Thermophoresis

As thermophoresis, thermal diffusion or the Ludwig- Soret effect the movement of particles due to a temperature gradient is called within a fluid in the natural sciences. Named is the effect of the German physiologist Carl Ludwig (1856 ) and the Swiss physicist and chemist Charles Soret (1879 ), which described the phenomenon. In most cases, the movement is from hot to cold, however, depends on the nature of the particles and the fluid, and a movement to the hotter region possible.

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

Basics

Explains the effect in gases as follows: On a dust particle bombarded from all sides in the Middle evenly air molecules - statistical fluctuations lead to Brownian motion, but the motion is random and undirected. However, if the particle is in a temperature gradient to make faster on the hot side than on the cold molecules - that is the particle experiences a net momentum in the direction of the cold side. The movement is still statistically, but the particle moves over long times towards cold.

In liquids, the whole is more difficult, because the theory of gases can not explain the migration of some large molecules to the heat source. There are already attempts at explanation by currents on the surface of the molecules or by changing the surface energy in different temperature conditions, but the matter is the subject of current research ( 2005). Theoretical approaches are based on the works of Lars Onsager and Eli Ruckenstein and of course on recent experimental research results.

Thermal diffusion in solids is again less understood than those in liquids.

In a binary mixture ( fluid consisting of two components ), the temporal evolution of the mole fraction ( = mole fraction ) describes a component with an extended diffusion equation (valid for the mole fraction and the mole fraction of the second component ). The first term on the right hand side describes the Fick's diffusion, the second thermal diffusion, which depends on the spatial variation of the temperature:

Here, the diffusion coefficient and the thermal diffusion coefficient. The quotient of the two coefficients

Called Soret coefficient. This is a measure of the separation of substances in the presence of a temperature gradient in the stationary state. In general, the Soret coefficient on the temperature and the mole fraction depends.

For a mixture of two gases, the kinetic theory of gases and can estimate the coefficients well. In contrast, there is no adequate theory for liquids, even the sign of the Soret coefficient can not predict here. This is a problem of statistical thermodynamics to describe the intermolecular interaction in a multi-component non-equilibrium system.

Applications

Since the thermal diffusion coefficient in most systems by a factor of 102 to 103 is smaller than the diffusion coefficient of gases dissolved electrolytes and non-electrolytes, thermophoresis probably not of particular importance to living organisms.

Applies the thermophoresis in the separation of isotopes in gases. Thus 84Kr and 86Kr or H37Cl H35Cl and in a vertical tube, which is heated by means of an electric wire along its axis, to be separated. The process will be supported in by convection, as the filament flowing component ascends while the other, which moves to the colder wall at the same time drops down. Through the interplay of these two processes results in a much more effective separation of the components, than would be expected solely on the basis of the thermal Diffusionskoeffizieten.

Different dust samplers using thermophoresis. An aerosol flow passes over a slide glass for a microscope, on which a heated wire is placed. Thermophoresis separates dust particles from the air stream quantitatively to the slide. Such a device is called thermal precipitator.

Accumulation ( bioaccumulation) of DNA molecules in solutions: Through clever design of a heated fluid chamber, it is possible to enrich by interaction of convection and thermophoresis DNA on a spot up to 1000 times.

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

In the analysis, the analytes are free in solution. The measurements can also be carried out in any buffers and complex biological fluids and allow for the measurement of in vivo -like conditions.

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

Origin of life

On the question of the origin of life: Perhaps the first biomolecules originated in the vicinity of hydrothermal vents in the deep sea. By convective mixing in hollow porous rock and enrichment by thermophoresis can the geologically short period of time it took for life to the emergence, possibly explain.