  The uptake and sorption of charged molecules by responsive polymer membranes
and hydrogels in aqueous solutions is of key importance for the development of
soft functional materials. Here we investigate the partitioning of simple
monoatomic (Na$^+$, K$^+$, Cs$^+$, Cl$^-$, I$^-$) and one molecular ion
(4-nitrophenolate; NP$^-$) within a dense, electroneutral
poly($N$-isopropylacrylamide) membrane using explicit-water molecular dynamics
simulations. Inside the predominantly hydrophobic environment water distributes
in a network of polydisperse water nanoclusters. The average cluster size
determines the mean electrostatic self-energy of the simple ions, which
preferably reside deeply inside them; we therefore find substantially larger
partition ratios $K\simeq\>$10$^{-1}$ than expected from a simple Born picture
using a uniform dielectric constant. Despite their irregular shapes we observe
that the water clusters possess a universal negative electrostatic potential
with respect to their surrounding, as is known for aqueous liquid-vapor
interfaces. This potential, which we find concealed in cases of symmetric
monoatomic salts, can dramatically impact the transfer free energies of larger
charged molecules because of their weak hydration and increased affinity to
interfaces. Consequently, and in stark contrast to the simple ions, the
molecular ion NP$^-$ can have a partition ratio much larger than unity,
$K\simeq\>$10-30 (depending on the cation type) or even $10^3$ in excess of
monovalent salt, which explains recent observations of enhanced reaction
kinetics of NP$^-$ reduction catalyzed within dense polymer networks. These
results also suggest that ionizing a molecule can even enhance the partitioning
in a collapsed, rather hydrophobic gel, which strongly challenges the
traditional simplistic reasoning.
