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Additional support for a revised Gibbs analysis.

Fredric M Menger1, Lei Shi, Syed A A Rizvi

  • 1Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, USA. menger@emory.edu

Langmuir : the ACS Journal of Surfaces and Colloids
|December 8, 2009
PubMed
Summary
This summary is machine-generated.

Many surfactants show small surface tension reduction at high molecular areas, suggesting gradual interface population rather than saturation. Micelle formation can obscure true interface saturation in surface tension measurements.

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Area of Science:

  • Physical Chemistry
  • Surface Science
  • Colloid Science

Background:

  • Gibbs adsorption equation predicts significant surface tension reduction at high molecular areas (>60 Ų/molecule) for common surfactants.
  • Observed small surface tension reductions contradict the expectation of a saturated air/water interface in the linear decline region of surface tension vs. ln[surfactant] plots.

Purpose of the Study:

  • To investigate the discrepancy between Gibbs-determined areas and observed surface tension reductions for surfactants.
  • To provide evidence for a gradually populating air/water interface in the linear region of surface tension plots.
  • To understand the role of micelle formation in obscuring interface saturation.

Main Methods:

  • Surface tension measurements using a Langmuir film balance.
  • Compression of surfactants (sodium docosanyl sulfate and docosanyltrimethylmmonium bromide) to specific molecular areas (50-65 Ų/molecule).
  • Analysis of surface tension versus natural logarithm of surfactant concentration ([surfactant]) plots.

Main Results:

  • Sodium docosanyl sulfate showed a surface tension reduction of only 7 mN/m at 50 Ų/molecule.
  • Docosanyltrimethylmmonium bromide exhibited a 15 mN/m reduction at 65 Ų/molecule.
  • These reductions are considerably smaller than the 30-40 mN/m drop typically observed in the Gibbs analysis range.

Conclusions:

  • The findings support the hypothesis of a gradually populating air/water interface, even at high molecular areas.
  • Competitive micelle formation can prematurely level surface tension plots, masking the true interface saturation point.
  • Standard Gibbs analysis may overestimate interface saturation due to micelle interference.