Biomedical Engineering Reference
In-Depth Information
DLS can be used to study structural changes associated with the volume fraction of the
dispersed phase in microemulsions, where changes are often the result of modified droplet
interactions. These interactions increase when droplet size increases, alcohol (co-surfactant)
chain length decreases and salinity decreases (Cazabat and Langevin, 1980; Bellocq et al .,
1984 ; Cazabat, 1985 ; Hou et al ., 1988). Furthermore, it can be used to confirm domain
diameter, as per Figure 5.5.
5.10.5 Nuclear magnetic resonance
This approach is widely used for the determination of the diffusion coefficients of the
various components in microemulsions, namely the oil, water and surfactant(s). These
measurements are useful in elucidating the structural changes occurring in microemulsions
with changes in composition or environmental conditions. It is also helpful in establishing
the presence of long-range order and particle anisotropy (Moulik and Paul, 1998).
Numerous authors have established structural transitions and diffusion coefficients in
microemulsions using NMR. Fanun ( 2008 b) reported structural transitions in water:propylene
glycol-sucrose esters-benzaldehyde:ethanol systems upon increasing volume fraction of
water. The diffusion measurements suggested a transition from water-in-oil microemulsions
via bicontinuous microstructure to oil-in-water microemulsions. In a follow-up study, Fanun
(2008a) suggested structural transitions occurring in water-sucrose laurate-ethoxylated
mono-di-glyceride-R(+)-limonene systems. The diffusion coefficient of water increased
and that of oil decreased upon increasing the volume fraction of water. Determination of
self-diffusion coefficients in water-octylglucoside-pentanol-decane microemulsions led
Parker and co-workers (1993) to report a gradual decrease in the mean curvature of the
interfacial film upon water addition at a fixed oil content. These changes suggested a phase
transition from o/w via bicontinuous to w/o microemulsions.
Microemulsions have been attracting considerable attention due to properties such as ease
of formation, thermodynamic stability, transparency and high solubilization capacity.
Microemulsions are excellent delivery systems for nutraceuticals (Rozner et al ., 2007 , 2008 )
and as vehicles for chemical reactions, such as acid autocatalysis (McIlwaine et al ., 2008 ).
They have also been used for the separation and purification of proteins, metal extraction
and as drug delivery systems (Hatton, 1989; Pileni, 1989; Garti et al ., 2006 ; Spernath and
Aserin, 2006 ; Kogan et al ., 2007 ).
5.11.1 Solubilization of poorly-soluble drugs
A key benefit of microemulsions is their ability to disperse food ingredients, such as
bioactives, flavors and preservatives, that are poorly water soluble in aqueous systems, as these
compounds can be conveniently solubilized into the oil droplets of o/w microemulsions (Weiss
et al ., 2006). For example, Garti and co-workers (2004) studied the solubilization of water-
insoluble and poorly oil-soluble nutrients - phytosterol, lutein and lycopene - in U-type
microemulsions and showed an increase in the solubility of these nutrients by factors of 15,
11 and 5, respectively, by incorporating polysorbate 80 (or polysorbate 60) in a limonene-
ethanol mixture.
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