Undulations in salt-free charged lamellar phases detected by small angle neutron scattering and neutron reflectivity

Neutron reflectivity of lamellar phases containing pentaethylene glycol n-d odecyl ether, sodium decylsulfonate, and D2O is measured at the solid-liqui d interface. The scattering length density profiles deduced from fits to th e reflectivity data show that undulations of the mixed surfactant bilayers close to the solid-liquid interface decrease as the molar ratio of ionic to nonionic surfactant increases. This is consistent with recent theoretical predictions. Small angle neutron scattering measurements of these samples s how both small angle scattering and quasi-Bragg peaks. As the molar ratio o f ionic to nonionic surfactant increases, the low-q scattering intensity de creases and higher order quasi-Bragg peaks appear. These changes in the bul k neutron scattering signal are consistent with the changes observed with r eflectivity.