Incongruent Diffusion (Negative Main Mutual Diffusion Coefficient) for a Ternary Mixed Surfactant System

Moments analysis of Taylor dispersion profiles is used to measure ternary mutual diffusion coefficients ( D ik ) for dilute aqueous sodium dodecyl sulfate (SDS) + dodecylsulfobetaine (DSB) mixed surfactant solutions at 25 °C and total surfactant concentrations from 10 to 30 mmol dm -3 . The fluxes of the SDS(1) and DSB(2) components are strongly coupled by the formation of SDS−DSB mixed micelles. At several compositions, cross-coefficient D 12 or D 21 is larger than main coefficients D 11 and D 22 . More remarkably, D 22 is negative at solute fractions of SDS from 0.55 to 0.75, reaching a minimum value of −0.056 × 10 -9 m 2 s -1 at 13 mmol dm -3 SDS + 7 mmol dm -3 DSB. This surprising result means that DSB concentration gradients drive “incongruent” fluxes of DSB, from lower to higher DSB concentrations. A possible explanation for the incongruent diffusion of DSB is suggested on the basis of the electric field generated by gradients in the surfactant concentration. Negative D ii values have been reported previously for concentrated, strongly nonideal aqueous acetic acid + chloroform solutions near a consolute point. Because the SDS + DSB solutions are dilute and far removed from phase boundaries, incongruent diffusion might be considerably more common than previously assumed.