Surfactant/nonionic copolymer interaction: a SLS, DLS, ITC, and NMR investigation

Area of Science:

• Colloid and Surface Chemistry
• Polymer Science
• Supramolecular Chemistry

Background:

• Nonionic diblock copolymers self-assemble into micelles in aqueous solutions.
• Anionic surfactants like sodium dodecyl sulfate (SDS) interact with polymer micelles.
• Understanding these interactions is crucial for designing advanced materials and formulations.

Purpose of the Study:

• To investigate the complex formation between a specific styrene-oxyethylene copolymer (S20E67) and SDS in aqueous solutions.
• To characterize the structural and thermodynamic changes occurring upon mixing.
• To elucidate the nature of the interactions and the resulting supramolecular assemblies.

Main Methods:

• Static and dynamic light scattering (SLS and DLS) to determine micelle size and aggregation.
• Isothermal titration calorimetry (ITC) to quantify binding thermodynamics and energy changes.
• Nuclear magnetic resonance (NMR) techniques (13C and self-diffusion) to probe molecular interactions and dynamics.

Main Results:

• At low SDS concentrations, copolymer-rich mixed micelles form, leading to decreased light scattering intensity and hydrodynamic radius.
• At higher SDS concentrations, these structures transition to surfactant-rich micelles, indicating a breakdown of the initial complexes.
• ITC data revealed dehydration and rehydration processes, supporting the formation and destruction of mixed micelles. Interactions extended to the C3 carbon of SDS.

Conclusions:

• The S20E67 copolymer and SDS form distinct, concentration-dependent supramolecular complexes in solution.
• The observed transitions are driven by the interplay of hydrophobic interactions, electrostatic forces, and surfactant aggregation.
• These findings provide insights into the self-assembly behavior of block copolymers and surfactants, relevant for formulation science.