Micellar Shape Driven Counterion Binding. Small-Angle Neutron Scattering Study of AOT Micelle

Abstract

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior in aqueous electrolyte medium, viz., the counterion binding constant (β) abruptly increases by 2-fold at about 0.015 mol dm(-3) NaCl concentration (c*), but not in sodium salicylate (NaSa) solution. Since counterions affect the structure and performance of ionic surfactants, ascertaining the cause for the sudden shift in the β value of AOT micelle is of fundamental importance. In this study the special counterion binding behavior of AOT micelle has been ascertained at 40 °C by carrying out surface tension, zeta potential, and fluorescence emission (pyrene probe) measurements. The results of the small-angle neutron scattering experiment carried out at 40 °C showed that at c* the shape of AOT micelle changes from prolate spheroid to rodlike in NaCl solution, but not in NaSa solution, thus establishing micellar shape change as responsible for the abrupt change in β value. The absence of sudden shift in β of AOT micelle in NaSa solution is attributed to the binding of salicylate coanion to AOT micelle through hydrophobic interaction.

Effect of sodium salicylate, sodium oxalate, and sodium chloride on the micellization and adsorption of sodium deoxycholate in aqueous solutions

“…The binding behavior of sodium ion to NaDC micelles is similar in the presence of sodium chloride and sodium salicylate (30-40°C temperature range), but different in the presence of oxalate, thereby indicating that an organic coanion depending on its structure may have an influence on the counterion binding behavior of deoxycholate micelles. By comparing the present system with the AOT + electrolyte system in which a correlation between the shift in counterion binding constant and micellar shape change has been reported [31], a change in the shape of the deoxycholate micelles from prolate ellipsoid to rod-like may be expected at about 0.038 mol kg À1 of Na 2 Ox.…”

“…In contradiction to this general view, this study has revealed that even coions having negligible or no hydrophobicity can interact with ionic micelle. Our recent report [31] on the binding of salicylate coanion to AOT micelle which inhibited a shape change of this micelle is another example of interaction of a coion having low hydrophobicity with ionic micelle. Thus, it may be concluded that in determining the effect of a coion on the micellization of an ionic micelle their (coion and ionic micelle) structures play an important role, and hence hydrophobicity of the coion is not the only controlling factor.…”

“…Thus, oxalate coanion depending on its concentration has a suppressing as well as enhancing effect on the binding of sodium ion to deoxycholate micelles. In aqueous AOT + NaCl solution, the sudden increase in the value of b at about 0.02 mol kg À1 NaCl was reported [31] to be due to the shape change of AOT micelles and similarly at about 0.038 mol kg À1 of Na 2 Ox the shape of the deoxycholate micelles may be expected to change from prolate ellipsoid to rod-like. At c à , oxalate ion may be undergoing hydrogen bonding with the hydroxyl groups of the deoxycholate ions of the two neighboring primary aggregates, thereby inducing formation of secondary aggregates and hence shape change.…”

Self-assembly of sodium bis(2-ethylhexyl) sulfosuccinate in aqueous solutions: Modulation of micelle structure and interactions by cyclodextrins investigated by small-angle neutron scattering

“…Due to its relatively low toxicity and high versatility, AOT finds applications as petroleum dispersant [8][9][10], as adjuvant or wetting agent in pesticide formulations [11], and as drug carrier in pharmaceutical formulations [12]. AOT associates in water to form micelles at low concentrations, and lyotropic mesophases, e.g., hexagonal or lamellar, at higher concentrations [2,[13][14][15]. The critical micelle concentration (CMC) of AOT in water has been obtained from surface tension measurements to be 2.6 mM (1.18 wt.%) [15].…”

Synergism between anionic double tail and zwitterionic single tail surfactants in the formation of mixed micelles and vesicles, and use of the micelle templates for the synthesis of nano-structured gold particles

“…The AOT can form micelles at 2.66 mM, and vesicles at 6.04 mM and above at 298 K [53]. Structural changes of the AOT micelles (ellipsoids and rods) by the influence of the counter-ions are possible [54]. Zwitterionic surfactants in combination with the cationic and the anionic surfactants can form supramolecular structures above their CMCs in the aqueous medium (such as spherical micelles, worm-like micelles, vesicles, etc.).…”