SPIN RELAXATION BY COLLECTIVE DIRECTOR FLUCTUATIONS AND MOLECULAR-DIFFUSION IN LAMELLAR PHASES - CONTINUUM THEORY OF RELAXATION ANISOTROPY AND DISPERSION

The orientation and frequency dependence of nuclear spin relaxation ra tes can provide detailed information about the amplitudes and rates of collective orientational fluctuations (director fluctuations) in liqu id crystals. In particular, the low-frequency spin relaxation rates fr om a lamellar phase reflect the membrane bending rigidity and the inte rmembrane forces. This information is contained in three spectral dens ity functions J(n)(omega), n=0,1,2. We have recently presented a conti nuum-mechanical theory for the second-order spectral density J(1)(omeg a). Here we extend the theory to the fourth-order spectral densities J (0)(omega) and J(2)(omega), which dominate the transverse relaxation r ate in the parallel and perpendicular configurations. These spectral d ensities have previously been considered in connection with director f luctuations in nematic phases, neglecting the elastic and hydrodynamic anisotropy of the phase. In lamellar phases, this anisotropy plays a crucial role and must be retained in the relaxation theory. Director f luctuations can be induced by elastic distortion modes as well as by m olecular translational diffusion. In a lamellar phase, these independe nt processes give rise to qualitatively different spin relaxation beha vior. In particular, J(0)(0) and J(2)(0) diverge in the limit of quenc hed disorder. The theoretical results presented here are directly appl icable to spin relaxation data from a variety of lamellar systems, inc luding phospholipid bilayers and sterically stabilized dilute lamellar phases. An analysis of published H-2 and P-31 relaxation data from ph ospholipid bilayer phases is presented, leading to a qualitatively dif ferent picture, from what has previously been deduced in terms of a fr ee membrane theory. (C) 1997 American Institute of Physics.