Surface tension of synthetic polyelectrolyte solutions at the air-water interface

Abstract

Surface tensions, γ, of synthetic polyelectrolytes are studied systematically at the air-water interface by the Wilhelmy method. The polyelectrolytes used are anionic ones: (1) polyacrylic acid and its sodium and tetraalkylammonium (alkyl = methyl, ethyl, n-propyl, and n-butyl) salts, (2) polymethacrylic acid and its sodium salt, (3) sodium poly(ethylene sulfonate), and (4) sodium and tetraalkylammonium salts of poly(styrene sulfonic acid). Cationic macroions are (5) copolymer of dimethyldiallylammonium chloride and sulfur dioxide, (6) poly-4-vinyl- N-alkylpyridinium halide, (7) copolymer of 4-vinyl- N-benzylpyridinium chloride (95 mole%) and 4-vinyl- N-n-cethylpyridinium bromide (5 mole%), (8) 3-3, 6-3, 6-6, and 6–10 type ionen polymers, and (9) quaternized polyethylenimine with methyl groups. In addition, (10) low-molecular-weight analogs of the methylated polyethylenimine and the bolaform-type electrolytes are studied. The surface activity of vinyl-type synthetic polyelectrolytes is low due to the fact that their conformations are inadequate to separate their groups sterically to hydrophilic and hydrophobic parts at the air-water interface. The γ values of the tetraalkylammonium salts are, however, low due to the arrangement of hydrophobic gegenions at the interface assisted by macroions. For ionen-type polymers, γ is low. This is because a folded conformation is formed rather easily at the interface. For many polyelectrolytes, γ begins to decrease above the critical concentration, m ∗ , at which the lateral and attractive intermacroion interaction occurs at the interface, and the two-dimensional orientation forms. Below m ∗ , the surface activity is negligible. The excess concentrations at the interface are evaluated using Manning theory for a polyelectrolyte solution.