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 the σ-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 Γ2 and τ with x2,b are dependent on the SAA model parameters, namely, the adsorption equilibrium constant (Kx) and the average aggregation number (n). With an increase in x2,b, Γ2 continuously increases when Kx < 2v1/[n(2n − 1)v2] (where v1 and v2 are the partial molar volumes of components 1 and 2, respectively); otherwise (i.e., Kx ≥ 2v1/[n(2n − 1)v2]), Γ2 initially increases and then decreases, showing a maximum on the Γ2-isotherm. When n ≥ 1, τ gradually decreases with an increase in x2,b; otherwise (i.e., n < 1), τ initially increases and then decreases, showing a maximum on the τ-isotherm. An increase in the adsorption trend leads to a decrease in both Γ2 and τ. This work provides a better understanding of the surface adsorption behavior of liquid mixtures.