Direct evidence of the enthalpy release being accompanied by first-order phase transitions of monolayers at the air/water interface by compression

Abstract

Insoluble monolayers at the air/water interface change their phases with isothermal compression. Previously, however, it has been difficult to detect the temperature change of the monolayers by the enthalpy release in their first-order phase transition regions because the amount of film materials on the water surface was much too small. We constructed a new instrument, which controlled the temperatures of the water surface and the air above the water within 0.02°C, to detect this temperature change. Direct evidence is given for the enthalpy release which is accompanied by the first-order phase transitions caused by the compression of monolayers of arachidic acid and stearic acid at the air/water interface, and the possibility is discussed that the so-called “solid films” of these monolayers do not correspond to the two-dimensional solids but to the first-order phase transition regions from two-dimensional liquids to solids.