Experimental indication of structural heterogeneities in fragile hydrogen-bonded liquids

We present the first experimental characterization in molecular fragile glass-formers of a "prepeak" that appears significantly below the main peak of the static structure factor. The temperature and density dependences of this prepeak are studied via elastic neutron scattering experiments under high pressure (up to 300 MPa) in m-toluidine and m-fluoroaniline. The prepeak intensity increases with decreasing temperature, but it remains constant with increasing pressure while its position and width stay roughly the same. These features are opposite to those observed for the "first sharp diffraction peak" of network glasses. The origin of the phenomenon is analyzed with the help of Monte Carlo simulations. We associate the prepeak with hydrogen-bond-induced heterogeneities (or clusters) whose limited size results from an exclusion effect between benzene rings that prevents the extension of a hydrogen-bond network. Implications for the dynamics of the liquid close to the glass transition are finally considered.