Abstract: The nature of pyridine adsorption by acid- and base-saturated montmorillonite from Otay, California (A.P.I, no. H-24) was studied by infrared adsorption. The spectrum for chemisorbed pyridine was found to be distinctively different from physically adsorbed pyridine; the latter gave a spectrum very similar to liquid pyridine. Upon exposure of the clay films to pyridine at low vapor pressures, only the pyridinium ion spectrum was obtained; but when the vapor pressure was increased, both chemisorbed and physically adsorbed pyridine was detected. The intensity of the spectrum for physically adsorbed molecules could be reduced by a vacuum, whereas the intensities of the peaks for the chemisorbed pyridinium ions were unaffected.
Interlayer water was replaced by adsorption of pyridine. Water, in turn, replaced physically adsorbed pyridine from the clay surface but was unable to replace chemisorbed pyridine.