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

Aggregate structures of aqueous nonionic Gemini surfactant solutions, [2,4,7,9-tetramethyl-5-decyne-4,7-diyl]bis[omega-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 (CS < 0.9 wt %), large clusters, with an average hydrodynamic radius (RH) > 40 nm, are found to coexist with monomers. At intermediate CS (0.9 < CS < 2 wt %), some clusters break down forming micelles, with an (RH) similar to 2-3 nm, while the remaining clusters coexist with micelles. Increasing CS 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.