Langmuir Monolayer Properties of Perfluorinated Double Long-Chain Salts with Divalent Counterions of Separate Electric Charge at the Air-Water Interface

The novel perfluorinated double long-chain salts with divalent counterions of separate electric charge, 1,1-(1,ω-alkanediyl)-bispyridinium perfluorotetradecane- carboxylate [CnBP(FC14)₂ : n = 2, 6, 10, 14], were newly synthesized and their interfacial behavior was investigated by Langmuir monolayer methods. Surface properties [surface pressure (π)−, surface potential (ΔV)−, dipole moment (μ)−area (A) isotherms] and morphological images of CnBP(FC14)₂ monolayers on a subphase of water and on various NaCl concentrations were measured by employing the Wilhelmy method, the ionizing electrode method, fluorescence microscopy (FM), and Brewster angle microscopy (BAM). CnBP(FC14)₂ formed a stable monolayer on water at 298.2 K, where these π−A isotherms shifted to a larger molecular area with increasing charge separation and had no transition point from a disordered phase to an ordered one. On the contrary, the π−A isotherms on NaCl solutions moved to the smaller areas, showed the transition and higher collapse pressures compared to the π−A isotherms on water. These results suggested that a sodium chloride subphase induced the condensation of CnBP(FC14)₂ molecules upon compression. In addition, it is quite noticeable that a dissociation of CnBP counterion from CnBP(FC14)₂ occurs on NaCl solutions, depending on the extent of charge separation. This phenomenon was supported by the changes of the limiting area, transition pressure, collapse pressure, repeated compression−expansion cycle curve, and ΔV behavior of perfluorotetradecanoic acid (FC14). Furthermore, temperature dependence of these monolayers was investigated, and an apparent molar quantity change on the phase transition was evaluated on 0.15 M NaCl. The morphological behavior of CnBP(FC14)₂ and FC14 monolayers was also confirmed by FM and BAM images.