Nano-emulsions Ostwald ripening

Unless adequately prepared (to control the droplet size distribution) and stabilised against Ostwald ripening (which occurs when the oil has some finite solubility in the continuous medium), nano-emulsions may lose their transparency with time as a result of increasing droplet size. 

Lack of knowledge on the mechanism of Ostwald ripening, which is perhaps the most serious instability problem with nano-emulsions. 

One of the main problems with nano-emulsions is Ostwald ripening, which results from the difference in solubility between small and large droplets. The difference in chemical potential of dispersed phase droplets between different sized droplets... 

All nano-emulsions showed an increase in droplet size with time, as a result of Ostwald ripening. Figure 9.10 shows plots of versus time for all the nanoemulsions studied. The slope of the lines gives the rate of Ostwald ripening w (m s ), which showed an increase from 2 x 10 to 39.7 x 10 m s as the surfactant concentration is increased from 4 to 8 wt%. This increase could be due to several factors (1) A decrease in droplet size increases the Brownian diffusion... 

Tab. 9.3. HLB temperature (Thlb), droplet radius r, Ostwald ripening rate (co) and oil solubility for nano-emulsions prepared using hydrocarbons with different alkyl chain length.

In contrast to the results obtained with hexadecane, addition of squalene to the O/W nano-emulsion system based on isohexadecane showed a systematic decrease in Ostwald ripening rate as the squalene content was increased. Figure 9.13 shows the results as plots of versus time for nano-emulsions containing varying amounts of squalene. Addition of squalene up to 20% based on the oil phase showed a systematic reduction in the rate (from 8.0 x 10 to 4.1 x 10 m s ). Notably, when squalene alone was used as the oil phase the system was very unstable and showed creaming within 1 hour. This indicates that the surfactant used is not suitable for the emulsification of squalene. 

As expected, nano-emulsions prepared using high-pressure homogenisation showed a lower Ostwald ripening rate than systems prepared using the PIT method. This is illustrated in Figure 9.17, which shows plots of versus time for the two systems. 

In this chapter, nano-emulsion formation by low-energy emulsification methods, with special emphasis on phase inversion methods and their relation to surfactant phase behavior will be discussed first. This will be followed by an analysis of nano-emulsion functional characteristics. The relation with their structure is discussed regarding the applications in which they are relevant. Prior to discussing nano-emulsion functional characteristics, the main nano-emulsion destabilization mechanism, Ostwald ripening, is described. 

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. 

Therefore, the only process that may produce coarsening of nano-emulsions is Ostwald ripening. It is described by the LSW theory, formulated by Lifshitz and Slezov [68] and independently by Wagner [69], Several authors have indicated that this theory can be applied to macroemulsions with reasonable accuracy [79,80], It has also been reported that the presence of microemulsion droplets in the continuous phase accelerates the Ostwald ripening rate by increasing the diffusion coefficient [80,81], However, this effect is relatively small because microemulsion droplets have much smaller diffusion coefficients than molecules. 

The LSW theory assumes that the droplets are separated by distances much larger than their diameters, the transport of the dispersed component is due to molecular diffusion, and the concentration of the dissolved species is constant except when adjacent to the droplet boundaries. These assumptions may not be completely valid for nano-emulsions because the strong Brownian motion may induce convective diffusion accelerating the diffusion rate, which would be slower if it were due only to molecular diffusion. However, it has been shown that convective contributions do not change the fundamental nature of Ostwald ripening processes [82],... 

---