From Monomers to Self-Assembled Monolayers: The Evolution of Molecular Mobility with Structural Confinements

TL;DR

This study investigates how structural confinement affects molecular mobility across monomers, oligomers, and self-assembled monolayers, revealing hindered motions in ordered structures and unique relaxation behaviors in SAMs.

Contribution

It provides new insights into molecular dynamics in SAMs, including the first observation of independent relaxation of polar endgroups and the impact of endgroup bulkiness on monolayer organization.

Findings

Monomers show diverse relaxation processes.

Oligomers and SAMs exhibit hindered local fluctuations.

Polar endgroups in SAMs relax independently, influenced by electrostatic interactions.

Abstract

The effect of structural constriction on molecular mobility is investigated by broadband dielectric spectroscopy (BDS) within three types of molecular arrangements: monomers, oligomers and self-assembled monolayers (SAMs). While disordered monomers exhibit a variety of cooperative and local relaxation processes, the constrained nanodomains of oligomers and highly ordered structure of monolayers exhibit much hindered local molecular fluctuations. Particularly, in SAMs, motions of the silane headgroups are totally prevented whereas the polar endgroups forming the monolayer canopy show only one cooperative relaxation process. This latter molecular fluctuation is, for the first time, observed independently from other overlapping dielectric signals. Numerous electrostatic interactions among those dipolar endgroups are responsible for the strong cooperativity and heterogeneity of the canopy relaxation process. Our data analyses also revealed that the bulkiness of dipolar endgroups can disrupt the organization of the monolayer canopy thus increasing their ability to fluctuate as temperature is increased.