Spectroscopic Characterization of Sulfonate Charge Density in Ion

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Spectroscopic Characterization of Sulfonate Charge Density in Ion-Containing Polymers Sarah B. Smedley, Tawanda James Zimudzi, Ying Chang, Chulsung Bae, and Michael A. Hickner J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06904 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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The Journal of Physical Chemistry

Spectroscopic Characterization of Sulfonate Charge Density in Ion-Containing Polymers Sarah B. Smedley1†, Tawanda J. Zimudzi1†, Ying Chang2, Chulsung Bae2, and Michael A. Hickner1* 1

Department of Material Science and Engineering, The Pennsylvania State University,

University Park, PA 16802, United States. 2

Department of Chemistry and Chemical Biology, New York State Center for Polymer

Synthesis, Rensselaer Polytechnic Institute, NY, 12180, United States. †

These authors contributed equally to this work

Corresponding Author *email: [email protected] Tel: (814) 867-1847

ACS Paragon Plus Environment

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ABSTRACT

The charge density and hydrogen bonding with water of five different polymer membranes functionalized with various sulfonate sidechain chemistries were investigated using FTIR techniques and DFT calculations. The peak position of the OD stretch of dilute HOD absorbed into the sulfonated poly(sulfone) membranes was studied using FTIR to compare the charge density of the sulfonate headgroup across the different samples which can ultimately be related to the acidity of the proton-form sulfonate moieties. The OD peak was deconvoluted to determine the percentage of headgroup-associated, intermediate, and bulk water. DFT modeling was used to calculate the charge density of each headgroup and visualize how the chemistry of the headgroup influenced the conformation of the side chain tether. FTIR-determined OD peak positions and charge density calculations demonstrated that a perflurosulfonate containing a thioether linkage produces the most acidic sulfonate headgroup. However, the amount of headgroup-associated water calculated for this sidechain was low due to the unique cis conformation of the thioether sidechain. The bi-perfluorosulfonate sidechain had very low calculated headgroup-associated water due its bulkiness and water molecules bridging the two sulfonates. These detailed insights on local hydration of sulfonate side chains will point the direction towards new functional headgroup designs.

INTRODUCTION Early studies of the vibrational spectra of electrolyte solutions revealed that the hydrogen bonding network of water was disrupted when ionized species were introduced into solution resulting in a hydrogen bond between water and the anion.1,2 By studying the vibrational


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frequency, v(OD), of isotopically dilute HOD in H2O, researchers were able to determine trends for various anions based on their size, charge and electronegativity. For instance, Giammanco, et al. studied the OD stretch frequency of dilute HOD in H2O/salt solutions using IR pump–probe spectroscopy and 2D IR vibrational echo spectroscopy and showed that replacing either ion of the salt pair causes a shift in the OD stretch frequency compared to the OD frequency in pure bulk water.3 They also demonstrated that the magnitude of the shift increased with larger, more polarizable anions or smaller, high charge density cations.3 Those intrinsic characteristics of ionic radius, charge, and electronegativity correlated to physical quantities which are commonly used to characterize hydrogen bonds, such as the frequency of the OD stretch and hydrogen bond distance measured by neutron or X-ray refraction.4,5 These studies concluded that there is a correlation between the OD frequency in dilute HOD and the length of the O-D···X- hydrogen bond. This observation has been confirmed by Bakker who found that as the electronegativity of the anion decreased, the OD frequency was more blueshifted,6 where NaI produced the largest blueshift of the sodium halide series compared to bulk water. This trend can also be described in terms of acidity, where the I- anion is considered the weakest conjugate base founded on its low electronegativity which is supported by HI being an extremely strong acid with an estimated pKa of -10 in water.7 The sulfonate anions in ion-containing polymers, such as NAFION® have been studied using vibrational spectroscopy to understand how water hydrogen bonding to the anionic sulfonate headgroup influences the mechanism of cation transport through the membrane.8–10

When

NAFION® is sufficiently hydrated with dilute HOD, which has a characteristic peak OD stretching frequency of 2509 cm-1, the peak is significantly blueshifted to ~2588 cm-1 indicating that the hydrogen bonding network is perturbed and that the bond between water and the anion is


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weak.8 The sulfonate anion in NAFION® is considered a superacid, by definition, as its pKa value is more negative than that of pure sulfuric acid.11 The increased acidity results from the perfluorinated tether of the sulfonate group; the fluorine atoms are highly electronegative and inductively withdraw electron density away from the sulfonate group decreasing its overall charge density. Moilanen, et al. were also able to characterize the water-polymer interactions by implementing a peak-fitting routine to extract hydrogen bonding population data from the OD peak. They were able to determine percentages of the total membrane sorbed water that were experiencing hydrogen bonds to other water molecules (bulk water) or hydrogen bonds to the sulfonate anion. Computational studies have revealed the differences in acidity or, more specifically, differences in proton dissociation between para-toluene sulfonic acid and triflic acid, and how the two moieties interacted with water. Paddison calculated the total charge on the oxygen atoms of each sulfonate group as it was exposed to an increasing number of water molecules and found that triflic acid had an overall lower sulfonate charge and as a result experienced a longer and therefore weaker hydrogen bond to water.12 The fluorine atoms of the triflic acid stabilized the excess negative charge better than the aromatic ring of para-toluene sulfonic acid. We were able to confirm these results in polymeric materials using FTIR measurements, and found that when two different sulfonated poly(styrene) films were hydrated with HOD, the sample containing a perfluorosulfonate displayed an OD peak frequency that was blueshifted by ~40 cm-1 compared to its aromatic sulfonate analog.13 It has been established by many methods that perfluorinated sulfonate groups are more acidic than the arylsulfonate moieties and that perfluorinated structures tend to lead to high conductivity proton exchange membranes. However, there has been less work on new perfluroinated sulfonate tether structures beyond simple perfluorinated


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linear structures. This new work greatly expands our understanding of how different tethered structures interact with water and fills in fundamental gaps on how a number of perfluorinated sulfonate tethers may enhance the properties of proton-conducting polymers.

EXPERIMENTAL SECTION In this study, we investigated novel sulfonate tether structures to better understand the waterpolymer interactions in a range of ion-containing polymer materials by using both FTIR and first principal computations. Thin membranes of sulfonated poly(sulfone) (PSU), Figure 1, were prepared and studied in manner that has been described and discussed previously.13 Films of sulfonated poly(sulfone), approximately15 µm thick, in sodium form were cast from dimethyl sulfoxide onto CaF2 windows, dried at 50 °C for 4 h, then at 80 °C for 6 h and placed in an FTIR transmission flow cell (Model 64100-F, New Era Enterprises, Vineland, NJ). Humid air containing 5 mol % D2O was flowed (20 std. cm3 s-1) through the cell while spectra were recorded every 5 minutes using a Bruker (Billerica, MA) VERTEX 70 spectrometer with a nitrogen-cooled wide band mercury-cadmium-telluride (MCT) detector. The use of a thin-film transmission cell caused some potential fringing of the IR spectrum which was observed at low absorbance. Wedge-cut gaskets and roughened film surfaces were used to avoid the parallel interfaces that tended to cause fringing in the transmission experiments. Humidification of the flowing air through the FTIR cell was achieved by mixing of fullyhumidified and dry air streams. Air at dewpoint was produced by a water sparging system, and was then mixed with a dry air stream at controlled mass flow ratios to yield a range of relative humidities. The relative humidity of the mixed stream was measured using an RH probe (Omega HX15-W, Omega Engineering, Inc., Stamford, CT) before being introduced to the measurement


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cell at the same temperature as the system. Each RH corresponded to a different hydration number (λ = mole of water/mole of sulfonate group) depending on the polymer sample. Hydration numbers were measured using a TA Instruments Q5000SA water vapor sorption analyzer and reported as a function of RH. Each spectrum was recorded at 2 cm-1 resolution and 100 scans. The hydrated spectra were obtained by using the dry polymer as a reference. Water vapor peaks occur above 3400 cm-1 and below 1900 cm-1 and as a result do not interfere with the OD stretch which occurs between 2700 cm-1 and 2400 cm-1. Spectra were extracted from 2700 cm-1 and 2400 cm-1 and baselined by setting the absorbance equal to 0 at those two points. Peak fitting was performed using Origin 8.0 (OriginLabs, Northhampton, MA) data analysis software. Three Gaussian peaks were used to fit the OD region from 2700 cm-1 to 2400 cm-1. One peak corresponding to bulk-like water was held constant for all samples and was centered at 2509 cm-1 with a constrained FWHM of 170 cm-1 (signature of HOD in bulk water), while the headgroup-associated and intermediate water peaks varied by sample. The peak positions and FWHM for headgroup-associated and intermediate water were determined by fitting the lowest RH samples with three peaks, the third peak being bulk water with peak position of 2509 cm-1 and FWHM of 170 cm-1. The peak shape (peak position and FWHM) of the head-group associated water was held constant throughout the remainder of the fitting at each RH. The intermediate peak position was held constant but the FWHM was allowed to vary. Once fit, the areas were corrected for non-Condon effects, see Supporting Information. Charge densities were calculated using Materials Studio (Dassault Systemes, BIOVIA Corp., San Diego, CA) to run DMol3 DFT calculations. Full geometry optimizations were performed using the general gradient approximation (GGA) with a PBE functional set. Electrostatic


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potential derived atomic charges were obtained by using the B3LYP method with a DNP basis set approximation (GGA) with a PBE functional set.

Figure 1. Repeat unit of the poly(sulfone) (PSU) backbone where R represents various tether structures. RESULTS AND DISCUSSION The deconvolution of the OD stretch peak of PSU-S4 is shown in Figure 2. Deconvolutions for the remaining samples can be found in Figure S1. The curve-fitting routine employed a threestate weighted-sum model where we consider water to be present in three different microenvironments within the polymer membrane. The first of which is a bulk-like state, where water is hydrogen bonded to other water molecules and has the same FTIR characteristics as


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bulk water. The vibrational parameters for this microenviroment are known; with a peak position of 2509 cm-1 and FWHM of 170 cm-1 with a peak shape that can be approximated as Gaussian.9 The second microenvironment is termed intermediate water, which encompasses water that is not directly interacting with the charged headgroup, but is still affected by its electric field. This lineshape is not known and is difficult to measure experimentally since there are likely a distribution of continuum states in this population.14 Lastly, water directly interacting with the charged headgroup, referred to as headgroup-associated water, will have unique vibrational properties depending on how the headgroup behaves as a conjugate base, here characterized by the charge density of the sodium salt form of the sulfonate.13 The peak position and FWHM of the intermediate and headgroup-associated water are shown in Table 1. These population peak parameters were then held constant, except the FWHM of the intermediate peak which was allowed to vary, for fitting of the higher hydration experiments on each sample. Table 1. Headgroup and intermediate water peak positions and FWHM determined by deconvolution of samples at 30% RH.

Sidechain

head-group peak position (cm-1)

FWHM (cm-1)

intermediate peak position (cm-1)

FWHM (cm-1)

S1 S2 S4 S5 S6

2617 2582 2621 2580 2601

79 114 54 123 57

2537 2520 2558 2515 2557

73 131 109 147 61

Deconvolution of the OD peak allows us to extract the populations of water in these three microenvironments to understand how the chemical structure of the tether affects the hydrogen bonding network of the water absorbed in the material.

However, for the population


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distributions to be accurate, the area of the peaks were corrected for the strong non-Condon effects in water as described in detail previously, see Supporting Information.13,15

0.024

PSU-S4 λ =5.27

0.09

PSU-S4 λ = 2.18 Absorbance

Intermediate

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.016

Bulk 0.008

Headgroup

0.000 2400

2500

2600

2700

0.06

0.03

0.00 2400

-1

Wavenumber (cm )

2500

2600

2700

Wavenumber (cm-1)

Figure 2. The OD stretch deconvolution PSU-S4 at a λ of 2.18 (Left) and 5.27(Right) discloses the population of water in different microenvironments. The charge density of the sulfonate headgroup can be characterized by the vibrational frequency of water that is directly hydrogen-bound to the oxygen atoms, in this case, the peak frequency of the headgroup-associated water. Table 2 shows the peak OD frequency of headgroup-associated water for each polymer sample. The samples are listed in order of increasing charge density according to the peak frequency, where the S4 sidechain has the lowest charge density and the S5 sidechain has the highest charge density. A higher peak frequency indicates water experiencing a weaker hydrogen bond to the headgroup. The S4 and S1 sidechains have lower charge density than the other samples due to their perfluorinated character. Paddison has shown that the perfluoromethylene groups are able to stabilize the negative charge of the sulfonate moiety better than an aromatic ring, like that of the S2 sidechain, causing the


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sulfonate group to have a decreased electron density and therefore behave as a weak conjugate base.12 The difference in charge density between S4 and S1 can be explained based on the differences between thioether and ether linkages. It is has been shown that sulfur’s ability to expand its octet can increase the acidity of an α-hydrogen compared to when an oxygen takes the same position.16,17 This effect can be observed in S4 where it displays a slightly higher headgroup-associated peak OD stretching frequency compared to S1, due to the ability of sulfur expand its octet and therefore more readily accept additional electron density relative to oxygen. The trend of charge density was confirmed by computing the partial charge of the sulfonate group using DMol3 DFT calculations.18,19 Table 1 lists the calculated total charge on the oxygen atoms of each tethered sulfonate at both high and low hydration levels. The charge density is directly related to the conjugate base character of the sulfonate group. When the negative charge is most concentrated on the oxygen atoms, a proton will interact more strongly with the sulfonate moiety therefore causing the sulfonate group to behave as a strong conjugate base. However, if the negative charge is less localized on the headgroup, the sulfonate will behave a weak conjugate base and consequently a stronger acid. At low hydration levels, S5 is the only tether that does not follow the charge density trend found from the experimental measurements, however at high hydration levels all calculations correlate to the charge density determined by FTIR. Interestingly, the fluorinated S5 tether does not display the same superacid characteristics as S1 and S4. This effect of a single perfluoromethylene tether compared to the addition of two or more perfluorinated carbons has been shown previously in calculation of the proton affinity of sulfonates with varying amounts of fluorination. It was found that perfluoroalkyl sulfonates experienced a large increase in proton affinity upon removal of a -CF2 group.20


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Table 2. Headgroup peak position determined by FTIR and the total charge of the oxygen atoms on the sulfonate headgroup determined computationally.

Sample

Measured headgroupassociated peak frequency (cm-1)

λ

Calculated total charge on oxygen atoms at λ= 1

Λ

Calculated total charge on oxygen atoms at λ= 5

PSU-S4

2621

1

-1.411

5

-1.305

PSU-S1

2617

1

-1.509

5

-1.505

PSU-S6

2601

1

-1.558*

5

-

PSU-S2

2582

1

-1.608

5

-1.664

PSU-S5

2580

1

-1.526

5

-1.698

*

Average of both sulfonate headgroups

Table 3 displays the population distributions of water in each of the three different microenvironments at both high and low hydration levels. These populations were obtained from the deconvoluted peak area of the dipole corrected OD stretch. Population distributions give insight into how the hydrogen bonding network is changing within the membrane as the material is hydrated. We have previously shown that strong acids will have a higher population of headgroup-associate water because of their greater dissociation and stronger ionic character compared to weaker acids.13,21,22 In this set of membranes, we observe a similar trend, but S4 and S6 have a very small population of water interacting with the headgroup at both high and low hydration levels. This effect can be explained based on the conformations of the tethers obtained from DFT calculations. The thioether bond in S4 causes the tether to take on a bent conformation where the sulfonate group is forced to be closer to the aromatic ring and therefore closer to the polymer backbone, displayed in Figure 3c and d. This bent or cis conformation causes the


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sulfonate to be crowded and therefore water is not able to interact with the oxygen atoms as readily as in the case of an extended side chain. Table 3. Deconvoluted peak positions and percentages of water in different microenvironments at low hydration and high hydrations.

Sample

λ

percent headgroupassociated water

percent intermediate water

percent bulk water

PSU-S4

2.2

20

49

31

PSU-S1

1.9

56

31

13

PSU-S6

2.1

39

29

32

PSU-S2

2.5

48

52

0

PSU-S5

1.8

56

44

0

PSU-S4

5.3

7

67

26

PSU-S1

5.3

33

67

0

PSU-S6

5.5

12

73

15

PSU-S2

5.0

49

45

6

PSU-S5

5.5

63

11

26

The S1 tether takes on a conformation that is linear, allowing the sulfonate to interact with water molecules from all directions. The dihedral angle in dimethyl sulfide (98.5°)23 is smaller compared to dimethyl ether (112.2°)24 which can be explained due to the increased size of the 3p orbitals on the sulfur atom compared to the 2p orbitals of the oxygen atom. The bond angle differences between these moieties causes drastic differences in water binding between S1 and S4 tethers. S4 can take on both cis and trans conformations, however, the calculated energies for the two states are equivalent.


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Figure 3. Fully optimized minimum energy structures of (A) S1 and (C) S4 with benzene in the plane and (B) S1 and (D) S4 with benzene out of the plane to show kinked structure of the tether. The differences in confirmation lead to differences in the amount of water than can associated with the sulfonate headgroup.

In the cis position, Figure 3c and d, the shortest distance from the ring to an oxygen atom of the head group is 4.855Å, whereas in the trans position, Figure 4a and b, this distance is 5.612 Å. The S1 tether has a lower energy in the trans conformation but, the fluorine groups will either orient perpendicular (Figure 3A and B) or parallel (Figure 4C and C) to the aromatic ring. In the parallel conformation, the shortest distance to an oxygen atom from the ring is 5.558 Å and in the perpendicular conformation this distance is 5.498 Å. The cis conformation of S4 causes the headgroup to be much closer to the backbone of the polymer, resulting in a more shielded


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headgroup and therefore reducing the population of headgroup-associated water determined from experimental OD peak deconvolution.

A)

B)

C)

D)

b

Figure 4. Fully optimized minimum energy structures of (a) S4 with benzene ring in the plane and (b) S4 with benzene out of the plane showing the trans conformation of the S4 tether. Fully optimized minimum energy structures of (c) S1 with aromatic ring in the plane and (d) S1 with aromatic ring out of the plane showing the alterative trans conformation of the S1 tether.

A similar argument can be made for the S6 tether, which has two sulfonate moieties in close proximity, shown in Figure 5. In one tether, there are two aromatic rings containing two perfluorosulfonate headgroups. Paddison investigated how the distance between two side chains in a short side-chain perfluorosulfonic acid membrane affected their conformation and protontransfer and found that as the distance (number of tetrafluoroethylene units in the backbone)


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Figure 5. Fully optimized minimum energy structure of the S6 tether. Two sulfonates per tether causes crowding where less water molecules can associate with the sulfonate headgroup.

increased so did the zero point energy of the system.25 It was also noted that cis conformation (sidechains on the same side of the backbone) was preferred to the trans conformation. Both results suggest that there is a source of long-range stabilization between the sulfonic acid groups that is lost as distance between the headgroups increases.25 With sufficiently short tetrafluorethylene segments between headgroups, two conformations were obtained, one where the sulfonate groups were separated by 11 Å and one where the sulfonates were at distance of 4.2 Å and formed two hydrogen bonds between the acid proton and the oxygen atoms of the adjacent headgroup.26 In our optimization of the S6 tether, we observed that the sulfur-sulfur bond distance is 6.448 Å, but there are no acidic protons in our model so there are no hydrogen bonds between the two sulfonate groups. When hydrated, one water molecule bridged the two sulfonate


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groups.

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Paddison reported similar results at short distances between headgroups.25,26 We

hypothesize that the small population of headgroup-associated water found experimentally is due to the short distance between the sulfonate groups which promotes the sharing of water molecules and does not allow water to fully access to the sulfonate group due to the distance between the headgroups and the bulkiness of the aromatic groups.

CONCLUSIONS We have examined five different ion-containing polymer membranes that contain various sulfonate tether structures both experimentally, using FTIR of dilute HOD, and computationally to determine how the chemical structure of the tether affects the acidity of the sulfonate headgroup.

Our findings suggest that a perfluorosulfonate containing a thioether linkage

produces the most acidic sulfonate based on the peak position of the headgroup-associated HOD peak and the overall charge density of the oxygen atoms of the headgroup that were calculated using DMol3 DFT modeling. Although the thioether S4 is the most acidic sulfonate group, our population distribution calculations showed that S4 did not display a large population of water interacting with the headgroup. We attributed the low hydration level of S4 to the unique low energy cis conformation of the thioether tether, causing the sulfonate headgroup to be more shielded from the water molecules by the backbone. A similar result was observed for the S6 tether, due to the close proximity of the sulfonate groups to one another and the overall bulkiness of the group, water molecules were not able to interact with the headgroup as readily as in the case of extended, isolated tethers. The conductivities of these samples were reported in Ref 27. While the IR experiments performed in this work on sodium-form materials do not provide direct observations of proton


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mobility or conductivity, some general connections can be drawn. The S6-type membrane had the highest conductivity of the samples in Ref. 27, consistent with its unusual structure and the high measured bulk water content of that sample. Surprisingly, the conductivity of S4 was second to that of S6. The unique kinked structure of the S4 thioether tether and its high bulk water content are key to understanding possible fundamental factors that lead to the high conductivity. While a direct IR-conductivity connection cannot be made, the present study clearly sheds some light on what might be happening with this material, which was left an open question in the previous work. Finally, the conductivity of the S1 sample was lower than that of S4 and the conductivities of S2 and S5 followed. These samples follow the expected charge density relationships on the headgroup, similar to what Paddison observed for an aryl sulfonate and triflic acid. Thus, while direct conductivity insights were not gained from the present study, some of the underlying factors towards high-performance proton-conducting headgroups were elucidated by studying how water interacted with this series of samples.

ASSOCIATED CONTENT Supporting Information. Additional peak deconvolutions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Email: [email protected] Tel: (814) 867-1847


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS C.B. thanks the National Science Foundation (CAREER DMR-0747667) and Rensselaer Polytechnic Institute for their generous support. This work was also supported by awards to M.A.H. from the National Science Foundation, Grants CBET-0803137 and the DMREF program via Grant CHE-1534326, and the Office of Naval Research, Grant N00014-10-1-0875. Infrastructure support was also provided by The Pennsylvania State University Materials Research Institute and the Penn State Institutes of Energy & the Environment. M.A.H. acknowledges the Corning Foundation and the Corning Faculty Fellowship in Materials Science and Engineering for support.

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TOC Graphic


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Spectroscopic Characterization of Sulfonate Charge Density in Ion

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