# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 3 Nanoemulsions
### Nanoemulsion Stability  
Nanoemulsions will eventually evolve according to several physicochemical mechanisms liable to affect any kind of emulsion (Figure 26), but with some particularities [93; 22; 94]. Generally speaking, the small droplet size of nanoemulsions confers a better physicochemical stability than conventional emulsions. Ostwald ripening or molecular diffusion is the main mechanism for nanoemulsion destabilization. Compared to conventional emulsions, nanoemulsions present better stability to gravitational separation (i.e., sedimentation or creaming) and droplet aggregation (i.e., flocculation and coalescence), but they are more susceptible to Ostwald ripening and chemical degradation [93]. In addition, since nanoemulsions are transparent or slightly turbid, ultraviolet (UV) and visible light can penetrate into them easily, which may trigger light-sensitive chemical degradation reactions [114].  
Figure 25: Comparison of the oil recovery after a waterflooding with CO\({}_{2}\) vs. nanoparticle–CO\({}_{2}\) foams. Images of initial oil in place (IOIP), secondary recovery, and CO\({}_{2}\) flood (**top left**); nanoparticle-stabilized CO\({}_{2}\) foam flooding (**top right**); cumulative oil recovery (**bottom left**); and an enlarged plot of the recovery after the waterflooding (**bottom right**). Figure reproduced from Ref. [104].  
Ostwald ripening is an important physical process which takes place because larger particles are thermodynamically more stable than smaller particles. Then, the mean size of the droplets in an emulsion increases over time as the system tries to lower its overall energy [93; 116]. The mechanism involved in this process is the Kelvin effect, whereby small droplets have higher local oil solubility than larger ones due to the difference in the Laplace pressures [96]. The difference in the chemical potential of dispersed phase droplets as a function of the droplet size was given by [PERSON] [95]:  
\[c(r)=c(\infty)e^{\frac{2\gamma V_{m}}{rRT}}, \tag{3}\]  
where \(c(\infty)\) represents the bulk phase solubility, \(c(r)\) is the solubility surrounding a particle of radius \(r\), and \(V_{m}\) is the molar volume of the dispersed phase. Furthermore, the quantity \(\left(\frac{2\gamma V_{m}}{rRT}\right)\) is referred to as the characteristic length. Theoretically speaking, the process should lead to the formation of one single drop. This does not take place in practice since the rate of this phenomenon decreases as the droplet size increases [95]. Ostwald ripening can be quantitatively studied using plots of the cube of the radius as a function of time \(t\) (Lifshitz-Slyozov-Wagner theory) (Figure 27) [117]:  
\[\omega=\frac{dr^{3}}{dt}=\frac{8}{9}\left[\frac{c(\infty)\gamma V_{m}D}{\rho RT }\right], \tag{4}\]  
where \(D\) is the diffusion coefficient of the disperse phase in the continuous phase and \(\rho\) is the disperse phase density. In order to prevent Ostwald ripening, several methods were proposed [116]. Firstly, the addition of a second disperse phase component that is insoluble or nearly insoluble in the continuous phase (e.g., [PERSON] [95] suggested A-B-A block copolymers). If the secondary component has zero solubility in the continuous phase, the size distribution does not deviate from the initial condition (\(\omega=0\)). Secondly, modifying the interfacial film at the oil/water (O/W) interface. According to the previous equations, a reduction in \(\gamma\) will lead to a decrease of this phenomenon. Polymeric  
Figure 26: Schematic diagram showing the most common instability mechanisms that take place in emulsions. Figure reproduced from Ref. [115].  
surfactants adsorbed at the O/W interface which do not desorb during ripening could lower the rate, and thus the ripening (Gibbs-Marangoni Effect) [118].