Significance This high-resolution X-ray study resolves the liquid–air interface structure for a long homologous series of room temperature ionic liquids (RTILs). RTILs are intensely studied for many potential “green” applications and for their intriguingly complex and rare combination of intermolecular interactions. Varying their cation’s alkyl chain length provides, therefore, an opportunity to tune the main interaction from mostly long-range electrostatic to mostly short-range van der Waals. This variation is found here to drive the interface structure from simple, to layered, to liquid crystalline. The quantitative results obtained constitute an accurate yardstick for testing simulations and theory, impact the bulk–surface structure relations in general, and provide currently scarce data for many RTIL applications, like batteries and supercapacitors.