Monolayer properties of alkylammonium perfluoroalkanoates on concentrated sodium chloride solution

Abstract

Surface pressure ( π)– and surface potential ( ΔV)–area ( A) isotherms for monolayers of alkylammonium perfluoroalkanoates (C m F 2 n+1 ) of four different chains length on pure water and 4.4 M NaCl solution subphase were measured at various temperatures by the Langmuir and the ionizing electrode methods, respectively. We found that the monolayer of C m F 2 n+1 salts with the total value of m+ n equal to 15 is stable at 298.2 K on 4.4 M NaCl subphase. As for butylammonium perfluorododecanoate (C 4F 23) of this short-hydro–long-fluoro alkyl chain salts, the π– A isotherms showed three phase transitions. The surface potential (or the surface dipole moment) changes enabled the estimation of the molecular orientation of the respective alkylammonium perfluoroalkanoate in the monolayer state. From the surface pressure and surface potential–area isotherms (surface dipole moment), we discuss the difference in hydro–fluoro chain lengths on the physical state of the monolayers.