Measuring entropy

A scanning tunneling microscope (STM), used to study changes in the shape of a single molecule at the atomic scale, impacts the ability of that molecule to make these changes; the entropy of the molecule is changed and, in turn, can be measured. The study led by Empa physicists is published in the current issue of "Nature Communications". 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'.