Interpretation of the Infrared Spectra of Metal-Stearate Salts

Abstract

Fourier transform infrared (FT-IR) spectra were measured of the sodium, zinc, and calcium metal salts of stearic acid. The normal mode analysis of sodium decanoate provides guidance on the group frequency-based band assignments for these metal stearates. The principal absorption bands are assigned to the vibrations of specific molecular groups, carboxylate and methylene (CH2), and to a lesser extent methyl species. Highly characteristic group frequencies without significant mechanical coupling are observed above 1500 cm–1. Below 1500 cm–1, the vibrational modes of the carboxylate and alkane chain exhibit mechanical coupling. The prominent carboxylate stretching vibrations differ based on the metal, metal coordination structure, and metal-stearate polymorphic form. The nature of the mechanical coupling of the CH2 wag is discussed in order to understand the empirical correlation between the number of observed infrared bands (1360–1160 cm–1 spectral region) and the number of carbons in the n-alkane chain.