Wetting agent or spreading agent = surfactant (surface active agent). Photo Flo 200 is a very good one.
Soap is one, but should not be sued due to additives and pH.
They come in 3 forms: Neutral (no charge in the molecule at all), Anionic (negative charge in molecule) and Cationic (positive charge in Molecule). The first two are best as they have no activity against emulsions but the last one can cause fog and stain. Photo Flo is neutral.