INVESTIGADORES
LONGINOTTI Maria Paula
congresos y reuniones científicas
Título:
Proton Solvation and Dynamics in Water-Acetone Bulk Mixtures and Mesoscopic Nanoclusters
Autor/es:
R. SEMINO; M. P. LONGINOTTI; D.LARÍA
Lugar:
Lille
Reunión:
Congreso; EMLG - JMLG annual meeting 2013; 2013
Institución organizadora:
European molecular liquids group
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)