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College Park, Maryland      June 6 - 10 , 2004

MP27: Size and shape of mixed micelles of nonionic surfactants: puzzles and resolution

R. Zhang, P. Somasundaran (Columbia University), C. Glinka, B. Hammouda (NIST Center for Neutron Research), P. Cummins (Estee Lauder), Q. Zhou (Columbia University)

Mixed surfactants are of considerable interest because of synergy in surface activity which depends on compatible molecular interactions. In this regard, in order to develop molecular structure-property relationships that control relevant colloidal phenomenon in mixed surfactant systems and to develop predictive theoretical models based on such relationships, it is necessary to acquire quantitative information on size and shape of micelles in these complex systems. Micellar shape has been correlated in this work with packing parameter for mixing and surfactant molecular structures in dilute solutions of binary nonionic polyethylene oxide nonyl phenol ethoxylated decyl ether (NP-10) / nonionic sugar-based n-dodecyl--D-maltoside (DM) system using a multi-pronged approach. The packing results suggest that micelles undergo a sphere to cylinder transition with an increase in DM percentage. However, experimental results from SANS (small angle neutron scattering) along with those from analytical ultracentrifuge, cryo-TEM, FT-PGSE (pulsed gradient spin echo)-NMR contradict this prediction. By comparing the energy changes due to packing using molecular modeling, NP-10 is found to prefer to form close packed geometry in water. In order to account for these effects, we modify the packing parameter by taking molecular conformation and mixing into consideration.

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