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III. MOLECULAR SIMULATIONS OF AQUEOUS
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aceton water
III. MOLECULAR SIMULATIONS OF AQUEOUS
ACETONE MIXTURES Molecular dynamics simulations have been conducted with the DL_POLY-2 program, 24 by using a system of N = 2048 molecules, with SPC/E water and TraPPE-UA ace- tone. A comparison of some of the force field available for the molecule is shown in Table I . It is seen that some of the force field parameters can differ quite a lot from one set to another, such as the partial charges. These details pro- duce large differences in the micro-heterogeneity, ranging from small micro-segregation (WS-SPC/E) to full demixing (OPLS/SPCE). There does not seem to be any clear rule that would explain such wide variety of behaviour. One exam- TABLE I. Acetone force fields. σ (Å) O C CH 3 OPLS 2.96 3.75 3.91 TraPPE-UA 3.05 3.82 3.75 WS 3.10 3.36 3.75 /k B (K) OPLS 105.75 52.87 80.57 TraPPE-UA 79.00 27.00 98.00 WS 67.35 39.69 104.30 q (e) OPLS − 0.424 0.3 0.062 TraPPE-UA − 0.564 0.662 − 0.049 WS − 0.565 0.565 0 ple of this situation is the sign of the partial charge on the hydrophobic CH 2 groups. These are positive for the OPLS model, which might explain why this model leads to demix- ing. On the other hand the TraPPE-UA has a negative charge, and yet it exhibits more micro-heterogeneity than the WS model which has zero partial charge. We have used the same simulation protocol as in all our previous publications. 14 , 21 All simulations have been con- ducted in the isobaric ensemble under ambient conditions of pressure of 1 atm and T = 300 K, with the use of Berend- sen thermostat and barostat, with relaxation times of 0.1 ps and 0.5 ps, respectively. The time step for the integration of motion was fixed at 2 × 10 −15 s in all the calculations. The cutoff of the LJ interaction was set to 8.5 Å, which is 1.7 times the size of the acetone molecule. The electrostatic interaction were handled with the Ewald summation techniques incorpo- rated within the DL_POLY code. Acetone mole fractions x ranging from 0 to 1 have been studied by steps of 0.1. For each concentration, the mixture were simulated starting from a random disposition of the molecules and equilibrated for 0.5 ns, then production runs were conducted for 3 ns. Figure 1 shows some snapshots of the system at x = 0.2, 0.5, and 0.8, where we can see that large micro-heterogeneity is present at all concentrations. This MH looks very similar to that shown in our earlier studies of the same mixtures, with- out showing any clear demixing tendencies as that reported earlier by us for x = 0.3 for the SPC/E-OPLS mixture. 7 We find that water has a Swiss-cheese-like structure at small x-values x < 0.5, which then gradually switches to linear and globular clusters, as clearly seen at x = 0.8. In contrast, ace- tone rich regions do not show a clear Swiss-cheese structure, hinting that acetone is more homogeneously distributed in the simulation box. Figure 2 shows the enthalpies and volumes, as well as ex- cess quantities, defined as A ex (x) = A(x) − (1 − x)A(x = 0) − xA(x = 1), where A(x) is either the enthalpy or the volume. Previous results from 7 are equally shown as well as the exper- imental data. 26 , 27 It is seen that the new TraPPE-UA model is in much better agreement with experimental values than any of the previous estimates. This is particularly true of the ex- cess enthalpies, which show the typical S-shaped curve that none of the previous model combinations could capture. It is difficult to infer the superiority of the TraPPE-UA force field from a simple comparison of the various parameters in Table I . The fact that this force field is initially designed to reproduce the entire phase behaviour of the neat ace- tone could be an indication. 13 Since it captures the micro- structure of this liquid across large regions of thermody- namic parameters, it may be able to better reproduce the “cor- rect” micro-heterogeneity of the aqueous mixture as well. However, we wish to emphasize that it also behaves like the TIP4P-FMKH mixture with respect to the large short range behaviour of the RKBI, leading to high transient KBI. Figure 3 shows the evolution of the water-water, acetone- acetone, and cross correlations. These appear quite similar to those reported earlier. 7 The major features are that the acetone-acetone short-range correlations change much less that those of water with concentration, clearly indicating the 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 134502-4 B. Keži´c and A. Perera J. Chem. Phys. 137, 134502 (2012) FIG. 1. Snapshots of the aqueous-acetone mixtures for acetone mole fractions x = 0.2 (left), x = 0.5 (middle) and x = 0.8 (right). Oxygen is shown in red, hydrogen in white, and carbon and CH 3 groups are in cyan. In all shots the acetone molecules are shown as semi-transparent. role of the hydrogen bonding in the restructuring of the mix- tures. Figures 4 and 5 show two examples of the procedure we have used to extrapolate the correlations by using the TS extension. Figure 4 shows the water-water correlations at Download 0.81 Mb. Do'stlaringiz bilan baham: |
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