Manipulating headgroups in Langmuir–Blodgett films through subphase pH variation

Cadmium arachidate Langmuir–Blodgett (LB) films, deposited at ‘normal’ (∼6.5) and ‘high’ (∼9.0) subphase pH on hydrophilic Si(0 0 1) have been studied by atomic force microscopy (AFM) and X-ray reflectivity techniques. Surface topography and electron density profiles (EDP) reveal absence of pinhole type defects in the ‘high pH’ LB film, but presence of ridges aligned roughly parallel to the direction of deposition. EDP of the films indicate nearly one extra Cd-ion per headgroup for ‘high pH’ films, suggesting clearly for the first time that more metal ions can be incorporated in LB films through subphase pH variation. Fourier transform infrared (FTIR) spectra indicate formation of bidentate bridging type headgroups consistent with one more metal ion per headgroup.