Water compressibility in confined spaces

In order to explore the compressibility of water sequestered in droplets surrounded by surfactant molecular films (reverse micelles), we custom-built a high-precision velocimeter. The apparatus comprises two ultrasonic small-sized cells, electronic generators and a computer-controlled digital scope. We carried out a systematic study of the stability and reproducibility of the experimental set-up which yields an accuracy of 10/sup -5/. The ultrasound velocity, the apparent volume, and compressibility of encapsulated water were determined in charged and uncharged molecular films as a function of water concentration. We observed a close relationship between the measured parameters of encapsulated water, the chemical nature of the surfactant molecular film and the number of water molecules in interaction. The values obtained are higher than that reported for bulk water and are assigned to the surfactant-water interactions arising between the head groups and water molecules.