Vladimír Raus

Activation of cellulose by 1,4-dioxane for dissolution in N,N-dimethylacetamide/LiCl

N,N-Dimethylacetamide/lithium chloride (DMAc/LiCl) mixture is a popular solvent system used for cellulose dissolution, analysis, and derivatization. However, a pre-treatment (activation) procedure is needed for most celluloses to dissolve readily in DMAc/LiCl. Here, an optimized version of the activation protocol based on solvent exchange to 1,4-dioxane was introduced. Its universality was demonstrated by successful activation and dissolution of six different celluloses (AVICEL, Sigmacell, cotton linters, Encell, Lincell, and Whatman paper). Dissolution times varied significantly for different cellulose types and also depended on factors such as the drying method employed or the water removal step inclusion/omission. Dioxane-activated celluloses were analyzed with a variety of methods. SEC measurements indicated low destructivity of the dioxane activation method. The infrared spectroscopy analysis showed that dioxane remained adsorbed on cellulose even after rigorous drying. In addition, upon dioxane activation, stagnation or a slight increase in the total order index of celluloses was observed. This observation was in accordance with the crystallinity index changes determined by solid-state NMR. Finally, scanning electron microscopy revealed disintegration of AVICEL particles and defibrillation of fibrous celluloses upon dioxane activation; Sigmacell remained apparently unchanged.

Structural changes in alginate-based microspheres exposed to in vivo environment as revealed by confocal Raman microscopy

A next-generation cure for type 1 diabetes relies on immunoprotection of insulin-producing cells, which can be achieved by their encapsulation in microspheres made of non-covalently crosslinked hydrogels. Treatment success is directly related to the microsphere structure that is characterized by the localization of the polymers constituting the hydrogel material. However, due to the lack of a suitable analytical method, it is presently unknown how the microsphere structure changes in vivo, which complicates evaluation of different encapsulation approaches. Here, confocal Raman microscopy (CRM) imaging was tailored to serve as a powerful new tool for tracking structural changes in two major encapsulation designs, alginate-based microbeads and multi-component microcapsules. CRM analyses before implantation and after explantation from a mouse model revealed complete loss of the original heterogeneous structure in the alginate microbeads, making the intentionally high initial heterogeneity a questionable design choice. On the other hand, the structural heterogeneity was conserved in the microcapsules, which indicates that this design will better retain its immunoprotective properties in vivo. In another application, CRM was used for quantitative mapping of the alginate concentration throughout the microbead volume. Such data provide invaluable information about the microenvironment cells would encounter upon their encapsulation in alginate microbeads.

Intermolecular Interactions in N,N-Dimethylacetamide without and with LiCl Studied by Infrared Spectroscopy and Quantum Chemical Model Calculations

The mixture of LiCl and N, N-dimethylacetamide (DMAc) is an important laboratory-scale solvent for cellulose. However, the mechanism of cellulose dissolution in DMAc/LiCl could not be fully established due to the limited knowledge about the interactions between DMAc and LiCl. To address this issue, we studied neat DMAc and DMAc/LiCl mixtures by ATR FTIR spectroscopy and quantum chemical model calculations. On the basis of the calculations, we newly assigned the bands at 1660 and 1642 cm-1 in the ν(C═O) region of the spectra to DMAc monomeric and dimeric structures. The latter are presumably stabilized by the C-H···O═C weak hydrogen bonds that prevail in both neat DMAc and DMAc/LiCl mixtures. The analysis of the concentrated (7.9 wt % of LiCl) DMAc/LiCl mixture revealed that only about half of DMAc molecules interact directly with LiCl. The resulting average stoichiometry of about 2.8:1 (DMAc:LiCl), indicating the predominance of [(DMAc)2-LiCl] and [(DMAc)3-LiCl] complexes, was found to be temperature independent. Conversely, the stoichiometry was considerably temperature sensitive for the diluted DMAc/LiCl mixture (2.6 wt % of LiCl), indicating that further DMAc molecules can be incorporated into the primary solvation shell of LiCl at higher temperatures. These results highlight the dynamic character of the DMAc/LiCl system that needs to be considered when studying the cellulose dissolution mechanism.