Sulfonate-hydronium

A possible explanation comes from X-ray analyses of the sulfonic acids [45]. All X-rayed crown ether crystals contained water and the sulfonic acid moiety was dissociated. Therefore in crystals of [45], macrocyclic ben-zenesulfonate anions and hydronium ions (sometimes hydrated) are present. The ions are bound to each other by hydrogen bonds. The size of the included water-hydronium ion cluster (varying by the number of solvating water molecules) depends on the ring size. In the 15-membered ring, HsO" was found, whereas in a 21-membered ring HsO and in the 27-membered ring were present. This means the sulfonic acid functions in [45] are... 

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.

Sulfonic acids are listed in narrow ranges above these apply only to anhydrous forms. Such acids hydrate readily to give bands that are probably a result of the formation of hydronium sulfonate salts, in the 1230-1120 cm1 range. 

A series of articles were published by Ennari et al. on MD simulation of transport processes in Poly(Ethylene Oxide) and sulfonic acid-based polymer electrolyte.136,137 The work was started by the determination of the parameters for the ions missing from the PCFF forcefield made by MSI (Molecular Simulations Inc.), to create a new forcefield, NJPCFF. In the models, the proton is represented as a hard ball with a positive charge. Zhou et al. used the similar approach to model Nation.138 The repeating unit of Nafion (Fig. 17) was optimized using ab initio VAMP scheme. The protons were modeled with hydronium ions. Three unit cell or molecular models were used for the MD simulation. The unit cell contains 5000 atoms 20 pendent side chains, and branched Nafion backbone created with the repeating unit. Their water uptakes or water contents were 3, 13, or 22 IEO/SO3, which correspond to the room temperature water uptakes at 50% relative humidity (RH), at 100% RH, and in liquid water respectively.18 The temperature was initially set at a value between 298.15 and 423.15 K under NVE ensemble with constant particle number, constant volume (1 bar), and constant energy. 

Figure 3.3.5 (A) Chemical structure of sulfonated perfluorinated polyethylene (Nafion ). (B) Schematic illustration of the microscopic structure of hydrated Nafion membrane perfluorinated polyethylene backbone chains form spherical hydrophobic clusters. Sulfonic end groups interface with water-filled channels and mediate the migration and diffusion of protons. The channels are filled with water and hydronium ions. Figure adapted from [4].

The last value can be lowered to 1300 kWh t-1, if the diaphragms are replaced by cation exchange membranes of the sulfonate type pretreated to conduct hydronium ions [3, p. 133]. 

Coon et al. [58] have reported the preparation of aftronium trifluoro-methane sulfonate based on the analogy of the related preparation of the perchlorate or fluoroborate. Hydronium trifluoromethanesulfonate is, however, difficult to separate from the nitronium salt... 

The first waters sorbed cause the sulfonate heads to dissociate, resulting in the formation of hydronium ions [26]. The water that hydrates the membrane forms counter-ion clusters localized on sulfonate sites with the sulfonate heads acting as nucleation sites [26]. Given the hydrophobic nature of the backbone, and the hydrophilic nature of the sulfonate heads, it is reasonable to consider that all water molecules sorbed by the membrane at this low water content are associated with the sulfonate heads. Moreover, the hydronium ions will be localized on the sulfonate heads, and, because the amount of water sorbed is insufficient for the formation of a continuous water phase, the conductivity will be extremely low. Figure 4.2a is a schematic... 

The coefficients Ai and A2 reflect the relative size of and D2M respectively. Since the fitted value of Ai is much smaller than A2, this implies that the resistance of the membrane to hydronium diffusion is much larger than to water. Given that hydronium ions have a net charge while water does not, this is reasonable since we expect the interaction forces between the membrane (with charged sulfonate heads) and the hydronium ions to be stronger than the forces between membrane and water. Sensitivity analysis shows [23] that regardless of the order of magnitude of Z)i2 the ratio of the values of and... 

Figure 10.2. Snapshots at different times of a jelly-bean representation of a simulated mixture of water and Nafion in protonated form (at water content A = 5). Dark Red regions denote polymer, grey, blue and yellow regions denote water, hydronium and sulfonate groups, respectively, in the aqueous phase. The indicated box corresponds to a length of 4.5 nm. Note that the jelly-bean surfaces hide a large number of molecules [72].

Figure 12.2. Global minimum energy structures determined by full optimization at the B3LYP/6-31G level of a trifluoromethanesulfonic acid molecule with different numbers of water molecules, showing the 0-0 and O-H (in brackets) distances, and revealing that with (a) 1 and (b) 2 water molecules, the proton does not dissociate with (c) 3, (d) 4, and (e) 5 water molecules, the proton is dissociated forming a contact hydronium ion-sulfonate pair. Redrawn from results first presented in Ref. [23].

The model computes an effective friction coefficient of a hydrated proton (assumed to be a structureless classical hydronium ion and labeled a) in a single pore of length L and cross-sectional radius R, filled with N water molecules each possessing a dipole moment p. All sulfonic acid groups are... 

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