Structural Transition Behavior of Potassium Salts of Saturated Fatty Acids, CH3(CH2)nCO2K (n=14, 16, 18, 20)

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Abstract
The structural transition behavior of potassium soaps (99% purity), CH3(CH2)nCO2K (n=14, 16, 18, 20), having low water contents of less than 10 wt% was investigated by both DSC and vibrational spectroscopies. Nine phases from crystal (I<80°C) to liquid crystal (260°C<IX) were observed for n=14 and 16 by thermal measurements. An additional phase I′ (I<I′<II) was found for n=18 and 20. The structural changes of the alkyl chains accompanied by these transitions were investigated by using IR and Raman methods. With increase in temperature, the itensities of the IR methylene progressive bands which were characteristic of all-trans conformation decreased and conformational defect bands ascribed to gt-, gg-, and gtg′ conformations, were observed in phases I′, II, and III for n=14–20. These indicate the onset of a partial melting of the chains in these phases. At the transition from phase III to IV, the chains entered liquid-like conformations. The result was also confirmed by the Raman spectra.