Diffusivities in 1-Alcohols Containing Dissolved H2, He, N2, CO, or CO2 Close to Infinite Dilution

Measurement of Diffusion of Atmospheric Gases in a Liquid Perfluoro Compound by Means of an Optical Technique

We report on accurate measurements of Fickian diffusion coefficients in binary mixtures consisting of hydrofluoroether (a perfluoro compound of methoxy-nonafluorobutane or HFE-7100) with dissolved atmospheric gases CO₂, N₂, and O₂ in the limit of an infinite dilution of the gas. We show that the use of optical digital interferometry (ODI) allows the determination of diffusion coefficients of dissolved gases with relatively small standard uncertainties for this class of experiments. In addition, we illustrate the ability of an optical approach to determine the gas concentration. We compare the capacity of four mathematical models, singly used in the literature, to obtain diffusion coefficients by applying them to the processing of a large amount of experimental data. We quantify their systematic errors and standard uncertainties. The temperature behavior of the diffusion coefficients measured in the range of 10 to 40 °C is consistent with the behavior of the same gases in other solvents available in the literature.

Fick Diffusion Coefficient in Binary Mixtures of [HMIM][NTf2] and Carbon Dioxide by Dynamic Light Scattering and Molecular Dynamics Simulations

Dynamic light scattering (DLS) experiments and equilibrium molecular dynamics (EMD) simulations were performed in the saturated liquid phase of the binary mixture of 1-hexyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide ([HMIM][NTf₂]) and carbon dioxide (CO₂) to access the Fick diffusion coefficient (D₁₁). The investigations were performed within or close to saturation conditions at temperatures between (298.15 and 348.15) K and CO₂ mole fractions (x_CO2) up to 0.81. The DLS experiments were combined with polarization-difference Raman spectroscopy (PDRS) to simultaneously access the composition of the liquid phase. For the first time in an electrolyte-based system, D₁₁ was directly calculated from EMD simulations by accessing the Maxwell-Stefan (MS) diffusion coefficient and the thermodynamic factor. Agreement within combined uncertainties was found between D₁₁ from DLS and EMD simulations for CO₂ mole fractions up to 0.5. In general, an increasing D₁₁ with increasing x_CO2 could be observed, with a local maximum present at a CO₂ mole fraction of about 0.75. The local maximum could be explained by an increasing MS diffusion coefficient with increasing x_CO2 over the entire studied composition range and a decreasing thermodynamic factor at x_CO2 above 0.7. Finally, PDRS and EMD simulations were combined to investigate the influence of the fluid structure on the diffusive process.

Simultaneous determination of multiple transport properties over a wide range of temperatures and pressures from the analysis of non-equilibrium fluctuations by the shadowgraph method

The present work demonstrates that by the analysis of the dynamics of non-equilibrium fluctuations using the shadowgraph method, the thermal diffusivity, the Fick diffusion coefficient, the kinematic viscosity, and the Soret coefficient of a binary mixture can be determined from a single thermodiffusion experiment. The study was performed for a mixture consisting of equal masses of 1,2,3,4-tetrahydronaphthalene and n-dodecane in a newly developed shadowgraph apparatus at temperatures up to 373 K and pressures up to 40 MPa. The obtained results are mainly discussed in the light of their uncertainties at varying thermodynamic states for evaluating the benefits, drawbacks, and potentials of the apparatus. The Fick diffusion coefficient and the thermal diffusivity obtained with average expanded uncertainties of 2.8% and 6.6% agree with literature data and measurements for the same mixture taken by heterodyne dynamic light scattering. Current limitations of the method are reflected by the distinctly larger uncertainties of the kinematic viscosity and the Soret coefficient. Corresponding reasons and potential measures to overcome the limitations are discussed.

Generalized Form for Finite-Size Corrections in Mutual Diffusion Coefficients of Multicomponent Mixtures Obtained from Equilibrium Molecular Dynamics Simulation

The system-size dependence of computed mutual diffusion coefficients of multicomponent mixtures is investigated, and a generalized correction term is derived. The generalized finite-size correction term was validated for the ternary molecular mixture chloroform/acetone/methanol as well as 28 ternary LJ systems. It is shown that only the diagonal elements of the Fick matrix show system-size dependency. The finite-size effects of these elements can be corrected by adding the term derived by Yeh and Hummer (J. Phys. Chem. B2004, 108, 15873–15879). By performing an eigenvalue analysis of the finite-size effects of the matrix of Fick diffusivities we show that the eigenvector matrix of Fick diffusivities does not depend on the size of the simulation box. Only eigenvalues, which describe the speed of diffusion, depend on the size of the system. An analytic relation for finite-size effects of the matrix of Maxwell–Stefan diffusivities was developed. All Maxwell–Stefan diffusivities depend on the system size, and the required correction depends on the matrix of thermodynamic factors.

Mutual and Thermal Diffusivities as well as Fluid-Phase Equilibria of Mixtures of 1-Hexanol and Carbon Dioxide

This work contributes to an improved understanding of the fluid-phase behavior and diffusion processes in mixtures of 1-hexanol and carbon dioxide (CO₂) at temperatures around the upper critical end point (UCEP) of the system. Raman spectroscopy and dynamic light scattering were used to determine the composition at saturation conditions as well as Fick and thermal diffusivities. An acceleration of the Fick diffusive process up to CO₂ mole fractions of about 0.2 was found, followed by a strong slowing-down approaching vapor-liquid-liquid equilibrium or critical conditions. The acceleration of the Fick diffusive process vanished at temperatures much higher than the UCEP. Experimental Fick diffusivity data were compared with predictions from equilibrium molecular dynamics simulations and excess Gibbs energy calculations using interaction parameters from the literature. Both theoretical methods were not able to predict that the thermodynamic factor is equal to zero at the spinodal composition, stressing the need for new methodologies under such conditions. Thus, new sets of temperature-dependent interaction parameters were developed for the nonrandom two-liquid model, which improve the prediction of the Fick diffusion coefficient considerably. The link between the Fick diffusion coefficient and the nonrandomness of the liquid phases is also discussed.