Investigations on Natural Surfactant obtained from Soap-Nuts through Spectrophotometric Interactions with Congo Red and Comparison with Commercial Surfactants

Abstract

A natural surfactant (NS) of plant-base was obtained from the fruit pericarp of soapnuts by using a simple and economical method. The interaction of this natural surfactant with direct dye, anionic dye, Congo red (CR) was studied spectrophotometrically in sub-micellar and micelle concentration range in aqueous solution. These interactions (CR-NS) were compared with that of CR-CTAB (cationic surfactant-cetyl trimethylammonium bromide, CTAB), CR-SDS (anionic surfactant-sodium dodecyl sulphate, SDS) and CR-TX 100 (neutral surfactant - Triton X-100, TX 100) and were useful to understand the nature of natural surfactant. The mechanism of formation of complex due to interactions between Congo red and natural surfactant was suggested. This spectrophotometric method was used for the determination of critical micelle concentration (CMC), at which the formation of micelles was started. The CMC values obtained spectrophotometrically for the natural surfactant was coincided with the experimental value available in the literature. A definite change in the absorbance maxima of Congo red in the presence of natural surfactant (micelles of natural surfactant) was also observed. The change in maxima was also interpreted in terms of pH and CMC. The equilibrium constant of interaction between Congo red and natural surfactant was calculated on the theoretical model. The stability of the complexes of Congo red with different surfactants like CTAB, SDS, TX 100 and natural surfactant may be written in increasing order as: CR-TX 100 > CR-CTAB > CR-NS > CR-SDS. The biodegradable, non-toxic, inexpensive, environmental friendly, renewable natural surfactant was suggested in place of synthetic surfactants.