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.

Fluorescence study on the structure of ionic liquid aggregates in aqueous solutions.

Although ionic liquids are a relatively novel class of materials, it is well documented that they form micelles through aggregation of cation aliphatic tails. However, anion self-assembly has not yet been reported. In this study, we analyzed the intrinsic fluorescence of p-toluenesulfonate groups (tosylate) as part of the ionic liquid 1-ethyl-3-methylimidazolium tosylate ([emim][TOS]) and p-toluenesulfonic acid (pTSA), in aqueous solution. pTSA was found to have overlapping monomer and excimer emissions for chromophore concentrations from 10(-3) to 1 M, whereas [emim][TOS], in the same conditions, showed monomer emission slightly broadened by much weaker excimer emission. These different photophysical behaviors of the same chromophore in the two compounds are explained by the formation of ion pairs by [emim][TOS], which can also be inferred from the loss of vibrational structure of the absorption spectra with respect to pTSA. Despite this different behavior regarding ion pairing, anion aggregation was observed in the excitation spectra of both pTSA and [emim][TOS]. While the absorption spectra corresponded to single chromophores, the excitation spectra changed from those characteristic of a single chromophore (below 10(-3) M) to red-shifted narrow bands (above 0.1 M) typical of J aggregates. Between those concentrations, the excitation spectra split into blue- and red-shifted bands with relative intensities that changed with concentration as the chromophores rearranged in their clusters from head-to-head to head-to-tail aggregates. Differences between the absorption and excitation spectra were ascribed to aggregation-induced fluorescence enhancement.

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.

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.

Determination of the parameters controlling swelling of chemically cross-linked pH-sensitive poly(N-vinylimidazole) hydrogels.

The number of variables controlling the behavior of ionic gels is large and very often some of them are unknown. The aim of this work is to interpret quantitatively the swelling behavior of pH sensitive gels, with the minimum number of simplifying assumptions. With this purpose, the equilibrium degree of swelling (S) and protonation (alpha) of chemically cross-linked poly(N-vinylimidazole) (PVI) immersed in aqueous salt solutions were measured as a function of the ionic strength (mu), in the whole range of pH. In acid solutions with pH in the range 0 to 4, imidazole moieties become protonated, and PVI behaves as a polyelectrolyte gel: S decreases upon increasing mu both for NaCl and for CaCl(2), with HCl as protonating acid. In aqueous solutions with larger pH, between 4 and 12, the hydrogel is practically neutral, and S increases as mu rises, showing a salting-in effect. From the quantitative analysis of these results, the following facts emerged. Protonation induces chain stiffness (as measured by the non-Gaussian factor) and worsening of the solvent quality of the aqueous media (as measured by the polymer-solvent interaction parameter). For alpha below 33%, swelling seems to be governed by the excess of mobile counterions inside the gel with respect to the bath, with a minor but still significantly negative contribution of the osmotic swelling pressure due to polymer-solvent mixing. Above 33% protonation, it is necessary to consider Manning counterion condensation to get parameters with physical meaning. The crossover between polyelectrolyte and salting-in effects corresponds to alpha and mu values with the same ionic and mixing contributions to the osmotic swelling pressure. The formation of ionic nonpermanent cross-links, with H(2)SO(4) as the protonating acid, was discarded.