Structural organization of cetyltrimethylammonium sulfate in aqueous solution: The effect of Na2SO4

We used dynamic light scattering (DLS), steady-state fluorescence, time resolved fluorescence quenching (TRFQ), tensiometry, conductimetry, and isothermal titration calorimetry (ITC) to investigate the self-assembly of the cationic surfactant cetyltrimethylammonium sulfate (CTAS) in aqueous solution, which has SO²⁻₄ as divalent counterion. We obtained the critical micelle concentration (cmc), aggregation number (N_agg), area per monomer (a₀), hydrodynamic radius (R_H), and degree of counterion dissociation (α) of CTAS micelles in the absence and presence of up to 1 M Na₂SO₄ and at temperatures of 25 and 40 °C. Between 0.01 and 0.3 M salt the hydrodynamic radius of CTAS micelle R_H≈16 Å is roughly independent on Na₂SO₄ concentration; below and above this concentration range R_H increases steeply with the salt concentration, indicating micelle structure transition, from spherical to rod-like structures. R_H increases only slightly as temperature increases from 25 to 40 °C, and the cmc decreases initially very steeply with Na₂SO₄ concentration up to about 10 mM, and thereafter it is constant. The area per surfactant at the water/air interface, a₀, initially increases steeply with Na₂SO₄ concentration, and then decreases above ca. 10 mM. Conductimetry gives α=0.18 for the degree of counterion dissociation, and N_agg obtained by fluorescence methods increases with surfactant concentration but it is roughly independent of up to 80 mM salt. The ITC data yield cmc of 0.22 mM in water, and the calculated enthalpy change of micelle formation, , Gibbs free energy of micellization of surfactant molecules, and entropy indicate that the formation of CTAS micelles is entropy-driven.