Dissolution of cellulosic particles: Population ensemble modeling informs efficient woody biomass processing

A major barrier to the efficient utilization of biomass is the recalcitrance to dissolution of semicrystalline cellulose. The present study addresses the kinetics of swelling and dissolution of cellulose particles in conditions emulating large-scale processing where the particles exhibit a distribution of size and degree of crystallinity. To this end, we have developed a model in which the behavior of a population of particles is obtained from an ensemble of individual cellulose particle dissolution models. The dissolution of individual solid cellulose particles is based on the relevant transport phenomena and kinetics and reveals decrystallization and disentanglement as two important and potentially rate-determinant steps in the process [1]. The average value or the number distribution of any intra-particle property captured by the individual particle model can be determined by simulation of a sufficient number of individual particles such that ensemble averages are independent of the particle numbers and the computed particle distributions are acceptably smooth. Using this population ensemble model, various cellulose particle size distributions and crystallinity distributions are analyzed for different dissolution parameters. The findings from this study would be useful for the rational design and optimization of pretreatment processes to reduce the particle size and degree of crystallinity, leading to enhanced woody biomass utilization [2]. References: [1] Ghasemi, M.; Alexandridis, P.; Tsianou, M., Cellulose dissolution: Insights on the contributions of solvent-induced decrystallization and chain disentanglement. Cellulose 2017, 24 (2), 571-590. DOI: 10.1007/s10570-016-1145-1. [2] Ghasemi, M.; Tsianou, M.; Alexandridis, P., Assessment of solvents for cellulose dissolution. Bioresource Technol. 2017, 228, 330-338. DOI: 10.1016/j.biortech.2016.12.049

Fundamental understanding of cellulose dissolution can improve the efficiency of biomass processing

A major barrier to the efficient utilization of biomass is the recalcitrance to dissolution of crystalline cellulose. The aim of this review is to provide an overview of the current understanding of the mechanism and kinetics of cellulose dissolution, with particular attention on how these findings can improve the efficiency of biomass processing. An improved fundamental understanding of cellulose dissolution can guide the rational selection of solvents and the optimization of processing conditions, thus leading to an enhanced utilization of biomass

Cellulose pretreatment and dissolution: Selection of solvent and processing conditions

Efficient utilization of biomass is hindered by the recalcitrance to dissolution of semicrystalline cellulose. Pretreatment is often used to alter the structure of cellulosic biomass in order to make cellulose more accessible to solvents and enzymes. The pretreatment involves physical and/or chemical processing which affects the degree of crystallinity and size of biomass particles. We examine here the effects of (i) solvent properties, pretreatment steps and temperature, and (ii) fiber diameter and degree of crystallinity, on the kinetics of cellulose swelling and dissolution. To this end we have combined (a) experimental results on cotton fiber swelling, change in crystallinity and dissolved amount when treated under different solvent conditions, with (b) a phenomenological model that accounts for the phenomena governing the dissolution of solid cellulose, e.g., solvent penetration, transformation from crystalline to amorphous domains, specimen swelling, and polymer chain untangling [1]. The insights obtained from this analysis would facilitate the rational selection of solvents and the design of pretreatment processes to reduce the size and degree of crystallinity of cellulosic biomass particles, leading to enhanced biomass utilization [2]. References: [1] Ghasemi, M.; Singapati, A. Y.; Tsianou, M.; Alexandridis, P., Dissolution of semicrystalline polymer fibers: Numerical modeling and parametric analysis. AIChE Journal 2017, 63 (4), 1368-1383. DOI: 10.1002/aic.15615. [2] Ghasemi, M.; Tsianou, M.; Alexandridis, P., Assessment of solvents for cellulose dissolution. Bioresource Technol. 2017, 228, 330-338. DOI: 10.1016/j.biortech.2016.12.049

Cellulose pretreatment and dissolution: Selection of solvent and processing conditions

Efficient utilization of biomass is hindered by the recalcitrance to dissolution of semicrystalline cellulose. Pretreatment is often used to alter the structure of cellulosic biomass in order to make cellulose more accessible to solvents and enzymes. The pretreatment involves physical and/or chemical processing which affects the degree of crystallinity and size of biomass particles. We examine here the effects of (i) solvent properties, pretreatment steps and temperature, and (ii) fiber diameter and degree of crystallinity, on the kinetics of cellulose swelling and dissolution. To this end we have combined (a) experimental results on cotton fiber swelling, change in crystallinity and dissolved amount when treated under different solvent conditions, with (b) a phenomenological model that accounts for the phenomena governing the dissolution of solid cellulose, e.g., solvent penetration, transformation from crystalline to amorphous domains, specimen swelling, and polymer chain untangling [1]. The insights obtained from this analysis would facilitate the rational selection of solvents and the design of pretreatment processes to reduce the size and degree of crystallinity of cellulosic biomass particles, leading to enhanced biomass utilization [2]. References: [1] Ghasemi, M.; Singapati, A. Y.; Tsianou, M.; Alexandridis, P., Dissolution of semicrystalline polymer fibers: Numerical modeling and parametric analysis. AIChE Journal 2017, 63 (4), 1368-1383. DOI: 10.1002/aic.15615. [2] Ghasemi, M.; Tsianou, M.; Alexandridis, P., Assessment of solvents for cellulose dissolution. Bioresource Technol. 2017, 228, 330-338. DOI: 10.1016/j.biortech.2016.12.049

Association Phenomena Involving Hydrophobically Modified Polymers. Electrostatic and Hydrophobic Contributions

The electrostatic and hydrophobic interactions in aqueous solutions of hydrophobically-modified water-soluble non-ionic or ionic polymers, their mixtures with a second polymer, nonionic or of opposite charge, and their mixtures with surfactants of opposite charge have been investigated. The relative strength of the electrostatic and hydrophobic contributions has been modulated by a variation of the polymer charge density and the solution pH, and by a variation in the degree and strength of the hydrophobic modifications, respectively. The repercussions of such interactions on the thermodynamics and dynamics of the systems under study have been quantified experimentally with the aid of phase behavior, rheology, and light scattering techniques, and have been rationalized theoretically in the context of dynamic scaling theory. The association between two oppositely charged hydrophobically modified polyelectrolytes can lead to the formation of soluble or insoluble mixtures depending on the mixture composition, the total polymer concentration, and the charge density of the polyions. The simultaneous presence of ionic and hydrophobic groups in both polymers results in a broad miscibility region (instead of the precipitate expected if only electrostatics were in effect), where the viscosity is three to four orders of magnitude higher than that of the solutions of the individual polymers. The physical networks obtained through the formation of mixed aggregates are especially reinforced in the proximity of phase separation as revealed by the existence of two relaxation modes, a fast diffusive mode and a slow mode (attributed to enhanced hydrophobic associations) in the time correlation data obtained from light scattering measurements. The network dynamics are strongly affected by the presence of the hydrophobic moieties and by the mixture composition, thus making the hydrophobically modified polyelectrolyte mixtures exhibit rheological properties of complex scaling behavior. However, the binary mixtures of unmodified polyelectrolytes as well as the single polyelectrolyte solutions exhibit a viscosity scaling behavior in good agreement with recent predictions of the dynamic scaling theory of polyelectrolytes in the semidilute regime. The interactions between polymers and surfactants of opposite charge in aqueous solutions can lead to the formation of transient networks, the rheological properties of which have been modulated via the addition of cyclodextrins. Cyclodextrins can form inclusion complexes with the hydrophobic moieties of surfactants, and thus they disrupt the polymer-surfactant network as evidenced by their ability to reverse the high viscosities exhibited by such networks

Molecular Organization in Exponentially Growing Multilayer Thin Films Assembled with Polyelectrolytes and Clay

Multilayer thin film assembly by the layer-by-layer (LbL) technique offers an inexpensive and versatile route for the synthesis of functional nanomaterials. In the case of polymer-clay systems, however, the technique faces the challenges of low clay loading and lack of tunability of the film characteristics. This is addressed in the present work that achieves exponential growth in clay-containing polyelectrolyte films having high clay loading and tailored properties. Our approach involves the incorporation of a weak polyelectrolyte and a clay with relatively high charge density and small particle size. The system of investigation comprises poly(diallyldimethylammonium chloride) (PDDA) as the polycation and laponite clay and poly(acrylic acid) (PAA) or poly(sodium-4-styrene sulfonate) (PSS) as polyanions that are used alternately to create multilayers. Successful high clay loading and exponential growth were achieved by two different approaches of polyanion incorporation in the multilayers. A progressive increase in the degree of ionization of PAA was shown to contribute to the exponential growth. Our findings also include novel pathways to manipulate thickness, surface topography, and clay content. The strategy presented here can lead to novel approaches to fabricate tailor-made nanomaterials for distinct applications

Dissolution of cellulosic particles: Population ensemble modeling informs efficient woody biomass processing

International audienceA major barrier to the efficient utilization of biomass is the recalcitrance to dissolution of semicrystalline cellulose. The present study addresses the kinetics of swelling and dissolution of cellulose particles in conditions emulating large-scale processing where the particles exhibit a distribution of size and degree of crystallinity. To this end, we have developed a model in which the behavior of a population of particles is obtained from an ensemble of individual cellulose particle dissolution models. The dissolution of individual solid cellulose particles is based on the relevant transport phenomena and kinetics and reveals decrystallization and disentanglement as two important and potentially rate-determinant steps in the process [1]. The average value or the number distribution of any intra-particle property captured by the individual particle model can be determined by simulation of a sufficient number of individual particles such that ensemble averages are independent of the particle numbers and the computed particle distributions are acceptably smooth. Using this population ensemble model, various cellulose particle size distributions and crystallinity distributions are analyzed for different dissolution parameters. The findings from this study would be useful for the rational design and optimization of pretreatment processes to reduce the particle size and degree of crystallinity, leading to enhanced woody biomass utilization [2]. References: [1] Ghasemi, M.; Alexandridis, P.; Tsianou, M., Cellulose dissolution: Insights on the contributions of solvent-induced decrystallization and chain disentanglement. Cellulose 2017, 24 (2), 571-590. DOI: 10.1007/s10570-016-1145-1. [2] Ghasemi, M.; Tsianou, M.; Alexandridis, P., Assessment of solvents for cellulose dissolution. Bioresource Technol. 2017, 228, 330-338. DOI: 10.1016/j.biortech.2016.12.049

Cyclodextrins and Surfactants in Aqueous Solution above the Critical Micelle Concentration: Where Are the Cyclodextrins Located?

Cyclodextrins (CDs) are known to bind surfactant molecules below the surfactant critical micelle concentration (CMC); however, interactions of CDs with surfactant micelles (above the CMC) are not well understood. In particular, direct structural evidence of the location of CDs in the different subphases found in micellar solutions is lacking. We have utilized small-angle neutron scattering (SANS) with contrast matching to probe the localization of α-cyclodextrin (α-CD) and 2-hydroxypropyl-β-cyclodextrin (HPβ-CD) in sodium dodecyl sulfate (SDS) micelles in aqueous (D₂O) solutions. SANS data from solutions containing either hydrogenated or deuterated surfactants were analyzed by considering three different scenarios pertaining to the localization of cyclodextrin, either all in solution or some in the micelle shell or some in the micelle core, and were simultaneously fitted using the core–shell prolate ellipsoid form factor and the Hansen–Hayter-based structure factor. The scenario that considered a fraction of CD to localize in the micelle core well described the SANS data from both hydrogenated and deuterated SDS-CD-D₂O solutions, while the other two scenarios did not. Among the various structural and interaction parameters obtained from this analysis, it emerged that the micelle core consisted of up to ∼10% HPβ-CD or ∼16% α-CD with respect to the total number of molecules (surfactants and CDs) present in the micelle at 25 mM SDS, and up to 14% HPβ-CD or 28% α-CD at 50 mM SDS. This is the first study that provides direct evidence on the location of cyclodextrin in the core of surfactant micelles. An improved understanding of CD interactions with surfactants and lipids would enable better strategies for drug encapsulation and delivery with CDs