ARTICLE pubs.acs.org/Langmuir
Quartz Crystal Microbalance with Dissipation Monitoring and Surface Plasmon Resonance Studies of Carboxymethyl Cellulose Adsorption onto Regenerated Cellulose Surfaces Zelin Liu,†,‡ Heejun Choi,†,§ Paul Gatenholm,|| and Alan R. Esker*,† †
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Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States Department of Chemical and Biological Engineering, Biopolymer Technology, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden
bS Supporting Information ABSTRACT: Adsorption of anionic polyelectrolytes, sodium salts of carboxymethyl celluloses (CMCs) with different degrees of substitution (DS = 0.9 and 1.2), from aqueous electrolyte solutions onto regenerated cellulose surfaces was studied using quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) experiments. The influence of both calcium chloride (CaCl2) and sodium chloride (NaCl) on CMC adsorption was examined. The QCM-D results demonstrated that CaCl2 (divalent cation) caused significantly greater CMC adsorption onto regenerated cellulose surfaces than NaCl (monovalent cation) at the same ionic strength. The CMC layers adsorbed onto regenerated cellulose surfaces from CaCl2 solutions exhibited greater stability upon exposure to flowing water than layers adsorbed from NaCl solutions. Both QCM-D and SPR results showed that CMC adsorption onto regenerated cellulose surfaces from CaCl2 solutions increased with increasing CaCl2 concentration up to the solubility limit (10 mM). Voigt-based viscoelastic modeling of the QCM-D data indicated that the CMC layers adsorbed onto regenerated cellulose surfaces had shear viscosities of ηf ≈ 103 N 3 s 3 m2 and elastic shear moduli of μf ≈ 105 N 3 m2. Furthermore, the combination of SPR spectroscopy and QCM-D showed that the CMC layers contained 9095% water. Adsorption isotherms for CMCs in CaCl2 solutions were also obtained from QCM-D and were fit by Freundlich isotherms. This study demonstrated that CMC adsorption from CaCl2 solutions is useful for the modification of cellulose surfaces.
’ INTRODUCTION Polyelectrolyte adsorption onto material surfaces has been identified as a promising method to create materials with tailored surface chemistry and greater hydrophilicity, adsorption capacity, and biocompatibility.1 Likewise, bacterial cellulose (BC) exhibits many excellent properties, including high mechanical strength, high crystallinity, and strongly bound water, that have prompted studies of possible biomedical applications such as artificial blood vessels for microsurgery and substrates for tissue engineering of cartilage.2 However, the fabrication of materials with properties comparable to those of natural bone from BC requires negatively charged groups on the BC surface to enhance the heterogeneous nucleation of apatite. Carboxymethyl cellulose (CMC), an inexpensive polysaccharide derivative that has been widely used in the paper-making, food, and textile industries, can be used to introduce negative charge onto cellulose fiber surfaces.3,4 A recent study produced cellulose/hydroxyapatite nanocomposites with CMC.5 Moreover, another study showed that monovalent salt drives CMC adsorption onto hydrophobic alkane thiol selfassembled monolayers (SAMs) on gold and indicated that Ca2þ with CMC might provide a generic mechanism for surface r 2011 American Chemical Society
modification.6 Although a negatively charged layer can be formed on BC surfaces through self-assembly,5 the heterogeneity of BC surfaces makes it difficult to obtain fundamental information about CMC adsorption. In contrast, cellulose surfaces regenerated from spin-coated trimethylsilylcellulose (TMSC) films have well-defined morphological and chemical characteristics.6 Although cellulose surfaces regenerated from TMSC do not contain cellulose I domains, whereas BC is a highly crystalline material with both amorphous and cellulose I domains,7 studies of CMC adsorption onto regenerated cellulose surfaces provide a fundamental understanding of how cellulose surface chemistry affects CMC adsorption. Polyelectrolyte adsorption onto solid surfaces has been quantitatively studied by quartz crystal microbalance with dissipation monitoring (QCM-D)1,810 and surface plasmon resonance (SPR) spectroscopy.11,12 Both QCM-D and SPR spectroscopy are surfacesensitive techniques that have been used to investigate the Received: February 17, 2011 Revised: May 16, 2011 Published: June 23, 2011 8718
dx.doi.org/10.1021/la200628a | Langmuir 2011, 27, 8718–8728
Langmuir adsorption of surfactants,13 polysaccharides,14 proteins,15,16 and DNA17 at liquid/solid interfaces. Adsorption of cationically modified starches onto cellulose surfaces was studied by QCMD by Kontturi et al.1 Modeling of QCM-D results can provide the mass and viscoelastic properties of the adsorbed layers.18 Furthermore, the combination of the QCM-D and SPR techniques has facilitated the determination of coupled water for hydrated layers.13 In this study, the adsorption of anionic polyelectrolytes, sodium salts of CMC, with different degrees of substitution (DS) onto cellulose surfaces from aqueous electrolyte solutions was probed. The cellulose surfaces were regenerated from spincoated TMSC films by hydrochloric acid (HCl) catalyzed hydrolysis in the gas phase. The influence of both calcium chloride (CaCl2) and sodium chloride (NaCl) on CMC adsorption was examined. The coupled water and viscoelastic properties of CMC layers were determined by the combination of QCM-D and SPR studies and Voigt-based viscoelastic modeling of the QCM-D data. Understanding how different salts affect CMC adsorption to create stable functional layers on cellulose surfaces was the focus of this work.
’ EXPERIMENTAL SECTION Materials. Sodium salts of CMC (weight-average molar mass of Mw ≈ 250000 g 3 mol1, DS = 0.9 and 1.2) were purchased from Aldrich and were used without further purification, as was calcium chloride (CaCl2, 99.9%). Sodium chloride (NaCl, 99.9%) was purchased from Mallinckrodt Baker, Inc. TMSC (DS = 2.71) was synthesized and kindly provided by the Heinze group at the Friedrich Schiller University of Jena, in Jena, Germany.19 Sulfuric acid (98%) and ammonium hydroxide (28% solution) were purchased from Fisher Chemical. Toluene (HPLCgrade) was purchased from EMD. Hydrogen peroxide (30% solution) was purchased from VWR International. Solutions of CMC were made from ultrapure water (Millipore, Milli-Q Gradient A-10, 18.2 MΩ 3 cm,
