The state of water and several transitions were examined in the systems n-decanephosphonic acid (DPA)-water and the sodium salts of DPA-water. Temperature - composition phase diagrams are reported. The results show that several liquid crystalline phases plus isotropic liquid, and two solid phases (a waxy solid phase and a crystalline phase) are formed. Several types of water were detected: bulk-like water, interfacial water and hydration water
