Effect of the cation on micelle formation by sulfonates in benzene

Abstract

The states of aggregation of dinonylnaphthalene sulfonates of ten cations (Li, Na, Cs, NH 4, Mg, Ca, Ba, Zn, Al, and H) have been studied in benzene by fluorescence depolarization, cryoscopy, viscometry, and densimetry. The micelles observed by fluorescence depolarization contained 9 to 14 acid residues each. Their aggregation number was usually independent of concentration and almost independent of the water content of the system, although moisture moderately increased the aggregation of the zinc salt. The relative insensitivity of the sulfonate micelle size to influences of the cation and moisture contrasts sharply with the behavior of the phenyl-stearate soaps, whose aggregation depends critically upon these variables. This indicates that the size of the dinonylnaphthalene sulfonate micelles depends primarily on the geometry of the acid residue. However, the apparent volume of the anion was inversely related to the coordinating tendency of the cation, suggesting that coordination forces are a factor in micelle stability. The acid is associated principally to the dimer, but in the presence of water some larger aggregates are formed. Viscosities of the sulfonate solutions exceeded those predicted by the Einstein relation for spherical particles. This anomaly corresponded to an asymmetry of the order of 2, or a solvation of 10% to 20%. It might also have resulted from surface irregularities of the micelles. Present evidence does not justify a choice among these explanations. Cesium dinonylnaphthalene sulfonate was shown cryoscopically to have an aggregation number of 6 or more. It differed from the other micelle-forming sulfonates, however, in not solubilizing enough Rhodamine B to permit determination of micelle size by the fluorescence depolarization technique. The micelle size found cryoscopically for the magnesium salt agreed with that derived from fluorescence depolarization measurements.