Selective hydration of the 'short-side-chain' perfluorosulfonic acid membrane. An ONIOM study

We evaluate the hydration of the short-side-chain perfluorosulfonic acid membrane through a comparative study of the energetics of an oligomeric fragment of the polymer using ONION! DFT/HF molecular orbital calculations. Extensive searches for minimum energy conformations of the three pendant side chain oligomeric fragment of the polymer, CF(3)(CF(-O(CF(2))(2)SO(3)H)(CF(2))(7))(2) CF( -O(CF(2))(2)SO(3)H)CF(3), at the B3LYP/63 IG**:HF/3-21G** level with from 6 to 9 explicit water molecules revealed that at the lower range of the examined hydration (i.e. 2 H(2)O/SO(3)H) the uniform hydration of the sulfonic acid groups results in the lowest energy and therefore most favorable state of the system. Our calculations showed, however, that as the degree of hydration is increased the energetic preference for uniform hydration decreases, disappearing altogether at 3 H(2)O/SO(3)H. Furthermore, we found that water distributions that facilitate a higher degree of dissociation and separation of the protons are important factors in stabilizing the fragments. These calculations provide a base line set of results for which the effects of distinct backbone and side chain chemistry maybe explored on hydration in minimally hydrated candidate polymer electrolyte membranes. (C) 2007 Elsevier B.V. All rights reserved.