Polymeric Surfactant P84/Polyoxometalate α-PW₁₂O₄₀^3-—A Model System to Investigate the Interplay Between Chaotropic and Hydrophobic Effects

Abstract

Low charge density nanometric ions were recently shown to bind strongly to neutral hydrated matter in aqueous solution. This phenomenon, called (super-)chaotropic effect, arises from the partial dehydration of both, the nano-ion and the solute, leading to a high gain in enthalpy. Here, we investigate the chaotropic effect of the polyoxometalate α-PW12O403- on the triblock copolymer P84: (EO)19(PO)43(EO)19 with (EO)19 the polyethoxylated and (PO)43 the polypropoxylated chains. The combination of phase diagrams, spectroscopic (nuclear magnetic resonance) and scattering (small angle neutron/X-ray scattering) techniques reveals that (i) below the micellization temperature of P84, PW12O403- exclusively binds to the propylene oxide moiety of P84 unimers and (ii) above the micellization temperature, PW12O403- mostly adsorbs on the ethylene oxide micellar corona. The preferential binding of the PW12O403- to the PPO chain over the PEO chains suggests that the binding is driven by the chaotropic effect and reinforced by the hydrophobic effect. At higher temperatures, the copolymer micellization leads to the displacement of PW12O403- from the PPO chain to the PEO chains. This study deepens the understanding of the subtle interplay between the chaotropic and hydrophobic effects in complex salt-organic matter solutions.