A MOVING-BOUNDARY TECHNIQUE FOR THE MEASUREMENT OF DIFFUSION IN LIQUIDS - TRITON-X-100 IN WATER

A modified Taylor dispersion technique is used to measure liquid-phase mutual diffusion coefficients D. Rather than inject a narrow band of solution of solute concentration C + DELTA-C into a carrier stream of composition C, the carrier stream is switched from a solution of composition C - (DELTA-C/2) to a solution of composition C + (DELTA-C/2), forming an initially-sharp moving boundary at the tube inlet. D is calculated from the refractive index profile across the broadened boundary at the tube outlet. Since the mean of concentration of the diffusing solute (C) is constant throughout the run, the calculated value of D accurately represents the differential value at C, even if relatively large concentration differences are used or if D is sensitive to composition. The advantages of the technique are illustrated by measuring the diffusion of aqueous Triton X-100, a nonionic surfactant. D is found to drop sharply as the concentration is raised through the critical micelle concentration near 0.15 g-L-1.