Diffusion of levodopa in aqueous solutions of hydrochloric acid at 25 °C

Graphical abstract: - Highlights: • Ternary mutual diffusion coefficients of aqueous L-dopa plus hydrochloric acid at 25 °C. • Diffusion of L-dopa driven by HCl gradients. • Coupled diffusion of L-dopa and HCl. - Abstract: Ternary mutual diffusion coefficients (D11, D22, D12 and D21) measured by the Taylor dispersion method are reported for aqueous solutions of {levodopa (L-dopa) + HCl} solutions at 25 °C and HCl concentrations up to 0.100 mol · dm−3. The coupled diffusion of L-dopa (1) and HCl (2) is significant, as indicated by large negative cross-diffusion coefficients. D21, for example, reaches values that are larger than D11, the main coefficient of L-dopa. Combined Fick and Nernst–Planck equations are used to analyze the proton coupled diffusion of L-dopa and HCl in terms of the binding of H+ ions to L-dopa and ion migration in the electric field generated by L-dopa and HCl concentration gradients