Measuring entropy

Chemical reactions, especially in biological systems, oftentimes involve macromolecules changing their shape – their “configuration” – for instance, by rotation or translational movements. To study what drives or impedes molecular mobility in more detail chemists and physicists turn to simplified model systems such as individual molecules adhering to a surface. These can then be investigated at temperatures just a few degrees above absolute zero (-273 degrees Celsius) using, for instance, a scanning tunneling microscope (STM), which can probe numerous physical properties of surfaces at the atomic level.

A well-known molecule for this kind of studies is dibutyl sulfide (DBS), a lengthy hydrocarbon with a central Sulphur atom, through which the molecule can be absorbed (attached) to a gold surface. Depending on the temperature the two “arms” rotate more or less easily about the central Sulphur “axis”. There are two physical parameters that are typically used to describe how free to move a molecule on a surface is: the energy barrier it has to overcome to carry out the movement in question – for chemical reactions this barrier is called activation energy –, and the attempt rate, which one can picture as the number of attempts made by the molecule to initiate the movement. And the higher the temperature, the more the two DBS arms rotate (because, at higher temperatures, they are more likely to overcome the energy barrier).

And not only that: when Gehrig and Penedo plotted the molecule’s attempt rates they obtained a map that looked almost identical to the one with the energy barriers. “This is when I thought: 'Hang on a second, nature is trying to tell us something',” recalls Hug. “Entropy is often thought of as a measure of disorder or randomness, but here it is determined by the number of shapes that the molecule could potentially take, as well as by the number of different ways that the molecule could meet the energy requirements to change its configuration,” explains Eric Hudson, associate professor of physics at PSU. “If the tip of the STM increases the energy required by the molecule to make a change in shape, it is also increasing entropy in the system. In essence, a transition requires a potentially large number of small-energy excitations to co-occur to overcome the energy barrier for a configuration change. The larger the number of excitations required, the more ways in which those excitations may be collected. This multiplicity gives rise to entropy.”

The research team was interested in understanding what drives a molecule’s ability to make changes to its shape — a common requirement of chemical reactions and biological processes., The results of the Empa-PSU team imply that entropy plays a decisive role for the dynamics of the molecule even at very low temperatures where a molecule’s degree of freedom (and thus its “configurational” entropy) is usually significantly reduced and entropy is considered to only play a minor role. “Although entropy is well understood in thermodynamics, it remains more difficult to grasp than other physical quantities”, admits Hug, “perhaps because it is not so much a “property” as a measure of information.” And maybe also because we tend to have a strong association between entropy and chaos – or the “dark side” of entropy –, be it in the kids’ room or on one’s desk.