Nano-emulsions flocculation

Nano-emulsions are transparent or translucent systems mostly covering the size range 50-200 nm [1, 2], They were also referred to as mini-emulsions [3, 4]. Unlike microemulsions (which are also transparent or translucent and thermodynamically stable, see Chapter 10), nano-emulsions are only kinetically stable. However, their long-term physical stability (with no apparent flocculation or coalescence) makes them unique and they are sometimes referred to as Approaching Thermodynamic Stability . 

Nano-emulsions, as nonequilibrium systems, tend to phase separation by some of the four mechanisms of disperse systems destabilization sedimentation or creaming and flocculation, as reversible mechanisms, and coalescence and Ostwald ripening, as irreversible ones. Nano-emulsions, due to the small characteristic size are stable against sedimentation or creaming. An adequate selection of surfactant molecules can protect nano-emulsions from flocculation and coalescence. In addition, the greater curvature as a result of the smaller droplet size does not favor flocculation or coalescence phenomena. These considerations leave the Ostwald ripening as the main destabilization mechanism of nano-emulsions. This fact has been experimentally confirmed in numerous studies and discussed in several reviews [63,1]. 

The main mechanisms of instability that are involved in leading to complete phase separation of emulsions are creaming [64], flocculation [65,66], coalescence [67], and Ostwald ripening [68,69]. However, nano-emulsions do not cream (or sediment) because the Brownian motion is larger than the small creaming rate induced by gravity. Practically, the creaming of droplets smaller than 1 im is stopped by their faster diffusion rate. 

With respect to flocculation of nano-emulsion droplets, it is not clear whether such droplets can adhere and form a thin flat film, as do large drops. On the one hand, because of their small size, the curvature is very high and the Laplace pressiue opposes deformation. On the other hand, thermal agitation of small droplets (Brownian motion) can increase collisions and enhance deformation [70]. Anyway, flocculation is achieved spontaneously if the profile of the interaction energy as a function of the separation distance has a minimum deep enough to overcome the thermal energy of the droplets. 

Radford, S.J., Dickinson, E. (2004). Depletion flocculation of caseinate-stabilized emulsions what is the optimum size of the non-adsorbed protein nano-particles Colloids and Surfaces A Physicochemical and Engineering Aspects, 238, 71-81. ... 

---