Assembly of new dioxygenated alkyl alkanoates at the air–water interface

New amphiphiles with various oxygen substituents (OH, OCH3, OCOCH3, OC(CH3)2O) attached to different positions in methyl octadecanoate have been synthesised. Their phase behaviour is determined from surface pressure–area isotherms (π–A isotherms) and Brewster angle microscopy (BAM). Some amphiphiles exhibit a two-phase coexistence region depending on the substituents and their position in the alkyl chain. The other amphiphiles show a plateau region due to a film collapse. Here, three-dimensional aggregates are formed, that can be visualised by BAM. Depending on the position of the vicinal oxygen substituents in the alkyl chain the surface pressure in the plateau region as well as the length of the plateau region differ in a characteristic way. The pressure–area isotherms are discussed and molecular models for the arrangement of the amphiphiles at the air–water interface are proposed.