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aceton water
IV. PERSPECTIVES AND CONCLUSION
The concept of molecular emulsions allows to match experimental KBI without the need of excessively large and lengthy simulations at the price of an ansatz on the form of the decay of the long range part of the correlation functions. We have previously demonstrated this point in the case of aqueous-Tbutanol mixtures, 14 and we conjecture here that many other systems that show strong micro-heterogeneous behaviour, and not necessarily aqueous mixtures, could benefit from this approach. It is however required to simulate these systems for a size that allows to see more clearly the decay of these short range molecular correlations, to the point that domain correlations start to show up. These nascent—yet incompletely calculated—correlations could be in fact distorted by the periodic boundary conditions, and therefore may not represent the real correlations in the same range of distances. Nevertheless, we think that, in view of the small values associated with these correlations, the TS extension allows one to capture the gross features of the domain-induced oscillatory decay. On a more fundamental level, micro-heterogeneity is a short to medium range feature that encompasses the mean segregated domain size, while concentration fluctuations are a thermodynamic property of the entire system. The corre- lation functions capture enhanced correlations in their short range features, due to self-segregation of species, while they contain information about concentration fluctuations in their long range limit. The KBI, which are integrals of the correla- tion functions, contain therefore both features, but in an intri- cate way. If one computes correlations in simulation cells that merely encompass one or two domain lengths, then the corre- sponding KBI are likely to show mostly the short range self- aggregation, hence leading to very large values of the KBI. It is therefore crucial to sample enough domain statistics in order to properly appreciate the concentration fluctuations in the thermodynamic limit. Our methodology allows to guess this limit through an ansatz on the form of the tail of the cor- relation functions. We stress once again that the present considerations do not apply to mixture models that show clear demixing ten- dencies. The issue of phase separating models is not very clear at present. It is obviously necessary to allow the system to grow a flat interface for a clear signature of phase separa- tion. Our previous experience indicates that phase separating models simulated with at least thousand particles show a clear interface after few hundreds of picoseconds. There are appar- ently no slow coarsening of domains that would take nanosec- ond time scales to turn into a flat interface. In other words, phase separation can be detected and should be rapidly de- tected within system sizes of about 2048 particles. It would be highly desirable to have a general argument about droplet nucleations versus domain segregation. The approach presented in this paper is still very much empirical, and it would be desirable to obtain a method to de- duce the domain size and correlation length associated with the fluctuations, within the thermodynamical data. From that perspective, we think that a better development of the molec- ular theory of liquids, in particular in what concerns a reliable calculation of the correlations, namely through the develop- ment of the integral equation theory, 33 would allow important progress, both with respect to matching the experimental data and also explaining more fundamental features, such as the role of the fluctuations and the domains, especially for these mixtures where it is rather difficult to distinguish one from the other. We believe that such considerations will allow a bet- ter understanding of the organisation capabilities of aqueous mixtures at a mesoscopic level. 1 S. Dixit, J. Crain, W. C. K. Poon, J. L. Finney, and A. K. Soper, Nature (London) Download 0.81 Mb. Do'stlaringiz bilan baham: |
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