Nuclear magnetic relaxation dispersion investigations of molecular dynamics in soft matter

Abstract

The work reported in this thesis is concerned with the investigation of the e_ect of details of molecular structure on the dynamic processes in liquid crystals and also with denaturant-protein interactions in water solutions through a study of dynamic processes of water molecules. These systems belong to the general class of complex uids, and the slow time scale of relevant motions is very conveniently accessed through frequency dispersion measurements on the nuclear relaxation processes using _eld cycling NMR methods. Role of order director uctuations in inuencing the stability of the nematic phase against the formation of a low temperature layered mesogenic structure has been well recognized. Appearance of di_ering polymorphism in a given homologous series of liquid crystal system, with only subtle changes in the molecular structure, has been a subject of considerable importance from the point of molecular engineering for application purposes. Investigation of resultant molecular processes, including the collective dynamics, in a well chosen set of such systems with speci_c focus on a detailed modeling of the relevant dynamics has been made possible with the advent of new technologies to probe relaxation phenomena at very low Larmor frequencies. Such experimentation provides the required dynamic range to attempt the above study over a wide frequency range. Thus the _rst part of the thesis deals with the experimental studies on two liquid crystal systems. Other complex phenomena which could be made accessible through such low frequency dispersion studies on spin-lattice and spin-spin relaxation processes has been the dynamics of water molecules which contain large and relatively very slowly tumbling guest molecules, like for example proteins. The dynamics of a given water molecule depends on the region in the solution that it is sampling during the time of observation. In this case it is customary to distinguish three types of dynamic environments that are accessible to the water molecules.