Small angle X-ray scattering from phase separating n-paraffin binary mixtures

Small angle X-ray scattering from binary mixtures of deuterated and non-deuterated n-paraffins (carbon numbers n, between 18 and 36), quickly quenched from the melt, has been studied in order to understand the demixing process. For a large difference in n, such as C-18/C-36 the components separate into two distinct phases. These are both dilute solid solutions but, for example, despite the presence of the second component, the monoclinic forms characteristic of very pure C30H62, C32H66 and C36H74 appear. For mixtures with a weaker demixing tendency, a number of distinct phases, typically five, have been found, reflecting the complex polymorphic behaviour of the pure phases in this region of composition. For close n values, solid solutions form and 'microphase separated' states from these have been characterized by the appearance of a set of diffraction lines which grows into the pattern over a period of days to months at 25 degrees C. We associate this with long density fluctuations in a lamellar 'microphase' structure. For the case of C30H62 : C36H74, the crystallographic symmetry of the system in the solid solution and microphase separated slates are evaluated from the diffraction. For the closest n values of all, a solid solution composed of rather imperfect crystals is stable for at least 5 months.