Thermophoresis

In the natural sciences, the movement of particles due to a temperature gradient within a fluid is called thermophoresis , thermal diffusion or the Ludwig-Soret effect . The effect is named after the German physiologist Carl Ludwig (1856) and the Swiss physicist and chemist Charles Soret (1879), who described the phenomenon. In most cases the movement is from hot to cold, but depending on the type of particles and fluid, movement to the hotter region is possible.

Thermal diffusion occurs in all substances; this effect can be clearly observed with aerosols and dust particles in the air (see black dust ). It can also be easily observed with simple gas or liquid mixtures, with polymers in solution or colloidal suspensions and also with magnetic fluids . Thermal diffusion is still the subject of current research.

Basics

The effect in gases is explained as follows: On average, air molecules patter uniformly on a dust particle from all sides - statistical fluctuations lead to Brownian motion , but the motion is statistical and undirected. However, if the particle is in a temperature gradient, faster molecules hit on the hot side than on the cold side - the particle experiences a net momentum in the direction of the cold side. The movement is still statistical, but the particle moves towards cold for a long time.

The whole thing is more difficult in liquids, because the theory for gases cannot explain the migration of some large molecules to the heat source. There have already been attempts to explain it by means of flows on the surface of the molecules or by changing the surface energy in different temperature states, but the matter is the subject of current research (2005). Theoretical approaches are based on the work of Lars Onsager and Eli Ruckenstein and of course also on current experimental research results.

Thermal diffusion in solids is less understood than that in liquids.

In a binary mixture (fluid consisting of two components) the development over time of the mole fraction (= molar fraction ) of a component can be described with an extended diffusion equation (applies to the mole fraction and the mole fraction of the second component is ). The first term on the right describes Fick's diffusion, the second the thermal diffusion, which depends on the spatial course of the temperature : ${\displaystyle \chi }$ ${\displaystyle \chi _{1}=\chi \in [0,1]}$ ${\displaystyle \chi _{2}=1-\chi }$ ${\displaystyle T}$

${\displaystyle {\frac {\partial \chi }{\partial t}}=\nabla \cdot (D\,\nabla \chi +D_{T}\,\chi (1-\chi )\,\nabla T)}$

Here is the diffusion coefficient and the thermal diffusion coefficient . The quotient of both coefficients ${\displaystyle D}$ ${\displaystyle D_{T}}$

${\displaystyle S_{T}={\frac {D_{T}}{D}}}$

is called the Soret coefficient. This is a measure of the material separation in the presence of a temperature gradient in the steady state. In general, the Soret coefficient depends on the temperature and the amount of substance.

For a mixture of two gases, the kinetic gas theory can estimate the coefficients and well. On the other hand, no adequate theory exists for liquids, and even the sign of the Soret coefficient cannot be predicted here. This is a problem in statistical thermodynamics to describe the intermolecular interaction in a multicomponent nonequilibrium system. ${\displaystyle D}$ ${\displaystyle D_{T}}$

Applications

Since the thermal diffusion coefficient in most systems is a factor of 10 ² to 10 ³ smaller than the diffusion coefficient for gases, electrolytes and dissolved non-electrolytes, thermophoresis is probably of no particular significance for living beings.

Microscale Thermophoresis. Measurement of the thermophoresis of a fluorescently labeled biomolecule. The normalized fluorescence in the heated laser spot is plotted against time. The IR laser is switched on at time t = 5s and induces a measurable change in fluorescence. The measured decrease in fluorescence is caused by two effects that differ in their relaxation time: the rapid temperature jump (time scale ≈ 3 s) and the thermophoretic change in concentration of the molecules (time scale ≈ 30 s). After switching off the IR laser (t = 35 s), an inverse temperature jump and the back diffusion of the molecules are observed.

Thermophoresis is used in the separation of isotopes in gases. Thus ⁸⁴ Kr and ⁸⁶ Kr or H ³⁷ Cl and H ³⁵ Cl can be separated in a vertical tube that is heated along its axis by means of an electric wire. The process is supported by convection , as the component flowing to the heating wire rises, while the other component, which moves to the colder wall, sinks down at the same time. The interaction of these two processes results in a much more effective separation of the components than would be expected based on the thermal diffusion coefficient alone.

Various dust samplers use thermophoresis. A stream of aerosol sweeps over a slide for a microscope, over which a heated wire is attached. Thermophoresis separates the dust particles from the air flow quantitatively on the slide. Such a device is called a thermal precipitator .

Accumulation ( bioaccumulation ) of DNA molecules in solutions: By cleverly designing a heated liquid chamber, it is possible to enrich DNA up to 1000 times in one spot through the interplay of convection and thermophoresis.

A more recent method, the thermophoresis optically generated ( English microscale thermophoresis ) is, by means of an infrared - laser in a liquid-filled glass capillary generates a defined temperature gradient is microscopic. The molecules in it are initially evenly distributed, but typically move from higher to lower temperatures within seconds.

During the analysis, the analytes are freely in solution. The measurements can also be carried out in any buffers and complex biological fluids and allow measurements under conditions similar to in vivo.

This method is used in the affinity determination between all types of biomolecules including proteins, DNA, and RNA and chemical compounds as well as in the determination of enzyme activities . The determination of the stability as well as the adsorption and aggregation behavior of biomolecules in blood serum and the biochemical investigation of purified proteins is possible.

Origin of Life

Regarding the question of the origin of life : Possibly the first biomolecules arose near hydrothermal springs in the deep sea . Convective mixing in cavities of porous rock and enrichment through thermophoresis may explain the short geological period that life needed to develop.

See also

• Thermo-osmosis

literature

• De Groot, Mazur: Non-Equilibrium Thermodynamics . Dover Publications, 1985, ISBN 0-486-64741-2
• Konstantin Morozov: Soret effect in molecular mixtures. In: Physical Review E. 79, 2009, doi: 10.1103 / PhysRevE.79.031204 .

Web links

• biosystems LMU Munich: Investigation of optically generated thermophoresis for the enrichment of DNA
• When molecules have to concentrate: Optical traps themselves collect nanoparticles in liquids

Individual evidence

1. ↑ Carl Ludwig: Diffusion between unequally heated places of equally composed solutions . Meeting report. Emperor. Akad. Wiss. (Mathem.-Naturwiss. Cl.), Vienna, 65, 1856, p. 539.
2. ^ Charles Soret: Sur l'état d'équilibre que prend, du point de vue de sa concentration, une dissolution saline primitivement homogène, dont deux parties sont portées à des températures différentes. In: Archives de Genève , 3e période, tome II, 1879, p. 48.
3. ^ ^{A b} A. Katchalsky, Peter F. Curran: Nonequilibrium thermodynamics in biophysics . Chapter 13: Systems with temperature gradients , p. 185. Harvard University Press, Cambridge 1965.
4. ^ K. Clusius, G. Dickel: Naturwissenschaften 26, 1938. S. 546.
5. ^ K. Clusius, G. Dickel: Z. Physik. Chem B44, 1939. pp. 397,451.
6. ↑ CJ Wienken et al .: Protein-binding assays in biological liquids using microscale thermophoresis. . In: Nature Communications . 1, 2010. doi : 10.1038 / ncomms1093 .
7. ↑ P. Baaske, C. Wienken, S. Duhr: Optically generated thermophoresis for bioanalytics . In: Biophotonics . 2009, pp. 22-24.
8. ↑ Duhr S, Braun D: Why molecules move along a temperature gradient . In: Proc. Natl. Acad. Sci. USA . 103, No. 52, December 2006, pp. 19678-19682. doi : 10.1073 / pnas.0603873103 . PMID 17164337 . PMC 1750914 (free full text).
9. ↑ P. Baaske, CJ Wienken, P. Reineck, S. Duhr, D. Braun: Optical thermophoresis for quantifying the buffer dependence of aptamer binding. In: Angewandte Chemie (International ed. In English). Volume 49, Number 12, March 2010, ISSN  1521-3773 , pp. 2238-2241, doi: 10.1002 / anie.200903998 , PMID 20186894 .
10. ^ Eugene V. Koonin: An RNA-making reactor for the origin of life. In: PNAS . Volume 104, 2007, pp. 9105-9106, doi: 10.1073 / pnas.0702699104 .