Transparency nanoemulsions

K. Nakabayashi,F. Amemiya, T. Fuchigami, K. Machida, S. Takeda, K. Tamamitsu, M. Atobe, Highly clear and transparent nanoemulsion preparation under surfactant-free conditions using tandem acoustic emulsification. Chem. Commun. 47, 5765-5767 (2011)... 

It is clear from Equation (15.1) that t decreases with a decrease in K that is smaller (np - no) than a decrease in No and a decrease in V. Thus to produce a transparent nanoemulsion one has to decrease the difference between the refractive index of the droplets and the medium (i.e. try to match the two refractive indices). If such matching is not possible then one has to reduce the droplet size (by high-pressure homogenization) to values below 50 nm. It is also necessary to use a nanoemulsion with a low oil volume fraction (in the region of 0.2). 

Phase inversion along the dilution path (by addition of water to the oil/surfactant mixture) followed for nanoemulsion preparation was confirmed by conductivity measurements, and was found to be essential for obtaining finely dispersed systems, as transparent dispersions were not obtained if the order of addition of the components was changed following an experimental path with no phase inversion (Figure 6.2). 

When a NAPL reaches the subsurface, it may by subject to mechanical forces that lead to the formation of a mixed NAPL-water micro-/nanoemulsion characterized by the presence of micro- and nanodroplets of organic compounds. These micro- and nanoemulsions are transparent or translucent systems, kinetically (nano-) or thermodynamically (micro-) stable, and display an apparent increase in aqueous solubility as compared to the intrinsic solubility of the NAPL itself (Tadros 2004). The very small droplet size (50-200 nm in the case of a nanoemulsion) causes a large reduction in the force of gravity, enabling the system to remain dispersed and... 

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]. 

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

TTie transparent nature of the system, their fluidity (at reasonable oil concentrations), as well as the absence of any thickeners may give nanoemulsions a pleasant aesthetic character and skin feel. 

An emulsion is a significantly stable suspension of particles of liquid of a certain size within a second, immiscible liquid. The term significantly stable means relative to the intended use and may range from a few minutes to a few years. Investigators in this field distinguish between three different types of emulsions, based upon the size of the dispersed particles (1) macroemulsions, the most well-known type, opaque emulsions with particles >400 nm (0.4 pm), easily visible under a microscope (2) microemulsions, transparent dispersions with particles <100 nm (0.1 pm) in size and (3) nanoemulsions (miniemulsions), a type that is blue-white, with particle sizes between those of the first two types (100-400 nm [0.1-0.4 pm]. Multiple emulsions (Matsumoto, 1976), in which the dispersed particles are themselves emulsions, have been the subject of considerable investigation. 

Nanoscale emulsions have gained technological interest because the transport efficiency of functional components in emulsion food systems is increased when droplets are in the nanoscale (Spernath and Aserin, 2006). In addition, these emulsions are transparent, and they have lower viscosity when compared to conventional emulsions, which make them suitable for use in beverages, for example. In recent years, considerable research effort was made to understand the physical properties, preparation, phase behavior, and stability of micro- and nanoemulsions. 

The nanoemulsion (<100 nm) is nsed for clear, transparent drinks/liquids, however, with possibility of coalescence and loss of clarity with time. 

Subsequently, Han et al. [34] reported IL-in-IL nanoemulsions for the first time. Assisted with a certain amount of surfactant AOT, the hydrophilic IL PAF and the hydrophobic IL 3-methyl-l-octylimidazolium hexafluorophosphate ([omimjPF ) formed [omimjPF -in-PAF microemulsions when the volume ratio of [omimjPF was relatively low. With the increase in volume ratio of [omimjPF, unstable [omim] PFj-in-PAF nanoemulsions were formed, which were transparent before phase separation. The conductivity of [omimjPF -in-PAF nanoemulsions was much lower than that of [omim]PFj-in-PAF microemulsions. Small-angle X-ray scattering (SAXS) experiment demonstrated that the microdroplets of the nanoemulsion were spherical and the gyration radii of the microdroplets decreased from 19.3 to 15.7 nm when the volume ratio of [omimjPF was reduced from 0.35 to 0.25. Utilizing this IL-in-IL nanoemulsion as the reaction medium, stable and crystallined metal-organic framework (MOF) nanorods were successfully synthesized, indicating potential applications of IL-in-IL nanoemulsions as well as microemulsions in other fields. 

Nanoemulsions are transparent or translucent systems in the size range 20-200 nm [35]. Whether the system is transparent or translucent depends on the droplet size, the volume fraction of the oil and the refractive index difference between the droplets and the medium. Nanoemulsions having diameters < 50 nm appear transparent when the oil volume fraction is < 0.2 and the refractive index difference between the droplets and the medium is not large. With increasing droplet diameter and oil volume fraction the system may appear translucent, and at higher oil volume fractions the system may become turbid. 

Nanoemulsions are only kinetically stable. They have to be distinguished from microemulsions (that cover the size range 5-50 nm) which are mostly transparent and thermodynamically stable. The long-term physical stability of nanoemulsions (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 nanoemulsions can be well understood from a consideration of their steric stabilization (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 stabilized against Ostwald ripening (that occurs when the oil has some finite solubility in the continuous medium), nanoemulsions may show an increasing droplet size and an initially transparent system may become turbid on storage. 

Nanoemulsioms are transparent or translucent systems. Due to their small droplet size, nanoemulsions are stable against creaming, or seimentation, flocculation, and coalescence. A main advantage is that high occlusive film may be formed on application to the skin. Another useful application is the ability to enhance penetration of actives (e.g., vitamins) into skin. This is due to their much higher surface area when compared to coarser emulsions (31). 

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