Micellar properties of sodium sulfopropyl alkyl maleates in aqueous solution - A 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 degrees 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 proportional to I-beta, with a unique exponent beta congruent to 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*. Th, micellar polydispersity is generally low for ionic strength I < I* and increases with I, in agreement with the theory of micellization.