Anion effects on calixarene monolayers: A hofmeister series study

Due to their amphiphilic structure, calixarenes adsorb at the air/water interface and form stable Langmuir films. We have explored the effect of salts on calix[6]- and calix[8]arene spreading isotherms at the air/water interface. A wide range of different potassium salts was used in the subphase: KCl, KI, KBr, KSCN, KNO₃, CH₃COOK, K₂SO₄, and K₃PO₄. The differences in Langmuir isotherms are due to the presence of different anions in the subphase, to the different conformations of the ligands at the interface, and to the different complexing affinities of calix[6]- and calix[8]arene for potassium ions. The two systems show a significant specific ion effect that can be discussed in terms of Hofmeister series. Characteristic monolayer parameters, e.g., limiting area (A _lim), collapse pressure (π_coll), modulus of compressibility (C_s^-1), and surface potential (ΔV), are discussed in terms of some physicochemical parameters that reflect dispersion forces: in particular, anion polarizabilities, lyotropic number (N), molar surface tension increment (σ), and partial molar volume (v _s).