Nucleation of crystalline films at liquid-solid interfaces is widespread in both natural and synthetic systems. Observing this process is challenging, because it is a consequence of unstable density fluctuations, making the structures and events that must be probed both transient in nature and small in extent. Thus, the application of high-speed, molecularly resolved AFM to systems that assemble on surfaces provides a powerful method to quantify the dynamics and extract the underlying physics. Here we illustrate the fundamental insights into the pathways and energetics of nucleation and self-assembly made possible by this method using results from two contrasting systems: gibbsite (Al(OH)3) films grown on muscovite mica (001) and 2D arrays of peptides that self-assemble on both MoS2 (0001) and graphite. The results on gibbsite show that a dynamic population of clusters is created by continuous attachment and detachment of adsorbed ions, consistent with the basic concept of classical nucleation theory (CNT). However, severe discrepancies emerge when the values of key thermodynamic parameters are extracted from the data. Comparison to kinetic Monte Carlo simulations shows that electrostatic interactions between positively charged aluminum hydroxide clusters and the negatively charged mica is the source of these discrepancies, modifying both the chemical potential of the accumulating ions and the edge tension of the clusters. These interactions stabilize a nanostructured surface mesophase, which evolves gradually via a process of cluster growth and coalescence not limited by formation of a critically sized nucleus. The results for peptides on MoS2 show that the high anisotropy of inter-peptide interactions drives 2D assembly to occur row-by-row, starting with 1D nuclei. In agreement with expectations of CNT, these 1D nuclei form without a free energy barrier and there is no critical size. However, on graphite, two new phenomena emerge. First, peptide rows become highly mobile, enabling to a molecular level form of growth by oriented attachment. Second, appearance of the final phase is preceded by formation of a metastable phase, which dissolves as the stable phase nucleates. This manifestation of Ostwald’s Rule of Stages cannot be rationalized with the usual thermodynamic arguments due to the barrier-free nature of 1D nucleation. Instead, we propose a rationale rooted in chemical kinetics and applied here to peptide dimer formation at the surface.

Nucleation and self-assembly at solid-liquid interfaces

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.