Carboxyl stretching vibrations of spontaneously adsorbed and LB-transferred calcium carboxylates as determined by FTIR internal reflection spectroscopy

The nature of adsorbed calcium carboxylates (oleate and stearate) has been investigated by in situlex situ Fourier transform infrared internal reflection spectroscopy (FTIR/IRS). Experimental results confirm that chemisorbed carboxylate is characterized by a singlet band near 1550 cm(-1) while physisorbed calcium dicarboxylate salt is characterized by a doublet band near 1540 and 1575 cm(-1). The singlet IR band at 1550 cm(-1) was attributed to the carboxylate groups associated with calcium ions in a bridging mode. On the other hand, the doublet IR absorbance band at 1540 and 1575 cm(-1), which is observed for adsorbed carboxylate groups at multilayer coverage and for bulk-precipitated calcium dicarboxylate salts, was assigned for the carboxylate groups associated with calcium ions in unidentate and bidentate modes, respectively. In this regard, the IR singlet at 1550 cm(-1) and the doublet at 1540 and 1575 cm(-1) are correlated to the coordination structure of carboxylate groups with calcium ions. Constrained carboxylate groups at a surface such as a chemisorbed layer at a calcium semisoluble mineral surface (carboxylate bridging coordination at the fluorite surface, for example) and/or a transferred Langmuir-Blodgett (LB) layer at a high-transfer pressure give rise to the IR singlet at 1550 cm(-1). Carboxylate groups associated with calcium ions in three-dimensional seven- or eight-fold coordination, which may contain carboxylate groups coordinated with calcium ions in different modes (i.e., unidentate and bidentate modes) are responsible for the IR doublet at 1540 and 1575 cm(-1). This assignment is in agreement with the observed IR doublet for bulk-precipitated calcium dicarboxylate salts, adsorbed/surface precipitated calcium carboxylate species at multilayer coverage, and transferred calcium distearate at a low-transfer pressure because under these conditions formation of the three-dimensional structure is possible. (C) 2002 Elsevier Science (USA).