Thanks guys..these all seem reasonable.
The back story is this happened to me in the past, when I went through photos that survived our moving van fire. Soaking in water worked..slowly and with varying levels of success (the emulsion kept wanting to peel away from the paper backing, so a few salvaged pictures are very thin). Now a colleague has stacks of stuck photos and I found myself curious as to whether there might be a better way.
I suggested warm water and photo flo, that seems like a useful additive. He doesn't know what photo flo is or where to get it, so instead I suggested a few drops of dish soap.