Ines F. Pierola

Removal of Hg(II) from acid aqueous solutions by poly(N-vinylimidazole) hydrogel

Caption of Hg(II) from acid aqueous solution by immersed poly(N-vinylimidazole) hydrogel particles was studied as a function of pH, counterion, and cation concentration. Fitting parameters to several sorption isotherms have been determined. Their values depend mostly on pH and less, on temperature and counterion, and suggest a large affinity of imidazole groups in the gel and mercury cations. Practically total removal (94.4%) of Hg(II) is achieved at pH = 2, with 10 g of dry gel per liter of solution, when cation concentration was as large as 15,000 ppm (0.075 M). Polymer protonation decreases about fourfold the cation affinity, supporting competitive protonation-complexation mechanisms. By its side, metal uptake decreases polymer protonation. Thermal stability of loaded gels decreases with respect to metal free hydrogels. Scanning electron micrographs reveal no changes in the gel morphology upon cation binding, but Tg increases significantly with the Hg(II) content of loaded gels and swelling decreases moderately, indicating the role of the cation as ionic crosslinker. Practically total elution of Hg(II) is achieved with 1 M HNO3 in consecutive loading-elution cycles.

Equilibrium swelling properties of poly(N-vinylimidazole-co-sodium styrenesulfonate) hydrogels

Abstract Hydrogels of N-vinylimidazole (VI) and sodium styrenesulfonate (SSS) were synthesized by radical crosslinking copolymerization. Contrary to expectation, for some SSS compositions, the equilibrium swelling (Seq) in deionized water shows a minimum with values close to those of collapsed states. In aqueous solutions of NaCl, Seq either remains constant, increases, or decreases with changes of the ionic strength, depending on the gel composition. These peculiarities are interpreted in terms of a sodium–proton exchange followed by the interaction of sulfonate groups with neighboring protonated imidazole rings, which behave as ionic crosslinks. The crosslink density due to ionic interactions was determined, as a function of composition, in deionized water, in methanol, and in 0.2 N NaCl aqueous solution.

pH‐dependence of the swelling capacity of poly(N‐vinylimidazole) hydrogels

The swelling capacity (S) of poly(N-vinylimidazole) (PVI) hydrogels in aqueous acid solutions was measured as a function of the pH, the nature of the acid, and the polymer network structure. In acid solutions, imidazole groups become protonated and the swelling capacity increases to reach a maximum corresponding to a degree of protonation equivalent to the Manning limit charge density. The variable controlling the degree of protonation, and therefore the swelling capacity, is the mass of dry polymer immersed in a given volume of solution, which was called the gel effective concentration. Divalent counteranions reduce significantly S with respect to the same ionic strength and degree of protonation, obtained with monovalent anions, while non-complexant divalent cations have no specific influence on the PVI swelling capacity. Results were discussed in terms of the effective charge density caused by counterion condensation on protonated PVI, the non-covalent crosslink density caused by ionic interactions with divalent anions, and the non-Gaussian character of chains between crosslinks.

Ion pairing and anion-driven aggregation of an ionic liquid in aqueous salt solutions.

A relevant question regarding ionic liquids is whether they exist in aqueous solution as totally dissociated ions or as ionic pairs, molecularly dispersed or forming part of aggregates. Several methods were employed here to evaluate these points by comparing the results of an ionic liquid, 1-ethyl-3-methylimidazolium tosylate ([emim][TOS]) with its model compound p-toluenesulfonic acid (pTSA). Conductivity measurements of [emim][TOS] and pTSA dissolved in deionized water support the existence of small amounts of aggregates for both compounds, with a larger extent in the first case. However, apparent molar volume measurements provide no clear evidence for aggregation in aqueous solution or in NaCl aqueous solutions. In contrast, the excimer emission and the absorption and excitation spectra prove the existence of aggregates of [emim][TOS] and pTSA anions in aqueous solution, with and without salt, giving in addition structural information about them. It was thus found that only [emim][TOS] forms ion pairs in deionized water, which dissociate in the presence of NaCl. J-aggregates of pTSA and [emim][TOS] anions, with slip angles that decrease with increasing concentration, were observed through the excitation spectra, and the roles of the anion and cation as well as the effect of the presence of NaCl were analyzed. Thanks to aggregation-induced emission enhancement, fluorescence is much more sensitive than conductivity or density measurements to detect aggregation.

Chemical Imaging of Phase-Separated Polymer Blends by Fluorescence Microscopy

Blends of poly(vinylacetate) (PVAc) and poly(cyclohexylmethacrylate) (PCHMA) labeled by copolymerization with 4-methacryloylamine-4′-nitrostilbene (Sb), with (1-pyrenylmethyl)methacrylate (Py), or with 3-(methacryloylamine)propyl-N-carbazole (Cbz) were prepared by casting dilute solutions in tetrahydrofurane (THF) or chloroform. Films about 10 μm thick were formed. Phase separation in two types of domains is observed by transmission optical microscopy (TOM) and epifluorescence microscopy (EFM): small craters of 1 to 10 μm placed at the polymer-air interface and larger domains, on the scale of 100 μm. The morphology of samples depends on the composition of the polymer blend and on solvent. The green fluorescence of Sb, the violet of Py, or the blue of Cbz provides imaging of the distribution of PCHMA in the different domains and in the matrix. It is thus observed that (i) superficial craters and large domains are formed mainly by PCHMA and (ii) the matrix is composed of PVAc in films cast from THF and it is a blend of the two polymers, homogeneous at the submicrometric scale, for chloroform. The emission intensity of Py, recorded by microfluorescence spectroscopy (MFS), yields a mapping similar to imaging detection. It is remarkable that in films cast from chloroform, the smaller domains are distributed with a 2D hexatic order disrupted by dislocations and disclinations, whereas in films cast from THF, a larger heterogeneity is found, denoting different mechanisms of solvent evaporation.

The pH inside a swollen polyelectrolyte gel: poly(N-vinylimidazole).

The pH inside a dissolved polyelectrolyte coil or a swollen ionic polymer network is not accessible to direct measurement. It is here calculated through a simple model, based on Donnan equilibrium, counterion condensation (for charge density exceeding the critical value), and balance of mobile ions, without any assumption on the pKa of the ionizable groups. The data needed for the calculation with this model are polymer concentration, pH value in the initial solution, and pH value in the bath at equilibrium. All three can be determined experimentally by a batch method where the polymer is immersed in a different pot for each starting pH. The model is applied to a sample system, namely, chemically cross-linked poly(N-vinylimidazole) immersed in acidic baths of different pH values. The imidazole units are basic and become protonated by the acid, thus changing the pH of the initial bath. The model shows how the pH developed inside the swollen gel is several units higher than the pH of the bath at equilibrium, both with or without the correction for counterion condensation. Consequently, when the pKa of the polyelectrolyte is determined in the usual way (with the pH measured in the external bath), it gives an apparent value that is several units below the pKa determined from the actual pH inside the swollen gel at equilibrium. The inclusion of the counterion condensation decreases very slightly the polymer basicity. Surface effects and intramolecular association between protonated and unprotonated imidazole rings are discussed to explain the pKa behavior in the limit of low degree of ionization.

SWELLING PROPERTIES OF POLY(N-VINYLIMIDAZOLE) HYDROGELS

The swelling capacity of poly(N-vinylimidazole) hydrogels in aqueous solutions and organic solvents was measured as a function of polymer network structure. In pure water, the swelling capacity decreases monotonously upon increasing the crosslinker ratio and the total comonomers concentration in the polymerization feed mixture. In methanol a similar behaviour was observed while in ethanol, the swelling capacity shows an abrupt change from large values to 0.01 grams of water per gram of dry gel, that is to say, PVI experience swollen-collapsed transitions with small changes of the gel crosslinking degree. Molecular weight of chains between crosslinks, determined from swelling measurements in methanol, scales with the product of the crosslinker and total comonomers concentrations in the feed mixture to m1.14-0.05. The dependence of the polymer solvent interaction parameter on the polymer concentration inside the swollen gel was also determined for water and ethanol.

Salting-in effect of ionic liquids on poly(N-vinylimidazole) hydrogels

This work attempts to study the interaction of two ionic liquids (1-ethyl-3-methylimidazolium tosylate and 1-hexyl-3-methylimidazolium chloride) with chemically crosslinked poly(N-vinylimidazole) in aqueous media, and to compare it with that of NaCl, a typical salting-out electrolyte. The three salts show a salting-in effect whose intensity was measured on the basis of the decrease of the polymer–solvent interaction parameter and the increase of swelling with increasing ionic strength. It was thus found that the salting-in effect is the same for the two salts with the same anion (chloride), while the intensity of the salting-in effect exerted by the tosylate ionic liquid is larger. The coefficient of selective sorption, which expresses the salt excess inside the swollen gel with respect to the external solution was determined by comparing the initial and equilibrium compositions of the immersion bath. These results are discussed in terms of the hydrophobic character of the ions involved.

Poly(N-vinylimidazole) Gels as Insoluble Buffers that Neutralize Acid Solutions without Dissolving

Typical buffers are solutions containing weak acids or bases. If these groups were anchored to insoluble gels, what would be their behavior? Simple thermodynamics is used to calculate the pH in two-phase systems that contain the weak acid or base fixed to only one of the phases and is absent in the other. The experimental reference of such systems are pH sensitive hydrogels and heterogeneous systems of biological interest. It is predicted that a basic hydrogel immersed in slightly acidic solutions should absorb the acid and leave the external solution exactly neutral (pH 7). This is in accordance with experimental results of cross-linked poly(N-vinylimidazole). The pH 7 cannot be obtained if the system were homogeneous; the confinement of the weak base inside the gel phase is a requisite for this neutral pH in the external solution. The solution inside the gel is regulated to a much higher pH, which has important implications in studies on chemical reactions and physical processes taking place inside a phase insoluble but in contact with a solution.

SOLVENT EFFECTS ON THE TAUTOMERIZATION OF 5′-DEOXYPYRIDOXAL. A PHOTOPHYSICAL STUDY

Abstract— Absorption and fluorescence spectra of 5′‐deoxypyridoxal (DPL) in various pure solvents and mixtures were recorded both at room temperature and over the range10–65°C. The areas under the absorption bands were analyzed to obtain the mole fraction (fN, fz) of two tautomers (the zwitterionic, Z, and neutral, N, forms) in the ground state. The following spectral parameters were determined from the fluorescence spectra: Stokes shift (Δv), fluorescence quantum yield of the neutral form (QN), fluorescence ratio of the neutral to the zwitterionic form (øN/øZ) and the rate constant of tautomerization (k1) from Z to N in the excited state. Some of these parameters (fN, Δv, QN, k1) were found to depend on the proton donor character of the solvent, whereas others (øN/øZ) depended on its dipole moment. Thus, the absorption and fluorescence spectra of DPL allow one to obtain information on the polarity and the concentration of –OH groups on its environment.