Compressibility measurements of water sequestered in spherical droplets

In order to explore the compressibility of water sequestered in droplets surrounded by molecular films, the authors have custom-built a high precision velocimeter. The apparatus comprises two ultrasonic small-sized cells, electronic generators and a computer-controlled digital scope. The authors have carried out a systematic study of the stability and reproducibility of the experimental set-up which yields an accuracy of 10^-5. They have measured the apparent compressibility of encapsulated water in charged and uncharged molecular films as a function of water content. They observed a close relationship between the compressibility of water encapsulated and the nature of the molecular film. In all cases studied the value obtained is higher than that reported for bulk water. These differences can be interpreted by the interactions arising between the charged part of the molecular film and water, by the physical properties of the system and by the confinement of water molecules