Modulation of ionization and structural properties of weak polyelectrolytes due to 1D, 2D, and 3D confinement

What is the impact of reducing the space available to molecules onto their properties is a fundamental question for capillary systems, molecular biology and transport, protein and material sciences. Possibly influenced by space restriction, ionization degree has rarely been studied for confined polyelectrolytes; Monte Carlo titrations and coarse-grained models are thus used to investigate structural and ionization changes induced on a single polyelectrolyte chain by confinement into slit (1D), cylindrical (2D), or spherical (3D) cavities. Four polyelectrolyte models differing in chain stiffness and the possible formation of charged hydrogen bonds (cHbonds) are studied. Low pH effective ionization constants (pKa) of confined chains are lower than for the free species if cHbonds can be formed. This is especially evident for 3D-confined stiff chains, a finding rationalized by the impact of global compression onto chain conformations. If no cHbonds are allowed, chain ionization is largely unaffected by 1D or 2D confinement, while it is depressed by 3D. Chain confinement Helmholtz energy (ΔAconf) was computed as a function of both pH and confining width (W) to gauge the impact of ionization-induced stiffening onto ΔAconf versus W behavior, the partition coefficient k(pH, w) governing absorption, and the average number of cHbond formed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1088–1102.