Modeling micro-heterogeneity in mixtures: The role of many body correlations

Microscopic origin of the scattering pre-peak in aqueous propylamine mixtures: X-ray and neutron experiments versus simulations.The structure of aqueous propylamine mixtures is investigated through X-ray and neutron scattering experiments, and the scattered intensities compared with computer simulation data. Both sets of data show a prominent scattering pre-peak, which first appears at propylamine mole fraction x ≥ 0.1 around scattering vector k ≈ 0.2Å −1 , and evolves towards k ≈ 0.8Å −1 for neat propylamine x = 1. The existence of a scattering pre-peak in this mixture is unexpected, specifically in view of its absence in aqueous 1-propanol or aqueous DMSO mixtures. The detailed analysis of the various atom-atom structure factors and snapshots indicates that significant micro-structure exists, which produces correlation pre-peaks in the atom-atom structure factors, positive for like species atoms correlations and negative for the cross species ones. The scattering pre-peak depends on how these two contributions cancel or not. The way the amine group bond with water, produce a pre-peak through the inbalance of the positive and negative scattering contributions, unlike 1-propanol and DMSO, where these 2 contributions compensate exactly. Hence molecular simulations demonstrate how chemical details influence the microscopic segregation in different types of molecular emulsions and can be detected or not by scattering experiments Russia S(k). This simple formula also holds more complex systems, such as micelles. The reason is that micelles look just like meso-atoms floating in a structureless solvent, as usually explained in various textbooks. In this case, the form factor F(k) refers to the micelle shape, and S(k) to micelle-micelle correlations. Since micelles are composite objects, with an underlying atomic sub-structure, the corresponding I(k) will exhibit 2 scattering peaks, a main peak k M positioned at the mean atomic size, and a pre-peak at k P < k M , related to the micelle shape and size 5,6 . Pre-peaks are equally found in neat alcohols, as I(k) reveals, in addition to a main peak at k M , their existence at k P ≈ 0.4 − 0.7Å −1 7-9 . The origin of such pre-peak has been traced back to the existence of short chain-like clustering of the hydroxyl head groups, with mean size d ≈ 10Å. These experimental results have been confirmed by computer simulation, both from snapshot and cluster analysis and study of the atom-atom correlation functions and corresponding structure factor 10-12 . These analyses clearly demonstrate that the pre-peak k P is related to both the size of the chains formed in the neat liquids and their density. If one applies the same type of analysis to spherical micellar systems, for example, which are made of surfactant molecules immersed in a solution made of solvent, ions and counterions, the existence of a scattering peak around k P ≈ 0.06Å −1 is commonly interpreted in terms of spherical mi-

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“…A simple model can further help explain the argument above 53 . We consider a model binary mixture which micro-segregates.…”

Section: Figmentioning

confidence: 96%

Microscopic origin of the scattering pre-peak in aqueous propylamine mixtures: X-ray and neutron experiments versus simulations

Almásy

Куклин

Požar

et al. 2019

Phys. Chem. Chem. Phys.

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Solid Surface Induced Anisotropic Clustering in Ethanol-Cyclohexane Binary Liquids Studied by Molecular Dynamics Simulations

et al. 2021

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Excess spectroscopy and its applications in the study of solution chemistry

Zhang

Wang

et al. 2020

Pure and Applied Chemistry

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AbstractCharacterization of structural heterogeneity of liquid solutions and the pursuit of its nature have been challenging tasks to solution chemists. In the last decade, an emerging method called excess spectroscopy has found applications in this area. The method, combining the merits of molecular spectroscopy and excess thermodynamic functions, shows the ability to enhance the apparent resolution of spectra, provides abundant information concerning solution structures and intermolecular interactions. In this review, the thinking and mathematics of the method, as well as its developments, are presented first. Then, research progress related to the exploration of the method is thoroughly reviewed. The materials are classified into two parts, small-molecular solutions and ionic liquid solutions. Finally, potential challenges and the perspective for further development of the method are discussed.

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Modeling micro-heterogeneity in mixtures: The role of many body correlations

Cited by 3 publications

References 44 publications

Microscopic origin of the scattering pre-peak in aqueous propylamine mixtures: X-ray and neutron experiments versus simulations

Microscopic origin of the scattering pre-peak in aqueous propylamine mixtures: X-ray and neutron experiments versus simulations

Solid Surface Induced Anisotropic Clustering in Ethanol-Cyclohexane Binary Liquids Studied by Molecular Dynamics Simulations

Excess spectroscopy and its applications in the study of solution chemistry

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