Biomedical Engineering Reference
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hydraulic fluid using SAXS and established the change in scattering behavior as a function
of the percentage of water, noting microstructural changes associated with water solubilizing
capacity. Using SAXS to analyze monolinolein-water-Pluronic F127 systems, de Campo
and co-workers (2004b) reported the existence of an intact inner w/o structure when dispersed
to form an emulsified w/o microemulsion - an o/w emulsion where water is microemulsified
within the inner oil droplets. These authors also reported the transition of an emulsified cubic
phase via an emulsified hexagonal phase to emulsified L 2 phase, establishing that temperature
reversibly influenced the internal structure of the dispersed particles.
Polizelli and co-workers (2006) used SAXS to distinguish structurally different regions L 2
(w/o microemulsions) and L 2
(dry lamellar structures) within the PTPD consisting of
soybean oil-water-monoolein:AOT. Fanun and co-workers (2008a) reported an increase in
the periodicity of microemulsions, and hence an increase in domain size, upon increasing
the volume fraction of water in water-sucrose laurate-ethoxylated mono-di-glyceride-R(+)-
limonene systems. Along with NMR results, they elucidated the structural transitions in the
microemulsions from water-in-oil via bicontinuous to oil-in-water dispersions.
5.10.2.2
Small angle neutron scattering
Small angle neutron scattering has been effectively used to study droplet shape, size and
polydispersity of microemulsions. For the purpose of simplifying structural analysis in
dispersions, droplets are considered spherical and monodispersed (Kotlarchyk and Chen,
1983 ; Toprakcioglu et al ., 1984). Eastoe and co-workers (1996) established that the alkyl
chain structure of the surfactant was critical to the penetration of cyclohexane/n-heptane
(oil phase) at the interface of o/w microemulsions. The study screened various surfactants
(phosphatidylcholines, dialkyldimethyl ammonium bromides and AOT) to establish their
effect on the oil penetration at the oil-water interface. Kellay and co-workers (1993) studied
interfacial film structure in AOT-alkane-brine mixtures at low AOT concentrations as a
function of brine salinity, noting the lamellar arrangement of octane and decane films. At the
highest salinity, they reported the existence of an oil/brine bicontinuous phase. Finally, Full
and Kaler (1994) studied the structure of styrene-dodecyltrimethylammoniumbromide-
brine microemulsions, noting the presence of swollen micelles upon the addition of styrene.
5.10.3 Cryo-transmission electron microscopy
Cryo-transmission electron microscopy is widely used to study the microstructure of micelles
(Clausen et al ., 1992; Zana and Talmon, 1993; Kaplun et al ., 1994 ; Danino et al ., 2000 ) and
microemulsions (Jahn and Strey, 1988; Strey, 1994). This technique is unique in providing
direct images of the structure (shape and size) at a resolution of ~1 nm (Jahn and Strey, 1988;
Danino et al ., 2000; Garti and Aserin, 2006). The procedure for studying microemulsions
using cryo-TEM involves four steps: sample preparation, freeze-fracturing, replication, and
viewing under the microscope (Jahn and Strey, 1988; Strey, 1994). Figure 5.5 shows the
structure of a representative microemulsion characterized via cryo-TEM, along with the
corresponding particle size distribution obtained via DLS.
One of the first attempts at distinguishing water and oil regions in microemulsions
using cryo-TEM was undertaken by Jahn and Strey (1988), who reported the first evidence
of bicontinuous structures, using water- n -octane- n -dodecylpentaethylene glycol micro-
emulsions as a model. Regev et al . (1996) reported the presence of different structures
based on the amount of water in microemulsions with cryo-TEM, including spherical
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