Nanoemulsion surfactants used

In contrast to the results obtained with hexadecane, the addition of squalane to the O/W nanoemulsion system based on isohexadecane showed a systematic decrease in Ostwald ripening rate as the squalene content was increased. The results are included in Figure 14.14, which shows plots of versus time for nanoemulsions containing varying amounts of squalane. The addition of squalane up to 20% based on the oil phase showed a systematic reduction in ripening rate (from 8.0 to 4.1 x 10 m s i). It should be noted that when squalane alone was used as the oil phase, the system was very unstable and showed creaming within 1 h. The results also showed that the surfactant used was unsuitable for the emulsification of squalane. 

Table 13.3 Various Surfactants Used in Nanoemulsion (Shinoda and...

This section, which is by no means exhaustive, will deal with the following topics (i) Surfactants used in cosmetic formulations, (il) Interaction forces between particles or droplets in a dispersion and their combination, (iil) Description of stability in terms of the interaction forces, (iv) Self-assembly structures and their role in stabilization, skin feel, moisturization and delivery of actives, (v) Use of polymeric surfactants for stabilization of nanoemulsions, multiple emulsions, liposomes and vesicles. 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

Further evidence for Ostwald ripening was obtained by using a more soluble oil, namely a branched hexadecane (Arlamol HD). The results are shown in Fig. 1.31 for nanoemulsions prepared using 4% surfactant. It can be seen that the more soluble oil (Arlamol HD) give a higher rate of Ostwald ripening when compared with a less soluble oil such as hexadecane. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

Unlike microemulsions (which require a high surfactant concentration, usually in the region of 20% and higher for a 20% microemulsion), nanoemulsions can be prepared using reasonable surfactant concentrations. For a 20% O/W nanoemulsion, a surfactant concentration in the region of 5% may be sufficient... 

In spite of the above difficulties, several companies have introduced nanoemulsions onto the market, and their benefits will be evaluated within the next few years. Nanoemulsions have been used in the pharmaceutical industry as drug-delivery systems [5], although the acceptance by customers of nanoemulsions as a new type of formulation depends on how they are perceived and their efficacy. With the advent of new instruments for high-pressure homogenisation, and the competition between various manufacturers, the cost of nanoemulsion production wiU surely and may even approach that of classic macroemulsions. Fundamental investigations into the role of surfactants in the process [6,7] will lead to optimised emulsifier systems such that a more economic use of surfactants will doubtless emerge. 

As most nanoemulsions are prepared using nonionic and/or polymeric surfactants, it is necessary to consider the interaction forces between droplets containing adsorbed layers (steric stabilisation). As this was described in detail in Chapter 10, only a summary will be given here [15, 16]. 

The results with isohexadecane are summarised in Table 14.2. As with the hexadecane system, the droplet size and polydispersity index were decreased with increases in surfactant concentration. Nanoemulsions with droplet radii of 25-80run were obtained at 3-8% surfactant concentration. It should be noted, however, that nanoemulsions could be produced at lower surfactant concentration when using isohexadecane, when compared to results obtained with hexadecane. This could be attributed to the higher solubility of isohexadecane (a branched hydrocarbon), the lower HLB temperature, and the lower interfacial tension. 

Figure 14.18 r versus time for nanoemulsion systems prepared using the PIT and Microfluidizer. 20/80 O/W ratio and 4wt% surfactant. 

These cationic SLN are formulated from a matrix lipid surfactants to stabilize the formulation and cationic lipids to charge the SLN surface positively. The cationic lipids employed are the same used in cationic liposomes for transfection [28]. Eormulation optimization studies revealed that both the cationic lipid and the matrix lipid influence transfection activity [27]. Comparable results were found for the oil component in cationic nanoemulsions for transfection [29]. Several formulations made from different cationic lipids and matrix lipids were tested for in vitro transfection efficiency (Eigure 6.8). The SLN Cp DOTAP made from the wax cetyl pahnitate and the cationic lipid N-[l-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium... 

Chausson et al. obtained nanoemulsions using a block copolymer with units of EO and caprolactone (CAP) as nonionic surfactant [87]. Peaks present in the MALDI spectrum of the EO-CAP copolymer are due... 

---