Molecular modeling of polymeric adsorbent media: the effects of counter-ions on ligand immobilization and pore structure

Resin structure impacts two-component protein adsorption and separation in anion exchange chromatography

The influence of the resin structure, on the competitive binding and separation of a two-component protein mixture with anion exchange resins is evaluated using conalbumin and green fluorescent protein as a model system. Two macroporous resins, one with large open pores and one with smaller pores, are compared to a resin with grafted polymers. Investigations include measurements of single and two-component isotherms, batch uptake kinetics and two-component column breakthrough. On both macroporous resins, the weaker binding protein, conalbumin, is displaced by the stronger binding green fluorescent protein. For the large pore resin, this results in a pronounced overshoot and efficient separation by frontal chromatography. The polymer-grafted resin exhibits superior capacity and kinetics for one-component adsorption, but is unable to achieve separation due to strongly hindered counter-diffusion. Intermediate separation efficiency is obtained with the smaller pore resin. Confocal laser scanning microscopy provides a mechanistic explanation of the underlying intra-particle diffusional phenomena revealing whether unhindered counter-diffusion of the displaced protein can occur or not. This study demonstrates that the resin's intra-particle structure and its effects on diffusional transport are crucial for an efficient separation process. The novelty of this work lies in its comprehensive nature which includes examples of the three most commonly used resin structures: a small pore agarose matrix, a large-pore polymeric matrix, and a polymer grafted resin. Comparison of the protein adsorption properties of these materials provides valuable clues about advantages and disadvantages of each for anion exchange chromatography applications.

Deconvolution of electroosmotic flow in hysteresis ion transport through single asymmetric nanopipettes

Unveiling the contributions of electroosmotic flow (EOF) in the electrokinetic transport through structurally-defined nanoscale pores and channels is challenging but fundamentally significant because of the broad relevance of charge transport in energy conversion, desalination and analyte mixing, micro and nano-fluidics, single entity analysis, capillary electrophoresis etc. This report establishes a universal method to diagnose and deconvolute EOF in the nanoscale transport processes through current-potential measurements and analysis without simulation. By solving Poisson, Nernst-Planck (PNP) with and without Navier-Stokes (NS) equations, the impacts of EOF on the time-dependent ion transport through asymmetric nanopores are unequivocally revealed. A sigmoidal shape in the I-V curves indicate the EOF impacts which further deviate from the well-known non-linear rectified transport features. Two conductance signatures, an absolute change in conductance and a 'normalized' one relative to ion migration, are proposed as EOF impact (factor). The EOF impacts can be directly elucidated from current-potential experimental results from the two analytical parameters without simulation. The EOF impact is found more significant in intermediate ionic strength, and potential and pore size dependent. The less-intuitive ionic strength and size dependence is explained by the combined effects of electrostatic screening and non-homogeneous charge distribution/transport at nanoscale interface. The time-dependent conductivity and optical imaging experiments using single nanopipettes validate the proposed method which is applicable to other channel type nanodevices and membranes. The generalizable approach eliminates the need of simulation/fitting of specific experiments and offers previously inaccessible insights into the nanoscale EOF impacts under various experimental conditions for the improvement of separation, energy conversions, high spatial and temporal control in single entity sensing/manipulation, and other related applications.

Insights into the Role of Counterions on Polyelectrolyte-Modified Nanopore Accessibility

Nanopores play a decisive role in different technologies from oil production, separation, and sensing to drug delivery or catalysis and energy conversion. In recent years, abilities to functionalize nanopores have advanced significantly. Thereby, nanopores functionalized with polyelectrolytes or responsive polymers show fascinating transport properties, such as gated or gradually controlled ionic permselectivity. Nonetheless, understanding the influence of external parameters such as ion type or concentration on nanopore performance, and thus on the mentioned applications, remains a challenge but is crucial for applications. In this work, the effect of different counterions on the wetting and ionic transport in poly(2-(methacryloyloxy)ethyltrimethylammonium chloride)-functionalized silica mesopores (pore diameter <10 nm) was experimentally and theoretically investigated. Static contact angles covered a range from 45 to almost 90° by exclusively changing the counterion. Ionic pore accessibility was also strongly dependent on the counterion present and was found to gradually change from accessible pores up to complete, pH-independent ion exclusion. On the basis of molecular theory calculations, these experimental observations were rationalized on the basis of ion binding between the [2-(methacryloyloxy)ethyl]trimethylammonium chloride monomers and the counterions. In addition, the theoretical framework provided a nanoscopic view into the molecular organization inside the pores, showing a strong dependence of ion concentration and ion distribution profiles along the pore radius in dependence of the present ions. The obtained insights on the role of counterion type and ion binding in nanopores are expected to have direct impact on the above-mentioned applications.

Modeling the construction of polymeric adsorbent media: effects of counter-ions on ligand immobilization and pore structure

Molecular dynamics modeling and simulations are employed to study the effects of counter-ions on the dynamic spatial density distribution and total loading of immobilized ligands as well as on the pore structure of the resultant ion exchange chromatography adsorbent media. The results show that the porous adsorbent media formed by polymeric chain molecules involve transport mechanisms and steric resistances which cause the charged ligands and counter-ions not to follow stoichiometric distributions so that (i) a gradient in the local nonelectroneutrality occurs, (ii) non-uniform spatial density distributions of immobilized ligands and counter-ions are formed, and (iii) clouds of counter-ions outside the porous structure could be formed. The magnitude of these counter-ion effects depends on several characteristics associated with the size, structure, and valence of the counter-ions. Small spherical counter-ions with large valence encounter the least resistance to enter a porous structure and their effects result in the formation of small gradients in the local nonelectroneutrality, higher ligand loadings, and more uniform spatial density distributions of immobilized ligands, while the formation of exterior counter-ion clouds by these types of counter-ions is minimized. Counter-ions with lower valence charges, significantly larger sizes, and elongated shapes, encounter substantially greater steric resistances in entering a porous structure and lead to the formation of larger gradients in the local nonelectroneutrality, lower ligand loadings, and less uniform spatial density distributions of immobilized ligands, as well as substantial in size exterior counter-ion clouds. The effects of lower counter-ion valence on pore structure, local nonelectroneutrality, spatial ligand density distribution, and exterior counter-ion cloud formation are further enhanced by the increased size and structure of the counter-ion. Thus, the design, construction, and functionality of polymeric porous adsorbent media will significantly depend, for a given desirable ligand to be immobilized and represent the adsorption active sites, on the type of counter-ion that is used during the ligand immobilization process. Therefore, the molecular dynamics modeling and simulation approach presented in this work could contribute positively by representing an engineering science methodology to the design and construction of polymeric porous adsorbent media which could provide high intraparticle mass transfer and adsorption rates for the adsorbate biomolecules of interest which are desired to be separated by an adsorption process.