Surfactants in nano-emulsions

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. 

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. 

Nano-emulsions are attractive for application in personal care and cosmetics as well as in health care due to the following advantages  

Applied Surfactants Principles and Applications. Tharwat F. Tadros Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30629-3 

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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]. 

Sole, I., Maestro, A., Gonzalez, C., Solans, C. and Gutierrez, J.M. (2006) Optimization of nano-emulsion preparation by low-energy methods in an ionic surfactant system. Langmuir, 22 (20), 8326-8332. 

N. Uson, MJ. Garcia, and C. Solans Formation of Water-in-Oil (W/O) Nano-Emulsions in a Water/Mixed Non-Ionic Surfactant/Oil Systems Prepared by a Low-Energy Emulsification Method. Colloid and Surfaces A Physicochem. Eng. Aspects 250, 415 (2004). 

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]. 

Nano-sized PtRu catalysts supported on carbon have been synthesized from inverse micro emulsions and emulsions using H2PtClg (0.025 M)/RuCl3 (0.025 M)/NaOH (0.025 M) as the aqueous phase, cyclohexane as the oil phase, and NP-5 or NP-9) as the surfactant, in the presence of carbon black suspended in a mixture of cyclohexane and NP-5-I-NP-9 [164]. The titration of 10% HCHO aqueous solution into the inverse micro emulsions and emulsions resulted in the formation of PtRu/C catalysts with average particle sizes of about 5 nm and 20 nm respectively. The RuPt particles were identified by X-ray diffraction. X-ray photoelectron, and BET techniques. All of the catalysts prepared show characteristic diffraction peaks pertaining to the Pt fee structure. XPS analysis... 

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. 

Unlike microemulsions (which require a high surfactant concentration, usually in the region of 20% and higher), nano-emulsions can be prepared using reasonable surfactant concentrations. For a 20% O/W nano-emulsion, a surfactant concentration in the region of 5-10% may be sufficient. 

Thus, emulsion formation is non-spontaneous and energy is required to produce the droplets. The formation of large droplets (few pm) as is the case for macroemulsions is fairly easy and hence high speed stirrers such as the Ultraturrax or Silverson Mixer are sufficient to produce the emulsion. In contrast the formation of small drops (submicron, as is the case with nano-emulsions) is difficult, requiring a large amount of surfactant and/or energy. 

Surfactants play major roles in the formation of nano-emulsions By lowering the interfacial tension, p is reduced and hence the stress needed to break up a drop is reduced. Surfactants prevent coalescence of newly formed drops. 

The role of surfactants on emulsion formation is detailed in Chapter 6 and the same principles apply to the formation of nano-emulsions. Thus, one must consider the effect of surfactants on the interfadal tension, interfacial elasticity, and interfacial tension gradients. 

Several procedures may be applied to enhance the efficiency of emulsification when producing nano-emulsions One should optimise the efficiency of agitation by increasing 6 and decreasing the dissipation time. The emulsion is preferably prepared at high volume faction of the disperse phase and diluted afterwards. However, very high rj) may result in coalescence during emulsification. Addition of more surfactant creates a smaller and possibly diminishes recoalescence. A surfactant mixture that shows a reduction in y compared with the individual components can be used. If possible, the surfactant is dissolved in the disperse phase rather than the continuous phase this often leads to smaller droplets. 

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]. 

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 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 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. 

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... 

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