Nano-emulsions stability

In this section, the functional characteristics of nano-emulsions and the relation with their structure are analyzed and discussed in relation to the applications in which these functional characteristics can be important. The section is divided according to the main functional characteristics determining the applications of nano-emulsions stability, droplet size, and solubilizing capacity. 

Nano-emulsion droplets are generally stabilized by surfactants. Although it is considered that surfactant molecules are adsorbed at the oil-water interface in the form of monolayers, other surfactant self-organizing structures such as multilayers may play an important role in nano-emulsion stability. In this context, the results of studies of the relation between nano-emulsion formation, stability, and phase behavior are very illustrative [14-16,47]. 

In this chapter, different methods for nano-emulsion formation, with special emphasis on low-energy emulsification methods, are discussed in Section 11. This is followed by a description of nano-emulsion stability (Section 111). Finally, the most relevant applications of nano-emulsions are reviewed... 

Izquierdo, P., Esquena, J., Tadros, T.F., Dederen, C., Garcia, M.J., Azemar, N. and Solans, C. (2002) Formation and stability of nano-emulsions prepared using the phase inversion temperature method. Langmuir, 18 (1), 26-30. 

P. Izquierdo, J. Esquena, T.F. Tadros, J.C. Dederen, M.J. Garcia, N. Azemar, and C. Solans Formation and Stability of Nano-Emulsions Prepared Using the Phase Inversion Method. Langmuir 18, 26 (2002). 

The emulsion polymerization methodology is one of the most important commercial processes. The simplest system for an emulsion (co)polymerization consists of water-insoluble monomers, surfactants in a concentration above the CMC, and a water-soluble initiator, when all these species are placed in water. Initially, the system is emulsified. This results in the formation of thermodynamically stable micelles or microemulsions built up from monomer (nano)droplets stabilized by surfactants. The system is then agitated, e.g., by heating it. This leads to thermal decomposition of the initiator and free-radical polymerization starts [85]. Here, we will consider a somewhat unusual scenario, when a surfactant behaves as a polymerizing comonomer [25,86]. 

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. 

Formation and stability of nano-emulsions prepared using the phase inversion temperature method, Langmuir 18, 26-30 (2002). 

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 . 

The inherently high colloid stability of nano-emulsions can be well understood from a consideration of their steric stabilisation (when using nonionic surfactants and/or polymers) and how this is affected by the ratio of the adsorbed layer thickness to droplet radius, as will be discussed below. 

Since most nano-emulsions are prepared using nonionic and/or polymeric surfactants, it is necessary to consider the interaction forces between droplets containing adsorbed layers (Steric stabilization). As this is detailed in Chapter 6, only a summary is given here [15, 16]. 

As an illustration. Figure 9.6 shows the variation of Gj with h at various ratios of 8/R. The depth of the minimum clearly decreases with increasing 8/R. This is the basis of the high kinetic stability of nano-emulsions. With nano-emulsions having a radius in the region of 50 nm and an adsorbed layer thickness of say 10 nm, 8/R is 0.2. This relatively high value (for macroemulsions 8/R is at least an order of magnitude lower) results in a very shallow minimum (which could be less than kT). 

The effect of HLB number on nano-emulsion formation and stability was investigated by using mixtures of Q2EO4 (HLB = 9.7) and Q2EO4 (HLB = 11.7). Two surfactant concentrations (4 and 8 wt%) were used and the O/W ratio was kept at 20/80. Eigure 9.14 shows the variation of droplet radius with HLB number. This figure shows that the droplet radius remain virtually constant in the HLB range... 

Ortiz, D. P. Baydaka, E.N. Yarranton H.W.(2010). Effect of surfactants on interfacial films and stability of water-in-oil emulsions stabilized by asphaltenes. Journal off Colloid and Interface Sdence,doi 10.1016/j.jcis.2010.08.032 Pryanto, S., Mansoori, G.A., Suwono, A., (2001). Measurement of property relationships of nano-structure micelles and coacervates of asphaltene in a pure solvent. Chemical Engineering Science, 56, 6933-6939... 

In this chapter we will start with a section on the raw materials used to produce HMI, the possible production methods of this product and its safety. The second section will give a short description of the solution properties of long-chain inulin and HMI. This is followed by a section on the interfacial properties of HMI at the air/liquid, liquid/liquid and solid/liquid interfaces. Particular attention will be given in describing the effectiveness of HMI as a stabilizer for various disperse systems, e.g. emulsions, nanoemulsions and latexes. The application of HMI in the formulation of emulsions, latex dispersions and nano-emulsions will be described in subsequent sections. 

In some systems, when the limit of stability of the phase with zero curvature is exceeded, a direct single phase (direct microemulsion, [22,59], or direct cubic liquid crystal [25,60]) instead of a two-phase region, is formed. Then, further addition of water leads the system to the region with two-phase equilibria (W + O), where kinetically (but not thermodynamically) stable dispersions (i.e., nano-emulsions) can form. This emulsification method could be classified as a self-emulsification method since once the system is in one-phase region with direct-type structure, no inversion takes place. 

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 majority of publications on nano-emulsions emphasize their potential applications, comparing them with conventional emulsions (macroemulsions) and microemulsions. With respect to macroemulsions, the main advantage of nano-emulsions would be the smaller droplet size, whereas with respect to microanulsions the main advantage would be the lower amount of surfactant needed for their stabilization. The possibilities of developing applications are mainly limited by stability [2], which is of conrse related to the structure of nano-emulsions, and can be considered a functional characteristic. On the other hand, the potential applications of nano-emulsions depend on their functional characteristics. 

It may seem that due to the different mechanisms and factors influencing the stability of nanoemulsions it is not possible to know a priori which will be the behavior of a defined system. However, the growing knowledge about a great variety of systems and a wise use of this knowledge would allow the formulation of nano-emulsions with enough stability for the applications intended. 

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