I need to know, How the structure of ionic liquid cations and anions as well as impurities (e.g. water) influence the stability of ionic liquids in application and in utilization conditions?
I am afraid there is no simple answer to your questions, it very much depends on the ionic liquid and the application. Water content is usually the least of your problems, as far as stability is concerned, but rather strong bases, or oxidizers, ot metals that catalyse decomposition.