Monolayer studies of perfluorostearic acid at air water interface

We report the formation of a monolayer of perfluorostearic acid (PFS, CF3(C F2)(16)COOH) using a mixed solvent of pentafluorobenzene (PFB) and methanol (weight ratio of methano/PFB =similar to 1 : 86) as a PFS spreading solven t. PFS formed very stable monolayer at air/water interface, which has 0.32 nm(2) of molecular area and 63.4 mN/m of collapse pressure on DI water. The presence of all the metal cations such as Na+, Ca2+, Co2+, Cu2+ and La3+ r esults in the expansion of the monolayer, the surface pressure (pi)-area (A ) curves of PFS on Ca2+ or La3+ subphase showing the most expanded isotherm s. Polyion complex formation between PFS and water-soluble polymers was obs erved. Especially, polyethyleneimine (PEI) formed a strong polyion complex with PFS. Changing the pH of the subphase controlled the degree of the comp lex formation between PFS and the PEI. The mixed monolayers of PFS and stea ric acid (SA) showed positive molecular area change on mixing due to repuls ion between PFS domains and the SA domains on the subphase. (C) 1999 Elsevi er Science S.A. All rights reserved.