Micellar structure in gemini nonionic surfactants from small-angle neutron scattering.

The size and shape of micelles formed by dimeric polyoxyethylene (nonionic gemini) surfactants having the structure (Cn-2H2n-3CHCH2(OCH2CH2)mOH)2(CH2)6 with alkyl and ethoxy chain lengths ranging from n = 12-20 and m = 5-30 have been determined using small angle neutron scattering (SANS). The surfactants are polydisperse in the hydrophilic groups but otherwise analogous to the widely studied monomeric poly(oxyethylene) alkanols. We find that longer ethoxylated chains are needed to confer solubility on the gemini surfactants and that these chains in the hydrophilic corona around the alkyl core of the micelles are reasonably well described as a homogeneous random coil in a good solvent. Spherical micelles are formed by the surfactants with the longest ethoxylated chains. Shorter chains lead first to rods and ultimately a vesicle dispersion. These solutions exhibit conventional cloud point behavior, and on warming, a sphere to rod transition can be observed. For the n = 20 and m = 15 surfactant, this shape transition is accompanied by a striking increase in viscosity at low concentration and gelation at higher concentrations.