Solubility determination of surface-active components from dynamic surface tension data

A new technique of utilizing the dynamic surface tension (ST) data for solubility determination was proposed, tested, and verified in this study. Dibutyl phthalate was chosen as the surface-active component and its solutions were prepared with 0, 1, and 5 wt.% aqueous methanol (MeOH) solutions. The solutions for ST measurement were prepared by stirring the aqueous DBP mixtures in a heated water bath (45 °C) for an extended duration of time. The data revealed that an increase in DBP concentration corresponded to a more rapid relaxation of dynamic ST and reached a lower equilibrium ST; similar to that observed amongst typical micelle forming surfactants (at C < cmc). Unlike surfactants, an increase in concentration beyond a certain value corresponded to near-identical relaxations of dynamic ST and the same equilibrium ST. For typical surfactants, although the equilibrium ST remained constant when concentration was increased at C > cmc, the dynamic ST relaxations became increasingly rapid. These inconsistencies in dynamic ST behavior pointed towards the absence of micelle formation amongst the DBP molecules. Based on the dynamic ST data, the solubility of DBP in water, 1 wt.%, and 5 wt.% MeOH_(aq) were found to be ∼6.1, ∼4.4, and ∼4.7 (10⁻⁸ mol/cm³), respectively.