Micellization and adsorption modeling of single and mixed nonionic surfactants

Abstract

The surface tension and heats of dilution of aqueous solutions of octyl-β-d-glucopyranoside (C8G1), tetraethylene glycol monooctyl ether (C8E4), pentaethylene glycol monooctyl ether (C8E5) and C8E5-C8G1 mixtures at 298.2 K were studied by tensiometry (ST) and isothermal titration calorimetry (ITC). Single surfactant micelle formation and growth were modeled using a multiple chemical equilibrium approach under the assumption that micelles are sphero-cylinders that can be thermodynamically described by three parameters, namely spherical and cylindrical contributions to the micelle molar Gibbs energy and the enthalpy of micellization. The two-dimensional virial equation of state was employed to correlate the surface tensions, which required two additional parameters: a 2D second virial coefficient and a reference monomer concentration. For the binary surfactant solutions, combining and mixing rules were suggested with no further parameters. For the single surfactants, the model correlated very well both ITC and ST experimental data. For mixed surfactants, ST data were also very well predicted, but this was not the case for ITC data. For the critical micelle concentrations, the model enthalpograms presented both positive and negative deviations from the experimental results, depending on the surfactant relative concentrations. Very low C8G1 concentrations promoted the demicellization of C8E5, which suggests strong interactions between monomer hydrophilic heads and the possible formation of small aggregates.