Micellization of a weakly charged surfactant in aqueous salt solution: self-consistent field theory and experiments

Álvaro González García, Elizabeth Maria Timmers, Noah Romijn, Shidong Song, Sheen Sahebali, Remco Tuinier (Corresponding author), Ilja Karina Voets (Corresponding author)

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Abstract

Self-consistent field (SCF) calculations and light scattering experiments were performed to study the pH and salt response of micelles composed of surfactants with a single weak acid group in aqueous salt solution. To this end, the common surfactant Brij 35 was oxidized to yield a polyoxyethylene alkyl ether carboxylic acid with a single terminal weakly charged carboxylic acid group in alkaline media. At low pH values, the micellar hydrodynamic radii (Rh) are independent of the salt concentration. By contrast, at pH values around the acid dissociation constant (pH ≈ pKa ± 1), the micellar radius decreases upon increasing pH until a salt-dependent plateau value is reached. The reduction in micellar size is more pronounced for lower salt concentrations. The SCF computations are in qualitative agreement with the experimental results and further reveal a limiting value for Rh corresponding approximately to the Debye length λD. Self-assembly into micelles is suppressed for low salt concentrations that would yield Rh < λD. Instead, the surfactants remain as unimers in solution. The results are summarized in a state diagram displaying the preferred surfactant configuration in solution as a function of RhD, pH and salt concentration.

Original languageEnglish
Pages (from-to)201-208
Number of pages8
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Volume561
DOIs
Publication statusPublished - 20 Jan 2019

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Keywords

  • Model systems
  • Self-assembly
  • Surfactants

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