We report on the role of a surfactant monolayer in the impact of small spheres on an air-water interface. We observe that an air cavity (splashing) is induced above a threshold impact velocity. We explore the dependence of this threshold velocity on the bath surface tension and on the bath viscosity using water-ethanol and water-glycerol mixtures, respectively. Interestingly, the threshold velocity for air entrainment is reduced by the presence of a stearic acid monolayer because the hydrophobic tails favor the forced entry of air during the sphere's impact. More generally, we show that this threshold velocity is determined by the wettability of the sphere by the bath, which can equivalently be controlled by tuning the bath properties (presence of a monolayer) or the sphere surface properties (via a surface treatment). These results provide insight into recently developed vesicle production techniques based on the impact of drops or jets on a lipid layer.