(92d) Spontaneous Emulsification of Octane/AOT/Brine Systems to Form Nanoemulsions

Nanoemulsions were formed by diluting Water/Oil (W/O or L₂) or Brine/Oil (B/O) microemulsions of a hydrocarbon (octane), anionic surfactant (Aerosol-OT or AOT) and water/brine in varying levels of excess brine. The water-continuous nanoemulsions were characterized by interfacial tension (IFT), dynamic light scattering (DLS) and electrophoresis. The mechanism of emulsification followed trends reported previously by Rang and Miller ¹using formulations of n-hexadecane, pure non-ionic surfactant C₁₂E₆ and n-octanol and by Nishimi and Miller ² 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.

Most work on spontaneous emulsification has till date, focused on determining the conditions under which a system/class of systems undergoes this phenomenon along with identifying the prevalent mechanism (interfacial turbulence or supersaturation) of emulsification. Often, the mean droplet size/size distribution at a select ?end? time point of the process is reported, but little attention is given to the initial mean size/size distribution of oil droplets and the mechanism by which droplet size evolves to the end time point. This understanding becomes particularly important in designing nanoemulsions for commercial applications. The present work provides information on evolution of the droplet size distribution of nanoemulsions formed when different W/O or B/O microemulsion formulations of octane/AOT/water/brine were diluted in varying levels of excess brine. The brine concentration constituting the excess phase was selected to correspond to different Winsor domains of the octane/AOT/brine system. Previous studies had established the crossover point (corresponding to Winsor III domain) to be at NaCl weight percent (ε) of 0.3. 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.³

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. Droplet growth of nanoemulsions prepared in water (ε = 0) could be explained using an Ostwald ripening mechanism with interfacially controlled mass-transfer. For nanoemulsions prepared at other brine levels corresponding to the Winsor I domain (0 ≤ ε ≤ 0.2), flocculation/coalescence appeared to be the underlying mechanism of droplet growth. However, at the cross-over salinity of ε = 0.3, nanoemulsions showed remarkable stability to growth where droplet size was measured to be consistently lower than 100 nm over 24 hours. The presence of birefringence observed under cross-polarizers indicated the existence of AOT in the lamellar phase. Stability to droplet growth could be attributed to lowering of octane solubilization and mass-transfer across lamellar phase coated oil droplets, as previously reported by Solans and coworkers ⁴ and Rang and Miller⁵. For nanoemulsions prepared at salinity levels 0.4 ≤ ε ≤ 0.99 (Winsor II domain), drops greater than 1 µm were consistently recorded for the first 5-7 hours after which droplet size decreased to values below 1 µm, indicating droplet shrinkage. The larger initial droplet size was rationalized from the initial formation W/O/W multiple emulsions as higher salinity promotes local formation of W/O emulsions. However, a large excess of water dictates it to be the continuous phase. Consequently, as the internal water droplets coalesced with the bulk aqueous phase, the observed drop size was found to decrease.

Electrophoresis studies showed nanoemulsions to be highly negative charged (-60 to -120 mV). The magnitude of charge decreased with increasing salinity, possibly from charge screening effects, but the charge on the emulsion droplets that formed at a specific salinity did not vary significantly with time. Electrostatics may thus be an important contributing factor to the general stability of the nanoemulsions. An important feature of the nanoemulsions produced via phase inversion and spontaneous emulsification is that the final mean droplet size was below 1 µm, 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.⁶

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. Izquierdo, P.;, Esquena, J.; Tadros, T. F.; Dederen, J. C.; Feng, J.; Garcia-Celma, M.J.; Azemar, N.; Solans, C. Langmuir. 2004, 20, 6594-6598.

5. Rang, M.J.; Miller, C.A. Journal of Colloid and Interface Science. 1999, 209, 179-192.

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