Can models of charged rods show features of undercooled liquids? S. Lago ⁎ , M. Cortada, M.D. Lobato, F. Gámez, M. Basoalto, J.M. Pedrosa, S. Calero Dpto. Sistemas Físicos, Químicos y Naturales, University Pablo de Olavide, Ctra. de Utrera Km. 1, Seville 41013, Spain Available online 22 January 2007 Abstract We have performed molecular dynamic simulations of linear charged particle models. They are defined by rods with discrete charges at the ends. Intermolecular interactions are described by a coulombic term plus a Kihara potential for the rest of interactions. Our simulations were carried out for low and high charged systems either at thermodynamic states corresponding to the liquid branch of vapor–liquid equilibrium curve of non-charged rods or at supercritical temperatures of the non-charged systems. Our results show that rods with null or small discrete charge behave as normal liquids: the viscosity increases with rod length and they present short range velocity self-correlation function (VSCF). However, for medium and large discrete charges VSCF shows a very long range tail and the apparent viscosity increases by several orders of magnitude with a non-Arrhenius dependence with temperature. In some cases a transition from Arrhenius to non-Arrhenius behavior for medium charges is observed. Our results strongly suggest that systems behave as normal liquids when the rods have null or small charges and show undercooled liquid behavior for larger discrete charges at low temperatures. © 2006 Elsevier B.V. All rights reserved. Keywords: Correlation functions; Non-Arrhenius viscosity; Diffusion coefficient 1. Introduction The study of the glassy state from a microscopic point of view has experienced a spectacular expansion over the last few years [1]. The classification of glasses as strong and fragile glasses according to the variation of their viscosity with temperature has supposed a landmark in the study of these systems [2]. During the last few years a number of systems composed by different substances, probably more than in all the preceding centuries, have been identified as glass formers. These systems do not freeze normally at the melting point, suffering a first order phase transition as the normal liquids do. On the contrary, they remain as liquids at temperatures lower than the melting point. Viscosity of undercooled liquids changes by several orders of magnitude in a range of a few Kelvin. In these cases, rotation and vibration degrees of freedom are only partially frozen showing an energy landscape with a huge number of accessible wells of different depth. When tempera- ture decreases even lower the freezing points, the number of accessible depths dramatically decreases and the diffusion coefficients also abnormally decrease. It is in these circum- stances that the well-known Stokes–Einstein relation stops fulfilling. Not only these features but also a few else are well enumerated by Casalini et al. [3] to define an undercooled liquid. The most extensively studied undercooled liquid in the last few years is undoubtedly water [4], not only due to its own relevance but also because it seems to show polyamorphism [5], including a second critical point at temperatures well below the normal freezing point. This polyamorphism is a new peculiarity of a liquid with a number of well-known peculiarities [6]. Furthermore, polymers are the typical examples of undercooled liquids but they present polydispersity or, in molecular terms, they are a mixture of molecules of different chain lengths. We restricted ourselves in this work to monodisperse substances where polymers are explicitly excluded but additional examples of organic [7,8] and inorganic [9] relevant molecules are known. In particular, it is well known that hydrocarbons or closely related compounds can remain during centuries forming asphalts or shoe-polish lakes as La Brea Lake not far from Los Angeles or Bermúdez Lake in Venezuela. Moreover, in marine spills, the appearance of “chapapote” (from nahuatl Chapopotl, asphalt) is a relevant environmental problem currently. The behavior of these undercooled liquids is far from being well understood from a molecular point of view. The Journal of Molecular Liquids 134 (2007) 136 – 141 www.elsevier.com/locate/molliq ⁎ Corresponding author. Tel.: +34 954349309; fax: +34 954349151. E-mail address: slagara@upo.es (S. Lago). 0167-7322/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2006.12.012