Influence of water-insoluble nonionic copolymer E6P39E6 on the microstructure and self-aggregation dynamics of aqueous SDS solution—NMR and SANS investigations

Abstract

The influence of water-insoluble nonionic triblock copolymer PEO–PPO–PEO [poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide)] i.e., E₆P₃₉E₆ with molecular weight 2800, on the microstructure and self-aggregation dynamics of anionic surfactant sodium dodecylsulfate (SDS) in aqueous solution (D₂O) were investigated using high resolution nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) measurements. Variable concentration and temperature proton (¹H), carbon (¹³C) NMR chemical shifts, ¹H self-diffusion coefficients, ¹H spin–lattice and spin–spin relaxation rates data indicate that the higher hydrophobic nature of copolymer significantly influenced aggregation characteristics of SDS. The salient features of the NMR investigations include (i) the onset of mixed micelles at lower SDS concentrations (<3 mM) relative to the copolymer-free case and their evolution into SDS free micelles at higher SDS concentrations (∼30 mM), (ii) disintegration of copolymer–SDS mixed aggregate at moderate SDS concentrations (∼10 mM) and still binding of a copolymer with SDS and (iii) preferential localization of the copolymer occurred at the SDS micelle surface. SANS investigations indicate prolate ellipsoidal shaped mixed aggregates with an increase in SDS aggregation number, while a contrasting behavior in the copolymer aggregation is observed. The aggregation features of SDS and the copolymer, the sizes of mixed aggregates and the degree of counterion dissociation (α) extracted from SANS data analysis corroborate reasonably well with those of ¹H NMR self-diffusion and sodium (²³Na) spin–lattice relaxation data.