Determination of the length of polymethylene chains in salts of saturated and unsaturated fatty acids by infrared spectroscopy

The length of polymethylene chains is determined by counting the number of, or measuring the position of, methylene vibration peaks in the 1070–710 cm−1 and/or the 1380–1170cm−1 regions of the IR spectrum of salts of fatty acids. Plotting this peak position against the phase relationship of the vibration in adjacent methylenes gives a curve which is independent of the chain length. (Thephase relationship, Φ/π=k/(m+1); where φ is the phase difference in radians between adjacent methylenes in a chain;m is the number of methylenes in the chain;k=1,2,3,…m, withk=1 generally assigned to the in-phase vibration.) Separate curves are obtained for methylene wagging and for two arrays of coupled twisting-rocking vibrations.