pH-dependent polyampholyte SDS interactions.

Area of Science:

• Physical Chemistry
• Polymer Science
• Surface Chemistry

Background:

• Investigating the behavior of synthetic polyelectrolytes (PELs) like poly(N,N'-diallyl-N,N'-dimethyl-alt-maleamic carboxylate) (PalH) in aqueous solutions is crucial for understanding their applications.
• Sodium dodecylsulfate (SDS) is a widely studied anionic surfactant whose interactions with polymers are complex and depend on various factors.

Purpose of the Study:

• To elucidate the intricate interaction mechanisms between SDS and the pH-tuneable polyelectrolyte PalH.
• To investigate the influence of pH and salt concentration (NaCl) on these interactions.
• To differentiate between molecular and micellar binding of SDS to the polyelectrolyte.

Main Methods:

• Potentiometric measurements using a surfactant-selective electrode.
• Isothermal titration calorimetry (ITC).
• Investigation across a range of pH values and in the presence and absence of NaCl.

Main Results:

• Potentiometry revealed complex interactions involving non-cooperative, electrostatic, and cooperative hydrophobic forces, which are pH-dependent in the absence of salt.
• ITC data indicated that initial SDS binding to PalH is exothermic (hydrophobic), followed by endothermic, entropy-driven electrostatic interactions.
• In the presence of NaCl, interactions become pH-independent due to charge screening, and SDS micelles bind to the polymer instead of individual molecules.

Conclusions:

• Polyelectrolyte conformational changes significantly influence SDS interaction mechanisms at different pH values without salt.
• Salt addition simplifies the SDS-PalH interaction by screening electrostatic forces and promoting micellar binding.
• The study highlights the transition from molecular to micellar binding of SDS to PalH as a function of salt concentration.