The molten phase conductance and the thermotropic phase transition in lead(II), zinc(II) and cadmium(II) 10-hydroxyoctadecanoates

Abstract

The electrical conductances and heats of phase changes in lead(II), zinc(II) and cadmium(II) 10-hydroxyoctadecanoates are measured. The soaps are prepared by metathesis in alcohol solution from 10-hydroxyoctadecanoic acid which in turn is prepared from the oxidation of cis-9-octadecenoic (oleic) acid with concentrated sulphuric acid. The Arrhenius plot for lead(II) 10-hydroxyoctadecanoate shows a maximum, a behaviour that had earlier been interpreted to be due to complex formation. A similar plot for zinc(II) 10-hydroxyoctadecanoate is linear while that for cadmium(II) 10-hydroxyoctadecanoate presents a discontinuity which is interpreted, with the aid of the differential scanning calorimetric (DSC) spectra and optical examination, to be suggestive of the formation of an intermediate phase between the solid and the liquid phases. The low conductances of the soaps in comparison with the literature data on some metal caboxylates indicate the existence of extensive intermolecular hydrogen bonding in the molten phase. Both lead(II) and zinc(II) 10-hydroxyoctadecanoates melt directly from the solid to the liquid phase while their cadmium(II) counterpart passes through the gel-like phase before melting into the liquid. The mesophase of cadmium(II) 10-hydroxyoctadecanoate is transformed to the liquid phase with a relatively low enthalpy change and the behaviour is associated with the cylindrical micekkar structures of the liquid phase that have their formative stage from the gel-like mesophase.