To mix or not to mix: 2D crystallization and mixing behavior of saturated and unsaturated aliphatic primary amides

Physisorbed monolayers based on relatively weak noncovalent interactions can serve as excellent model systems for understanding crystallization of materials in reduced dimensionality. Here we employ a powerful combination of scanning tunneling microscopy (STM), differential scanning calorimetry (DSC), and computational modeling to reveal two-dimensional (2D) crystallization and mixing behavior of saturated and unsaturated (cis as well as trans) aliphatic primary amides. The foundation of the present work is laid by DSC measurements, which reveal characteristic adsorption and mixing behavior of aliphatic amides. These results are further supported by STM visualization of the adlayers. STM reveals, at submolecular resolution, the adsorption as well as the two-component 2D phase behavior of these molecules at the liquid-solid interface. The saturated and trans-unsaturated amides exhibit random mixing in view of their size and shape complementarity. Binary mixtures of saturated and cis-unsaturated amides, on the other hand, display unprecedented mixing behavior. The linear saturated and bent cis-unsaturated amide molecules are found to mix surprisingly better at the liquid-solid interface than might have been expected on account of the dissimilarity in their shapes. Strong, directional intermolecular hydrogen-bonding interactions as well as the relative stabilization energies of the adlayers are responsible for such unusual mixing behavior. Computational modeling provides additional insight into all the possible interactions in 2D assemblies and their impact on stabilization energies of the supramolecular networks. This study provides a model for understanding the effect of nanoscale cocrystallization on the thin film structure at interfaces and demonstrates the importance of molecular geometry and hydrogen bonding in determining the coadsorption behavior.