Bruno Medronho

Rationalizing cellulose (in)solubility: reviewing basic physicochemical aspects and role of hydrophobic interactions

Despite being the world’s most abundant natural polymer and one of the most studied, cellulose is still challenging researchers. Cellulose is known to be insoluble in water and in many organic solvents, but can be dissolved in a number of solvents of intermediate properties, like N-methylmorpholine N-oxide and ionic liquids which, apparently, are not related. It can also be dissolved in water at extreme pHs, in particular if a cosolute of intermediate polarity is added. The insolubility in water is often referred to strong intermolecular hydrogen bonding between cellulose molecules. Revisiting some fundamental polymer physicochemical aspects (i.e. intermolecular interactions) a different picture is now revealed: cellulose is significantly amphiphilic and hydrophobic interactions are important to understand its solubility pattern. In this paper we try to provide a basis for developing novel solvents for cellulose based on a critical analysis of the intermolecular interactions involved and mechanisms of dissolution.

Brief overview on cellulose dissolution/regeneration interactions and mechanisms

The development of cellulose dissolution/regeneration strategies constitutes an increasingly active research field. These are fundamental aspects of many production processes and applications. A wide variety of suitable solvents for cellulose is already available. Nevertheless, most solvent systems have important limitations, and there is an intense activity in both industrial and academic research aiming to optimize existing solvents and develop new ones. Cellulose solvents are of highly different nature giving great challenges in the understanding of the subtle balance between the different interactions. Here, we briefly review the cellulose dissolution and regeneration mechanisms for some selected solvents. Insolubility is often attributed to strong intermolecular hydrogen bonding between cellulose molecules. However, recent work rather emphasizes the role of cellulose charge and the concomitant ion entropy effects, as well as hydrophobic interactions.

Shear-induced transitions between a planar lamellar phase and multilamellar vesicles: continuous versus discontinuous transformation.

The shear-induced transitions between an oriented lamellar phase and shear-induced multilamellar vesicles (MLVs) in a nonionic surfactant system were studied by deuterium rheo-NMR spectroscopy as a function of time in start-up experiments at several temperatures and shear rates. By starting from an initial state of oriented lamellae and observing the transformation to the final steady state of MLVs and vice-versa, two different mechanisms were found, depending on the direction of the transition and the initial state. The transition is continuous when MLVs are formed, starting from the oriented lamellar phase. On the other hand, a discontinuous nucleation-and-growth process with a coexistence region is observed when transforming MLVs into an oriented lamellar phase.

Dissolution state of cellulose in aqueous systems. 1. Alkaline solvents

The understanding of the state of dissolution of cellulose in a certain solvent is a critical step forward in the development of new efficient solvent systems for cellulose. Nevertheless, obtaining such information is not trivial. Recently, polarization transfer solid-state NMR (PTssNMR) was shown to be a very promising technique regarding an efficient and robust characterization of the solution state of cellulose. In the present study, combining PTssNMR, microscopic techniques and X-ray diffraction, a set of alkaline aqueous systems are investigated. The addition of specific additives, such as urea or thiourea, to aqueous NaOH based systems as well as the use of an amphiphilic organic cation, is found to have pronounced effects on the dissolution efficiency of cellulose. Additionally, the characteristics of the regenerated material are strongly dependent on the dissolution system; typically less crystalline materials, presenting smoother morphologies, are obtained when amphiphilic solvents or additives are used.

Shear-Induced Defect Formation in a Nonionic Lamellar Phase

(2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta approximately gamma(n), with n approximately -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.

Amphiphilic Molecules in Drug Delivery Systems

Numerous drug delivery colloidal systems are formulated using polymers or surfactants or a mixture of both, typically due to their self-assembly properties. Molecular self-assembly creates the possibility to dissolve and protect drugs from adverse external environments. Therefore, it is important to understand the interactions behind the self-assembly phenomena of surfactant and polymer molecules, polymer-polymer and polymer-surfactant mixtures. A number of colloidal structures used in drug delivery formulations such as micelles, vesicles, liquid crystalline phases, microemulsions, polymer gels, aerosols, polymer-polymer and polymer-surfactant complexes will be illustrated in this chapter and their main physicochemical properties will be highlighted, keeping in mind their relevance to the drug delivery research field.

Cellulose dissolution in an alkali based solvent: influence of additives and pretreatments

The distinction between thermodynamic and kinetics in cellulose dissolution is seldom considered in the literature. Therefore, herein an attempt to discuss this topic and illustrate our hypotheses on the basis of simple experiments was made. It is well-known that cellulose can be dissolved in a aqueous sodium hydroxide (NaOH/H2O) solvent at low temperature but it is here shown that such an alkaline solvent can be considerably improved regarding solubility, stability and rheological properties as a whole if different additives (salts and amphiphilic molecules) are used in the dissolution stage. This work probes new aqueous routes to dissolve cellulose, thereby improving the potential to commercially dissolve cellulose in an inexpensive and environmentally friendly manner.

Segmental order parameters in a nonionic surfactant lamellar phase studied with 1H–13C solid-state NMR

A lyotropic nonionic lamellar system composed of pentaethyleneglycol mono n-dodecyl ether and D(2)O was studied using natural abundance (13)C NMR under magic-angle spinning. Applying a two-dimensional recoupling method proposed by Dvinskikh (R-PDLF), (1)H-(13)C dipolar couplings were estimated over a range of temperatures (300-335 K), thus enabling analysis of structural changes in the liquid crystalline system. The results obtained are used to correlate the conformation and mobility of local sites in the surfactant molecule with overall changes in the lamellar structure.

Gels of Catanionic Vesicles and Hydrophobically Modified Poly(ethylene glycol)

The association between hydrophobically modified polyethylene glycol (HM‐PEG) and sodium dodecyl sulphate/didodecyldimethylammonium bromide (SDS/DDAB) catanionic vesicles is studied by rheology, 1H‐NMR, and cryo‐TEM techniques. Phase behavior studies were performed as a function of polymer weight fraction and vesicle volume fraction. At sufficiently high polymer weight fraction and/or vesicle volume fraction, a gel phase is detected, indicating the presence of a three‐dimensional network. Phase separation was also identified, with a dilute solution coexisting with a bluish precipitate (gel fraction). Vesicle‐polymer binding is expected to occur via the insertion of the polymer hydrophobes into the vesicle bilayer. Each vesicle can act as a cross‐linking point in the network structure. The size of the vesicles is found to be important in terms of the storage modulus and viscosity of the polymer‐vesicle mixed solutions. On the other hand, changes in the vesicle total net charge do not influence the association and the rheological properties. We also address the question of how the length of the hydrophobic modification can influence the rheology of the system and, as expected, larger hydrophobic groups increase the cross‐linking and the relaxation time.

Homogeneous length scale of shear-induced multilamellar vesicles studied by diffusion NMR.

A recently developed protocol for pulsed gradient spin echo (PGSE) NMR is applied for the size determination of multilamellar vesicles (MLVs). By monitoring the self-diffusion behavior of water, the technique yields an estimate of the homogeneous length scale λ(hom), i.e. the maximum length scale at which there is local structural heterogeneity in a globally homogeneous material. A cross-over between local non-Gaussian to global Gaussian diffusion is observed by varying the experimentally defined length- and time-scales. Occasional observation of a weak Bragg peak in the PGSE signal attenuation curves permits the direct estimation of the MLV radius in favorable cases, thus yielding the constant of proportionality between λ(hom) and radius. The microstructural origin of the Bragg peak is verified through Brownian dynamics simulations and a theoretical analysis based on the center-of-mass diffusion propagator. λ(hom) is decreasing with increasing shear rate in agreement with theoretical expectations and results from (2)H NMR lineshape analysis.