Shape and Structure Formation of Mixed Nonionic-Anionic Surfactant Micelles
Michael Ludwig1, Ramsia Geisler1, Sylvain Prévost2
1Soft Matter at Interfaces, Institute for Condensed Matter Physics, Technical University of Darmstadt, D-64289 Darmstadt, Germany.
Molecules (Basel, Switzerland)
|July 24, 2021
Summary
This study investigates nonionic-anionic surfactant mixtures using small-angle neutron scattering (SANS). Mixed micelles form readily, and at high concentrations, excluded volume effects dictate micelle structure regardless of charge.
Area of Science:
- Colloid and Surface Science
- Materials Science
- Physical Chemistry
Background:
- Understanding surfactant self-assembly is crucial for designing advanced materials.
- Nonionic and anionic surfactants exhibit distinct behaviors, but their mixtures offer tunable properties.
- Poly-ethylene oxide (PEO) headgroups in nonionic surfactants (Tween20, BrijL23) allow comparison of branched vs. linear structures.
Purpose of the Study:
- To investigate the self-assembly and intermicellar interactions in aqueous mixtures of nonionic (Tween20, BrijL23) and anionic (sodium dodecyl sulfate, SDS) surfactants.
- To determine the effect of mixing ratios and concentrations on the formation and structure of mixed micelles.
- To compare the influence of branched versus linear nonionic surfactant headgroups on micelle formation and interactions.
Main Methods:
- Small-angle neutron scattering (SANS) was employed to probe the structure of surfactant solutions.
- SANS data were analyzed using a core-shell model for micelle form factors.
- Intermicellar interactions were modeled using hard-sphere Percus-Yevick (HS-PY) or rescaled mean spherical approximation (RMSA) structure factors.
Main Results:
- Mixed micelles form at concentrations below the critical micelle concentration of SDS.
- At high volume fractions (ϕ ≲ 0.45), excluded volume effects dominate intermicellar structuring, irrespective of micelle charge.
- Almost spherical mixed micelles are observed across all mixing ratios, except for equimolar ratios (X≈ 0.4-0.6) where specific sulfate-EO interactions cause increased micelle size and ellipticity.
Conclusions:
- Nonionic-anionic surfactant mixtures readily form mixed micelles with tunable surface charge.
- Excluded volume effects are the primary driver of intermicellar structure at high concentrations.
- The ability to control micelle size and charge offers potential for creating novel colloidal systems.
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