Proton Solvation and Transport in Hydrated Nafion

Abstract

Proton solvation properties and transport mechanisms have been studied in hydrated Nafion using the self-consistent multistate empirical valence bond (SCI-MS-EVB) method that includes the effects excess proton charge defect delocalization and Grotthuss proton hopping. It was found that sulfonate groups influence excess proton solvation, as well as the proton hydration structure, by stabilizing a more Zundel-like (H(5)O(2)(+)) structure in their first solvation shells. Hydrate proton-related hydrogen bond networks were observed to be more stable than networks with water alone. Diffusion rates, Arrhenius activation energies, and transport pathways were calculated and analyzed to characterize the nature of the proton transport. Diffusion rate analysis suggests that a proton-hopping mechanism dominates the proton transport for the studied water loading levels and that there is a clear degree of anticorrelation with the vehicular transport. The activation energy drops quickly with an increasing water content when the water loading level is smaller than ∼10 H(2)O/SO(3)(-), which is consistent with experimental observations. The sulfonate groups were also found to affect the proton hopping directions. The temperature and water content effects on the proton transport pathways were also investigated.

Biological Water Dynamics and Entropy: A Biophysical Origin of Cancer and Other Diseases

“…Verdel et al [106,107] attributed increased proton transfer deduced from conductivity measurements to the "autothixotropic" phenomenon (weak gel-like behavior) of water, which supposedly develops spontaneously with time, where ions and hydrophilic surfaces seem to play an important role. Voth et al [105,108,109] have shown that sulfonate groups in the sulfonated fluoropolymer Nafion influence excess proton solvation, as well as the proton hydration structure, by stabilizing a more Zundel-like (H 5 O 2 + ) structure in their first solvation shells [110]. The sulfonate groups were also found to affect the proton hopping directions.…”

“…Studies with Nafion, used in proton exchange membrane-based fuel cells [108,109,111], and with carbon nanotubes (CNTs) functionalized with CF 3 SO 3 H groups [112], have provided additional insights. At low water content, the sulfonated side chains of Nafion form isolated hydrophilic regions.…”

“…Moreover, the pH is thought to be acidic adjacent to the aqueous interphase [7] where the protomerism of nanomolecular ensembles of CD water produces long-range, dynamic, charge-separation by means of the Grotthuss phenomenon [103][104][105][106]108], The in vivo toxicity of hydrated aluminum sulfate to our bodies, whether orally, parenterally, or topically, may prove to be both autocatalytic and systemic, by virtue of interference with the Grotthuss phenomenon and the Josephson phenomenon [243,244]. To wit, insoluble aluminum hydroxide might readily form a floc precipitate responsible for colloid removal in vivo, via the following reaction [241,242] …”

Ion transport in sulfonated polymers

“…[4][5][6] Proton transport in aforementioned sulfonated polymers is commonly understood through a combination of vehicular and hopping motions of the excess protons in the presence of water medium. 52,53 Both experimental and simulation results have suggested that the proton hopping motion increases faster than the vehicular motion as hydration levels increase since the proton diffusion coefficient is much greater than diffusion coefficient of the surrounding water medium. 10,52 At low hydration levels, most of the water molecules and the hydronium ions (H 3 O þ ) are strongly bound to the ÀSO 3 H groups and the proton transport is strongly coupled to and impeded by the structure of the membrane.…”

“…52,53 Both experimental and simulation results have suggested that the proton hopping motion increases faster than the vehicular motion as hydration levels increase since the proton diffusion coefficient is much greater than diffusion coefficient of the surrounding water medium. 10,52 At low hydration levels, most of the water molecules and the hydronium ions (H 3 O þ ) are strongly bound to the ÀSO 3 H groups and the proton transport is strongly coupled to and impeded by the structure of the membrane. 10 In this respect, the most common route to achieving high conductivity from sulfonated polymers is the solvation of ionic domains with water molecules to facilitate proton dissociation from the ÀSO 3 H groups.…”