NMR RELAXATION BEHAVIOR OF THE HEAD GROUP OF SODIUM 4-(1'-HEPTYLNONYL)BENZENESULFONATE IN LAMELLAR LIQUID-CRYSTALS

Deuterium NMR relaxation-time constants T1Q and T1Z from spectra of a phenyl-ring-deuterated double-tailed surfactant, sodium 4-(1-heptylnon yl)benzenesulfonate (SHBS), in liquid crystals formed with water were obtained from 0 to 80-degrees-C. The relaxation-time constants were de termined for the 15-degrees and 90-degrees orientations of the powder sample. These data are compared to predicted relaxation-time constants from two different models for molecular reorientation, including (I) phenyl rings undergoing uniaxial reorientation about the 1'-4' axis an d (II) phenyl rings rotating rapidly about the 1'-4' axis of a molecul e which is reorienting about a space-fixed director axis. Both models include an adjustable parameter that describes molecular motion about the 1'-4' axis that results from either strong collisions or small ste p free rotational diffusion. The results from both models indicate the rate of reorientation about the 1'-4' axis of the phenyl ring is appr oximately 3 times the nuclear Larmor frequency ofthe deuterium nucleus , 30 MHz, at room temperature. This rate increases with increasing tem perature. The best fits of the data were determined using model II. Th e predicted rate constants for motion about this axis differ slightly for each model, whereas the type of motion predicted by the two models is very different. Model I indicates reorientation about the 1'-4' ax is is more like strong collisions. Model II indicates that the motion is more like small step rotational diffusion.