Surface Proton Conduction in Biology and at Monolayers

In spite of diverging predictions of surface proton diffusion coefficients, the studies quoted above provide consistent accounts of the impact of monolayer composition, reduced dimensionality, and interfacial ordering on proton dynamics. Altogether, there is ample evidence for efficient surface proton transport, which is sensitive to the packing density and chemical nature of acid headgroups. Surface pressure, surface electrostatic potential, and lateral proton conductivity increase dramatically upon monolayer compression below a critical area with typical values in the range of 25 to 40 K per SG. This critical area corresponds to a nearest-neighbor separation distance of SG of 6.5-7 A (Leite et al., 1998 Mitchell, 1961). 

A targeted experimental study on interfacial proton transport with a view to designing PEM with improved water retention and proton conductivity was reported by Matsui et al. (2011). The authors performed surface pressure and conductivity 

Proton Conduction at Simulated Surfaces Theory and Computation 

A tryiuoromethane sulfonic acid monohydrate (TAM) solid was explored by Eikerling et al. (2003). Although this system does not resemble a structure for surface proton conduction in the PEM, it allows studying correlation effects at high density of triflic acid groups, the sidechain head groups in PFSA ionomer membranes, and under conditions of low hydration. 

Higher hydrates of triflic acid were studied by Hayes, Paddison, and Tuckerman using path integral Car-Parrinello MD (Hayes et al., 2009, 2011). The larger amount of water available per acidic proton led to the formation of larger proton complexes and more versatile proton defect structures. Formation of such defects involves local proton transfer events. The observed defects correspond to local structures. They are energetically expensive to form, and it seems unlikely that they could propagate in the crystal. 

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