Analytical procedure
4 mL of the acetate buffer solution (pH 5) containing of carmine
(so that its final concentration would be in the range of 0.04–
5 lgmL1) was transferred to a 10 mL centrifuge tube. Then
1 mL of 40% (w/v) of Triton X-100 and 0.75 g of Na2CO3 salt were
added to this solution. After dissolving the salt, the mixture was
then placed in a thermostat bath at 55 C for 15 min. The phase
separation was accelerated by centrifuging the test tube for
5 min at 4000 rpm. The surfactant-rich phase became a viscous
and was collected at the upper of the tube. Therefore, the aqueous
phase was carefully removed using a syringe with a long needle
that passed through the surfactant-rich phase. The
surfactant-rich phase was diluted with water up to 1 mL. The
absorbance of the solution was measured at 513 nm. A blank solution
(without carmine) was also submitted to the same procedure
and measured in parallel to the samples.