Micellar Properties of Sodium Sulfopropyl Alkyl Maleates in Aqueous SolutionA Time-Resolved Fluorescence Quenching Study

The aggregation behavior of four sodium sulfopropyl alkyl maleates with alkyl chain carbon numbers 12, 14, 16, and 18 has been investigated in aqueous solution as a function of surfactant concentration, added sodium chloride concentration, and temperature by means of electrical conductivity and time-resolved fluorescence quenching (TRFQ). Critical micelle concentration and micelle ionization degree decrease upon increasing chain length, exhibiting the classical behavior for straight alkyl chain surfactants, whereas the Krafft temperature, T K, increases. The aggregation numbers have been found to decrease with increasing temperature between T K and 65 °C, with the magnitude of decrease becoming higher for longer hydrophobic chains. The surfactants show micellar growth with surfactant and salt concentration. Whereas the increase of the aggregation number N with ionic strength I is generally weak and follows in good approximation the power law relation N ∝ I β, with a unique exponent β ≅ 0.13 below a surfactant-dependent characteristic threshold ionic strength, I *, N steeply increases beyond I *. The values of N indicate that the micelles are probably spheroidal below I *. The micellar polydispersity is generally low for ionic strength I < I * and increases with I, in agreement with the theory of micellization.