(410e) Characteristics of Spontaneously Formed Nanoemulsions

Physicochemical characteristics of nanoemulsions formed upon contacting Water/Oil (W/O or L2) microemulsions of a hydrocarbon Octane, anionic surfactant Aerosol-OT (AOT) and Water with excess water/brine were investigated using interfacial tension (IFT) and light scattering measurements. The mechanism of emulsification follows trends observed in earlier studies by Rang and Miller¹ using formulations comprised of n-hexadecane, pure non-ionic surfactant C₁₂E₆, and n-octanol and by Nishimi and Miller² using L2-phase microemulsions of Octane, AOT and Water. In these systems, diffusion caused phase inversion from oil-continuous to water-continuous systems, resulting in regions where supersaturation in oil led to nucleation of droplets.

The present work extends the study of Nishimi and Miller² by investigating the nature of nanoemulsions formed by phase inverting L2 microemulsions of Octane: AOT: Water in water and brine. The cross-over from Winsor I to Winsor II regions, corresponding to Winsor III, occurred at a salt weight percent (ε) of 0.3.³ This was confirmed by a minimum IFT value of 0.01 mN/m measured between octane and the nanoemulsion formed at ε = 0.3, when compared to IFTs measured between octane and nanoemulsions formed at other levels of salinity. Nanoemulsions in our studies were thus prepared at salinity levels in the range (0 ≤ ε ≤ 0.99) to encompass investigations in all three Winsor regions. Droplet size measurements of nanoemulsions over time periods of 24 hours revealed three distinct trends of size evolution that correlated with salinity levels defining the three Winsor regimes. For nanoemulsions formed at salinity levels 0 ≤ ε < 0.3 (Winsor I domain), the mean size of octane drops was seen to grow from initial levels of 150-250 nm to the order of 1 µm by flocculation and/or coalescence. Nanoemulsions formed at a salinity value of ε = 0.3 (cross-over Winsor III domain) showed remarkable stability towards coalescence with resulting droplets having values consistently lower than 200 nm, even at time periods of 24 hours. When nanoemulsions were formed at salinity levels 0.3 4

References:

1. Rang, M. J.; Miller, C. A. Prog. Colloid Polym. Sci. 1998, 109, 101

2. Nishimi, T., Miller, C.A. Langmuir. 2000, 16, 9233-9241

3. Maugey, M., Bellocq, A-M. Langmuir. 1999, 15, 8602-8608

4. Srivastava, V.K., Kini, G.C., Rout, D.K. Journal of Colloid and Interface Science. 2006, 304, 214-221.