By Andrés Martínez, Ana Leonor Rivera, and Víctor M. Castaño of the CFATA
Emulsions are currently among the most widely used materials and have a long history from the research and development standpoint. Size matters in emulsions, and tiny dispersed particles account for the properties of ancient and modern materials. More specifically, practical applications already exist for nanoemulsions, including cosmetics, pharmacology as drug delivery systems or detectors, disinfectants, food technology, beverages, agronomy, and fuel.1
Nanoemulsions are metastable dispersions of nanodroplets (2–200 nm) into another immiscible liquid. The droplet structure determines the properties of nanoemulsions, which depend not only on composition but also on the preparation (emulsifying path, agitation or emulsification time). Nanoemulsions are thermodynamically unstable systems with a relatively high kinetic stability and a spontaneous tendency to separate into their constituent phases. They can even be distinguished by visual inspection: nanoemulsions are clear and exhibit very little light scattering, despite significant refractive index contrast, even at high droplet volume fractions.
From the standpoint of industrial applications, the leading property of nanoemulsions is their large interfacial area-to-volume ratio, which makes the Laplace pressure and the Young’s elastic modulus of nanoemulsions considerably larger than those of ordinary emulsions. In contrast with common emulsions, nanoemulsions have a surprisingly strong elasticity at low droplet volume fractions and enhanced diffusive transport and stability. The rheological properties of nanoemulsions depend strongly on the droplet size and their interaction. Moreover, nanoemulsions exhibit enhanced shelf stability against gravitationally driven creaming over microscale emulsions.
Although a good portion of the literature on nanoemulsions indicates that they can be stable for many years, the small droplet size causes nanoemulsions to break down by the Ostwald ripening mechanism in relatively short time periods, due to deformations of the droplets to a foam-like structure.
On the other hand, the bioavailability of lipophilic components encapsulated within nanoemulsions may be appreciably higher than the same components encapsulated within conventional emulsions, which allows more applications, such as carrier or transport substances. In particular, oil-in-water nanoemulsions have proven to be a cost-effective alternative as diluents for transportation of heavy crude oil.2 A typical transport of nanoemulsions is composed of 70% crude oil, 30% aqueous phase, and 500–2000 ppm of a stabilizing surfactant formulation. Furthermore, surfactants, which can be biodegradable substances, may be injected into a well bore to affect the emulsification in the pump or tubing for the production or extraction of heavy crude oil as oil-water nanoemulsions. Recently, the possibility for enhanced oil recovery has been explored3 by using CO2-induced nanoemulsions. However, these particular proposal results are too expensive and complicated to be developed at large-scale and/or in-field applications. Nevertheless, this finding opens the possibility of using oil-water nanoemulsions based on a natural surfactant (thus, biodegradable) to remove crude oil from the sea.
Our group has a lot of experience working with novel nanoemulsions.1 We have studied the possibility of using them as alternative high-power fuels by using a spontaneous emulsification method with a biodegradable surfactant—obtained from a Mexican plant—as an effective stabilizing agent. The surfactant is completely miscible in water and fuel and allows a very high water content (up to 95%) in commercial gasoline. Furthermore, these nanoemulsions have been tested as fuels in a two-engine cycle, showing an increase in the octane index of about 300% as compared to the base commercial fuels. The potential use of this technology with crude is certainly worth exploring, not only for spills, but also for transport and stabilization.
- A. Rosas, A.L. Rivera, V.M. Castaño, Fuel-Water Nanoemulsions A Revie, Nano Trends 4 (2), 1–38 (2008).
- D.P. Rimmer, A.A. Gregoli, J.A. Hamshar, E. Yildirim, in Pipeline Emulsion Transportation for Heavy Oils, L.L. Schramm, Ed. (American Chemical Society, Washington, DC, 1992), pp. 295–312.
- 3. J.L. Zhang, B. Han, C. Zhang, W. Li, X. Feng, Nanoemulsions Induced by Compressed Gases, (Angewandte Chemie, Wiley InterScience, MA, 2008), 3012–3015.