engleski

The nano-structuration of associated liquids and their mixtures

Associated liquids such as alcohols, water or amides show surprising amount of local order, and yet are not structured similarly. For example, methanol contains a fair amount of chain-like clusters, while tertbutanol looks almost entirely ordered into small globular clusters. In contrast, water and amides do not have such clearly defined clustering, yet they are also ordered by the same H-bond interaction. Since these liquids are isotropic, the differences in order cannot be described in terms of any one body function, hence should be searched in the pair correlation functions. Local order should contributes as a pre-peak in the corresponding structure factor. How can one depict their type of local order from the pair correlation function? Mixtures of associated liquids are also strongly structured at the level of the nanometer, and could be called nano-emulsions. While this local heterogeneity can be seen from the mole fraction dependence of the experimentally obtained Kirkwood-Buff integrals for aqueous alcohols, how does one explain that these quantities look surprisingly ideal for several aqueous amides mixtures? Yet, both types of mixtures exhibit the same micro-heterogeneity. Such questions are answered by exploring the structure of such liquids and mixtures by Molecular Dynamics simulations, in terms of specific features of the pair correlation functions, concentration fluctuations and the associated correlation length, the existence of new length scale introduced through the local heterogeneity. We equally discuss how the existence of two length scales and relaxation times, one due to the molecular motion and the other due to the local structure, depend on the modeling of the force field parameters and how they affect the simulations themselves.

structure; associated liquids; molecular dynamics

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