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

Abstract

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(Ionic Liquid) Semi-Interpenetrating Network Multi-Responsive Hydrogels

“…This is due to the Na + and Cl − ions competing for water with the linear p(NiPAAm), enhancing the tendency of p(NiPAAm) to form polymer-polymer interactions and to more readily precipitate as the globular form. This is the same phenomenon that appears when salting-out proteins from a solution [ 7 , 36 ]. A similar effect of the salt concentration is expected with the p(NiPAAm-BSP-AA) chains when the sIPN is hydrated in aqueous salt solutions.…”

“…At higher salt concentrations, the hydrogels primarily shrink in size due to the interactions that start taking place between the dissolved salt ions and the charged polymer chains, while the white light irradiation shows reduced influence. This shrinking behaviour is known as the polyelectrolyte effect [ 7 , 26 ]. For each hydrogel composition, the shrinking profile is very similar as the salt concentration is increased (13.95 ± 0.21 mm² in 0.5% w/w NaCl, 12.56 ± 0.23 mm² in 1% w/w NaCl and 10.00 ± 0.09 mm² in 5% NaCl, respectively) ( Figure 9 ).…”

“…The results indicate that F − induces the maximum area shrinking for every hydrogel, while I − has the least influence ( Table 4 ). The reason behind this shrinking effect is two-fold: the dissolved salts have a shrinking effect on the PIL matrix due to the polyelectrolyte effect [ 7 ], while also having the same effect on the linear p(NiPAAm-BSP-AA) chains [ 22 , 32 , 36 ]. The shrinking effect occurs because of an electrostatic screening that appears between the charged PIL chains and the ions present in the solution that affects the way water is distributed around the polymer chains and salt ions, thus directly contributing to the water intake of the hydrogels [ 37 ].…”

“…Stimuli-responsive polymers are defined as polymers that exhibit a conformation change in a reliable, reproducible and useful manner when exposed to a variation in their external environment [ 1 , 2 ]. Many types of stimuli can be employed, including temperature, pressure, magnetic and electric field, lighting conditions, pH and the presence of other chemical species in their local medium [ 1 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. Hydrogels comprise a particularly interesting class of stimuli-responsive materials due to their biomimetic nature [ 6 ].…”

“…Hydrogels comprise a particularly interesting class of stimuli-responsive materials due to their biomimetic nature [ 6 ]. These materials are crosslinked polymer networks that absorb large quantities of water and, when under the influence of specific stimuli, change their water absorption properties, which in turn translate into a hydrogel volume variation [ 7 , 14 ]. Extensively studied examples of such materials include N -isopropylacrylamide (NiPAAm) hydrogels, which possess a well-documented thermally induced shrinking behaviour associated with a lower critical solution temperature (LCST) at ca.…”