Aggregation of Oligomeric Surfactant Constructed by Four-Armed Carboxylic Acid Sodium and Cationic Surfactant.

Abstract

A star-shaped oligomeric-like surfactant with variable oligomeric degrees has been formed with a four-arm carboxylate salt (4EOCOONa) and cationic single chain surfactant dodecyl trimethylammonium bromide (DTAB). The aggregation of the 4EOCOONa/(DTAB)n complexes has been investigated by surface tension, electrical conductivity, isothermal titration microcalorimetry, ζ potential, dynamic light scattering, 1H NMR spectroscopy, and steady-state fluorescence measurements. The calorimetric result shows that 4EOCOONa interacts strongly with DTAB and each 4EOCOONa molecule binds with six DTAB molecules, wherein four DTAB molecules electrostatically bind to one 4EOCOONa molecule and additional two DTAB molecules further bind to the 4EOCOONa/(DTAB)n complex by hydrophobic interaction. The critical micelle concentration (CMC) of the 4EOCOONa/(DTAB)n complexes is remarkably lower than the CMC of DTAB, similar to synthesized star-shaped oligomeric surfactants. The micelle properties of the DTAB/4EOCOONa mixtures depend on the component changes of the 4EOCOONa/(DTAB)n complexes. By increasing the DTAB/4EOCOONa molar ratio and/or concentration, the DTAB/4EOCOONa mixtures gradually form the complexes of 4EOCOO(DTA)13-, 4EOCOO(DTA)22-, 4EOCOO(DTA)3-, 4EOCOO(DTA)4, and 4EOCOO(DTA)62+, and the corresponding aggregates are small anionic micelles with loose molecular packing, and nearly nonionic or positively charged small micelles with more compact packing. Moreover, the positive charge of the small micelles increases with the increase of the concentration and the DTAB/4EOCOONa molar ratio. Therefore, constructing oligomeric-like surfactants by adding appropriate organic salts into conventional surfactants is a convenient method to achieve desired properties of surfactant aggregates.