Biomedical Engineering Reference
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to form a hydrocarbon solution with solubilized water. Phase reversal takes place at the
Phase Inversion Temperature (PIT), that is, the temperature range in which an o/w
microemulsion inverts to a w/o type (Flanagan and Singh, 2006).
Polydispersity is a measure of the uniformity of a droplet size distribution and typically
varies from 0.0 to 1.0 (unit-less) (Constantinides and Yiv, 1995), where values of 0.000 to
0.02 indicate a monodisperse or nearly monodisperse distribution, values of 0.02-0.08 are
common for narrowly-distributed droplet sizes, and values higher than 0.08 indicate broad
distributions. Despite microemulsions being polydispersed, they are frequently interpreted
as being monodispersed to simplify analysis.
Polydispersity can be measured using techniques such as dynamic light scattering (DLS)
and time-resolved fluorescence quenching (TRFQ). Also, small angle light scattering has
been used in various systems, for example, water-AOT- n -hexane microemulsions (Ricka
et al ., 1991). Understanding the importance of polydispersity can provide insight into the
overall stability of microemulsions. Ideally, microemulsions should be monodispersed, with
a narrow size distribution. An increase in breadth usually implies reduced stability, and is
often observed when the composition of a microemulsion system is altered, for example,
with the addition of a bioactive compound to the microemulsion formulation or with changes
in environmental (pH, temperature, ionic strength, etc.) conditions. For example,
Constantinides and co-workers (Constantinides and Yiv, 1995) studied the influence of
temperature and dilution of w/o microemulsions on polydispersity and mean particle
diameter using DLS, reporting a significant change in these parameters above 60 °C, which
was used as an indicator of the impact of storage conditions on system stability.
Most microemulsions consist of an organic phase (usually oil), an aqueous phase, surfactant
and a co-surfactant. However, in some studies, co-surfactants can be eliminated by the judi-
cious combination of high and low HLB (hydrophilic lipophilic balance) surfactants that are
structurally complementary (Constantinides and Scalart, 1997; Polizelli et al ., 2006 ). For
instance, surfactants with bulkier head groups [e.g., polysorbates or polyoxyethylene lauryl
ether (von Corswant and Soderman, 1998)] will tend to form o/w microemulsions whereas
surfactants with bulkier tails [e.g., AOT (sodium bis(2-ethylhexyl) sulfosuccinate)] will tend
towards w/o microemulsion formation (Gaonkar and Bagwe, 2002). Thus, combining such
surfactants can promote a wide range of compositions leading to microemulsion
5.7.1 Organicphase
Many food-based oils, including mineral, soy, corn, cottonseed and sunflower, have been
used to generate microemulsions, although not necessarily for food applications (Hamdan
et al ., 1995 ; von Corswant et al ., 1997 ; Gaonkar and Bagwe, 2002 ; Garti, 2003 ; Flanagan
et al ., 2006). Another commonly used oil is mineral oil (Hamdan et al ., 1995 ; Garti, 2003 ),
given its ability to form well-structured dispersed phases (Gaonkar and Bagwe, 2002).
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