SDS Micelles at High Ionic Strength. A Light Scattering, Neutron Scattering, Fluorescence Quenching, and CryoTEM Investigation

Abstract

Sodium dodecyl sulfate (SDS) in 0.8 M NaCl in D2O has been studied by small-angle neutron scattering (SANS), dynamic light scattering (DLS), and time-resolved fluorescence quenching (TRFQ) measurements in the concentration range from 10 to 80 mM and at temperatures from 25 to 45°C. Examination by cryotransmission electron microscopy revealed the presence of a variety of structures, from broad band-like or lace-like aggregates to multiconnected threads. It is suggested that these structures are formed at the air–solution interface. To verify that they are not present in the bulk solution, SANS, DLS, and TRFQ were studied on the same solutions. The SANS results confirm, on the length scale available using this technique, a local cylindrical structure for the micelles. Assuming a rod model, with the slow mode corresponding to translational diffusion and the fast mode to the rotational dynamics of cylindrical SDS micelles, the DLS data at 25°C in 0.8 M NaCl would correspond to a hydrodynamic lengthL≈ 150 nm, employing a radius of about 2 nm. The TRFQ shows a transition from small micelles at high temperature and low surfactant concentration to long structures at low temperature and high concentration.