 50°C to ensure that NaDP remained in solution at the lower end of the pH range. Parallel experiments were run in sodium lauryl sulfate, NaLS, to check for special effects of the head groups of micellized NaDP in the indicator equilibria. None were found. NaDP micelles behave "normally", and the assumptions of the model are well-obeyed provided (a) measured values of the pH are used instead of assuming that the buffer maintains the pH of the aqueous phase constant at all salt concentrations and (b) the concentrations of sodium ions in the aqueous phase are converted to activities by using the mean ion activity coefficient of NaCl. Limitations of the approach and sources of error are discussed.

Original languageEnglish (US)
Pages (from-to)4690-4698
Number of pages9
JournalJournal of physical chemistry
Issue number16
Publication statusPublished - Jan 1 1988


All Science Journal Classification (ASJC) codes

  • Engineering(all)
  • Physical and Theoretical Chemistry

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