Chemistry Reference
In-Depth Information
TABLE 6.2
Effect of Stabilizer Concentration
a
Concentration of Silica
Nanocolloids (wt %)
Internal Nanostructure
of ISAsomes
Average Radius of
ISAsomes 〈
R
〉 (nm)
PT : TC
50 : 50
0.5
L
2
140
50 : 50
1.0
L
2
286
50 : 50
1.75
L
2
203
100 : 0
0.5
Pn3m
140
100 : 0
1.0
Pn3m
142
a
Dynamic light scattering (DLS) results for 100 : 0 and 50 : 50 mixtures of PT : TC as a function of
silica nanocolloid concentration.
(Pn3m) were seen to increase in both Laponite-stabilized ISAsomes (Fig. 6.5b)
and F-127-stabilized samples at high pH (Fig. 6.5d) (Muller et al., 2010b). In
conclusion, increasing the pH can change the structure and hydration of the
LC phases and release trapped molecules. For many potential applications
there might be substantial benefi ts in stabilizing cubosomes using clay-disk-
like particles instead of F-127.
In recent work, Salonen et al. (2010b) also used spherical silica nanocolloids
for stabilizing ISAsomes. These colloids were 25 nm (Ludox TM 50, Du Pont)
and carrying negative charge that was partially screened by sodium counter-
ions. The pH of a 50-wt % stock solution was measured to be 8.85 at 25°C. As
observed for Laponite (Table 6.1), the silica particles also affected the stability
of monoglyceride-based cubosomes with Pn3m nanostructure in case of DU/
TC. However, these nanocolloids were successfully utilized for stabilizing the
ISAsomes in the PT/TC system (Table 6.2).
The infl uence of the concentration of stabilizing silica nanocolloids on the
LC nanostructure was also investigated (Salonen et al., 2010b) in order to
understand the fundamental difference between the stabilization pathways
of different internal structures of ISAsomes and to quantify this effect (Table
6.2). Far fewer silica particles were required to stabilize cubic phases than L
2
phases of similar size. Moreover, the concentration of silica particles was also
found to control the size of L
2
-based ISAsomes but not that of cubosomes.
Further, free nanoparticles coexisted with the cubosomes.
Very recent observations show that the addition of sodium dodecyl sulfate
increases the stability of ISAsomes prepared from silica particles (diameter
18 nm) (Dulle et al., in preparation). Subsequent addition of C
12
E
5
showed
even more interesting results with enhanced charge and stability as well as a
decrease in the free silica particles in the dispersion. ISAsomes with hexagonal
nanostructure were observed under cryo-transmission electron microscopy
(TEM) (Fig. 6.6) and were seen to be covered and stabilized with silica par-
ticles on the surface. Residual nonadsorbed stabilizer particles were also
observed as monomers or as aggregates.
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