Quantitative Static Secondary Ion Mass Spectrometry of pH Effects on Octadecylamine Monolayer Langmuir-Blodgett Films

A modified method to obtain quantitative information from molecular ion signals from static secondary ion mass spectrometry analysis is reported. The technique utilizes both integration and curve-fitting procedures. This method has been applied to study solution cast and Langmuir-Blodgett (L-B) films of octadecylamine on polycrystalline silver. The L-B films have been prepared with subphase pH values ranging from 1 to 11. The results reported test the validity of the newly developed quantitative approach and allow conclusions about the secondary ion formation mechanisms. In particular, the mechanisms involved with the formation of secondary (M + Ag)⁺ and (M + H)⁺ ions are discussed. A surface titration curve is developed, and this allows the quantitative evaluation of local pH at the surface of these films. For L-B monolayers, it is found that the integrated peak intensity ratio of (M + Ag)⁺/Ag⁺ is independent of the subphase pH value while that of (M + H)⁺/Ag⁺ decreases rapidly as the pH value increases from 1 but remains relatively constant for pH values over 7.