Proton diffusivity

Figure 8. Three types of polarization of Mn02 (1) J]c (H+ solid), due to proton diffusion in solid (2) rja, due to the solution-solid interface (3) 7/t (ApH), due to a pH change of the electrolyte in the pores.

Penke, B Kinsey, S Gibbs, SJ Moerland, TS Locke, BR, Proton Diffusion and T1 Relaxation in Polyacrylamide Gels A Unified Approach Using Volume Averaging, Journal of Magnetic Resonance 132, 240, 1998. 

Figure 8.31. Principle of a Polymer Electrolyte Membrane (PEM) fuel cell. A Nation membrane sandwiched between electrodes separates hydrogen and oxygen. Hydrogen is oxidized into protons and electrons at the anode on the left. Electrons flow through the outer circuit, while protons diffuse through the...

These measurements showed that in-plane lateral proton diffusion was facilitated at air-water interfaces on which stearic acid monolayers were formed, with a surface diffusion coefficient that depended critically on the physical state of the monolayer, and which was at most ca. 15% of the magnitude in bulk solution. These promising initial studies... 

Figure 11 Schematic of mucosal membrane sodium-proton exchanger and chloride-bicarbonate exchanger responsible for pH homeostasis in enterocyte cytosol. Microclimate pH is maintained by mucosal slowing of proton diffusion away from the lumenal membrane.

Once formed, the protons diffuse through the platinum layer and enter deep into the layer of semi-permeable membrane. They travel from the left-hand side of the membrane to its right extremity in response to a gradient in concentration. (Movement caused by a concentration gradient will remind us of dye diffusing through a saucer of water, as described on p. 129.)... 

More recent quantum-based MD simulations were performed at temperatures up to 2000 K and pressures up to 30 GPa.73,74 Under these conditions, it was found that the molecular ions H30+ and OH are the major charge carriers in a fluid phase, in contrast to the bcc crystal predicted for the superionic phase. The fluid high-pressure phase has been confirmed by X-ray diffraction results of water melting at ca. 1000 K and up to 40 GPa of pressure.66,75,76 In addition, extrapolations of the proton diffusion constant of ice into the superionic region were found to be far lower than a commonly used criterion for superionic phases of 10 4cm2/s.77 A great need exists for additional work to resolve the apparently conflicting data. 

The transition pressure of 75 GPa is much higher than the 30 GPa predicted earlier.65 This difference is likely caused by the use of a much smaller basis set (70 Ry) by Cavazzoni et al. Our results are also in disagreement with simple extrapolations of the proton diffusion constant to high temperatures.77... 

The 0-0 and H-H RDFs (not shown) indicate that no 0-0 or H-H covalent bonds are formed during the simulations at all densities. The g(Roti) shows a lattice-like structure at 115 GPa, which is consistent with proton diffusion via a hopping mechanism between lattice sites.65 At 34 GPa, the coordination number for the first peak in g(RQH) is 2, which indicates molecular H20. Between 95 GPa and 115 GPa, however, the coordination number for the first peak in g(RQH) becomes four, which indicates that water has formed symmetric hydrogen bonds where each oxygen has four nearest-neighbor hydrogens. 

Figure 4. Schematic illustration of correlated proton transfers in pure liquid imidazole leading to proton diffusion but not proton conductivity (see text).

Presently, there is no direct proof for such a mechanism in pure imidazole (e.g., by NMR) however, the observation that the ratio of the proton diffusion and conduction rates virtually coincide with the Boltzmann factor (i.e., exp(— E e)/ kT)), where is the electrostatic separation energy of two unit charges in a continuum of dielectric constant e) is a strong indication. 

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

S. Nomura, Y. Yang, C. Inoue and T. Chida, Observation and evaluation of proton diffusion in porous media by the pH-imaging microscopy using a flat semiconductor pH sensor, Anal. Sci., 18 (2002) 1081-1084. 

Ab initio methods provide elegant solutions to the problem of simulating proton diffusion and conduction with the vehicular and Grotthuss mechanism. Modeling of water and representative Nation clusters has been readily performed. Notable findings include the formation of a defect structure in the ordered liquid water cluster. The activation energy for the defect formation is similar to that for conduction of proton in Nafion membrane. Classical MD methods can only account for physical diffusion of proton but can create very realistic model... 

Pyranine has been used to study the proton dissociation and diffusion dynamics in the aqueous layer of multilamellar phospholipid vesicles [101], There are 3-10 water layers interspacing between the phospholipid membranes of a multilamellar vesicle, and their width gets adjusted by osmotic pressure [102], Pyranine dissolved in these thin layers of DPPC and DPPC+cholesterol multilamellar vesicles were used as a probe for the study. Before the photoreleased proton escapes from the coulombic cage, the probability of a proton excited-anion recombination was found to be higher than in bulk. This was attributed to the diminished water activity in the thin layer. It was found that the effect of local forces on proton diffusion at the timescale of physiological processes is negligible. 

Very humid mortars and concretes, due to proton diffusion, exhibit no sharp carbonation, i.e., pH limit. 

Fast proton diffusion at the surface was modeled by the random exchange of Zr02 and ZrO(OH)2 groups over the entire growing film. 

---