Dynamic properties of micellar solutions: I. Effects of short-chain alcohols and polymers on micellar stability

Abstract

The effect of n-alkanols from C1 to C5 on both equilibrium and dynamic properties of sodium dodecyl sulfate (SDS) micellar solutions has been investigated using electrical conductivity and pressure-jump methods. Methanol and ethanol were found to decrease the conductance of micellar solutions due to the lowering of dielectric constant of solvent. Higher alkanols from C3 to C5 may decrease or increase the conductance of micellar solutions depending on the concentration of SDS. At low surfactant concentration, a minimum of conductance was observed. This has been attributed to both effects of decrease in surfactant monomer concentration (CMC) and the increase in counterion dissociation of micelles upon the penetration of alcohols into micelles. At higher surfactant concentration, the conductance was found to increase directly due to the dominance of the latter effect. Unlike the complex change of conductance, the slow relaxation rate constant, 1/τ2, has been found to increase monotonically upon the addition of all alkanols except pentanol. The labilizing effect (i.e., decreasing τ2) of alkanols increases with increasing alkanol chain length from C1 to C3 and levels off at C4. The labilizing effect has also been found to vary with SDS concentration depending upon the mechanism of slow relaxation process. Based on the Aniansson and Wall theory, the labilizing effect of alkanols has been attributed to two factors: (1) the stabilization of micelle nuclei either due to a decrease of micelle nucleus size, or due to an increase of micelle nucleus population; (2) the decrease of activation energy for the formation of micelle nuclei. In view of the changes in both slow relaxation time and CMC upon the addition of alkanols, the concept of micelle stability has been discussed in terms of the “thermodynamic stability” and the “kinetic stability” of micelles. Finally, the effect of polymers on the slow relaxation process was also investigated. The slow relaxation rate constant 1/τ2 was found to increase by three orders of magnitude as the concentration of polyvinylpyrrolidone increased up to 8% (w/w). Based on the molecular model of polymer-micelle complex proposed by B. Cabane (J. Phys. Chem.81, 1639, 1977), the effect of polymer has been interpreted in terms of the polymer being the nucleating agent for micelle nuclei. The result verifies the concept that the rate-limiting step in micelle formation and dissolution process is the formation of micelle nuclei.