Counterion Effects on the Structure and Solubilization Properties of Anionic Detergents. Role of Hydrophobic Cations in Stabilizing Micelles and Attenuating Their Solvent Properties

Article abstract of DOI:10.1021/j100338a070

A series of alkyl sulfate detergents has been investigated in the presence of the cations Na⁺, methylviologen(2+) (MV²⁺), 4-(cyanomethyl)pyridinium(+) (CMP⁺), and tetramethylammonium (TMA⁺).The binding of these ions to the aqueous micellar assemblies has been measured through studies of luminescence quenching with the extramicellar probe, RuL3^4-, where L=4,4'-dicarboxy-2,2'-bipyridine.A general comparison of the alkyl sulfate aggregates with the ''nonquenching cations'' Na⁺ and TMA⁺ shows that the latter ion reduces the critical micelle concentration but at the same time depresses the ability of the detergent assemblies to ''bind'' or solubilize the hydrophobic quencher cations MV²⁺ or CMP⁺.The reduced binding ability of the TMA⁺ aggregates compared to that of the corresponding Na⁺ soaps shows up largely in the form of a reduced favorable ΔS^o for the solubilization in the case of the formed.The results are in accord with a picture of the TMA⁺ micelle as being more stable and more disordered than the corresponding assembly with Na⁺ as the counterion.