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| Home > Publications database > Thermophoresis of charged colloidal spheres and rods |
| Conference Presentation (After Call) | FZJ-2015-05504 |
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2015
Please use a persistent id in citations: http://hdl.handle.net/2128/9150
Abstract: Thermophoresis or Thermal diffusion, which is also known as the Ludwig–Soret effect, is the phenomenon where mass transport is induced by a temperature gradient in a multi-component system. So far there is no microscopic understanding for fluids. In the recent years the « heat of transfer » concept has been successfully applied to non-polar systems, but in aqueous systems the situations is complicated due to charge effects and strong specific cross interactions so that this concept fails.Recently Dhont and Briels [1] calculated the double-layer contribution to the single-particle thermal diffusion coefficient of charged, spherical colloids with arbitrary double-layer thickness. In this approach three forces are taken into account, which contribute to the total thermophoretic force on a charged colloidal sphere due its double layer: i) the force FW that results from the temperature dependence of the internal electrostatic energy W of the double layer, ii) the electric force Fel with which the temperature-induced non-spherically symmetric double-layer potential acts on the surface charges of the colloidal sphere and iii) the solvent-friction force Fsol on the surface of the colloidal sphere due to the solvent flow that is induced in the double layer because of its asymmetry. This concept has successfully been used to describe the Soret coefficient of Ludox particles as function of the Debye length [2] (cf. Fig. 1). The surface charge density of the Ludox particles is independently obtained from electrophoresis measurements, the size of the colloidal particles is obtained from electron microscopy, and the Debye length is calculated from the ion concentration. Therefore the only adjustable parameter in the comparison with theory is the intercept at zero Debye length, which measures the contribution to the Soret coefficient of the solvation layer and possibly the colloid core material.Later the concept was extended for charged colloidal rods [3]. As model system we used the charged, rod-like fd-virus. The wild type fd-virus has a contour length L of 880 nm, a radius R of 3.4 nm, and a persistence length LP of 2.2 µm. The Soret coefficient of the fd-viruses increases monotonically with increasing Debye length (cf. Fig. 1), while there is a relatively weak dependence on the rod-concentration when the ionic strength is kept constant. Additionally to the intercept at zero Debye length we used the surface charge density as an adjustable parameter. Experimentally we found a surface charge density of 0.0500.003 e/nm2, which compares well the surface charge density, of 0.0660.004 e/nm2, which has been determined by electrophoresis measurements taking into account the ion condensation.All experiments so far have been performed with the so-called infrared thermal diffusion forced Rayleigh scattering technique [4], with a writing wavelength of 980 nm, which corresponds to an absorption band of water with an approximate optical density equal to OD=0.5 cm-1. This method uses the refractive index contrast between the different components and is therefore typically limited to binary mixtures. In order to study also biological colloids in buffer solutions we are presently developing a microscopic cell with heated wires. First results for some fluorescently labelled polystyrene lattices in the microwire cell are presented in comparison with thermal diffusion forced Rayleigh scattering measurements. Figure 1: (A) Soret coefficient, ST, as function of the Debye length for Ludox particles and the wild type fd-virus. (B) TEM image of the Ludox particle and (C) the corresponding size distribution. (D) TEM image of the fd-virus.REFERENCES[1] J.K.G. Dhont and W.J. Briels, Eur. Phys. J. E 25, 61(2008).[2] H. Ning, J.K.G. Dhont, and S. Wiegand, Langmuir, 24, 2426(2008).[3] Z. Wang, H. Kriegs, J. Buitenhuis, J.K.G. Dhont, and S. Wiegand, Soft Matter, 9, 8697(2013).[4] S. Wiegand, H. Ning, and H. Kriegs, J. Phys. Chem. B, 111, 14169(2007).
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