The influence of the strength of intermolecular interactions on mass diffusive processes remains poorly understood for mixtures of associative liquids with dissolved gases. For contributing to a fundamental understanding of the interplay between liquid structures and mass diffusivities in such systems, dynamic light scattering, Raman spectroscopy, and molecular dynamics simulations were used in this work. As model systems, binary mixtures consisting of the gases hydrogen, helium, nitrogen, carbon monoxide, or carbon dioxide dissolved in ethanol, 1-hexanol, or 1-decanol were selected. Experiments and simulations were performed at macroscopic thermodynamic equilibrium close to infinite dilution of solute for temperatures between 303 and 423 K. The Fick diffusion coefficients and self-diffusivities of the gas solutes increase with increasing temperature, decreasing alkyl chain length of the 1-alcohols, and decreasing molar mass of the solutes except for helium and hydrogen showing the opposite behavior. The analysis of the liquid structure of the mixtures showed that the fraction of hydrogen-bonded alcohol molecules decreases with increasing alkyl chain length and temperature. From the obtained structure–property relationships, a new correlation was developed to predict mass diffusivities in binary mixtures consisting of n-alkanes or 1-alcohols with dissolved gases close to infinite dilution within 10% on average.

In the present study, dynamic light scattering (DLS) experiments and molecular dynamics (MD) simulations were used for the investigation of the molecular diffusion in binary mixtures of liquids with dissolved gases at macroscopic thermodynamic equilibrium. Model systems based on the n-alkane n-hexane or n-decane with dissolved hydrogen, helium, nitrogen, or carbon monoxide were studied at temperatures between 303 and 423 K and at gas mole fractions below 0.06. With DLS, the relaxation behavior of microscopic equilibrium fluctuations in concentration and temperature is analyzed to determine simultaneously mutual and thermal diffusivity in an absolute way. The present measurements document that even for mole gas fractions of 0.007 and Lewis numbers close to 1, reliable mutual diffusivities with an average expanded uncertainty ( k = 2) of 13% can be obtained. By use of suitable molecular models for the mixture components, the self-diffusion coefficient of the gases was determined by MD simulations with an averaged expanded uncertainty ( k = 2) of 7%. The DLS experiments showed that the thermal diffusivity of the studied systems is not affected by the dissolved gas and agrees with the reference data for the pure n-alkanes. In agreement with theory, mutual diffusivities and self-diffusivities were found to be equal mostly within combined uncertainties at conditions approaching infinite dilution of the gas. Our DLS and MD results, representing the first available data for the present systems, reveal distinctly larger mass diffusivities for mixtures containing hydrogen or helium compared to mixtures containing nitrogen or carbon monoxide. On the basis of the broad range of mass diffusivities of the studied gas-liquid systems covering about 2 orders of magnitude from about 10 to 10 m·s, effects of the solvent and solute properties on the temperature-dependent mass diffusivities are discussed. This contributed to the development of a simple semiempirical correlation for the mass diffusivity of the studied gases dissolved in n-alkanes of varying chain length at infinite dilution as a function of temperature. The generalized expression requiring only information on the kinematic viscosity and molar mass of the pure solvent as well as the molar mass and acentric factor of the solute represents the database from this work and further literature with an absolute average deviation of about 11%.

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.

The present contribution provides experimental data for the liquid viscosity and surface tension of n-alkane based model systems at temperatures up to 573 K. The fundamental advantage of the used surface light scattering (SLS) method lies in its application in thermodynamic equilibrium without calibration in a contactless way. The investigated systems comprise the pure fluids n-dodecane (n-C 12 H 26 ) and n-octacosane (n-C 28 H 58 ), their binary mixture at a n-C 12 H 26 mole fraction of about 0.3, and the commercially available hydrocarbon wax SX-70 representing a multicomponent mixture of n-alkanes with a broad chain length distribution. For the first time, it could be demonstrated that the SLS method can simultaneously access the liquid viscosity and surface tension of such medium-to long-chained n-alkane systems close to saturation conditions over a broad temperature range from 323 to 573 K. Typical measurement uncertainties of 2% based on a coverage factor k = 2, i.e., a level of confidence of more than 95%, were obtained. Over the entire temperature range, a simple polynomial equation for the dynamic viscosity and a modified van der Waals equation for the surface tension represent the measured data of the pure and binary systems well. The present investigations improve the data situation for hydrocarbon systems in the high-temperature range where no measurement results exist in literature.

This work contributes to an improved understanding of the fluid-phase behavior and diffusion processes in mixtures of 1-hexanol and carbon dioxide (CO 2 ) 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 CO 2 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.

Ionic liquids (ILs) are interesting working fluids in many areas of chemical and energy engineering such as gas separation, catalysis, or energy storage. For the optimum design of related processes and apparatuses, the diffusion coefficient of gases dissolved in ILs is necessary, yet is often only known for a few selected gases. In this work, the thermal and mutual diffusivities in binary mixtures consisting of the gases hydrogen, helium, nitrogen, carbon monoxide, carbon dioxide, or krypton dissolved in the homologous series of the ILswere investigated by dynamic light scattering (DLS) and equilibrium molecular dynamics (EMD) simulations for temperatures between (298 and 423) K at conditions close to infinite dilution of the dissolved gas. Here, the Fick diffusion coefficient determined by DLS can be compared with the self-diffusion coefficient of the dissolved gases predicted by EMD simulations. For the latter, selected force fields (FFs) for the ILs available in literature were tested, for which the most suitable FF was modified by scaling the partial charges of all atoms with an IL-specific factor using experimental data for density, viscosity, and the self-diffusion coefficient of the pure ILs. On the basis of the modified FF, agreement of the calculated gas self-diffusion coefficients and the experimental Fick diffusion coefficients with an average absolute relative deviation of 12% was found. The Fick diffusion coefficient increases with decreasing molecular weight of the dissolved gas, with the exception of hydrogen and helium, which show an inverse behavior. In contrast to the trends observed for binary mixtures of the above gases dissolved in n-alkanes or 1alcohols of varying alkyl chain length, the diffusion coefficients of the mixtures with the different ILs investigated in this study were found to be not significantly affected by the solvent viscosity.

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Fluctuations in a fluid are strongly affected by the presence of a macroscopic gradient making them long-ranged and enhancing their amplitude. While small-scale fluctuations exhibit diffusive lifetimes, larger-scale fluctuations live shorter because of gravity, as theoretically and experimentally well-known. We explore here fluctuations of even larger size, comparable to the extent of the system in the direction of the gradient, and find experimental evidence of a dramatic slowing-down in their dynamics. We recover diffusive behaviour for these strongly-confined fluctuations, but with a diffusion coefficient that depends on the solutal Rayleigh number. Results from dynamic shadowgraph experiments are complemented by theoretical calculations and numerical simulations based on fluctuating hydrodynamics, and excellent agreement is found. The study of the dynamics of non-equilibrium fluctuations allows to probe and measure the competition of physical processes such as diffusion, buoyancy and confinement.PACS numbers: 05.70.Ln, 42.30.Va It is well established that fluctuations are long-ranged in systems out-of-equilibrium [1-3], even far from critical points where the long-range behaviour is observed also in equilibrium conditions [4]. In a binary fluid mixture subject to a stabilizing (vertical) temperature or concentration gradient, the coupling between the spontaneous velocity fluctuations and the macroscopic gradient results in giant concentration fluctuations in the quiescent state [3, 5]. Gravity quenches the intensity of fluctuations with length scales larger than a characteristic (horizontal) size 2π/q s related to the dimensionless solutal Rayleigh number Ra s of the system [5, 6]:where β s = ρ −1 (∂ρ/∂c) is the solutal expansion coefficient, ρ the fluid density, g the gravity acceleration, c the concentration (mass fraction) of the denser component of the fluid, ∇c the modulus of the concentration gradient, D the mass diffusion coefficient, ν the kinematic viscosity, and q s a characteristic solutal wave vector. Vertical boundaries suppress fluctuations larger than the confinement length L in the direction of the gradient [3, 7]. Gravity also accelerates the dynamics of the fluctuations for wavenumbers smaller than q s via buoyancy effects, leading to non-diffusive decay of large-scale fluctuations [8].The dynamics of concentration non-equilibrium fluctuations (c-NEFs) in the presence of a vertical concentration gradient in a binary liquid mixture can be characterized in terms of the Intermediate Scattering Function (ISF or, equivalently, normalized time correlation function) f (q, t), with f (q, 0) = 1. At first approximation the ISF can be modeled by a single exponential with decay time τ (q) depending on the analysed wave vector q.Available theories accounting for the simultaneous presence of diffusion (d) and gravity (g) [9, 10], but not for confinement, predict for a stable configuration (Ra s < 0):where the wave vector is expressed in its dimensionless formq = qL and τ s = L 2 /D is the typical so...