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Molecular dynamics of glass-forming liquids: Structure ... - Chimica

Molecular dynamics of glass-forming liquids: Structure ... - Chimica

Molecular dynamics of glass-forming liquids: Structure ... -

JOURNAL OF CHEMICAL PHYSICS VOLUME 116, NUMBER 14 8 APRIL 2002 Molecular dynamics of glass-forming liquids: Structure and dynamics of liquid metatoluidine Riccardo Chelli, Gianni Cardini, Piero Procacci, Roberto Righini, and Salvatore Califano a) Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy and European Laboratory for Nonlinear Spectroscopy (LENS), Via Nello Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy Received 25 October 2001; accepted 7 January 2002 A molecular dynamics simulation has been performed to investigate the structure and the dynamics of liquid and supercooled metatoluidine. H-bonding and clustering has been shown to dominate the structure of the liquid. An extensive analysis of the cluster formation and of its relation with the characteristic prepeak of the static structure factor is given. It is shown that molecular association is strongly driven not only by H-bonds, but also by methyl–methyl interactions which favor specific cluster configurations. The liquid–glass transition has been followed through a calculation of the temperature variation of the molar specific heat and a mechanism has been suggested for the structural changes occurring at the phase transition. The librational dynamics of the system has been studied and recent optical Kerr effect measurements have been perfectly reproduced in a wide time regime. © 2002 American Institute of Physics. DOI: 10.1063/1.1454993 I. INTRODUCTION In the last decade, Angell’s classification 1 of glassforming liquids as strong, intermediate or fragile became usual in the scientific community. Strong liquids are characterized by relatively strong and directional intermolecular interactions, whereas fragile liquids present weaker and nondirectional interactions. Both strong and fragile liquids show a well-defined short-range order, evident as a first coordination shell in the radial distribution functions. An interesting feature of H-bonded systems with strong and directional H-bonds is the persistence of order up to distances of a few nanometers, the so-called intermediaterange order. Fragile liquids with weak H-bonds have in many cases no intermediate-range order. However, several experimental data reveal the existence of intermediate-range order even in fragile molecular liquids which do not form H-bonds. 2,3 The origin of this phenomenon and its connection with the molecular structure and intermolecular interactions is currently the object of investigation. For glassforming molecular liquids the balance between intermolecular interactions and molecular shape is actually crucial in determining the preferential occurrence of molecular networking 4 or of clustering phenomena 5,6 in the liquid and glass phases. An even more complex problem is the relationship between the liquid–glass transition and the dynamical processes occurring in the supercooled liquid. Although it is indeed well established that the relaxation times of several dynamical properties increase of orders of magnitude as the liquid–glass transition is approached, the interpretation of these processes remains at a phenomenological level and no link has yet been found with the microscopic dynamics. a Author to whom correspondence should be addressed. Electronic mail: califano@lens.unifi.it The structure and the dynamics of glass-forming materials is normally investigated by X ray and neutron diffraction and by spectroscopic techniques. Since many experimental techniques give information on the macroscopic observables, their correlation with the microscopic structure and dynamics is very indirect. For this reason, models of the liquid structure are normally used for the interpretation of the experimental data. From this point of view, computer simulations using realistic potential models represent actually a very efficient theoretical tool for the interpretation of structural and dynamical properties of complex systems, as they can furnish, at the same time, detailed microscopic information and their links to the experimental observables. Metatoluidine is a typical example of fragile glassforming liquids and, for this reason has been the subject of a considerable number of experimental and theoretical works aimed to understanding its structure 7–11 and dynamics 12–21 in the condensed phases. The structure of liquid metatoluidine has been investigated by X rays 7 and neutron scattering 8–11 experiments and the interpretation of the data has been supported by Monte Carlo simulations. 9 These studies were aimed at rationalizing the relationship between clustering phenomena and intermediate-range order as revealed by structural features of the static structure factor at low wave vectors. The dynamics of metatoluidine has been studied by light scattering techniques including Brillouin 13,15 and Raman spectroscopy, 12,19 by dielectric relaxation 14 and more recently by time resolved optical Kerr effect 16 OKE and transient grating experiments. 22 The abundance of experimental data on the structural and dynamical properties of this key molecular system and the lack of a microscopic interpretation of the relaxation processes involved in the overall dynamics, has thus prompted us to undertake a molecular dynamics study of metatoluidine in a temperature range covering both the normal, the super- 0021-9606/2002/116(14)/6205/11/$19.00 6205 © 2002 American Institute of Physics Downloaded 27 Mar 2002 to 150.217.154.29. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/jcpo/jcpcr.jsp

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