Salt effect on the complex formation between polyelectrolyte and oppositely charged surfactant in aqueous solution

Area of Science:

• Physical Chemistry
• Polymer Science
• Colloid Science

Background:

• Investigating the interactions between charged polymers and surfactants is crucial for understanding complex fluid behavior.
• Sodium carboxymethylcellulose (NaCMC) is an anionic polyelectrolyte, and dodecyltrimethylammonium bromide (DTAB) is a cationic surfactant.
• The influence of ionic strength on polyelectrolyte-surfactant complexation is a key area of research.

Purpose of the Study:

• To elucidate the complex formation between NaCMC and DTAB under varying sodium bromide (NaBr) concentrations.
• To investigate the role of salt concentration in modulating the critical surfactant concentration (C1) and complexation behavior.
• To understand the underlying mechanisms governing salt-induced enhancement or inhibition of NaCMC/DTAB complex formation.

Main Methods:

• Microcalorimetry for thermodynamic measurements.
• Turbidimetric titration to determine phase boundaries and concentrations.
• Steady-state fluorescence and fluorescence polarization to probe molecular interactions and aggregation.

Main Results:

• NaCMC/DTAB complex formation is significantly influenced by NaBr concentration.
• At moderate NaBr concentrations (0.00-0.20 M), DTAB forms micelle-like aggregates on NaCMC chains above C1.
• At 0.23 M NaBr, DTAB first forms micelles, then aggregates with NaCMC; at 0.25 M NaBr, complexation is inhibited due to electrostatic screening.

Conclusions:

• Sodium bromide concentration critically affects NaCMC/DTAB complex formation, with an optimal range for enhancement.
• A decrease in C1 is observed at NaBr concentrations below 0.20 M, indicating salt-enhanced complexation.
• The observed salt-enhancing effect results from a balance between electrostatic screening and increased polyelectrolyte-surfactant interactions due to micelle growth.