Self-assembly of colloidal nanoparticles at water-oil interfaces offers an efficient way to construct multi-dimensional arrays. Self-assembly is generally induced by using adsorbing molecular “modifiers” or co-solvants to remove the nanoparticles’ surface charge. Here, it is shown that CTAB, which is commonly used in bulk-quantities in colloidal synthesis, can induce self-assembly of negatively charged colloidal nanoparticles at water-oil interfaces, even at sub-micromolar levels but it does this by providing charge-screening rather than removing the surface charge. Since this is a physical effect, CTAB can promote assembly of nanoparticles regardless of their morphology or material composition. In the specific case of nanoparticles which allow CTAB to adsorb, such as Au, the self assembly mechanism switches from charge-screening to chemical-adsorption as the concentration is increased. This both explains previous observations of spontaneous interfacial assembly and opens up further possibilities for deliberately constructing functional nanoparticle-arrays without the need for additional modifiers or co-solvents.