Effect of the hydrophilic size on the structural phases of aqueous nonionic gemini surfactant solutions

Aggregate structures of aqueous nonionic Gemini surfactant solutions, α,α′-[2,4,7,9-tetramethyl-5-decyne-4,7-diyl] bis[ω-hydroxyl-polyoxyethylene] with three different length polyoxyethylenes (i.e., 10, 20, and 30 ethylene oxide monomers, denoted from now on as S-10, S-20, and S-30, respectively), are investigated using small angle neutron scattering, dynamic light scattering, and fluorescence spectroscopy. For S-10 at low surfactant concentrations (C _s < 0.9 wt %), large "clusters", with an average hydrodynamic radius 〈RH〉 > 40 nm, are found to coexist with monomers. At intermediate C _s (0.9 < C _s < 2 wt %), some clusters break down forming micelles, with an (R _H) ∼2-3 nm, while the remaining clusters coexist with micelles. Increasing C _s further (>2 wt %) results in a pure micellar phase with little or no clusters present. S-20 and S-30 mixtures, on the other hand, differ from S-10 in that irrespective of surfactant concentration, large clusters and small monomers/dimers are found to coexist, while there is no direct evidence for the presence of micelles.