Proton Magnetic Resonance Absorption in Anhydrous Sodium Stearate

Abstract

The proton magnetic resonance absorption in anhydrous sodium stearate has been investigated over the temperature range -182OC t o 200' C. An abrupt change in the line width and the second moment has been observed between 113" C and 114" C, which corresponds to a known phase transition. The results suggest that a t temperatures below 114" C the motion of the hydrocarbon-chain portion of the sodium stearate molecule is chiefly one of oscillation or rotation about the chain longitudinal axis. The hydrocarbon chains may be free t o move about other axes a t temperatures above 114" C, although the molecule as a whole retains a fixed position in the crystal lattice. INTRODUCTIONThe various mesomorphic phase transitions in sodium salts of long-chain fatty acids have been exteilsively studied by several physical methods (1). X-Ray, infrared, and nuclear nlag~letic resonance studies have indicated that the lower temperature transitions may represent the onset of rapid reorientation in the paraffin-chain part of the sodium soap lllolecule (2, 3, 4). Of particular interest are the phase transitions curd-supercurd a t 89-92" C, supercurd-subwaxy a t 110-117" C, and subwaxy-waxy a t 125-135" C (4). The followil~g is a proton magnetic resonance investigation of these transitions in sodium stearate, and is an extension and correction of the work in a previous communication (4). E X P E R I M E N T A LThe sodium stearate was prepared from Eastman Kodalc white label stearic acid, f.p. 68.1" C, which had been purified by repeated crystallization a t -20" in acetone (5). The purified acid, f.p. 69.2" C, was dissolved in hot ethanol and titrated t o a phenolphthalein end point with an aqueous ethanol solution of sodium hydroxide. The sodium stearate precipitate was filtered, dried in air, and melted under vacuum t o remove residual solve~lts and t o render it anhydrous (6). This compact, fused soap was remelted under vacuum in sample tubes; dry nitrogen was admitted t o the system and the tubes were sealed off.The nuclear magnetic resonance spectrometer and the nlethods of temperature coiltrol have been described previously (7). The temperature of the sample was measured with a ther~nocouple inserted in a well in the side of the sample tube. T h e line width was talcen as the separation in gauss between points of maximum and rninimunl slope. The second moments of the resonance lines were obtained from the experimental derivative curves (8) by numerical integration and were corrected for the effects of modulatio~l (9). RESULTSThe proton magnetic resonance line widths, measured from -68" C t o 130" C, are given by the open circles in Fig. 1. The line width was also measured a t -182" C where it is lG.l gauss. Particularly notable is the extensive decrease in the line width a t temperatures above 30" C, and the abrupt change from a width of 2.8 gauss a t 113" C to a width determined by the field homogeneity a t 114" C. This abrupt narrowing coi~lcides with 'Manuscript

Nuclear Magnetic Resonance Studies of Molecular Motion in a Number of Stearates and Oleates

“…Second moments on the low temperature side of the transition are in the range of about 16-20 G2, which is very similar to the values on the low temperature side of the crystal-crystal phase transitions of the alkali metal stearates, and on the high temperature side they have decreased t o about 12 G~. abrupt, and correspond to a change in crystal structure (6,(11)(12)(13). Although the gradualness of this transition in the oleates makes one doubt that the transition involves a phase change, the motional changes in the hydrocarbon chain seem to be very similar to those of the hydrocarbon chains of the stearates in their low temperature crystal phase change.…”

“…The higher temperature transition is the more abrupt one, leading at the high temperature side to second moments of about 1 G 2 and line widths which are determined by depth of modulation. In terms of n.m.r., the nature of this transition is very similar to the crystalline to waxy phase transitions of the alkali metal stearates (6, 1 [1][2][3][4][5][6][7][8][9][10][11][12][13]. The lower temperature transition, which is apparently absent in caesium oleate and not especially well defined in rubidium and sodium oleates, is a more gradual one in terms of the n.m.r.…”

Proton Magnetic Resonance Absorption in Copper(ii) Stearate

“…The nuclear magnetic resonance spectrometer and associated apparatus have been described previously (10). Figure 1 shows the variations of the proton magnetic resonance absorption line width of copper(I1) stearate as a function of temperature.…”

Proton Magnetic Resonance Absorption in Ethyl Stearate

“…If this motion occurs in ethyl stearate the second moment should be reduced to 23.1 gauss?. T h e results show that the methyl end groups probably do not rotate a t -77' C or even at 0' C, and it is suggested that the difference between the rigid lattice and experimental second moments a t these temperatures is due to torsional oscillation about the long axis of each chain (5,16). At 24" C the proton resonance absorption line has a complicated structure and it is apparent that motion other than torsioilal oscillation is taking place.…”

“…These narrow peaks are thought to be due to the somewhat unhindered and random motion of molecules or parts of molecules in crystal lattice defects (6). Since the correlation frequency of the motion associated with such narrow peaks is usually in the order of several megacycles (14,16), it seems likely that this motion contributes to the radio-frequency dielectric dispersion observed in /?-phase ethyl stearate.…”