"Concentration-in-Control" self-assembly concept at the liquid-solid interface challenged.

Area of Science:

• Surface science
• Supramolecular chemistry
• Materials science

Background:

• Self-assembled molecular networks (SAMNs) are crucial for fundamental research and applications.
• Controlling SAMN polymorphs is typically achieved by adjusting molecular building block concentration at liquid-solid interfaces.
• Solute concentration significantly influences self-assembly, often leading to high-density networks at high concentrations and low-density at low concentrations.

Purpose of the Study:

• To investigate an atypical concentration-dependent self-assembly behavior at a solution-solid interface.
• To explore the self-assembly of heptanoic acid (HA) on highly oriented pyrolytic graphite (HOPG).
• To understand the underlying mechanisms of inverse concentration-dependent self-assembly.

Main Methods:

• Scanning Tunneling Microscopy (STM) to visualize molecular network formation.
• UV-vis spectroscopy to detect preaggregation of solute molecules.
• Analysis of molecular density in self-assembled monolayers.

Main Results:

• Observed formation of a low-density porous network at high HA concentrations on HOPG.
• Observed formation of a high-density compact network at low HA concentrations on HOPG.
• Evidence of solute molecule preaggregation in HA solution, explaining the inverse behavior.

Conclusions:

• The study demonstrates an inverse concentration-dependent self-assembly behavior, contrary to conventional models.
• Preaggregation of solute molecules in solution is identified as the key factor driving this atypical self-assembly.
• Results provide insights into controlling molecular packing density in self-assembled monolayers.