SPECTROSCOPIC AND ELECTROKINETIC STUDY OF PH-DEPENDENT IONIZATION OF LANGMUIR-BLODGETT-FILMS

The ionization of the spectroscopic probe molecule 4-heptadecyl-7-hydroxycoumarin (HHC) cast in Langmuir-Blodgett (L-B) films with the neutral diluent molecules poly(vinyl octadecyl ether) (PVOE) and octadecan-1-ol (OA) has been examined via the techniques of attenuated internal reflection spectroscopy and streaming potential measurements. For the HHC/PVOE system, cast onto fused silica made hydrophobic with octadecyltrichlorosilane (OTS), the anomalous titration results obtained earlier from this laboratory were confirmed by both techniques. A detailed theoretical examination, including consideration of the possible effects of ion binding, probe aggregation, and probe-substrate interactions, suggested that the most likely explanation of the HHC/PVOE results was that the OTS-treated surface was porous. For the HHC/OA system, cast onto fused silica made hydrophobic by esterification with dodecan-1-ol, the titration results were in agreement with the behavior predicted by the Gouy-Chapman equation and established a pK(a) of ca. 8.9 for HHC in the L-B film in the absence of any electrostatic potential from the probe/diluent film.