pH Responsive Nanofiltration Membranes for Sugar Separations

Article pubs.acs.org/IECR

pH Responsive Nanofiltration Membranes for Sugar Separations Heath H. Himstedt,† Hongbo Du,‡ Kathryn M. Marshall,† S. Ranil Wickramasinghe,‡ and Xianghong Qian*,‡ †

Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523, United States Ralph E. Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States

‡

ABSTRACT: Nanofiltration membranes were surface modified by grafting poly(acrylic acid) (PAA) nanochains using a UV initiated free radical polymerization method. These pH sensitive membranes were then used to separate monosaccharides, disaccharides, and their mixtures from filtration feed streams. It was found that pH dramatically affects the flux, rejection, and selectivity using these modified pH sensitive nanofiltration membranes. Membrane performance could be enhanced not only by the conformational changes of the grafted surface layer due to protonation and deprotonation of the acid groups on the polymer chains but also from the different interactions between the sugar molecules and the neutral PAA chain or its negative conjugate. Classical molecular dynamics (MD) simulations were conducted to investigate the specific interactions between glucose/sucrose and the neutral/negatively charged PAA chains. The simulation results show that both sugar size and PAA charge affect significantly the sugar-polymer interactions.

1. INTRODUCTION Nanofiltration is a pressure-driven membrane filtration process capable of removing small solutes such as divalent salts, sugars, and low molecular weight proteins. Features of nanofiltration membranes include greater than 99% rejection of multivalent ions but lower rejection of monovalent ions. Up to 90% rejection of species with molecular weights between 150 and 300 g mol−1 is observed.1 Though initially developed as low pressure reverse osmosis membranes for softening of surface and ground waters, today nanofiltration membranes find numerous applications in the textile,2 pulp and paper,3 food,4,5 and dairy6−8 industries. In addition, a number of investigators have focused on the use of nanofiltration for concentration and recovery of oligosaccharides and monomer sugars in aqueous streams.9−15 Fractionation of aqueous streams containing sugars and larger oligosaccharides has also been investigated.16−18 Fractionation of aqueous feed streams containing monosaccharaides and disaccharides such as glucose, xylose, and sucrose is challenging using nanofiltration membranes.19 Aydoğan et al.20 investigated fractionation of sucrose and glucose. Glucose rejection was less than sucrose rejection, and separation factors of around 2.33−2.43 were obtained for the experimental conditions investigated. This may be explained by the fact that the molecular weight of glucose is 180 g mol−1 while that of sucrose is 342 g mol−1. Sjomon et al.21 investigated the fractionation of xylose and glucose from concentrated monosaccharide solutions and found that limited separation of hexose and pentose sugars is possible. Generally speaking, fractionation of sugars with molecular weights within a factor of 2, such as glucose, xylose, and sucrose, requires careful optimization of operating conditions such as transmembrane pressure, pH, and feed flow rate. Stimuli responsive membranes change their physiochemical properties because of changes in environmental conditions such as pH, temperature, ionic strength, and so forth.22 These physicochemical changes can lead to changes in membrane © XXXX American Chemical Society

performance. In our earlier studies we developed pH responsive nanofiltration membranes by grafting poly(acrylic acid) (PAA) chains from the surface of commercially available thin film composite polyamide nanofiltration membranes.23 Grafting PAA chains from the surface of the membrane lead to a decrease in permeate flux compared to the unmodified membrane. In addition, at feed pH values above the pKa of the grafted PAA chains, the carboxylic groups are deprotonated, and the chains swell because of electrostatic repulsion. Swelling of the grafted polymer chains lead to a further decrease in permeate flux. Thus, the permeate flux could be modulated by changing the feed pH. Decrease in permeate flux at higher pH values seems to contradict results reported by Mänttäri et al.24 who observed that for the unmodified NF 270 nanofiltration membranes the permeate flux increased in alkaline solutions; however, this increase in flux was not significant until pH above 8. This observation is consistent with the fact that the polyamide barrier layer in these nanofiltration membranes consists of a three-dimensional network of polymer chains and associated free volume. Nanofiltration membranes do not possess real pores. Transport is via diffusion through a polymeric selective layer; thus, discussion of pore size is not appropriate.1,12,14,15,19,23,24 Minor changes in free volume can lead to observable changes in permeability. Thus, under alkaline conditions, proton dissociation from the carboxylic groups in the barrier layer of the unmodified membrane leads to the swelling of the cross-linked polymer barrier layer and thus an increase in free volume which results in higher permeate fluxes.20,22−25 As demonstrated in our previous work,23 for low density, noncross-linked, PAA chains grafted from the surface of the Received: March 27, 2013 Revised: June 5, 2013 Accepted: June 7, 2013

A

dx.doi.org/10.1021/ie400982p | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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acrylic acid chains to better understand the observed rejection data.

barrier layer, the effect appears to be opposite. At pH values below the first pKa of the grafted PAA chains, the polymer chains are neutral and will adopt a random coil-like conformation. PAA has multiple COOH groups, and the acid dissociation constants are typically different from its monomer value because of the change in electrostatic environment. Further, it becomes progressively more difficult to deprotonate two or more acid groups on the same polymer chain. At pH values above the first pKa value of the PAA, the polymer chains become negatively charged. Consequently, these negatively charged groups are strongly hydrated. In addition, the electrostatic repulsion between the charged groups will cause polymer chains, which are only physically tethered to the membrane surface, to repel each other leading to significant swelling of the membrane barrier layer compared to the barrier layer of the unmodified membrane. Moreover, it is well understood that hydration free energies for charged residues are much larger than the corresponding neutral species. This indicates a strong interaction between the negatively charged PAA chains and the solvent water molecules leading to the likely formation of a highly ordered and structured water layer around these negatively charged polymer chains. As a result, the thickness of the membrane surface layer increases significantly. This increases the resistance to flow through the membrane leading to a decrease in the flux at pH above its first pKa. The increase in the permeate flux above pH 8 in the cross-linked base membrane is likely to be more dominated by the increase in free volume. Besides intrinsic properties of the nanofiltration membranes, rejection of uncharged species such as monosaccharide and disaccharide depends on the size, shape of the solute species at the feed conditions as well as the hydrophobicity/hydrophilicity of the solute species.1,6,9,12 In addition, the interactions between the solute species and the membrane also play an important role. Stronger interaction generally leads to greater passage of the solute, thus a lower rejection. As expected, Mänttäri et al.24 observed lower glucose rejections for alkaline feed streams, that is, when the free volume of the barrier layer was greater. For our PAA grafted membranes under alkaline conditions, that is, when the resistance to permeate flow was greater, we observed higher glucose rejection.23 Interestingly, glucose rejection for the modified membranes under acidic conditions, when the PAA chains are protonated and adopt a more collapsed conformation, was in fact lower than that for the unmodified membrane. Here we demonstrate that under relatively low operation pressure, the specific interactions between the sugar molecules and the membrane barrier layer appear to affect the rejection significantly. Therefore it is possible to tailor the surface properties of the membrane to achieve separations of similar solute species based on their specific interactions with the membrane materials. The work presented here extends our previous studies by investigating fundamental principles underlying the fractionation of glucose and sucrose by pH responsive nanofiltration membranes. Experimentally, UV initiated free radical polymerization has been used to grow poly(acrylic acid) chains from the surface of NF 270 nanofiltration membranes. We show that for modified membranes glucose rejection can be higher than sucrose rejection even though the molecular weight of glucose is about half that of sucrose. Computationally, classical molecular dynamic (MD) simulations have been conducted to investigate the interactions between the sugar molecules and

2. EXPERIMENTAL SECTION 2.1. Materials. NF 270 membranes (Dow, Midland, MI) were supplied by the manufacturer. These composite membranes consist of a thin semiaromatic poly(piperazinamide) barrier layer supported by polysulphone and polyester support layers.25 Unless otherwise noted, all chemicals were reagent grade. Benzophenone, potassium chloride, sodium phosphate monobasic, and sodium phosphate dibasic were purchased from ThermoFischer Scientific (Waltham, MA); acrylic acid (AA), D-glucose, sucrose (D(+)Saccharose), sodium azide, sulfuric acid, and hydrochloric acid (37%) from Sigma-Aldrich Corp. (St. Louis, MO); and ethanol from Pharmco-Aaper (Brookfield, CT). All DI water was obtained from a Siemens/ELGA Purelab Ultra deionizer and filter (SCMK2) from Siemens Water Technologies (Warrendale, PA). 2.2. Surface Modification. Forty-five millimeter diameter membrane discs were cut from a larger membrane sheet using a hammer and die. Membranes were washed, dried, and modified as described in detail previously.23 Briefly, benzophenone was dissolved to saturation in DI water at room temperature. Acrylic acid (AA) monomer was added to yield 1% or 2% by weight reaction solutions. The reaction solution was then purged with nitrogen for 15 min to remove oxygen. The membrane discs were placed in the benzophenone-saturated monomer solution and incubated in the monomer solution for 15 min before the Hoenle (Gräfelfing, Germany) UV irradiation system was turned on. The average UV intensity during irradiation was measured to be ∼13 mW/cm2. Membranes in 1% AA solution were irradiated for 15 min while membranes in 2% AA solution were irradiated for either 10 or 15 min. Increasing monomer concentration and polymerization time lead to greater grafting degrees.23 Following modification, the membranes were washed, dried, and stored in zip top bags containing 0.01 wt % sodium azide solution to prevent bacterial growth during storage. Even though the PAA chains are only physically attached to the membrane surfaces, the modified membranes appear to be stable after repeated washing and filtration experiments. 2.3. Membrane Characterization. The zeta potential of unmodified and modified membranes was measured using streaming potential analysis as described in detail previously.26,27 The measurements were always started at pH 10 in a 10−3 M aqueous KCl solution. 1 N HCl was added dropwise to obtain multiple pH values over the range 10−2. The streaming current was measured and converted to the zeta potential using the Helmholtz−Smoluchowski model. ζ=

η ·κ ·ΔESP εo·εr ·ΔP

(1)

In eq 1 the symbols are defined as follows: η, viscosity; κ, conductivity; εr, permittivity of the test solution; εo, permittivity of free space; ΔESP, streaming potential; ΔP, transmembrane pressure. All zeta potential measurements were performed in tangential flow mode, and the average value of four measurements at each pH is reported. 2.4. Membrane Flux and Rejection. A 50 mL stirred cell 8050 (Millipore, EMD, Billerica, MA) with an active membrane area of 13.4 cm2 was used for all filtration experiments. When B



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dissociation constants for second or third acid groups resulting from the electrostatic repulsion from the first acid dissociation.38 The MD simulations were conducted in water for one sugar molecule interacting with either the neutral trimer or its negative conjugate base. The unit cell consisted of one glucose or sucrose molecule and one neutral or deprotonated AA trimer surrounded by about 3500 water molecules. In unit cells containing negatively charged AA trimers, one Na+ counterion was introduced to the system to neutralize the charge. The atomic charges of glucose, sucrose, and AA trimers were calculated using the exchange-correlation potential B3LYP and basis set 6-31G*//HF/6-31G* with Gaussian09 according to the RESP protocol29 based on the optimized structures. Geometry optimizations were performed in water with implicit IEFPCM30 solvent model after constrained geometry optimization in the gas phase. The TIP3P water model was used for the MD simulations, and the recently developed general AMBER force field (GAFF)31 was used for all the organic molecules. The force field parameters compatible with TIP3P water model were used for the Na+ ion.32 The optimized geometries were used as the initial structures for the MD simulations. Periodic boundary conditions (PBC) were applied. The simulations were conducted under constant temperature (300 K) and constant pressure (1 atm) using a Langevin−Hoover scheme.33 The pairwise short-range electrostatic and van der Waals interactions were calculated for up to a distance of 12 Å. Longrange electrostatic interactions were calculated using the Ewald summation.34 A time step of 2 femtoseconds (fs) was used for these MD simulations. The electrostatic and van der Waals interactions between glucose (or sucrose) and AA trimer (or its anion) were determined as a function of simulation time. In classical MD simulations, hydrogen-bonding interaction is approximated as a special type of electrostatic interaction. The hydration water molecule numbers were calculated within 3.5 Å from the surface of the molecular complexes as was done previously.35 Additional details on the computational method can be found in our earlier publications.35,36

removed from storage, the membranes were briefly washed with water before use. Membrane samples were first wet with a 50% ethanol/water solution and a 20 mM phosphate buffer.23 The pH of the buffer depended on the pH of the feed stream to be tested. Experiments were conducted at three representative pH values at 3.15, 6.05, and 7.25. These three pH values correspond to the pH below the pKa of the grafted poly(acrylic acid) chains, the pH of DI water, and the pH value above the pKa of the grafted chains, respectively. The membranes were then allowed to equilibrate in the buffer for 2 h before filtration experiments commenced. For filtration studies, a constant pressure of 3.1 bar was used, and all filtration experiments were conducted at room temperature. Permeate mass was recorded every 30 s via a balance connected to a computer, and the average flux was calculated over 10 min intervals. Permeate samples (three 10 μL samples) were collected every 20 min to determine sugar concentration. At least five membranes were tested for each modification, and average results for both flux and rejection are presented. In all figures, the size of the data points represents the statistical spread of data. Feed streams were prepared by dissolving glucose, sucrose, or equal amounts of the two in the 20 mM phosphate buffers. For feed streams consisting of only one sugar, the sugar concentration was 20 mM. The total sugar concentration of the binary sugar mixtures was either 20 mM or 40 mM total. Sugar concentration was measured with a Hewlett-Packard (Houston, TX) Series 1050 HPLC equipped with a HPX-87C column heated to 55 °C. The mobile phase was 0.01 N sulfuric acid, the injection volume was 10 μL, and the flow rate was 0.6 mL min−1. The instrument was calibrated in six different sugar solutions ranging from 1 M to 1 mM. Sugar rejection was calculated using eq 2, and the membrane selectivity was calculated from eq 3 using the feed and permeate concentrations of each sugar in the two component mixture. Selectivity greater than 1 corresponds to greater glucose rejection than sucrose. R=1−

αS/G =

Cpermeate Cfeed

(2)

3. RESULTS AND DISCUSSION 3.1. Membrane Characterization. Our previous work describes the methods used to characterize modified membranes.23 Briefly, ATR-FTIR and XPS indicated the growth of a peak representing carboxylic groups with increasing degree of grafting because of the presence of grafted PAA. FESEM analysis indicated that surface modification leads to a change in appearance of the barrier layer of the nanofiltration membrane. Qualitatively, increasing degrees of grafting lead to greater changes in the appearance of the barrier layer. Finally, contact angle measurements indicted that the contact angle of the membrane increases with increasing grafting degree from 17° for the unmodified membrane to 38° for membranes modified with 2% acrylic acid monomer solutions for 15 min. Figure 1 gives the variation of zeta potential for unmodified and modified membranes as a function of pH. Modified membranes are represented by 1:15, 2:10, and 2:15 which refer to the weight (%) AA monomer in solution during polymerization and the polymerization time (min), respectively. The zeta potential of all membranes decreased, that is, became more negative, with increasing pH, and the zeta potential was always negative above pH ∼ 3, as discussed elsewhere.19,24,37 Furthermore it is seen that the zeta potential is more negative for the modified membranes compared to the unmodified

CSucrose_permeate/CSucrose_feed CGlucose_permeate/CGlucose_feed

(3)

2.5. Computational Method. To understand the rejection and selectivity of glucose and sucrose for both modified and unmodified membranes, classical atomistic molecular dynamics (MD) simulations were conducted to elucidate the interactions between the sugar molecules and AA trimers grafted from the membrane surface. MD simulations were conducted for glucose and sucrose interacting with the neutral and deprotonated AA trimers at 300 K for about 40 ns (ns) in aqueous solution using NAMD, a highly parallel and efficient classical MD code.28 Only neutral or negatively charged acrylic acid oligomers with three monomer units were investigated since both glucose and sucrose are relatively small molecules, and their interactions with AA are likely localized. This is due to the fact that both van der Waals and hydrogen bonding interaction are short-range interactions. Electrostatic interaction is relatively longer range and decays with 1/r where r is the distance between the point charges. However, only the short-range interaction is specific and thus most relevant to our study here. In the case of the negatively charged AA trimer, only one carboxylic acid group on the trimer was deprotonated due to the decreased C



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Figure 1. Variation of membrane zeta potential as a function of pH for Results are given for 1:15 (○), 2:10 (▲), and 2:15 (▽) modified membranes, as well as unmodified membranes (■). Data points are sized to contain variance between experiments.

membrane at the same pH value because of the negative charges on the PAA chains at higher pH,23 and greater grafting degree leads to a more negative zeta potential at higher pH values. At greater grafting degree, the polymer chains possess more acidic groups. For polymer chains longer than the electrostatic correlation length that is typically less than 20 Å, where the electrostatic interaction energy becomes negligible, acid groups could simultaneously dissociate with very little effects on each other. The zeta potential is therefore more negative for longer chains at the same pH value. 3.2. Membrane Flux and Rejection of 20 mM Glucose and Sucrose. Membrane performance for 20 mM glucose solutions was presented in our earlier work.23 The results are presented in modified form in Figures 2 and 3 to compare these results with results for sucrose only and mixed glucose/sucrose feed streams which are the focus of this study. Unmodified membranes as well as membranes modified at 1:15, 2:10, and 2:15 conditions are presented. The flux data show a steady decrease over the 100−120 min batch mode filtration experiment periods. The decline in flux over time is likely caused by the increased concentration of the solute as well as the more severe concentration polarization as the filtration time increases. The rejection data also show a similar trend. However, rejection exhibits much larger variation over time among experiments. This is because rejection is more sensitive to the operation condition as well as to the interfacial properties. Rejection is affected not only by the concentration and concentration polarization, but also by the specific interactions between the solute molecules and the membrane barrier layers. Our subsequent results will show that this interaction varies dramatically for the glucose or sucrose interacting with the neutral or charged polymer chains. Whether the PAA polymer chain is neutral or negatively charged depends strongly on the pH of the solution. Since the flux decreases over time, it is only meaningful to compare the results at the similar filtration times or to compare only the range of flux variation over time. For example, the flux values are approximately 19, 18, and 16 LMH measured at 20 min filtration time for membrane modification condition 1:15 and for the experiments conducted at pH 3.15, 6.05, and 7.25, respectively, as shown in Figure 2. It is clear that increase in pH leads to a decrease in flux for the same membrane modification

Figure 2. Flux for feed streams containing 20 mM glucose in (A) 3.15, (B) 6.05, and (C) 7.25 pH buffers. Results are given for 1:15 (○), 2:10 (▲), and 2:15 (▽) modified membranes, as well as unmodified membranes (■). Data points are sized to contain variance between experiments.

condition. On the other hand, for the same pH condition at 7.25, the flux values are approximately 16, 14, and 10 LMH at membrane modification conditions 1:15, 2:10, and 2:15, respectively. Clearly increasing grafting degree leads to a decrease in flux because of the additional resistance of the grafted nanolayer. At constant feed pressure, the flux is lower at pH values above the pKa of the grafted poly(acrylic acid) chains, and the effect is more marked with increasing grafting degree. Figure 3 indicates that for modified membranes, glucose rejection also depends on pH. Glucose rejection is between 35 and 45% at pH 3.15 for the three different modification conditions during the entire filtration time period. It increases to between 50 and 65% at pH 6.05 and 55−75% at pH 7.25. This increase in rejection at higher pH value agrees with the measured flux decrease for the modified membranes at the corresponding conditions. Rejection is higher when the grafted D



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Figure 3. Rejection for feed streams containing 20 mM glucose in (A) 3.15, (B) 6.05, and (C) 7.25 pH buffers. Results are given for 1:15 (○), 2:10 (▲), and 2:15 (▽) modified membranes, as well as unmodified membranes (■). Data points are sized to contain variance between experiments.

Figure 4. Flux for feed streams containing 20 mM sucrose in (A) 3.15, (B) 6.05, and (C) 7.25 pH buffers. Results are given for 1:15 (○), 2:10 (▲), and 2:15 (▽) modified membranes, as well as unmodified membranes (■). Data points are sized to contain variance between experiments.

PAA is negatively charged at pH values above the first pKa of the polymer. At pH values below the pKa, here at pH 3.15, rejection is actually less than the unmodified membrane. Even though surface modification with PAA chains leads to substantial decrease in glucose rejection at pH 3.15, the differences in rejection between the three different modification conditions are actually rather small with a slight higher rejection value at higher grafting degree. The differences in rejection for both modified and unmodified membranes at higher pH are small. Results for 20 mM sucrose solutions are given in Figures 4 and 5. Analogous to Figures 2 and 3, results are given for the

unmodified membrane and membranes modified using three different conditions. Comparing the flux data between glucose and sucrose, it can be seen that sucrose follows the same trend as glucose. Flux decreases with increasing grafting degree. The flux of the modified membranes depends on pH. Flux decreases with increasing operating pH value. As with glucose, the pH effect is more marked with increasing grafting degree. Protonation and deprotonation of the carboxylic groups of the grafted poly(acrylic acid) depends not only on the pH of the solution, but also on the pKa values of the polyacrylic chains. E



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groups, multiple pKa values exist.38−40 Here only the first pKa value is considered as subsequent ionization of additional carboxylic groups will require much higher pH because of the strong electrostatic repulsion between the negative charges on the PAA chains. Since the polymer chains are not bonded chemically to the membrane surface, the negatively charged chains will likely expand significantly because of charge repulsion. The swelling from polymer chain expansion will increase the barrier layer thickness, thereby increasing the resistance for the water flow. It is also possible that the negative charges on the dissociated carboxylic groups will promote stronger interaction between the water molecules and the charged residues on the polymer chain leading to a much more structured water layer around the grafted poly(acrylic acid) chains. This could also lead to a greater resistance to permeate flow resulting in the lower fluxes observed at pH 7.25 as shown in Figures 2 and 4. Figure 5 gives rejection data for 20 mM sucrose solutions. Comparing Figures 3 and 5, it can be seen that for the unmodified membrane, sucrose rejection at the same conditions is always greater than glucose rejection. Since the molecular weight of sucrose is 342 g mol−1 and of glucose is 180 g mol−1, this difference is due to the larger size of sucrose. However, more careful examination of Figures 3 and 5 indicate some important differences. For unmodified membranes, rejection of glucose is independent of buffer pH while sucrose rejection appears to decrease slightly at higher pH values. The decrease in sucrose rejection at higher pH values likely comes from increased free volume in the polyamide barrier layer because of the electrostatic repulsion among COO− groups present,24,41 which is likely to affect the passage of the larger size molecules more than the smaller ones. More importantly, our MD simulation results will show later that sucrose interacts more strongly with negatively charged polymer chains than with the uncharged polymers leading to the decrease in its rejection value at higher pH. Results for modified membranes indicate that sucrose rejection decreases at higher pH, and the highest grafting degrees have the lowest sucrose rejection. Importantly, for modified membranes the effect of pH on sucrose rejection is completely opposite to the effect on glucose rejection, which increased at higher pH values. Consequently, at higher pH, modified membranes show greater rejection of glucose than sucrose. Figures 3 and 5 also indicate that for all membranes rejection appears to decrease at longer times. Since experiments were conducted in dead end mode, this is most likely due to the increase in concentration of the rejected species in the feed reservoir. The effect is more noticeable for conditions that result in higher rejection. 3.3. Rejection of Glucose/Sucrose Mixtures. A mixture of 10 mM glucose and 10 mM sucrose (20 mM total sugar concentration) was filtered at various pH values. No statistical difference in the flux was observed when filtering mixtures of sucrose and glucose compared to pure sucrose or glucose; thus flux data for the mixture are not included. Figure 6 indicates that as with the single sugar solutions, sugar rejection is strongly dependent on solution pH for the modified membranes. For the unmodified membrane, sucrose rejection is higher than glucose rejection, which is to be expected considering sucrose is larger than glucose. As observed for single sugar solutions, glucose rejection is independent of feed pH, but sucrose rejection decreases slightly at higher pH values.

Figure 5. Rejection for feed streams containing 20 mM sucrose in (A) 3.15, (B) 6.05, and (C) 7.25 pH buffers. Results are given for 1:15 (○), 2:10 (▲), and 2:15 (▽) modified membranes, as well as unmodified membranes (■). Data points are sized to contain variance between experiments.

Our earlier calculations show that the pKa of polymethacrylic acid is substantially different to its monomer value.38−40 A similar result is expected for PAA. Our calculations for methacrylic acid indicate that the first pKa value of poly(acrylic acid) could be up to two pH units higher than the monomeric acrylic acid which has a pKa of 4.87. Consequently, at pH 3.15 the grafted poly(acrylic acid) chains should be completely protonated while at pH 6.05 some deprotonation is likely. At pH 7.25 the degree of deprotonation will be much greater. Since each poly(acrylic acid) chain contains multiple carboxylic F



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larger sucrose molecule. On the other hand, the smaller glucose molecule is less likely to affect the passage of sucrose. Since Figures 3 and 5 indicate that for modified membranes the dependence of glucose and sucrose rejection on pH is opposite, membrane selectivity for sucrose in a glucose/sucrose mixture should also depend on feed pH. Figure 6 does indicate that sucrose selectivity depends on feed pH for modified membranes, and the effect increases with increasing grafting degree. Sugar selectivity was calculated at 20 min intervals for the corresponding values using eq 3 for solutions containing a mixture of 10 mM glucose and 10 mM sucrose. The average selectivities over the entire 120 min experiment period are presented in Table 1. Average selectivities over 120 min Table 1. Sucrose Selectivity for 20 mM and 40 mM Total Concentration Sugar Mixturesa pH 3.15 control 1:15 2:10 2:15

0.62 0.72 0.65 0.87

± ± ± ±

control 1:15 2:10 2:15

0.68 0.82 0.78 0.93

± ± ± ±

pH 6.05

20 mM, 3.1 bar 0.02 0.62 ± 0.05 0.01 1.36 ± 0.10 0.02 2.66 ± 0.13 0.01 2.31 ± 0.22 40 mM, 3.1 bar 0.03 0.69 ± 0.04 0.07 1.51 ± 0.16 0.08 2.93 ± 0.17 0.10 2.57 ± 0.28

pH 7.25 0.77 1.40 2.76 2.35

± ± ± ±

0.03 0.08 0.34 0.19

0.83 1.66 3.01 2.64

± ± ± ±

0.05 0.12 0.40 0.24

a

Selectivity greater than 1 corresponds to a higher rejection of glucose than sucrose. Results are given for control (unmodified) and modified membranes.

filtration time for feed streams containing 40 mM total sugar concentration (50:50 sucrose: glucose) are also given in Table 1. The statistical errors are also provided. Table 1 indicates that selectivity increases with the increasing pH of the feed solution. At higher pH, selectivity greater than 1 can be obtained indicating glucose is rejected preferentially over sucrose even though the molecular weight of glucose is about half that of sucrose. At 40 mM total sugar concentration selectivities are higher than 20 mM total sugar concentration in agreement with the observation that the presence of sucrose leads to greater glucose rejection. For modified membranes, selectivity generally increases with increasing grafting degree with a maximum observed for the 2:10 modified membranes. This is probably because some fluctuations for glucose rejection at the three membrane modification conditions were observed. However, sucrose rejection decreases with increasing grafting degree. 3.4. Computational Results and the Mechanisms Underlying pH Dependence of Selectivity. Complementary classical MD simulations were conducted for the sugar molecules interacting with protonated and deprotonated AA trimers in aqueous solutions mimicking the lower and higher experimental pH conditions, respectively. Figures 7 (A−D) give electrostatic and van der Waals interaction energies between the sugar molecules and the AA trimers during the course of 40 ns MD simulations. Our results show that both glucose and sucrose have relatively weak interactions with the protonated neutral AA trimer. The sporadic stronger interactions occur when close contact between the sugar molecule and the trimer via hydrogen

Figure 6. Rejection for feed streams containing equal amounts of glucose and sucrose (20 mM total) in (A) 3.15, (B) 6.05, and (C) 7.25 pH buffers. Open symbols are glucose, filled symbols are sucrose. Results are given for 1:15 (○, ●), 2:10 (Δ, ▲), and 2:15 (□, ■) modified membranes, as well as unmodified membranes (▽,▼). Data points are sized to contain variance between experiments.

Comparing Figures 3 and 6 it can be seen that glucose rejection for the unmodified membrane is slightly higher for the mixed glucose/sucrose feed compared to the feed stream containing only glucose. Comparing Figures 5 and 6 it can be seen that sucrose rejections in the mixed feed and sucrose only feed stream are the same. Aydoğan et al.20 also observed that glucose rejection is increased in the presence of low concentrations of sucrose. This slight increase in the glucose rejection in the glucose/sucrose mixture is likely due to the partial blockage of the free volume for glucose transport by the G



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Figure 7. Electrostatic and van der Waals contributions to the interaction energies between glucose or sucrose and neutral or negatively charged PAA trimers. Energy contribution from hydrogen bonding interaction is included in the electrostatic interaction energies. (A) glucose-neutral AA trimer, (B) sucrose-neutral AA trimer, (C) glucose-negatively charged AA trimer, (D) sucrose-negatively charged AA trimer.

Figure 8. Total number of water molecules associated with the first hydration shells of glucose and sucrose. Larger decreases occur because of hydrogen bonding interactions. (A) glucose-neutral AA trimer, (B) sucrose-neutral AA trimer, (C) glucose-negatively charged AA trimer, (D) sucrose-negatively charged AA trimer.

bonding is established. Hydrogen bonding interaction energy manifests itself mainly through electrostatic interaction energy in classical MD simulations. During the entire simulation period, only one instance of hydrogen bonding interaction was observed between sucrose and the protonated AA trimer at around 27 ns and a lifetime of less than several hundred picoseconds (ps). For glucose interaction with the protonated trimer, stronger and longer interactions were found. Hydrogen

bonding interactions occurred at least twice at around 15 and 30−35 ns simulation times and much longer lifetimes. The interactions between the sugar molecules and the neutral or charged trimers could also be observed by the total number of water molecules in the first hydration shell of sugar− AA trimer complexes as shown in Figure 8 (A−D). When a hydrogen bonding interaction occurs, the number of water molecules associated with the complex drops significantly. In the case of sucrose interacting with the neutral AA trimer, the H



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Figure 9. Snapshots of the complex structures formed between glucose, sucrose, neutral and negatively charged AA trimer. (A) glucose-neutral AA trimer, (B) sucrose-neutral AA trimer, (C) glucose-negatively charged AA trimer, (D) sucrose-negatively charged AA trimer.

number of hydration water molecules decreases from an average 75 to as low as 55. In the case of glucose interacting with the protonated AA trimer, the number of hydration water molecules decreases from average 60 to as low as 40. The drop in hydration water number corresponds to the stronger interaction occurring as seen in Figure 7 (A,B) for both glucose and sucrose. Some of the conformational structures of the complexes formed are given in Figure 9 (A,B). These results indicate that sucrose has a very weak interaction with the neutral AA trimer while glucose appears to have a stronger hydrogen bonding interaction with the neutral AA trimer. Thus, glucose will preferentially associate with the grafted nanostructure leading to a higher glucose concentration at the membrane surface. This in turn could explain the lower glucose rejection for modified membranes compared to the unmodified membrane at pH 3.15. Figure 7 (C,D) shows both the electrostatic and the van der Waals interaction energies for glucose and sucrose with negatively charged AA trimer. For glucose, the interaction energy appears to be slightly stronger with the negatively charged trimer than with the neutral AA trimer; however, sucrose interacts much more frequently and more strongly with the negatively charged AA trimer than with the neutral trimer. The interaction of sucrose is also much stronger than the corresponding glucose interaction. Figure 8 (C,D) indicates that for sucrose, strong and frequent hydrogen bond interaction with the charged trimer leads to more frequent drops in the number of water molecules in the first hydration shell compared to glucose. Figure 9 (C,D) gives the corresponding hydrogen bonded structures formed between the negatively charged AA trimer, glucose, and sucrose. The simulation results indicate that sucrose interacts more strongly with the deprotonated, negatively charged form of grafted poly(acrylic acid) nanostructure leading to a higher

sucrose concentration at the membrane surface at higher pH. This will lead to greater passage of sucrose compared to glucose at higher pH values, as observed experimentally (Figure 6). Figure 1 indicates that the membrane surface charge becomes more negative at higher pH values and at higher grafting degrees. At higher pH, the stronger interaction between sucrose and the negatively charged AA trimer compared to the corresponding glucose-trimer interaction leads to lower rejection of sucrose than glucose. This appears to change the membrane selectivity for the two sugars. The reason why glucose and sucrose molecules interact differently with the protonated and deprotonated PAA trimers is not entirely clear from our current simulations. Sucrose has a higher solubility in water and is more strongly solvated by the water molecules than glucose leading to its weaker interaction with the neutral trimer. However, when the polymer chain is negatively charged which promotes stronger hydrogen bonding interaction with the sugar molecules, sucrose is able to dehydrate partially and form strong hydrogen bonds with the polymer chains. Subsequently the stronger van der Waals interaction between the larger sucrose molecule and the polymer chains leads to a stronger interaction between the sucrose molecule and the negatively charged trimers. Our results highlight the importance of membrane surface properties and the interactions between membrane, solvent, and solute during nanofiltration. Here low-pressure nanofiltration experiments were conducted to exploit surface interactions between the grafted nanolayer, glucose, and sucrose in solution. Our results indicate the importance of optimizing membrane surface chemistry when developing advanced membranes. I



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(2) van der Bruggen, B.; Daems, B.; Wilms, D.; Vandecasteele, C. Mechanisms of retention and flux decline for the nanofiltration of dye baths from the textile industry. Sep. Purif. Technol. 2001, 22−23, 519− 528. (3) Nyström, M.; Nuortila-Jokinen, J. M. K.; Mänttäri, M. J. Nanofiltration in the pulp and paper industry. In Nanofiltration − Principles and Applications; Schäfer, A. I., Fane, A. G., Waite, T. D., Eds.; Elsevier: Oxford, U.K., 2005. (4) Bargeman, G.; Timmer, M.; van der Horst, C. Nanofiltration in the food industry. In Nanofiltration − Principles and Applications; Schäfer, A. I., Fane, A. G., Waite, T. D., Eds.; Elsevier: Oxford, U.K., 2005. (5) Vincze, I.; Stefanovits-Bányai, E.; Vatai, G. Using nanofiltration and reverse osmosis for the concentration of seabuckthorn (Hippophae rhamnoides L.) juice. Desalination 2006, 200, 528−530. (6) Atra, R.; Vatai, G.; Bekassy-Molnar, E.; Balint, A. Investigation of ultra- and nanofiltration for utilization of whey protein and lactose. J. Food Eng. 2005, 67, 325−332. (7) Vourch, M.; Belannec, B.; Chaufer, B.; Dorange, G. Nanofiltration and reverse osmosis of model process waters from the dairy industry to produce water for reuse. Desalination 2005, 172, 245−256. (8) Himstedt, H. H.; Hestekin, J. A. Membrane Separation Techniques in the Dairy Industry. In Modern Applications in Membrane Science and Technology; Escobar, E., van der Bruggen, B., Eds.; ACS Symposium Series, American Chemical Society: Washington, D.C., 2011. (9) García-Martín, N.; Perez-Magariño, S.; Ortega-Heras, M.; González-Huerta, C.; Mihnea, M.; González-Sanjosé, M. L.; Palacio, L.; Prádanos, P.; Hernández, A. Sugar reduction in white and red musts with nanofiltration membranes. Desal. Water Treat. 2011, 27, 167−174. (10) García-Martín, N.; Perez-Magariño, S.; Ortega-Heras, M.; González-Huerta, C.; Mihnea, M.; González-Sanjosé, M. L.; Palacio, L.; Prádanos, P.; Hernández, A. Sugar reduction in musts with nanofiltration membranes to obtain low alcohol-content wines. Sep. Purif. Technol. 2010, 76, 158−170. (11) Li, W.; Li, J.; Chen, T.; Chen, C. Study on nanofiltration for purifying fructo-oligosaccharides: I. Operation modes. J. Membr. Sci. 2004, 245, 123−129. (12) Bargeman, G.; Vollenbroek, J. M.; Straatsma, J.; Schroën, C. G. P. H.; Boom, R. M. Nanofiltration of multi-component feeds. Interactions between neutral and charged components and their effect on retention. J. Membr. Sci. 2005, 247, 11−20. (13) Wang, X.; Zhang, C.; Ouyang, P. The possibility of separating saccharides from a NaCl solution by using nanofiltration in diafiltration mode. J. Membr. Sci. 2002, 204, 271−281. (14) Kimura, S.; Sourirajan, S. Transport characteristics of porous cellulose acetate membranes for reverse osmosis separation of sucrose in aqueous solutions. Ind. Eng. Chem. Proc. 1968, 7, 548−554. (15) Rodrigues, C.; Cavaco Morão, A. I.; de Pinho, M. N.; Geraldes, V. On the prediction of permeate flux for nanofiltration of concentrated aqueous solutions with thin-film composite polyamide membranes. J. Membr. Sci. 2010, 346, 1−7. (16) Nabetani, H.; Nakajima, M.; Watanabe, A.; Nakao, S.; Kimura, S. Prediction of the flux for the reverse osmosis of a solution containing sucrose and glucose. J. Chem. Eng. Jpn. 1992, 25, 575−580. (17) Botelho-Cunha, V. A.; Mateus, M.; Petrus, J. C. C.; de Pinho, M. N. Tailoring the enzymatic synthesis and nanofiltration fractionation of galacto-oligosaccharides. Biochem. Eng. J. 2010, 50, 29−36. (18) Goulas, A. K.; Grandison, A. S.; Rastall, R. A. Fractionation of oligosaccharides by nanofiltration. J. Sci. Food Agric. 2003, 83, 675− 680. (19) Nyström, M.; Butylina, S.; Platt, S. NF retention and critical flux of small hydrophilic/hydrophobic molecules. Membr. Technol. 2004, Oct, 5−8. (20) Aydoğan, N.; Gürkan, T.; Yilmaz, L. Effect of operating parameters on the separation of sugars by nanofiltration. Sep. Sci. Technol. 1998, 33, 1767−1785.

4. CONCLUSIONS Polyacrylic acid chains were grafted from the surface of commercially available nanofiltration membranes using UV initiated free radical polymerization. At higher pH values the grafted chains change their conformation and swell because of deprotonation of carboxylic groups present. This leads to a decrease in permeate flux, which was pH dependent. The flux is higher when the grafted polymer chains are neutral at pH values below the first pKa of the grafted chains. Below the first pKa the chains are neutrally charged and adopt a more compact conformation. The switch between more swollen and collapsed conformations is reversible. Rejection of two model sugars, glucose and sucrose was investigated. Though the molecular weight of glucose is about half that of sucrose, rejection of sucrose is less than that of glucose at pH values above the pKa when the grafted poly(acrylic acid) chains are deprotonated, negatively charged, and swollen. In addition, rejection of glucose at lower pH values, when the grafted polyacrylic chains are protonated and neutrally charged, is less than the unmodified membrane. Classical MD simulations indicate that hydrogen bonding interaction between the sugar molecules and the grafted polyacrylic chains in the neutral and negatively charged states is drastically different. Glucose interacts more frequently than sucrose with the neutral poly(acrylic acid) chains while the reverse is true for the negatively charged chains. This in turn leads to a higher average glucose concentration at the membrane surface at low pH values but higher sucrose concentrations at higher pH values, explaining the experimentally observed changes in rejection and selectivity with pH. Our work demonstrates the importance of the specific interactions between the solute species and the membrane materials. It is possible to tailor the membrane surface properties or the separation conditions so that a stronger interaction with one of an otherwise similar group of molecule species occurs, resulting in selective separation of the species.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 479-575-8401. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding was provided by a United States Department of Defense, National Defense Science and Engineering Graduate (NDSEG) Fellowship, and the United States National Science Foundation (CBET 0651646). Some of the filtration experiments were performed by Ms. Sarah Williams at Colorado State University.

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ABBREVIATIONS pAA = poly(acrylic acid) ; UV = ultraviolet; NF = nanofiltration; 1:15, 2:10, 2:15 = shorthand for modification conditions. The first number (1 or 2) represents the weight % of monomer in the reaction solution. The second number (10 or 15) represents the length of UV exposure in minutes

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REFERENCES

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Characterization of polymeric nanofiltration membranes for systematic analysis of membrane performance. J. Membr. Sci. 2006, 278, 418−427.

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