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An investigation of proton magnetic resonance in stearic acid, sodium stearate and potassium stearate Grant, Rowland Frederick

Abstract

The proton magnetic resonance absorption has been measured in stearic acid, anhydrous sodium stearate and anhydrous potassium stearate. The temperature dependence of the widths and second moments of the proton magnetic resonance absorption lines of these compounds is discussed. The proton magnetic resonance absorption in the C-form of stearic acid has been measured between -188°C and the melting point. The results suggest that the methyl end groups may be free to rotate about their C₃ axes above -77°C and that pre-melting becomes significant above 40°C. The proton magnetic resonance absorption in anhydrous sodium stearate has been investigated between -182°C and 200°C. An abrupt change in the line width and second moment has been observed between 113°C and 114°C, the supercurd to subwaxy phase transition. The results suggest that, at 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 to move about other axes above 114°C, although the molecule as a whole retains a fixed position in the crystal lattice. The proton magnetic resonance absorption in anhydrous potassium stearate has been measured from -190°C to 182°C. Abrupt changes in the line width have been observed between 55°C and 62°C and at 171°C; these changes correspond to known phase transitions. The second moment results suggest that some torsional oscillation may occur about the longitudinal axes of the hydrocarbon chain portion of the potassium stearate molecule, at temperatures below 55°C. Above 62°C, contact between the molecules may have decreased enough to allow some lateral motion of the hydrocarbon chains.

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