Influence of CTAB and SDS on the properties of oil-in-water nano-emulsion with paraffin and span 20/Tween 20

Oil-in-water (o/w) nano-emulsions with paraffin as an oil phase and Sorbitan monooleate (Span 20)/polyoxyethylene sorbitan monooleate (Tween 20) as emulsifiers were prepared using the emulsion inversion phase (EIP) method at 25 ℃. The properties of the nano-emulsions were investigated in detail as a function of emulsifier content and the addition of ionic surfactants including cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). The droplets of the nano-emulsions become smaller with the increasing concentration of Tween 20/Span 20 and the polydispersity of the droplets decreases. Similarly, the mean droplet size also decreases with the addition of both CTAB and SDS. The zeta potential of the nano-emulsion droplet without SDS or CTAB was found to be negative. Upon the addition of SDS, a more negative value was obtained which leads to an increased electrostatic interactions between droplets and improves the stability of the nano-emulsions via lowering the Ostwald ripening rate. Upon the addition of CTAB, however, a less negative zeta potential was induced which weakens the electrostatic interactions between droplets and lowers the stability of the nano-emulsions. These results indicate that electrostatic interaction is the main factor determining the stability of the nano-emulsions. Interfacial rheological measurements indicated that the maximum values of dilational moduli of both Tween 20/SDS and Tween 20/CTAB mixed adsorption layers at paraffin oil/water interface are lower than that of single adsorption layer of Tween 20. Our results give new insights of the nano-emulsions containing mixed surfactants and may serve as guidelines for preparation of new nano-emulsion systems for practical applications.