Chemical relaxation and equilibrium studies of aqueous solutions of laurylsulfate micelles in the presence of divalent metal ions

Abstract

Equilibrium methods (conductivity, activity and density) as well as chemical relaxation methods (ultrasonic absorption, T-jump, p-jump and shock-tube) have been used to investigate aqueous micellar solutions of several divalent metal laurylsulfates (MLS 2) and of sodium laurylsulfate (NaLS) in the presence of divalent metal ions. The conductivity and activity data clearly show that divalent metal counterions are more bound to laurylsulfate (LS) micelles than Na + ions. The density data, however, indicate that the binding of M 2+ to LS micelles gives rise to a very small volume change and, therefore, that this binding does not proceed beyond the stage of an outer-outer sphere complex formation between counterions and micelle ionic head groups. The kinetic measurements show the existence of two relaxation processes which, as for NaLS, appear to be due to the exchange of LS ions between the micelles proper (fast process; relaxation time τ 1) and to the micelle formation-dissolution equilibrium (slow process; relaxation time τ 2). The kinetic results have been interpreted in terms of Aniansson and Wall [ J. Phys. Chem. 78, 1024 (1974); 79, 857 (1975)] theory. The results concerning the fast process indicate that as for detergents with monovalent counterions the association of LS ions to a micelle proper is close to diffusion controlled. The LS micelles formed in the presence of divalent counterions are less polydisperse than with monovalent counterions. The rate constant for the dissociation of one LS ion from a micelle proper is smaller in the presence of Ni 2+ or Co 2+ ions than with Na +. This observation has been related to the stronger association of M 2+ relatively to Na +, to LS micelles. Most of the results relative to the slow relaxation process have been explained in terms of an association of counterions to the micelle nucleus and/or a decrease of aggregation number of the micelle nucleus upon increasing detergent concentration or addition of divalent metal chlorides (MCl 2). The data concerning the change of τ 2 of NaLS solutions upon addition of MCI 2 or mixing to MLS 2 have been qualitatively interpreted in terms of these two effects in addition to the change of cmc which prevails at low concentration of added MCl 2 or MLS 2.