Molecular dynamics simulation study of polyelectrolyte adsorption on cellulose surfaces

The adsorption of two polyelectrolyte ((carboxy methyl) cellulose and poly(acrylate) in water on crystalline cellulose is studied in this work. The multi-component problem has been splitted up into simulations of solutions of the polyelectrolyte (polyanions including sodium counterions) in water, into simulations of the interface of crystalline cellulose towards water. Finally polyelectrolyte-cellulose systems were studied. Molecular dynamics simulations of diluted (≈ 2.5 weight percent) aqueous solutions of two polyelectrolytes, namely sodium (carboxy methyl) cellulose (CMC) and sodium poly(acrylate) (PAA) have been performed. Water and counterions were taken into account explicitly. For CMC the substitution pattern is important. Simulations of CMC oligomers resulted in two different structures: One molecule takes a stretched conformation, while the other one takes a globule-like, collapsed state. In an additional simulation, which starts from a linear state, the second CMC molecules collapses possibly due to solute-solute hydrogen bonding. PAA is stretched during the whole simulation. On a local atomistic scale, CMC and PAA have different hydrogen-bond properties. The COO− groups of PAA can only act as hydrogen bond acceptors, but due to the high negative charge density, there are still more water molecules assembled around PAA than around CMC. There are 0.029 bonds/amu respectively 0.019 bonds/amu to water for the two CMC oligomers, but more than twice as many for PAA: 0.083 bonds/amu. Beside intermolecular hydrogen bonding, there is a significant amount of intramolecular H-bonding for CMC, which is influenced by the COO− groups, which act as strong Hacceptor. In contrast to hydroxyand carboxylic groups, ether oxygen are hardly involved in hydrogen bonding. The water-cellulose interface shows common features for both simulations of the 11--0 and 110 surface of cellulose Iβ. We have simulated a cellulose-crystal with the 11--0 and 110 surface exposed to water. Both interfaces are stable with respect to surface reconstruction and water penetration, at least on the nanosecond time scale and in the absence of defects. Both show essentially the crystal structure of cellulose Iβ, also in the adsorption patterns for water and argon. In spite of the presence of OH groups, both surfaces are non-hydrophilic and lipophilic: Water molecules are not attracted to the surface, as they are equally happy in bulk water. On the other hand, the presence of the surface perturbs the water structure sufficiently to create free volume, in which an argon can dissolve more easily than in the denser bulk water. The wider spacing of cellulose chain allows the 110 surface to expose more of the hydrophobic grooves where lipophilic adsorption takes place. In contrast, the protruding OH groups are less affected by chain packing, so the behavior towards water is similar for both surfaces. We did a direct comparison of the adsorption properties of poly(acrylate) and (carboxy methyl) cellulose on different monoclinic cellulose surfaces. Four systems have been simulated: 110-CMC, 11--0-CMC and 110-PAA and 11--0-PAA. Every systems consists of the water solvent, a cellulose-crystal and the polyelectrolyte. As found in experiment, CMC does adsorb onto the cellulose surface, and PAA does not. This is due to a high number of hydrogen bonds from the cellulose surface to the CMC-oligomers. There are hardly any hydrogen bonds between cellulose and PAA. One (carboxy methyl) cellulose is flat on the surface, whereas other oligomers take a end-onthe-surface conformation. The anhydroglucose unit which is in contact with the cellulose crystal aligns parallel to the cellobiose-units of the crystal (200-plane).