Significance Peptoid nanosheets are an emerging class of 2D nanomaterials that have the potential for use in a variety of applications ranging from molecular sensors to artificial enzymes. Because peptoids are highly designable polymers, nanosheets provide a general platform on which to display an enormous diversity of functionalities. Nanosheets are known to form through a unique monolayer compression mechanism, catalyzed by the air–water interface. Here we demonstrate that nanosheets can be formed via adsorption of peptoids at an oil–water interface. Using vibrational sum frequency spectroscopy, we show that electrostatic interactions are essential in the formation of an ordered peptoid monolayer at the interface, a critical intermediate in the nanosheet assembly pathway. These findings open the door for enhancing the complexity and functionality of 2D nanomaterials.