Surface, interfacial and thermodynamic aspects of the Rhamnolipid-salt systems

Abstract

Rhamnolipids have been exploited for fluid–fluid and solid–fluid interactions in various applications due to their excellent surface and interfacial properties. The present work deals with the rhamnolipid-salt system and measured the variation in the thermodynamic and physicochemical properties. A systemic investigation has been carried out; first measured the critical micelle concentration (CMC) of rhamnolipid (200 mg/L) and then the effect of salts over various physiochemical properties of biosurfactant was evaluated. Further, the concentration of rhamnolipid was varied with the selected salts (NaCl, Na2CO3, NaNO3) at optimum concentrations. The physio-chemical characterization of biosurfactant-salt systems was conducted by performing surface tension, particle size/zeta potential, contact angle and microscopic analyses using a tensiometer, dynamic light scattering, goniometer and microscope, respectively. The results demonstrated that the CMC values of rhamnolipid in the biosurfactant-salts system were reduced to 40–60 mg/L. The addition of electrolyte in the rhamnolipid reduced the CMC and surface tension values significantly, which altered the wettability and formed agglomerated structure. The contact angle of rhamnolipid decreased from 76° to 58° in the presence of salt. The reductions in surface tension and CMC value of rhamnolipid have been correlated with the physio-chemical properties of biosurfactant-salt systems. Further, Gibb’s standard free surface energies of rhamnolipid micellization and adsorption at the air–water interface were estimated to predict the thermodynamical behaviour. At CMC values, the surface excess of rhamnolipid was 2.09 × 10−6 mol/m2, which reduced to 1.93 × 10−6 mol/m2 for rhamnolipid-salt systems.