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Revisiting aqueous-acetone mixtures through the concept
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- I. INTRODUCTION
Revisiting aqueous-acetone mixtures through the concept
of molecular emulsions Bernarda Keži´c 1,2 and Aurélien Perera 1 1 Laboratoire de Physique Théorique de la Matière Condensée (UMR CNRS 7600), Université Pierre et Marie Curie, 4 Place Jussieu, F75252, Paris Cedex 05, France 2 Department of Physics, Faculty of Sciences, University of Split, Nikole Tesle 12, 21000 Split, Croatia (Received 17 July 2012; accepted 14 September 2012; published online 2 October 2012) Aqueous acetone mixtures represent a good example of perfectly miscible liquids in reality, and that hard to mix in silico. This is related to a key problem in molecular simulations, which is to distin- guish between strongly micro-segregated mixtures from phase-separated ones. The Kirkwood-Buff integrals of in silico aqueous mixtures are often found to be dramatically higher than the experimen- tal ones, hinting at a possible underlying phase separation. This is the case for many combinations of the force field models chosen for water or for acetone. Herein, we demonstrate, through a proper handling of the tail of the correlation functions, that these high values of the Kirkwood-Buff integrals represent in fact a transient regime within the segregated spatial domains, and that they asymptoti- cally settle down to values in much better agreement with the experimental ones. The concept central to this new approach is that of molecular emulsions, where the long range part of the correlations is modulated by the micro-segregated domains, and that it is necessary to take into this modulation in order to recover the correct thermodynamical properties. © 2012 American Institute of Physics. [ http://dx.doi.org/10.1063/1.4755816 ] I. INTRODUCTION In the past decade, the molecular simulation stud- ies of aqueous mixtures put into evidence the fact that these mixtures were highly micro-segregated. 1 – 3 This micro- heterogeneity (MH) found in the simulation often leads to very high values of the integrals of the correlation functions— the so-called Kirkwood-Buff integrals 4 (KBI)—excessively higher than the KBI obtained from the experiments. From these results, one may ask whether micro-heterogeneity is a genuine feature of real aqueous mixtures, or an artifact pro- duced by inaccurate force field models and/or computer sim- ulations themselves. 5 , 6 The aqueous acetone mixtures were found to be almost archetypal of this situation. 7 Many com- binations of acetone and water models, namely, OPLS (op- timized potentials for liquid state) 8 and FHMK (Ferrario, Haughney, McDonald, and Klein) 9 for acetone and SPC/E (extended single point charge) 10 and TIP4P (transferable inter-molecular potential 4-point charges) 11 almost invariably produced KBI 6–7 times larger than the experimental values. In particular, it was found that, in the case of the SPC/E-OPLS mixture model, the strong clustering that was apparent for system sizes of N = 864 molecules, translated into a clear phase separation with N = 2048 molecules 7 for acetone mole fraction around 0.3. Such findings throw a serious doubt in the ability of these force fields to reproduce the miscibility of this particular system. Weerasinghe and Smith 12 introduced a new model of acetone (WS) that is miscible with SPC water and also keeps the KBI within the experimental range. How- ever, the KBI obtained by this model have the wrong shape when plotted versus acetone mole fraction x: they increase monotonously until x = 1, instead of having a clear maxi- mum at x = 0.6. Nevertheless, this model allows a consider- able reduction of the micro-segregation, quite visible through a comparative analysis of the snapshots, thereby hinting that reducing the micro-heterogeneity could be an option in or- der to match the experimental KBI by molecular simulations techniques. This result leads one to question the status of the other mixture models that do not show clear demixing like the OPLS-SPC/E combination, but nevertheless produce very high KBI through the computer simulations. In the present work, we revisit this problem with the aim to show that the strong micro-heterogeneous nature of the simulated mixtures is in fact not an obstacle, since it rep- resents only the short range nature of micro-segregated sys- tems, and that the key point is to describe properly the asymp- totic tail part of the correlation functions. For this purpose, we use herein another recent force-field for acetone, the TraPPE- UA 13 force field, together with the SPC/E water. This force field has been built in order to reproduce accurately the liquid- gas coexistence of various acetone mixtures. 13 We find that this particular model combination shows perfect miscibility at all acetone mole fractions and nevertheless leads to very high apparent KBI values. The fact that so many good model combinations almost inevitably lead to very high KBI leads one to question if this could not be an artifact of the simula- tions, and in particular if the size of few thousand molecule is appropriate for this particular calculation. Instead of handling very large system sizes directly through simulations, we use herein the concept of “molecu- lar emulsions” which we have recently put forward. 14 , 15 This concept is borrowed from the physics of micro-emulsions and the fact that oil-water domains modulate the long range part of the correlations, which in turn produce a pre-peak in the 0021-9606/2012/137(13)/134502/6/$30.00 © 2012 American Institute of Physics 137, 134502-1 Downloaded 02 Oct 2012 to 134.157.8.133. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions 134502-2 B. Keži´c and A. Perera J. Chem. Phys. 137, 134502 (2012) scattering intensity. 16 Since micro-segregation also produces water and solute domains, albeit much smaller than those found in micro-emulsions, it seems reasonable to infer that these domains should also modulate the long range part of the correlations, and therefore produce a pre-peak in the site-site structure factors. By introducing an ansatz on the asymptotic form of the correlations, 14 , 15 based on the Teubner-Strey (TS) approach to micro-emulsions, 16 we are able to account for these long range domain oscillations that are missing from the current molecular simulations. This procedure allows us to take into account long range negative contributions to the KBI, which lower the currently obtained high values to fig- ures that match the experimental values. This suggests that the current simulations take into account only the correlations in the first segregated domain, and that much larger system sizes would be required. The remaining of the paper is set as follows. In Sec. II we recall the concept of molecular emulsions and the conse- quence in the shape of the correlation functions. In Sec. III we show how this new concept solves the problem posed by the micro-heterogeneity in molecular simulations. In Sec. IV we outline some perspectives and conclude. Download 0.81 Mb. Do'stlaringiz bilan baham: |
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