Hydrated protons structural models

Further, the coarse grained nature of the algorithm will allow the extension of modeling of proton transport in bulk water to PFSA membranes because hydrated protons form similar Zundel-ion-like stractme and Eigen-ion-like structure with the oxygen of the sulfonate groups, which can be easily integrated into the RMD formalism. 

The present study has provided a critical test of the importance of cross relaxation involving both water and protein protons in hydrated protein systems. In addition it has successfully demonstrated that the temperature dependence of both the water and protein relaxation is dominated by motions in the water phase and not by motions in the solid phase such as methyl group rotations. While a detailed analysis has not yet been attempted, it appears that a picture of water in the interfacial regions around a protein that is consistent with the NMR relaxation data is one characterized by fast if slightly anisotropic motion. Structural models for water-protein interactions must be consistent with this very fluid character of water in this interfacial region-, however, it is also important to recognize that the time scale appropriate to the present experiments is still long when compared to the rotational correlation times or diffusion times usually associated with water in the pure liquid state. 

From the definition of acidity in solution as presented in Eq. (7.4), it is clear that in order to compute AG° one must know the proton solvation energy. However, from the previous discussion on the structural models for the hydrated proton one may anticipate some difficulties. For example, how many water molecules should be considered in the calculation of the proton solvation energy In other words How large should one take the H (H20) cluster One reasonable approach should be to examine the convergence of... 

The hydrated proton in liquid water, certainly the most important positive ion in solution, has been the subject of great interest and continuous study. A widely accepted model assumes that the proton is contained in the symmetric structure H30 (H20)3. Since the arguments in support of the structure have dealt with the isolated H30 (H20)3 ion, results obtained for the gas-phase proton hydrates should have validity in confirming or reputing the proposed structure. The pertinent results from the mass spectrometric clustering equilibria measurements are shown in Fig. 12. The equilibrium concentrations of the clusters clearly show that the ion H904 is not exceptionally stable since its concentration fits in the... 

Free hydrated protons produced by acid dissociation move inside of porous domains, relative to the interfacial layer of anionic surface groups. Self-consistent theories and coarse-grained molecular simulations can help to rationalize this structural picture. After demonstrating consistency with existing data on structure and transport, these modeling approaches could be employed as predictive tools in polymer design. 

The important impact of these experimental insights for molecular modeling is that the development of structure versus property relations of PEMs does not require multiscale approaches going all the way to the macroscopic scale. Rather, the main job is done if one arrives at the scale of several 10s of nanometers. Notably, operation at low hydration emphasizes even more the importance of (sub)nanoscale phenomena controlled by explicit interactions in the polymer-water-proton system. 

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