J. Am. Chem. Soc. 135, 14854–14862 (2013)

Credit: © 2013 ACS

Understanding the self-assembly of small molecules into predictable patterns on surfaces — particularly at the liquid/solid interface — is important in the bottom-up synthesis of functional nanostructures. That these processes occur spontaneously reveals that the associated overall free-energy change is positive. There are, however, a large number of factors to the overall process, both enthalpic and entropic in nature, all competing with each other, and understanding their subtle balance is challenging. Now, Markus Lackinger from the Technical University of Munich and co-workers have created a Born–Haber cycle that allows them to assess the enthalpic contributions.

Lackinger and co-workers decided to apply their process to the widely studied assembly of terephthalic acid on a graphite surface from a solution of nonanoic acid. The overall enthalpy change, which is associated with molecules in solution assembling on the surface in a solvated monolayer, is difficult to measure directly. The Born–Haber approach avoids this by combining several more easily measured enthalpies: those of substrate dissolution, sublimation and deposition from a vacuum in an unsolvated monolayer. Each of these values could be assessed experimentally and they compared well with values obtained by molecular mechanics and molecular dynamics simulations. A further important contribution in the overall process is dewetting of the surface — where molecules of the solvent need to make way for the substrate — and this had to be evaluated solely through simulations.

Several surprising conclusions arise from this treatment of self-assembly thermodynamics, and they can begin to explain observed phenomena. For example, the enthalpy barrier to desorption into solution is only a small fraction of that in a vacuum — a fact that explains the mobility of large substrates in a solvated monolayer. At the same time as reducing the enthalpic benefit of self-assembly, however, solvation also reduces the entropic penalty: solvent molecules removed from the surface in the process of dewetting gain entropy, and interactions between solvent and substrate in solution mean that the solution state is not as disordered as it may first seem.