Micellar shape driven counterion binding. Small-angle neutron scattering study of AOT micelle

Area of Science:

• Colloid and Surface Chemistry
• Physical Chemistry
• Materials Science

Background:

• Ionic surfactants like sodium dioctylsulfosuccinate (AOT) exhibit unique counterion binding behaviors in electrolyte solutions.
• Understanding these behaviors is crucial as counterions influence micelle structure and performance.

Purpose of the Study:

• To investigate the cause of the abrupt two-fold increase in the counterion binding constant (β) for AOT micelles in NaCl solutions at a critical concentration (c*).
• To compare this behavior with AOT micelles in sodium salicylate (NaSa) solutions.

Main Methods:

• Surface tension measurements
• Zeta potential measurements
• Fluorescence emission spectroscopy using a pyrene probe
• Small-angle neutron scattering (SANS) experiments

Main Results:

• AOT micelle shape transitions from prolate spheroid to rod-like at c* in NaCl solutions, correlating with the abrupt increase in β.
• No significant micellar shape change or abrupt shift in β was observed for AOT in NaSa solutions.
• The binding of salicylate anions via hydrophobic interactions is proposed to stabilize the AOT micelle structure in NaSa solutions.

Conclusions:

• Micellar shape transition is the primary driver for the sudden change in counterion binding of AOT in NaCl solutions.
• The absence of this transition in NaSa solutions, due to salicylate coanion binding, prevents the abrupt shift in counterion binding constant.