STRUCTURE OF THE LOWEST TEMPERATURE CRYSTALLINE PHASE AND ITS ANNEALING EFFECT OF POTASSIUM PENTADECANOATE

The structure and the thermal behavior of potassium pentadecanoate (99% Purity), CH3(CH2)13CO2K, having low water content were investigated by differential scanning calorimetry, IR and Raman spectroscopies, and X-ray diffraction. Six phases were observed by thermal measurements. The molecular and crystal structures of the lowest temperature crystalline phase (phase I) and its annealing effect were studied. The molecular conformation was confirmed to be all-trans, comparing IR observed frequencies in the methylene progressive region with calculated ones. The crystal structure was speculated to be analogous to the B-form in even-numbered potassium soaps from analyzing the X-ray powder pattern. When a specimen was annealed in the temperature range from 110 to 150-degrees-C, a new structural modification appeared. The molecular conformation of the annealed specimen in phase I was speculated as slightly differed from all-trans. The crystal structure of the annealed one was different from both the A- and B-forms. The annealed specimen was unstable and converted to the initial one over a few days.