Mechanical Properties of Fatty Acid Monolayers on the Water Surface Based on Surface Pressure Relaxation and Area Creep Measurements

Abstract

The surface pressure relaxation and area creep experiments were carried out for the myristic acid (C₁₄), stearic acid (C₁₈), and behenic acid (C₂₂) monolayers on the water surface at 293K. The myristic acid monolayer is in an amorphous state on the water surface at 293K. In the case of π(t=0) =17 mN·m^-1 for the myristic acid monolayer, little change of π was observed in the experimental range of 1000 sec. The large relaxation was observed both above and below 17 mN·m^-1. On the other hand, the stearic and behenic acid monolayers are in crystalline states on the water surface at 293K. In the case of π(t=0)=26 mN·m^-1 for the crystalline stearic acid monolayer, large relaxation of the surface pressure was not observed up to 100 sec. Also, in the case of π(t=0)=25mN·m^-1 for the crystalline behenic acid monolayer, the surface pressure did not change within 1000 sec. The transmission electron microscopic(TEM)observation revealed that this stability of monolayer might be related to homogeneity of monolayer. Electron diffraction (ED) measurement also revealed that the origin of relaxation for crystalline monolayer at high surface pressure can be attributed to the collapse of monolayer and the sintering among crystalline domains of monolayer. On the other hand, the area creep measurement revealed that the magnitudes of constant pressure π₀ which showed the smallest decrease of area were 17, 23, and 25 mN·m^-1 for the myristic, stearic, and behenic acid monolayers, respectively. These surface pressure (π₀) values are almost equal to the surface pressure π(t=0) at which the monolayer showed fairly small surface pressure relaxation.