Characterization of air-water monolayers using pressure-area isotherms in conjunction with UV-visible spectroscopic methods: The effect of pH on the ionization of 4-octadecyloxy-1-naphthoic acid and 4-heptadecyl-7-hydroxycoumarin monolayers

Abstract

The ionization characteristics of 4-octadecyloxy-1-naphthoic acid (ONA) and 4-heptadecyl-7-hydroxycoumarin (HHC) air-water films have been examined using pressure-area (Π-A) isotherms and fluorescence from the molecules on different pH subphases. When ONA was spread on subphases at pH levels less than ≈ 8.6 multilayer films were formed at the air-water interface. Spreading ONA on subphases with pH greater than 8.6 yielded monolayer films, although clusters/ aggregates of ONA molecules existed in these films. The effect of this aggregation was to shift the apparent pKa of the lipoidal acid to greater than nine, some five pH units greater than the pKa of ONA in pure water. HHC showed similar b..