Molecular dynamics of lyotropic liquid crystals: A time-resolved fluorescence study
The rotational dynamics and orientational order of lyotropic liquid crystals (lipid/water mixtures) have been studied by using frequency-domain fluorescence spectroscopic technique. The lipid/water mixtures exhibit polymorphic phases of various packing symmetries, e.g., lamellar liquid crystalline (La) , inverted hexagonal (Hn) , inverted cubic (CQ) and other intermediate phases. Recently a rotational diffusion model was developed to describe the fluorescence anisotropy decay behavior of fluorophores in the lamellar liquid crystals. By considering an extra mode of curvature related rotational diffusion, this molecular model was extended into studying the hexagonally packed liquid crystals. By embedding lipid-like fluorophores (diphenylhexatriene-labeled phosphatidylcholine) into the liquid crystals and applying the newly developed diffusion model, different molecular dynamics parameters, such as rotational diffusion constants and orientational order parameters of the lipids in different phases were obtained from the frequency-domain fluorescence data. These parameters, particularly the curvature related rotational diffusion (hopping) constant (D„) , are very sensitive to the lamellar to non-lamellar phase transitions (e.g., L„-HQ) . From the lateral diffusion constant of the lipids as obtained from the fluorescence excimer formation measurement and the above D„, the average interfacial curvature of the liquid crystals in the non-lamellar phases was further estimated and compared favorably with that from the X-ray diffraction measurement. Based on the recovered order parameters, local orientational distribution functions of the lipids with respect to the normal of the lipid/water interface of the liquid crystals were reconstructed. The results revealed that the most probable orientation of the lipids in liquid crystals shifts progressively toward the normal of the lipid/water interface as the system approaches the L„ to Hn phase transition. The correlation between the molecular dynamics parameters obtained from this study and the lateral stress in the lipid layers was also discussed.