Hydration shell stripping governs ion rejection in PAP[5] water channels

Peptide appended pillar[5]arene (PAP[5]) molecules are artificial water channels with permeability comparable to aquaporin proteins. Although PAP[5] embedded block copolymer membranes have been extensively studied for water desalination, a comprehensive understanding of ion rejection characteristics of these channels is lacking. In this study, we compute the adiabatic work W required to transfer Na⁺ and Cl⁻ ions from the bulk solution to the pore center, and thereby obtain a rejection efficiency. We obtain the work using a novel method that eliminates the effects of residual stress. We find work values close to those obtained for carbon nanotubes of similar diameter, which are capable of perfect ion rejection. To pass through the channel, ions must strip off their hydration shells, which incurs a large free energy penalty. The W values for Na⁺ and Cl⁻ are three orders of magnitude greater than the work done by pressure head of 10⁷ Pa as an ion passes through the pore, resulting in rejection efficiencies for both Na⁺ and Cl⁻ of practically 100 percent.