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

The peak-parking (PP) method permits the measurement of molecular diffusivities (Dm) in solutions. Dm is first measured for benzene in pure methanol and acetonitrile (ACN), using an empty open tube. This yields an effective axial diffusion coefficient (Dax,m) equal to Dm 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 (Ym), defined as the ratio Dax,m/Dm, which accounts for the influence of tortuosity and constriction of the interparticulate space in packed columns on axial molecular dispersion. The value obtained for Ym, 0.74–0.75, is constant, irrespective of the solvent. Then, PP experiments were made with the same apparatus and column to measure Dm for benzene, toluene, and ethylbenzene in aqueous solutions of methanol and ACN. The values of Dm obtained by correcting the experimental Dax,m values with Ym 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 Dm.