Björn Stenqvist

The mechanism of cellulose solubilization by urea studied by molecular simulation

We used molecular dynamics simulation to model the effect of urea and thiourea on the solvent quality of aqueous solutions with respect to cellulose. A model system consisting of a periodically replicated cellulose molecule of effectively infinite degree of polymerization immersed in aqueous (thio-)urea solution was considered. Kirkwood-Buff theory, which relates the pair distribution functions to the concentration derivatives of the chemical potential, allowed the solubilization effect to be quantified in terms of the preferential binding of urea over water to the cellulose molecule. We found that urea is preferentially adsorbed on the hydrophobic faces of the anhydroglucose rings but has the same affinity as water to the hydroxyl groups. Thus, the simulations suggest that urea acts primarily by mitigating the effect of the hydrophobic portions of the cellulose molecule.

On the dissolution of cellulose in tetrabutylammonium acetate/dimethyl sulfoxide : a frustrated solvent

We have found that the dissolution of cellulose in the binary mixed solvent tetrabutylammonium acetate/dimethyl sulfoxide follows a previously overlooked near-stoichiometric relationship such that one dissolved acetate ion is able to dissolve an amount of cellulose corresponding to about one glucose residue. The structure and dynamics of the resulting cellulose solutions were investigated using small-angle X-ray scattering (SAXS) and nuclear magnetic resonance techniques as well as molecular dynamics simulation. This yielded a detailed picture of the dissolution mechanism in which acetate ions form hydrogen bonds to cellulose and causes a diffuse solvation sheath of bulky tetrabutylammonium counterions to form. In turn, this leads to a steric repulsion that helps to keep the cellulose chains apart. Structural similarities to previously investigated cellulose solutions in aqueous tetrabutylammonium hydroxide were revealed by SAXS measurement. To what extent this corresponds to similarities in dissolution mechanism is discussed.

Assembling oppositely charged lock and key responsive colloids: A mesoscale analog of adaptive chemistry

Oppositely charged thermoresponsive particles with complementary geometries are assembled into adaptive colloidal molecules. We have seen a considerable effort in colloid sciences to copy Nature’s successful strategies to fabricate complex functional structures through self-assembly. This includes attempts to design colloidal building blocks and their intermolecular interactions, such as creating the colloidal analogs of directional molecular interactions, molecular recognition, host-guest systems, and specific binding. We show that we can use oppositely charged thermoresponsive particles with complementary shapes, such as spherical and bowl-shaped particles, to implement an externally controllable lock-and-key self-assembly mechanism. The use of tunable electrostatic interactions combined with the temperature-dependent size and shape and van der Waals interactions of these building blocks provides an exquisite control over the selectivity and specificity of the interactions and self-assembly process. The dynamic nature of the mechanism allows for reversibly cycling through various structures that range from weakly structured dense liquids to well-defined molecule-shaped clusters with different configurations through variations in temperature and ionic strength. We link this complex and dynamic self-assembly behavior to the relevant molecular interactions, such as screened Coulomb and van der Waals forces and the geometrical complementarity of the two building blocks, and discuss our findings in the context of the concepts of adaptive chemistry recently introduced to molecular systems.

Direct summation of dipole-dipole interactions using the Wolf formalism

We present an expanded Wolf formalism for direct summation of long-range dipole-dipole interactions and rule-of-thumbs how to choose optimal spherical cutoff (Rc) and damping parameter (α). This is done by comparing liquid radial distribution functions, dipole-dipole orientation correlations, particle energies, and dielectric constants, with Ewald sums and the Reaction field method. The resulting rule states that ασ < 1 and αRc > 3 for reduced densities around ρ(∗) = 1 where σ is the particle size. Being a pair potential, the presented approach scales linearly with system size and is applicable to simulations involving point dipoles such as the Stockmayer fluid and polarizable water models.

Cellulose-Water Interactions: Effect of electronic polarizability

Understanding cellulose-water interactions is important for advancing current technology, not the least in developing effective dissolution methods for wooden fibers. Here we study the effect of electronic polarization on cellulose-water interactions by all-atom computer simulations. We show that induced dipoles on both interfacial water and cellulose hydroxyl groups are significant and may influence cellulose/co-solute interactions. The non-polarizable SPC/E water model yields remarkably similar solvent radial distribution functions as the polarizable POL3 model while orientational correlations differ slightly. For the present study we have developed a polarizable cellulose force field, based on the popular GLYCAM parameters, as well as tested the Wolf technique for handling long range dipolar interactions in polarizable, all-atom Monte Carlo simulations. (Less)