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A study of molecular motion in potassium caproate, caprylate, and caprate and lithium stearate by proton magnetic resonance. Janzen, Wayne Roger


Proton magnetic resonance has been measured in the temperature range -196° to 295°C for potassium caprylate, from -196° to 230°C for lithium stearate, and at -196° and 27°C for potassium caproate. Theoretical second moments for potassium caproate, caprylate and caprate at -196°C were computed using various molecular parameters and were compared with experimental values. Unfortunately the theoretical values were sufficiently alike for any one soap so that it could not be decided which of the parameters were applicable. The results do show, however, that the end methyl group rotates in potassium caproate at -196°C and probably does so in potassium caprylate and caprate. A sharp decrease in line width and second moment takes place in potassium caprylate between 50° and 55°C and between 283° and 286°C. The first transition corresponds to a known crystal phase change at 55°C. The second moment results suggest that some torsional oscillation about the longitudinal axes of the hydrocarbon chain portion of the potassium caprylate molecules takes place below 50° C. Above 55°C torsional oscillation of large amplitude or possibly even rotation of the hydrocarbon chain occurs. The transition between 283° and 286°C corresponds to onset of motion in the hydrocarbon chain restricted only by continued ordering of the polar end groups in the ionic layer of the soap. The proton magnetic resonance results in lithium stearate indicate transitions at 115°, 171+°, and 225°C. These pmr transitions correspond to known phase transitions. The second moment results suggest that the methyl group of the hydrocarbon chain in lithium stearate begins to rotate between-183° and -136°C. The second moment above 115°C is approximately equivalent to that estimated for rotation of the hydrocarbon chain about its long axis. Above 171°C very extensive motion of the chains occurs, although they are still held in position by the ionic layer. The ionic layer begins to break up in the region 215° to 218°C, with the compound becoming an isotropic liquid at about 225°C.

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