  Glass transition and relaxation of the glycerol-water binary mixture system
are studied over the glycerol concentration range of 5 - 85 mol% using the
highly sensitive technique of spin probe ESR. For the water rich mixture the
glass transition, sensed by the spin probe, arises from the vitrified
mesoscopic portion of the binary system. The concentration dependence of the
glass transition temperature manifests a closely related molecular level
cooperativity in the system. A drastic change in the mesoscopic structure of
the system at the critical concentration of 40 mol% is confirmed by an
estimation of the spin probe effective volume in a temperature range where the
tracer reorientation is strongly coupled to the system dynamics.
