Rhamnolipid–Metal Ions (CrVI and PbII) Complexes: Spectrophotometric, Conductometric, and Surface Tension Measurement Studies

Abstract

AbstractThe formation of rhamnolipid complexes with metal ions of chromium (VI) and lead (II) has been studied spectrophotometrically, conductometrically, and by surface tension measurement. The values of the critical micelle concentration (CMC) of rhamnolipid, obtained by spectroscopic, conductometric, and surface tension measurements, were 5.2 × 105, 5.0 × 105, and 5.3 × 105 mol.dm−3, respectively, which are in close agreement. An increase in CMC on increasing metal ion content in the rhamnolipid solution and a shift in λmax in the spectra of rhamnolipid indicated the formation of a complex between rhamnolipid and both the metal ions, namely, chromium (VI) and lead (II). The values of stability constants for the {rhamnolipid–chromium (VI)} and {rhamnolipid–lead (II)} complexes have been determined by spectroscopic data and were as 0.58 × 104 and 0.50 × 104 at 308 K, respectively.The thermodynamic parameters for micellization, namely, free energy change (∆Gmic), entropy change (∆Smic), and heat enthalpy change (∆Hmic), have been determined by conductivity measurements. An increase in the negative value of ∆Gmic and a decrease in the value of ∆Smicon increasing metal ion content in the surfactant solution indicated lower micellization of rhamnolipid in the presence of metal ions. The electrostatic attractions and entrapment of chromium (VI) in the micelles of the biosurfactants were found to be responsible for {rhamnolipid–lead (II)} and {rhamnolipid–chromium (VI)} complexes, respectively.