This is a question related to analog photography. Photographic films have a suspension of silver halide crystals in them, and these crystals have adsorptive properties which are used in this application. I have read that when there are $\ce{Ag+}$ and $\ce{Br-}$ ions in solution along with a crystal of $\ce{AgBr}$, $\ce{Br-}$ ions preferentially bind to the surface of the crystal relative to $\ce{Ag+}$ ions. This creates a net negative charge on the silver halide crystal which plays a role in adsorptive properties.
I want to know why $\ce{Br-}$ ions preferentially bond to the silver halide crystals.
I thought it might have something to do with the Frenkel defect, because of which the $\ce{Ag+}$ ions move from the surface to inner interstitial sites, thus creating a negative charge on the surface. But this would mean that silver halide crystals attract $\ce{Br-}$ ions not $\ce{Br-}$ ions.
