The construction of phase diagram allows to find out the right
proportion of components for the existence of microemulsions
(Piao et al., 2010). The diagrams were constructed using wate
titration method. The samples were defined as microemulsions
when they appeared as clear liquids (Fig. 2). Three microemulsions
were selected from the phase diagrams and their composition was
reported in Table 3.
3.2.1. C1 (Tween 20/Ethanol)
The phase diagram obtained was reported in Fig. 2. In this case,
the system is not a mixture of two surfactants, but rather between a
surfactant and ethanol. Tween 20 forms more easily microemulsions
in the presence of a short-chain alcohol and this latter
acts as cosolvent. A weight ratio surfactant/ethanol 10/90 g/g was
used. During the titration, we observed a net color change and the
formation of a transparent system. The area corresponding to the
microemulsion is very similar to that already reported for a
different drug (Hirunpanich & Sato, 2009), but in our study, the area
corresponding to the formation of gels was also inserted, due to the
presence of birefringent structures with a certain viscosity.
3.2.2. C2 (Tween 20/Cremophor EL)
The microemulsion is present for both weight ratio of Tw20/
CremEL 1/2 and 1/1 and their area of existence are similar.
Furthermore, it can be concluded that Tw20/Cremophor EL 1/2 is
the best formulation (C2, Fig. 2) because the system showed good
stability, also after dilution. The ratio oil/Smix is maintained around
10/90.
3.2.3. C3 (Lecitin/Tween 80)
From the analysis of the phase diagrams the best formulation
resulted Lc/Tw80 with a molar ratio of 1/4, because it provides a
more distinct color change and a wider area of existence (Fig. 2). So