Surface concentration or surface excess, which one dominates the surface tension of multicomponent mixtures?

Abstract

This work is to answer an important question: which parameter, surface concentration or surface excess, dominates the surface tension of multicomponent mixtures? Two series of solutions have been chosen to inspect the dependence of solution surface tension on surface concentration and surface excess of the solutes. The solutions are separately 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) in 3-hydroxypropionitrile and tetrabutylphosphonium bromide (TBPBr) in formamide. The surface tension data of these two series of binary solutions have been respectively correlated to the Gibbs surface excess and the surface concentration of the solutes. Both surface excess and surface concentration are experimentally determined by Neutral Impact Ion Scattering Spectroscopy (NICISS). The surface tension of POPC solutions is linearly dependent on the surface concentrations of POPC; whereas, the surface tension of TBPBr solutions depends piecewise linearly on surface concentration but irregularly upon surface excess of TBPBr. The intrinsic linear relationships between surface tension and surface concentration found in those two series of solutions are consistent to those results found in previous work. This observation evidences that the dominant factor to the surface tension of solutions is the surface concentration of solute to which both the surface fraction and the molecular orientation are subject.