Scattering vector dependence of the small-angle scattering from mixtures of hydrogenated and deuterated organic solvents

In connection with a contrast variation small angle neutron scattering (SANS) study of water-in-oil microemulsions, mixtures of hydrogenated and deuterated n-decane and similar mixtures of iso-octane have been measured as backgrounds. For the pure deuterated and hydrogenated solvents the spectra were flat but for all the mixtures the large molecular size of the alkanes give rise to a scattering vector dependence in the small angle scattering region. An explanation of this can be given based on the scattering contributions which have been identified for mixtures of water. The intensity is proportional to x(x - 1) where x is the volume fraction of deuterated molecules. Using the analogy to scattering from polymer blends of hydrogenated and deuterated chains, the scattering spectra are analysed using the model for random gaussian coils. For the solvent mixtures investigated in the present study, the radius of gyration was determined to be 3.8 Å for decane and 2.9 Å for iso-octane, independent of the fraction of deuterated molecules.