CHAIN-LENGTH AND THE COSOLUBILITY OF N-PARAFFINS IN THE SOLID-STATE

Within the constraints of rectangular layer chain packing and the orthorhombic perpendicular methylene subcell imposed by the crystal structure, a chain length series of n-paraffins from C28H58 to C60H122 was used to form binary solids. If chain length differences fall within limits similar to those found in earlier analyses, solid solutions are formed. As indicated by theoretical calculations of the melting point line, they are nearly ideal in their behavior. Outside of this chain length difference eutectics are formed, with liquidus curves also corresponding closely to the freezing point depression of an ideal liquid solution. Depending on chain length difference, the eutectic solid will be composed of an incommensurate layer structure (crystallized from a metastable solid solution) coexisting with the longer chain component. If no cosolubility is possible, a mixture of the two pure components separates. A boundary domain also exists before strict eutectic separation when a critical temperature is reached. Here, at the miscibility gap, metastable solid solutions fractionate, with a structure very similar to that of a secondary eutectic solid for binary combinations where limited solid solubility still exists. However, the incommensurate solid changes continuously with concentration unlike the mixed lamellar stacking in the eutectic, which has a constant composition. It is therefore possible to describe a progression of crystalline arrays between the stable solid solution and fully separated eutectic. © 1990, American Chemical Society. All rights reserved.