pubs.acs.org/langmuir © 2009 American Chemical Society
Force Pulling of Single Cellulose Chains at the Crystalline Cellulose-Liquid Interface: A Molecular Dynamics Study
Malin Bergenstrahle,*,† Esben Thormann,‡ Niklas Nordgren,‡ and Lars A. Berglund† †
Department of Fibre and Polymer Technology, Royal Institute of Technology, Teknikringen 58, SE-10044 Stockholm, Sweden and ‡Department of Chemistry, Surface Chemistry, Royal Institute of Technology, :: Drottning Kristinas vag 51, SE-10044 Stockholm, Sweden Received November 26, 2008. Revised Manuscript Received January 23, 2009
Pulling single cellulose molecules from a crystalline cellulose surface has been modeled by molecular dynamics (MD) simulations of the experimental procedure used in atomic force microscopy (AFM). Specifically, the aim of the study was to investigate cellulose interactions at desorption. Simulations were performed in both water and the organic solvent cyclohexane. Moreover, the effects of initial octamer conformation and orientation with respect to the surface chains were studied. A strong effect from the solvent was observed. In cyclohexane, normal forces of 200-500 pN and energies of 43.5 ( 6.0 kJ/mol glucose unit were required to pull off the octamer. The normal forces in water were substantially lower, around 58 pN, and the energies were 18.2 ( 3.6 kJ/mol glucose unit. In addition, the lateral components of the pull-off force were shown to provide information on initial conformation and orientation. Hydrogen bonds between the octamer and surface were analyzed and found to be an important factor in the pull-off behavior. Altogether, it was shown that MD provides detailed information on the desorption processes that may be useful for the interpretation of AFM experiments.
1. Introduction Cellulose is the main structural reinforcing polymer in plants and trees and thus is one of the most abundant polysaccharides on earth. In nature, cellulose self-assembles into hierarchical crystalline fiber structures, exhibiting outstanding mechanical properties considering the inherent low weight of the material. Cellulose is therefore utilized in numerous applications ranging from textiles and papermaking to the fields of biotechnology and medicine. Polysaccharides in general, being a renewable resource, have prompted growing interest in incorporating them into areas where traditionally synthetic polymers or metals have been used, such as in composite materials. For these purposes, it is important to understand how cellulose is structured and how it interacts with other materials and changes in the surrounding environment. Many studies are enlightening in terms of understanding the overall nanoscopic interactions1-4 whereas the specific nature of the interactions between single molecular species has been less extensively investigated. The desorption of a single polymer chain from a surface has in several cases been experimentally studied by the use of an atomic force microscope (AFM). In such cases has a polymer, specifically or nonspecifically, attached to the tip of an AFM cantilever been pulled loose from a surface. The restoring
force has been monitored as a function of the tip-surface separation as described in detail elsewhere.5-12 This experimental technique can indirectly provide information about the (single-molecule) interfacial energy10,12 and to some extent also the conformation of the adsorbed polymer (e.g., partially adsorbed looplike conformation or fully adsorbed flat conformation5,6). However, these experiments also suffer from some limitations. For instance, adhesion between the AFM tip and the surface makes it difficult to study the desorption of short chains (
