Adsorption of Carboxymethyl Cellulose onto Titania Particle Films

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Adsorption of Carboxymethyl Cellulose onto Titania Particle Films Studied with In Situ IR Spectroscopic Analysis Bea Botka, Alexander James McQuillan, Marta Krasowska, and David A Beattie Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01011 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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Adsorption of Carboxymethyl Cellulose onto Titania Particle Films Studied with In Situ IR Spectroscopic Analysis

Bea Botka1 #, A. James McQuillan2, Marta Krasowska1,3, David A. Beattie1,3* 1

Future Industries Institute, University of South Australia, Mawson Lakes, South Australia 5095, Australia 2

3

Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand

School of Information Technology and Mathematical Sciences, University of South Australia, Mawson Lakes, South Australia 5095, Australia

* Correspondence and requests for materials should be addressed to D.A.B. (email: [email protected]) # Current address: Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, 1525 Budapest, Hungary

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Abstract Adsorption of carboxymethyl cellulose (CMC) in aqueous solution onto a titania nanoparticle film has been studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). CMC was adsorbed onto the positively charged titania surface in neutral, partially charged, and fully charged state. The response of the adsorbed polyelectrolyte layer was monitored upon changing the electrolyte pH and ionic strength. The degree of dissociation of the CMC increased upon adsorption onto the titania surface, and changed with the surface coverage. Ionic strength change was observed to influence the degree of dissociation of the adsorbed CMC similar as when in solution. No significant peak shifts were observed in the spectrum of the adsorbed CMC during adsorption or in response to changing solution conditions, therefore inner sphere complexation between the carboxyl groups and the titania could not be confirmed. The effect of ion identity on the adsorption process was studied using soft and hard cations, and mono- and divalent cations. The presence of a divalent counterion was observed to cause changes in the carboxymethyl vibrations, which can be related to formation of intra- or interchain linkages.

Keywords Titania; carboxymethyl cellulose; ATR-IR; adsorption


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Introduction Titania (TiO2) and carboxymethyl cellulose (CMC) are both widely used additives in everyday products. Titania is a mineral commonly used as a pigment in paper-making, paints, food and cosmetics.1 Carboxymethyl cellulose is a prevalent additive in these products, mainly serving as a thickener, emulsion or dispersion stabiliser or as a dispersing/suspension agent.2, 3 The analysis of the interactions at the solid-liquid interface between such a polymer and mineral nanoparticles like titania can provide a better understanding of the use of these components in multi-phase mixtures and products. Attenuated total reflectance infrared spectroscopy (ATR-IR) provides an in situ tool to study the adsorption of polymers at the solid-liquid interface, and generates valuable information about dynamic processes, structure of the adsorbed molecules, and their interaction with the surface.4 Typically a thin, porous, immobile particle film is formed on the surface of an ATR internal reflection element and placed in contact with a dilute solution containing the adsorbing molecules.5 The evanescent wave predominantly probes the particle film and the interface, which is beneficial as water absorbs strongly in the mid infrared region. Adsorption of carboxymethyl cellulose onto hematite and titania was studied in great detail by Hoogendam et al. using reflectometry, dynamic light scattering, electrophoretic mobility and depletion measurements.6-9 This series of publications discussed the influence of ionic strength (IS) and pH, as well as the kinetics of the adsorption process. Theoretical modelling of the kinetics of polyelectrolyte adsorption onto charged surfaces was also addressed by the group of Hoogendam10. While these studies provide valuable information regarding the adsorption process, they do not provide direct experimental evidence of the nature of binding or structural changes of the adsorbed species due to interaction with the surface. The potential for ATR-IR spectroscopy 3 ACS Paragon Plus Environment

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to contribute to this particular issue was highlighted in the work of Beaussart et al., who investigated the adsorption of various forms of dextrin onto titania using in situ ATR-IR spectroscopy.11 By comparing carboxymethylated and non-carboxymethylated dextrins at different pH, the authors outlined the importance of the carboxyl group in the interaction with the surface. This work also included preliminary investigation of the adsorption of CMC onto titania at pH 9 and 3 (reported in the supporting information of that paper). Polyelectrolyte adsorption onto charged surfaces can be enhanced if a pH or an ionic strength cycle is applied.10, 12 When CMC is adsorbed at low pH, and the pH of the solution is increased molecules partially desorb, but still higher amounts remain on the surface than would adsorb at high pH. This similarly applies to ionic strength. Adsorption at higher ionic strength, followed by lowering ionic strength results in higher adsorbed amount than adsorption at low ionic strength. Sedeva et al., using a quartz crystal microbalance, studied the adsorption of CMC onto a self-assembled hydrophobic thiol monolayer coated onto a gold surface.13 The authors observed that the amount of hydration water retained in the polyelectrolyte layer changes significantly upon changing the pH and the ionic strength of the solution, indicating that the structure of the layer dynamically reacts to these changes. In this work, we have systematically investigated the binding mechanism of CMC onto a titania surface as function of pH, and the changes occurring to the adsorbed polyelectrolyte upon pH and ionic strength change as stimulus. We compare our results to the extant literature for this system, and the conclusions reached from those earlier works using less direct methodologies.


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Materials and Methods Materials Sodium-carboxymethyl cellulose (Na-CMC), having a molar mass of 90 000 g mol-1 was purchased from Sigma Aldrich (Australia). A schematic diagram of the molecular structure is given in Figure 1. The degree of carboxymethyl substitution is approximately 0.7 based on the information provided by the supplier. Electrolyte solutions were prepared from NaCl, KCl (both 99% purity, Chem-Supply, Australia), and CaCl2*2H2O (99% purity, Univar, Ajax Finechem, Australia). NaCl and KCl were purified by calcination at 550 °C for 8 hours prior to use. CaCl2 was purified three times by purging the solution with high purity N2 gas (99.999% purity, BOC, Australia) for 15 minutes, and cleaning off the surface of the solution using a glass pipette. Volumetric grades of NaOH and KOH (Merck, Australia) and HCl (Scharlau, Spain) were used for pH adjustment of the solutions. Aeroxide P25 titania nanoparticles were received from Degussa (Evonik Industries, Germany).14 The particles are of 90% anatase and 10% rutile phase and have a mean diameter of 25 nm.15 The isoelectric point (pHIEP) of the P25 titania nanoparticles is pH 7. The pHIEP was determined by measuring the zeta potential as a function of pH in 0.001 and 0.005 M KCl electrolyte solutions, using a Malvern Zeta Sizer Nano (see Figure S.1 in the Supporting Information). Solutions All solutions were prepared in Milli-Q water (Millipore, USA) of resistivity 18.2 MΩ cm, and total organic content below 5 ppb. Water was purged for 45 minutes with ultrapure dried nitrogen stream through a glass porous frit before solution preparation to remove dissolved CO2. Electrolyte solutions were pH adjusted using 1 - 0.01 M NaOH solution, and 4 - 0.002 M HCl solution to pH 2.0, 4.0, 5.4 and 9. 2000 mg L-1 CMC stock solutions were prepared by dissolving the Na-CMC at 5 ACS Paragon Plus Environment

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native pH in NaCl, KCl and CaCl2 electrolyte solutions. Stock solutions were stirred overnight and diluted to 100 mg L-1 using the pH adjusted electrolytes. After dilution, the pH of the CMC solution was readjusted to the desired value. Each solution was used within 24 hours from preparation. Concentrated solutions of CMC were also prepared as a reference to study the polymer in solution. CMC was used in the form of a Na salt, providing approx. 0.03 M added Na ion concentration for 1000 mg L-1 CMC concentration, which was taken into account for the ionic strength of these solutions.

Figure 1: Left: Schematic drawing of a fragment of CMC for identification of carbon atoms.16 Right: The scheme of carboxymethyl cellulose, showing the hydrogen bonds between O3H3…O5 and O2H2…O6. Where the side groups are not substituted H bonds can form in between O6H6…O3.17

Particle Films Particle films were prepared on the surface of the internal reflection element (IRE) using Aeroxide P-25 titania nanoparticles. Prior to deposition of the particle film the IRE was polished using OP-


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U colloidal silica gel, having 0.04 μm grain size, on MD-Nap-T polishing cloth (both Streuers, Australia), then rinsed with water and polished again on a clean wet polishing cloth. Afterwards the IRE was rinsed again and dried under pure dry nitrogen stream. The titania dispersion for drop casting was prepared by dispersing 1 mg L-1 P25 titania in Milli-Q water followed by 15 minutes of sonication. 75 μL (ZnSe IRE) or 20 μL (diamond IRE) of this dispersion was drop cast onto the surface of the clean IRE and dried for 15-25 minutes under nitrogen stream. Films were hydrated for at least 1 hour prior to commencing the measurement by flowing Milli-Q water over them. The deposited films were stable under the applied flow conditions. ATR-FTIR Spectroscopy FTIR measurements were performed on a dry air purged Varian 670-IR spectrometer using a liquid nitrogen cooled linearized mercury cadmium telluride (MCT) detector (Agilent Technologies, USA). Attenuated total reflection (ATR) was measured using two different accessories. The FastIR single reflection ATR accessory (Harrick Scientific, USA), equipped with a ZnSe internal reflection element having 45° incident angle was used for measurements in the pH range of 4 to 9. Titania films were drop coated onto the surface of the IRE, solutions were delivered into the flow cell by a peristaltic pump (Masterflex, L/S, John Morris Scientific, Australia) via Tygon tubing (Masterflex L/S 13, Cole-Parmer, USA). The Harrick FastIR ambient flow cell used for the measurements has an internal volume of approximately 0.1 mL. The flow cell design facilitates quick exchange of solutions, therefore it is ideal for the stimulus response studies. For initial film cleaning 0.5 mL min-1 flow rate was applied, for the adsorption and desorption studies the flow rate was decreased to 0.1 mL min-1. For the pH dependent adsorption studies (pH 2 - 5.4) a DuraSamplIR accessory was used with a triple reflectance diamond IRE and equipped with a custom-made flow cell, described in earlier 7 ACS Paragon Plus Environment

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work.15 The particle film preparation and the film cleaning procedure was practically the same as for the other flow cell, but the flow speeds (1 mL min-1 for film cleaning, 0.3 mL min-1 for adsorption studies) and the titania solution drop size were adjusted to better match the dimensions of the cell. An O-ring made of AFLAS® fluoropolymer was used with both flow cells in contact with the solution to ensure chemical inertness and compatibility in the applied pH range. The spectrometer was allowed to purge for at least 30 minutes after placement of the ATR accessory before commencing the measurements. Water vapour spectra were recorded on the clean IRE and were used where necessary to correct the sample spectra. Background spectra were recorded on a clean titania film in the same electrolyte solution as the sample spectra. As the presented absorbance spectra of the adsorbed species are the difference spectra of the sample and background spectra, the substantial absorptions of water are almost entirely cancelled. As reference for the particle film measurements, solution spectra were recorded of concentrated CMC solutions on a clean IRE. After recording spectra of the CMC solution, clean electrolyte was flown over the IRE until the flow cell volume was sufficiently flushed and spectra of the CMC adsorbed onto the surface of the IRE was recorded. To obtain the spectra of bulk solution CMC only, the spectra of adsorbed species was subtracted from the one measured on the concentrated solution. Spectra were recorded using Resolutions Pro software. Spectra were calculated at 4 cm-1 resolution from 128 co-added scans for kinetic studies and 512 co-added scans for bulk solution studies. Kinetic spectra were collected every 1 minute, which consisted of 46 s of measurement time and 14 s delay in between measurements.


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Data Processing Spectra were processed using a Matlab script as described below. An ATR correction was applied to the spectra where appropriate. The effective penetration depth in ATR spectroscopy depends on the wavelength, therefore when peak intensities in different regions are compared, one has to carefully evaluate which correction is appropriate to use depending on the measurement configuration. Harrick and du Pré18 give two limiting cases: the first is the thin film approximation, where the thickness of the sample on the surface of the IRE is much smaller than the effective penetration length, the second is the semi-infinite case, used mostly for bulk samples, where the thickness of the sample is significantly larger than the effective penetration depth. This latter case is applicable to the solution spectra measured as a reference for the particle film studies. When the polymers are adsorbed onto the surface of a particle film, neither of these approximations are valid. Titania particle films formed on the surface of ZnSe and diamond are typically 1 - 2 μm thick, which is comparable to the penetration depth. The appropriate correction in this case depends on the effective refractive index of the particle film (volume fraction of particles compared to water within the film) and the film thickness.19, 20 Therefore, while it is appropriate to compare changes in peak intensity ratios quantitatively on polymer films that are formed on the same titania film, assuming that the adsorbed CMC layer is sufficiently thin, care must be taken when solution and adsorbed layer spectra on the surface of a particle film are compared. We applied a linear ATR penetration depth correction on the bulk solution spectra.20, 21 The spectra of the polymer adsorbed onto the titania particle film were not corrected.


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The water vapour contribution was automatically subtracted where necessary by finding a least square fit of the second derivative (SD) spectra of the sample and clean water vapour spectra in the 1800 - 2000 cm-1 range. Peak intensities were calculated by summing the absorbance in the wavenumber regions denoted on the relevant figures, measured from a common baseline. The adsorbed amount was calculated as A(1050-1070 cm-1), the delta dissociation as A(1410-1430 cm-1) / [A(1234-1250 cm-1) + A(1410-1430 cm-1)]. For calculation of the adsorbed amount a linear baseline between 971 and 1194 cm-1 was used, and for the dissociation ratios, 1194 and 1800 cm-1 baseline points were used. SD analysis was used to identify components of intrinsically overlapping peaks.22, 23 SD spectra were calculated on spectra smoothed using Savitzky-Golay algorithm using 7 points with a cubic polynomial. Even though the spectra were water vapour corrected, SD analysis was applied in regions which are free from water vapour vibrations, because narrow linewidth peaks are significantly enhanced in the second derivative spectra.24 This effect, combined with the smoothing can result in the appearance of broader extra peaks arising from the water vapour that might be mistaken with spectral features. Where water vapour affected areas are displayed, the scaled second derivative spectrum of water vapour is also included.

Results CMC Adsorption at Various pH CMC is a polyelectrolyte, and so the adsorption process is governed by the balance of nonelectrostatic interactions and electrostatic forces between the polyelectrolyte and the surface. Dissociation of polyelectrolytes depends on salt concentration, where the apparent dissociation


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constant 𝑝𝐾1/2 𝑎𝑝𝑝 (i.e. the pH at which the polymer has half of its functional groups protonated, and half deprotonated) of CMC changes from 3.9 to 3.2 in the 0.01 M and 0.5 M NaCl concentration range. CMC was adsorbed onto the positively charged titania surface in three different forms: as a neutral polymer (pH 2), partially charged polyelectrolyte (pH 4) and in its fully dissociated form (pH 5.4). The interaction between the oppositely charged polymer and surface initially facilitates adsorption, but once the surface charge is compensated, the electrostatic barrier due to the adsorbed polyelectrolyte hinders further adsorption. When non-electrostatic interactions are absent or weak, electrostatic interactions between polymer and substrate dominate adsorption. As a result, in the high salt concentration regime, the adsorption can be reduced due to increased screening of any attractive polymer-substrate electrostatic interactions. When the role of non-electrostatic interactions is more prominent, with increasing salt concentration, screening enhances the adsorption of polyelectrolytes, as the dominant effect of the high salt is to reduce inter- and intrapolymer charge repulsion.25 Hoogendam et al. showed that the adsorption of CMC onto rutile and hematite is a screening-enhanced process.6 For our experiments we used 0.5 M salt concentration both to facilitate the attainment of a high adsorbed amount, and to reach equilibrium within the timeframe of the experiment.10


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Figure 2: Infrared spectra of CMC adsorbed onto titania and in solution at pH 2 (top panel), 4 (middle panel) and 5.4 (bottom panel). Adsorbed species spectra were recorded on titania films exposed to 100 mg L-1 CMC solution in 0.5 M NaCl for 90 mins. Solution spectra were recorded on 5000 mg L-1 CMC solution in 0.5 M NaCl.

Figure 2 shows the spectra of CMC adsorbed onto the titania nanoparticles compared to the solution spectra, which were also recorded at the same pH. Figure 3 shows spectra of adsorbed CMC acquired for two of these pH values: 2 and 5.4 (spectra for pH 4 are given in Figure S.2 in the supporting information). The spectra contain significant amounts of detail, so it is prudent to consider the spectra in reference to the structure of the polymer (Figure 1), and previously published studies on the spectroscopy of CMC. Experimental and theoretical vibrational spectra 12 ACS Paragon Plus Environment

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of pristine and carboxymethylated cellulose in solid form can be found in the literature.26-29 Andrianov calculated the vibrational modes of a monomeric fragment of the 2,6-carboxymethyl cellulose molecule for different conformations.16 The pH dependence of the vibrational spectra of CMC in aqueous solution is discussed in other published work.30 The carboxymethyl substitution of the hydroxyl groups is not homogeneous along the polymer. C2 and C6 sites are more likely to be substituted than C3 sites, but the relative populations depend on the synthesis method. The relative number of unsubstituted, mono-, di, and tri-substituted sites vary with the average number of substitution.31-33 Peaks in the 1000 - 1500 cm-1 region of the vibrational spectra of pristine and carboxymethyl cellulose arise mostly from overlapping and mixed vibrations16, 26, and therefore care must be taken to ensure that peaks used for semi-quantitative analysis are distinct and assignable. The most notable changes in the spectrum of solution CMC upon pH change occur to vibrations of the carboxylic group. Deprotonation of the carboxylic groups at high pH results in disappearance of the C=O and C-O stretch modes located around 1732 and 1243 cm-1, while new modes related to the symmetric and antisymmetric stretch of the deprotonated carboxyl group appear at 1417 and 1583 cm-1. Second derivative analysis reveals that the 1417 peak consists of two peaks centred at 1412 and 1423 cm-1 both becoming more intense at high pH (see spectrum of adsorbed CMC, initially adsorbed at pH 2, then exposed to a rinse of native pH water, in Figure S.3 in the supporting information). The dominant contribution of these modes most likely originated from the symmetric stretching of the deprotonated carboxyl group 16, 34. The intensity of the peak at 1325 cm-1 also increases.


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Figure 3: IR spectral data of adsorbed CMC during CMC adsorption onto titania at pH 2 (neutral form, top panels) and pH 5.4 (fully dissociated form, bottom panels). Left panels show the integrated absorbance between 1050 and 1070 cm-1 (normalised to value at the end of the adsorption step), which is proportional to the amount, and the change of dissociation throughout the adsorption and rinse phase (see text for details of the calculation). Note: in each plot, the dark shades of the colours used are the data points for absorbance (left y-axis), and the lighter shades are those for the change in dissociation (right y-axis). Grey data points are indicative of times when the composition of the solution in the flow cell is undergoing change. The right panels show representative spectra at different stages of the adsorption. Spectra are normalized to the absorbance at 1061 cm-1. The colour bar indicates the flow time in minutes.

The amount of adsorbed CMC was monitored using the absorption of the glycoside bridge (integrated absorbance in the region 1050 - 1070 cm-1).11,

16

To qualitatively monitor the

dissociation of the carboxyl groups during adsorption, the integrated absorbance values of the 1234 - 1250 cm-1 region (C-O stretch of the protonated carboxyl group) and the 1410 - 1430 cm-1 region 14 ACS Paragon Plus Environment

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(symmetric stretch of the deprotonated carboxyl group) were used. The parameter given in the plots is termed  Dissociation, and the numerical value is obtained from the following expression: A(1410-1430 cm-1) / [A(1234-1250 cm-1) + A(1410-1430 cm-1)]. If the carboxyl peaks were the only contributions to these regions of the spectra this ratio would be equal to 1 when the polymer is fully deprotonated and equal to 0 when the polymer is fully protonated. However, as other modes also contribute to the absorbance in the monitored areas, the calculated value is only semiquantitative, but is still a viable and useful indication of the changes in the degree of dissociation of the adsorbed polymer. Spectra were recorded once the solution was flowing to capture the onset of the absorption, but complete solution exchange in the flow cell is delayed. Datapoints in these regions are masked with grey.  Dissociation values were only calculated for time points when start of the polymer adsorption was confirmed. It should be noted that the two spectral regions chosen for this calculation are less commonly used for the purpose of quantifying degree of dissociation. The antisymmetric COO- stretch (1583 cm1)

and the C=O stretch (1732 cm-1) peaks are more often used to determine the degree of

dissociation. However, these peaks are close to the water bending mode at 1635 cm-1, and their intensities and peak shapes are influenced by a contribution from the water bending mode that is largely independent of the adsorption process, and that is due to changes in the hydration of the titania particle film itself. This altered film hydration results in difficulties in objectively correcting this region to reveal the changes in the antisymmetric COO- stretch and C=O stretch (see Figures S.4 and S.5 in the supporting information for spectra that display this variation in film hydration and the alteration of the spectral region for the antisymmetric COO- stretch and C=O stretch). The spectra and analysis presented in Figures 2 and 3 indicate that the carboxyl groups of the CMC are more dissociated in the adsorbed form than at the same pH in solution, but no significant shift 15 ACS Paragon Plus Environment

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of the carboxyl group vibrations (C-O stretch mode at 1243 cm-1, and the symmetric stretch of the deprotonated carboxyl group appear at 1417 cm-1) was detected due to the interaction with the surface. Furthermore, comparison of CMC spectra at different stages of the adsorption process (Figure 3) reveals, that the extent of protonation of the adsorbed CMC depends on the surface coverage, when CMC is adsorbed in its neutral form. Another critical observation is that, upon rinsing with electrolyte of the same pH, CMC did not desorb in the timeframe of the measurement. One smaller detail that can be seen in the spectra in Figure 3 is an alteration of the profile for the peaks attributed to the pyranose ring of the CMC polymer for adsorbed layers at pH 2 relative to pH 5.4. The peak at 1119 cm-1 on the protonated CMC (pH 2) appears downshifted upon protonation at pH 5.4 (at pH 9, this peak is located at 1110 cm-1). In general, the 1000-1200 cm-1 region consist of stretching vibrations of the pyranose ring, C-O modes mixed with C-C-H and OC-H bending vibrations. It was shown on pristine and modified celluloses that change of the intramolecular hydrogen bonding can cause changes in the relative peak intensities in this region.29, 35

Second derivative analysis (Figure S.3) of CMC spectra acquired with a polymer layer exposed

to a changing pH (adsorbed at pH 2, then exposed to native water pH) shows that the apparent peak shift is caused by changing contribution of subpeaks at 1134, 1119 and at 1101 cm-1. These modes originate from mixed vibrations located on the carboxymethyl side groups involving the O6 and O2 oxygens, and are predicted to be sensitive to conformational changes of these groups.16 Conformational changes of the carboxyl side groups are connected to changes of the intramolecular hydrogen-bonding pattern. We also investigated changes for CMC adsorbed in its fully dissociated state and then exposed to a subsequent decrease of the pH of the electrolyte, and compared the acquired spectrum to that obtained for direct adsorption at low pH. Figure 4 shows that the adsorbed CMC is more 16 ACS Paragon Plus Environment

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dissociated when pH is decreased after high pH adsorption, than when directly adsorbed at low pH. In its fully dissociated state the CMC adsorbs in a lower amount due to the intermolecular repulsion caused by the ionised carboxylate groups than in the neutral state. As the pH of the electrolyte is decreased less adsorbed molecules are available to compensate the increased surface charge of the titania, which in turn facilitates dissociation of the carboxyl groups. The same effect plays a role during the initial part of the adsorption process. At low surface coverage the adsorbed molecules are more dissociated (Figure 3).

Figure 4: Infrared spectra of CMC adsorbed in neutral (pH 2), partially charged (pH 4) and in fully charged form (pH 5.4) from 100 mg L-1 CMC solution in 0.5M NaCl. The film adsorbed at pH 5.4 was subsequently rinsed with pH 4 and pH 2 electrolyte. Spectra are normalized to the absorbance at 1061 cm-1. The peak marked with asterisk is the bending mode of water. The peaks due to protonated and deprotonated carboxyl groups that are unperturbed by the influence of the water mode are highlighted with grey shading.


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pH and Ionic Strength Cycling: The adsorbed CMC layer on the titania surface was exposed to two types of stimuli, changing pH and ionic strength, and these stimuli were cycled. The initial CMC layer was adsorbed in both cases from 0.1 M NaCl solution at pH 4, containing 100 m g L-1 CMC. Once quasi equilibrium was reached, an ionic strength cycle (rinsing with 0.1 and 0.01 M electrolyte solution at pH 4) or a pH cycle (rinsing with 0.1 M electrolyte solution, at pH 4 and 9) was performed. Both of these cycles are expected to cause significant change in the structure of the adsorbed CMC layer.13 Lowering the ionic strength of the solution reduces screening, causing increased self-repulsion within the adsorbed CMC layer, which can also result in desorption.12 Increasing the pH not only results in self-repulsion of the CMC layer due to increased dissociation, but at pH 9 the titania surface is also negatively charged. Upon lowering the ionic strength from 0.1 to 0.01 M (Figure 5) at pH 4, we did not observe desorption, showing that the attractive forces between CMC and the titania surface dominate over self-repulsion. The adsorbed layer appears more dissociated at higher ionic strength, similarly as in solution, as the increased screening facilitated dissociation.7 The polymers in contact with the surface are more dissociated at both ionic strengths, but the presence of the surface does not influence significantly the increase of the degree of dissociation upon increasing salt concentration. Sedeva et al. investigated a pH cycle on CMC layers adsorbed at pH 9 on a model hydrophobic surface, cycling between pH 9 and pH 4, studied using QCM.13 The pH cycle in this work (Figure 6) started with adsorbing CMC at pH 4, because it was previously shown that CMC does not absorb onto anatase at pH 9.11 This altered direction of switch (initially adsorbed at pH 4 rather than pH 9) relative to the work of Sedeva et al. raises the possibility of desorption, which should occur if the pH is raised significantly above the point of zero charge of the titania film.6 We observed that 18 ACS Paragon Plus Environment

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upon rinsing the film at pH 9, approximately 40% of the molecules desorbed (Figure 6). As the pH was decreased to pH 4 again, the adsorbed CMC layer appeared more dissociated compared to the original one. The cause of the surface induced dissociation is the same as discussed earlier. After desorption at high pH, less molecules are available to compensate the surface charge of the titania film. The contribution of the 1101 cm-1 peak also increased, which is potentially related to change in the conformation of the adsorbed CMC. No further desorption was observed at the second pH 9 rinse step.


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Figure 5: Ionic strength cycle. The top panel shows the adsorbed amount (absorbance) and change in dissociation. Note: in the plot, the dark shades of the colours used are the data points for absorbance (left y-axis), and the lighter shades are those for the change in dissociation (right y-axis). Grey data points are indicative of times when the composition of the solution in the flow cell is undergoing change. Bottom panel is a comparison of the IR spectra of CMC adsorbed onto titania and in solution at 0.1 and 0.01 M ionic strength at pH 4. Spectra are normalized to the absorbance at 1061 cm-1. Note: an axis break was used to remove a period of time when an air bubble was trapped in the flow cell at the end of the first 0.1 M NaCl rinse cycle. The experiment continued once the air bubble was displaced.


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Figure 6: pH cycle The top panel shows the adsorbed amount (absorbance) and change in dissociation. Note: in the plot, the dark shades of the colours used are the data points for absorbance (left y-axis), and the lighter shades are those for the change in dissociation (right y-axis). Grey data points are indicative of times when the composition of the solution in the flow cell is undergoing change. Bottom panel shows comparison of the IR spectra at different stages of the cycle. Spectra are normalized to the absorbance at 1061 cm-1.


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Ion Effects: The effect of different counterions on the adsorbed CMC layer was investigated by adsorbing CMC in its fully dissociated form to titania (pH 5.4) and subsequently rinsing the layer with an electrolyte of the same ionic strength containing a different cation. K+, Na+ and Ca2+ were used for these experiments, the anion was always Cl-. Spectra of CMC adsorbed in the presence of different counterions look almost identical; minor shifts occur, but they are mostly not significant compared to the spectral resolution. Minor changes occur around 1100 - 1134 cm-1, which can reflect changes in the conformation of the carboxymethyl groups. The most prominent difference observed upon ion exchange is that, in the presence of Ca2+ ions, the intensity ratio of the 1412 and 1425 cm-1 peaks change. This most likely indicates a changed environment of the carboxymethyl side groups.


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Figure 7: Effect of cations on the spectra of adsorbed CMC. Top panel: absorbance. Bottom panel: second derivative of absorbance. Spectra are normalized to the absorbance at 1061 cm-1. CMC was adsorbed from 0.5 M electrolyte at pH 5.4. Comparison of CMC direct after adsorption from NaCl, KCl and CaCl2, or adsorption followed by a rinse with electrolyte of a different cation (X→Y). The figure also displays second derivative of water vapour spectra.


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Discussion Binding Several groups have studied the surface complex formation of small carboxylic acids with mineral surfaces, monitoring changes in the separation of the symmetric and antisymmetric stretching of the carboxylate compared to the free ions in solution.19, 36-38 These complexes can be divided to three main classes. (i) solvent-surface hydration-separated ion pairs, (ii) surface hydration-shared ion pairs, and (iii) inner sphere complexes. The former two can also be referred as outer sphere complexation.38 In the case of solvent-surface-hydration separated ion pairs (i) the interacting functional group is solvated by water, therefore the separation of the carboxylate stretching frequencies are practically the same as in case of solution species. Inner sphere complexation (iii) on the other hand means a significantly altered environment, therefore depending on the nature of the complexes formed, the vibrational frequency of the symmetric and antisymmetric carboxylic stretches change. An empirical set of rules for the classification of bulk coordination complexes of small carboxylic acids were developed by Deacon and Philips39 based on comparison of the infrared spectra and structural information of several acetate complexes: (a) if the separation of the symmetric and antisymmetric stretching of the carboxylate (ΔνCOO-) is significantly larger than that of the ionic value (>200 cm-1), the complexation is monodentate, except when the uncoordinated oxygen atom forms a hydrogen bond to other ligands via pseudo-bridging, and in this case the separation can be significantly lower; (b) very small ΔνCOO- (

Adsorption of Carboxymethyl Cellulose onto Titania Particle Films

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