Abstract
Counter-ion binding and mobility in aqueous solutions of partially hydrophobic ionene oligoions is studied here by a combination of all-atomic molecular dynamics (MD) simulations and NMR (19F and 81Br nuclei) measurements. We present results for 12, 12–ionenes in the presence of different halide ions (F−, Cl−, Br− and I−), as well as their mixtures; the latter allowing us to probe counter-ion selectivity of these oligoions. We consolidate both structural and dynamic information, in particular simulated radial distribution functions and average residence times of counter-ions in the vicinity of ionenes and NMR data in the form of counter-ion chemical shift and self-diffusion coefficients. On one hand, previously reported enthalpy of dilution and mixing measurements show a reverse counter-ion sequence for 12, 12–ionenes with respect to their less hydrophobic 3, 3– and 6, 6– analogues. On the other hand, the current MD and NMR data, reflecting the counter-ion binding tendencies to the ionene chain, give evidence for the same ordering as that observed by MD for 3, 3–ionenes. This is not seen as a contradiction and can be rationalized on the basis of increasing chain hydrophobicity, which has different consequences for enthalpy and ion-binding. The latter is reflecting free energy changes and as such includes both enthalpic and entropic contributions.
Highlights
Understanding polyelectrolyte solutions is of importance for basic sciences, most notably for chemistry and biology, as well as, for various technologies.[1,2,3,4,5] In last decades we witness a revival of interest in polyelectrolyte studies, resulting in new materials and applications, ranging from electronics to medicine
We consolidate both structural and dynamic information, in particular radial distribution functions and average residence times in the vicinity of 12, 12–ionenes obtained by simulation on one hand, chemical shift and counter-ion self-diffusion coefficients measured by NMR on the other
Notice that 6, 6– and 3, 3–ionene radial distribution function (RDF) were collected during the molecular dynamics (MD) simulations presented in Ref. 29 but have not been published so far
Summary
Understanding polyelectrolyte solutions is of importance for basic sciences, most notably for chemistry and biology, as well as, for various technologies.[1,2,3,4,5] In last decades we witness a revival of interest in polyelectrolyte studies, resulting in new materials and applications, ranging from electronics to medicine (see for example Refs. 6–8). Experimental studies have been performed for x, y equal to 3,3–, 4,5–, 6,6–, and 6,9– to 12,12–ionenes, that is for increasingly more hydrophobic polyelectrolytes.[16,17,18,19,20,21,22,23] Among physico–chemical properties we measured enthalpies of dilution and mixing, osmotic coefficients, dielectric relaxation, conductivities and transport numbers These solutions have been examined using NMR, X-ray and neutron scattering methods.[24,25] Experimental studies were accompanied by molecular dynamics (MD) simulations.[17,26,27,28,29]. Special attention is paid to the nature of counter-ions and their binding competition to ionene chains in aqueous solutions We consolidate both structural and dynamic information, in particular radial distribution functions and average residence times in the vicinity of 12, 12–ionenes obtained by simulation on one hand, chemical shift and counter-ion self-diffusion coefficients measured by NMR on the other. This may modify the solvation of embedded charges and influence the potential of mean force between the macromolecules and ions in solution.[21,31]
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