Perera Aurélien
Laboratoire de Physique Théorique de la Matière Condensée (UMR CNRS 7600), Université Pierre et Marie Curie, 4 Place Jussieu, F75252, Paris cedex 05, France.
Phys Chem Chem Phys. 2017 Oct 25;19(41):28275-28285. doi: 10.1039/c7cp05727j.
Aqueous mixtures of small molecules, such as lower n-alkanols for example, are known to be micro-segregated, with domains in the nano-meter range. One consequence of this micro-segregation would be the existence of long range domain-domain oscillatory correlations in the various atom-atom pair correlation functions, and subsequent pre-peaks in the corresponding atom-atom structure factors, in the q-vector range corresponding to nano-sized domains. However, no such pre-peak have ever been observed in the large corpus of radiation scattering data published so far on aqueous mixtures of small n-alkanols. By using large scale simulations of aqueous-1propanol mixtures, it is shown herein that the origin for the absence of scattering pre-peak resides in the exact cancellation of the contributions of the various atom-atom correlation pre-peaks to the total scattered intensity. The mechanism for this cancellation is due to the differences in the long range oscillatory behaviour of the correlations (beyond 1 nm), which are exactly out-of-phase between same species and cross species. This is similar to the charge order observed in ionic melts, but differs from room temperature ionic liquids, where the segregation is between charged and neutral groups, instead of species segregation. The consequences of such cancellation in the experimental scattering data are examined, in relation to the possibility of detecting micro-segregation through such methods. In the particular case of aqueous-1propanol mixtures, it is shown the X-ray scattering leads an exact cancellation, while this cancellation in neutron scattering is seen to depend on the deuteration ratio between solvent and solute.
已知小分子的水性混合物，例如低级正烷醇，会发生微相分离，形成纳米级别的区域。这种微相分离的一个结果是，在各种原子对相关函数中存在长程区域间振荡相关性，以及在对应于纳米尺寸区域的q向量范围内，相应原子对结构因子中出现后续的预峰。然而，在迄今为止发表的关于低级正烷醇水性混合物的大量辐射散射数据中，从未观察到这样的预峰。通过对1-丙醇水性混合物进行大规模模拟，本文表明散射预峰缺失的原因在于各种原子对相关预峰对总散射强度的贡献恰好相互抵消。这种抵消的机制是由于相关性的长程振荡行为（超过1纳米）存在差异，相同物种和不同物种之间的振荡行为恰好相位相反。这类似于在离子熔体中观察到的电荷有序，但与室温离子液体不同，在室温离子液体中，分离发生在带电和中性基团之间，而不是物种分离。本文研究了这种抵消在实验散射数据中的后果，以及通过此类方法检测微相分离的可能性。在1-丙醇水性混合物的特定情况下，结果表明X射线散射导致了完全抵消，而中子散射中的这种抵消则取决于溶剂和溶质之间的氘化率。
