Molecular mobility in solid sodium dodecyl sulfate.

Here we report on the molecular mobility in solid sodium dodecyl sulfate (SDS), a commonly used surfactant, as measured by high-resolution neutron scattering in the temperature range 175-400 K. While the quasielastic data showed the presence of dynamical motion at and above 210 K, the fixed energy window (FEW) data indicated that the dynamics is present even at lower temperatures. The FEW data showed that the dynamics evolves monotonically with increasing temperature until 360 K where a dynamical transition takes place. A similar signature is also found in the differential scanning calorimetry data. The analysis of the quasielastic neutron scattering (QENS) data shows that at T < 360 K, SDS molecules undergo fractional reorientational motion about the molecular axis. With the increase in temperature the mobility progresses from the tail toward head of the molecular chain. While 11% of the molecule is mobile at 210 K, at 350 K it grows to 60% indicating a gradual melting. The same set of parameters obtained from analysis of the QENS data describes the elastic scan data as well. Above the transition, at T > 360 K, the observed dynamics correspond to localized translational diffusion of the hydrocarbon chain of the SDS molecule. It is interesting to note that the dynamical behavior of SDS in the high-temperature chain melt state is very similar to that observed for the monomer dynamics in SDS micelles.