Microstructure and concentration fluctuations in alcohol-Toluene and alcohol-Cyclohexane binary liquids: A small angle neutron scattering study

The origin of concentration fluctuations in a series of binary liquids is examined by small angle neutron scattering in relation with H-bonded micellar clusters and the aggregation of alcohol-rich domains, which is also related to the well-known observation of a prepeak in diffraction spectra of alcohols. The results suggest that concentration fluctuations do not arise from size variation and re-arrangement of the mesoscopic domains but from the micellar-clusters themselves. We evaluate the scattering intensity at small angles and deduce the Ornstein-Zernike correlation lengths, the Bhatia-Thornton concentration fluctuations and the Kirkwood-Buff integrals. An alternative approach, based on the Guinier approximation was applied, indicating that the observed inhomogeneity could be related to spherical particles with diameter comparable to the H-bonded multimers. We compare the structure factor of different systems when varying the molecular interactions: the alcohol-solvent interaction has been tuned with Toluene and Cyclohexane aprotic solvents, and the amphiphilic character of the alcohol, by going from Tert-butanol to Ethanol and Methanol.