Phase behaviour of a mixed ionic/nonionic surfactant system used to prepare stable oil-in-water paraffin emulsions

The phase behaviour of a mixed surfactant system was studied in order to determine the role of surfactants in the stability of paraffin emulsions. The study was carried out by means of phase diagrams. The different phases were characterized by Small and Wide Angle X-ray Scattering (SAXS/WAXS), Polarized Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC). The results showed that the nonionic and ionic surfactants, used in industry for paraffin emulsions, possess high Krafft points and both form lamellar interdigitated gel structures. Phase behaviour of the water/mixed surfactant pseudoternary system indicated that, lamellar liquid crystalline aggregates are formed at very diluted surfactant concentrations (≈98 wt% water), even at low nonionic/ionic surfactant weight ratio (10/90). Therefore, lamellar liquid crystalline aggregates coexist with excess water, at the surfactant compositions used to obtain stable paraffin emulsions (2–4 wt% of mixed surfactant). Theses aggregates could contribute to the high kinetic stability of the paraffin emulsions.

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► Surfactants form Lβ phases at room temperature, preventing desorption at the oil–water interface.
► In the mixed surfactant system, Lα phase is observed at low surfactant concentrations.
► Multilayers consisting of lamellar aggregates may enhance emulsion stability.