Highly concentrated oil-in-water (O/W) emulsions stabilized by catanionic surfactants

• Paraffin oil-in-water emulsions were constructed by catanionic surfactants.
• Surfactants with a pseudo-gemini structure can not induce emulsion formation.
• The emulsions are viscoelastic.
• Upon temperature rising, an unusual viscosity increase was observed.

The abilities of a series of catanionic surfactants (catanionics) to emulsify paraffin oil/water (PO/H2O) mixtures have been investigated. It was found that catanionics with head-to-tail structures are efficient emulsifiers, including tetradecyltrimethylammonium laurate (TTAL), decyltrimethylammonium laurate (D10TAL) and 1-methyl-3-decyl-immidazolium laurate (1-M-3-D10-ImL). Stable oil-in-water (O/W) emulsions were obtained by simple hand-shaking at low water content (down to 6% by volume), as confirmed by visual inspection, optical microscopy observations, conductivity measurements and confocal fluorescence microscopy observations. The emulsion droplets are closely-packed, imparting the samples viscoelastic properties as proved by rheological measurements. The emulsions could not be destroyed at elevated temperatures and interestingly, a slight increase of viscoelasticity was observed. Specifically for TTAL which was prepared by acid-base neutralization between tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA), the influence of the composition of the ion-pair was also investigated. While the properties of the emulsions can be only slightly influenced at LA-rich side, significant influence was noted at TTAOH-rich side. We have also checked the emulsifying ability of a pseudo-gemini catanionic surfactant, bis(tetradecyltrimethylammonium) adipate ((TTA)2A), and found it could not emulsify PO/H2O mixtures under the same experimental conditions, indicating that the architecture of the catanionics also has a profound influence.

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