Proton conduction in aqueous systems
Proton transfer in aqueous media is an extremely widespread and important process in nature and technology. For bulk water and bulk ice strong indications have been found that protons are not transported by ordinary diffusion, but by special conduction mechanisms. Recent theoretical and experimental work demonstrated that the extremely fast motion of the protonic charge through bulk liquid water involves the so-called Grotthuss mechanism illustrated in the figure.
- Grotthuss mechanism of proton conduction in liquid water. Note that in order to move the proton charge, only permutations of covalent and hydrogen bonds are required.
The Grotthuss mechanism involves an ongoing exchange of covalent and hydrogen bonds between O and H atoms, leading to a net displacement of the positive charge. Hence, in this mechanism, only the charge of the proton and not its mass is being transported, which explains its high mobility in water. Another characteristic of this mechanism is that the charge of the proton is strongly delocalized over several hydrogen atoms and the transfer of the proton can involve tunneling. Proton transfer in water is thus very comparable to the transport mechanism of holes in a semiconductor. In complete analogy to hole transport in a semiconductor, proton transfer involves the flow of electrons in the opposite direction. As a result, the effective mass of the aqueous excess proton is much lower than that of a hydrogen atom.
Recently, we studied the dynamics of the proton in water by resonantly exciting the O-H stretching mode of the H9O4+ (Eigen) hydration structure of the proton in water and probing the subsequent absorption change over a broad frequency range with femtosecond mid-infrared pulses. In addition to a remarkably short lifetime of 120 fs (shorter than for any other vibration in liquid water), we observe the interconversion between the H9O4+ (Eigen) and H5O2+ (Zundel) hydration structures of the proton. This interconversion is found to occur on even shorter (<100 fs) timescales. Hence, the proton rapidly rattles between the oxygen atoms of two neighboring water molecules. This rattling constitutes an essential step of proton transport in water. We also studied the proton transfer in liquid water using THz dielectric relaxation spectroscopy. We find that there are ~4 water molecules strongly bound to the proton, indicating that the proton is predominantly present in the form of an H9O4+ (Eigen) complex. We also observed that the transfer of the proton charge to other water molecules involves the motion of no less than 15 water molecules.
- Schematic representation of the mechanism of proton transfer from an acid to a base. The ongoing fluctuations in the hydrogen-bond structure of water lead to short-living connections of hydrogen-bonded water molecules. During the short lifetime of this connection, the proton charge is quickly conducted from the acid to the base via a collective interconversion of O-H bonds and hydrogen bonds.
We also studied the mechanism of acid-base reactions in water with femtosecond visible-pump mid-IR probe spectroscopy on an aqueous system of a photoacid and an accepting base. The conventional view of this reaction is that the acid and the base have to diffuse into direct contact to enable proton transfer. However, we found that proton transfer occurs primarily via Grotthuss conduction, through a hydrogen-bonded ‘water wire’ of 2-4 water molecules which connects the photoacid with the base. We also found that the excitation of the second excited state of the OH stretch vibration leads to a strong delocalization of the hydrogen atom along the O-H…O hydrogen bond between two water molecules. An important implication of this finding is that this second excited vibrational state forms, energetically, the most favourable transition state for the autodissociation of water, i.e. the process in which two water molecules split spontaneously into H3O+ and OH–.
In view of the fact that proton transfer in bulk water involves the collective restructuring of many water molecules, it is to be expected that the rate and mechanism of proton conduction will be strongly dependent on the nearby presence of solutes and surfaces. We will study the effect of ions and hydrophobic molecular groups on the proton transfer. In many cases proton conduction takes place in extremely small water volumes. Examples are proton transfers in small embedded water pools within proteins or through water channels of trans-membrane peptides like ATP-ase and gramicidin, or through the channels of nafion membranes in hydrogen fuel cells. The confinement of water in small channels and volumes will have a profound effect on the rate and mechanism of proton transfer. We will study the influence of nanoconfinement on proton conduction for several systems including nafion membranes and protein channels.