Chemical Charge Density Research Articles

Taylor dispersion analysis using capacitively coupled contactless conductivity detector

Taylor dispersion analysis (TDA) is a simple and absolute method to determine the hydrodynamic radius of solutes that respond to UV or fluorescence detections. To broaden the application range of TDA, it is necessary to develop new detection modes. This study aims to study capacitively coupled contactless conductivity detector (C4D) for the analysis of charged macromolecules. The detection sensitivities and hydrodynamic radii were compared for a C4D detector and a UV detector on positively or negatively charged polymers responding both to UV and C4D (poly-L-lysine and poly(acrylamide-co-2-acrylamido-1-methyl-propanesulfonate). The influence of the composition of the background electrolyte on the detection sensitivity has been studied and optimized for C4D detection. The influence of the molar mass and of the polymer chemical charge density on the C4D and UV sensitivities of detection have been investigated based on well-characterized copolymers samples of different molar masses and charge densities. The advantages and disadvantages compared to UV detection, as well as the range of applicability of C4D detection in TDA were identified. C4D detection can be an alternative method for sizing charged polymers of reasonable molar mass (typically below 105 g mol−1) that do not absorb in UV. A decline in the sensitivity of detection in C4D was observed for higher molar masses.

Switchable ionic selectivity of membranes with electrically conductive surface: Theory and experiment

Nanoporous membranes with electrically conductive surface represent an important class of stimuli–responsive materials. The variation of surface potential provides a powerful tool for adjusting ionic selectivity, conductivity, and rejection. This work is devoted to the theoretical and experimental study of switchable ionic selectivity. The one–dimensional Space charge (SC) and two–dimensional Uniform potential (UP) models are first generalized to constant surface potential case taking into account the Stern layer with inner (iHp) and outer (oHp) Helmholtz planes. The ionic selectivity is investigated experimentally by measuring the membrane potential at zero current for C–Nafen membranes prepared from alumina nanofibers with conductive carbon coating. The evolution of charging current is used to determine the dependence of surface charge density and differential capacitance on the applied potential. These data are fitted by the UP and SC models to find the Stern layer capacitance. It is shown that the variation of surface potential results in a continuous change of ionic selectivity from anion to cation. The membrane potential data are fitted by the UP and SC models using the chemical charge density and concentration boundary layer thickness as fitting parameters. It allows to determine the potential, at which the membrane becomes non–selective. The SC and UP models provide close results for membrane potential and surface charge density and demonstrate a good agreement with the experimental data. The UP model overestimates the solution velocity and ion concentrations at the membrane surface, while it underestimates the ion fluxes and iHp/oHp potentials. This work essentially extends our understanding of ion transport in stimuli–responsive membranes operated by the electric field. The results can be applied in the area of nanofiltration, (reverse) electrodialysis, electrochemical sensors, and nanofluidic devices.

Separation and Characterization of Highly Charged Polyelectrolytes Using Free-Solution Capillary Electrophoresis.

The characterization of statistical copolymers of various charge densities remains an important and challenging analytical issue. Indeed, the polyelectrolyte (PE) effective electrophoretic mobility tends to level off above a certain charge density, due to the occurrence of Manning counterion condensation. Surprisingly, we demonstrate in this work that it is possible to get highly resolutive separations of charged PE using free-solution capillary electrophoresis, even above the critical value predicted by the Manning counterion condensation theory. Full separation of nine statistical poly(acrylamide-co-2-acrylamido-2-methylpropanesulfonate) polymers of different charge densities varying between 3% and 100% was obtained by adjusting the ionic strength of the background electrolyte (BGE) in counter electroosmotic mode. Distributions of the chemical charge density could be obtained for the nine PE samples, showing a strong asymmetry of the distribution for the highest-charged PE. This asymmetry can be explained by the different reactivity ratios during the copolymerization. To shed more light on the separation mechanism, effective and apparent selectivities were determined by a systematic study and modeling of the electrophoretic mobility dependence according to the ionic strength. It is demonstrated that the increase in resolution with increasing BGE ionic strength is not only due to a closer matching of the electroosmotic flow magnitude with the PE electrophoretic effective mobility, but also to an increase of the dependence of the PE effective mobility according to the charge density.

Effect of Dendrigraft Generation on the Interaction between Anionic Polyelectrolytes and Dendrigraft Poly(l-Lysine).

In this present work, three generations of dendrigraft poly(l-Lysine) (DGL) were studied regarding their ability to interact with linear poly (acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonate) (PAMAMPS) of different chemical charge densities (30% and 100%). Frontal analysis continuous capillary electrophoresis (FACCE) was successfully applied to determine binding constants and binding stoichiometries. The effect of DGL generation on the interaction was evaluated for the first three generations (G2, G3, and G4) at different ionic strengths, and the effect of ligand topology (linear PLL vs. dendrigraft DGL) on binding parameters was evaluated. An increase of the biding site constants accompanied with a decrease of the DGL-PAMAMPS (n:1) stoichiometry was observed for increasing DGL generation. The logarithm of the global binding constants decreased linearly with the logarithm of the ionic strength. This double logarithmic representation allowed determining the extent of counter-ions released from the association of DGL molecules onto one PAMAMPS chain that was compared to the total entropic reservoir constituted by the total number of condensed counter-ions before the association.

The Effect of Molar Mass and Charge Density on the Formation of Complexes between Oppositely Charged Polyelectrolytes.

The interactions between model polyanions and polycations have been studied using frontal continuous capillary electrophoresis (FACCE) which allows the determination of binding stoichiometry and binding constant of the formed polyelectrolyte complex (PEC). In this work, the effect of the poly(l-lysine) (PLL) molar mass on the interaction with statistical copolymers of acrylamide and 2-acrylamido-2-methyl-1-propanesulfonate (PAMAMPS) has been systematically investigated for different PAMAMPS chemical charge densities (15% and 100%) and different ionic strengths. The study of the ionic strength dependence of the binding constant allowed the determination of the total number of released counter-ions during the formation of the PEC, which can be compared to the total number of counter-ions initially condensed on the individual polyelectrolyte partners before the association. Interestingly, this fraction of released counter-ions, which was strongly dependent on the PLL molar mass, was almost independent of the PAMAMPS charge density. These findings are useful to predict the binding constant according to the molar mass and charge density of the polyelectrolyte partners.

Prediction of Polyelectrolyte Complex Stoichiometry for Highly Hydrophilic Polyelectrolytes

The interaction between two hydrophilic polyelectrolytes of opposite charges was investigated using poly(l-lysine) (PLL) as the polycation and a library of copolymers of acrylamide and 2-acrylamido-2-methyl-1-propanesulfonate (P(AM-co-AMPS)) with various chemical charge densities as polyanions. The formation of polyelectrolyte complexes (PECs) was comparatively studied by varying different parameters, such as the mixing order, the P(AM-co-AMPS) chemical charge density and the initial polycation to polyanion molar ratio. PECs were then characterized in terms of charge stoichiometry and of stability toward ionic strength. The results showed a strong dependency of precipitated PEC stoichiometry on the P(AM-co-AMPS) chemical charge density and the initial polycation to polyanion molar ratio. In contrast, PEC stoichiometry was not affected by the mixing order of the two polyelectrolyte partners. A general rule capable of predicting the PEC stoichiometry is proposed.

Determination of Polymer Effective Charge by Indirect UV Detection in Capillary Electrophoresis: Toward the Characterization of Macromolecular Architectures

This work deals with the determination of polymer effective charge based on the sensitivity of detection in capillary electrophoresis using indirect UV detection. In this detection mode, the polyelectrolyte (solute) displaces a certain quantity of probe contained in the background electrolyte and having the same charge as the solute. This quantity of displaced probe is directly correlated to the effective charge of the solute. Contrary to other electrophoretic methods generally used for monitoring changes in effective charge, this methodology is not based on the effective mobility of the polyelectrolyte (i.e., migration times) but on the sensitivity of detection (i.e., peak areas). Experimental values of effective charge obtained for statistical copolymers of poly(acrylamide-co-sodium 2-acrylamido-2-methylpropanesulfonate), for homopolymers of poly(diallyldimethylammonium chloride), poly(acrylic acid), and poly(metacrylic acid), are compared to the Manning theory of charge condensation. Interestingly, this methodology can be used for the characterization of macromolecular architecture since the effective charge of copolymers highly depends on the repartition of the charged monomers along the polymer chain. As an example, this methodology can easily distinguish statistical copolymers from diblock copolymers of similar chemical charge densities. Experiments were carried out on diblock copolymers of poly(acrylic acid)-b-poly(ethylene oxide) (PAA-b-PEO) and poly(methacrylic acid)-b-poly(ethylene oxide) (PMAA-b-PEO). The variation of the sensitivity of detection of statistically charged copolymers in the indirect UV mode with the chemical charge density is also discussed in detail.