Abstract
We used atomistic molecular dynamics (MD) simulations to study polyelectrolyte brushes based on anionic α,L-glutamic acid and α,L-aspartic acid grafted on cellulose in the presence of divalent CaCl2 salt at different concentrations. The motivation is to search for ways to control properties such as sorption capacity and the structural response of the brush to multivalent salts. For this detailed understanding of the role of side-chain length, the chemical structure and their interplay are required. It was found that in the case of glutamic acid oligomers, the longer side chains facilitate attractive interactions with the cellulose surface, which forces the grafted chains to lie down on the surface. The additional methylene group in the side chain enables side-chain rotation, enhancing this effect. On the other hand, the shorter and more restricted side chains of aspartic acid oligomers prevent attractive interactions to a large degree and push the grafted chains away from the surface. The difference in side-chain length also leads to differences in other properties of the brush in divalent salt solutions. At a low grafting density, the longer side chains of glutamic acid allow the adsorbed cations to be spatially distributed inside the brush resulting in a charge inversion. With an increase in grafting density, the difference in the total charge of the aspartic and glutamine brushes disappears, but new structural features appear. The longer sides allow for ion bridging between the grafted chains and the cellulose surface without a significant change in main-chain conformation. This leads to the brush structure being less sensitive to changes in salt concentration.
Highlights
It is well known that chemical modifications of surfaces of a material allow for tuning and controlling many of their properties and structure
We used molecular dynamics (MD) simulations to investigate the structure of brushes based on α,L-oligomers of anionic amino acids grafted onto the surface of nanocrystalline cellulose immersed in water and with multivalent salt solutions
We demonstrated the formation of different structures of organomineral complexes by polyaspartic and polyglutamic acids in the presence of calcium salt solutions [32]
Summary
It is well known that chemical modifications of surfaces of a material allow for tuning and controlling many of their properties and structure. We used MD simulations to investigate the structure of brushes based on α,L-oligomers of anionic amino acids (glutamic and aspartic acid) grafted onto the surface of nanocrystalline cellulose immersed in water and with multivalent salt solutions. The transition from the osmotic brush mode to the quasi-neutral mode (where steric interactions start to play a significant role) should occur at a lesser degree of surface modification for glutamic acid grafted chains. Specific interactions between the grafted chains and cellulose play an important role in determining the brush structure; both amino acid and cellulose have active groups, which can be involved. Specific interactions between the grafted chains and cellulose play an important role in determining the brush structure; both amino acid and cellulose have active groups, which can be invol3voefd in specific interactions, H-bonds and dipole–dipole interactions in particular.
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