On a phthalate ester's adsorption kinetics: DBP in 5 wt% aqueous MeOH solution

Abstract

Although phthalate esters such as dibutyl phthalate (DBP) have attracted much attention in the literature, the surface-active nature of DBP was not well understood until now; particularly, the adsorption kinetics of DBP when dissolved in solvents other than water. In this study, the adsorption kinetics of DBP in a 5 wt% aqueous methanol solution was investigated. A pendant bubble tensiometer was utilized for measuring the dynamic and equilibrium surface tension (ST) of DBP solutions at nine bulk concentrations ranging from 0.37 to 4.0 (10−8 mol/cm3). Theoretical simulations were performed using the Langmuir and Frumkin models, for a mixed-controlled and diffusion-controlled adsorption process, to investigate the adsorption kinetics of DBP molecules. The results showed the Frumkin model better described the equilibrium ST data. Using the Frumkin model, a comparison between the dynamic ST data and theoretical ST profiles revealed that the adsorption of DBP onto the air-water interface was of mixed-control; thereby indicating a non-negligible intermolecular interaction amongst the adsorbed DBP molecules. In addition, a new approach, by evaluating the ratio between the adsorption rate and desorption rate, of examining the effect of solvent on DBP's adsorption kinetics was briefly discussed as well.