Dormin is a brand name for Diphenhydramine, which goes most commonly by the brand name, Benadryl. I'ts a sedative antihistamine. It's used as an adulterant in street Heroin for a few reasons. It has sedative effect in its own right, which can easily convince a user that their dope is actually stronger than it is. It also will contribute to a pereivedly stronger rush. It's also fairly cheap and easy to acquire on a wholesale scale.
Diphenhydramine Hydrochlodride, the most commercially pervasive form of the drug, is freely soluble in water. The binders, fillers and so on also contained in the capsule/tablet could be a totally different story.