Formation of liquid crystal/gel emulsions to nano-emulsions constructed by polyalkoxylated fatty alcohol (PAFA)-based mixed surfactant systems

Abstract

In the construction of ternary phase diagrams, the polyalkoxylated fatty alcohol (PAFA)-based mixed surfactant systems including PAFA-AS (alkyl sulfonate), PAFA-CB (cocamidopropyl betaine) and PAFA-APG (alkyl polyglucosides) were used to develop self-standing liquid crystal/gel emulsions containing rapeseed oil methyl esters (ROME) and water. The formation of liquid crystal/gel emulsions are observed at semi-dilute regions of the phase diagrams. A pre-emulsion was chosen from each of PAFA-AS, PAFA-CB and PAFA-APG systems for minor modification with sodium silicate. Upon aqueous dilution of the modified pre-emulsions to weight fractions (Φ_w) of 0.8 and 0.6 and with an isothermal shaker agitation, the samples demonstrate dramatic increases in apparent viscosity with flow resistance and shear thinning behaviour. In oscillatory amplitude study, the emulsions show linear viscoelastic (LVE) plateau (G’>G”) and strain softening region (G”>G’) indicating the samples promote a viscoelastic behaviour. Further affirmation by Cole-Cole plots reveal the emulsion samples behave as a Maxwell fluid. The optical microscope study verifies the emulsions of PAFA-AS, PAFA-CB and PAFA-APG systems comprising of multilamellar vesicles, bicontinuous cubic phase and multilamellar phase, respectively. Upon aqueous dilution of the liquid crystal/gel emulsions with an isothermal agitation, the formation of nano-emulsion droplets is confirmed by transmission electron microscopy (TEM) and dynamic light scattering studies. The nano-emulsions display spherical and elliptical shapes with mean droplet sizes are in the range of 158.37 to 206.43 nm and zeta potential values are in the range of –12.07 to –32.79 mV.

Graphical Abstract

Keywords:

• Polyalkoxylated fatty alcohol
• green surfactant
• liquid crystal/gel emulsions
• Maxwell fluid
• nano-emulsion

Acknowledgments

The main author (C.J. Lim) would like to gratefully acknowledge the Ministry of Higher Education (MOHE) Malaysia for supporting him MyPhD scholarship (MyBrain15 program). Thank to Ajinomoto Omnichem and Oleon from Belgium, for providing complimentary samples of PAFA and ROME for this research.