3.4. Solubility of curcumin into microemulsions
The amount of drug solubilized in the selected microemulsions
was reported in Table 5. The solubility of curcumin was improved
considerably by all formulations. Among the three microemulsions,
C1 showed the highest solubilization capacity (14.57 mg/ml). Also
C3 microemulsion increased the solubility of drug to 5.15 mg/ml.
The effect on solubility also affected all other curcuminoids
(demethoxycurcumin and bisdemethoxycurcumin) probably due
to the different co-surfactants used in the formulations.
In the case of C1 formulation, this is probably due to the presence
in the system of ethanol. In fact, the penetration of alcohol in
the interfacial film determined a greater fluidity of the interface
thus allowing long hydrophobic chains of the surfactant to move
freely. Consequently, the penetration of oils between lipophilic
structures was facilitated and thus increased the solubility of curcumin.
In addition, ethanol acts as a co-solvent. Finally, from the
studies of phase solubility, it was observed the maximum solubility
of curcumin in tocopherol acetate oil phase and Tween 20 as surfactant,
respectively. These two components showed a synergic
effect when they are present in the same formulation.
Curcumin solubility in microemulsion C3 was slightly higher
than ones in the every single constituent of microemulsion: it is
increased approximately 5-folds compared to that one in olive oil,
about 7 times in comparison with lecithin and about 1.5 times
compared to Tween 80 (Table 2). This is probably due to the ability
of curcumin of solubilizing at the interface.
The solubility of curcumin in the C2 microemulsion was the
lowest, and this despite the fact that this substance showed a rather