Peak parking-moment analysis: A strategy for the measurement of molecular diffusivity in liquid phase

摘要

The peak-parking (PP) method permits the measurement of molecular diffusivities (D-m) in solutions. D-m is first measured for benzene in pure methanol and acetonitrile (ACN), using an empty open tube. This yields an effective axial diffusion coefficient (D-ax,D-m) equal to D-m because there is no tortuosity nor constriction in the flow channel. The same measurements made for the same combinations of solute and solvents, using a column packed with non-porous silica particles provides the obstructive factor (Y-m), defined as the ratio D-ax,D-m/D-m, which accounts for the influence of tortuosity and constriction of the interparticulate space in packed columns on axial molecular dispersion. The value obtained for Y-m, 0.74-0.75, is constant, irrespective of the solvent. Then, PP experiments were made with the same apparatus and column to measure D-m for benzene, toluene, and ethylbenzene in aqueous solutions of methanol and ACN. The values of D-m obtained by correcting the experimental D-ax,D-m values with Y-m were compared with values previously reported and those estimated by several literature correlations. They were in good agreement with each other. The average relative error is estimated at 4.5-10%, demonstrating that the PP method is practically effective for experimental measurements of D-m.