Effects of ammonium surfactants with different hydrocarbon chain lengths (C8, C12, C16, and C18) on the colloidal forces between bitumen and silica were studied by atomic force microscopy. The results showed that the chain length of the ammonium surfactants had a significant impact on both the long-range interaction and adhesion forces. With the addition of surfactants with relative short chains of C8 and C12 in the solutions, the long-range repulsive force decreased or even became strong attractive force, while it became repulsive again in solutions of surfactants with long chains of C16 and C18. It was further observed that addition of Ca²⁺ in various surfactants solutions would either depress or enhance the colloidal interactions based on the surfactant chain lengths. It was believed that variation of the interaction behaviors resulted from the mono-layer or bilayer adsorption of various surfactant molecules on the negatively charged surfaces of bitumen and silica, which affected the surface wettability and the surface charge characteristics and then greatly changed the colloidal interactions. The findings indicated that, to have a high bitumen recovery and good froth quality, the surfactant type and concentration of the di-valent metal ions in the oil sand processing slurry must be well considered.