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Characteristics of Spontaneously Formed Nanoemulsions

Gautam C. Kini1, Sibani Lisa Biswal1, M. S. Wong2, and Clarence Miller1. (1) Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, (2) Departments of Biological & Chemical Engineering and Chemistry, Rice University, Departments of Biological & Chemical Engineering and Chemistry, Rice University, Houston, TX 77251

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 Ám 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 < ε ≤ 0.99 (Winsor II domain), drops greater than 1 Ám were consistently recorded for the first 5-7 hours, after which droplet size decreased to values below 1 Ám, indicating droplet break up. This trend is counter-intuitive to what the DLVO theory predicts for simple O/W emulsions and may be attributed to initial formation of W/O/W multiple emulsions as higher salinity promotes local formation of W/O emulsions, but the large excess of water dictates that water is the continuous phase. As internal water drops coalesce with the bulk aqueous phase, the observed drop size decreases. An important feature of the nanoemulsions produced via phase inversion and spontaneous emulsification is that the final mean droplet size was below 1 Ám, all achieved without requirements of external energy inputs. This highlights the significance of surfactant phase-behavior in engineering high surface area, nano-sized emulsions with potential applications in numerous fields such as detergency, cosmetics, personal care and enhanced oil recovery.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.