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

Abstract

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,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,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,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.