Relationship between Morphology and Na+ Content of Langmuir-Blodgett Mesoscopic Tapes Induced by a Change of pH

Langmuir-Blodgett (LB) films of a short chain carboxyazobenzene derivative on mica exhibit nearly straight tape structures 1 nm thick, 80 nm wide, and over several μm long at pH 7.0. Lowering pH causes the tapes to be wavy and irregular without coagulation. The fraction of straight tapes at a constant salt concentration increases smoothly till pH 7, then drops sharply. Above 8, no transfer occurs. The fraction of Na per molecule in the film also increases with pH and becomes 0.5 at pH 7. The wavy tapes are found to become straight even with a slight ionization. An importance of the charge distribution rather than the total charge density is suggested.