Influence of Liquid Structure on Fickian Diffusion in Binary Mixtures of n-Hexane and Carbon Dioxide Probed by Dynamic Light Scattering, Raman Spectroscopy, and Molecular Dynamics Simulations

Abstract

This study contributes to a fundamental understanding of how the liquid structure in a model system consisting of weakly associative n-hexane ( n-CH) and carbon dioxide (CO) influences the Fickian diffusion process. For this, the benefits of light scattering experiments and molecular dynamics (MD) simulations at macroscopic thermodynamic equilibrium were combined synergistically. Our reference Fickian diffusivities measured by dynamic light scattering (DLS) revealed an unusual trend with increasing CO mole fractions up to about 70 mol %, which agrees with our simulation results. The molecular impacts on the Fickian diffusion were analyzed by MD simulations, where kinetic contributions related to the Maxwell-Stefan (MS) diffusivity and structural contributions quantified by the thermodynamic factor were studied separately. Both the MS diffusivity and the thermodynamic factor indicate the deceleration of Fickian diffusion compared to an ideal mixture behavior. Computed radial distribution functions as well as a significant blue-shift of the CH stretching modes of n-CH identified by Raman spectroscopy show that the slowing down of the diffusion is caused by a structural organization in the binary mixtures over a broad concentration range in the form of self-associated n-CH and CO domains. These networks start to form close to the infinite dilution limits and seem to have their largest extent at a solute-solvent transition point at about 70 mol % CO. The current results not only improve the general understanding of mass diffusion in liquids but also serve to develop sound prediction models for Fick diffusivities.

Diffusivities in Binary Mixtures of n-Hexane or 1-Hexanol with Dissolved CH4, Ne, Kr, R143a, SF6, or R236fa Close to Infinite Dilution

“…The present work provides a link to previous investigations ,,,, studying the Fick diffusion coefficient D 11 as well as the structure–property relationships in the liquid phase of binary mixtures consisting of a liquid and a dissolved gas close to infinite dilution. As solvents, nonpolar n -alkanes, polar 1-alcohols, or bis­(trifluoromethylsulfonyl)­imide ([NTf 2 ] − )-based ionic liquids (ILs) with varying carbon chain lengths were investigated.…”

“…Dynamic light scattering (DLS) has been proven to be an excellent tool for the simultaneous and accurate determination of a and D 11 in liquids with dissolved gases under various thermodynamic conditions. − In contrast to conventional techniques, DLS does not require calibration and is applied at the macroscopic thermodynamic equilibrium. Therefore, no macroscopic gradient, which may lead to a convective/advective mass transport besides the molecular one, must be applied.…”

Viscosity, Interfacial Tension, and Density of Binary-Liquid Mixtures of n-Hexadecane with n-Octacosane, 2,2,4,4,6,8,8-Heptamethylnonane, or 1-Hexadecanol at Temperatures between 298.15 and 573.15 K by Surface Light Scattering and Equilibrium Molecular Dynamics Simulations

“…The atomistic RDFs from n -hexadecane, on the other hand, show a more defined first peak, followed by a local minimum and then a second peak. This shows the ability of linear molecules to form large structures with a periodic arrangement. − From the CE–CE RDFs, the presence of two peaks within the first maxima, between 0.4 and 0.5 nm, can be seen. The two peaks here represent different configurations due to the trans - or gauche -conformation of the linear alkanes. − In this case, the first peak within the first maxima located at around 0.4 nm represents the trans -conformations, allowing the molecules to interact at very short distances.…”

“…This shows the ability of linear molecules to form large structures with a periodic arrangement. − From the CE–CE RDFs, the presence of two peaks within the first maxima, between 0.4 and 0.5 nm, can be seen. The two peaks here represent different configurations due to the trans - or gauche -conformation of the linear alkanes. − In this case, the first peak within the first maxima located at around 0.4 nm represents the trans -conformations, allowing the molecules to interact at very short distances. The second peak around 0.5 nm represents the gauche -conformations, where the intermolecular distance is increased due to the flipping of dihedral angles, leading to less linear molecules.…”

Effective Thermal Conductivity of Nanofluids: Measurement and Prediction

“…For the characterization of particle size and its distribution as well as the dispersion stability of the nanofluids in the same fluid state as it is investigated in connection with the effective thermal conductivity, the diffusion coefficients were obtained by dynamic light scattering (DLS). For details on the principles of the technique [52] and its application for the determination of several thermophysical properties of various types of fluids [53][54][55][56] and dispersed systems [57][58][59], the reader is referred to the respective literature. In the following, only the information relevant for the present investigations is given.…”