INQUIMAE   12526
INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Proton Solvation and Dynamics in Water-Acetone Bulk Mixtures and Mesoscopic Nanoclusters
Autor/es:
ROCÍO SEMINO; MARÍA PAULA LONGINOTTI; DANIEL LARIA
Lugar:
Lille
Reunión:
Conferencia; EMLG - JMLG annual meeting 2013; 2013
Resumen:
Proton transfer is an ubiquitous phenomena and it is of paramount relevance in solutions chemistry. In aqueous solution, proton diffusion is controlled by the well-known Grotthuss mechanism [1], which is intimately related with the dynamics of hydrogen bonds, in the picosecond timescale. This mechanism considers not the individual diffusion of a tagged proton, but a translocation of the average position of the excess charge which requires successive spatial rearrangements along chains of hydrogen bonds. In this work, I will present the result of two different studies. On the one hand, a theoretical [2] and experimental analysis of the modifications that take place in the solvation structure and in the proton transfer dynamics for different water-acetone bulk mixtures, covering almost all of the concentration range. On the other hand, a theoretical analysis of the structure and dynamics of proton in water-acetone mesoscopic nanoclusters [3]. The theoretical approach is in both cases based on results from molecular dynamics experiments using a multistate empirical valence bond hamiltonian model that naturally includes a proton translocation mechanism [4]. For all mixtures studied, we have verified that the structure of the first solvation shell of the H3O+ moiety remains practically unchanged, compared to the one observed in pure water. This shell is composed by three water molecules acting as hydrogen bond acceptors, with no evidence of hydrogen bond donor-like connectivity. Rates of proton transfer and proton diffusion coefficients as a function of water-acetone relative concentrations show a transition region, in the vicinity of xw ∼ 0.8, where the concentration dependences of the two magnitudes change at a quantitative level. A crude estimate shows that, at this tagged concentration, the volumes ?occupied? by the two solvents become comparable. The origins of this transition is rationalized in terms of modifications operated in the nearby, second solvation shell, which in acetone rich solutions normally includes at least one acetone molecule. Our results would suggest that one possible mechanism controlling the proton transfer in acetone-rich solutions is the exchange of one of these tagged acetone molecules by nearby water ones. This exchange would give rise to Zundel-like structures, exhibiting a symmetric, first solvation shell composed exclusively by water molecules, and would facilitate the transfer between neighbouring water molecules along the resonant complex. A conductivity study of HCl and LiCl water - acetone mixtures confirmed the xw ~ 0.8 transition observed in our simulations. Moreover, we found that the ratio of HCl and LiCl conductivities at infinite dilution is unity for solutions where acetone is the main component, and up to xw ~ 0.25, where the Grotthuss mechanism activates, gradually becoming more and more important. Previous studies of conductivity measurements in different organic solvents aqueous mixtures show the activation of Grotthuss mechanism at similar values of xw, supporting a geometrical interpretation. Finally, for water-acetone mesoscopic aggregates, we have found that there are cases in which water and acetone do not mix, and the cluster is composed of a water core partially coated by acetone. Proton is found in the surface of the water core, and has at least three acetone molecules in its second solvation shell. Proton dynamics is much slower than in bulk mixtures of the same composition, reaching the nanosecond timescale. The proton transfer mechanism seems to be similar to the one described for bulk mixtures, with the extra ingredient of a proton "immersion" in the water core, searching for more hydrated structures in order to be able to transfer. REFERENCES [1] Grotthuss C. J. T., ? Temperature dependence of the latera hydrogen bonded clusters of molecules at the free water surface?, Annales des Chemie LVIII, 54 (1806) [2] Semino R., Laria D., ?Excess protons in water-acetone mixtures?, Journal of Chemical Physics 136, 194503 (2012) [3] Semino R., Martí J., Guàrdia E., Laria D., ?Excess protons in mesoscopic water-acetone nanoclusters?, Journal of Chemical Physics 137, 194301 (2012) [4] Schmitt U., Voth G. A., ?The computer simulation of proton transport in water?, Journal of Chemical Physics 111, 9361-9381 (1999)