Interaction of Poly(ethylenimine) with Perfluoro Fatty Acid at the Air/Water Interface

The effect of poly(ethylenimine) (PEI) dissolved in water on the surface pressure-area isotherm of perfluorotetradecanoic acid (PFTDA) at the air/water interface was investigated. PEI was confirmed to stabilize the PFTDA monolayers over a wide range in pH such that the collapse pressure of the PFTDA monolayer attained 70 mN/m upon interaction with PEI. The attenuated total reflectance IR spectroscopy revealed that PFTDA and PEI could be cotransferred to a solid substrate in a Y-type at neutral to weakly basic conditions and in a Z-type at highly basic conditions. From the frequency difference between the antisymmetric and symmetric stretching modes of the carboxylate group, the carboxylate group was demonstrated to interact ionically with PEI. These observations led us to propose an interaction scheme for PFTDA with PEI at the air/water interface. At higher pH, more PEI molecules should be present near the air/water interface for efficient interaction with the carboxylate group of PFTDA-. This is as if the headgroup of PFTDA becomes bulkier and heavier upon increase in the solution pH.