This work contributes to the characterization of long linear and branched alkanes and alcohols via the determination of their thermophysical properties up to temperatures of 573.15 K. For this, experimental techniques including surface light scattering (SLS) and molecular dynamics (MD) simulations were used under equilibrium conditions to analyze the influences of chain length, branching, and hydroxylation on liquid density, liquid viscosity, and surface tension. For probing these effects, 12 pure model systems given by the linear alkanes n-dodecane, n-hexadecane, n-octacosane, n-triacontane, and n-tetracontane, the linear alcohols 1-dodecanol, 1-hexadecanol, and 1,12dodecanediol, the branched alkanes 2,2,4,4,6,8,8-heptamethylnonane (HMN) and 2,6,10,15,19,23-hexamethyltetracosane (squalane), and the branched alcohols 2-butyl-1-octanol and 2-hexyl-1-decanol were investigated at or close to saturation conditions at temperatures between 298.15 and 573.15 K. Based on the experimental results for the liquid densities, liquid viscosities, and surface tensions with average expanded uncertainties (k = 2) of 0.061, 2.1, and 2.6%, respectively, the performance of the three commonly employed force fields (FFs) TraPPE, MARTINI, and L-OPLS was assessed in MD simulations. To improve the simulation results for the bestperforming all-atom L-OPLS FF at larger temperatures, a modified version was suggested. This incorporates a temperature dependence for the energy parameters of the Lennard-Jones potential obtained by calibrating only against the experimental liquid density data of n-dodecane. By transferring this approach to all other systems studied, the modified L-OPLS FF shows now a distinctly better representation of the equilibrium and transport properties of the long alkanes and alcohols, especially at high temperatures.

This work is a continuation of previous studies focusing on the influence of intermolecular interactions on the diffusive mass transport in mixtures consisting of liquids with dissolved gases by determining the Fick diffusion coefficient of the mixtures or self-diffusion coefficient of the gas solutes. Dynamic light scattering, Raman spectroscopy, and molecular dynamics simulations are applied to study the interplay between the liquid structure and diffusive mass transport in binary mixtures consisting of methane, neon, krypton, sulfur hexafluoride, and the two refrigerants R143a and R236fa dissolved in n-hexane or 1-hexanol. Experiments and simulations were performed at the macroscopic thermodynamic equilibrium close to infinite dilution of the solute at temperatures between 303 and 423 K. The obtained Fick diffusion coefficients increase with increasing temperature and are always smaller in mixtures based on 1-hexanol compared to those of n-hexane. For both solvents, a decreasing molar mass of the solutes leads to increasing Fick diffusion coefficients with the exception of methane and neon showing the opposite behavior. Next to a general discussion and comparison with the literature, the present diffusivity data are compared with values predicted by a semiempirical model, which was previously developed to predict mass diffusivities in binary mixtures consisting of n-alkanes or 1-alcohols with dissolved gases close to infinite dilution.

The present work reports Fick diffusion coefficients D 11 in the saturated liquid phases of binary mixtures of cyclohexane, n-hexadecane, noctacosane, or n-hexanoic acid and dissolved carbon dioxide (CO 2 ) as a function of composition. D 11 is experimentally determined by dynamic light scattering (DLS) and predicted by equilibrium molecular dynamics (EMD) simulations within or close to saturation conditions at temperatures between 303 and 423 K and CO 2 mole fractions x CO2 up to 0.99. Thermal diffusivity data obtained simultaneously by DLS are reported as well. The mixture composition of the liquid phases is experimentally determined by polarization-difference Raman spectroscopy (PDRS). The combination of PDRS experiments and EMD simulations, where the latter allows a direct insight into or the detection of changes of the fluid structure, is used to investigate the influence of the phase behavior and the resulting fluid structure on D 11 . The behavior of the experimentally determined D 11 and that obtained from EMD simulations by the independent calculation of the Maxwell−Stefan diffusivity and the thermodynamic factor show generally good agreement. Distinct structural changes are reflected by the thermodynamic factors, which are between about 0.35 and 1.4. D 11 as a function of x CO2 at a given temperature is nearly constant in mixtures of n-hexanoic acid and CO 2 due to the opposing kinetic and thermodynamic contributions to D 11 . Pronounced composition-dependent behaviors are observed in the other mixtures due to the proximity to a liquid−liquid miscibility gap or the mixtures' critical line.

Diffusive mass transport in binary mixtures of a liquid solvent and dissolved helium (He) or krypton (Kr) close to infinite dilution is investigated by determining the Fick diffusion coefficient D 11 experimentally with dynamic light scattering (DLS). Equilibrium molecular dynamics (EMD) simulations are used to access the solute self-diffusion coefficient D 1, which is equal to D 11 at the limit of infinite dilution. To address how the molecular characteristics of the solvent influence molecular diffusion, a wide range of different solvents is considered, including n-decane, n-hexadecane, n-octacosane, ethanol, 1-decanol, cyclohexane, benzene, 2-methylpentane, 2,2-dimethylbutane, ethyl butanoate, and n-hexanoic acid. Mixtures are investigated between (303 and 448) K and up to 6.5 MPa. The average expanded experimental uncertainty (k = 2) of D 11 from DLS experiments is 8.6%, and the average expanded statistical uncertainty (k = 2) of D 1 from EMD simulations is 5.8%. Solvent force fields (FF) used in EMD simulations to describe interactions within and between molecules are primarily based on the all-atom optimized potentials for liquid simulation (OPLS) FF, and a temperature-dependent modification developed within our research group is applied. The average absolute relative deviation of the simulated D 1 with respect to the experimental D 11 is 14%. Results from DLS and EMD show that diffusive mass transport in mixtures containing dissolved He for a given solvent is (20–50)% greater than in those with Kr. In comparison to mixtures based on linear alkanes, those based on branched alkanes have larger D 11, while mixtures based on oxygenated and cyclic components have smaller D 11.

In the present study, the influence of dissolved gases on the viscosity and interfacial tension of n-hexadecane is investigated using surface light scattering (SLS) experiments and equilibrium molecular dynamics (EMD) simulations. In detail, binary mixtures of n-hexadecane with the solutes hydrogen, helium, methane, water, nitrogen, carbon monoxide, and carbon dioxide are studied in the temperature range between (298 and 573) K and for two solute mole fractions up to 0.2. With SLS, the liquid viscosity and interfacial tension of the binary mixtures were accessed with average expanded uncertainties (coverage factor k = 2) of 2.3 and 1.9%, respectively. By comparing the thermophysical properties of the binary mixtures with the ones of pure n-hexadecane, the influence of the dissolved gases is discussed. For the two lightest gases hydrogen and helium as well as for the solute water, only a small influence of the dissolved gas could be found. For the other gases, a decrease of up to 30% is found for both the viscosity and interfacial tension. While the results for the interfacial tension of the binary mixtures from EMD simulations agree well with the results from SLS measurements, the simulations fail to predict the influence of the dissolved gas on the viscosity accurately. Finally, the enrichment of the solute molecules at the vapor−liquid interface analyzed using EMD simulations is linked to the interfacial tension results.