An electrostatic mechanism for the coupled diffusion of polymer molecules and ionic micelles. Aqueous poly(ethylene glycol) + sodium dodecyl sulfate solutions

Abstract

Taylor dispersion is used to measure ternary mutual diffusion coefficients for aqueous PEG200 [poly(ethylene glycol), average molecular weight 200 g mol⁻¹] + NaDS (sodium dodecyl sulfate) and PEG3400 + NaDS solutions at 25 °C. Diffusion coefficients are reported for solutions containing 1 or 3 wt% PEG and NaDS at concentrations from 0.0035 to 0.25 mol dm⁻³. The PEG200 and NaDS fluxes are weakly coupled, but PEG3400 concentration gradients can drive substantial coupled fluxes of NaDS (up to 17 mol NaDS per mol of PEG3400 driven by its own gradient), an order of magnitude larger than the coupled fluxes predicted by thermodynamic and excluded-volume theories of coupled diffusion. An electrostatic mechanism is proposed to account for the strong interactions between PEG and NaDS fluxes based on the association of PEG molecules and NaDS micelles, which reduces the charge density on the micelles and releases bound Na⁺ counterions. The diffusion of relatively mobile free Na⁺ ions down PEG concentration gradients generates an electric field which pulls along a coupled flux of the oppositely-charged NaDS micelles to maintain electroneutrality. The measured coupled fluxes of NaDS are in qualitative agreement with predictions made using Nernst–Planck equations and increases in free Na⁺ ion activity with added PEG measured with a sodium ion-selective electrode.