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Ionization of Cellobiose in Aqueous Alkali and the Mechanism of Cellulose Dissolution
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2016 (Engelska)Ingår i: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 7, nr 24, s. 5044-5048Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Cellulose, one of the most abundant renewable resources, is insoluble in most common solvents but dissolves in aqueous alkali under a narrow range of conditions. To elucidate the solubilization mechanism, we performed electrophoretic NMR on cellobiose, a subunit of cellulose, showing that cellobiose acts as an acid with two dissociation steps at pH 12 and 13.5. Chemical shift differences between cellobiose in NaOH and NaCl were estimated using 2D NMR and compared to DFT shift differences upon deprotonation. The dissociation steps are the deprotonation of the hemiacetal OH group and the deprotonation of one of four OH groups on the nonreducing anhydroglucose unit. MD simulations reveal that aggregation is suppressed upon charging cellulose chains in solution. Our findings strongly suggest that cellulose is to a large extent charged in concentrated aqueous alkali, a seemingly crucial factor for solubilization. This insight, overlooked in the current literature, is important for understanding cellulose dissolution and for synthesis of new sustainable materials.

Ort, förlag, år, upplaga, sidor
2016. Vol. 7, nr 24, s. 5044-5048
Nationell ämneskategori
Kemi
Identifikatorer
URN: urn:nbn:se:kth:diva-199740DOI: 10.1021/acs.jpclett.6b02346ISI: 000390087000014Scopus ID: 2-s2.0-85006725083OAI: oai:DiVA.org:kth-199740DiVA, id: diva2:1067861
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QC 20170123

Tillgänglig från: 2017-01-23 Skapad: 2017-01-16 Senast uppdaterad: 2017-06-29Bibliografiskt granskad
Ingår i avhandling
1. Ions interacting with macromolecules: NMR studies in solution
Öppna denna publikation i ny flik eller fönster >>Ions interacting with macromolecules: NMR studies in solution
2017 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

Specific ion effects, identified for more than hundred years, play an important role in a wide range of phenomena and applications. Several mechanisms such as direct ion interaction with molecules have been suggested to explain these effects, but quantitative experimental evidence remains scarce. Electrophoretic NMR (eNMR) has been emerging as a very powerful tool for studying molecular association and ionic transport in a variety of systems. Yet its potential in studying specific ion effect has been unexplored. In this thesis, eNMR was in part developed further as an analytical method and was in part used as one of the main techniques to study ions interacting with macromolecules in aqueous and non-aqueous solutions.

The complexation of a large group of cations with poly ethylene oxide (PEO) in methanol was studied with eNMR. The binding of monovalent ions was demonstrated not to follow the Hofmeister order; multivalent cations except barium all showed negligible complexation. As a unifying feature, only cations with surface charge density below a threshold value were able to bind suggesting that ion solvation is critical. The binding mechanism was examined in greater detail for K+ and Ba2+ with oligomeric PEO of different chain lengths. Those two cations exhibited different binding mechanisms. K+ was found to bind to PEO by having at least 6 repeating units wrap around it while retaining the polymer flexibility. On the other hand, Ba2+ (and, to some extent, (BaAnion)+) needs a slightly shorter section to bind, but the molecular dynamics at the binding site slow considerably.

The binding of anions with poly (N-isopropylacrylamide) in water was quantified at low salt concentration with eNMR and the binding affinity, though very weak, followed the Hofmeister order. This result indicates the non-electrostatic nature of this specific ion effects. The increase of binding strength with salt concentration is well described by a Langmuir isotherm.

The specific ion binding to a protein, bovine serum albumin (BSA), was also studied at pH values where BSA has either net positive and negative charges. Our results show that anions have the same binding affinity irrespective of the surface charge while the binding strength of cations is reversed with the change in net surface charge. This indicates different binding mechanisms for cations and anions.

The ionization of cellobiose in alkaline solutions was measured quantitatively by eNMR. The results show a two-step deprotonation process with increasing alkaline strength. Supported by results from 1H-13C HSQC NMR and MD simulation, ionization was proposed to be responsible for the improved solubility of cellulose in alkaline solution. eNMR was also used to characterize the effective charge of tetramethylammonium ions in a variety of solvents. In solvents of high polarity, the results agree well with predictions based on Onsager’s limiting law but for nonpolar solvents deviations were found that were attributed to ion pair formation. 

Ort, förlag, år, upplaga, sidor
KTH Royal Institute of Technology, 2017. s. 61
Serie
TRITA-CHE-Report, ISSN 1654-1081 ; 2017 : 12
Nyckelord
electrophoretic NMR, diffusion NMR, specific ion effects, Hofmeister, ion binding
Nationell ämneskategori
Fysikalisk kemi
Forskningsämne
Kemi
Identifikatorer
urn:nbn:se:kth:diva-201732 (URN)978-91-7729-282-1 (ISBN)
Disputation
2017-03-22, F3, Lindstedtsvägen 26, Stockholm, 10:00 (Engelska)
Opponent
Handledare
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QC 20170216

Tillgänglig från: 2017-02-16 Skapad: 2017-02-15 Senast uppdaterad: 2017-05-30Bibliografiskt granskad

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