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A study of premelting in the C-forms of stearic, palmitic, myristic and lauric acids by infrared spectroscopy and in anhydrous sodium stearate by high resolution nuclear magnetic resonance spectroscopy. Barr, Matthew Ronald

Abstract

A general discussion of melting and premelting is given. Crystal structures and phase transitions in the C-forms of stearic, palmitic, myristic and lauric acids and in anhydrous sodium stearate are also discussed. Simple theories of infrared and nuclear magnetic resonance spectrometry are presented. Experimental difficulties in the use of solid phase infrared spectra are discussed. A detailed study is made of the temperature behaviour of the infrared spectra of the fatty acids in the region from 750 to 700 cm¯¹ and over the range from about 70ºC below the melting points to about 20ºC above them. Three overlapping peaks of different intensity are resolved - 720 cm¯¹, about 727 cm¯¹, and a peak at higher frequency the position of which varies in the different acids. The complete disappearance of the 720 cm¯¹ band slightly below the melting point in each acid is taken to indicate a transition, not previously reported, to a disordered phase in which there is considerable molecular motion as evidenced by the phase's liquid-like spectra. The presence of three peaks in the region is discussed on the basis of a simple theoretical expression derived by Snyder for the position of fundamental methylene rocking vibrations. The crystallinities of the acids are estimated from an expression, based on the work of Stein and Sutherland, which involves the apparent integrated absorption intensities of the resolved 720 and 727 cm¯¹ bands. The extent of premelting is determined. Extensive premelting giving the spectra a liquid-like, but not completely liquid character is found to take place within about 2ºC of the melting point. The high resolution nuclear magnetic resonance spectra of anhydrous sodium stearate were taken at the limit of their experimental application. The spectra show, however, that the subwaxy, waxy, superwaxy and subneat phases of anhydrous sodium stearate form a structurally similar group (from about 120 to 235ºC) with liquid-crystalline properties, while the neat and melt phases also form a structurally similar group (from 235º C upwards) but with the properties of a liquid. The information indicates extensive premelting beginning in the vicinity of 120ºC, below which temperature the salt is essentially crystalline, which reaches a climax at 235ºC, above which temperature the salt is essentially liquid.

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