3.2.3. Physical and chemical stability of curcumin in diluted NADES
Samples of curcumin in GS and MC3 diluted 10–50 times in MilliQ
water showed a rapid precipitation of curcumin. No solubilized
curcumin could be detected in GS diluted 10 times after 30 min while
48 ± 1% still remained in solution in GS diluted 50 times. After 1 h
complete precipitation was observed also in this sample. Dilution of
MC3 did not induce complete precipitation of curcumin. An equilibrium
concentration of ~ 0.011 mg/ml was maintained for at least 1 h independent
of dilution factor. Dilution of samples GS and MC3 (1:10) in phosphate
buffer at pH 8.0 (resulting in pH 8.0 and 1.8, respectively) induced
degradation of curcumin but no precipitation was observed. The
curcumin degradation half-life according to 1st order kinetics (r ≥ 0.91
except for one MC3 sample with r = 0.86) was calculated to 3.4 ±
0.9 h and 40.8 ± 8.8 h in GS and MC3, respectively.
3.2.4. Photostability studies
The curcumin photodegradation half-life according to 1st order kinetics
(r N 0.95) was calculated to 4.3 ± 0.7 h in GS at a curcumin concentration
of 0.010 mg/ml. Under similar conditions a sample of
curcumin (0.006 mg/ml) in MC3 was decomposed to a concentration
below the detection limit of the HPLC system after 10 min exposure
(i.e., b3 × 10−8 M after minimum dilution, see Section 2.2). The
curcumin concentration was therefore increased six times in order to
calculate the degradation half-life. The resulting photodegradation
half-life (0.0