Surface Aggregation Adsorption of Binary Solutions Between Diiodomethane, Furfural, and N,N-Dimethylformamide

The surface tensions (σ) of binary solutions of diiodomethane (DIM, 1)–furfural (FA, 2), DIM (1)–N,N-dimethylformamide (DMF, 2), and FA (1)–DMF (2) were determined at 25 °C over the entire bulk composition range, and the surface adsorption behavior was analyzed using the surface aggregation adsorption (SAA) model proposed recently. In particular, by combining the SAA model with the Gibbs adsorption equation, the changes in the Gibbs surface excess (Γ2) and the adsorption layer thickness (τ) with the bulk composition (x2,b) were investigated. The SAA model combined with the modified Eberhart model can well describe theσ-isotherms of the three binary solutions. The surface adsorption trends of component 2 in DIM–FA, DIM–DMF, and FA–DMF decrease in turn. The change trends of Γ2andτwithx2,bare dependent on the SAA model parameters, namely, the adsorption equilibrium constant (Kx) and the average aggregation number (n). With an increase inx2,b, Γ2continuously increases whenKx< 2v1/[n(2n− 1)v2] (wherev1andv2are the partial molar volumes of components 1 and 2, respectively); otherwise (i.e.,Kx≥ 2v1/[n(2n− 1)v2]), Γ2initially increases and then decreases, showing a maximum on the Γ2-isotherm. Whenn≥ 1,τgradually decreases with an increase inx2,b; otherwise (i.e.,n< 1),τinitially increases and then decreases, showing a maximum on theτ-isotherm. An increase in the adsorption trend leads to a decrease in both Γ2andτ. This work provides a better understanding of the surface adsorption behavior of liquid mixtures.