New insight into surface tension inverted curvature for liquid-liquid and solid-liquid binary mixtures: The special case of hexamethylenetetramine in water

Surface tension of most binary liquid systems decreases concavely up with increasing solute concentration. In a lesser extent, surface tension curvature is inverted and decreases concavely down. In this report, inverted curvature is explained through a Modified Extended Langmuir (MEL) isotherm that relates solute bulk concentration and surface coverage through a single lyophobic parameter β, and through the Volmer-Margules (VM) model developed by our group that provides activity coefficients from surface tension data. According to the MEL isotherm, inverted curvature is due to the weak adsorption of the solute at low concentrations, making the surface coverage a concave up function of solute bulk concentration. The MEL model was tested with different liquid-liquid binary mixtures and with the liquid-solid system of water + hexamethylenetetramine (HMT), with partial miscibility hindering its description. The VM model fits HMT surface tension data remarkably well, providing similar activity coefficients to those obtained from isopiestic data. The activity coefficient at infinite dilution of HMT in water is smaller than one, ratifying the weak adsorption of HMT due to its affinity to water in bulk solution. The positive Gibbs adsorption energy computed from the VM model confirms the conclusions, providing a new meaning to the inverted curvature.