Reciprocating Motion of a Self-Propelled Object on a Molecular Layer with a Local Minimum and a Local Maximum Isotherm

The mode change of a simple autonomous motor depending on the nature of an N-acyl-p-nitroaniline, (CnANA, CH3(CH2)n−2CONHφNO2, n= 8, 14, 16, 18, or 22) monolayer on water was investigated. A camphor disk was floated on a molecular layer of CnANA, which gave a characteristic surface pressure (Π) vs area (A) isotherm. The nature of the camphor motion changed depending on the Π vs Aisotherm, and in particular reciprocating motion was observed for C14ANA, C16ANA, and C18ANA, which gave a Π vs Aisotherm with a local minimum and a local maximum. The characteristic motion of a camphor disk is discussed in relation to the Π vs Aisotherm of CnANA and the influence of the interaction between molecules on the driving force of motion. Reciprocating motion was qualitatively reproduced by numerical calculation.