Difference in Surface Properties between Insoluble Monolayer and Adsorbed Film

Difference in Surface Properties between Insoluble Monolayer and Adsorbed Film from Kinetics of Water Evaporation and BAM Image

Yoshikiyo Moroi,* Muhammad Rusdi, and Izumi Kubo

Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University-Ropponmatsu, Fukuoka 810-8560, Japan, and Faculty of Engineering, Soka University, Tokyo 192-8577, Japan

Received: May 19, 2003
http://pubs.acs.org/cgi-bin/abstract.cgi/jpcbfk/2004/108/i20/abs/jp0306287.html

In Final Form: March 1, 2004

Abstract:

The evaporation rate of water molecules across three kinds of interfaces (air/water interface (1), air/surfactant solution interface (2), and air/water interface covered by insoluble monolayer (3)) was examined using a remodeled thermogravimetric balance. There was no difference in both the evaporation rate and the activation energy for the first two interfaces for three types of surfactant solutions below and above the critical micelle concentration (cmc). This means that the molecular surface area from the Gibbs surface excess has nothing to do with the evaporation rate. In the third case, the insoluble monolayer of 1-heptadecanol decreased the evaporation rate and increased the activation energy, indicating a clear difference between an insoluble monolayer and an adsorbed film of soluble surfactant. This difference was substantiated by BAM images, too. The images of three surfactant solution interfaces were similar to that of just the water surface, while distinct structures of molecular assemblies were observed for the insoluble monolayer. The concentration profile of water molecules in an air/liquid interfacial region was derived by Fix's second law. The profile indicates that a definite layer just beneath the air/liquid interface of the surfactant solution is made mostly of water molecules and that the layer thickness is a few times the root-mean-square displacement of the water molecules. The thickness was found to be more than a few nanometers, as estimated from several relaxation times derived from the other kinetics than evaporation of amphiphilic molecules in aqueous systems and a maximum evaporation rate of purified water.