Self-Propulsion of a Benzoic Acid Disk Reflecting the Mesoscopic State of an Amphiphilic Molecular Layer

Abstract

In this study, we developed a novel self-propulsion system using the mesoscopic state of an amphiphilic molecular layer. A benzoic acid (BA) disk and 4-stearoyl amidobenzoic acid (SABA) were used as the self-propelled object and amphiphile, respectively. The BA disk was driven by the difference in surface tension around it on the aqueous surface, and its motion was influenced by the intermolecular interactions between BA and SABA. Simultaneous Brewster angle microscopy and surface pressure versus area isotherm measurements were performed to evaluate the meso- and macroscopic states of the SABA molecular layer at different temperatures (T) in the aqueous phase. At T = 293 K and 10 ≤ A ≤ 16.8 Ų molecule⁻¹, the BA disk exhibited linear reciprocating motion due to the homogeneously distributed SABA molecular layer. Conversely, the heterogeneous distribution of SABA domains at T = 303 K and 10 ≤ A ≤ 12.6 Ų molecule⁻¹ led to ring-shaped reciprocating motion. The SABA molecular layer, which was irreversibly compressed via BA disk motion, acted as a boundary for the BA disk motion. The findings of this study should advance the programming of self-propulsion at the molecular level via the mesoscopic state.