Characterization and structure elucidation of sodium di-n-alkylsulfosuccinate surfactants in water

The micellar structures and properties of a homologous series of double-tailed surfactants were determined using a number of techniques. Phase studies and conductivity, electromotive force and ²³Na nuclear magnetic resonance measurements were performed on aqueous solutions of the dihexyl, diheptyl, dioctyl, dinonyl and didecyl members of the sodium di-n-alkylsulfosuccinates in order to determine the change in the degree of cooperativity of micellization that occurred as a function of the surfactant chain length. Small-angle neutron scattering experiments were conducted to determine the size and shape of the sulfosuccinate micelles as a function of surfactant and salt concentration. The change in micellar size that occurred upon the addition of a counterion complexing agent to solutions of anionic surfactants was ascertained using small-angle neutron scattering.

The dinonyl and didecyl members of the sulfosuccinates were found to have limited solubility in water whereas the dihexyl, diheptyl and dioctyl members possessed fairly extensive isotropic micellar solution regions; results indicated that the latter members formed normal micelles in water over a fairly wide concentration range. Aggregation of these surfactants in water was characterized by lower optimum aggregation numbers and higher extents of counterion dissociation. The micelles consisted of a disordered hydrocarbon core surrounded by a hydrated shell. Compared to micelles formed from single-tailed surfactants, the micelles of the di-n-alkylsulfosuccinates experienced more rapid growth upon increasing surfactant concentration and upon addition of salt. As the micellar solubility limit was approached, a transition in micellar shape from spheres to ellipsoids occurred for the dihexyl and diheptyl members. The addition of the cryptand, 4,7,13,16,21,2A-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane, C222, to sodium dodecylsulfate solutions resulted in the formation of smaller, more highly charged micelles whereas the effect of C222 upon the structure of micelles formed by sodium di-n-heptylsulfosuccinate was concentration dependent. The observed differences in micellar structure and behavior between single- and double-tailed surfactants were explained in terms of headgroup areas and hydrocarbon-water contact.