Article pubs.acs.org/Langmuir
Adsorption of Carboxymethyl Cellulose on Polymer Surfaces: Evidence of a Specific Interaction with Cellulose Rupert Kargl,*,† Tamilselvan Mohan,‡ Matej Bračič,‡ Martin Kulterer,† Aleš Doliška,‡ Karin Stana-Kleinschek,*,‡,§ and Volker Ribitsch†,§ †
Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstraße 28/III, AT-8010 Graz, Austria Laboratory for Characterization and Processing of Polymers, Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia
‡
ABSTRACT: The adsorption of carboxymethyl cellulose (CMC), one of the most important cellulose derivatives, is crucial for many scientific investigations and industrial applications. Especially for surface modifications and functionalization of materials, the polymer is of interest. The adsorption properties of CMC are dependent not only on the solutions state, which can be influenced by the pH, temperature, and electrolyte concentration, but also on the chemical composition of the adsorbents. We therefore performed basic investigation studies on the interaction of CMC with a variety of polymer films. Thin films of cellulose, cellulose acetate, deacetylated cellulose acetate, polyethylene terephthalate, and cyclo olefin polymer were therefore prepared on sensors of a QCM-D (quartz crystal microbalance) and on silicon substrates. The films were characterized with respect to the thickness, wettability, and chemical composition. Subsequently, the interaction and deposition of CMC in a range of pH values without additional electrolyte were measured with the QCM-D method. A comparison of the QCM-D results showed that CMC is favorably deposited on pure cellulose films and deacetylated cellulose acetate at low pH values. Other hydrophilic surfaces such as silicon dioxide or polyvinyl alcohol coated surfaces did not adsorb CMC to a significant extent. Atomic force microcopy confirmed that the morphology of the adsorbed CMC layers differed depending on the substrate. On hydrophobic polymer films, CMC was deposited in the form of larger particles in lower amounts whereas hydrophilic cellulose substrates were to a high extent uniformly covered by adsorbed CMC. The chemical similarity of the CMC backbone seems to favor the irreversible adsorption of CMC when the molecule is almost uncharged at low pH values. A selectivity of the cellulose CMC interaction can therefore be assumed. All CMC treated polymer films exhibited an increased hydrophilicity, which confirmed their modification with the functional molecule. functional molecules.13−17 Some of these surface modification methods with CMC are based on the assumption that there is a selective CMC−cellulose interaction which originates from structural similarities between the polyelectrolyte and the biopolymer's surface. Such assumptions were corroborated by interaction studies of CMC on, for instance, cellulose fibers18−20 or spin coated regenerated cellulose model films from trimethylsilyl cellulose21,22 by applying a quartz crystal microbalance (QCM-D) and surface plasmon resonance (SPR).23 Even though it was shown in a couple of previous studies that increased ionic strength of especially bivalent cations lead to irreversible deposition of CMC on cellulose and other materials, evidence of a specific selectivity for cellulose surfaces is still missing. This study therefore aims at providing indirect evidence of this selectivity by comparing the influence
1. INTRODUCTION The water-soluble negatively charged polyelectrolyte carboxymethyl cellulose (CMC) is one of the most important cellulose derivatives.1 It is widely used in industry as a thickener, as an additive in washing powders, for paper making, or as a flotation agent.2 In many of these applications, the understanding of interfacial properties and adsorption processes on solid surfaces plays a key role. For that reason, the interaction of dissolved CMC with solid surfaces was studied by several authors in the past. These studies tried to elucidate the interaction mechanism of CMC with, for instance, hydrophobic and hydrophilic minerals,3−5 talc,6,7 metal particles,8 or hydrophobic self-assembled monolayers.9 Besides adsorption on inorganic materials, the interaction of CMC with cellulose surfaces was investigated in detail and utilized within the last years. The specific properties of CMC and its seemingly selective interaction with cellulose have been used to increase the paper strength,10 to inhibit vessel picking in paper making,11 to modify textile fibers,12 or to irreversibly attach © 2012 American Chemical Society
Received: May 23, 2012 Revised: July 3, 2012 Published: July 3, 2012 11440
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min, subsequent immersing in water twice, and drying with nitrogen gas. For the coating of the QCM-D gold crystals with polyvinyl alcohol (PVA, 99+% hydrolyzed, Mw: 85−124 kDa, Sigma-Aldrich, Austria), 1 g of PVA was dissolved in 100 mL of pure water at 90 °C. The solutions were filtered using 0.2 μm nylon filters. This solution was applied to the QCM-D cell (flow rate: 0.1 mL min−1) for 30 min and rinsed with pure water for 30 min. Following this, the crystals were used for the adsorption studies of CMC as described in section 2.4. 2.2. ATR-IR Measurements. The films on the QCM-D gold crystals were subjected to ATR-IR measurements using a PerkinElmer Spectrum GX Series-73565 FTIR-spectrometer with 32 scans at a resolution of 4 cm−1 and a scan range between 4000 and 650 cm−1. 2.3. Thickness Measurements. The thickness of all films was determined by optical thickness measurements with the Sarfus technique on SiO2 surfs. This method is based on optical reflectance microscopy on nonreflecting substrates (Surfs). The intensity and color of the reflected light is increased, when a coating is deposited on these substrates.28 These changes are proportional to the films thickness, which can be calculated by comparison with a standard of known thickness. A detailed description of this method can be found elsewhere.29 The thickness of each polymer film was determined on SiO2 Surfs on at least three independent samples with three measurements on each sample. An average thickness and standard deviation was calculated from these values. 2.4. QCM-D Measurements. The QCM-D method is based on the piezoelectric effect of quartz crystals. In this study, a QCM-D model E4 from Q-Sense (Gothenburg, Sweden) was used. In the QCM-D method, the oscillation frequency of a quartz crystal is reduced when mass is deposited on the surface. For rigid coatings, the change in frequency depends linearly on the change of mass. This dependence can be expressed with the following Sauerbrey equation.
of the solid substrate on the adsorption properties of dissolved CMC at different pH values. In order to show that not only complexation of carboxylic groups with bivalent ions and screening of charges and decrease of solubility by increased ionic strenghts is the reason for the irreversible immobilization of CMC, a pH dependent study without additional electrolytes except hydrochloric acid was conducted in this work. Five different polymeric model films which strongly differ in their chemical composition were manufactured by spin coating on sensors of a quartz-crystal microbalance. Pure cellulose (CE), cellulose acetate (CA),24 partially deacetylated cellulose acetate (DCA), polyethylene terephthalate (PET), and cyclo olefin polymer (COP) model films were prepared and characterized with infrared spectroscopy, contact angle measurements, and atomic force microscopy. To further elucidate the influence of the wettability and surface structure on the deposition of CMC, hydrophilic silicon dioxide and polyvinylalcohol (PVA) coated surfaces were included in the study. The pH dependent interaction and irreversible deposition of CMC was quantified with QCM-D and supported by atomic force microscopy (AFM). Contact angle measurements gave further insights into the wettability of the modified polymeric surfaces. This study should verify the existence of a selective cellulose CMC interaction via the comparison of the interaction of CMC with a variety of polymer films. Furthermore, it should show how polymeric films can be modified by a simple adsorption/coating procedure with this seminatural polysaccharide in order to alter their surface properties and introduce new functionalities.
Δf = − C
2. EXPERIMENTAL SECTION 2.1. Substrate Cleaning and Film Preparation. The thin polymer films used in this study were prepared by spin coating on different substrates. As substrates either silicon wafers (100 nm SiOx, Silchem, Freiberg, Germany), Surfs (SiO2-surface, Nanolane, France), gold coated QCM-D crystals (QSX-301, LOT-Oriel, Germany), or SiO2 coated QCM-D crystals (QSX-303, LOT-Oriel, Germany) were used. The Surfs and QCM-D crystals were used as received. Trimethylsilyl cellulose (kindly provided by the Friedrich Schiller University Jena, synthesized as published elsewhere,25 Mw: 174 kDa, DSTMS: 2.55) solutions were prepared in toluene, (Sigma-Aldrich, Austria) at a concentration of 1 wt %. Cellulose acetate (CA) thin films were prepared from solutions of 1 wt % cellulose acetate (CA), (acetyl content 38 wt % Mw: 30 kDa, Sigma-Aldrich, Austria) in 1,4-dioxane (Sigma-Aldrich, Austria). Cyclo olefin polymer solutions were prepared at a concentration of 1 wt % COP (Zeonor1060R, Zeon Chemicals, Germany) in o-xylene (98%, Sigma-Aldrich, Austria). Polyethylene terephthalate (PET, Mylar A, DuPont, Taijing Films, Luxembourg) solutions were prepared by dissolving PET in 1,1,2,2tetrachloroethane (≥98%, Fluka, Austria) at 80 °C under reflux until a clear solution was obtained. After cooling to room temperature, the PET solutions were filtered through a 0.2 μm Acrodisc GHP filter. All polymer solutions except PET were spin coated by placing 50 μL on the static substrates and spinning at 4000 rpm with an acceleration of 2500 rpm s−1 for 60 s. PET solutions were spin coated by placing 50 μL on the static substrate and spinning at 2500 rpm at an acceleration of 2500 rpm s −1 for 60 s. A detailed procedure on the preparation of PET films is published elsewhere.26 For the regeneration of TMSC to pure cellulose (CE), each TMSC coated substrate was placed into a 20 mL polystyrene Petri-dish with a diameter of 5 cm. Two milliliters of 10 wt % HCl were place beside the substrate, and the Petri-dish was covered with its cap. TMSC was regenerated to pure cellulose during the exposure to HCl vapors for 10 min. A detailed description of this procedure is described elsewhere.21,27 Deacetylated cellulose acetate films (DCA) were prepared by incubating the spin coated CA films in 15 mL of 0.1 M KOH for 20
Δm n
(1)
where Δf describes the frequency shift, Δm the mass change, C the Sauerbrey constant (17.7 ng Hz−1), and n the number of the overtone of oscillation. A detailed description of the QCM-D method can be found elsewhere.30−32 Equation 1 is not applicable if coatings are not rigid enough and do not sufficiently couple to the oscillating crystal. In order to determine the rigidity of the coating, the dissipation factor of the crystal can be measured simultaneously.33 This factor is described as the ratio of the energy that is stored in the system and the energy that is lost during one oscillation cycle (eq 2). The higher this dissipation factor, the less rigid and the more swollen the film or an adsorbed layer is.
D=
Elost 2πEstored
(2)
For all pH dependent adsorption studies of CMC on the polymeric films, CMC solutions of 0.2 wt % CMC (DSCOONa: 0.7, Mw 90 kDa, Sigma-Aldrich, Austria) were prepared in pure water (resistivity ≥18 MΩ cm) and stirred overnight. The pH (either 2 or 4) was adjusted with 0.1 M HCl, and the solutions were filtered using 0.45 μm PVDF (polyvinylidenefluoride) syringe filters. Native solutions of CMC were taken for the measurements at pH 7. Except the pH adjustments, no electrolyte was added to the CMC solutions. Prior to adsorption, the coated QCM-D gold crystals were mounted into the measurement chamber and equilibrated with pure water (constant flow rate 0.1 mL min−1). After that, the starting frequencies were scanned and the film was equilibrated with water at the pH investigated. All frequencies and dissipation values were set to zero, and the measurement was started (constant flow rate 0.1 mL min−1). After 10 min, the CMC solution with the desired pH value was introduced into the QCM-D chamber for 60 min. Afterward, the films were rinsed with pure water (15 min), 10 mM NaHCO3 buffer (pH 7, 30 min), pure water (15 min), 3 M NaCl (15 min), and finally pure water (15 min) again. During the whole procedure, the flow rate was kept constant at 0.1 mL min−1. The interaction of dissolved poly(acrylic acid) sodium salt (0.2 wt % in 11441
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water, Mw: 2100 g mol−1, Aldrich, Austria) with cellulose model films was performed in the same way as the CMC studies. For the analysis of all data, the final frequency shifts of the third overtone after all rinsing steps were compared. All measurements were conducted three times on independently prepared surfaces. An average frequency and dissipation shift was calculated from these values. 2.5. Contact Angle Measurements. In order to determine the infuence of CMC deposition on the wettability, all polymer films were subjected to contact angle measurements with pure water (≥18 MΩ cm). Contact angles were determined on pure polymer films and on CMC modified ones. All measurements were performed on films that were obtained from the QCM-D studies. As a comparison, untreated films and films treated with pure water at different pH values in the QCM-D cell were used for contact angle measurements. A contact angle device from dataphysics OCA 15plus was used (Dataphysics, Germany). For the measurements, 3 μL drops were deposited on the substrates and the static contact angle was measured by the instruments software via a drop shape analysis using the Young− Laplace equation as a basis. At least three drops per film on two independent substrates were measured, and an average value and standard deviation were calculated. The surface free energy of hydration (ΔGiw) as a measure of hydrophilicity was calculated from the contact angle data according to eq 3, where γTw describes the total surface tension of water (72.8 mJ m−2)34 and θw is the contact angle of a sessile drop of water on a solid surface. A detailed description of the derivation and use of this equation can be found elsewhere.34,35 ΔGiw = − γwT(1 + cos θw)
(3)
Figure 1. ATR-IR spectra of polymer films on QCM-D gold crystals.
2.6. Atomic Force Microscopy. The surface morphology of the thin polymer films was determined with atomic force microscopy in tapping mode with an Agilent 5500 AFM multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA, USA). The images were scanned using silicon cantilevers (ATEC-NC-20, Nanosensors, Germany) with a resonance frequency of 210−490 kHz and a force constant of 12−110 N m−1. All measurements were performed at room temperature under ambient air. An image size of 1 × 1 μm2 was taken from all samples. The films that were treated with pure water at pH 2 and with CMC solutions at pH 2 in the QCM-D chamber were subjected to AFM measurements.
Table 1. Optical Sarfus Thickness of the Polymer Films, Spin Coated on SiO2 Surfaces polymer CE DCA CA PET COP
3. RESULTS AND DISCUSSION 3.1. Film Characterization. 3.1.1. ATR-IR Measurements and Film Thickness. The ATR-IR spectra of the spin coated polymer films are depicted in Figure 1. Pure cellulose thin films that were regenerated from TMSC show a typical spectrum (OH, 3000 cm−1; CH, 2600 cm−1; C−O, 1100 cm−1) which is comparable with literature values.21,27,36 Deacetylated cellulose acetate (DCA) thin films give OH-vibrations at 3000 cm−1 concomitantly with a reduced CO vibration at 1600 cm−1. This confirms that CA thin films were partially deacetylated to DCA by treatments with potassium hydroxide solutions.37 Even though the appearance of OH vibrations is obvious, the film still consists to a considerable amount of cellulose acetate. Therefore, the DCA film can be regarded as a CA film with a partial cellulose character. Pure CA, in contrast, does not show pronounced OH vibrations at 3000 cm−1 but gives absorption bands which are typical for cellulose acetate.38 Thin films of polyethylene terephthalate are characterized by CH vibrations (2900 cm−1) and a typical fingerprint region of a PET film.39 COP gives a spectrum showing mainly CH vibrations (2900 cm−1). Table 1 shows the optical Sarfus thickness of the thin polymer films that were spin coated on silicon dioxide Surfs. The thickness values of all films are below 100 nm. Deviations in the films thickness result from either different spin coating parameters or solution properties of the polymers. Interestingly, it could be shown that the film thickness of CA was reduced by
thickness (nm) 28.1 59.0 69.4 35.2 38.8
± ± ± ± ±
1.3 0.4 0.6 1.7 0.2
10 nm upon conversion to DCA. Cleavage of acetyl groups seems to reduce the thickness either upon increase in density or by dissolution of the film surface. The results from the ATR-IR and thickness measurements clearly demonstrate that closed, nanometric thin films of diffent polymeric materials were successfully prepared on QCM-D gold crystals and Surfs. Subsequently, these films were used for the CMC adsorption studies with QCM-D and for contact angle and AFM measurements. 3.2. QCM-D Measurements. 3.2.1. Interaction of CMC with Cellulose Films. The frequency and dissipation shifts of the third overtone of oscillation (Δf 3, ΔD3) that resulted from the interaction of CMC with a pure cellulose film at three different pH values are depicted in Figure 2. At neutral pH, minor interaction of CMC with the solid surface can be detected. This is reflected by low changes in frequency after introduction of the CMC solution. More CMC is deposited on cellulose at lower pH values with a maximum at pH 2. For the measurements at pH 7 and 4, the frequency signals are stable after approximately 5 min of CMC incubation. In contrast, the frequency changes gradually with time when CMC is incubated at pH 2. The stronger interaction of CMC with the cellulose surface at lower pH can be attributed to the reduced charge and solubility of the polyelectrolyte. Since the carboxylic groups of CMC have an isoelectric point of 3.2,40,41 the molecule is almost fully protonated at pH 2 and partially charged at pH 4. 11442
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−50.6 Hz; ΔD3, 21.6 × 10−6). The treatment of cellulose thin films with CMC solutions at pH 2 obviously allows the immobilization of high amounts of the polyelectrolyte. Protonation reduces the charges and the solubilty and enhances the interaction of cellulose with CMC. The QCM-D instrument is operated at different overtones of oscillation. From these overtones, qualitative and quantitative information about the rigidity of the bound layers can be obtained.42,43 A strong deviation between the overtones of frequency or dissipation reflects a low coating rigidity. When the overtone values are similar, a higher rigidity of the coating can be assumed. Figure 3 shows the overtones f 3 to f 9 and the
Figure 2. Interaction of CMC with a cellulose film at pH 2, 4, and 7. The frequency and dissipation changes of the third overtone of the oscillation are shown.
Even though no precipitates were formed during pH adjustments at the preparation of the CMC solutions, the solubility of CMC is lower at a pH below the pK-value. This can sufficiently explain the higher affinity of CMC to cellulose at pH 4 and 2 before rinsing with water. After rinsing with pure water (pH 7), differences in the behavior of CMC can be observed. At pH 7 and 4, the frequency of oscillation is increased, which can be interpreted as a desorption. This is also reflected in reduced dissipation values. Contrary to that, frequency is reduced and dissipation is increased for surfaces that were incubated with CMC at pH 2. A reasonable explanation is the deprotonation of the bound CMC layer during rinsing with neutral water. This causes tremendous swelling and therefore negative frequency shifts and highly positive dissipation values. This is an evidence that, compared to higher pH values, more CMC remained on the surface after incubation at pH 2 and subsequent rinsing with pure water. When the surfaces are rinsed with 10 mM NaHCO3, further deprotonation of the bound layer occurs, which results in desorption and swelling. This desorption is more pronounced on surfaces modified at pH 2, where the CMC was less charged at the beginning and where more material was irreversibly immobilized. Rinsing with pure water causes again density changes of the liquid in the QCM-D cell and, most likely, simultaneous desorption and swelling, owing to the increased solubility of deprotonated CMC. Remarkable density changes of the liquid in the QCM-D cell can then be observed when the layers are rinsed with 3 M NaCl. These changes are completely reversible after rinsing with pure water, indicating minor desorption of CMC at high ionic strength. Overall, the interaction of CMC with a spin coated solid cellulose model film strongly depends on the pH of the applied solution. The QCM-D results suggest that irreversible deposition of CMC takes place at pH values of 4 and 2. A highly swollen layer of CMC is immobilized on the cellulose surface. This is especially pronounced after a treatment with CMC at pH 2, where the final frequency and dissipation shifts are the highest. (Δf 3,
Figure 3. Interaction of CMC with a cellulose film at pH 2. The frequency and dissipation changes of four overtones of the oscillation are shown.
corresponding dissipation values for the interaction steps of CMC with cellulose at pH 2. From a qualitative point of view, the differences in the overtones for both dissipation and frequency increase after the rinsing steps with water and buffer. This clearly indicates that the immobilized CMC layer is highly hydrated and coupling to the oscillating crystal is reduced. These deviations in the overtones remain, even after exposure to pure water. When the overtones deviate to a high extent, the Sauerbrey equation highly underestimates the adsorbed mass. One can therefore assume that higher masses of CMC are bound to the cellulose surface than the Sauerbrey equation would suggest. 3.2.2. Interaction of CMC with CA, DCA, and SiO2. The influence of the surface composition on the interaction with dissolved CMC was further investigated using spin coated films of cellulose acetate (CA) and deacetylated cellulose acetate (DCA) with a partial cellulose structure. The interaction of CMC with a hydrophilic nonpolysaccharide surface was further studied using pure silicon dioxide QCM-D crystals. As shown in Figure 4, CMC does not interact with the surface of SiO2 coated QCM-D crystals at pH 2. This is reflected by minor frequency and dissipation changes when CMC is introduced into the QCM-D chamber. Even though the solution state of CMC is the same as that in the experiments on the cellulose model films, CMC is not deposited on the hydrophilic SiO2 11443
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Figure 5. Final frequency shifts (f 3) from the deposition of CMC on different surfaces. The results from three pH values are shown.
DCA, CA, and PET. Lower pH values lead to higher amounts of deposited material. The dissipation values of all experiments follow the same trend as the frequency data (Figure 6), and high frequency shifts lead to larger changes in dissipation, caused by high water contents.13,23,43 Figure 4. Interaction of CMC with SiO2, cellulose acetate (CA) and deacetylated cellulose acetate (DCA) at pH 2. The frequency and dissipation changes of the third overtone ( f 3; D3) of the oscillation are shown.
surface. The high hydrophilicity and chemical dissimilarity seem to prevent the polymer adsorption on this substrate. The rootmean-square surface roughness of SiO2 and freshly prepared cellulose coated crystals, which is an important parameter in polymer adsorption, does not differ significantly (rms-roughness: SiO2, 1.1 nm; CE, 0.78 nm).21,44 Furthermore, the sensitivity of SiO2 crystals is higher than those of polymer coated ones because the rigid SiO2 layer completely couples with the quartz crystal oscillator. Significant changes of frequency and dissipation on SiO2 surfaces are only observable when 3 M NaCl is rinsed over the crystals. This can be attributed to density differences of the salt solutions since the shifts are completely reversible. A very different behavior can be observed on CA films. Higher interaction occurs, and both frequency and dissipation shifts are visible, indicating the deposition of CMC. However, films with a partial cellulose character (DCA) behave in the same way as pure cellulose films (section 3.2.1). A higher interaction at the beginning and an irreversible deposition of CMC and pronounced swelling (dissipation increase) when rinsed with pure water show that CMC is deposited on DCA substrates. With respect to the frequency shift, the amount of irreversibly deposited material on cellulose films is much higher than that on all other materials. The comparison of the interaction of CMC with DCA, CA, SiO2, and pure cellulose films leads to the conclusion that there has to be a significant contribution of a specific interaction of CMC with solid cellulose. Especially, the irreversibilty of the deposition can be interpreted as a specific interaction of parts of unsubstituted glucose within the CMC chain with the solid cellulose surface. The chemical compostion of the surface seems to play a crucial role in this interaction. 3.2.3. Comparison of the Different Materials. The frequency shifts on the different surfaces after the final rinsing steps with water are summarized in Figure 5. The dependency of the deposited amount on the pH is pronounced for CE,
Figure 6. Final dissipation shifts (D3) from the deposition of CMC on different surfaces at pH 2, 4, and 7.
For CE and DCA, which both have a cellulose structure, significantly more CMC is deposited at pH 2 and 4. The hydrophobic polymer films PET and COP show minor interaction with CMC from solution, which can be attributed to their difference in chemical composition and a low wettability by water. These low interactions of water with the hydrophobic polymer surfaces together with the strong solvation of CMC in solution most likely prevent the polyelectrolyte from interacting with COP and PET. A sole interaction that is based on apolar forces is therefore not the only driving force for the deposition of CMC. QCM-D crystals that were coated with PVA did not significantly interact with CMC. This is a proof that the ability to form hydrogen bonds does not necessarily enhance the amount of deposited CMC. From that data one can conclude that the mechanism of immobilization is not only based on the formation of hydrogen bonds but also on other interaction forces. A reasonable explanation for the observed low deposition of CMC on PVA is the fact that PVA has a higher affinity to water than the cellulose surface. Therefore, the solvated CMC molecules are not able to compete with adsorption sites on the strongly hydrated PVA surface. The fact that cellulose surfaces are not only hydrophilic but bear nonpolar parts in 11444
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CA films with KOH successfully cleaved the ester bonds and produced a more hydrophilic material with available OHgroups at the surface. PET and COP are hydrophobic synthetic polymers, with PET being the one with a higher polarity in its chemical structure. It is worth noting that treatments with pure water at pH values of 2 and 4 lead to a slight increase in the hydrophilicity for all materials. This can be explained by the adsorption/ incorporation of water or a partial surface hydrolysis of CE, DCA, CA, and PET. However, the stability of the surfaces under the treatment conditions was sufficient, and no leaching or destruction could be observed by optical thickness measurements. Any treatment with CMC solutions leads to reduced contact angles of water on every polymeric material. On hydrophobic surfaces such as PET and COP, the influence of the CMC deposition on the increased hydrophilicity is higher. Even though QCM-D suggests minor deposition of CMC on those surfaces, low amounts of CMC can already influence the wettability to a high extent. The films of CE and DCA are already hydrophilic, and changes in the wettability with water are therefore less pronounced. Nevertheless, the general trend that more CMC is deposited at lower pH values is also reflected in the contact angles of water and visible on each polymeric material. As a measure of hydrophilicity and for comparative reasons, one can calculate the surface free energy of hydration (ΔGiw) from the water contact angle data. Lower water contact angles result in more negative values of ΔGiw. The general trend of increasing hydrophilicities after treatment with CMC solutions is therefore also reflected in the quantitative value ΔGiw (pure films: CE, −137.5 mJ m2; DCA, −125.5 mJ m2; CA, −113.2 mJ m2; PET, −82.9 mJ m2; COP, −59.6 mJ m2; CMC pH 2 treated films: CE, −142.9 mJ m2; DCA, −135.5 mJ m2; CA, −124.9 mJ m2; PET, −124.1 mJ m2; COP, −87.7 mJ m2). Atomic force microscopy measurements revealed interesting details about the deposited CMC layers. On hydrophobic substrates (COP and PET), CMC is deposited in a particle-like structure (Figure 8). The hydrophobic character seems to favor this form of deposition, since the wettability of water is lower and spreading of the strongly hydrated CMC molecules during attachment is less favorable.3 Hydrophilic surfaces (CE, DCA) do not show this structure, and most of the CMC seems to be more homogeneously distributed on the surface. On CA
their molecular structure therefore seems to be important for the irreversible deposition of CMC.45,46 The low deposition of CMC on PVA surfaces is also an indication that the formation of acid catalyzed ester bonds between the polymer surface OH-groups and the carboxylic groups of the CMC molecules is negligible. Even though there is no experimental proof which excludes the formation of such bonds, there are several arguments against it. First, CMC is also deposited on cellulose at pH 4. Second, solutions of poly(acrylic acid) sodium salt, which were tested as a control, are not deposited on CE surfaces at pH 2 (data not shown). These findings together further confirm that the presence of a cellulose backbone of the dissolved polymer is an important factor for the relative selective interaction of CMC with CE. 3.3. Wettability and Surface Morphology. The wettability data of films that were treated with pure water or CMC solutions at pH 2, 4, and 7 in the QCM-D cells are shown in Figure 7.
Figure 7. Contact angle of water on surfaces treated with CMC solutions and with water at pH 2, 4, and 7.
The hydrophilicity of pure films follows a trend that can be expected from the chemical composition. The most hydrophilic material is CE, followed by DCA, which is more hydrophilic than nondeacetylated CA. This also proves that treatments of
Figure 8. Morphology of surfaces that were treated with water at pH 2 (upper row) and CMC solutions at pH 2 (lower row). The z-scale is 20 nm. 11445
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Langmuir surfaces, less particles of CMC are visible, which can be attributed to the intermediate hydrophilicity of the substrate. Overall AFM measurements confirmed the stability of the coated polymer films, the differences in the morphology of the deposited CMC layers, and, indirectly, the findings from contact angle measurements.
ACKNOWLEDGMENTS
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REFERENCES
Prof. Thomas Heinze from the University of Jena is highly acknowledged for providing the TMSC. The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 214653.
4. CONCLUSION Polymeric thin films of cellulose, cellulose acetate (CA), deacetylated cellulose acetate (DCA), polyethylene terephthalate (PET), and cyclo olefin polymer (COP) were successfully prepared on sensors of a quartz crystal microbalance and silicon dioxide surfaces by spin coating. These films are stable, homogeneous, and representative for the respective polymeric material, as confirmed by ATR-IR and thickness measurements. The films could be used to measure and compare the interation with solutions of carboxymethyl cellulose (CMC) in dependence of the pH value. The pH value was varied from 7 to 2. On pure cellulose films, carboxymethyl cellulose is irreversibly attached at lower pH values of 2 and 4. Rinsing with water, 10 mM bicarbonate buffer, and 3 M NaCl solution could not remove these bound layers of CMC. The layers are highly hydrated and are therefore causing high dissipation values of the QCM-D sensors. At pH values below the pK value of the carboxylic groups of the polyelectrolyte, high amounts can be immobilized irreversibly on cellulose without any additional electrolyte. A similar behavior is observed on films with a partial cellulose character (DCA). On deacetylated cellulose acetate, the pH dependent CMC immobilization follows the same trend as on pure cellulose films even though the absolute frequency and dissipation shifts are lower. This is an indirect evidence that pure or partial cellulose surfaces favor the irreversible deposition of CMC when the molecule is protonated and therefore almost uncharged. A further evidence of the importance of the molecular surface structure is given by the fact that hydrophilic silicon dioxide and polyvinylalcohol coated QCM-D crystals did not strongly interact with CMC solutions at different pH values. Low adsorption of CMC was also found on hydrophobic cellulose acetate, PET, and COP films. On PET and COP films, CMC is deposited in a particle like structure which can be explained by their hydrophobic character. The strong variation of the pH dependent adsorption properties of CMC on very different polymeric materials leads to the conculsion that there has to be a relatively selective interaction of carboxymethyl cellulose with solid cellulose surfaces. The selectivity and the simplicity of the surface modification process without the addition of a bivalent cation are of importance in the applicability of cellulose, CMC, and other polymeric materials.
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AUTHOR INFORMATION
Corresponding Author
*K.S.-K.: e-mail,
[email protected]. R.K.: e-mail, rupert.
[email protected]; phone, 0043 316 380 5413; fax, 0043 316 380 9850. Notes
The authors declare no competing financial interest. § Member of the European Polysaccharide Network of Excellence (EPNOE). 11446
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