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

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 (PI-A) isotherms and fluorescence from the molecules on different pH subphases. When ONA was spread on subphases at pH levels less than almost-equal-to 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 pK(a) of the lipoidal acid to greater than nine, some five pH units greater than the pK(a) of ONA in pure water. HHC showed similar behaviour to ONA with HHC aggregates formed in the air-water monolayers, although not to the same extent as for the ONA system. The combination of PI-A isotherms and emission spectroscopy is shown to be a powerful strategy for characterising air-water monolayers.