Collective and local molecular dynamics in the lyotropic mesophases of decylammonium chloride: ¹H and ²H NMR study

The collective and individual dynamics of decylammonium chloride (DACl) molecules in water environment were investigated as a function of surfactant concentration and temperature. In the presence of water the DACl forms a variety of self-assembled structures, ranging from isotropic micellar systems to lyotropic liquid crystalline phases of hexagonal, nematic, and lamellar types. In order to characterize the complex molecular dynamics that occur in the DACl -water system, we applied ¹H and ²H NMR techniques that cover the whole frequency range between 1  kHz and 30  MHz . The slow molecular dynamics were studied by ¹H NMR fast-field-cycling T₁ measurements and pulse-frequency dependence of ²H NMR transverse relaxation time, performed by means of the Carr-Purcell-Meiboom-Gill sequence. We detected a well-expressed contribution of order director fluctuations, i.e., layer undulations, with characteristic ω_L⁻¹ frequency dependence of T₁⁻¹ in the lamellar phase. Its presence indicates a relatively weak impact of interactions between neighboring DACl layers. The frequency dependence of proton T₁⁻¹ in the hexagonal phase exhibits a different type of frequency dispersion, T₁⁻¹∼ω_L^−1.32 . The increase in the exponent is explained with the quasi-one-dimensional character of fluctuations in elongated cylinders. Further, the T₁ and T₂ relaxation times of deuterons selectively attached to the C2 and C7 segments of the hydrocarbon chains of DACl were measured at a Larmor frequency of 30.7  MHz , providing quantitative information about local molecular dynamics.