Boric acid ant poison
Boric acid is soluble in water, not oil.  Pick an emulsifier and add it to your ant poison. 

Boric acid needs to remain acidic to retain its effectiveness as an ant killer.  Ants deaths due to boric acid poisoning is a topic still up for debate; however, it has been suggested boric acid is a neuro-toxin and that the acid damages one of the organs in an ants digestive system. 

Peanut butter is usually slightly acidic; however, your particular mixture may be rendering the boric acid inert.  Solutions of sugar and boric acid with a high pH have been known to pass right through ants doing little harm. Perhaps using an acidic additive will increase the bait's effectiveness.

Boric acid in a 1-2% concentration may be ideal if the entire ant colony is targeted. To target the entire colony, enough poison should be used so the ant does not die before it makes it back it its colony.