Simone Wiegand

Thermal nonequilibrium phenomena in fluid mixtures

Concepts, Theory, and Computer Simulations.- Computer Simulations of Thermal Diffusion in Binary Fluid Mixtures.- Thermodiffusion in the Critical Region.- On the Theory of the Soret Effect in Colloids.- Principle of Entropy Maximization for Nonequilibrium Steady States.- A Comprehensive Theory of the Soret Effect in a Multicomponent Mixture.- Thermodiffusion and Nonlinear Heat Equation.- Nonequilibrium Concentration Fluctuations in Binary Liquid Systems Induced by the Soret Effect.- Thermodiffusion in Ionic Solids -Model Experiments and Theory.- Hip, Hip, Soret!.- Experimental Techniques and Special Systems.- Measurement of Transport Coefficients by an Optical Grating Technique.- A Survey of the Thomaes Flow Cell Method for the Soret Coefficient.- Validity Limits of the FJO Thermogravitational Column Theory.- Determination of Thermodiffusion Parameters from Thermal Field-Flow Fractionation Retention Data.- Thermodiffusion of Polymer Solutions in Convectionless Cells.- Convection and Porous Media.- Soret Effect and Free Convection: A Way to Measure Soret Coefficients.- Convective Patterns in Binary Fluid Mixtures with Positive Separation Ratios.- Convective Instability of Magnetized Ferrofluids: Influence of Magnetophoresis and Soret Effect.- On the Soret-Driven Thermosolutal Convection in a Vibrational Field of Arbitrary Frequency.- Thermodiffusion in Porous Media and Its Consequences.- Soret Effect and Mixed Convection in Porous Media.- Soret Effect in Multicomponent Flow Through Porous Media: Local Study and Upscaling Process.

Benchmark values for the Soret, thermal diffusion and diffusion coefficients of three binary organic liquid mixtures

With the aim of providing reliable benchmark values, we have measured the Soret, diffusion and thermal diffusion coefficients of the three binary mixtures of dodecane, isobutylbenzene and 1,2,3,4 tetrahydronaphthalene for a concentration of 50 wt% at a temperature of 25C. The experimental techniques applied by the five participating laboratories are transient holographic gratings, annular and parallelepipedic thermogravitational columns, and vertical parallelepipedic columns with velocity amplitude determination by laser doppler velocimetry. The systems have also been studied in a annular thermogravitational column filled with a porous medium in the gap. There is a good agreement between the different experiments with deviations of the order of a few per cent in most cases (8.5% at most). The numerical values are tabulated in the paper.

Sign change of the Soret coefficient of poly(ethylene oxide) in water/ethanol mixtures observed by thermal diffusion forced Rayleigh scattering

Soret coefficients of the ternary system of poly(ethylene oxide) in mixed water/ethanol solvent were measured over a wide solvent composition range by means of thermal diffusion forced Rayleigh scattering. The Soret coefficient S(T) of the polymer was found to change sign as the water content of the solvent increases with the sign change taking place at a water mass fraction of 0.83 at a temperature of 22 degrees C. For high water concentrations, the value of S(T) of poly(ethylene oxide) is positive, i.e., the polymer migrates to the cooler regions of the fluid, as is typical for polymers in good solvents. For low water content, on the other hand, the Soret coefficient of the polymer is negative, i.e., the polymer migrates to the warmer regions of the fluid. Measurements for two different polymer concentrations showed a larger magnitude of the Soret coefficient for the smaller polymer concentration. The temperature dependence of the Soret coefficient was investigated for water-rich polymer solutions and revealed a sign change from negative to positive as the temperature is increased. Thermodiffusion experiments were also performed on the binary mixture water/ethanol. For the binary mixtures, the Soret coefficient of water was observed to change sign at a water mass fraction of 0.71. This is in agreement with experimental results from the literature. Our results show that specific interactions (hydrogen bonds) between solvent molecules and between polymer and solvent molecules play an important role in thermodiffusion for this system.

How does the chain extension of poly (acrylic acid) scale in aqueous solution? A combined study with light scattering and computer simulation

This work addresses the question of the scaling behavior of polyelectrolytes in solution for a realistic prototype. We show results of a combined experimental (light scattering) and theoretical (computer simulations) investigation of structural properties of the sodium salt of poly (acrylic acid) (Na-PAA). Experimentally, we determined the molecular weight (MW) and the hydrodynamic radius (RH) by static and dynamic light scattering for six different Na-PAA samples in aqueous NaCl-containing solution (0.1–1 mol/l) of polydispersity DP between 1.5 and 1.8. On the computational side, three different variants of a newly developed mesoscopic force field for Na-PAA were employed to determine RH for monodisperse systems of the same MW as in the experiments. The force field effectively incorporates atomistic information and one coarse-grained bead corresponds to one PAA monomer. We find that RH matches with the experimental data for all investigated samples. The effective scaling exponent for RH is found to be ar...

Soret and mass diffusion measurements and molecular dynamics simulations of n-pentane–n-decane mixtures

Soret coefficients and mass diffusion coefficients of three states of the n-pentane–n-decane mixture have been measured by thermal diffusion forced Rayleigh scattering (TDFRS) and are compared with molecular dynamics simulations values. Both equilibrium (EMD), synthetic (S-NEMD), and boundary driven (BD-NEMD) nonequilibrium techniques have been applied to compute the phenomenological and the transport coefficients relevant to the Soret effect. It is found that statistical error on cross-coefficients using equilibrium and dynamical S-NEMD is too high to enable any comparison with experiments, whereas stationary S-NEMD and BD-NEMD methods have statistical error less than ≈35%. S-NEMD simulations have been carried out in the center-of-mass reference frame and the resulting transport coefficients transformed to the center-of-volume frame of reference. The mass diffusion coefficients are sensibly affected by this transformation and show the same weight fraction dependence as the experimental value, although a difference of roughly a factor of 1.4 is found. The Soret coefficients are, as expected, unaffected by the frame of reference transformation and a good agreement between experiment and simulations is found.

The liquid-liquid phase transition in ionic solutions: Coexistence curves of tetra-n-butylammonium pricrate in alkyl alcohols

We report coexistence curves of the liquid–liquid phase transition in ionic solutions. The phase diagrams of tetra-n-butylammonium pricrate solutions in a series of alkanols (2-propanol, 1-decanol, 1-dodecanol, 1-tridecanol and 1-tetradecanol) are determined either by measuring the refractive index in the two phase region in one sample of near-critical composition as a function of the temperature, or by direct observation of the composition dependent phase separation temperatures. With the exception of the 2-propanol system, the critical points are in accordance with the predictions by the restricted primitive model. The coexistence curves are analyzed in terms of different composition variables, of which the volume fraction seems to be the most appropriate one. For the volume fraction, deviations from asymptotic Ising behavior are observed which are equally well described by a critical exponent slightly different from the Ising value or by Wegner corrections. Although the deviations are quite small, they...

Near‐critical light scattering of an ionic fluid with liquid–liquid phase transition

We report on the first static and dynamic light scattering measurements of near‐critical behavior of an ionic fluid. The model system tetra‐n‐butylammonium picrate‐tridecanol exhibits a liquid–liquid phase transition with an upper critical point near 342 K. The correlation lengths and scattering intensities scale with temperature corresponding to mean‐field behavior. The data are also concordant with nonclassical 3D‐Ising exponents if suitable Wegner corrections are applied. The spherical model recently suggested to apply for ionic fluids can be ruled out, thus resolving a current controversy in the literature.

Negative thermodiffusion of polymers and colloids in solvent mixtures

Results on thermodiffusion of poly(ethylene oxide) and colloidal boehmite (γ-AlOOH) rods in ethanol/water mixtures are presented. Data were obtained using thermal diffusion forced Rayleigh scattering. The sign of the Soret coefficient of the boehmite rods changes from positive to negative with increasing water content, i.e., at sufficiently high water content the colloidal particles move to higher temperatures. The sign of the Soret coefficient of the poly(ethylene oxide) in ethanol/water mixtures is negative, i.e., the poly(ethylene oxide) molecules move to higher temperatures, whereas in pure water the sign is positive. To our knowledge this is the first time that a sign change has been observed for polymers in solution. The analysis of the static light scattering on poly(ethylene oxide) allows the determination of the preferentially solvating solvent. In the investigated concentration range the preferentially solvating solvent is ethanol, in spite of being the poorer solvent for poly(ethylene oxide).

Thermal diffusion and molecular diffusion values for some alkane mixtures: a comparison between thermogravitational column and thermal diffusion forced rayleigh scattering.

In the present work we studied the thermal diffusion behavior of n-decane in various alkanes by thermogravitational column (TC) technique and the thermal diffusion forced Rayleigh scattering (TDFRS) method. The investigated lighter alkanes compared to n-decane are n-pentane, n-hexane, n-heptane, n-octane, and the heavier ones are n-tetradecane, n-pentadecane, n-hexadecane, n-heptadecane, n-octadecane, and n-eicosane. The binary mixture n-decane/ n-pentane we investigated at several different concentrations; all other mixtures were only investigated at a mass fraction of 50%. Even for the volatile n-pentane/ n-decane mixture the deviations between the thermal diffusion coefficients determined by the different methods agreed within the error bars. Typically the agreement between the two methods was in the order of 5%. In comparison to recently published TC and TDFRS data we found deviations in the order of 30% up to 40%. We analyze and discuss the possible reasons for the discrepancies for the present and the past publications.

Thermal-diffusive behavior of a dilute solution of charged colloids.

Thermal diffusion of a dilute solution of charged silica colloidal particles (Ludox) is studied by a holographic grating technique. The Soret coefficient of the charged colloids is measured as a function of the Debye screening length and the surface charge density of the colloids. The latter is varied by means of variation of the pH. The experimental Soret coefficients are compared with several theoretical predictions. The surface charge density is independently obtained from electrophoresis measurements, the size of the colloidal particles is obtained from electron microscopy, and the Debye length is calculated from ion concentrations. The only adjustable parameter in the comparison with theory is therefore the intercept at zero Debye length, which measures the contribution to the Soret coefficient of the solvation layer and possibly the colloid core material.