Effect of tetraalkylammonium salts on the micellar behavior of lithium dodecyl sulfate: A conductometric and tensiometric study

The effect of quaternary ammonium bromides R4NBr (R = H, CH3, C2H5, C3H7 and C4 H9) on the micellar properties of lithium dodecyl sulfate (LDS) micelles has been studied by conductometric and tensiometric techniques. The degree of the conuterion binding, β, is found to increase with the increasing size of the alkyl chain length of the tetraalkylammonium salts. The thermodynamics of micelle formation of ionic surfactant LDS in the presence of 0.001M of different quaternary salts have been thoroughly assessed by the critical micelle concentration (CMC) measurement in the 288.15-318.15K-temperature range at 10K intervals conductometrically. Results show a shallow minimum of the CMC values typically between 300 to 305K for LDS micelles in presence of various salts. The standard free energy of micellization, ΔG0m has been found to decrease upon growth of temperature. The standard enthalpy change on aggregation was calculated by using the Gibbs-Helmholtz equation. Tensiometric results show a significant reduction in the solution surface tension with increasing electrolyte concentration. The surface excess of LDS and its minimum area at the air-water interface in the presence of different R4NBr environment has been determined. The micellar solution of LDS has been found to exhibit a cloud point phenomenon in the presence of Bu4NBr. The occurrence of cloud point phenomenon is accounted for in terms of the penetration of butyl chains at the surface layer of one micelle into another due to the hydrophobic effect.