Hartmut Kriegs

Thermal diffusion of a stiff rod-like mutant Y21M fd-virus.

We investigated the thermal diffusion phenomena of a rodlike mutant filamentous fd-Y21M virus in the isotropic phase by means of an improved infrared thermal-diffusion-forced Rayleigh scattering (IR-TDFRS) setup optimized for measurements of slowly diffusing systems. Because this is the first thermal diffusion study of a stiff anisotropic solute, we investigate the influence of the shape anisotropy on the thermal diffusion behavior. The influence of temperature, fd-Y21M concentration, and ionic strength in relation with the thermodiffusion properties is discussed. We characterize and eliminate the effect of these parameters on the absolute diffusion of the rods and show that diffusion determines the behavior of the Soret coefficient because the thermal diffusion coefficient is constant in the investigated regime. Our results indicate that for the thermal diffusion behavior structural changes of the surrounding water are more important than structural changes between the charged macroions. In the investigated temperature and concentration range, the fd-Y21M virus is thermophobic for the low salt content, whereas the solutions with the high salt content change from thermophobic to thermophilic behavior with decreasing temperature. A comparison with recent measurements of other charged soft and biological matter systems shows that the shape anisotropy of the fd-virus becomes not visible in the results.

Thermal diffusion of oligosaccharide solutions: the role of chain length and structure.

We investigated the chain length dependence of the thermodiffusion behavior of oligosaccharides by the infrared thermal diffusion forced Rayleigh scattering (IR-TDFRS) technique. Three disaccharides, sucrose, cellobiose and maltose, two trisaccharides, melezitose and raffinose, and a tetrasaccharide, stachyose, have been studied. We determined the thermal diffusion (D(T)), mass diffusion (D), and Soret (S(T)) coefficient as a function of temperature and concentration. While monosaccharides in water accumulate at the cold side in the investigated temperature (20-50 degrees C) and concentration (0.2-0.6667 wt) range, oligosaccharides enrich on the warm side with decreasing temperature or increasing sugar concentration. Additionally, we determined the kinematic viscosity (nu), the density (rho), and the thermal expansion coefficient (alpha) of the mixtures in order to check the linear correlation between D(T) and the ratio alpha/nu, which has been recently found for aqueous solutions of monosaccharides and for alkane mixtures. Finally, we found that D(T) and D decay with increasing chain length of the oligosaccharides in the whole studied range of temperatures, in contradiction with experimental results for nonpolar mixtures and theoretical predictions.

Thermophoresis of charged colloidal rods

The thermal diffusion behavior of dilute solutions of very long and thin, charged colloidal rods (fd-virus particles) is studied using a holographic grating technique. The Soret coefficient of the charged colloids is measured as a function of the Debye screening length, as well as the rod-concentration. The Soret coefficient of the fd-viruses increases monotonically with increasing Debye length, while there is a relatively weak dependence on the rod-concentration when the ionic strength is kept constant. An existing theory for thermal diffusion of charged spheres is extended to describe the thermal diffusion of long and thin charged rods, leading to an expression for the Soret coefficient in terms of the Debye length, the rod-core dimensions, and the surface charge density. The thermal diffusion coefficient of a charged colloidal rod is shown to be accurately represented, for arbitrary Debye lengths, by a superposition of spherical beads with the same diameter of the rod and the same surface charge density. The experimental Soret coefficients are compared with this and other theories, and are contrasted against the thermal diffusion behaviour of charged colloidal spheres.

Development of a thermogravitational microcolumn with an interferometric contactless detection system.

We present a new type of thermogravitational (TG) column, a so-called TG microcolumn with transparent windows and a very small sample volume of less than 50 μL. The TG microcolumn has a planar geometry with a thickness of 0.523 ± 0.004 mm, a height of 30 mm, and a width of 3 mm. The concentration difference between two points at different heights is measured with an interferometer using active phase control. From the concentration difference we can determine the thermal diffusion coefficient, D(T), using the refractive index variation with concentration, which has to be determined independently. We studied the three binary mixtures of dodecane, isobutylbenzene, and 1,2,3,4-tetrahydronaphthalene with a concentration of 50 wt % at a temperature of 298 K. The thermal diffusion coefficients agree within a few percent with the proposed benchmark values. In addition we investigated also the binary mixture toluene/n-hexane and compare the results with literature values. For the investigated mixtures the typical measurement times were between 30 min and 2 h with an applied temperature difference of ΔT = 6 K.

Flow Velocity Profiles and Shear Banding Onset in a Semidilute Wormlike Micellar System under Couette Flow

Velocity profiles in Couette flow are measured in a wormlike micellar solution made of cetyltrimethylammonium bromide (CTAB), sodium salicylate (NaSal), and water, at R (= [NaSal]/[CTAB]) = 2 and at R = 4; [CTAB] = 100 mM. Velocity profiles were obtained by using a two-incident beam laser Doppler technique. Profiles reveal that one of the micellar solutions (R = 2) becomes heterogeneous a long time after flow inception, even at very low imposed shear rates. However, profiles do not correspond to what is expected for gradient shear banding, because the fluid splits in one section close to the moving cylinder where the local mean velocity depends linearly on the gap position and in a second section presenting important velocity fluctuations. Close to the static cylinder, there is a third section where the fluid does not flow; it behaves like a slipping block. On the other hand, at high imposed shear rates, the former slipping block flows and presents a linear profile. Here, velocity profiles are consistent with gradient shear banding. The onset of shear banding was observed. The picture of two stable shear bands separated by a thin steady interface is not always valid. Inhomogeneous flow could be observed, although it cannot be classified as shear banding. In addition, conditions can be found where, as shear rate is increased and before shear banding appears, instead of a thin interface, a fluctuating intermediate band can be observed. On the contrary, for the R = 4 solution, the flow never becomes heterogeneous even at high shear rates. Flow curves were measured in a Couette cell under shear rate control in two cases, when stress is sensed with the moving cylinder and when it is sensed with the static cylinder of the cell. Differences between the flow curves can be explained by using the velocity profiles for both solutions.

Influence of temperature and charge effects on thermophoresis of polystyrene beads

Abstract.We study the thermodiffusion behavior of spherical polystyrene beads with a diameter of 25 nm by infrared thermal diffusion Forced Rayleigh Scattering (IR-TDFRS). Similar beads were used to investigate the radial dependence of the Soret coefficient by different authors. While Duhr and Braun (Proc. Natl. Acad. Sci. U.S.A. 104, 9346 (2007)) observed a quadratic radial dependence Braibanti et al. (Phys. Rev. Lett. 100, 108303 (2008)) found a linear radial dependence of the Soret coefficient. We demonstrated that special care needs to be taken to obtain reliable thermophoretic data, because the measurements are very sensitive to surface properties. The colloidal particles were characterized by transmission electron microscopy and dynamic light scattering (DLS) experiments were performed. We carried out systematic thermophoretic measurements as a function of temperature, buffer and surfactant concentration. The temperature dependence was analyzed using an empirical formula. To describe the Debye length dependence we used a theoretical model by Dhont. The resulting surface charge density is in agreement with previous literature results. Finally, we analyze the dependence of the Soret coefficient on the concentration of the anionic surfactant sodium dodecyl sulfate (SDS), applying an empirical thermodynamic approach accounting for chemical contributions.Graphical abstract

Peptide model systems: Correlation between thermophilicity and hydrophilicity

In recent years, the response of biomolecules to a temperature gradient has been utilized to monitor reactions of biomolecules, but the underlying mechanism is not well understood due to the complexity of the multicomponent system. To identify some underlying principles, we investigate the thermal diffusion of small amide molecules in water systematically. We re-analyze previous measurements of urea and formamide and compare the results with acetamide, N-methylformamide, and N,N-dimethylformamide, amides with a lower hydrophilicity. It turns out that less hydrophilic substances do not show the typical temperature dependence of water soluble macromolecules. Analyzing temperature and concentration dependent measurements using an empirical expression originally derived for nonpolar mixtures, we find that the so-called isotope contribution depends strongly on the hydrophilicity of the solute. This can be qualitatively understood by comparing with molecular dynamic simulations of Lennard-Jones fluids. The hydrophobic/hydrophilic balance also influences the structure in the fluid and with that the thermal expansion coefficient, which correlates with the thermal diffusion coefficient. Furthermore, we observe a clear correlation of the temperature and concentration dependence of the Soret coefficient with the hydrophilicity, which can be quantitatively described by the partition coefficient log P.