We propose a mean field model with two states to describe the monolayers of bipolar amphiphilous molecules (acido-basic, acid-ester, diacid, molecules, etc.) at the air–water interface. We show that the asymmetry and the width of the coexistence curve are related to intermolecular parameters (proportion of molecules in each state, interaction energy) and also to intramolecular parameters such as the curving of the aliphatic chains with increasing density that can even bring one of the polar heads to leave the surface, and with the variation of entropy in passing from one molecular state to the other. This last factor is also solely responsible for the coexistence curve asymmetry near the critical point.[Journal translation]