Binding of Cd ²⁺ to self-assembled bilayers bearing pyridine terminal groups: Attenuated total reflection fourier transform infrared spectroscopic studies

Various aspects of the adsorption of Cd ²⁺ on self-assembled monolayers (SAMs) of 4-heptadecylpyridine (HDpy) and 7-tridecyl-4-methyl-l,10- bipyridine (TMbipy) supported on an octadecylsilane (ODS)-modified Ge prism, have been examined both ex situ (dry) and in situ (in the presence of aqueous solutions) using attenuated total reflection Fourier transform infrared spectroscopy. In direct analogy with the behavior found for a variety of genuine pyridine (py) and bipyridine (bipy) metal ion complexes, Cd ²⁺ binding to both SAMs led to shifts in the skeletal vibrational modes of the corresponding uncoordinated ligands in the region 1650-1400 cm ^-1, toward higher energies. Analysis of spectra acquired ex situ for HDpy/ODS/Ge and TMbipy/ODS/Ge before and after exposure to 0.1 mM [Cd ²⁺] yielded values for the fraction of uncoordinated py, θ(py _un), and uncoordinated bipy, θ(bipy _un), of about 0.5 and about 0.1, respectively. The features attributed to the py _un groups for spectra collected for HDpy/ODS/Ge in situ and ex situ were found to be virtually identical, making it possible to isolate by graphical means the most prominent band of py _co centered at about 1615 cm ^-1 for HDpy/ODS/Ge collected in situ. The resulting bands for pure py _co and py _un in situ were then used to deconvolute spectra recorded in situ for HPpy/ODS/Ge in contact with Cd ²⁺containing solutions in the range 10 nM < [Cd ²⁺] < 0.1 mM, from which information regarding the adsorption isotherm was obtained.