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Related Experiment Videos

Selective counterion condensation in ionic micellar solutions.

V K Aswal1, P S Goyal

  • 1Spallation Neutron Source Division, Paul Scherrer Institut, CH-5232 PSI Villigen, Switzerland.

Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics
|June 6, 2003
PubMed
Summary
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Adding KBr salt significantly increases micelle size in cetyltrimethylammonium bromide (CTABr) and chloride (CTACl) solutions, unlike KCl. This is due to selective counterion condensation affecting micelle structure.

Area of Science:

  • Physical Chemistry
  • Colloid and Surface Science
  • Materials Science

Background:

  • Surfactant micelles are crucial in various applications, and their properties are influenced by counterions and additives.
  • Understanding micellar behavior is key to controlling solution properties and developing new materials.

Purpose of the Study:

  • To investigate the effect of different salt counterions (Br- and Cl-) on the size and structure of cationic surfactant micelles (CTABr, CTACl).
  • To explore the role of selective counterion condensation in determining micellar properties.
  • To compare the influence of KBr versus KCl on micelle formation and size.

Main Methods:

  • Small-angle neutron scattering (SANS) experiments were performed on micellar solutions.
  • Experiments involved varying concentrations of cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium chloride (CTACl) with potassium bromide (KBr) and potassium chloride (KCl).

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  • Measurements were conducted at different equimolar surfactant-to-salt concentrations and temperatures.

    • Main Results:

    • Micelle size significantly increased with the addition of KBr, while KCl had a less pronounced effect.
    • In equimolar CTABr/KCl solutions, micellar sizes were larger than in CTACl/KBr solutions.
    • Selective counterion condensation was identified as the mechanism explaining the observed differences in micellar size.

    Conclusions:

    • The type of counterion (Br- vs. Cl-) and its interaction with the surfactant headgroup and other ions critically influence micelle size.
    • Selective condensation of counterions, particularly the replacement of Cl- by Br- in CTACl/KBr systems, leads to larger micelles.
    • Similar phenomena were observed for anionic surfactants, suggesting a general principle of counterion-salt interactions in micellar systems.