Biomedical Engineering Reference
In-Depth Information
Scheme 12.5
binding affi nity of the coordinating surfactant at low temperature. Consequently,
nonhydrolytic sol-gel reactions are generally performed at high temperatures
(200-300 °C), so that the coordinating surfactant will bind reversibly on the metal
cation (Scheme 12.5 ).
12.3.1.3
Microemulsions
Microemulsions are defi ned as clear, thermodynamically stable dispersions con-
sisting of, at least, a ternary mixture of water, a surfactant or a mixture of surface-
active agents, and oil [63]. Depending on the proportion of suitable components
and the hydrophile-lypophile balance (HLB) value of the surfactant used, the
formation of microdroplets may be either in the form of oil-swollen micelles dis-
persed in the aqueous phase, or as water-swollen micelles dispersed in oil.
Microemulsion methods can be classifi ed as either “ normal micelle ” (also
known as oil - in - water ( o/w ) methods, or as “ reverse micelle ” (also known as water -
in-oil (w/o) methods. Surfactants must be used in both cases, with their concentra-
tion being maintained above the so- called critical micelle concentration ( CMC );
this is defi ned as the concentration of surfactants above which micelles are spon-
taneously formed.
Pileni and coworkers have exploited the synthesis of many different ferrite
nanoparticles in functionalized micelles (i.e., the reactants are the counterions of
the surfactants) [64, 65]. In a typical procedure, ferrous dodecylsulfate (previously
prepared by the reaction of FeCl
2
with sodium dodecyl sulfate , SDS ) [66] was solu-
bilized in a methyl amine aqueous solution and stirred for 3 h at 50 °C. The black
precipitate of magnetite was separated, purifi ed, and redispersed in water. The
resultant particles had a mean diameter of 7.4 nm. Experiments performed at dif-
ferent ferrous dodecyl sulfate concentrations and at different temperatures showed
that the particle size can vary between 3.7 and 11.6 nm, with the size being con-
trolled by changing either the precursor concentration (by a factor of 4) or the
