Conformational Dynamics of Surfactant in a Mesolamellar Composite Studied by Local Field NMR Spectroscopy

Ordered mesostructured materials possess unique surface, structural, and bulk properties that lead to important practical applications. Mesostructured organic inorganic composites are also of broad interest for fundamental studies of confinement effects and surface interaction on structural and dynamic properties of organic molecules. In the present study, solid state dipolar C-13-H-1 NMR spectroscopy is applied to quantitatively characterize the conformational dynamics of a surfactant in a mesolamellar composite. By applying dipolar recoupling and separated local field spectroscopy techniques, the motion of surfactant molecules was studied in a wide range of mobilities from an essentially immobilized rigid state to a highly flexible and anistropically tumbling state. From the analysis of the measured heteronudear dipolar couplings, the orientational order parameters of C-H bonds along the surfactant chain were determined. The study shows that in surfactant bilayers in AIPO layered structure at room temperature the highly ordered chains in all-trans conformation undergo fast rotation about the molecular axis. In a higher temperature phase, the order parameter is gradually decreasing toward the chain end due to conformational transitions; however, the dynamics of the segment in the vicinity of the headgroup is only slightly affected. The conformational dynamics in the surfactant bilayers confined between solid inorganic sheets is also compared to that in fluid bilayers in an aqueous lamellar phase.