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Effect of polycation charge density on polymer conformation at the clay surface and consequently on pharmaceutical binding
Journal of Colloid and Interface Science 552 (2019) 517–527
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Regular Article
Effect of polycation charge density on polymer conformation at the clay surface and consequently on pharmaceutical binding Hagay Kohay a, Itzhak I. Bilkis b, Yael G. Mishael a,⇑ a b
Department of Soil and Water Science, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel Institute of Biochemistry, Food Science and Nutrition, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 24 April 2019 Revised 22 May 2019 Accepted 24 May 2019 Available online 25 May 2019 Keywords: Adsorption Polycation Charge density Clay Montmorillonite Composite Conformation Structure Pharmaceuticals
a b s t r a c t Polycation conformation upon adsorption to a negatively charged surface can be modulated by its charge density. At high charge density monomers directly interact with the surface in a ‘trains’ conformation and as charge density decreases a high degree of monomers dangle into solution in a ‘loops and tails’ conformation. In this study, the conformations of polycations upon adsorption to montmorillonite, as a function of polycation charge (20, 50 and 100% of the monomers, denoted as P-Q20, P-Q50 and P-Q100) were characterized and in accordance with their conformation, the adsorption of non-ionic and anionic molecules by the composite was tested. As expected, the adsorption of the nonionic pharmaceuticals increased to a composite with a ‘loops and tails’ conformation, due to both conformation and chemical properties. On the other hand, the anionic molecules, gemfibrozil and diclofenac, preferably adsorbed to composites with higher charge density (Q50 or Q100 composites). However, they showed different tendency toward the composites, i.e. higher adsorption of diclofenac by Q100 composite vs. higher adsorption of gemfibrozil by Q50 composite. To elucidate the differences in adsorption between these two pharmaceuticals, density functional theory calculations were employed. Both gemfibrozil and diclofenac were found to be better stabilized by methyl pyridinium sites (prevail in Q100 composite). The preferable adsorption of gemfibrozil by Q50 composite was explained by the presence of ‘loops and tails’ conformation enabling additional adsorption sites and diverse monomer-target molecule orientations. Ó 2019 Published by Elsevier Inc.
Abbreviations: MMT, Montmorillonite; PVPcS, poly-4-vinylpyridine-co-styrene (styrene/pyridine ratio 1/10); HPVPcS, protonated PVPcS; P-Q100/Q50/Q20, PVPcS which was methylated by methyl iodide in 100, 50 or 20% of its pyridine sites, respectively.; Q100/50/20, composites which are based on MMT and P-Q100/Q50/Q20, respectively; PY, pyridine monomer; HPY, pyridinium (protonated PY); 4MPY, N-methyl pyridinium (methylated PY); GMF, Gemfibrozil; DCF, Diclofenac; CBZ, Carbamazepine; LMG, Lamotrigine. ⇑ Corresponding author at: Dept. Soil and Water Sci., The Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot 76100, Israel. E-mail address: [email protected] (Y.G. Mishael). https://doi.org/10.1016/j.jcis.2019.05.079 0021-9797/Ó 2019 Published by Elsevier Inc.
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1. Introduction Polymer characteristics at the solid-liquid interface plays an important role in many applications such as water treatment, drug delivery systems (biocompatible surfaces), battery technology, membrane distillation and oil extraction [1–4]. The adsorption of polyelectrolytes to oppositely charged surfaces and the functionalities of such materials has been thoroughly described [5–15]. One of the aspects explored is polyelectrolyte conformation on the surface. Polyelectrolyte conformation is dominated by the degree of monomers directly interacting with the surface; a high degree of interaction resulting in a ‘trains’ conformation whereas a low degree of interaction resulting in polymer segments which are not only interacting with the surface, but also dangling into solution as ‘loops and tails’. The adsorption of a copolymer with high affinity (sticky monomers) and low affinity monomers was described theoretically [16] and experimentally [15,17–23]. In contrast to the theoretical studies which assume that the system reaches equilibrium, experimental studies suggest that the system is stocked in a non-equilibrium state [7,21,24,25] in which additional factors such as the initial conformation received and adsorption kinetics may affect the conformation. Among other parameters, the effect of polymer charge density, surface charge density, polymer concentration and solution composition, on polymer conformation at the adsorbed state, were addressed experimentally [15,17–22]. The effect of the charge density of the polymer on adsorption is expressed in terms of conformation and loading, which is governed by the surface charge compensation. Accordingly, high charge density polymers form relatively flat conformations with low loading, while low charge density polymers tend to form ‘loops and tails’ conformations along with higher loading [7,8,11,19,26,27]. These conformational changes of the polycation were visualized by AFM on mica surface. Minko et al. [28], and Borkovec et al. [29], displayed the transition from more extended to more globular conformation as charge density decreased (controlled by pH). These states (‘trains’ vs. ‘loops and tails’) are not absolute and even at high charge density, polycation may have monomers ‘‘dangling” into solution as can be explained by entropic considerations [22,24]. In addition to charge density, polyelectrolyte concentrations and high Ionic strength of the solution, are additional factors that promote a ‘loops and tails’ conformation [9,10,20,25,30–35]. Recently, polymer-clay composites have been developed to bind a third component for various applications [36–39]. Despite the extensive literature on polymer adsorption to surfaces and the ongoing interest in the applicability of the composites, only a few of the studies addressed the polymer conformation and the consequent binding affinity and capacity of the composites. Séquaris et al. [40], reported that as the added concentration of non-ionic polymer increased, the conformation was inclined to be more ‘loops and tails’. The effect of polymer conformation on the adsorption of anionic surfactant and non-ionic molecule was tested. While the adsorption capacity of the anionic surfactant substantially increased in the presence of ‘loops and tails’, the adsorption of the non-ionic molecule was less affected by this conformation. In the case of conformational changes which governed by the charge density of the polymer, the overall effect will also be determined by the ability of each monomer to interact with the target molecule [41]. In order to correlate between conformation and functionality, the chemical interactions between the monomers and the target molecule, need to be defined. Tsuzuki et al. [42], tested the interaction between a benzene ring and pyridine (PY), pirydinium (HPY) and N-methyl pyridinium (4MPY) monomers by ab-initio calculations. They demonstrated that higher energy stabilization was
obtained through interactions with the charged monomers and that the extent of stabilization was dependent on monomerbenzene orientation. The polycation conformation on the clay can affect the adsorption either by exposing more adsorption sites or by enabling better access to adsorption sites. While the ability of polymer in a ‘trains’ conformation to interact with other molecules is restricted, a polymer in a ‘loops and tails’ conformation has more degrees of freedom enabling varied orientations between the monomers and target molecules. The overall adsorption will be determined by the interplay between the interactions in accordance to monomer chemistry in addition to conformation considerations. In the current study we aimed to test the effect of polymer conformation upon adsorption, ‘trains’ vs. ‘loops and tails’, on its ability to adsorb target molecules with diverse chemical properties. Accordingly, the pyridine (PY) monomers of poly-4-vinylpyridinco-styrene (PVPcS) were methylated to three degrees, 100, 50 and 20%, (denoted P-Q100, P-Q50 and P-Q 20, respectively). The effect of polycation charge level on its adsorption to a negatively charged clay-mineral, montmorillonite (MMT), was studied and the conformations of the polymers in the different polymer-clay composites were characterized. The adsorption of four pharmaceuticals (non-ionic and negatively charged), also considered as potential contaminants with only partial degradation in conventional water treatment plants [43–46], by the composites was tested. The adsorption was correlated to the molecular interactions with the monomers and to the polycation conformation at the interfaces. An in-depth understanding of the relation between polymer conformation at surfaces and its ability to bind a third component would contribute to improve and govern adsorption and desorption processes which can be useful in many fields. 2. Materials Wyoming Na-montmorillonite SWy-2 (MMT) was obtained from the Source Clays Repository of the Clay Mineral Society (Columbia, MO); cation capacity (CEC) and specific surface area were 76.4 meq/100 g and 756 m2/g, respectively. Poly-4vinylpyridine-co-styrene (PVPcS) (Mw = 1200–1500 KDa, PVP: Styrene ratio-9:1), Diclofenac sodium (DCF), Gemfibrozil (GMF), Carbamazepine (CBZ), (all > 97% purity), and methyl iodide (95%) were purchased from Sigma-Aldrich Israel Ltd. Lamotrigine (LMG) (>99%) was purchased from EnzoBiochem Inc. (New York). 3. Methods 3.1. Polycations preparation and characterization Protonated PVPcS (HPVPcS) solution was prepared in acidified distilled water (ADW) solution by adding H2SO4 (95% purity) in stoichiometric concentrations of pyridine. Methylation of PVPcS was preformed according to previous studies [47,48]; briefly, PVPcS was freeze-dried, solubilized in N,N-Dimethylformamude (DMF) for 24 h and methyl iodide was added in 0.2, 0.5 and 1.5 M ratios in order to form P-Q20, P-Q50 and P-Q100 polycations, respectively. The reaction mixture was refluxed for 12 h at 60 °C and was washed thoroughly with hexane. The solvent was removed under reduced pressure and the sample was lyophilized. P-Q100 and P-Q50 were dissolved in distilled water (DW), while P-Q20 was dissolved in ADW (pH 3). The polycation solutions were dialyzed (MWCO 14,000 da) against DW and ADW respectively to remove residual unreacted components (methyl iodide). A sample from each polycation solution was lyophilized and the
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weight distribution of the elements (C,H,N,I) was analyzed (see below). The methylation was assured by NMR and the methylation level was calculated by the N/C molar ratio, by FTIR and by XPS measurements (see below). The total organic carbon (TOC) of the solutions was measured upon filtration by PTFE syringe filter (AXIVA) 0.45 lm and the molar concentration of each polycation was calculated based on carbon/average monomer ratio in each case. 3.2. Polycations stability To test whether polycation stability depends on pH, vials with exactly the same concentration of each polycation (3 repetitions) but at different pH’s (3 and 7) were prepared. The solutions were agitated, filtered through PTFE syringe filter and the concentration was measured by UV–Vis spectrophotometer (see below). In addition, in order to compare this procedure to the one applied with the composites; the concentration of the polycations at the tested pH was also measured upon centrifugation (12000g for 10 min, 18 °C). Good correlation was found between both procedures (filtration/centrifugation). 3.3. Polycation adsorption and composite preparation MMT clay suspension of (0.5%) was added to polycation (P-Q20, P-Q50, P-Q100, HPVPcS) solutions (final concentrations were 0.1–2.5 mmol/L for polycation monomers and 0.17% for MMT suspention). The clay-polycation suspensions were agitated (for at least 4 h, reaching equilibrium); suspensions were centrifuged (12000g for 10 min, 18 °C) and supernatant was separated. The polycation and iodide concentrations in supernatants were measured and calculated against calibration curve using UV–Vis spectrophotometer at a wavelength of 223–226 [49] and 255–262 nm [50] (depending on iodide interference in this range) for iodide and polycation, respectively (Fig. S4). The polycation and iodide adsorption was calculated accordingly. In order to avoid the effect of the different pH’s on polycation charge and subsequently on its conformation and functionality, the composites solutions were adjusted to pH 6.5–7 by adding equal preparation volume of tap water (pH-7, EC = 0.75 mS/cm, DOM concentration and organic contamination–negligible) to the composite precipitates which were re-suspended, agitated to reach equilibrium, centrifuged and the supernatant was separated as was mentioned above.
3.4. Calculation of the dangling monomer’s percentage and the internal division of the dangling monomers The percentage of 4MPY sites for each polycation upon adsorption were calculated based on XPS measurements, (50.2 and 100% for P-Q50 and P-Q100 respectively, see Fig. S2). Since for both polycations styrene consists 10% of the monomers, the values from the XPS measurements were multiplied by 0.9 (under ‘4MPY sites’ column in Fig. 2d). The percentage of the dangling monomers (Fig. 2c) was calculated by the sum of three components: un-anchored 4MPY monomers (Ua4MPY) PY (exist only for Q50) and styrene monomers (Eq. (1)). Based on literature [51] the last two are not expected to form substantial interactions with the clay. Ua4MPY was calculated based on 4MPY- iodide interactions; under the assumption that direct interaction of 4MPY to MMT includes ion exchange in which the counter ion (Iodide) is released, while dangling 4MPY sites used as anion exchange sites for iodide. Based on that, Ua4MPY percentage was calculated by the ratio Ia/TPa, in which Ia is Iodide adsorbed and TPa is the total polymer adsorbed (both are expressed as mmol/g).
Dangling monomersð%Þ ¼ Ua4MPY þ PY þ Styrene ¼
Ia 100 þ Py þ 10 TPa
ð1Þ
The percentage of PY monomers as a component in the dangling monomers (only for Q50) was calculated as follows: PYð%Þ ¼ 100 4MPYsites styrene ¼ 100 45:2 10 ¼ 44:8%. 3.5. Polycation release upon washes The composites (0.28–0.6 mmol of polycation monomers/g clay) were prepared as described above. A preparation volume of DW (Q100, Q50), ADW (Q20, HPVPcS composite) or salted water (1 M NaCl) was added to the precipitates. The composites were re-suspended, agitated for 24 h, centrifuged (12000g for 10 min, 18 °C) and polycation concentration in the supernatant was measured against calibration curves at the same condition (pH and salt). The release was calculated accordingly. 3.6. Polycation and composite characterization 3.6.1. Zeta potential measurements Zeta potential values of MMT (0.17%) and the composites as function of polycation loading (0–1.33 mmol/g) were calculated
Fig. 1. Adsorption isotherms of (a) P-Q100 and P-Q50 polycations (pH = 6) and (b) HPVPcS and P-Q20 polycations (pH = 3) by MMT (1.7 g/L), Concentrations of the polycation are expressed by average mmol monomer/g. Three characteristics adsorption points along the isotherm are demonstrated with arrows on P-Q100 isotherm.
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Fig. 2. (a) FTIR spectra of the polycations upon adsorption by MMT (b) Zeta potential of the composites as function of polycation loading, tested at pH 6.5–7. (c) Monomers (%) dangling as function of polycation loading. (d) Distribution between dangling vs. anchored segments of P-Q100 and P-Q50 polycation upon adsorption to MMT for composites at maximum adsorption.
from the mobility of the particles based on Smoluchowski equation using a Zetasizer Nanosystem (Malvern Instruments, Southborough, MA).
3.6.2. FTIR measurements FTIR spectra were obtained for MMT, polycations at pH 6 and composites (0.53–0.59 mmol/g). Pellets were prepared from dried polycation or composite mixed with KBr (2:98 ratio). FTIR spectra were recorded at room temperature in the range of 500–4000 cm1 using FTIR spectrometer (Nicolet Magna-IR-550, Madiso WI).
3.6.3. X-ray diffraction (XRD) measurements The basal (d 001) spacings of MMT and composites (0.53– 0.59 mmol/g) were measured by XRD before and after heat treatment (360°, 4 h). In addition, the basal spacing of the composites was measured upon adsorption of DCF and GMF on Q100 (86 and 2.95 mmol/g for DCF and GMF, respectively) and Q50 composites (49 and 6.4 mmol/g for DCF and GMF, respectively). Samples were prepared on a round glass slide, 1–2 mL of the suspension (0.17% MMT) were placed and left to sediment (oriented sample) for one day. The basal spacing was measured before and after heat treatment at 360 °C using an X-ray diffractometer (Philips PW1830/3710/3020) with Cu Ka radiation, k = 1.54 A°.
3.6.4. Elemental analysis (C, H, N, and Iodide) Determination of C, H and N was performed using the PerkinElmer 2400 series II Analyzer. A combustion method (950– 1000 °C) to convert the sample elements to simple gases was applied. The system uses a steady-state, wave front chromatographic approach, to separate the controlled gases which are detected as a function of thermal conductivity. Determination of iodide was done using the oxygen-flask combustion method (Schoniger application) for the decomposition of organic samples, and subsequent potentiometric titration by the 835 Titrando Metrohm Titroprocessor and by ion chromatography analysis using a Dionex IC system. The methylation level of polycations (P-Q20 - P-Q100) was calculated based on C/N molar ratio and was confirmed by I/ N ratio (small differences were observed). 3.6.5. XPS measurements The surface chemical analyses were obtained using XPS with a Kratos Axis Ultra instrument (Kratos Analytical Ltd., Manchester, UK). The XPS spectra were acquired using a monochromated Al Ka (1486.6 eV) X-ray source with 90° takeoff angle (normal to analyzer). The pressure in the analytical chamber was maintained at 2109 Torr. The high-resolution XPS spectra were obtained with pass energy 20 eV and step size 0.1 eV. Data analyses were performed using Vision processing software (Kratos Analytical Ltd) and CasaXPS (Casa Software Ltd). The peak fitting was done using
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Gaussian – Lorenzian (30%) ratio. The binding energies were calibrated using C 1s peak as 285.0 eV. The polycations adsorption and the distribution between pyridine and pyridinium were calculated by analyzing the N1s spectra of the samples. The peaks at 399–399.5 and 402–402.5 eV were attributed to un-charged pyridine and positively charged pyridinium monomers (HPY and 4MPY), respectively [19]. P-Q100 was considered as polycation with complete charged sites (based on element analysis and FTIR measurements) and the other samples were compared to its pyridiniun/pyridine ratio.
state). 2. Working in adequate concentration for efficient analysis by HPLC.
3.6.6. BET measurements The BET surface area was measured by NOVAÒ e-Series Surface Area Analyzers (Quantachrome Instruments). Composites were added to 9 mm tubes and temperature was elevated gradually (20 °C, 1 h in each step) from 20 to 120 °C under vacuum. Samples were weighted and reinserted to the instrument for adsorption and desorption measurements. The gas used was nitrogen and isotherms were obtained between relative pressures of 0–0.9. The analysis of BET was conducted by the International Organization for standardization (ISO) recommendation, the linearity range of adsorption (up to P/P0 = 0.3, depends on its linearity), in all cases C was confirmed to have positive value.
3.7.4. Calculation of the interaction between DCF/GMF and polycation segments using DFT method The interactions between DCF or GMF and 4MPY or PY (monomer and trimer) were calculated using B3LYP density functional theory (DFT) with 6-31G* basis set (Spartan’16 Wavefunction, Inc. Irvine, CA), all calculations were conducted in the presence of water as a solvent. A geometry optimization was applied to each of the components and for the complex; the extent of stabilization was calculated (Table S5) and the interactions between the components were assessed accordingly. For the complexes, at least three different starting orientations were applied according to the expected interactions, in order to reach the most stable orientation.
3.7. Pharmaceuticals adsorption by the composites
4. Results and discussion
3.7.1. Pharmaceuticals analysis DCF, GMF, CBZ and LMG were analyzed by HPLC (Agilent Technologies 1200 series) (Table 1). HPLC column was LiChroCARTR 250 (150 for LMG) 4 PurospherR STAR RP-18 (5 lm), the flow rate was 1.0 mL/min (0.9 mL/min for LMG).
The effect of polycation charge level on its conformation upon adsorption to the clay platelets was explored. PVPcS was charged (100, 50 and 20% denoted P-Q100, P-Q50 and P-Q20) by methylation of the pyridine (PY) monomers to form N-methyl-pyridinium (4MPY) using relevant stoichiometric concentrations of CH3I [47,52]. PVPcS can also be charged, under acidic conditions, by protonating the PY monomers (Pka of poly 4-vinylpyridine is in the range of 4–4.6 [53,54]) to form HPVPcS; in addition to the methylated polycations, the adsorption of HPVPcS to MMT was studied as a control. The charge levels of the polycations were evaluated and confirmed by FTIR, element analysis and XPS measurements (Fig. S1 a,b).
3.7.2. Pharmaceuticals adsorption by the composites and MMT Composites of three loadings were chosen for each polycation: Low (0.29–0.3 mmol/g), Intermediate (0.53–0.6 mmol/g) and High loading (which is close to the saturation level and is differ for each polycation- Table S3). The results are expressed as means ((3 repetitions), SD in all cases was in the range of ± 0–0.03). Upon composite preparation (see above), pharmaceuticals were added to the centrifuge tubes containing composite or MMT (1.67 g/L) sediments. The composites were re-suspended and the tubes were agitated for 24 h. Supernatants were separated by centrifugation (12000g for 10 min) and prior to HPLC analysis were filtered with PTFE syringe filter (AXIVA) 0.45 lm pore size. Except GMF (13–20% reduction), the filtration didn’t substantially affect the pharmaceutical concentration (less than 5% reduction in all cases). The calibration curve was built using filtered solution to achieve maximum precision. The experiments were performed at least in triplicate and samples were kept in darkness in order to avoid photo degradation (DCF). One-way ANOVA tests were used at the second point (Intermediate) to determine the statistical differences among groups (p < 0.05). The adsorption of CBZ (4.2 mM), LMG (3.9 mM), DCF (135 mM) and GMF (40 mM) on the composites and on MMT was tested. These concentrations of the pharmaceuticals were chosen based on two considerations: 1.Working under pharmaceutical saturated solution in all cases (40–87% un-adsorbed
3.7.3. Adsorption isotherms of DCF and GMF by Q100 and Q50 composites (Fig. 6) GMF (up to 48 mM) and DCF (up to 507 mM) were added to the composites (0.53 and 0.59 mmol/g for Q100 and Q50 composite, respectively) and equilibrated for 24 h. DCF and GMF concentrations in supernatants were measured by HPLC as mentioned above and their adsorption isotherms were constructed.
4.1. Composites fabrication and characterization The adsorption of the polycations by MMT was measured at equilibrium (Fig. 1). While the adsorption of P-Q100 and P-Q50 polycations was measured at their native pH (pH = 6), the solution pH of P-Q20 and HPVPcS was adjusted to 3 due to solubility limitations (Fig. S1c). In all cases adsorption reached a plateau with the maximum adsorption values decreasing with an increase in polycation charge level [30]; that can be explained by the tendency to compensate the surface charge on one hand and by the limitation of the steric repulsion between the polycation chains on the other hand [19,24]. The adsorption of P-Q100 was the lowest and did not even reach the cation exchange capacity (CEC) of MMT, (0.8 mmol/g). On the other hand, HPVPcS and P-Q20, with similar low charge levels, exhibited high adsorption (1.33–1.46 mmol/ g); while P-Q50 with intermediate charge level, exhibited moderate values of adsorption (0.86 mmol/g). In all cases, polycation des-
Table 1 HPLC conditions and limit of quantification for each pharmaceutical. Molecule
Detector
Mobile phase
Wavelength/kexcitation-kemission (nm)
Limit of quantification (LOQ) (mg/L)
DCF GMF CBZ LMG
diode-array Fluorescence diode-array diode-array
acetonitrile/acidic water (0.1% formic acid) 80/20 acetonitrile/acidic water (0.1% formic acid) 80/20 acetonitrile/ distilled water 50/50 P.buffer(pH 8, 0.025 M):ACN:MeOH = 70/16/14
276 220–300 286 210
0.05 0.025 0.05 0.05
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orption, measured by rinsing with distilled (neutral pH/pH 3) and salted (1 M NaCl) water, was negligible (less than 2.5%) (Table S1); indicating strong interactions between the oppositely charged polycation and MMT surface. To shed light on polycation conformation upon adsorption, the affinity of each of the monomer residues (PY, 4MPY and HPY) to the clay should be discussed. Styrene and PY monomers have low affinity to MMT surface [51], while pyridinium monomers (HPY and 4MPY) have higher affinity to the clay due to electrostatic attraction. In order to avoid the effects evolved from different pH, all composites, including the HPVPcS and Q20 composites, which were prepared at an acidic pH, were thoroughly rinsed with tap water, reaching pH 6.5–7. Upon adsorption, the presence of PY monomers is dominant in the case of PVPcS, as expressed by the FTIR peak at 1608 cm1 (Fig. 2a), it decreases gradually for P-Q20 and P-Q50 and obviously does not exist in the case of P-Q100, Indicating that both PY and HPY/4MPY (expressed at 1637 and 1643 cm1, respectively) monomers exist at the adsorbed state. Accordingly, the amount of ‘low affinity’ monomers (Table S2) can be calculated using the polycation loading (Fig. 1) and the degree of charged monomers at the adsorbed state which can be received by the XPS results (Fig. S2c). As the charge level of the polycation decreases (100– 20%) the concentration of the ‘low affinity’ monomers increases (0.06–0.81 mmol/g). Furthermore, since average distance between charged sites on MMT is about 11.5 A° [21] and the distance between two adjacent charged nitrogen (P-Q100) is about 9 A°, at least part of the charged monomers may also dangle into solution, suggesting higher amount of detached monomers [21,22,24]. Zeta potential of the composites will be affected by polycation charge density through both the loading and conformation on the clay. For all composites, zeta potential was less negative as polycation loading increased (Fig. 2b). In all cases the number of compensated sites was lower than the CEC (Table S2). However, while for Q20 and PVPcS composites, Zeta potential was slightly negative to neutral; for Q100 and Q50 composites, the values were positive due to the screening of the clay charge by the charge of the polycations [55]. Similar zeta values were obtained for Q100 and Q50 composites at their high loading, despite the lower charge compensation found for Q50. This discrepancy can be explained by the conformational differences (more looped for Q50) affecting the shear plan in which the Zeta values are measured [56,57]. The Q50 and Q100 composites represent a unique case since they have similar zeta values (at high loading) but different conformation. Hence, we precisely defined the anchored vs. dangling monomers in each case, considering also the charged monomers that are not directly interact with the clay by using the molar ratio between the adsorbed counter ion (I) and the adsorbed polycation (Fig. 2 c,d). The percentage of monomer dangling as function of polymer loading on the clay was obtained (Fig. 2c). The degree of monomers dangling to the solution was high and constant for all Q50 composites (70–75%). In contrast, Q100 composites exhibited an increase in the degree of dangling monomers, ranging from 30% of the monomers at low loadings to 55% at higher loadings. A deeper observation was taken into the composites at their highest loadings (Fig. 2d). Higher amount of 4MPY sites which directly interact with the clay were found for Q100 in comparison to Q50 composite (0.27 vs.0.19 mmol/g, respectively). Accordingly, the amount of overall monomers dangling into solution is 2.1 fold higher in the case of Q50 composite enabling more active adsorption sites. The internal distribution of the dangling monomers displays 37% more sites of 4MPY in the case of Q100 composite, while the amount of the uncharged sites (styrene and PY) in Q50 composite is 8 fold higher. An even higher degree of dangling monomers is expected
in the case of PVPcS and Q20 composites but could not be determined due to the low pH which excluded the iodide. These conformation differences also affect the BET surface area of the composites which represent merely the external surfaces. The BET of the composites was higher, 31.6 ± 2.6 and 38.6 ± 1.1 m2/g for Q100 and Q50, respectively, than the BET of MMT (22.7 m2/g) [50]. The contribution of the organic segments to the surface area is known to be low [58–60], hence the increased BET surface area for Q50 composite, can be attributed to the exposure of more clay surfaces due to loosely associated polymer conformation. Indeed, C values, associated to the energy of monolayer adsorption, were higher in the case of Q50 in comparison to Q100 (69 vs. 57), indicating that more clay surfaces are unveiled [59]. The conformation was also expressed in the interlayer space of the composites. In all cases the polycations intercalated between the clay platelets as was demonstrated by X-ray diffraction measurements (Fig. S3a,b). However, upon heating [61], higher dspace was observed for Q50, Q20 and PVPcS composites in comparison to Q100 (1.47 ± 0.02 vs. 1.34 nm, respectively), suggesting a more extended conformation of the low charged density polymers also in the interlayer space. To conclude, the degree of polycations adsorption was negatively correlated to their charge density and in all cases polycation release was negligible. Based on iodide exchange, FTIR, XPS, zeta potential, XRD and BET, the conformation of polycations and composites structure can be depicted. For high charge density polycation a more flatted conformation, described as ‘trains’, was displayed. Alternatively, for low charge density polycation, a conformation with more ‘loops and tails’ was suggested. The polycations also affect the MMT structure resulting in higher inter-layer space for low charge density polycations. The effect of polycation conformation and composite structure on their ability to adsorb pharmaceuticals was explored in depth and is discussed below. As polycation charge density decreases, its conformation shifts towards ‘loops and tails’ and the composite surface is more hydrophobic (less charged monomers). Hence, the adsorption of non-ionic molecules by such composites should increase due to both interaction and conformation considerations (Fig. 3). To test this hypothesis, the adsorption of two non-ionic molecules, carbamazepine (CBZ) and lamotrigine (LMG), by the composites was tested (The chemical structures of the pharmaceuticals appear in Fig. 4). In order to distinguish between these two parameters, the interactions between the composites and anionic molecules were tested. In this case, the anionic molecules would preferably interact with the high charge density polycation due to electrostatic interactions. However, the ‘loops and tails’ conformation may also promote adsorption, enabling more adsorption sites with more varied polymer-pharmaceutical orientation (Fig. 3). In order to test the magnitude of electrostatic interactions vs. ‘loops and tails’ conformation, the adsorption of two negatively charged molecules, diclofenac (DCF) and gemfibrozil (GMF) by the composites, was tested. The adsorption of the pharmaceuticals was studied at three characteristics loadings along the polycation adsorption isotherms (Table S3) denoted as: Low - complete adsorption for all polycations (0.29–0.3 mmol/g), Intermediate 0.53–0.6 mmol/g, High – close to the saturation level of the polycation. The adsorbed amounts (lmol/g) of the pharmaceuticals by the composites are within a range of two orders of magnitude (Fig. 5). The adsorption of LMG and CBZ, the nonionic molecules, to the composites, was relatively low and similar, 0.1–0.3 and 0.26–0.6 mmol/g, respectively, due to merely week non-ionic, hydrophobic interactions. In contrast, the adsorption of the anionic molecules, GMF and DCF, was mainly electrostatic and therefore higher, 0.46–6.33 and 6.1–49.6 mmol/g, respectively, with a magni-
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Fig. 3. The expected contribution of both conformation and chemical interactions to the adsorption of non-ionic and anionic molecules by composites of polycations with diverse charge densities.
Fig. 4. Molecular structures of pharmaceuticals: diclofenac (DCF), gemfibrozil (GMF), lamotrigine (LMG) and carbamazepine (CBZ). Physical-chemical properties of the molecules are specified in Table S4.
tude higher in the case of DCF. The substantial higher adsorption of DCF is addressed below. As expected, for both non-ionic molecules, CBZ and LMG, a clear trend of preferable adsorption to the low charge density composites was observed (PVPcS > Q20 > Q50 > Q100 > MMT) (Fig. 5a,b) due to both an increase in hydrophobicity and a more dangling conformation. The adsorption of CBZ and LMG did not dramatically change as a function of polycation loading; indicating limited adsorption sites of these non-ionic molecules on the composites. The anionic molecules, DCF and GMF, exhibited an opposite trend (Fig. 5c, d). The adsorption of DCF and GMF correlates to the zeta potential of the sorbents: MMT < PVPcS < Q20 < Q50, Q100. The adsorption of DCF to Q100 composite was substantially higher (also when expressed as (mmol/mmol) (Fig. S5)) than to Q50, while, surprisingly the adsorption of GMF was higher in the case of Q50. To further explore the opposite adsorption trends of DCF and GMF, their adsorption isotherms to Q50 and Q100 composites (0.53–0.59 mmol/g), were studied (Fig. 6). Upon adding low con-
centrations of DCF or GMF there was no difference in their adsorption by Q50 and Q100 composites. However, at high pharmaceutical concentrations, the differences are clear; higher capacity of DCF to Q100 composite vs. higher capacity of GMF to Q50 composite. In order to further elucidate the differences between DCF and GMF adsorption; both aspects, magnitude of adsorption (50– 85 lmol/g vs. 3–6.4 lmol/g, respectively) and higher adsorption towards Q100 or Q50 composite, respectively, were evaluated. Accordingly, the different sites on the composites were studied and basic DFT calculations, to assess the interaction between the anionic molecules and the polycation’s monomers, were performed. The capacity of DCF (to all composites) may be attributed to its ability to access not only the external surfaces but also to access sites within the clay platelets, enabling up to 20 fold more adsorption sites [62]. In order to evaluate the ability of the pharmaceuticals to intercalate, the basal spacing of the composites, before and after adsorption of GMF and DCF (at max. pharmaceutical adsorption) was measured by XRD (Fig. 7). In both cases the basal spacing was 1.54 ± 0.01 nm, and upon pharmaceutical adsorption no change of the peak position was observed except of additional shoulder in the case of DCF adsorbed by Q50 composite (1.27 nm). However, upon DCF adsorption the diffraction peaks of both composites were lower and wider, in comparison to the bare composites, and in comparison to the diffraction of GMF adsorbed to the composites. Wider and lower intensity peaks, are attributed to a transition from a crystalline structure to more amorphic one [63,64], which can be more accurately determined by measuring the peak width at its half height (Fig. 7, insert), [65]. A more amorphic structure may be induced by DCF intercalation in between the clay platelets [63,64]. The ability of DCF vs. GMF to intercalate in the composite may be explained by the correlation with the size of the molecules (263.4 and 277.9 A3, respectively) and by the lower aspect ratio of GMF. The suggested intercalation of DCF may explain its higher adsorption to all composites. However, it does not explain the higher adsorption of DCF to Q100 vs. Q50 which is in contrast to the higher adsorption of GMF to Q50 vs. Q100. Basically, the
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Fig. 5. (a) CBZ (4.2 mM) (b) LMG (3.9 mM) (c) DCF (135 mM) (d) GMF (40 mM) adsorption on MMT and on the composites (PVPcS, Q20-Q100) at three regions of the adsorption isotherms (see Fig. 1).
Fig. 6. (a) GMF (up to 48 mM) and (b) DCF (up to 507 mM) adsorption isotherms on Q50 and Q100 composites (0.53–0.59 mmo/g).
4MPY sites are expected to promote interactions with both anionic molecules i.e., higher adsorption to Q100 is expected. However, GMF adsorption to Q50 is higher suggesting that may be, unexpectedly, its interactions with PY are preferable. In order to assess the interactions between the adsorbates (DCF and GMF) and adsorbents (4MPY and PY) quantum chemical calcu-
Fig. 7. The XRD spectra of: (a) Q100 composites (0.53 mmol/g), b. Q50 composites (0.59 mmol/g). In each graph the effect of maximum adsorbed amount of DCF and GMF is demonstrated (see Fig. 6). The width at half height of the main peaks is specified in the inserted table.
lations were conducted (Fig. 8 and Table S5). The calculations were based on DFT method [66], (Spartan’ 16 Wavefunction, Inc. Irvine,
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that polycation conformation may affect its ability to interact with molecules. Séquaris et al. [40], in one of the few studies in this context, correlated the adsorption conformation of non-ionic polymer to composite ability to interact with anionic surfactant. At low loadings the adsorption of sodium dodecyl sulfate was low which were correlated to a ‘trains’ conformation and proximity to the anion exclusion volume. At higher loadings the adsorption substantially increased along with a more extended conformation of the polymer. All the more so, in the current study the adsorption trend doesn’t match the fact that zeta potential of Q50 (20 mV) is significantly lower than that of Q100 (39 mv) at the studied loading (0.53–0.59 mmol/g); meaning that the effect of charge, as expressed by zeta potential, plays a role but is not the exclusive factor governing the adsorption of anionic molecules. Based on the conformation proposed and on the expected interaction calculated; we suggest that the adsorption of , may be restricted by the trains conformation of Q100 which limits its ability to interact with GMF in a suitable orientation. On the other hand, more loosely associated polycation monomers, as was described in the case of Q50; may enable more versatile binding options; thus, increasing the adsorption. The reason that GMF is more affected by the conformation can be explained by its accessibility mostly to the external surfaces in which the conformational effect is more dominant or by a more specific orientating needed in order to achieve a stable interaction, both options needed to be further investigated. 5. Conclusions
Fig. 8. Geometry optimization by DFT calculation of the complexes DCF/GMF4MPY/PY in water. (a) summary of the product-reactants energy differences, (for detailed values, see Table S5). (b–e) DCF (b,d) or GMF (c,e) most stable orientation received upon interaction with 4MPY (b,c) or PY (d,e).
CA), and included geometry optimization and estimation of energy stabilization considering the effect of water. The differences in stabilization energy- DE, which indicate the strength of the chemical interaction, was substantially higher for both DCF and GMF interactions with 4MPY (monomer and trimer) in comparison to their stabilization by PY (Fig. 8a). The contribution of p-p interactions or charge transfer between 4MPY and DCF or GMF was refuted due to too high energy gap between 4MPY LUMO and DCF GMF HOMO. Alternatively, the final orientations indicate that H-bonds, which need specific orientation, play a major role (Fig. 8b). Based on that, it can be concluded that higher adsorption of GMF by Q50 composite can’t be explained by a preferable interaction between GMF and PY sites. The DE received from GMF interactions with 4MPY (monomer and trimer) was higher (more negative) in comparison to DE of DCF interactions with 4MPY for their most stable orientation (Fig. 8a). The stabilization of GMF was higher also for PY; however, the difference DE4MPY- DEPY calculated for GMF is still more negative in comparison to DCF, indicating the dominancy of the ionic interaction between 4MPY and GMF. Higher affinity of GMF to both Q50 and Q100, in comparison to the affinity of DCF, is also expressed by its higher Langmuir affinity values (KL) calculated from the adsorption isotherm (Table S6). Although the interaction of GMF with both monomers are higher than the interactions of DCF, the capacity of the composites towards DCF is higher, strengthening the assumption that DCF is also exposed to internal adsorption sites. Yet to be explained is the preferable adsorption of GMF by Q50 composite (in comparison to its adsorption by Q100). We suggest
The effect of composite structure and polycation conformation on its ability to adsorb non-ionic and anionic pharmaceuticals was studied. The non-ionic molecules, CBZ and LMG, exhibited preferable adsorption to low charge density composites which was explained by both interaction and conformation considerations. The anionic molecules, DCF and GMF, displayed substantial differences in terms of capacity along with preferable adsorption to different sorbent (Q100 and Q50 composite, respectively). The higher capacity for DCF was attributed to its ability to penetrate into the interlayer space in counterpart with the polycation. Both molecules were expected to preferably adsorb to Q100 composite as was demonstrated by DFT calculations; however, GMF displayed higher adsorption to Q50 composite. Based on polycation conformation and the expected interaction, we suggest that the confined polycation conformation of Q100 (‘trains’) may limit the adsorption of GMF while dangling polycation conformation of Q50 composite, enables more adsorption sites and diverse monomer-GMF orientations resulting in more efficient adsorption of GMF. Acknowledgments This research was supported by the Israeli ministry of science, technology and space (grant 821-0142-06). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.079. References [1] M. Rosoff, Nano-Surface Chemistry, Rosoff Marcel DekkerNew York 2001, 2002, pp. 213–242. 10.1201/9780203908488. [2] P.A. Williams, Handbook of Industrial Water Soluble Polymers, Blackwell Pub, 2007. 10.1017/CBO9781107415324.004. [3] P. Meneghetti, S. Qutubuddin, Synthesis, thermal properties and applications of polymer-clay nanocomposites, Thermochim. Acta. 442 (2006) 74–77, https://doi.org/10.1016/j.tca.2006.01.017.
526
H. Kohay et al. / Journal of Colloid and Interface Science 552 (2019) 517–527
[4] J.A. Prince, G. Singh, D. Rana, T. Matsuura, V. Anbharasi, T.S. Shanmugasundaram, Preparation and characterization of highly hydrophobic poly(vinylidene fluoride) - clay nanocomposite nanofiber membranes (PVDFclay NNMs) for desalination using direct contact membrane distillation, J. Memb. Sci. 397–398 (2012) 80–86, https://doi.org/10.1016/j. memsci.2012.01.012. [5] C. Breen, The characterisation and use of polycation-exchanged bentonites, Appl. Clay Sci. 15 (1999) 187–219, https://doi.org/10.1016/S0169-1317(99) 00024-1. [6] F.T. Hesselink, On the theory of polyelectrolyte adsorption, J. Colloid Interface Sci. 60 (1977) 448–466, https://doi.org/10.1016/0021-9797(77) 90309-5. [7] L. Eriksson, B. Alm, P. Stenius, Formation and structure of polystyrene latex aggregates obtained by flocculation with cationic polyelectrolytes, Colloids Surf. A Physicochem. Eng. Asp. 70 (1993) 47–60, https://doi.org/10.1016/09277757(93)80495-Z. [8] M.A.G. Dahlgren, A. Waltermo, E. Blomberg, P.M. Claesson, L. Sjoestroem, T. Aakesson, B. Joensson, Salt effects on the interaction between adsorbed cationic polyelectrolyte layers: theory and experiment, J. Phys. Chem. 97 (1993) 11769–11775, https://doi.org/10.1021/j100147a033. [9] W. Barford, R.C. Ball, C.M.M. Nex, A non-equilibrium configuration theory of polyelectrolyte adsorption, J. Chem. Soc., Faraday Trans. 1 82 (10) (1986) 3233, https://doi.org/10.1039/f19868203233. [10] W. Barford, R.C. Ball, Towards a complete configurational theory of nonequilibrium polymer adsorption, J. Chem. Soc., Faraday Trans. 1 83 (8) (1987) 2515, https://doi.org/10.1039/f19878302515. [11] G. Durand-Piana, F. Lafuma, R. Audebert, Flocculation and adsorption properties of cationic polyelectrolytes toward Na-montmorillonite dilute suspensions, J. Colloid Interface Sci. 119 (1987) 474–480, https://doi.org/ 10.1016/0021-9797(87)90293-1. [12] R.I.S. Gill, T.M. Herrington, The effect of surface charge on the flocculation of Kaolin suspensions with cationic polyacrylamides of varying molar mass but similar cationic character, Colloids Surf. 25 (1987) 297–310, https://doi.org/ 10.1016/0166-6622(87)80310-4. [13] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some watersoluble cationic copolymers toward silica suspensions: a semiquantitative interpretation of the role of molecular weight and cationicity through a ‘‘patchwork” model, J. Colloid Interface Sci. 97 (1984) 120–136, https://doi. org/10.1016/0021-9797(84)90280-7. [14] J. Meadows, P.A. Williams, M.J. Garvey, R.A. Harrop, G.O. Phillips, Enhanced polyelectrolyte adsorption, Colloids Surf. 32 (1988) 275–288, https://doi.org/ 10.1016/0166-6622(88)80022-2. [15] H. Tanaka, L. Ödberg, L. Wågberg, T. Lindström, Adsorption of cationic polyacrylamides onto monodisperse polystyrene latices and cellulose fiber: effect of molecular weight and charge density of cationic polyacrylamides, J. Colloid Interface Sci. 134 (1990) 219–228, https://doi.org/10.1016/0021-9797 (90)90269-T. [16] T. Ge, M. Rubinstein, Strong selective adsorption of polymers, Macromolecules 48 (2015) 3788–3801, https://doi.org/10.1021/acs.macromol.5b00586. [17] L. Zhu, R. Zhu, L. Xu, X. Ruan, Influence of clay charge densities and surfactant loading amount on the microstructure of CTMA–montmorillonite hybrids, Colloids Surf. A Physicochem. Eng. Asp. 304 (2007) 41–48, https://doi.org/ 10.1016/j.colsurfa.2007.04.019. [18] S.A. Sukhishvili, S. Granick, Polyelectrolyte adsorption onto an initially-bare solid surface of opposite electrical charge, J. Chem. Phys. 109 (1998) 6861, https://doi.org/10.1063/1.477253. [19] O.J. Rojas, M. Ernstsson, R.D. Neuman, P.M. Claesson, Effect of polyelectrolyte charge density on the adsorption and desorption behavior on Mica, Langmuir 18 (2002) 1604–1612, https://doi.org/10.1021/la0155698. [20] G.M. Lindquist, R.A. Stratton, The role of polyelectrolyte charge density and molecular weight on the adsorption and flocculation of colloidal silica with polyethylenimine, J. Colloid Interface Sci. 55 (1976) 45–59, https://doi.org/ 10.1016/0021-9797(76)90007-2. [21] R. Denoyel, G. Durand, F. Lafuma, R. Audebert, Adsorption of cationic polyelectrolytes onto montmorillonite and silica: microcalorimetric study of their conformation, J. Colloid Interface Sci. 139 (1990) 281–290, https://doi. org/10.1016/0021-9797(90)90463-X. [22] M.A.C. Dahlgren, P.M. Claesson, R. Audebert, Interaction and adsorption of polyelectrolytes on mica, Nord. Pulp Pap. Res. J. 8 (2007) 62–67, https://doi. org/10.3183/npprj-1993-08-01-p062-067. [23] P.M. Claesson, E. Poptoshev, E. Blomberg, A. Dedinaite, Polyelectrolytemediated surface interactions, Adv. Colloid Interface Sci. 114–115 (2005) 173–187, https://doi.org/10.1016/J.CIS.2004.09.008. [24] K. Lowack, C.A. Helm, Molecular mechanisms controlling the self-assembly process of polyelectrolyte multilayers, Macromolecules 31 (1998) 823–833, https://doi.org/10.1021/ma9614454. [25] J. Gregory, S. Barany, Adsorption and flocculation by polymers and polymer mixtures, Adv. Colloid Interface Sci. 169 (2011) 1–12, https://doi.org/10.1016/ j.cis.2011.06.004. [26] M.R. Bohmer, O.A. Evers, J.M.H.M. Scheutjens, Weak polyelectrolytes between two surfaces: adsorption and stabilization, Macromolecules 23 (1990) 2288– 2301, https://doi.org/10.1021/ma00210a027. [27] T.K. Wang, R. Audebert, Adsorption of cationic copolymers of acrylamide at the silica—water interface: hydrodynamic layer thickness measurements, J. Colloid Interface Sci. 121 (1988) 32–41, https://doi.org/10.1016/0021-9797 (88)90405-5.
[28] Y.R., S. Minko, AFM Single Molecule Experiments at the SolidLiquid Interface: In Situ Conformation of Adsorbed Flexible Polyelectrolyte Chains, 2005. 10.1021/JA0558239. [29] L.J. Kirwan, G. Papastavrou, M. Borkovec, S.H. Behrens, Imaging the coil-toglobule conformational transition of a weak polyelectrolyte by tuning the polyelectrolyte charge density, Nano Lett. 4 (2004) 149–152, https://doi.org/ 10.1021/nl034912l. [30] P. Linse, Adsorption of weakly charged polyelectrolytes at oppositely charged surfaces, Macromolecules 29 (1996) 326–336, https://doi.org/ 10.1021/ma950539v. [31] J. Gregory, Polymer adsorption and flocculation in sheared suspensions, Colloids Surf. 31 (1988) 231–253, https://doi.org/10.1016/0166-6622(88) 80196-3. [32] E.G.M. Pelssers, M.A.C. Stuart, G.J. Fleer, Kinetic aspects of polymer bridging: equilibrium flocculation and nonequilibrium flocculation, Colloids Surf. 38 (1989) 15–25, https://doi.org/10.1016/0166-6622(89)80139-8. [33] F. Lafuma, T.K. Wang, G. Durand, R. Audebert, Adsorption and flocculation behavior for systems of copolymers and particles of opposite charge, Colloids Surf. 31 (1988) 255–257, https://doi.org/10.1016/0166-6622(88)80197-5. [34] T. Cosgrove, T. Obey, B. Vincent, The configuration of sodium poly(styrene sulfonate) at polystyrene/solution interfaces, J. Colloid Interface Sci. 111 (1986) 409–418, https://doi.org/10.1016/0021-9797(86)90044-5. [35] J. Meadows, P. Williams, M. Garvey, R. Harrop, G. Phillips, Characterization of the adsorption—desorption behavior of hydrolyzed polyacrylamide, J. Colloid Interface Sci. 132 (1989) 319–328, https://doi.org/10.1016/0021-9797(89) 90247-6. [36] S. Wang, Z. Jia, X. Zhou, D. Zhou, M. Chen, D. Xie, Y. Luo, D. Jia, Preparation of a biodegradable poly(vinyl alcohol)-starch composite film and its application in pesticide controlled release, J. Appl. Polym. Sci. 134 (2017) 45051, https://doi. org/10.1002/app.45051. [37] P. Liu, Polymer modified clay minerals: a review, Appl. Clay Sci. 38 (2007) 64– 76, https://doi.org/10.1016/j.clay.2007.01.004. [38] C. Viseras, C. Aguzzi, P. Cerezo, M.C. Bedmar, Biopolymer–clay nanocomposites for controlled drug delivery, Mater. Sci. Technol. 24 (2013) 1020–1026, https:// doi.org/10.1179/174328408X341708. [39] E.I. Unuabonah, A. Taubert, Clay-polymer nanocomposites (CPNs): adsorbents of the future for water treatment, Appl. Clay Sci. 99 (2014) 83–92, https://doi. org/10.1016/j.clay.2014.06.016. [40] J.-M. Séquaris, A. Hild, H.D. Narres, M.J. Schwuger, Polyvinylpyrrolidone adsorption on Na-Montmorillonite. Effect of the polymer interfacial conformation on the colloidal behavior and binding of chemicals, J. Colloid Interface Sci. 230 (2000) 73–83, https://doi.org/10.1006/jcis.2000.7046. [41] G.J. Churchman, Formation of complexes between bentonite and different cationic polyelectrolytes and their use as sorbents for non-ionic and anionic pollutants, Appl. Clay Sci. 21 (2002) 177–189, https://doi.org/10.1016/S01691317(01)00099-0. [42] Seiji Tsuzuki, A. Masuhiro Mikami, S. Yamada, Origin of Attraction, Magnitude, and Directionality of Interactions in Benzene Complexes with Pyridinium Cations, 2007, 10.1021/JA071372B. [43] N. Paxéus, Removal of selected non-steroidal anti-inflammatory drugs (NSAIDs), gemfibrozil, carbamazepine, beta-blockers, trimethoprim and triclosan in conventional wastewater treatment plants in five EU countries and their discharge to the aquatic environment, Water Sci. Technol. 50 (2004) 253–260. [44] Y. Zhang, S.-U. Geißen, C. Gal, Carbamazepine and diclofenac: removal in wastewater treatment plants and occurrence in water bodies, Chemosphere 73 (2008) 1151–1161. [45] C. Tixier, H.P. Singer, S. Oellers, S.R. Muller, Occurrence and fate of Carbamazepine, Clofibric Acid, Diclofenac, Ibuprofen, Ketoprofen, and Naproxen in surface waters, Environ. Sci. Technol. 37 (2003) 1061–1068. [46] R. Gurke, M. Rößler, C. Marx, S. Diamond, S. Schubert, R. Oertel, J. Fauler, Occurrence and removal of frequently prescribed pharmaceuticals and corresponding metabolites in wastewater of a sewage treatment plant, Sci. Total Environ. 532 (2015) 762–770, https://doi.org/10.1016/j. scitotenv.2015.06.067. [47] K. Ishizu, Y. Kashi, T. Fukutomi, T. Kakurai, Synthesis of AB and BAB poly (styrene-b-4-vinylpyridine) and solution properties of their quaternized compounds, Die Makromol. Chemie. 183 (1982) 3099–3107, https://doi.org/ 10.1002/macp.1982.021831222. [48] Poly(styrene-b- N-methyl 4-vinyl pyridine iodide) Procedure, (n.d.). (accessed January 28, 2015). [49] E.M. Kosower, Additions to Pyridinium Rings. I. 1-Methylpyridinium Iodide, J. Am. Chem. Soc. 77 (1955) 3883–3885, https://doi.org/10.1021/ja01619a060. [50] E.B. Hughes, H.H.G. Jellinek, B.A. Ambrose, Pyridine. Ultraviolet absorption spectrum and dissociation constant, J. Phys. Chem. 53 (1948) 410–414, https:// doi.org/10.1021/j150468a009. [51] R.A. Baker, M.-D. Luh, Pyridine sorption from aqueous solution by montmorillonite and kaolinite, Water Res. 5 (1971) 839–848, https://doi.org/ 10.1016/0043-1354(71)90020-0. [52] H. Kohay, A. Izbitski, Y.G. Mishael, Developing polycation-clay sorbents for efficient filtration of Diclofenac: effect of dissolved organic matter and comparison to activated carbon, Environ. Sci. Technol. 49 (2015) 9280–9288, https://doi.org/10.1021/acs.est.5b01530. [53] M. Satoh, E. Yoda, T. Hayashi, J. Komiyama, Potentiometric titration of poly(vinylpyridines) and hydrophobic interaction in the counterion
H. Kohay et al. / Journal of Colloid and Interface Science 552 (2019) 517–527
[54]
[55]
[56]
[57]
[58]
[59]
binding, Macromolecules 22 (1989) 1808–1812, https://doi.org/10.1021/ ma00194a051. L. Franck-Lacaze, P. Sistat, P. Huguet, Determination of the pKa of poly (4vinylpyridine)-based weak anion exchange membranes for the investigation of the side proton leakage, J. Memb. Sci. 326 (2009) 650–658, https://doi.org/ 10.1016/J.MEMSCI.2008.10.054. D. Zadaka, A. Radian, Y.G. Mishael, Applying zeta potential measurements to characterize the adsorption on montmorillonite of organic cations as monomers, micelles, or polymers, J. Colloid Interface Sci. 352 (2010) 171– 177, https://doi.org/10.1016/j.jcis.2010.08.010. H. Bouhamed, S. Boufi, A. Magnin, Alumina interaction with AMPS–MPEG random copolymers, J. Colloid Interface Sci. 261 (2003) 264–272, https://doi. org/10.1016/S0021-9797(03)00100-0. Q. Ran, P. Somasundaran, C. Miao, J. Liu, S. Wu, J. Shen, Effect of the length of the side chains of comb-like copolymer dispersants on dispersion and rheological properties of concentrated cement suspensions, J. Colloid Interface Sci. 336 (2009) 624–633, https://doi.org/10.1016/j.jcis.2009.04.057. K. Boki, N. Kawasaki, K. Minami, H. Takahashi, Structural analysis of collagen fibers by nitrogen adsorption method, J. Colloid Interface Sci. 157 (1993) 55– 59, https://doi.org/10.1006/jcis.1993.1157. L.M. Mayer, Extent of coverage of mineral surfaces by organic matter in marine sediments, Geochim. Cosmochim. Acta 63 (1999) 207–215, https://doi.org/ 10.1016/S0016-7037(99)00028-9.
527
[60] W.K. Lowen, E.C. Broge, Effects of dehydration and chemisorbed materials on the surface properties of amorphous silica, J. Phys. Chem. 65 (1961) 16–19, https://doi.org/10.1021/j100819a006. [61] E. Abramova, I. Lapides, S. Yariv, Thermo-XRD investigation of monoionic montmorillonites mechanochemically treated with urea, J. Therm. Anal. Calorim. 90 (2007) 99–106, https://doi.org/10.1007/s10973-007-8482-0. [62] S. Diamond, Surface areas of clay minerals as derived from measurements of glycerol retention, Clays Clay Miner. 5 (1956) 334–347, https://doi.org/ 10.1346/CCMN.1956.0050128. [63] R.A. Vaia, K.D. Jandt, E.J. Kramer, E.P. Giannelis, Microstructural evolution of melt intercalated polymerorganically modified layered silicates nanocomposites, Chem. Mater. 8 (1996) 2628–2635, https://doi.org/10.1021/ cm960102h. [64] K.G. Bhattacharyya, S. Sen Gupta, Kaolinite, montmorillonite, and their modified derivatives as adsorbents for removal of Cu(II) from aqueous solution, Sep. Purif. Technol. 50 (2006) 388–397, https://doi.org/10.1016/j. seppur.2005.12.014. [65] R.L. Frost, L. Rintoul, Lattice vibrations of montmorillonite: an FT Raman and Xray diffraction study, Appl. Clay Sci. 11 (1996) 171–183, https://doi.org/ 10.1016/S0169-1317(96)00017-8. [66] M. Orio, D.A. Pantazis, F. Neese, Density functional theory, Photosynth. Res. 102 (2009) 443–453, https://doi.org/10.1007/s11120-009-9404-8.
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Regular Article
Effect of polycation charge density on polymer conformation at the clay surface and consequently on pharmaceutical binding Hagay Kohay a, Itzhak I. Bilkis b, Yael G. Mishael a,⇑ a b
Department of Soil and Water Science, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel Institute of Biochemistry, Food Science and Nutrition, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 24 April 2019 Revised 22 May 2019 Accepted 24 May 2019 Available online 25 May 2019 Keywords: Adsorption Polycation Charge density Clay Montmorillonite Composite Conformation Structure Pharmaceuticals
a b s t r a c t Polycation conformation upon adsorption to a negatively charged surface can be modulated by its charge density. At high charge density monomers directly interact with the surface in a ‘trains’ conformation and as charge density decreases a high degree of monomers dangle into solution in a ‘loops and tails’ conformation. In this study, the conformations of polycations upon adsorption to montmorillonite, as a function of polycation charge (20, 50 and 100% of the monomers, denoted as P-Q20, P-Q50 and P-Q100) were characterized and in accordance with their conformation, the adsorption of non-ionic and anionic molecules by the composite was tested. As expected, the adsorption of the nonionic pharmaceuticals increased to a composite with a ‘loops and tails’ conformation, due to both conformation and chemical properties. On the other hand, the anionic molecules, gemfibrozil and diclofenac, preferably adsorbed to composites with higher charge density (Q50 or Q100 composites). However, they showed different tendency toward the composites, i.e. higher adsorption of diclofenac by Q100 composite vs. higher adsorption of gemfibrozil by Q50 composite. To elucidate the differences in adsorption between these two pharmaceuticals, density functional theory calculations were employed. Both gemfibrozil and diclofenac were found to be better stabilized by methyl pyridinium sites (prevail in Q100 composite). The preferable adsorption of gemfibrozil by Q50 composite was explained by the presence of ‘loops and tails’ conformation enabling additional adsorption sites and diverse monomer-target molecule orientations. Ó 2019 Published by Elsevier Inc.
Abbreviations: MMT, Montmorillonite; PVPcS, poly-4-vinylpyridine-co-styrene (styrene/pyridine ratio 1/10); HPVPcS, protonated PVPcS; P-Q100/Q50/Q20, PVPcS which was methylated by methyl iodide in 100, 50 or 20% of its pyridine sites, respectively.; Q100/50/20, composites which are based on MMT and P-Q100/Q50/Q20, respectively; PY, pyridine monomer; HPY, pyridinium (protonated PY); 4MPY, N-methyl pyridinium (methylated PY); GMF, Gemfibrozil; DCF, Diclofenac; CBZ, Carbamazepine; LMG, Lamotrigine. ⇑ Corresponding author at: Dept. Soil and Water Sci., The Robert H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem, Rehovot 76100, Israel. E-mail address: [email protected] (Y.G. Mishael). https://doi.org/10.1016/j.jcis.2019.05.079 0021-9797/Ó 2019 Published by Elsevier Inc.
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1. Introduction Polymer characteristics at the solid-liquid interface plays an important role in many applications such as water treatment, drug delivery systems (biocompatible surfaces), battery technology, membrane distillation and oil extraction [1–4]. The adsorption of polyelectrolytes to oppositely charged surfaces and the functionalities of such materials has been thoroughly described [5–15]. One of the aspects explored is polyelectrolyte conformation on the surface. Polyelectrolyte conformation is dominated by the degree of monomers directly interacting with the surface; a high degree of interaction resulting in a ‘trains’ conformation whereas a low degree of interaction resulting in polymer segments which are not only interacting with the surface, but also dangling into solution as ‘loops and tails’. The adsorption of a copolymer with high affinity (sticky monomers) and low affinity monomers was described theoretically [16] and experimentally [15,17–23]. In contrast to the theoretical studies which assume that the system reaches equilibrium, experimental studies suggest that the system is stocked in a non-equilibrium state [7,21,24,25] in which additional factors such as the initial conformation received and adsorption kinetics may affect the conformation. Among other parameters, the effect of polymer charge density, surface charge density, polymer concentration and solution composition, on polymer conformation at the adsorbed state, were addressed experimentally [15,17–22]. The effect of the charge density of the polymer on adsorption is expressed in terms of conformation and loading, which is governed by the surface charge compensation. Accordingly, high charge density polymers form relatively flat conformations with low loading, while low charge density polymers tend to form ‘loops and tails’ conformations along with higher loading [7,8,11,19,26,27]. These conformational changes of the polycation were visualized by AFM on mica surface. Minko et al. [28], and Borkovec et al. [29], displayed the transition from more extended to more globular conformation as charge density decreased (controlled by pH). These states (‘trains’ vs. ‘loops and tails’) are not absolute and even at high charge density, polycation may have monomers ‘‘dangling” into solution as can be explained by entropic considerations [22,24]. In addition to charge density, polyelectrolyte concentrations and high Ionic strength of the solution, are additional factors that promote a ‘loops and tails’ conformation [9,10,20,25,30–35]. Recently, polymer-clay composites have been developed to bind a third component for various applications [36–39]. Despite the extensive literature on polymer adsorption to surfaces and the ongoing interest in the applicability of the composites, only a few of the studies addressed the polymer conformation and the consequent binding affinity and capacity of the composites. Séquaris et al. [40], reported that as the added concentration of non-ionic polymer increased, the conformation was inclined to be more ‘loops and tails’. The effect of polymer conformation on the adsorption of anionic surfactant and non-ionic molecule was tested. While the adsorption capacity of the anionic surfactant substantially increased in the presence of ‘loops and tails’, the adsorption of the non-ionic molecule was less affected by this conformation. In the case of conformational changes which governed by the charge density of the polymer, the overall effect will also be determined by the ability of each monomer to interact with the target molecule [41]. In order to correlate between conformation and functionality, the chemical interactions between the monomers and the target molecule, need to be defined. Tsuzuki et al. [42], tested the interaction between a benzene ring and pyridine (PY), pirydinium (HPY) and N-methyl pyridinium (4MPY) monomers by ab-initio calculations. They demonstrated that higher energy stabilization was
obtained through interactions with the charged monomers and that the extent of stabilization was dependent on monomerbenzene orientation. The polycation conformation on the clay can affect the adsorption either by exposing more adsorption sites or by enabling better access to adsorption sites. While the ability of polymer in a ‘trains’ conformation to interact with other molecules is restricted, a polymer in a ‘loops and tails’ conformation has more degrees of freedom enabling varied orientations between the monomers and target molecules. The overall adsorption will be determined by the interplay between the interactions in accordance to monomer chemistry in addition to conformation considerations. In the current study we aimed to test the effect of polymer conformation upon adsorption, ‘trains’ vs. ‘loops and tails’, on its ability to adsorb target molecules with diverse chemical properties. Accordingly, the pyridine (PY) monomers of poly-4-vinylpyridinco-styrene (PVPcS) were methylated to three degrees, 100, 50 and 20%, (denoted P-Q100, P-Q50 and P-Q 20, respectively). The effect of polycation charge level on its adsorption to a negatively charged clay-mineral, montmorillonite (MMT), was studied and the conformations of the polymers in the different polymer-clay composites were characterized. The adsorption of four pharmaceuticals (non-ionic and negatively charged), also considered as potential contaminants with only partial degradation in conventional water treatment plants [43–46], by the composites was tested. The adsorption was correlated to the molecular interactions with the monomers and to the polycation conformation at the interfaces. An in-depth understanding of the relation between polymer conformation at surfaces and its ability to bind a third component would contribute to improve and govern adsorption and desorption processes which can be useful in many fields. 2. Materials Wyoming Na-montmorillonite SWy-2 (MMT) was obtained from the Source Clays Repository of the Clay Mineral Society (Columbia, MO); cation capacity (CEC) and specific surface area were 76.4 meq/100 g and 756 m2/g, respectively. Poly-4vinylpyridine-co-styrene (PVPcS) (Mw = 1200–1500 KDa, PVP: Styrene ratio-9:1), Diclofenac sodium (DCF), Gemfibrozil (GMF), Carbamazepine (CBZ), (all > 97% purity), and methyl iodide (95%) were purchased from Sigma-Aldrich Israel Ltd. Lamotrigine (LMG) (>99%) was purchased from EnzoBiochem Inc. (New York). 3. Methods 3.1. Polycations preparation and characterization Protonated PVPcS (HPVPcS) solution was prepared in acidified distilled water (ADW) solution by adding H2SO4 (95% purity) in stoichiometric concentrations of pyridine. Methylation of PVPcS was preformed according to previous studies [47,48]; briefly, PVPcS was freeze-dried, solubilized in N,N-Dimethylformamude (DMF) for 24 h and methyl iodide was added in 0.2, 0.5 and 1.5 M ratios in order to form P-Q20, P-Q50 and P-Q100 polycations, respectively. The reaction mixture was refluxed for 12 h at 60 °C and was washed thoroughly with hexane. The solvent was removed under reduced pressure and the sample was lyophilized. P-Q100 and P-Q50 were dissolved in distilled water (DW), while P-Q20 was dissolved in ADW (pH 3). The polycation solutions were dialyzed (MWCO 14,000 da) against DW and ADW respectively to remove residual unreacted components (methyl iodide). A sample from each polycation solution was lyophilized and the
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weight distribution of the elements (C,H,N,I) was analyzed (see below). The methylation was assured by NMR and the methylation level was calculated by the N/C molar ratio, by FTIR and by XPS measurements (see below). The total organic carbon (TOC) of the solutions was measured upon filtration by PTFE syringe filter (AXIVA) 0.45 lm and the molar concentration of each polycation was calculated based on carbon/average monomer ratio in each case. 3.2. Polycations stability To test whether polycation stability depends on pH, vials with exactly the same concentration of each polycation (3 repetitions) but at different pH’s (3 and 7) were prepared. The solutions were agitated, filtered through PTFE syringe filter and the concentration was measured by UV–Vis spectrophotometer (see below). In addition, in order to compare this procedure to the one applied with the composites; the concentration of the polycations at the tested pH was also measured upon centrifugation (12000g for 10 min, 18 °C). Good correlation was found between both procedures (filtration/centrifugation). 3.3. Polycation adsorption and composite preparation MMT clay suspension of (0.5%) was added to polycation (P-Q20, P-Q50, P-Q100, HPVPcS) solutions (final concentrations were 0.1–2.5 mmol/L for polycation monomers and 0.17% for MMT suspention). The clay-polycation suspensions were agitated (for at least 4 h, reaching equilibrium); suspensions were centrifuged (12000g for 10 min, 18 °C) and supernatant was separated. The polycation and iodide concentrations in supernatants were measured and calculated against calibration curve using UV–Vis spectrophotometer at a wavelength of 223–226 [49] and 255–262 nm [50] (depending on iodide interference in this range) for iodide and polycation, respectively (Fig. S4). The polycation and iodide adsorption was calculated accordingly. In order to avoid the effect of the different pH’s on polycation charge and subsequently on its conformation and functionality, the composites solutions were adjusted to pH 6.5–7 by adding equal preparation volume of tap water (pH-7, EC = 0.75 mS/cm, DOM concentration and organic contamination–negligible) to the composite precipitates which were re-suspended, agitated to reach equilibrium, centrifuged and the supernatant was separated as was mentioned above.
3.4. Calculation of the dangling monomer’s percentage and the internal division of the dangling monomers The percentage of 4MPY sites for each polycation upon adsorption were calculated based on XPS measurements, (50.2 and 100% for P-Q50 and P-Q100 respectively, see Fig. S2). Since for both polycations styrene consists 10% of the monomers, the values from the XPS measurements were multiplied by 0.9 (under ‘4MPY sites’ column in Fig. 2d). The percentage of the dangling monomers (Fig. 2c) was calculated by the sum of three components: un-anchored 4MPY monomers (Ua4MPY) PY (exist only for Q50) and styrene monomers (Eq. (1)). Based on literature [51] the last two are not expected to form substantial interactions with the clay. Ua4MPY was calculated based on 4MPY- iodide interactions; under the assumption that direct interaction of 4MPY to MMT includes ion exchange in which the counter ion (Iodide) is released, while dangling 4MPY sites used as anion exchange sites for iodide. Based on that, Ua4MPY percentage was calculated by the ratio Ia/TPa, in which Ia is Iodide adsorbed and TPa is the total polymer adsorbed (both are expressed as mmol/g).
Dangling monomersð%Þ ¼ Ua4MPY þ PY þ Styrene ¼
Ia 100 þ Py þ 10 TPa
ð1Þ
The percentage of PY monomers as a component in the dangling monomers (only for Q50) was calculated as follows: PYð%Þ ¼ 100 4MPYsites styrene ¼ 100 45:2 10 ¼ 44:8%. 3.5. Polycation release upon washes The composites (0.28–0.6 mmol of polycation monomers/g clay) were prepared as described above. A preparation volume of DW (Q100, Q50), ADW (Q20, HPVPcS composite) or salted water (1 M NaCl) was added to the precipitates. The composites were re-suspended, agitated for 24 h, centrifuged (12000g for 10 min, 18 °C) and polycation concentration in the supernatant was measured against calibration curves at the same condition (pH and salt). The release was calculated accordingly. 3.6. Polycation and composite characterization 3.6.1. Zeta potential measurements Zeta potential values of MMT (0.17%) and the composites as function of polycation loading (0–1.33 mmol/g) were calculated
Fig. 1. Adsorption isotherms of (a) P-Q100 and P-Q50 polycations (pH = 6) and (b) HPVPcS and P-Q20 polycations (pH = 3) by MMT (1.7 g/L), Concentrations of the polycation are expressed by average mmol monomer/g. Three characteristics adsorption points along the isotherm are demonstrated with arrows on P-Q100 isotherm.
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Fig. 2. (a) FTIR spectra of the polycations upon adsorption by MMT (b) Zeta potential of the composites as function of polycation loading, tested at pH 6.5–7. (c) Monomers (%) dangling as function of polycation loading. (d) Distribution between dangling vs. anchored segments of P-Q100 and P-Q50 polycation upon adsorption to MMT for composites at maximum adsorption.
from the mobility of the particles based on Smoluchowski equation using a Zetasizer Nanosystem (Malvern Instruments, Southborough, MA).
3.6.2. FTIR measurements FTIR spectra were obtained for MMT, polycations at pH 6 and composites (0.53–0.59 mmol/g). Pellets were prepared from dried polycation or composite mixed with KBr (2:98 ratio). FTIR spectra were recorded at room temperature in the range of 500–4000 cm1 using FTIR spectrometer (Nicolet Magna-IR-550, Madiso WI).
3.6.3. X-ray diffraction (XRD) measurements The basal (d 001) spacings of MMT and composites (0.53– 0.59 mmol/g) were measured by XRD before and after heat treatment (360°, 4 h). In addition, the basal spacing of the composites was measured upon adsorption of DCF and GMF on Q100 (86 and 2.95 mmol/g for DCF and GMF, respectively) and Q50 composites (49 and 6.4 mmol/g for DCF and GMF, respectively). Samples were prepared on a round glass slide, 1–2 mL of the suspension (0.17% MMT) were placed and left to sediment (oriented sample) for one day. The basal spacing was measured before and after heat treatment at 360 °C using an X-ray diffractometer (Philips PW1830/3710/3020) with Cu Ka radiation, k = 1.54 A°.
3.6.4. Elemental analysis (C, H, N, and Iodide) Determination of C, H and N was performed using the PerkinElmer 2400 series II Analyzer. A combustion method (950– 1000 °C) to convert the sample elements to simple gases was applied. The system uses a steady-state, wave front chromatographic approach, to separate the controlled gases which are detected as a function of thermal conductivity. Determination of iodide was done using the oxygen-flask combustion method (Schoniger application) for the decomposition of organic samples, and subsequent potentiometric titration by the 835 Titrando Metrohm Titroprocessor and by ion chromatography analysis using a Dionex IC system. The methylation level of polycations (P-Q20 - P-Q100) was calculated based on C/N molar ratio and was confirmed by I/ N ratio (small differences were observed). 3.6.5. XPS measurements The surface chemical analyses were obtained using XPS with a Kratos Axis Ultra instrument (Kratos Analytical Ltd., Manchester, UK). The XPS spectra were acquired using a monochromated Al Ka (1486.6 eV) X-ray source with 90° takeoff angle (normal to analyzer). The pressure in the analytical chamber was maintained at 2109 Torr. The high-resolution XPS spectra were obtained with pass energy 20 eV and step size 0.1 eV. Data analyses were performed using Vision processing software (Kratos Analytical Ltd) and CasaXPS (Casa Software Ltd). The peak fitting was done using
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Gaussian – Lorenzian (30%) ratio. The binding energies were calibrated using C 1s peak as 285.0 eV. The polycations adsorption and the distribution between pyridine and pyridinium were calculated by analyzing the N1s spectra of the samples. The peaks at 399–399.5 and 402–402.5 eV were attributed to un-charged pyridine and positively charged pyridinium monomers (HPY and 4MPY), respectively [19]. P-Q100 was considered as polycation with complete charged sites (based on element analysis and FTIR measurements) and the other samples were compared to its pyridiniun/pyridine ratio.
state). 2. Working in adequate concentration for efficient analysis by HPLC.
3.6.6. BET measurements The BET surface area was measured by NOVAÒ e-Series Surface Area Analyzers (Quantachrome Instruments). Composites were added to 9 mm tubes and temperature was elevated gradually (20 °C, 1 h in each step) from 20 to 120 °C under vacuum. Samples were weighted and reinserted to the instrument for adsorption and desorption measurements. The gas used was nitrogen and isotherms were obtained between relative pressures of 0–0.9. The analysis of BET was conducted by the International Organization for standardization (ISO) recommendation, the linearity range of adsorption (up to P/P0 = 0.3, depends on its linearity), in all cases C was confirmed to have positive value.
3.7.4. Calculation of the interaction between DCF/GMF and polycation segments using DFT method The interactions between DCF or GMF and 4MPY or PY (monomer and trimer) were calculated using B3LYP density functional theory (DFT) with 6-31G* basis set (Spartan’16 Wavefunction, Inc. Irvine, CA), all calculations were conducted in the presence of water as a solvent. A geometry optimization was applied to each of the components and for the complex; the extent of stabilization was calculated (Table S5) and the interactions between the components were assessed accordingly. For the complexes, at least three different starting orientations were applied according to the expected interactions, in order to reach the most stable orientation.
3.7. Pharmaceuticals adsorption by the composites
4. Results and discussion
3.7.1. Pharmaceuticals analysis DCF, GMF, CBZ and LMG were analyzed by HPLC (Agilent Technologies 1200 series) (Table 1). HPLC column was LiChroCARTR 250 (150 for LMG) 4 PurospherR STAR RP-18 (5 lm), the flow rate was 1.0 mL/min (0.9 mL/min for LMG).
The effect of polycation charge level on its conformation upon adsorption to the clay platelets was explored. PVPcS was charged (100, 50 and 20% denoted P-Q100, P-Q50 and P-Q20) by methylation of the pyridine (PY) monomers to form N-methyl-pyridinium (4MPY) using relevant stoichiometric concentrations of CH3I [47,52]. PVPcS can also be charged, under acidic conditions, by protonating the PY monomers (Pka of poly 4-vinylpyridine is in the range of 4–4.6 [53,54]) to form HPVPcS; in addition to the methylated polycations, the adsorption of HPVPcS to MMT was studied as a control. The charge levels of the polycations were evaluated and confirmed by FTIR, element analysis and XPS measurements (Fig. S1 a,b).
3.7.2. Pharmaceuticals adsorption by the composites and MMT Composites of three loadings were chosen for each polycation: Low (0.29–0.3 mmol/g), Intermediate (0.53–0.6 mmol/g) and High loading (which is close to the saturation level and is differ for each polycation- Table S3). The results are expressed as means ((3 repetitions), SD in all cases was in the range of ± 0–0.03). Upon composite preparation (see above), pharmaceuticals were added to the centrifuge tubes containing composite or MMT (1.67 g/L) sediments. The composites were re-suspended and the tubes were agitated for 24 h. Supernatants were separated by centrifugation (12000g for 10 min) and prior to HPLC analysis were filtered with PTFE syringe filter (AXIVA) 0.45 lm pore size. Except GMF (13–20% reduction), the filtration didn’t substantially affect the pharmaceutical concentration (less than 5% reduction in all cases). The calibration curve was built using filtered solution to achieve maximum precision. The experiments were performed at least in triplicate and samples were kept in darkness in order to avoid photo degradation (DCF). One-way ANOVA tests were used at the second point (Intermediate) to determine the statistical differences among groups (p < 0.05). The adsorption of CBZ (4.2 mM), LMG (3.9 mM), DCF (135 mM) and GMF (40 mM) on the composites and on MMT was tested. These concentrations of the pharmaceuticals were chosen based on two considerations: 1.Working under pharmaceutical saturated solution in all cases (40–87% un-adsorbed
3.7.3. Adsorption isotherms of DCF and GMF by Q100 and Q50 composites (Fig. 6) GMF (up to 48 mM) and DCF (up to 507 mM) were added to the composites (0.53 and 0.59 mmol/g for Q100 and Q50 composite, respectively) and equilibrated for 24 h. DCF and GMF concentrations in supernatants were measured by HPLC as mentioned above and their adsorption isotherms were constructed.
4.1. Composites fabrication and characterization The adsorption of the polycations by MMT was measured at equilibrium (Fig. 1). While the adsorption of P-Q100 and P-Q50 polycations was measured at their native pH (pH = 6), the solution pH of P-Q20 and HPVPcS was adjusted to 3 due to solubility limitations (Fig. S1c). In all cases adsorption reached a plateau with the maximum adsorption values decreasing with an increase in polycation charge level [30]; that can be explained by the tendency to compensate the surface charge on one hand and by the limitation of the steric repulsion between the polycation chains on the other hand [19,24]. The adsorption of P-Q100 was the lowest and did not even reach the cation exchange capacity (CEC) of MMT, (0.8 mmol/g). On the other hand, HPVPcS and P-Q20, with similar low charge levels, exhibited high adsorption (1.33–1.46 mmol/ g); while P-Q50 with intermediate charge level, exhibited moderate values of adsorption (0.86 mmol/g). In all cases, polycation des-
Table 1 HPLC conditions and limit of quantification for each pharmaceutical. Molecule
Detector
Mobile phase
Wavelength/kexcitation-kemission (nm)
Limit of quantification (LOQ) (mg/L)
DCF GMF CBZ LMG
diode-array Fluorescence diode-array diode-array
acetonitrile/acidic water (0.1% formic acid) 80/20 acetonitrile/acidic water (0.1% formic acid) 80/20 acetonitrile/ distilled water 50/50 P.buffer(pH 8, 0.025 M):ACN:MeOH = 70/16/14
276 220–300 286 210
0.05 0.025 0.05 0.05
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orption, measured by rinsing with distilled (neutral pH/pH 3) and salted (1 M NaCl) water, was negligible (less than 2.5%) (Table S1); indicating strong interactions between the oppositely charged polycation and MMT surface. To shed light on polycation conformation upon adsorption, the affinity of each of the monomer residues (PY, 4MPY and HPY) to the clay should be discussed. Styrene and PY monomers have low affinity to MMT surface [51], while pyridinium monomers (HPY and 4MPY) have higher affinity to the clay due to electrostatic attraction. In order to avoid the effects evolved from different pH, all composites, including the HPVPcS and Q20 composites, which were prepared at an acidic pH, were thoroughly rinsed with tap water, reaching pH 6.5–7. Upon adsorption, the presence of PY monomers is dominant in the case of PVPcS, as expressed by the FTIR peak at 1608 cm1 (Fig. 2a), it decreases gradually for P-Q20 and P-Q50 and obviously does not exist in the case of P-Q100, Indicating that both PY and HPY/4MPY (expressed at 1637 and 1643 cm1, respectively) monomers exist at the adsorbed state. Accordingly, the amount of ‘low affinity’ monomers (Table S2) can be calculated using the polycation loading (Fig. 1) and the degree of charged monomers at the adsorbed state which can be received by the XPS results (Fig. S2c). As the charge level of the polycation decreases (100– 20%) the concentration of the ‘low affinity’ monomers increases (0.06–0.81 mmol/g). Furthermore, since average distance between charged sites on MMT is about 11.5 A° [21] and the distance between two adjacent charged nitrogen (P-Q100) is about 9 A°, at least part of the charged monomers may also dangle into solution, suggesting higher amount of detached monomers [21,22,24]. Zeta potential of the composites will be affected by polycation charge density through both the loading and conformation on the clay. For all composites, zeta potential was less negative as polycation loading increased (Fig. 2b). In all cases the number of compensated sites was lower than the CEC (Table S2). However, while for Q20 and PVPcS composites, Zeta potential was slightly negative to neutral; for Q100 and Q50 composites, the values were positive due to the screening of the clay charge by the charge of the polycations [55]. Similar zeta values were obtained for Q100 and Q50 composites at their high loading, despite the lower charge compensation found for Q50. This discrepancy can be explained by the conformational differences (more looped for Q50) affecting the shear plan in which the Zeta values are measured [56,57]. The Q50 and Q100 composites represent a unique case since they have similar zeta values (at high loading) but different conformation. Hence, we precisely defined the anchored vs. dangling monomers in each case, considering also the charged monomers that are not directly interact with the clay by using the molar ratio between the adsorbed counter ion (I) and the adsorbed polycation (Fig. 2 c,d). The percentage of monomer dangling as function of polymer loading on the clay was obtained (Fig. 2c). The degree of monomers dangling to the solution was high and constant for all Q50 composites (70–75%). In contrast, Q100 composites exhibited an increase in the degree of dangling monomers, ranging from 30% of the monomers at low loadings to 55% at higher loadings. A deeper observation was taken into the composites at their highest loadings (Fig. 2d). Higher amount of 4MPY sites which directly interact with the clay were found for Q100 in comparison to Q50 composite (0.27 vs.0.19 mmol/g, respectively). Accordingly, the amount of overall monomers dangling into solution is 2.1 fold higher in the case of Q50 composite enabling more active adsorption sites. The internal distribution of the dangling monomers displays 37% more sites of 4MPY in the case of Q100 composite, while the amount of the uncharged sites (styrene and PY) in Q50 composite is 8 fold higher. An even higher degree of dangling monomers is expected
in the case of PVPcS and Q20 composites but could not be determined due to the low pH which excluded the iodide. These conformation differences also affect the BET surface area of the composites which represent merely the external surfaces. The BET of the composites was higher, 31.6 ± 2.6 and 38.6 ± 1.1 m2/g for Q100 and Q50, respectively, than the BET of MMT (22.7 m2/g) [50]. The contribution of the organic segments to the surface area is known to be low [58–60], hence the increased BET surface area for Q50 composite, can be attributed to the exposure of more clay surfaces due to loosely associated polymer conformation. Indeed, C values, associated to the energy of monolayer adsorption, were higher in the case of Q50 in comparison to Q100 (69 vs. 57), indicating that more clay surfaces are unveiled [59]. The conformation was also expressed in the interlayer space of the composites. In all cases the polycations intercalated between the clay platelets as was demonstrated by X-ray diffraction measurements (Fig. S3a,b). However, upon heating [61], higher dspace was observed for Q50, Q20 and PVPcS composites in comparison to Q100 (1.47 ± 0.02 vs. 1.34 nm, respectively), suggesting a more extended conformation of the low charged density polymers also in the interlayer space. To conclude, the degree of polycations adsorption was negatively correlated to their charge density and in all cases polycation release was negligible. Based on iodide exchange, FTIR, XPS, zeta potential, XRD and BET, the conformation of polycations and composites structure can be depicted. For high charge density polycation a more flatted conformation, described as ‘trains’, was displayed. Alternatively, for low charge density polycation, a conformation with more ‘loops and tails’ was suggested. The polycations also affect the MMT structure resulting in higher inter-layer space for low charge density polycations. The effect of polycation conformation and composite structure on their ability to adsorb pharmaceuticals was explored in depth and is discussed below. As polycation charge density decreases, its conformation shifts towards ‘loops and tails’ and the composite surface is more hydrophobic (less charged monomers). Hence, the adsorption of non-ionic molecules by such composites should increase due to both interaction and conformation considerations (Fig. 3). To test this hypothesis, the adsorption of two non-ionic molecules, carbamazepine (CBZ) and lamotrigine (LMG), by the composites was tested (The chemical structures of the pharmaceuticals appear in Fig. 4). In order to distinguish between these two parameters, the interactions between the composites and anionic molecules were tested. In this case, the anionic molecules would preferably interact with the high charge density polycation due to electrostatic interactions. However, the ‘loops and tails’ conformation may also promote adsorption, enabling more adsorption sites with more varied polymer-pharmaceutical orientation (Fig. 3). In order to test the magnitude of electrostatic interactions vs. ‘loops and tails’ conformation, the adsorption of two negatively charged molecules, diclofenac (DCF) and gemfibrozil (GMF) by the composites, was tested. The adsorption of the pharmaceuticals was studied at three characteristics loadings along the polycation adsorption isotherms (Table S3) denoted as: Low - complete adsorption for all polycations (0.29–0.3 mmol/g), Intermediate 0.53–0.6 mmol/g, High – close to the saturation level of the polycation. The adsorbed amounts (lmol/g) of the pharmaceuticals by the composites are within a range of two orders of magnitude (Fig. 5). The adsorption of LMG and CBZ, the nonionic molecules, to the composites, was relatively low and similar, 0.1–0.3 and 0.26–0.6 mmol/g, respectively, due to merely week non-ionic, hydrophobic interactions. In contrast, the adsorption of the anionic molecules, GMF and DCF, was mainly electrostatic and therefore higher, 0.46–6.33 and 6.1–49.6 mmol/g, respectively, with a magni-
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Fig. 3. The expected contribution of both conformation and chemical interactions to the adsorption of non-ionic and anionic molecules by composites of polycations with diverse charge densities.
Fig. 4. Molecular structures of pharmaceuticals: diclofenac (DCF), gemfibrozil (GMF), lamotrigine (LMG) and carbamazepine (CBZ). Physical-chemical properties of the molecules are specified in Table S4.
tude higher in the case of DCF. The substantial higher adsorption of DCF is addressed below. As expected, for both non-ionic molecules, CBZ and LMG, a clear trend of preferable adsorption to the low charge density composites was observed (PVPcS > Q20 > Q50 > Q100 > MMT) (Fig. 5a,b) due to both an increase in hydrophobicity and a more dangling conformation. The adsorption of CBZ and LMG did not dramatically change as a function of polycation loading; indicating limited adsorption sites of these non-ionic molecules on the composites. The anionic molecules, DCF and GMF, exhibited an opposite trend (Fig. 5c, d). The adsorption of DCF and GMF correlates to the zeta potential of the sorbents: MMT < PVPcS < Q20 < Q50, Q100. The adsorption of DCF to Q100 composite was substantially higher (also when expressed as (mmol/mmol) (Fig. S5)) than to Q50, while, surprisingly the adsorption of GMF was higher in the case of Q50. To further explore the opposite adsorption trends of DCF and GMF, their adsorption isotherms to Q50 and Q100 composites (0.53–0.59 mmol/g), were studied (Fig. 6). Upon adding low con-
centrations of DCF or GMF there was no difference in their adsorption by Q50 and Q100 composites. However, at high pharmaceutical concentrations, the differences are clear; higher capacity of DCF to Q100 composite vs. higher capacity of GMF to Q50 composite. In order to further elucidate the differences between DCF and GMF adsorption; both aspects, magnitude of adsorption (50– 85 lmol/g vs. 3–6.4 lmol/g, respectively) and higher adsorption towards Q100 or Q50 composite, respectively, were evaluated. Accordingly, the different sites on the composites were studied and basic DFT calculations, to assess the interaction between the anionic molecules and the polycation’s monomers, were performed. The capacity of DCF (to all composites) may be attributed to its ability to access not only the external surfaces but also to access sites within the clay platelets, enabling up to 20 fold more adsorption sites [62]. In order to evaluate the ability of the pharmaceuticals to intercalate, the basal spacing of the composites, before and after adsorption of GMF and DCF (at max. pharmaceutical adsorption) was measured by XRD (Fig. 7). In both cases the basal spacing was 1.54 ± 0.01 nm, and upon pharmaceutical adsorption no change of the peak position was observed except of additional shoulder in the case of DCF adsorbed by Q50 composite (1.27 nm). However, upon DCF adsorption the diffraction peaks of both composites were lower and wider, in comparison to the bare composites, and in comparison to the diffraction of GMF adsorbed to the composites. Wider and lower intensity peaks, are attributed to a transition from a crystalline structure to more amorphic one [63,64], which can be more accurately determined by measuring the peak width at its half height (Fig. 7, insert), [65]. A more amorphic structure may be induced by DCF intercalation in between the clay platelets [63,64]. The ability of DCF vs. GMF to intercalate in the composite may be explained by the correlation with the size of the molecules (263.4 and 277.9 A3, respectively) and by the lower aspect ratio of GMF. The suggested intercalation of DCF may explain its higher adsorption to all composites. However, it does not explain the higher adsorption of DCF to Q100 vs. Q50 which is in contrast to the higher adsorption of GMF to Q50 vs. Q100. Basically, the
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Fig. 5. (a) CBZ (4.2 mM) (b) LMG (3.9 mM) (c) DCF (135 mM) (d) GMF (40 mM) adsorption on MMT and on the composites (PVPcS, Q20-Q100) at three regions of the adsorption isotherms (see Fig. 1).
Fig. 6. (a) GMF (up to 48 mM) and (b) DCF (up to 507 mM) adsorption isotherms on Q50 and Q100 composites (0.53–0.59 mmo/g).
4MPY sites are expected to promote interactions with both anionic molecules i.e., higher adsorption to Q100 is expected. However, GMF adsorption to Q50 is higher suggesting that may be, unexpectedly, its interactions with PY are preferable. In order to assess the interactions between the adsorbates (DCF and GMF) and adsorbents (4MPY and PY) quantum chemical calcu-
Fig. 7. The XRD spectra of: (a) Q100 composites (0.53 mmol/g), b. Q50 composites (0.59 mmol/g). In each graph the effect of maximum adsorbed amount of DCF and GMF is demonstrated (see Fig. 6). The width at half height of the main peaks is specified in the inserted table.
lations were conducted (Fig. 8 and Table S5). The calculations were based on DFT method [66], (Spartan’ 16 Wavefunction, Inc. Irvine,
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that polycation conformation may affect its ability to interact with molecules. Séquaris et al. [40], in one of the few studies in this context, correlated the adsorption conformation of non-ionic polymer to composite ability to interact with anionic surfactant. At low loadings the adsorption of sodium dodecyl sulfate was low which were correlated to a ‘trains’ conformation and proximity to the anion exclusion volume. At higher loadings the adsorption substantially increased along with a more extended conformation of the polymer. All the more so, in the current study the adsorption trend doesn’t match the fact that zeta potential of Q50 (20 mV) is significantly lower than that of Q100 (39 mv) at the studied loading (0.53–0.59 mmol/g); meaning that the effect of charge, as expressed by zeta potential, plays a role but is not the exclusive factor governing the adsorption of anionic molecules. Based on the conformation proposed and on the expected interaction calculated; we suggest that the adsorption of , may be restricted by the trains conformation of Q100 which limits its ability to interact with GMF in a suitable orientation. On the other hand, more loosely associated polycation monomers, as was described in the case of Q50; may enable more versatile binding options; thus, increasing the adsorption. The reason that GMF is more affected by the conformation can be explained by its accessibility mostly to the external surfaces in which the conformational effect is more dominant or by a more specific orientating needed in order to achieve a stable interaction, both options needed to be further investigated. 5. Conclusions
Fig. 8. Geometry optimization by DFT calculation of the complexes DCF/GMF4MPY/PY in water. (a) summary of the product-reactants energy differences, (for detailed values, see Table S5). (b–e) DCF (b,d) or GMF (c,e) most stable orientation received upon interaction with 4MPY (b,c) or PY (d,e).
CA), and included geometry optimization and estimation of energy stabilization considering the effect of water. The differences in stabilization energy- DE, which indicate the strength of the chemical interaction, was substantially higher for both DCF and GMF interactions with 4MPY (monomer and trimer) in comparison to their stabilization by PY (Fig. 8a). The contribution of p-p interactions or charge transfer between 4MPY and DCF or GMF was refuted due to too high energy gap between 4MPY LUMO and DCF GMF HOMO. Alternatively, the final orientations indicate that H-bonds, which need specific orientation, play a major role (Fig. 8b). Based on that, it can be concluded that higher adsorption of GMF by Q50 composite can’t be explained by a preferable interaction between GMF and PY sites. The DE received from GMF interactions with 4MPY (monomer and trimer) was higher (more negative) in comparison to DE of DCF interactions with 4MPY for their most stable orientation (Fig. 8a). The stabilization of GMF was higher also for PY; however, the difference DE4MPY- DEPY calculated for GMF is still more negative in comparison to DCF, indicating the dominancy of the ionic interaction between 4MPY and GMF. Higher affinity of GMF to both Q50 and Q100, in comparison to the affinity of DCF, is also expressed by its higher Langmuir affinity values (KL) calculated from the adsorption isotherm (Table S6). Although the interaction of GMF with both monomers are higher than the interactions of DCF, the capacity of the composites towards DCF is higher, strengthening the assumption that DCF is also exposed to internal adsorption sites. Yet to be explained is the preferable adsorption of GMF by Q50 composite (in comparison to its adsorption by Q100). We suggest
The effect of composite structure and polycation conformation on its ability to adsorb non-ionic and anionic pharmaceuticals was studied. The non-ionic molecules, CBZ and LMG, exhibited preferable adsorption to low charge density composites which was explained by both interaction and conformation considerations. The anionic molecules, DCF and GMF, displayed substantial differences in terms of capacity along with preferable adsorption to different sorbent (Q100 and Q50 composite, respectively). The higher capacity for DCF was attributed to its ability to penetrate into the interlayer space in counterpart with the polycation. Both molecules were expected to preferably adsorb to Q100 composite as was demonstrated by DFT calculations; however, GMF displayed higher adsorption to Q50 composite. Based on polycation conformation and the expected interaction, we suggest that the confined polycation conformation of Q100 (‘trains’) may limit the adsorption of GMF while dangling polycation conformation of Q50 composite, enables more adsorption sites and diverse monomer-GMF orientations resulting in more efficient adsorption of GMF. Acknowledgments This research was supported by the Israeli ministry of science, technology and space (grant 821-0142-06). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.079. References [1] M. Rosoff, Nano-Surface Chemistry, Rosoff Marcel DekkerNew York 2001, 2002, pp. 213–242. 10.1201/9780203908488. [2] P.A. Williams, Handbook of Industrial Water Soluble Polymers, Blackwell Pub, 2007. 10.1017/CBO9781107415324.004. [3] P. Meneghetti, S. Qutubuddin, Synthesis, thermal properties and applications of polymer-clay nanocomposites, Thermochim. Acta. 442 (2006) 74–77, https://doi.org/10.1016/j.tca.2006.01.017.
526
H. Kohay et al. / Journal of Colloid and Interface Science 552 (2019) 517–527
[4] J.A. Prince, G. Singh, D. Rana, T. Matsuura, V. Anbharasi, T.S. Shanmugasundaram, Preparation and characterization of highly hydrophobic poly(vinylidene fluoride) - clay nanocomposite nanofiber membranes (PVDFclay NNMs) for desalination using direct contact membrane distillation, J. Memb. Sci. 397–398 (2012) 80–86, https://doi.org/10.1016/j. memsci.2012.01.012. [5] C. Breen, The characterisation and use of polycation-exchanged bentonites, Appl. Clay Sci. 15 (1999) 187–219, https://doi.org/10.1016/S0169-1317(99) 00024-1. [6] F.T. Hesselink, On the theory of polyelectrolyte adsorption, J. Colloid Interface Sci. 60 (1977) 448–466, https://doi.org/10.1016/0021-9797(77) 90309-5. [7] L. Eriksson, B. Alm, P. Stenius, Formation and structure of polystyrene latex aggregates obtained by flocculation with cationic polyelectrolytes, Colloids Surf. A Physicochem. Eng. Asp. 70 (1993) 47–60, https://doi.org/10.1016/09277757(93)80495-Z. [8] M.A.G. Dahlgren, A. Waltermo, E. Blomberg, P.M. Claesson, L. Sjoestroem, T. Aakesson, B. Joensson, Salt effects on the interaction between adsorbed cationic polyelectrolyte layers: theory and experiment, J. Phys. Chem. 97 (1993) 11769–11775, https://doi.org/10.1021/j100147a033. [9] W. Barford, R.C. Ball, C.M.M. Nex, A non-equilibrium configuration theory of polyelectrolyte adsorption, J. Chem. Soc., Faraday Trans. 1 82 (10) (1986) 3233, https://doi.org/10.1039/f19868203233. [10] W. Barford, R.C. Ball, Towards a complete configurational theory of nonequilibrium polymer adsorption, J. Chem. Soc., Faraday Trans. 1 83 (8) (1987) 2515, https://doi.org/10.1039/f19878302515. [11] G. Durand-Piana, F. Lafuma, R. Audebert, Flocculation and adsorption properties of cationic polyelectrolytes toward Na-montmorillonite dilute suspensions, J. Colloid Interface Sci. 119 (1987) 474–480, https://doi.org/ 10.1016/0021-9797(87)90293-1. [12] R.I.S. Gill, T.M. Herrington, The effect of surface charge on the flocculation of Kaolin suspensions with cationic polyacrylamides of varying molar mass but similar cationic character, Colloids Surf. 25 (1987) 297–310, https://doi.org/ 10.1016/0166-6622(87)80310-4. [13] F. Mabire, R. Audebert, C. Quivoron, Flocculation properties of some watersoluble cationic copolymers toward silica suspensions: a semiquantitative interpretation of the role of molecular weight and cationicity through a ‘‘patchwork” model, J. Colloid Interface Sci. 97 (1984) 120–136, https://doi. org/10.1016/0021-9797(84)90280-7. [14] J. Meadows, P.A. Williams, M.J. Garvey, R.A. Harrop, G.O. Phillips, Enhanced polyelectrolyte adsorption, Colloids Surf. 32 (1988) 275–288, https://doi.org/ 10.1016/0166-6622(88)80022-2. [15] H. Tanaka, L. Ödberg, L. Wågberg, T. Lindström, Adsorption of cationic polyacrylamides onto monodisperse polystyrene latices and cellulose fiber: effect of molecular weight and charge density of cationic polyacrylamides, J. Colloid Interface Sci. 134 (1990) 219–228, https://doi.org/10.1016/0021-9797 (90)90269-T. [16] T. Ge, M. Rubinstein, Strong selective adsorption of polymers, Macromolecules 48 (2015) 3788–3801, https://doi.org/10.1021/acs.macromol.5b00586. [17] L. Zhu, R. Zhu, L. Xu, X. Ruan, Influence of clay charge densities and surfactant loading amount on the microstructure of CTMA–montmorillonite hybrids, Colloids Surf. A Physicochem. Eng. Asp. 304 (2007) 41–48, https://doi.org/ 10.1016/j.colsurfa.2007.04.019. [18] S.A. Sukhishvili, S. Granick, Polyelectrolyte adsorption onto an initially-bare solid surface of opposite electrical charge, J. Chem. Phys. 109 (1998) 6861, https://doi.org/10.1063/1.477253. [19] O.J. Rojas, M. Ernstsson, R.D. Neuman, P.M. Claesson, Effect of polyelectrolyte charge density on the adsorption and desorption behavior on Mica, Langmuir 18 (2002) 1604–1612, https://doi.org/10.1021/la0155698. [20] G.M. Lindquist, R.A. Stratton, The role of polyelectrolyte charge density and molecular weight on the adsorption and flocculation of colloidal silica with polyethylenimine, J. Colloid Interface Sci. 55 (1976) 45–59, https://doi.org/ 10.1016/0021-9797(76)90007-2. [21] R. Denoyel, G. Durand, F. Lafuma, R. Audebert, Adsorption of cationic polyelectrolytes onto montmorillonite and silica: microcalorimetric study of their conformation, J. Colloid Interface Sci. 139 (1990) 281–290, https://doi. org/10.1016/0021-9797(90)90463-X. [22] M.A.C. Dahlgren, P.M. Claesson, R. Audebert, Interaction and adsorption of polyelectrolytes on mica, Nord. Pulp Pap. Res. J. 8 (2007) 62–67, https://doi. org/10.3183/npprj-1993-08-01-p062-067. [23] P.M. Claesson, E. Poptoshev, E. Blomberg, A. Dedinaite, Polyelectrolytemediated surface interactions, Adv. Colloid Interface Sci. 114–115 (2005) 173–187, https://doi.org/10.1016/J.CIS.2004.09.008. [24] K. Lowack, C.A. Helm, Molecular mechanisms controlling the self-assembly process of polyelectrolyte multilayers, Macromolecules 31 (1998) 823–833, https://doi.org/10.1021/ma9614454. [25] J. Gregory, S. Barany, Adsorption and flocculation by polymers and polymer mixtures, Adv. Colloid Interface Sci. 169 (2011) 1–12, https://doi.org/10.1016/ j.cis.2011.06.004. [26] M.R. Bohmer, O.A. Evers, J.M.H.M. Scheutjens, Weak polyelectrolytes between two surfaces: adsorption and stabilization, Macromolecules 23 (1990) 2288– 2301, https://doi.org/10.1021/ma00210a027. [27] T.K. Wang, R. Audebert, Adsorption of cationic copolymers of acrylamide at the silica—water interface: hydrodynamic layer thickness measurements, J. Colloid Interface Sci. 121 (1988) 32–41, https://doi.org/10.1016/0021-9797 (88)90405-5.
[28] Y.R., S. Minko, AFM Single Molecule Experiments at the SolidLiquid Interface: In Situ Conformation of Adsorbed Flexible Polyelectrolyte Chains, 2005. 10.1021/JA0558239. [29] L.J. Kirwan, G. Papastavrou, M. Borkovec, S.H. Behrens, Imaging the coil-toglobule conformational transition of a weak polyelectrolyte by tuning the polyelectrolyte charge density, Nano Lett. 4 (2004) 149–152, https://doi.org/ 10.1021/nl034912l. [30] P. Linse, Adsorption of weakly charged polyelectrolytes at oppositely charged surfaces, Macromolecules 29 (1996) 326–336, https://doi.org/ 10.1021/ma950539v. [31] J. Gregory, Polymer adsorption and flocculation in sheared suspensions, Colloids Surf. 31 (1988) 231–253, https://doi.org/10.1016/0166-6622(88) 80196-3. [32] E.G.M. Pelssers, M.A.C. Stuart, G.J. Fleer, Kinetic aspects of polymer bridging: equilibrium flocculation and nonequilibrium flocculation, Colloids Surf. 38 (1989) 15–25, https://doi.org/10.1016/0166-6622(89)80139-8. [33] F. Lafuma, T.K. Wang, G. Durand, R. Audebert, Adsorption and flocculation behavior for systems of copolymers and particles of opposite charge, Colloids Surf. 31 (1988) 255–257, https://doi.org/10.1016/0166-6622(88)80197-5. [34] T. Cosgrove, T. Obey, B. Vincent, The configuration of sodium poly(styrene sulfonate) at polystyrene/solution interfaces, J. Colloid Interface Sci. 111 (1986) 409–418, https://doi.org/10.1016/0021-9797(86)90044-5. [35] J. Meadows, P. Williams, M. Garvey, R. Harrop, G. Phillips, Characterization of the adsorption—desorption behavior of hydrolyzed polyacrylamide, J. Colloid Interface Sci. 132 (1989) 319–328, https://doi.org/10.1016/0021-9797(89) 90247-6. [36] S. Wang, Z. Jia, X. Zhou, D. Zhou, M. Chen, D. Xie, Y. Luo, D. Jia, Preparation of a biodegradable poly(vinyl alcohol)-starch composite film and its application in pesticide controlled release, J. Appl. Polym. Sci. 134 (2017) 45051, https://doi. org/10.1002/app.45051. [37] P. Liu, Polymer modified clay minerals: a review, Appl. Clay Sci. 38 (2007) 64– 76, https://doi.org/10.1016/j.clay.2007.01.004. [38] C. Viseras, C. Aguzzi, P. Cerezo, M.C. Bedmar, Biopolymer–clay nanocomposites for controlled drug delivery, Mater. Sci. Technol. 24 (2013) 1020–1026, https:// doi.org/10.1179/174328408X341708. [39] E.I. Unuabonah, A. Taubert, Clay-polymer nanocomposites (CPNs): adsorbents of the future for water treatment, Appl. Clay Sci. 99 (2014) 83–92, https://doi. org/10.1016/j.clay.2014.06.016. [40] J.-M. Séquaris, A. Hild, H.D. Narres, M.J. Schwuger, Polyvinylpyrrolidone adsorption on Na-Montmorillonite. Effect of the polymer interfacial conformation on the colloidal behavior and binding of chemicals, J. Colloid Interface Sci. 230 (2000) 73–83, https://doi.org/10.1006/jcis.2000.7046. [41] G.J. Churchman, Formation of complexes between bentonite and different cationic polyelectrolytes and their use as sorbents for non-ionic and anionic pollutants, Appl. Clay Sci. 21 (2002) 177–189, https://doi.org/10.1016/S01691317(01)00099-0. [42] Seiji Tsuzuki, A. Masuhiro Mikami, S. Yamada, Origin of Attraction, Magnitude, and Directionality of Interactions in Benzene Complexes with Pyridinium Cations, 2007, 10.1021/JA071372B. [43] N. Paxéus, Removal of selected non-steroidal anti-inflammatory drugs (NSAIDs), gemfibrozil, carbamazepine, beta-blockers, trimethoprim and triclosan in conventional wastewater treatment plants in five EU countries and their discharge to the aquatic environment, Water Sci. Technol. 50 (2004) 253–260. [44] Y. Zhang, S.-U. Geißen, C. Gal, Carbamazepine and diclofenac: removal in wastewater treatment plants and occurrence in water bodies, Chemosphere 73 (2008) 1151–1161. [45] C. Tixier, H.P. Singer, S. Oellers, S.R. Muller, Occurrence and fate of Carbamazepine, Clofibric Acid, Diclofenac, Ibuprofen, Ketoprofen, and Naproxen in surface waters, Environ. Sci. Technol. 37 (2003) 1061–1068. [46] R. Gurke, M. Rößler, C. Marx, S. Diamond, S. Schubert, R. Oertel, J. Fauler, Occurrence and removal of frequently prescribed pharmaceuticals and corresponding metabolites in wastewater of a sewage treatment plant, Sci. Total Environ. 532 (2015) 762–770, https://doi.org/10.1016/j. scitotenv.2015.06.067. [47] K. Ishizu, Y. Kashi, T. Fukutomi, T. Kakurai, Synthesis of AB and BAB poly (styrene-b-4-vinylpyridine) and solution properties of their quaternized compounds, Die Makromol. Chemie. 183 (1982) 3099–3107, https://doi.org/ 10.1002/macp.1982.021831222. [48] Poly(styrene-b- N-methyl 4-vinyl pyridine iodide) Procedure, (n.d.).
H. Kohay et al. / Journal of Colloid and Interface Science 552 (2019) 517–527
[54]
[55]
[56]
[57]
[58]
[59]
binding, Macromolecules 22 (1989) 1808–1812, https://doi.org/10.1021/ ma00194a051. L. Franck-Lacaze, P. Sistat, P. Huguet, Determination of the pKa of poly (4vinylpyridine)-based weak anion exchange membranes for the investigation of the side proton leakage, J. Memb. Sci. 326 (2009) 650–658, https://doi.org/ 10.1016/J.MEMSCI.2008.10.054. D. Zadaka, A. Radian, Y.G. Mishael, Applying zeta potential measurements to characterize the adsorption on montmorillonite of organic cations as monomers, micelles, or polymers, J. Colloid Interface Sci. 352 (2010) 171– 177, https://doi.org/10.1016/j.jcis.2010.08.010. H. Bouhamed, S. Boufi, A. Magnin, Alumina interaction with AMPS–MPEG random copolymers, J. Colloid Interface Sci. 261 (2003) 264–272, https://doi. org/10.1016/S0021-9797(03)00100-0. Q. Ran, P. Somasundaran, C. Miao, J. Liu, S. Wu, J. Shen, Effect of the length of the side chains of comb-like copolymer dispersants on dispersion and rheological properties of concentrated cement suspensions, J. Colloid Interface Sci. 336 (2009) 624–633, https://doi.org/10.1016/j.jcis.2009.04.057. K. Boki, N. Kawasaki, K. Minami, H. Takahashi, Structural analysis of collagen fibers by nitrogen adsorption method, J. Colloid Interface Sci. 157 (1993) 55– 59, https://doi.org/10.1006/jcis.1993.1157. L.M. Mayer, Extent of coverage of mineral surfaces by organic matter in marine sediments, Geochim. Cosmochim. Acta 63 (1999) 207–215, https://doi.org/ 10.1016/S0016-7037(99)00028-9.
527
[60] W.K. Lowen, E.C. Broge, Effects of dehydration and chemisorbed materials on the surface properties of amorphous silica, J. Phys. Chem. 65 (1961) 16–19, https://doi.org/10.1021/j100819a006. [61] E. Abramova, I. Lapides, S. Yariv, Thermo-XRD investigation of monoionic montmorillonites mechanochemically treated with urea, J. Therm. Anal. Calorim. 90 (2007) 99–106, https://doi.org/10.1007/s10973-007-8482-0. [62] S. Diamond, Surface areas of clay minerals as derived from measurements of glycerol retention, Clays Clay Miner. 5 (1956) 334–347, https://doi.org/ 10.1346/CCMN.1956.0050128. [63] R.A. Vaia, K.D. Jandt, E.J. Kramer, E.P. Giannelis, Microstructural evolution of melt intercalated polymerorganically modified layered silicates nanocomposites, Chem. Mater. 8 (1996) 2628–2635, https://doi.org/10.1021/ cm960102h. [64] K.G. Bhattacharyya, S. Sen Gupta, Kaolinite, montmorillonite, and their modified derivatives as adsorbents for removal of Cu(II) from aqueous solution, Sep. Purif. Technol. 50 (2006) 388–397, https://doi.org/10.1016/j. seppur.2005.12.014. [65] R.L. Frost, L. Rintoul, Lattice vibrations of montmorillonite: an FT Raman and Xray diffraction study, Appl. Clay Sci. 11 (1996) 171–183, https://doi.org/ 10.1016/S0169-1317(96)00017-8. [66] M. Orio, D.A. Pantazis, F. Neese, Density functional theory, Photosynth. Res. 102 (2009) 443–453, https://doi.org/10.1007/s11120-009-9404-8.