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Phenolic Pollutant Uptake Properties of Molecular Templated Polymers Containing #-Cyclodextrin Michael K Danquah, Riak C Aruei, and Lee D. Wilson J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01819 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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The Journal of Physical Chemistry

Phenolic Pollutant Uptake Properties of Molecular Templated Polymers Containing β-Cyclodextrin

Michael K. Danquah, 1 Riak C. Aruei, 1 and Lee D. Wilson1* 1

University of Saskatchewan, Department of Chemistry, 110 Science Place, Thorvaldson

Building (Room 165), Saskatoon, Saskatchewan, S7N 5C9 Canada

*

Corresponding Author: L. D. Wilson, Email: [email protected]

Tel. +1-306-966-2961, Fax. +1-306-966-4730

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ABSTRACT: Templated (T) and non-templated (NT) cross-linked materials containing βcyclodextrin (β-CD) and epichlorohydrin (EPH) were prepared at variable β-CD: EPH ratios (1:15, 1:20, and 1:25) in the presence and absence of a molecular template (toluene). The structural characterization of the materials was carried out using spectroscopy (FT-IR, solids 13C NMR, and SEM) and thermogravimetric analysis (TGA). The adsorption properties were studied with phenol-based adsorbates (TNP; 2,4,6-trinitrophenol and PNP; p-nitrophenol) at equilibrium and dynamic conditions. The monolayer adsorption capacity (Qm) varied for the T-polymer/TNP systems (Qm = 0.10 to 0.95 mmol/g), and NT-polymer/TNP systems (Qm = 0.23 to 0.83 mmol/g). The range of Qm values for the T-polymer/ PNP systems (0.26 to 0.62 mmol/g) exceeded that of the NT-polymer/ PNP systems (0.23 to 0.40 mmol/g). The kinetic uptake profiles for the polymers and phenolphthalein (phth) are reliably described by the pseudo-first order (PFO) model. The β-CD inclusion site accessibility for the polymers varied from 15-20%, according to the level of cross-linking, where the accessibility of the T-polymers was greater than the NTpolymers. The structural characterization and phenol adsorption properties provide complementary support for the role of tunable polymer morphology in adsorption processes. The role of two-site binding is demonstrated for linear and globular polymer materials according to the unique adsorption properties with phenols of variable size and hydrophile-lipophile character.


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1. INTRODUCTION The fate and transport of phenolic species such as p-nitrophenol (PNP) provide insight on the fate and transport of waterborne contaminants due to their variable solubility profiles in aquatic environments. The build-up of such species in ground and surface water supplies relate to the uncontrolled release of industrial effluents from various sources. The widespread use of phenolic additives in textile dyes, explosives, agrochemicals, and pharmaceuticals represent an emerging concern to human and ecosystem health due to buildup in surface and groundwater supplies.

1–4

The utility of model systems such as PNP and trinitrophenol (TNP) relate to their variable solubility profile4,5 and mobility in aquatic environments, where the occurrence of such species4 has revealed the need to develop advanced materials for remediation of aquatic environments. Recent studies on sorbent materials indicate that favorable host-guest molecular interactions6–10 between CDs and phenols occur, especially in the case of CD polymer-based materials. More recently, studies focused on elucidation of the role of the inclusion and the interstitial binding site contributions in CD-based polymers have gained attention.11–13 The structure and adsorption properties of cross-linked materials containing β-CD are strongly influenced in systems where the role of β-CD inclusion sites varies incrementally due to their favorable binding affinity.14–18 Synthetic modification where the secondary structure of CD-based polymers can be controlled by the degree of branching and linear segments yields materials with notable differences in physicochemical properties such as solubility, hydrophile-lipophile balance (HLB), and inclusion site accessibility. CD-based polymers with tunable secondary structure have potentially diverse applications in catalysis, chemical separations, and adsorptive removal of organic and inorganic pollutants. 14,15,18–36, In particular, cross-linked polymers that contain βCD and EPH are of continued interest due to their unique host-guest chemistry, tunable



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physicochemical properties and facile synthesis for diverse applications in polymer science.8,23,25–27,29,31,37,38 CD-based polymers have utility as molecular sensors, molecular sieves for chemical separations, catalytic agents, and controlled-release systems.9,10 ,12,39–43 The ability to design CD polymers with variable structure and textural properties have widened the scope of adsorption-based applications due to the unique role of dual mode binding (CD inclusion and interstitial) sites for such materials.26 The synthetic modification of β-CD-EPH polymers depend on several parameters: reactant ratios (CD to EPH), temperature, pH, mixing rate/reagent addition, and reaction time16,17. Recent studies have shown that the rate of cross-linker addition (drop-wise versus rapid addition) affects the polymer morphology, as evidenced by the kinetics of polymerization.44

Molecular

templation 10,19,41 plays a key role on the accessibility of the sorption sites according to reports on molecular imprinted polymers (MIPs). Systematic structural studies indicate that MIPs may adopt unique secondary and tertiary structure according to the formation of linear versus globular polymers due to the role of molecular templates.18 Polymer branching in the case of dendrimer materials yields polymers with variable textural and sorption properties, as evidenced by the sorptive selectivity of pollutants

19,35,41,45

. The assumption that the CD cavity serves as the

primary binding site for CD-EPH polymers is restrictive since the role of interstitial domains must also be considered, in accordance with steric effects and other binding sites for such crosslinked materials.18 In this study, water insoluble linear and globular β-CD-EPH polymers were prepared by a molecular templation strategy18 to assess the role of polymer morphology and adsorption properties in aqueous solution.

In particular, the study of templated versus non-templated

polymers where toluene acted as a molecular template and the relative role of inclusion and


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interstitial sites on the adsorption properties of various phenols in aqueous solution was studied. The materials were prepared at variable CD-EPH mole ratios and structurally characterized by spectroscopy (SEM and IR/NMR) and thermal gravimetric analysis (TGA). The adsorption properties were studied at equilibrium and dynamic conditions using batch and one-pot kinetic methods.44 This study provides new insight on the role of structure and adsorption properties for synthetically modified CD-polymers with a unique linear and globular morphology obtained by a molecular templation strategy.

2.

EXPERIMENTAL SECTION 2.1 Materials. β-cyclodextrin (β-CD) (≥ 97%), epichlorohydrin (EPH) (≥ 99%), acetone,

toluene, p-nitrophenol (PNP), 2,4,6-trinitrophenol (TNP), sodium hydroxide, sodium bicarbonate, phenolphthalein (phth) were purchased from Sigma-Aldrich and used without purification unless specified otherwise. 2.2 Methods Synthesis of Templated Polymer (TCD-EPH). β-CD (2.00 g, 1.76 mmol) was added to 3.2mL 35% NaOH in a 25-mL round-bottom flask equipped with a magnetic stirrer bar with stirring for 20 min at 65 °C, followed by drop-wise addition of toluene (0.19-mL, 1.76 mmol) with vigorous stirring for 30 min. EPH (2.07-mL, 26.4 mmol) was added drop-wise to the reaction mixture and stirred for 1 h (cf. Scheme 1A). A white gel was formed after cooling for 1 h followed by precipitation with acetone (50-mL). The product was washed several times with Millipore water to remove unreacted components. The product was Soxhlet extracted with acetone for 24 h, followed by oven-drying at 50 °C for 12 h. The white solids were crushed and



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sieved through 40-mesh screen and stored in glass vials. The same procedure was repeated for the synthesis of TCD-EPH polymers with variable EPH ratios at 1:20 and 1:25. Synthesis of Non-Templated Polymer (NTCD-EPH). Non-templated (NT) polymers were prepared using a similar protocol as outlined above, except that toluene was omitted as a molecular template, as illustrated in Scheme 1B.

(A)

(B)

Scheme 1. Synthesis of templated (T) TCD-EPH polymer (A) and non-templated (NT) NTCDEPH polymer (B) in the presence and absence of toluene. Note that the degree of polymerization is arbitrary and toluene serves as the molecular template and functions as a noncovalent structure directing agent (SDA). Table 1. Synthetic Yield (%) and Naming System for Templated (T) and Non-Templated (NT) β-CD/EPH Polymers at Variable Molar Ratio According to Scheme 1 Polymer ID NTCD-EPH15 NTCD-EPH20 NTCD-EPH25

β-CD: EPH (mole ratio) 1: 15 1: 20 1: 25

Product Yield (%) 83 57 56 6

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TCD-EPH15 TCD-EPH20 TCD-EPH25

1: 15 1: 20 1: 25

92 78 63

2.3 Polymer Characterization. UV-vis spectrophotometric analysis of the phenol solutions was performed with a Cary 100 Varian spectrophotometer. The UV-vis absorbance was measured at 295 K over the range 300-500 nm. Absorbance measurements of TNP and PNP were recorded at the wavelength maxima (λmax) for each dye at 355 nm and 400 nm, respectively. Infrared (IR) spectroscopy was performed on Bio-Rad FTS-40 spectrophotometer to yield diffuse reflectance infrared Fourier transform (DRIFT) spectra. Samples (ca. 5 mg) were mixed with 10 mg of spectroscopic grade KBr and corrected relative to a background spectrum of KBr in reflectance mode. The spectral resolution was 4 cm−1 over the 400−4000 cm−1 region. The intensity of the IR spectra was normalized for each sample using spectra intensity of β-CD at 1020 cm-1. Solid-state 13C NMR spectra were obtained using a wide-bore (89 mm) 8.6 T Oxford superconducting magnet system equipped with a 4 mm CP-MAS (cross-polarization with magic angle spinning) solids probe. An Avance DRX360 console and workstation running Top Spin 1.3 (Bruker Bio Spin Corp.; Billerica, MA) was used to control the acquisition parameters using standard pulse programs. The polymers were packed in 4-mm-outer-diameter zirconium oxide rotors capped with Teflon MAS rotor caps. Acquisition was carried out with MAS at 8 kHz along with a 2 s recycle delay and 750 µs contact time for all experiments. Thermogravimetric analysis (TGA) curves were recorded with a Q50 TA Instrument analyzer with aluminum sample pans by heating to 30 °C and allowed to equilibrate for 5 min prior to heating at 58 °C min−1 up to 500 °C in a nitrogen atmosphere. Scanning Electron Microscopy (SEM) was used to map the surface morphology with a SEM (Model SU8000, HI-0867-0003) at the following instrumental



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conditions: acceleration voltage 5 kV, working distance (8.7 mm) and variable magnification (5k× to 30k×). 2.4 Sorption Analyses. PNP and TNP were used as model adsorbates under batch conditions. Stock solutions of the PNP and TNP were prepared in aqueous solution at pH 9 and 4.5 with 0.1 M NaHCO3 buffer and Millipore water, respectively. A fixed amount of the adsorbent (10 mg) was added to a fixed volume (7-mL) of dye solution at variable concentration (0.2-5 mM) and equilibrated in a horizontal shaker for 24 h. The mixture was centrifuged at 2000 rpm for 5 min and the supernatant sampled. The initial concentration (Co) of the adsorbate dye before and after sorption (Ce) was measured. The molar absorptivity of phenolphthalein (phth), PNP and TNP was calculated from the calibration curves to be; Ɛ = 28,423 Lmol-1cm-1 (pH = 10.5; λmax = 552 nm), Ɛ = 18,478 Lmol-1cm-1 (pH = 9; λmax = 400 nm), Ɛ= 14,880 Lmol-1cm-1(pH = 4.5; λmax = 355 nm), respectively, in agreement with other reports.4,5,21,46 The adsorbate uptake (Qe; mmolg-1) at equilibrium was determined by eq. 1, where m is the mass of the adsorbent and V is the volume of adsorbate solution.

Qe =

(Co − Ce )V m

………………………………………………………………………………………………………………………….. (1)

The equilibrium uptake was analyzed by the Sips isotherm (cf. eq. 2) where Qm (mmol/g) is the sorption capacity of a single adsorbed monolayer onto the adsorbent, Ks (mM-1) refers to the adsorption affinity constant at equilibrium, and ns is a heterogeneity constant.

Qm ( K s Ce ) ns Qe = ………………………………………………………………………………………………………………………… (2) 1 + ( K s Ce ) ns


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2.5 One Pot Sorption Kinetics. The sorption kinetics of the CD-EPH polymers were measured using a one pot method with phenolphthalein (phth). Briefly, 120 mg of polymer was added to a folded filter paper that was clipped at both ends, similar to a tea bag sachet configuration. The system was immersed in 120-mL solution ([phth]=33 µM buffered with 0.1 M NaHCO3 at pH 10.5) equipped with a teflon stirrer bar. 3-mL aliquots of solution were taken at variable time intervals during continuous stirring at 200 rpm. The time dependent isotherm data was fit using two kinetic models, a pseudo-first order (PFO; eq. 3) and a pseudo-second order (PSO; eq. 4) model. According to eq. 3 and 4, Qt and Qe are the uptake of phenol at time (t) and time intervals that approach pseudo-equilibrium conditions, respectively. The rate constants for the PFO and PSO models are k1 and k2, respectively.

Qt = Qe (1 − e − k1t )

…………………………………………………………………………………………………………………..

(3)

Qt =

Qe 2 k2t 1 + Qe k2t

………………………………………………………………………………………………………………………………

(4)

2.6 Accessibility of β-CD inclusion Sites. The determination of the accessibility of β-CD inclusion sites was adapted from a method reported by Mohamed et al.12 In brief, a stock phth solution in ethanol was used to prepare aqueous buffer solutions. The ethanol/water (0.04%; v/v) solution afforded greater solubility of phth where freshly made solutions were analyzed within 24 h to ensure that absorbance changes due to instability was minimized to minimize experimental error. An aqueous solution (7-mL) containing phth (~3.6 × 10-5 M) was added to each vial with variable dosage of polymer. The mixtures were equilibrated for 24 h and centrifuged. The optical absorbance of the supernatant was measured at λmax = 552 nm, where 9 ACS Paragon Plus Environment


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details for the calculation of the β-CD inclusion site accessibility was reported by Mohamed et al.12

3.

RESULTS AND DISCUSSION

The preparation of linear and globular water soluble CD-EPH polymers via a molecular templation approach with toluene was previously reported by Koopmans and Ritter,18 where the rheological and light scattering properties were studied. The present study differs in several ways since the aim of the present work was focused on the preparation of water insoluble polymers with variable morphology using the molecular templation approach above. The role of molecular templates on the structure and adsorption properties of such polymers is unreported to the best of our knowledge. To address the foregoing knowledge gap, several linear and globular CD: EPH polymers were prepared at variable composition (1:15, 1:20, and 1:25), as outlined in Scheme 1. As well, the polymer adsorption properties were studied in aqueous solution using a combination of batch equilibrium and a one pot kinetic method to study the adsorption isotherms of various phenolate (phenolphthalein, TNP and PNP) species. The synthetic yield for the polymers varied (57-92%) according to Table 1 and the structural characterization of the templated (T-) and nontemplated (NT-) polymers are further described in the following section.

3.1 FTIR Spectral Results. Figure 1 shows the IR spectra of NT- and T- polymers prepared at variable β-CD and EPH mole ratios. The peaks at 3600-3400 cm-1 (-O-H), 2900-2800 cm-1 (CH) and 1200–1000 cm-1 (C-O-C) vibrational bands are common to β-CD and its cross-linked forms. The C-O-C band at 1040 cm-1 corresponds to the new alkoxy bond formed via crosslinking of EPH which undergoes attenuation with greater EPH content. At greater cross-linking ratios the stoichiometric ratio, the hydroxyl groups of β-CD are either cross-linked or have


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limited reactivity due to steric effects. At levels beyond the stoichiometric ratio, EPH may undergo self-reaction to form EPH based polymers.16,26,41 By comparison, the T-polymers show different trend in the IR results, where C-O-C band appears broader with lower intensity. The use of toluene as a molecular template to yield T-polymers is related to the proper orientation of βCD that enforces hydrogen bonding that resembles the linear morphology adopted by the templated polymers (cf. Scheme 1A). Moreover, self-polymerization of EPH may account for the broad C-O-C peak. The IR band at 3600-3400 cm-1 corresponds to -OH groups on the polymer that undergo broadening due to cross-linking, agglomeration and hydrogen bonding effects.47 The IR band at 2900 cm-1 for β-CD shows splitting into a doublet for the cross-linked forms and relates to the asymmetric stretching vibration of CH and CH2 (2900 - 2800 cm-1). For both NTand T-polymers, a weak band is noted near 1600 cm-1 which becomes sharper for the NTpolymers as the EPH content increases. By contrast, an opposite trend occurs for the T-polymers as the EPH content increases, where the IR signature was assigned to adsorbed water.48 The FTIR spectra for the T- and NT-polymers reveal vibrational signatures at 3400 cm-1 (-O-H), 2900 cm-1 (C-H) and 1600 cm-1 (adsorbed water), in agreement with other reports.16,41,47,48

NTCD-EPH25

TCD-EPH25

NTCD-EPH20

TCD-EPH20

NTCD-EPH15

TCD-EPH15

β−CD β−

β−CD β−

4000 3500 3000 2500 2000 1500 1000

500

4000

3500

3000

2500

2000

1500

1000

500

wavenumber (cm-1)

wavenumber (cm-1)



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Figure 1. FTIR spectra of β-CD and cross-linked forms with different CD-EPH molar ratios (1:15, 1:20, and 1:25) for NT- and T- polymers, where the spectral intensity were normalized using the band at 1072 cm-1.

3.2 NMR Spectroscopy Results

Figure 2. 13C NMR CP-MAS spectra of the templated and non-templated polymer materials at variable β-CD-EPH mole ratios (1: 15, 1: 20, and 1:25). Figure 2 shows the

13

C NMR CP-MAS spectra of the T- and NT- polymers at increasing

EPH content (β-CD: EPH; 1: 15, 1: 20, and 1:25). The broad NMR signatures between 110 ppm to 60 ppm are in agreement with the trend in chemical shift values reported for the 13C NMR CP12 ACS Paragon Plus Environment

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MAS spectra of β-CD.18 However, broader spectral lines are observed with partial band overlap due to cross-linking of EPH at different carbon sites (C2-, C3-, and C6-) of β-CD (Figure 2). The 13

results are in Figure 2 agree with other reports on

C NMR studies for β-CD-EPH

polymers.25,26,41

3.3 TGA Results

B

A 100

3 o Deriv. Weight (%/ C)

80 Weight (%)

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


β−CD NTCD-EP15 NTCD-EP20 NTCD-EP25 TCD-EPH15 TCD-EPH20 TCD-EPH25

60

40

20

β−CD NTCD-EPH15 NTCD-EPH20 NTCD-EPH25 TCD-EPH15 TCD-EPH20 TCD-EPH25

2

1

0

0 0

100

200

300

400

500

0

100

200

300

400

500

o Temperature ( C)

o Temperature ( C)

Figure 3. (A) TGA profiles (weight loss vs temperature) and (B) DTA plots (weight loss/°C vs temperature) of TGA for β-CD, templated (TCD-EPH) and non-templated (NTCD-EPH) polymers with variable EPH content. The TGA profiles in Figure 3A-B provides further support that cross-linking occurs between β-CD and EPH. The thermal event around 75 – 100 °C is attributed to evaporative loss of adsorbed water on the hydrophilic exterior of the β-CD; whereas, the absence of water loss for the polymers relates to the method of preparation and extensive washing with methanol to remove residual water. The variable thermal stability for the T- and NT-polymer can be inferred from the high temperature thermal events, as follows: NTCD-EPH25 ˃ NTCD-EPH20 ˃ NTCDEPH15 ˃ TCD-EPH15 ˃ TCD-EPH20 ˃ TCD-EPH25. The observed trend indicates that NTpolymers have greater thermal stability over the T-polymers according to the molar ratios of 13 ACS Paragon Plus Environment


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EPH and the polymer morphology. The greater thermal stability of the globular polymers is inferred due to their less efficient packing and insulative effects due to particle voids versus more densely packed linear polymers, in accordance with molecular templation effects that likely contribute to the thermal stabilization effects of the materials.19

3.4 SEM For Β-CD and Templated (T) vs Non-Templated (NT) Β-CD-EPH Polymers The SEM images for β-CD and the polymers are shown in Figure 4A-E. The images reveal that the polymers possess variable morphology that differs markedly with native β-CD. The SEM result for β-CD shows a relatively smooth and regular surface topology while the cross-linked polymers (T- and NT-) have variable surface roughness and the presence of granular to fibril structures. NT-polymers show an increasingly dense and compact morphology as the cross-linker ratio increases (1:15 to 1:25). Overall, the SEM results for the polymers show limited pore structure that correspond with the low specific surface area (SSA) (< 5 m2/g) of such materials in the solid state, as reported by Pratt et al. (cf. Table II, in ref 41) and Yu et al.47 by nitrogen gas isotherm results. The SEM results for the T-polymers reveal a distinctive morphology as compared with the NT-polymers. The T-polymers show a greater abundance of linear fibril structures in Figure 4D and 4E; whereas the NT-polymers show greater evidence of granular structures with minor contributions due to fibrils, in agreement with the formation of globular polymer materials. T-polymers possess a linear morphology that favor hydrogen bonding between polymer units that contributes to fibril formation.10,49 The formation of granular structures is consistent with the globular morphology of NT-polymers and the tendency for such systems to undergo non-specific aggregation that differs from the fibril-like morphology of the T-polymers. The trends observed for the SEM results provide evidence that templation and non-


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templation leads to polymers with variable morphology, in agreement with the results reported by Koopmans and Ritter.18

Figure 4. SEM images of polymers; (A) SEM image of native β-CD, (B) SEM image of NTCDEPH15, (C) SEM image of NTCD-EPH25, (D) SEM image of TCD-EPH15, and (E) SEM image of TCD-EPH25 (acceleration voltage 5 kV, working distance (8.7 mm) and variable magnification (5k× to 30k×)). The yellow bar (inset) represents a 5 micron length scale.

3.5

Sorption of PNP and TNP at Equilibrium. The dye sorption method is a

versatile approach for assessing the structure and surface chemical properties of materials with variable morphology and texture.11,12 In the case of T- and NT-polymers, Koopmans and Ritter18 reported that toluene templation leads to the formation of linear and globular CD-EPH polymers (cf. Scheme 1). Based on the relationship of polymer morphology and textural properties, it can be inferred that the adsorption properties are dependent on the structure of the T- and NT-



polymer systems. Figure 5 shows the sorption isotherms of CD-EPH polymers with TNP and PNP systems at variable pH at 295 K.

0.7

0.5 0.4

NTCD-EPH15 NTCD-EPH20 NTCD-EPH25 TCD-EPH15 TCD-EPH20 TCD-EPH25 TNP

0.4

Qe (mmol/g)

0.6

Qe (mmol/g)

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

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0.3 0.2

0.3

NTCD-EPH15 NTCD-EPH20 NTCD-EPH25 TCD-EPH15 TCD-EPH20 TCD-EPH25 PNP

0.2

0.1

0.1 0.0

0

1

2

3

Ce (mmol)

4

5

0.0

0

1

2

3

4

5

6

Ce (mmol)

Figure 5. Adsorption isotherms for CD-EPH polymer/phenol systems at 295K: (A) TNP at pH 4.5 and (B) PNP at pH 9. In general, there is a gradual nonlinear increase in Qe as Ce increases with variable uptake for the polymer/phenol (TNP and PNP) systems. At the pH conditions herein, the adsorbates exist as phenolate anions according to the variable pKa (PNP; 7.16 and TNP; 0.38)4,5 due to the electron withdrawing effects of the NO2 group. Variable electron density or HLB character of the phenols is the results of substituent effects where the binding affinity with the polymers can be used to infer the type of adsorption sites involved.25,26,29,30 In general, there is an increase in the Qm value as the cross-linker content increases for the polymer/TNP systems, as follows: 1:25 ˃ 1:20 ˃ 1:15. In general, the linear (T-) polymers display greater monolayer sorption capacity relative to the globular (NT-) polymers due to the relative accessibility of the adsorption sites, in accordance with the accessible surface area (SA) of linear and globular materials. Variable accessibility of the CD inclusion and interstitial sites is a consequence of steric effects as the CD:EPH ratio increases.11,12 The variable surface accessibility relates to increasing degree of substitution as the EPH content of the polymer increases, where it is noted that all –OH groups of 16 ACS Paragon Plus Environment

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β-CD undergo reaction at the 1:10.5 CD:EPH mole ratio if one neglect steric effects. In practice, incremental cross-linking can occur at the primary and secondary annular hydroxyl groups of βCD (cf. Scheme 3 in Ref 12) up to a certain limit depending on the steric bulk of the cross-linker unit. The extent of cross-linking is known to affect the polymer morphology, as reported elsewhere,26 in accordance with the above SEM results. Phenol adsorption can occur at the CD cavity (inclusion) and the linker (interstitial) domains, where the uptake at these sites depend on the relative accessibility and affinity of the binding sites. The reduced uptake of phenols by the non-templated CD-EPH polymers is consistent with the reduced accessibility of the dual binding sites, in agreement with the globular and highly branched morphology of the CD-EPH polymers (Scheme 1). The greater uptake of TNP over PNP relates to the -NO2 substituent effects, in agreement with differences in their pKa and water solubility. The lower electron density for the phenyl moiety of TNP versus PNP contributes to variable –OH/pi and –CH/pi interactions, along with variable ion-dipole interactions of the phenol with the CD inclusion and interstitial polymer binding sites. The greater equilibrium uptake of TNP over PNP parallels the three-fold difference in the 1:1 CD/phenol binding constants (cf. Table 2) at pH 9.37,50 The binding affinity for the Tand NT-polymers decrease as the EPH content increases according to the following trend: EPH15 ˃ EPH20 ˃ EPH25, in accordance with the decreasing binding contribution of the CD inclusion sites due to steric effects. Independent results indicate that cross-linked CD polymers afford dual binding sites (inclusion and interstitial), according to their composition and accessibility.26 In cases where binding at the interstitial domains is favored, the adsorption of the NT-polymers (globular form) exceed that for T-polymers (linear form). The accessibility of the CD inclusion vs polymer interstitial binding sites account for differences in the adsorption properties of the T- and NT-polymers. According to Scheme 1, the NT-polymers have a greater



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density of interstitial binding sites over polymers with a linear morphology. Since binding occurs at the dual sites for the adsorption process, a weighted contribution of the two binding sites (inclusion and interstitial) relate to the site availability for the NT- and T-polymers. The role of variable CD inclusion site accessibility varied from 15-25% as the EPH content increased, especially for globular NT-polymers. By contrast, reduced steric effects occur for linear Tpolymers due to the greater accessibility of dual binding sites. Evidence of greater steric effects is noted for the globular versus the linear polymers with similar EPH content for the polymer/dye systems where the Sips heterogeneity parameter (ns) deviates from unity, especially for polymers with greater EPH content. Evidence that multiple sorption sites are present can be inferred for values of ns ≠ 1, in agreement dual mode binding of such cross-linked CD polymers.51 Figure 5B illustrates the adsorption isotherms of CD-EPH polymers with PNP at pH 9 and 295 K. The sorption profiles show parallel trends as for TNP, except that the uptake is lower for PNP. The trends in uptake for the NT- and T-polymer parallel the greater CD site accessibility of the linear versus globular polymers. Pratt et al. 41 reported the isotherm profiles of CD-EPH/PNP systems at pH 10.3 by where minor variations in uptake were observed despite the variable cross-linking at the 1:15 and 1:25 ratios. The nearly constant uptake can be understood by the inverse relationship of the role of the inclusion and interstitial sites, where greater uptake at the interstitial sites occur as the level of cross-linking increases. With higher EPH content beyond the stoichiometric excess (˃10.5 equivalents EPH), greater steric effects occur at the inclusion sites since the available hydroxyl groups of β-CD undergo cross-linking with EPH or are sterically inaccessible. The binding of phenols with the polymers is strongly influenced by the role of steric effects at the β-CD cavity sites. PNP may bind to both the inclusion and the


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interstitial regions of the polymer due to the variable hydrophile-lipophile character of these binding sites. Previous binding studies at variable pH conditions indicate a lower complex stability of the β-CD/PNP system when PNP is non-ionized. The difference in binding affinity was related to polarizability effects and other solvation processes that favored complex formation with the phenolate anion.52,53 Differences in the binding affinity of CD polymer/phenol complexes for PNP and TNP are understood according to differences in polarizability. Table 2 shows the 1:1 binding constants of phenolates with β-CD. The offset in binding affinity for PNP and TNP relate to the inductive effects for the nitro-substituent, as described above. Substituent effects for PNP and TNP are known to influence the thermodynamic and kinetics of hydration in bulk solution versus adsorption at the polymer binding sites, in accordance with hydrophobic effects. Although TNP and PNP have similar water solubility,4,5 differences in their electron density due to the –NO2 substituent effects influence the polymer adsorption properties with each phenol. The stability of CD host-guest complexes for TNP and PNP can be inferred due to the role of electron donor-acceptor (EDA) interactions, in agreement with the trends observed herein for the CD-EPH polymer systems.54

Table 2. 1:1 Binding Constants (K1:1) of β-CD/ Phenolate Complexes37,50,51 Substrate pH condition K1:1/ M-1 PNP 9 650 PNP

Phenolic Pollutant Uptake Properties of Molecular ... - ACS Publications

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