Thermodynamics of interfaces extended to nanoscales

At the nanometer scale, the physical and thermodynamic laws that govern the properties of matter are often changed. Thus, confined water condenses more easily and two miscible liquids may no longer be, or vice versa, in a nanochannel. A researcher from the LCH (CNRS / ENS Lyon / Université Claude Bernard) recently set about extending classical thermodynamic theory to model essential properties such as surface tension, which cannot be measured at this scale. This important theoretical breakthrough appeared in the journal PNAS and should make it possible to optimize many systems used in nanotechnology.

The drops of water on a water lily, the bubbles of champagne or the water rising through a straw are all external manifestations of a physicochemical magnitude called surface tension. Studied by Pierre-Simon de Laplace or Thomas Young from the beginning of the 19th century, it is to Josiah Willard Gibbs that we owe the thermodynamic laws that describe the surface tension of matter, never questioned until now. In the age of microfluidics where liquids flow and react in channels the size of a hair, nanoporous materials where the exchange surfaces are larger than the material itself, or even nanomedicine which transports nanomedicines encapsulated in nanospheres, there are many applications where matter is found confined in geometries close to the size of the molecules themselves. At this scale, the role played by the surface becomes preponderant and the classical thermodynamic laws which made it possible to describe the macroscopic system are no longer necessarily valid. What is more, the direct measurement of quantities such as surface tension is often not possible, although knowledge or modeling would allow the properties of many nanosystems to be optimized.

In a recent study, Wei DONG, a researcher at the Chemistry Laboratory (CNRS / ENS Lyon / Université Claude Bernard), therefore set out to extend Gibbs' theory to take into account the predominant surface effects at the nanometric scale. His theoretical work shows that it is then necessary to introduce new concepts, such as two distinct surface tensions, one differential and the other integral, instead of just one as in Gibbs theory. This theoretical breakthrough was recently published in the journal PNAS and should provide a better understanding of the behavior of nanoscale systems. In addition, the measurement of macroscopically accessible quantities, such as the integral absorption (notion associated with the integral surface tension) of matter in a nanoporous system for example, should make it possible to deduce the differential surface tension of this system thanks to the proposed model. Future research will now focus on its experimental verification of this model.