The influence of the size of the hydrophilic group on the miscibility of zwitterionic and nonionic surfactants in mixed monolayers and micelles

The aim of this work was to study the surface properties of nonionic and zwitterionic surfactants (namely dodecyl-beta-D-glucoside-DG, dodecyl-beta-D-maltoside-DM, and dodecylsulfobetaine-DSB) and the miscibility of these compounds in mixed monolayers and micelles. A thermodynamic description of the interactions between surfactants in the mixed aggregates was provided by the Motomura theory. The obtained results indicate that the investigated compounds mix nonideally both in the mixed adsorbed layers and in micelles. The calculated values of the excess free energies of mixing prove that the interactions between molecules, in the mixed monolayers and micelles, are more attractive or less repulsive than those in the respective one-component aggregates. However, the values of the excess free energy of the components mixing in the monolayers and micelles are more negative for the DSB/DG system than for the DSB/DM system. This indicates that the investigated zwitterionic surfactant interacts more strongly with DG than with DM. This results from the difference in the size of the polar heads of nonionic surfactants. (C) 2007 Elsevier Inc. All rights reserved.