Phase diagrams and optical properties of binary liquid crystalline systems of cobalt (II) caprylate with lithium and lead caplylates

The concentration and temperature ranges of liquid crystal and glass formation in binary systems of cobalt (II) caprylate with either lithium or lead caplylates have been studied by differential thermal analysis and optical polarization microscopy. Absorption spectra of Co(II) cations in mesophases and glasses of these binary caprylate systems have been analyzed as a function of temperature and composition to get information on the coordination of Co(II) ions.For the binary system of lead–cobalt caprylate, we found that the Co(II) ions are octahedrally coordinated over the range of temperatures and compositions studied, whereas in the binary lithium–lead caprylate system, the Co(II) ions can exist in both octahedral and tetrahedral coordination depending on composition and temperature. Thermochromic behavior was observed for all compositions studied here, from blue to violet and pink, depending on the composition and temperature of the system studied. An unusual increase of optical density with decreased Co(II) ion concentration has been observed for the Co–Li caprylate binary system up to 50 mol%. An increasingly ionic Co(II) coordination environment as Li ion concentration increases facilitates a concomitant increase in the proportion of more optically dense tetrahedrally coordinated Co(II) ions.

Research highlights
► Optical properties of cobalt caprylate binary glasses and liquid crystals
► Cobalt–lithium caprylate and Cobalt lead caprylate are studied
► Phase diagrams for binary mixtures constructed
► Optical properties of binary mixtures are related to coordination numbers