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, (C(n)ANA, CH3(CH2)(n-2)CONH phi NO2, n = 8, 14, 16, 18, or 22) monolayer on water was investigated. A camphor disk was floated on a molecular layer of C(n)ANA, which gave a characteristic surface pressure (Pi) vs area (A) isotherm. The nature of the camphor motion changed depending on the Pi vs A isotherm, and in particular reciprocating motion was observed for C(14)ANA, C(16)ANA, and C(18)ANA, which gave a Pi vs A isotherm with a local minimum and a local maximum. The characteristic motion of a camphor disk is discussed in relation to the Pi vs A isotherm of C(n)ANA and the influence of the interaction between molecules on the driving force of motion. Reciprocating motion was qualitatively reproduced by numerical calculation.