Thermodynamic parameters and counterion binding to the micelle in binary anionic surfactant systems

Area of Science:

• Physical Chemistry
• Colloid and Surface Science
• Solution Chemistry

Background:

• Understanding counterion binding to micelles is crucial for modeling surfactant solutions.
• Mixed micellization in anionic surfactant systems requires accurate thermodynamic modeling.
• Previous models did not fully account for competitive binding of different cations.

Purpose of the Study:

• To investigate competitive counterion binding of sodium and calcium ions to micelles.
• To model mixed micellization in sodium dodecylsulfate (NaDS)/sodium decylsulfate (NaDeS) and NaDS/sodium 4-octylbenzenesulfonate (NaOBS) systems.
• To accurately model the activity of species in solution by determining critical micelle concentration (CMC) and equilibrium micelle compositions.

Main Methods:

• Thermodynamic modeling using regular solution theory to determine CMC and micelle compositions.
• Investigation of mixed micelle ideality using the regular solution parameter β(M).
• Analysis of counterion binding coefficients using an equilibrium model.

Main Results:

• The NaDS/NaOBS system exhibited ideal mixed micelle behavior (β(M)=0).
• The NaDS/NaDeS system showed slight synergistic interaction in mixed micelles (β(M)=-1.05).
• Total counterion binding remained constant at approximately 0.65 charge negation, irrespective of competing ion concentrations.

Conclusions:

• Regular solution theory effectively models mixed micelle formation and counterion binding.
• Competitive binding of sodium and calcium ions does not alter the overall degree of counterion binding.
• A simple equilibrium model accurately quantifies counterion binding coefficients.