Hydration, sulfonate groups

These studies showed that sulfonate groups surrounding the hydronium ion at low X sterically hinder the hydration of fhe hydronium ion. The interfacial structure of sulfonafe pendanfs in fhe membrane was studied by analyzing structural and dynamical parameters such as density of the hydrated polymer radial distribution functions of wafer, ionomers, and protons water coordination numbers of side chains and diffusion coefficients of water and protons. The diffusion coefficienf of wafer agreed well with experimental data for hydronium ions, fhe diffusion coefficienf was found to be 6-10 times smaller than the value for bulk wafer. 

Figure 27. Defective structure of solid trifluoromethane-sulfonic acid hydrate (CF3S0sH H20)4 found using ab initio molecular dynamics (AIMD see Section 2.2.3 for a description of the technique), showing two hydronium ions hydro-gen-bonded to sulfonate groups (as found in the perfect structure) but, more importantly, two shared protons (one between two sulfonate groups and the other as part of a Zundel ion see text). Note that the energy of the defective structure is only --30 kj/mol higher than that of the perfect structure.

Nafion is a novel and unique family of polymers which consist of a perfluorinated backbone and short pendant chains terminated by sulfonic groups. When swollen in water, the structure of Nafion is believed to resemble that of an inverse micelle (Fig. 3) [16-18], The hydrated SOf head groups are clustered together in a... 

Fig. 9. Formation of trimeric aggregates of 1,1-ionic surfactants with sulfonate groups. (Hydration and intermolecular interaction. New York Academic 1969]...

Water that is bound to the surfactant aggregates plays a role in interactions with the polar groups. It was shown that micelles can bind considerably more water than is accounted for by hydration of the alkali metal ions. This was taken as evidence that the carboxylate or sulfonate groups bind water through ion-dipole interaction and/or hydrogen bonds. Fatty carboxylic acid and carboxylate groups are linked by hydrogen bonds (AHA, A(HA)2 ), A(HA)n ). These effects have been demonstrated in many other surfactant systems (Ekwall, 1969 Ekwall and Mandell, 1967 Ekwall and Solyom, 1967 Ekwall et al., 1972). 

Along the walls of the aqueous domain, where the sulfonate groups are tethered, hydration of the reactants (either the hydro-nium ion or the water molecule) is possible by the oxygen atoms in SOs" because hydrated protons form Zundel-ion-like and Eigen-ion-like configurations with the end groups. This reaction can be represented by the following equation... 

The relative importance of each of these three reactions is likely a strong function of water content. In order for the bulk-like reaction (Eq. 16a) to take place, one must be at high degrees of hydration, where there is bulk-like water within the membrane. From electronic stracture calculation we are aware that a minimum of three water molecules is required for the dissociation of protons from the sulfonic acid end group, it is likely that the reaction in Eq. (16c) is important only at very low water contents. The reaction in which oxygen atoms of the sulfonate groups act as part of the solvation shell, (Eq. 16b) is likely relevant across a range of intermediate hydration levels. 

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. 

After the sample was hydrated to about 3.5% (19 hours), which corresponds to 3.6 moles H O per sulfonate group, water absorbances were observed in the IR spectrum at 3550,cm. The broad absor-... 

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