Synopsis

Why Heat Moves Molecules in Solution

Physics 7, s57
Researchers have developed a complete theory for thermophoresis—the process by which molecules in a liquid move under the influence of temperature—and tested it under a wide range of conditions.
M. Reichl et al., Phys. Rev. Lett. (2014)

Charged molecules in water move from warmer to colder regions, an effect called thermophoresis that is part of a technique that allows researchers to monitor the binding of candidate drugs to proteins. Despite the technique’s popularity, no one has come up with a complete theory of thermophoresis. Now, Dieter Braun of the Ludwig Maximilian University of Munich, Germany, and colleagues have developed a theory that includes previously neglected contributions to the effect, as they report in Physical Review Letters.

The new theory builds on the so-called capacitor model, previously developed by Braun and other colleagues, in which a charged macromolecule such as a wadded-up DNA strand acts like a spherical capacitor that drifts toward cooler regions to reduce its electric field energy. But the capacitor model alone could not account for some experiments. Braun and his colleagues have now added three additional contributions to their thermophoresis theory and tested it under a wider range of conditions. The first new contribution results from a difference in the temperature-induced motions of positive and negative ions in the solution, which leads to a charge imbalance that generates a weak electric field. The second contribution represents the weak dependence of the diffusion constant on temperature, and the third contribution comprises various non-ionic effects, which the researchers model with a simple empirical formula.

The team measured thermophoresis of fluorescently labeled DNA and RNA strands of various lengths and with a range of ions in the solution. Their model correctly predicted the temperature and concentration dependence of the effect, including nontrivial phenomena, such as a peak in the electrophoresis temperature dependence. – David Ehrenstein


Subject Areas

Biological PhysicsPhysical Chemistry

Related Articles

Reversing Flow in Charged Membranes
Chemical Physics

Reversing Flow in Charged Membranes

The process of osmosis is predicted—under certain conditions—to act in the opposite direction within charged membranes. Read More »

A Less Invasive Approach to Rheology Measurements
Biological Physics

A Less Invasive Approach to Rheology Measurements

Researchers have demonstrated a method of probing a soft material’s properties that could allow them to capture those properties more accurately and for smaller systems than current methods. Read More »

Bacteria That Shove Harder, Move Further
Biological Physics

Bacteria That Shove Harder, Move Further

Simulations show that the harder bacteria in a swarm push against one another, the more likely they are to go on long “walks.” Read More »

More Articles