Development of mesoporous polysaccharide/sol-gel composites with two different templating agents: Surfactants and choline chloride-based deep eutectic solvents

Abstract

Mesoporous sorbent composites, evolving from previous work on microporous composites of polyanionic polysaccharides were developed with the purpose of increasing the sorptive features of the materials. Using the widely successful classical surfactant micelle approach, it was observed, in this particular case, that the composites remained essentially microporous. The alternative consisted on the application of deep eutectic solvents (DES). The most common DES (choline chloride + neutral hydrogen bond donor), were tested because of their advantages over other possibilities such as imidazolium-based ionic liquids: lower cost, easy in-house preparation, safe constituents and water stability and solubility. Possible mechanisms underlying the observed mesoporosity were discussed. The surface area ranged between 76 and 267 m 2 /g and the average pore size was in the range 3-5 nm. DES had not a negative effect on synthesis yields and, in the case of fucoidan, composites bearing a higher content of the biopolymer were produced. As a consequence and in line with the initial expectations these new composites revealed highly enhanced Pb (II) sorptive features comparatively to their microporous predecessors: chondroitin sulfate composites-up to a 5 fold capacity enhancement; fucoidan composites-up to a 3.5 fold capacity enhancement. The highest capacity was observed for the fucoidan composite prepared with choline chloride-ethyleneglycol DES, 79 mg Pb (II)/g, which is slightly above the highest value (77 mg Pb (II)/g) found in the literature for Pb (II) sorbents based on polysaccharides, sol-gels or their composites.

Cation‐bioimprinted mesoporous polysaccharide/sol–gel composites prepared in media containing choline chloride‐based deep eutectic solvents

“…The textures observable by SEM, from both CS and Fuc imprinted composites, reveal a structure formed by the aggregation of sub-microparticles, as was also observed for the corresponding NIC. 4 However, clearly, the CIC presented finer patterns, in agreement with smaller mesopores measured by N 2 sorption. The mesopores themselves, predominating in the range of 2.9-3.6 nm, are too small to evaluate at the SEM resolution.…”

“…The degree of swelling (S w ) of composites was determined by a method previously described. 4 The difference between the wet mass (W t ) and the dry mass (W 0 ), after 24 h of agitation in the deionized water (pH 7), in comparison to initial weight (dry mass, approx. 30 mg) was used to calculate the degree of swelling (S w ) using the following equation:…”

“…A recent series of reports described the preparation of sorbents consisting on sulfated polysaccharide [chondroitin sulfate (CS) or fucoidan (Fuc)]/polysiloxane composites. [1][2][3][4] The preparation process relied on a covalent crosslinking scheme involving a modified siloxane coupler that contains the glycidyloxypropyl group. 1 This coupler attaches to saccharides through the hydroxyl groups, leaving other groups intact, such as carboxylate and sulfate.…”

“…[9][10][11][12] Aiming at higher sorption capacities for metallic cations, mesoporous versions of the initial composites were attempted. 4 A network of mesopores would greatly increase the binding surface and therefore render many more binding points accessible to sorbates. Commonly, mesoporous templating in sol-gel processes employs surfactant micelles as template [13][14][15][16][17] ; however, this approach was ineffective due to interference of the biopolymers with the formation of surfactant micelles.…”

“…4 The most probable mechanism underlying the mesoporosity templating by DES is associated with the aging stage when water is evaporated and DES remains as filler dispersed at the mesoscale within the gel network, while it densifies (late condensation) without shrinkage. 4,18 The mesoporous versions of the initial composites exhibited much higher capacities-up to fivefold enhancement for the CS composites and up to 3.5-fold for the Fuc composites. The binding strength was even greater in most of the cases.…”