Research Topics

Experimental Methods

Proton Transfer Dynamics in Nanoscopic Water and Bulk Liquids

A topic closely related to our studies of the dynamics and properties of nanoscopic water is our investigations of the influence of nanoconfinement of processes that occur in a nanoconfined liquid environment. A fundamentally important process in chemistry, biology, and in technological applications is proton transfer. We are addressing the question of how the dynamics of proton transfer and proton diffusion is influenced by nanoscopic confinement. We are studying proton transfer dynamics in reverse micelles and in Nafion fuel cell membranes.

Over the past three decades, a significant amount of effort has been directed towards understanding the structure and proton transport properties of Nafion, a polyelectrolyte membrane. Nafion is the most commonly used membrane separator in polymer electrolyte membrane fuel cells. The reasons for Nafion’s success as a fuel cell membrane lie in its robust structure and high proton conductivity, allowing it to function in the harsh conditions required for fuel cell operation without quickly deteriorating. Nafion is a polymer consisting of a fluorocarbon backbone with pendent side-chains terminated with sulfonate groups. The extreme difference in the polarity of the fluorocarbon backbone and the sulfonate groups causes the polymer to segregate itself into hydrophilic and hydrophobic domains. The hydrophilic regions are quite hygroscopic and readily absorb water, which hydrates the sulfonate groups. The water contained in Nafion’s nanoscopic “channels” is the medium that allows protons to diffuse from the anode to the cathode of a fuel cell.

The nature of proton dynamics inside a fuel cell membrane, a reverse micelle, and other nanoscopic systems can be studied using photoacids to generate protons. The left hand portion of figure 7 is a schematic illustration of how a photoacid can be used to study proton dynamics. HPTS (see figure 7) is a common photoacid. In the ground state, the hydroxyl does not dissociate to any significant extent. When the molecule is electronically excited with a fs light pulse, the acidity of the hydroxyl group increases dramatically. In its ground state HPTS has a pKa of 7.7 and is almost fully protonated in neutral water. Upon electronic excitation, the pKa of HPTS drops by approximately 7 units, and it rapidly transfers its phenolic proton to the surrounding water. The ability of the water to solvate the proton and move it away from the excited HPTS molecule depends strongly on the hydrogen bond dynamics, which are closely related to the reorientational dynamics, of water in the vicinity of the photoacid. In addition to its ability to transfer a proton to the surrounding water upon photoexcitation, HPTS has the added advantage that its -3 formal charge partitions it in the center of the aqueous domain in both AOT reverse micelles and Nafion fuel cell membranes. Both AOT and Nafion have sulfonate head groups that force the HPTS into the center of the nanoscopic water system.

We are using a combination of ultrafast transient absorption spectroscopy and time correlated single photon counting fluorescence measurements to study the dynamics of protons in both bulk solvents and in nanoscopic water environments. As discussed in the section Dynamics of Water and Nanoscopic Water, nanoconfinement of water has a substantial effect on water hydrogen bond dynamics. Because proton transfer and transport dynamics depend strongly on the solvents hydrogen bond dynamics, we are observing the substantial influence of nanoconfinement of proton dynamics.

In the transient absorption experiments, the full time dependent spectrum is taken using a time delayed white light probe following a ~400 nm pump pulse. Form the full spectrum, details of the nature of the initial proton transfer and solvation can be worked out. Figure 8 shows the short time behavior at a single wavelength for bulk water and various size AOT reverse micelles. Similar behavior is observed in Nafion membranes. This short time behavior is related to the initial proton transfer, solvation, and separation of the initially form contact ion pair, that is the separation of the hydronium ion from the deprotonated HPTS. The data in figure 8 demonstrate that as the size of the water nanopool is decreased, the proton solvation and separation dynamics slow significantly. Thus, proton transfer in nanoscopic water systems, such as crowded biological environments, is vastly different from proton transfer in bulk water.

The photoacid experiments can also be used to study long range proton transport dynamics. Once the proton has left the vicinity of the deprotonated HPTS, there is some probability that it will diffuse back and undergo geminate recombination. The result is that the long time scale (nanoseconds) fluorescence signal depends on the nature of proton (hydronium ion) diffusion. Figure 9 shows HPTS time correlated single photon counting fluorescence data on a log plot for bulk water and for w0 = 7 AOT reverse micelles. On a log plot, a power law appears as a straight line. It has been shown theoretical that in a three dimensional bulk liquid, e.g., water, proton diffusion will produce a t^-3/2 power law. As can be seen from the data, this is what is observed following the initial solvation and contact ion pair dynamics. The result in the AOT reverse micelles as well as Nafion membranes is dramatically different. As seen in figure 9, the data are still a power law, but now the power law is t^-0.8. We have observed this same -0.8 exponent over a range of sizes of AOT and a range of hydration levels (channel diameters) in Nafion. Thus, nanoscopic confinement not only changes the short time solvation dynamics, but it also has a substantial influence on the proton transport, which is important for the functioning of fuel cells. To date the -0.8 has not been explained.