Structure and mesomorphism of silver alkanoates

Silver salts of the higher straight-chain alkanoic acids (silver soaps) and especially silver(I) behenate and silver(l) stearate are being used as the silver source in thermographic and photothermographic materials. The silver(l) alkanoates exhibit a complicated thermal behavior, and different phase transitions can be detected between the solid, crystalline state and the onset point of thermal decomposition. The phase transitions are due to a gradual breakdown of the layerlike structure of these compounds, and some of the phases are viscous mesophases. The silver alkanoates of the homologous series between silver propionate (C3H5O2Ag) and silver tetracosanoate (C24H47O2Ag) have been prepared, as well as silver hexacosanoate (C26H51O2Ag) and silver octacosanoate (C28H55O2Ag). The light stability of the compounds increases with increasing chain length. The room-temperature X-ray diffractograms indicate that the silver alkanoates consist of a bilayer structure. The thermal properties of these silver-containing metallomesogens were investigated by high-temperature X-ray diffraction, differential scanning calorimetry, and polarizing optical microscopy. The number and the nature of the phase transitions depends on the alkyl chain length. The thermal decomposition of the silver alkanoates with long alkyl chains proceeds in a liquid-crystalline state (mesophase), and leads to the formation of silver. The thermal behavior of silver behenate is discussed in detail.