Chapter 8
Downloaded by CORNELL UNIV on June 26, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1019.ch008
Hydroxypropyl Xylan Self-Assembly at Air/Water and Water/Cellulose Interfaces Abdulaziz Kaya1, Daniel A. Drazenovich1, Wolfgang G. Glasser2, Thomas Heinze3, and Alan R. Esker1,* 1
Department of Chemistry and the Macromolecules and Interfaces Institute, and 2Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 3 Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraße 10, Jena, 07743 Germany
Hydroxypropylation of polysaccharides is one strategy for enhancing aqueous solubility. The degree of hydroxypropyl substitution can be controlled through the pH of the hydroxypropylation reaction. Surface tension measurements of aqueous solutions of hydroxypropyl xylan (HPX), synthesized from barley husk xylans, by the Wilhelmy plate technique show that surface tension changes (∆γ = γwater – γHPX(aq)) increase and critical aggregation concentrations generally decrease with increasing degree of substitution. Hence, even though hydroxypropyl substitution is necessary to induce aqueous solubility, excessive hydroxypropylation promotes aggregation in water. While surface tension studies reveal HPX affinity for the air/water interface, surface plasmon resonance spectroscopy studies indicate that HPXs do not adsorb significantly onto model regenerated cellulose surfaces (submonolayer coverage). Likewise, the HPXs do not show significant adsorption onto hydroxyl-terminated selfassembled monolayers of 11-mercapto-1-undecanol (SAMOH). In contrast, HPX does adsorb (~monolayer coverage) onto methyl-terminated self-assembled monolayers of These results show 1-dodecanethiol (SAM-CH3). hydroxypropylation is a sound approach for creating soluble xylan derivatives, suitable for further chemical modification.
© 2009 American Chemical Society In Model Cellulosic Surfaces; Roman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
173
174
Downloaded by CORNELL UNIV on June 26, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1019.ch008
Introduction Cellulose is one of the most important natural polymers and is used extensively in the textile and paper industries (1). In nature, cellulose is located in the core of plant cell walls (2) and is associated with hemicellulose and lignin in a hierarchial (composite) superstructure (3). Hemicelluloses, which serve as a matrix for the cellulose superstructure, are lower molar mass polysaccharides containing short side chains (4). These polysaccharides consist of various five (D-xylose, L-arabinose) and six carbon (D-glucose, D-galactose, D-mannose etc.) sugars (5). Xylans are the most common hemicelluloses and are considered to be the second most abundant biopolymer in land plants (6). Structurally, xylans are a class of heteropolysaccharides consisting of poly(anhydroxylose) with varying degrees of 4-O methyl glucuronic acid, acetyl groups, and anhydroarabinose substituents depending on the source and isolation procedures used to obtain the xylan (7). During the past several years, the need for effective biomass utilization has renewed interest in the exploitation of xylans as sources of biopolymers. This interest is aided by the fact that xylans are readily available as organic wastes from renewable forest and agricultural residues, such as wood meal and shavings, stems, stalks, hulls, cobs, and husks (8). Even though the isolation of xylans from biomass is relatively easy, the potential application of xylans has not yet been completely realized (8–11). Possible reasons for the lack of xylan utilization as a material stream include a shortage of high molar mass xylans on an industrial scale (9), heterogeneity of xylan structures within even a single plant (8), and the partial degradation of hemicelluloses during pulping processes (12). Another complication hindering widespread use of xylans is that they are usually difficult to dissolve in aqueous media and aprotic solvents even when they are isolated by aqueous extraction. Hence, investigations of xylan solution properties and molecular weight determinations are difficult (13). The substitution of a xylan’s hydroxyl groups by alkoxy or acetoxy groups enhances solubility in water and/or organic solvents (11). Therefore, chemical modification of xylans provides one avenue to make soluble xylans for molecular weight determinations and producing materials with interesting physical properties (11, 14–18). Glaudemans and Timmel prepared xylan acetate that was completely soluble in chloroform and chloroform–ethanol mixtures. These polymers had a degree of polymerization of ~200 (14). In addition to xylan acetates, other esters of xylans, such as benzoate, caprate, laurate, myristate, and palmitate have been synthesized (15). In another study, xylans fully substituted with carbamate groups showed thermoplastic behavior at high temperatures (16). Likewise, Jain et al. prepared water-soluble hydroxypropyl xylans and acetoxypropyl xylans that showed thermoplastic behavior and solubility in most organic solvents (11). Trimethylammonium-2-hydroxypropyl xylan prepared from beechwood and corn cob xylan show promise for papermaking additives by improving the strength of bleached hardwood kraft pulp and unbleached thermomechanical pulp, and by increasing the retention of fiber fines (17, 18). The enhancement of pulp properties by some xylan derivatives provides strong incentive for studying xylan self-assembly onto model cellulose and
In Model Cellulosic Surfaces; Roman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Downloaded by CORNELL UNIV on June 26, 2012 | http://pubs.acs.org Publication Date (Web): December 20, 2009 | doi: 10.1021/bk-2009-1019.ch008
175 cellulose fiber surfaces. Mora et al. investigated xylan retention on cellulose fibers and concluded that the driving force for xylan aggregate sorption and retention on cellulose fibers was hydrogen bonding between cellulose fibers and the xylans (19). Henrikkson et al. also invoked hydrogen bonding along with changes in colloidal stability to explain the adsorption behavior of autoclaved xylans onto cellulose fibers at elevated temperatures under alkaline conditions (20). In another study, it was observed that commercial birch xylan adsorbed slowly and irreversibly onto model cellulose surfaces at pH 10 (21). However, it was argued that the driving force for adsorption was a combination of weak van der Waals’ attractions and an entropically favorable release of solvent molecules when the polymer chains adsorbed. Recently, Esker et al. have shown that cationic and hydrophobic modification of xylan enhances xylan adsorption onto regenerated cellulose films prepared by the Langmuir–Blodgett technique (22). This result demonstrates that the hydrophobic forces and electrostatic interactions also influence xylan self-assembly onto cellulose surfaces. In this study, the adsorption of hydroxypropyl xylans (HPXs) onto model surfaces is studied as a function of the degree of hydroxypropyl (HP) substitution (DS). The source of the “parent” xylans for the HPX derivatives is barley husks (Hordeum spp.) (11). HPX self-assembly at the air/water interface is probed through the Wilhelmy plate technique, whereas surface plasmon resonance (SPR) spectroscopy studies allow quantification of HPX adsorption onto regenerated cellulose, and self-assembled monolayers (SAMs) of 11mercaptoundecanol (SAM-OH) and 1-dodecanethiol (SAM-CH3) on gold substrates. These studies provide insight into molecular factors influencing HPX self-assembly at surfaces and potential use of further-derivatized water-soluble HPX derivatives to modify surfaces and interfaces.
Experimental Materials Ultrapure water was used in all experiments (Millipore, Milli-Q Gradient A10, 18.2 MΩ·cm,
