For four conforming structures of the quaternized polystyreneblock-poly(ethylene-ran-butylene)-block-polystyrene membrane (QSEBS), (a) tetramethylammonium hydroxide (TMA + OH − ), (b) benzyltrimetylammonium hydroxide, and (c and d) QSEBS segments with 1 and 2 side chains (DBQSEBS), spatial distribution, bond distances, and charge-density profiles were obtained with density functional theory (DFT) and compared with structural simulations of DBQSEBS for two different hydration levels. Results for the TMA + OH − showed that its constituent ions stay metastable in the vicinity of each other and are joined by donor−acceptor interactions. Also, simulations of the other conforming structures show that, in the absence of water, spatial distribution as well as charge-density profiles of trimethylammonium hydroxide do not change with respect to isolated TMA + OH − , demonstrating that the QSEBS chain is a thermodynamically stable backbone to support the functional group, which is in agreement with the literature. When hydrated, simulations of DBQSEBS for water uptake of 4 show that there is a partial dissociation of hydroxide ions due to donor−acceptor interactions acting competitively on them. For water uptake of 6, this dissociation is completed, and hydroxide ions conform to hypercoordinated structures similar to the square-planar arrangement described for pure water medium, but with some structural differences associated with location, type, and interactions among the molecules involved.

The transport via structural diffusion of hydroxide ions through a segment of the functionalized polystyrene-block-poly(ethyleneran-butylene)-block-polystyrene (QSEBS) membrane is analyzed at two hydration levels using ab initio molecular dynamics (AIMD). First, dynamic simulations are carried out to identify and describe the characteristics of structural diffusion with respect to (a) hydration of the conductive polymer and (b) location and solvation pattern of hydroxide ions. Then, hydroxide diffusivity and conductivity are estimated and compared with data from simulations in pure water and experimental conductivity for hydrated QSEBS. A strong influence was found of the hydration and location of hydroxide ions in the polymeric system in the characteristics and frequency of charge-transfer events. As an example of this, hydroxide ions having high coordination numbers or located in dry zones are the molecules with lowest number of structural diffusion events that contribute effectively to hydroxide displacement (nonrattling events) and have the highest mean-lifetimes. Calculated diffusion coefficients and hydroxide conductivities showed a coherent tendency and magnitude with respect to hydration and conductivity in bulk water. Also, simulations show that as hydration increases, so does the number of nonrattling charge-transfer events and contribution to hydroxide mobility by structural diffusion. This not only corroborates the consistency of the developed simulations with respect to proposed physical models for anionexchange membranes in the literature, but also represents an important contribution to the detailed understanding of transport phenomena at the atomic scale and the role of cationic functional chains of the polymer in the development of transport mechanisms at such scale and their contribution to hydroxide mobility.

The presence of cations on injection fluids used during polymer flooding leads to viscosity losses of the polymeric solution and reduces its drag capacity. Thus, understanding the mechanisms of this chemical degradation is crucial to improving the efficiency of these treatments. This study focused on obtaining physical insights into the mechanisms involved in chemical degradation by molecular dynamics simulations. To do this, the interaction energies between a variety of cations present at polymer flooding (Na + , Ca 2+ , Fe 2+ , and Fe 3+ ) and partially hydrolyzed polyacrylamide (HPAM) were calculated. First, several potentials for ion description were evaluated to guarantee a proper description of the ion hydration. Then, multiple simulations were carried out to understand the effect of each ion individually and the synergic effect of a mixture of ions (brine) on the HPAM chain shrinking. The radius of gyration of the HPAM chain was used as an evaluation parameter of the chain shrinking. The results indicate that multivalent cations have a stronger interaction with the polymer than the monovalent cations, exhibiting smaller interaction distances and higher interaction energies. These interaction energies are related to the ionic radius of the cations and their charge. Smaller cations get close enough to avoid the repulsion between charged monomers, being the Coulombic interactions the most important (two-third of the total interaction energy). Thus, the strongest interaction energies with the HPAM correspond to multivalent cations, which reduce the radius of gyration of the HPAM since they can interact with two carboxylate oxygen simultaneously. Interestingly was found a high dependence of the concentration of Fe 3+ cations in the interaction with HPAM; at high concentrations, the cations cannot get close enough to interact with the polymer, but in low concentrations, the cations present the strongest interaction. These findings contribute to understanding the mechanisms that macroscopically are related to viscosity losses in the solution by the cation effect.

This study is focused on evaluating the aggregation behavior of four asphaltene molecules (PA3-type) dissolved in three different solvents. Hence, solution models composed of asphaltenes dissolved in toluene, n-heptane, and mixtures of n-heptane and toluene (known as heptol), were evaluated with molecular dynamics (MD) simulations. For MD calculations, four PA3-type structures were chosen to evaluate the effect of the location of one heteroatom in the polyaromatic core on the aggregation behavior. The first structure consists of only a CH structure, the second has sulfur, the third has nitrogen, and the last one contains oxygen. The heteroatom was replaced in the same location in one of the five-membered rings in the center of the molecule to test the different chemical effects induced by these heteroatoms. Molecules with the heteroatoms showed a higher aggregation tendency in n-heptane than in heptol-50 and toluene, with an average size of 12.8, 6.2, and 2.4 molecules per aggregate, respectively, whereas the PA3 molecule with no heteroatom forms less and smaller aggregates. The inclusion of only one heteroatom increases the polarity and the planarity of the molecule, increasing the attractive interaction energies (140−160% in n-heptane) and the overall aggregation tendency. The molecule with the sulfur heteroatom forms bigger and more stable aggregates than molecules with oxygen and nitrogen. In conclusion, the inclusion of heteroatoms in the PA3 structure increases the polarity and planarity of the molecule, improving the aggregation tendency in all solvents, notoriously in n-heptane.

In this chapter are described the characteristics of transport of hydroxide ions through hydrated polymeric materials with potential application in alkaline fuel cells are described. First, it is made a brief description of anion-exchange membrane fuel cells (AEMFCs), their evolution and key characteristics. Then, this chapter presents a detailed classification of the different types of polymers that have been proposed for AEMFCs and their state of development. After that, mechanisms involved in the transport of hydroxide ions through hydrated anion-exchange membranes are described and discussed, making emphasis in the theoretical approaches applied for their study and their implementation and representability in global transport models. In the final section, it is summarized the key features of the chapter and is made a brief discussion about challenges and future work required for the consolidation of this promising technology.