engleski

Why aqueous piperidine has a scattering pre-peak and pyridine not? Insights from Kirkwood-Buff integrals,scattering data and computer simulations

Pyridine is a benzene cycle with a nitrogen N atom replacing one methylene, while piperidine is a similar molecule with an half-amine group NH in place of the N of pyridine. Both molecules are fully miscible in water, and their KBI are quasi identical in shape, which can be explained by the similarity between the 2 molecules. However, the KBI of aqueous pyridine have twice the amplitude of those of aqueous piperidine. This difference can be arguably explained by the fact that piperidine may be more amphiphile than pyridine, because of the half amine group instead of a simple nitrogen. An additional difference comes from the fact that aqueous piperidine has a scattering pre-peak while aqueous pyridine has large k=0 raise instead. This feature points to the intriguing link between the scattering pre-peak at k≠0 and k=0 raise, the latter usually related to concentration fluctuations. Computer simulations show that both mixtures are very strongly micro-heterogeneous, and posing the usual problem of demixing-like behaviour. We track down the pre-peak feature to the fact that piperidine segregated domains tend to have a high surface coverage of H atoms from the NH half-amine group, which produces an extra-scattering which does not get canceled in the final scattering data. Hence, it is the water mingling on the surface piperidine domains which is responsible for the scattering pre-peak.

pyridine ; piperidine ; computer-simulations ; KBI ; neutron-scattering ; pre-peak

nije evidentirano