Nano-emulsions droplet size

O/W nano-emulsions with droplet radii in the range 26-66 nm could be obtained at surfactant concentrations between 4 and 8%. The nano-emulsion droplet size and polydispersity index decreases with increasing surfactant concentration. 

All the authors report a decrease in the nano-emulsion droplet size when the surfactant concentration increases, since there is more surfactant available to stabilize more interfaces, and therefore smaller droplets can be obtained. In general, it is observed in the phase diagrams that the region with phases with planar structure, that precedes the region where nano-emulsions are formed, extends to higher water concentrations when the surfactant/oil ratio increases, because when more surfactant is present, it can dissolve more water inside the bicontinuous or liquid crystal structure (as an example, see Eigure 21.6). 

The conditions for obtaining O/W nano-emulsions with a minimum droplet size and consequently low polydispersity by phase inversion emulsification methods (PIT and PIC) can be summarized as follows A bicontinuous microemulsion or a lamellar liquid crystalline phase (D or L , respectively), with all the oil dissolved, must be formed immediately before reaching the final two-phase region where the nano-emulsions form. These are composition conditions necessary but not sufficient, because the kinetics of incorporation of oil to these phases or the coalescence of droplets can make the nano-emulsion droplet size also dependent on preparation variables such as mixing rate and aqueous phase addition rate for the PIC method, or cooling rate for the PIT method. 

Much of the work in this area has been done in emulsions having a droplet size of more than 1 pm, and the application of submicron (nano) emulsions in encapsulation of oils and flavors is relatively new in the literature. Some works have been carried out to determine the influence of submicron emulsions produced by different emulsification methods on encapsulation efficiency and to investigate the encapsulated powder properties after SD for different emulsion droplet sizes and surfactants. The process has been referred to as nanoparticle encapsulation since a core material in nanosize range is encapsulated into a matrix of micron-sized powder particles (Jafari et al., 2008). This area of research is developing. Some patents were filed in the past describing microemulsion formulations applied to flavor protection (Chung et al., 1994 Chmiel et al., 1997) and applications in flavored carbonated beverages (Wolf and Havekotte, 1989). However, there is no clear evidence on how submicron or nanoemulsions can improve the encapsulation efficiency and stability of food flavors and oils into spray-dried powders. 

Morales et al. [50] systematically studied the relation between the emulsion droplet sizes and the phase behavior at the HLB temperatures for different compositions in the water/CigEg/mineral oil system. Figure 21.2 shows the fish-shaped phase diagram obtained as a function of temperature and surfactant concentration for a fixed oil/(water -f oil) weight ratio, = 0.2. Several nano-emulsions were prepared at different concentrations of the CigEg surfactant, from 1 to 9 wt%, observing a sharp decrease in emulsion droplet sizes when the CigEg concentration was around 3-4 wt%... 

From the earlier studies on nano-emulsion, their potential application for the preparation of nanoparticles with similar sizes as those of the nano-emulsion droplets has been claimed. In an early work about the preparation of nanoparticles from nano-emulsions (referred as miniemulsions), Ugelstadt [3] proposed a polymerization mechanism with nucleation in the miniemulsion droplet, as a different mechanism to that of macroemulsion polymerization, where nucleation takes place mainly in micelles. This mechanism can result in particles as one-to-one copy from nano-emulsion droplets, and this possibility is deeply discussed in a review by Asua [18] (Figure 21.12). 

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. 

Control of the particle size while retaining precise control over the release rate is enabled by compartmentalization of the sol-gel solution into droplets of definite size. This can be achieved by emulsification of the sol-gel solution by mixing it with a solution composed of a surfactant and a non-polar solvent (Figure 2.13). When an active molecule is located in the aqueous droplet of a W/O emulsion, encapsulation occurs as the silicon precursors polymerize to build an oxide cage around the active species. By changing the solvent-surfactant combination, the particle size can be varied from 10 nm to 100 pm as the size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol-gel reaction (Figure 2.13). 

The results sustained the opinion that the synthesis of nano-particles in microemulsion (w/o) is to be preferred. In common emulsions, both phases spontaneously separate from each other while microemulsions are thermodynamically stable, do not segregate and appear to be transparent. This can be explained with the size of the water droplets [14,11]. Microemulsions are also characterized by the so called dynamic exchange process . The emulsion droplets in such emulsions constantly integrate and disintegrate, thus exchanging substance between each other. 

Thus, droplets prepared close to the PIT will be smaller than those prepared at lower temperatures. These droplets are relatively unstable towards coalescence near the PIT, although by rapid cooling of the emulsion the smaller size can be retained. This procedure may be applied to prepare mini (nano) emulsions. 

Depending on the process conditions, the relative size of the dispersed and emulsion droplets, and the properties of the fuel, the final particles may be either micro or nano-sized and either hollow or solid fully-filled. 

Nano-anulsions constitute an attractive alternative for preparing polyurethane nanoparticles because of their small droplet size and, consequently, very large surface area and high kinetic stabiUty. - Nano-emulsions are a class of emulsions with a uniform and extremely small droplet size, usually ranging between 20 and 200 nm. They can be classified as oil-in-water (OAV) or water-in-oil (W/O) nano-emulsions if the internal phase is constituted by oil or aqueous droplets dispersed in aqueous or oily external phase, respectively. Therefore both hydrophobic and hydrophilic... 

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. 

Two methods may be applied for the preparation of nano-emulsions (covering the droplet radius size range 50-200 nm). Use of high-pressure homogenisers (aided by appropriate choice of surfactants and cosurfactants) or application of the phase inversion temperature (PIT) concept. 

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... 

In all cases, there is an increase in nano-emulsion radius with increasing f (0/S). However, when using the high-pressure homogeniser, the droplet size can be maintained to below 100 nm at high J (0/S). With the PIT method, there is a rapid increase in r with increase in i (0/S) when the latter exceeds 7. 

One of the main advantages of nano-emulsions is the high occlusive film that may be formed on application to the skin. The small size droplets can enter the rough surface of the skin and the droplets may form a close packed structure on the skin surface. This is particularly the case when the droplets have high viscosity or are solid-like . Another useful application of nano-emulsions is the ability to enhance penetration of actives (e.g. vitamins, antioxidants, etc.) into the skin. This is due to their much higher surface area when compared with coarser emulsions. 

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