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Effects of small ionic amphiphilic additives on reverse microemulsion morphology
Marios Hopkins Hatzopoulos1, Craig James1, Sarah Rogers2
1School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
Journal of Colloid and Interface Science
|March 6, 2014
Summary
Hydrotrope structure influences water-in-oil microemulsion shapes. More hydrophobic hydrotropes, like sodium octanoate, induce sphere-to-ellipsoid transitions, correlating with their aggregation concentration.
Area of Science:
- Colloid and Surface Science
- Supramolecular Chemistry
- Materials Science
Background:
- Ionic hydrotropic additives can alter water-in-oil (w/o) microemulsions stabilized by anionic surfactants like Aerosol-OT.
- Previous studies indicated a sphere-to-cylinder (ellipsoid) transition, but the underlying mechanisms were unclear.
- Understanding these transitions is crucial for controlling microemulsion nanostructure.
Purpose of the Study:
- To systematically investigate the effect of hydrotrope chemical structure on w/o microemulsion morphology.
- To explore the relationship between hydrotrope hydrophobicity and the observed structural transitions.
- To correlate microemulsion structure changes with the bulk aqueous phase critical aggregation concentration (cac) of hydrotropes.
Main Methods:
- Formulation of w/o microemulsions with varying water content and hydrotrope concentrations.
- Phase behavior studies conducted as a function of water content, additive type, and temperature.
- Small-angle neutron scattering (SANS) used to determine nanodroplet shapes (spheres, ellipsoids, cylinders) and sizes.
Main Results:
- Hydrotrope structure significantly impacts microemulsion morphology; aromatic hydrotropes (sodium benzoate) induced transitions, while saturated cyclic ones (sodium cyclohexanoate) did not.
- Increasing alkyl chain length of linear hydrotropes enhanced hydrophobicity and induced sphere-to-ellipsoid transitions (e.g., sodium heptanoate, sodium octanoate).
- The observed structural changes were directly linked to hydrotrope hydrophobicity, as indicated by their cac values.
Conclusions:
- Microemulsion nanostructure can be effectively tuned by the chemical structure and hydrophobicity of ionic hydrotropes.
- Hydrotrope hydrophobicity, quantified by cac, is a key factor driving sphere-to-ellipsoid transitions in w/o microemulsions.
- This study provides a clearer understanding of the origins of hydrotrope-induced structural changes in microemulsions.
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