Investigation of the Adsorption Processes of Fluorocarbon and

Article pubs.acs.org/JPCC

Investigation of the Adsorption Processes of Fluorocarbon and Hydrocarbon Anions at the Solid−Solution Interface of Macromolecular Imprinted Polymer Materials Abdalla H. Karoyo and Lee D. Wilson* Department of Chemistry, University of Saskatchewan, Saskatoon S7N 5C9, Canada ABSTRACT: This study details the isotherm adsorption properties of cross-linked polymers containing β-cyclodextrin (β-CD), hereafter referred to as macromolecular imprinted materials (MIMs). The isotherm sorption parameters of the MIMs are reported with perfluorooctanoic acid (PFOA), octanoic acid (OA), and perfluorooctanesulfonate (PFOS) anions. The Sips and BET models describe variable solid-solution interfacial interactions for the MIMs with perfluorocarbon (PFC) and hydrocarbon (HC) anions, respectively. Typical monolayer adsorption occurs for the MIMs/OA system via the hydrocarbon alkyl tail of OA. Atypical adsorption is observed for PFOA and is driven via interactions between the MIMs surface with the carboxylate headgroup, as evidenced by the formation of multilayer adsorption and aggregation of the PFC alkyl chain at elevated levels. Molecular level support for the unique interfacial adsorption phenomenon is provided by several complementary methods: contact angle, infrared (IR) spectral shifts and intensity variations, differential scanning calorimetry (DSC), and isothermal titration calorimetry (ITC). Isotherm studies reveal the formation of monolayers of a hydrocarbon surfactant anion (OA) vs self-assembled multilayer aggregates for PFC anion species (PFOA and PFOS) at the MIMs solid−solution interface. The surface interactions are interpreted as a balance between headgroup electrostatic interactions and hydrophobic effects of the amphiphile chain at the adsorbent−solution interface.

1. INTRODUCTION The uptake of perfluorocarbon (PFC) and hydrocarbon (HC) surface active agents from aqueous solution onto synthetically modified polymer surfaces has been widely reported using solid−solution adsorption isotherms.1−15 The adsorption process of such anionic surfactants onto polymer surfaces was reported to result in the formation of unique types of molecular aggregates such as micelles/hemimicelles and layered structures.15−17 The driving force of such interactions at the solid− solution interface depends on the mutual compatibility of interactions between the polymer adsorbent surface, adsorbate species, and solvent medium.18−20 The surface activity of PFC and HC surfactants varies markedly as evidenced by the systematically lower critical micelle concentration (CMC) of PFCs, indicating that self-assembly via micelle formation occurs more readily for PFC surfactants. The formation of host−guest complexes between cyclodextrins (CDs) and surfactants results in a reduction of the surface activity of such guests, especially for PFC surfactants over their HC counterparts.21 CDs are a group of structurally related macrocyclic host molecules comprised of six (α-), seven (β-), and eight (γ-CD) glucopyranose units with α-(1 → 4)linkages that are formed during bacterial digestion of starch.22,23 α-, β-, and γ-CDs have been reported to form stable host−guest complexes with surfactants at concentrations (Cs) below23−32 and above34 the CMC of the surfactant. An understanding of © 2016 American Chemical Society

the host−guest chemistry of a CD with a surfactant micellar phase (Cs > CMC) in solution is further complicated due to the occurrence of multiple equilibria, surfactant aggregation vs host−guest complex formation of a dispersed surfactant. Competitive equilibria of aggregates and host−guest complex formation depend on the relative magnitude of the equilibrium constant for such processes. For instance, the propensity of selfassembly of sodium perfluorononanoate (SPFN) was observed to occur along with the formation of 1:2 CD/SPFN inclusion complexes, according to an apparent molar volume and NMR studies.30,31 Thus, multiple equilibria may result in the competitive formation of CD inclusion complexes or selfassembly since such processes lower the Gibbs energy of amphiphilic systems in solution. Competitive equilibria in solution may extend to polymer-based CD systems because the structural dynamics and accessibility of the CD inclusion site are influenced by the topology and the cross-linker density. The interaction between surfactants with CD polymers can occur within the cavity (inclusion sites) or onto the surface domains of the polymer framework (interstitial sites). The formation of micelles/hemimicelles may occur at the interstitial sites of the Received: December 14, 2015 Revised: March 2, 2016 Published: March 3, 2016 6553

DOI: 10.1021/acs.jpcc.5b12246 J. Phys. Chem. C 2016, 120, 6553−6568

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The Journal of Physical Chemistry C

Scheme 1. Reaction between β-CD (Toroid) and 1,6-Hexamethylene Diisocyanate (HDI) to Form the MIM-x Materials, where x = 1, 3, or 6a

Note that for the 1:1 mol ratio, the cross-linker is attached at the primary annular hydroxyl sites (−C6H2−OH) of β-CD due to the greater reactivity of these groups. a

adsorption of such carboxylate anions onto the MIMs resulted in the formation of inclusion complexes, micelles, aggregates, and mono/multilayer structures, in accordance with the variable CMC, pKa, and headgroup structure of each anionic surfactant system. The characterization of the adsorptive process between an adsorbate in solution with a solid-phase adsorbent requires complementary spectroscopic (e.g., NMR, FT-IR, and Raman) and other surface-sensitive characterization such as DSC, ITC, and contact angle.55−57 Raman and IR spectroscopy, along with DSC, may provide insight into the nature of the surface interactions that occur in such adsorbent/ adsorbate systems, while contact angle and ITC methods can provide molecular level insight on the adsorption process. Herein, we report spectroscopic (FT-IR), thermal analyses (DSC), contact angle, and calorimetric (ITC) studies to characterize the nature of the adsorption at the solid (adsorbent)−solution (adsorbate) interface. The adsorption properties of perfluorooctanesulfonate (PFOS) are compared to provide insight into the nature of the interactions for MIMs with alkyl carboxylate surfactants (PFOA and OA).

host cavity when inclusion binding sites have attenuated accessibility. Modified polymer surfaces that contain CDs have been recently reviewed35−37 and are unique because of their wide ranging adsorption properties toward PFC and HC guest molecules. More recently, polymer materials containing CDs have been studied for the adsorption of alkyl carboxylates.38 The adsorption properties of such macromolecular imprinted materials (MIMs) containing a CD with a bifunctional crosslinker are tunable according to their composition, as shown by uptake selectivity toward amphiphilic adsorbates with variable properties (e.g., size, CMC, and hydration properties). In a previous report,15 the adsorption properties of MIMs with PFC and HC carboxylate anions were found to display switchable binding behavior at the polymer−solution interface according to the nature of the adsorbate. Adsorption of HC and FC alkyl carboxylates at the MIMs interface in aqueous solution may occur within the CD inclusion sites or at the interstitial (noninclusion) sites of the cross-linker domains where the formation of well-defined inclusion complexes, aggregates, micelles, monolayers, bilayers, and multilayers is possible. Selfassembly of multilayer structures through the adsorption of a surfactant on a surface is highly relevant to industrial applications such as coatings, paint technology, dispersionflocculation, and oil recovery processes.39 Multilayer structures at interfaces are currently recognized as important structural features for biological applications (e.g., cell patterning, drug delivery, and antibacterial coatings)40 where continued research in this area is anticipated. The unique interactions for PFC/HC surfactants with solidphase adsorbent materials have been inferred from thermodynamic adsorption parameters,12,15,41−45 where general aspects of the adsorption of HC surfactants at solid−solution interfaces have been reviewed.46,47 Spectroscopic studies have been used to characterize interfacial interactions between simple organic acids or inorganic molecules with various solid-phase adsorbent surfaces.48−53 By comparison, there are sparse examples of studies that have characterized the adsorption of PFC surfactants onto solid-phase adsorbents. Giao and Chorover54 reported a FT-IR study for the adsorption of PFOA and PFOS onto hematite surfaces from aqueous solution. The limited reports for such PFC surfactants may be related to challenges associated with the preparation of mixed systems containing PFC guests and HC-based adsorbents, due to the nonideal mixing behavior in such systems. In a previous isotherm study15 of PFOA and OA onto MIMs adsorbents, several modes of binding were interpreted from the equilibrium isotherm parameters (cf. Scheme 2 in ref 15). The

2. MATERIALS AND METHODS 2.1. Materials. Perfluorooctanoic acid (PFOA; 96%) and perfluorooctanesulfonic acid (PFOS; 98%) were purchased from SynQuest Laboratories USA (Alachua, FL). Octanoic acid (OA) was purchased from VWR Canada Ltd. Hexamethylene diisocyanate (HDI), β-CD hydrate (99%), sodium hydroxide (NaOH), and potassium hydrogen phosphate (KH2PO4) were purchased from Sigma-Aldrich Canada Ltd. (Oakville, ON). All materials were used as received unless specified otherwise. 2.2. Methods. 2.2.1. Synthesis of Macromolecular Imprinted Materials (MIMs). The methods for the synthesis of MIMs containing β-CD and HDI were reported previously15,58 (cf. Scheme 1). Two types of MIMs (MIM-1, water-soluble; and MIM-6, water insoluble) that differ according to the relative content of β-CD and HDI (1:1 and 1:6, respectively) were used in this study. Some adsorption results for MIM-3, as well as granular activated (GAC), will also be presented for comparison with the results of this study. In MIM-x, the numeric values where x = 1, 3, or 6 represent the relative mole quantity of the HDI diisocyanate linker units in relation to the moles of β-CD which is unity. 2.2.2. Preparation of MIMs/Surfactant Mixtures. Samples of MIM-6 in the solid phase were equilibrated with aqueous solutions of PFOA (pH ∼ 4) and OA (pH ∼ 8.5) at several different concentrations to obtain mixtures that correspond to monolayer and multilayer surface coverage. The determination of saturation of the monolayer was accounted for by the 6554


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surfactants with the MIMs was determined as the ratio (Qe/ Qm) of the amount of adsorbed species (Qe; mmol/g) relative to the monolayer sorption capacity (Qm; mmol/g). Qe was determined experimentally according to eq 1 where Co refers to the initial adsorbate (surfactant) concentration prior to sorption; m is the mass (g) of the adsorbate; and V is the volume (L) of the solution.

physicochemical properties of the surfactants in Table 1 and the inflection points in the isotherm results in Figure 1 (cf. Figure 3 Table 1. Physicochemical Properties66−70 of the Various MIM Adsorbents and the Alkyl Carboxylate Surfactants adsorbent

SA (m2/g)

CD content (mol %)

CD accessibility (%)

GAC MIM-1 MIM-3 MIM-6

1100a 3b 5b NR

87.1 69.2 52.9

100 4.78 ≈0

HDI Content (mol %)

adsorbates

molecular formula

pKa

CMC (mM)

12.9 38.7 77.4 water solubility (g/L at 25 °C)

PFOA OA

CF3(CF2)6COOH CH3(CH2)6COOH

2.5 4.9

8.7−10.5 350

3.4 0.68

Qe =

(Co − Ce) × V m

(1)

Adsorption isotherms for PFOS were expressed as plots of Qe (mmol/g) vs Ce (mmol/L), where the experimental sorption data were evaluated using the Brunauer−Emmet− Teller (BET) model.59 All equations and other parameters were defined elsewhere.15 A hybrid quadrupole-TOF (Q-star) MS/ MS mass spectrometer with electrospray ionization (ESI-MS) in negative-ion mode was used to evaluate the Ce values for the PFC surfactants. The residual level of unbound OA was estimated by back-titration using 0.02 M HCl solution and bromophenol blue as the indicator. The favorable water solubility of MIM-1 precluded the study of solid−solution isotherms for the MIM-1/OA system since the Ce values of the unbound OA could not be deconvoluted from the bound species using back-titration, as in the case of insoluble MIMs. 2.2.4. FT-IR Spectroscopy. Fourier transform-IR spectra were obtained on the hydrated samples in reflectance mode using a Bio-Rad FTS-40 spectrophotometer with a resolution of 4 cm−1. All spectra were obtained with spectroscopic grade KBr which constituted ∼80% (w/w) of the total sample. 2.2.5. Differential Scanning Calorimetry (DSC). Differential scanning calorimetry (DSC) of the MIMs systems before and after adsorption of adsorbate (PFOA or OA) was achieved using a TA Q20 thermal analyzer over a temperature range of 30−150 °C. The scan rate was set at 10 °C/min, and dry nitrogen gas was used to regulate the sample temperature and sample compartment gas purging. Samples were analyzed as hydrated solids (cf. Section 2.2.2) in hermetically sealed aluminum pans with a small exhaust hole punched in the lid of the pan to allow release of exhaust gases. Sample weights ranged from 10 to 12 mg. 2.2.6. Contact Angle. Contact angle measurements were measured using a CT Scan vintage 50 contact angle goniometer at ambient temperature (22 ± 1 °C). Liquid drops containing

a

Supplier derived BET surface area. bBET surface area and the CD accessibility (%) values were estimated from a previous report.66

in ref 15). In the case of PFOA, 100 mg of MIM-6 was added to three separate 250 mL beakers containing 100 mL of 7 mM (multilayer), 4 mM (mono/multilayer), and 1 mM (monolayer) PFOA in Milli-Q water at ambient pH conditions (pH ≈ 4), respectively. As for OA, the following monolayer concentrations were used: 18 mM, 4 mM, and 1 mM at pH 8.5, which corresponds to a pH value above the pKa of OA. The adjustment of pH was achieved using small volumes of 0.1 mM NaOH solution. The polymer/solution mixtures were stirred for 24 h and filtered through Whatman no. 2 filter paper followed by several washings of the filtrate with Milli-Q water to remove excess surfactant. The resulting equilibrated solid polymer samples were analyzed as hydrated solids using DSC and FT-IR spectroscopy. For the sake of discussion and clarity, the MIM-6 (M6) samples were spiked with the different levels of PFOA and OA; hereafter denoted as M6-PFOA-7, M6PFOA-4, M6-PFOA-1, and M6-OA-18, M6-OA-4, M6-OA-1. The numeric designation refers to the relative concentrations of the surfactant anions (mM) described above. 2.2.3. Adsorption Isotherms. Adsorption isotherms for PFOA and OA were expressed as plots of adsorbent surface coverage (θ) against the equilibrium concentration of unbound (residual) adsorbate species (Ce; mM) in aqueous solution. The surface coverage (θ) for the adsorption isotherms of the

Figure 1. Sorption isotherms of (a) MIMs/PFOA and (b) MIMs/OA systems expressed as surface coverage (θ) against equilibrium concentration (Ce). 6555


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Figure 2. Sorption isotherms of PFOS with various adsorbents; (a) MIM-1 and (b) MIM-6 at pH ∼ 3 and 295 K. The “best-fit lines” correspond to the BET model. Insets: The sorption isotherms of PFOA onto (a) MIM-1 and (b) MIM-6 surfaces are compared where the circled regions at low Ce show favorable adsorbate−adsorbent interactions.

similar levels of PFOA (7, 4, and 1 mM) and OA (18 and 1 mM) in Milli-Q water were placed on the surface of an air-dried film of MIM-6. Thin films with ca. 20 μm thickness were prepared by drop-casting 1 mL of DMSO solution containing ∼0.5 mg of MIM-6 onto a 0.5″ × 1.5″ silicon wafer to afford relatively smooth and uniform films.60 The use of silicon wafers as substrates for contact angle measurements was previously reported.55 Average contact angles (θ) were estimated from quadruple measurements with 4 drops of each liquid, where 10 μL drops were placed onto the films using a 250 μL syringe, and contact angle measurements were made on both sides of the drops. The reported θ value represents the static θ value at equilibrium measured within 1 min of droplet addition to the surface. The average θ values of water, PFOA, and OA were measured as controls where θ values were relatively constant to ±1°. 2.2.7. Isothermal Titration Calorimetry (ITC). ITC measurements were carried out at 25 °C on a Calorimetry Sciences Corp. (CSC) calorimeter using native β-CD and the soluble polymer adsorbent (MIM-1) as the host substrate for comparison. Since the use of MIM-6 for ITC measurements is not practical due to its relative water insolubility, MIM-1 was used as the model adsorbent to obtain the thermodynamic data in homogeneous solution. The thermodynamic parameters derived for the binding interactions between MIM-1 and the surfactant are expected to provide insight on the adsorption process for the adsorbent/surfactant system. As well, the host− guest binding affinity of MIM-1 can be compared with native βCD and its complexes with PFOA and OA surfactant systems. OA samples for ITC measurements were prepared in phosphate buffer at pH 8.5, while samples for PFOA were prepared at ambient pH conditions (pH ≈ 4) in Milli-Q water. The ITC experiments were measured by injecting the anionic surfactant solution (OA, or PFOA; 6 mM) as the titrant, at 200 s intervals between injections and 200 s standard equilibration time, using a 250 μL injection syringe into a stainless steel sample cell (1.430 mL) containing the host substrates (β-CD ≈ 0.3 mM or MIM-1 ≈ 0.4 mg/mL) with stirring at 300−400 rpm. The integrated heat effect of each injection was corrected by subtraction of the corresponding heats of dilution of surfactant solutions in Milli-Q water and phosphate buffer, respectively. In the case of the OA titrations, the heats of

dilution for the phosphate buffer into the host substrates were also subtracted. The experimental data obtained from the ITC titration were analyzed using CSC commercial BindWorks 3.1 software. A single binding model with an independent set of multiple binding sites was used for the nonlinear regression. The enthalpy of binding (ΔH), binding constant (Ki), and the number of binding sites (n) were estimated from the software using the independent and the multiple (β-CD/PFOA) models, respectively. The change in standard Gibbs free energy (ΔG°) and the corresponding change in standard entropy (ΔS°) were obtained using eq 2. ΔG° = − RT ln K i = ΔH ° − T ΔS°

(2)

3. RESULTS AND DISCUSSION In a previous report of the adsorption properties of alkyl carboxylates (PFOA and OA) onto MIMs, two modes of binding at the noninclusion sites of the polymer interface were first reported (cf. Scheme 2 in ref 15). Adsorption of PFOA occurs via dipolar interactions between the carboxylate headgroup and the polar domains on the MIMs surface which favor the formation of bilayers and multilayer aggregates, as described previously.15 By contrast, the hydrocarbon alkyl chain of OA is adsorbed at the MIMs surface as a well-defined monolayer. The details of the adsorption properties of these surfactants with the MIMs surface sites are outlined further in section 3.1 according to the isotherm results for the surfactants. The molecular level details of the adsorption of surfactants onto the MIMs is further evaluated using spectroscopic and calorimetric evidence in section 3.2. 3.1. Surface Binding Properties of MIMs. 3.1.1. Surface Coverage of PFOA and OA. Adsorption isotherms were used to characterize the adsorption properties of the HC and PFC anionic surfactants with the MIMs adsorbents at variable levels of cross-linking. The results in Figure 1 show the sorption isotherms of MIMs/PFOA and MIMs/OA systems are expressed as surface coverage (θ) against the equilibrium residual concentration (Ce; mM) of PFOA (a) and OA (b) in aqueous solution. The binding of the PFOA anion onto the MIMs (Figure 1a) surface leads to coverage greater than unity, in accordance with the formation of a multilayer isotherm. By contrast, Figure 1b shows that the adsorption of OA onto the 6556


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mmol/g), according to the BET model estimate. The greater Qm value according to the Sips model for the MIM-1/PFOA system (2.63 mmol/g) exceeds that for GAC (1.35 mmol/g), in accordance with the contribution of multilayer adsorption and inclusion binding for MIM-1. The greater Qm values for the MIM-1/PFOA system compared to the highly cross-linked adsorbents such as MIM-3 and -6 further illustrate the contribution of inclusion site binding to the overall surfactant uptake (cf. Scheme 2). The greater formation of multilayers for PFCs is consistent with the presence of noninclusion domains with moderate binding affinity and polar sites, including the low surface area of the MIMs materials. By contrast, GAC has fewer polar binding sites but greater surface area (cf. Table 1) relative to the MIMs adsorbents. The amide linkages of the noninclusion sites are hypothesized to play a key role in the binding of PFOA onto the MIMs surface due to their dipolar character and the reduced hydration of PFC anions.64,71 The minimum equilibrium saturation concentration (Ce,sat; mM) of the adsorbates for monolayer or multilayer profiles was estimated from the respective models (Sips/BET) in Table 2. In general, full coverage of the PFOA monolayer occurs at Ce ∼ 1.0 mM, and multilayers are formed at Ce > 1.5 mM with saturation when Ce ≥ 3.5 mM. In the case of OA, monolayer saturation occurs at Ce ∼ 10 mM. The adsorption affinity constants (Keq; M−1) in Table 2 were estimated from the “bestfit” parameters from the Sips model. The Keq values for PFOA/ MIMs are ca. 3-orders of magnitude greater than the corresponding OA/MIMs system. Adsorption processes of surface-active species are strongly governed by hydrophobic effects and electrostatic interactions that depend on the hydration of the surfactant headgroup, in agreement with the offset in CMC values for PFOA and OA (cf. Table 1). The adsorption of PFOA and OA by MIMs likely involves van der Waals interactions via the surfactant alkyl chain. However, secondary electrostatic interactions of the headgroup of PFOA or the terminal CF3 group have been reported according to 19F NMR results for the α-/β-CF2 and CF3 group.26,30,72 By contrast, the OA HC alkyl chain is apolar where its greater CMC reveals its lower hydrophobic character relative to PFOA. The greater pKa of OA (≈5.0) relative to PFOA (≈2.5) indicates differences in the charge density and hydration properties of the carboxylate anion of each surfactant. Thus, the greater Keq for PFOA (Table 2) suggests that dipolar interactions between the carboxylate headgroup and polar surface groups (−NH, −OH) at the MIMs interface are likely more pronounced due to its reduced hydration on account of its weaker conjugate base character. In the absence of steric effects due to cross-linking, the binding of PFOA with MIMs at the inclusion sites of β-CD is favored, as described above. In contrast to PFOA, the stronger conjugate base character of OA contributes to greater hydration of the headgroup with attenuated interactions with the MIMs interface. The HC alkyl tail of OA interacts with the MIMs surface for both inclusion and noninclusion sites, especially when there is no steric hindrance at the inclusion sites. Adsorption of OA at the crosslinker domains is likely to occur, as evidenced by the adsorption of the p-nitrophenolate anion (PNP) with CD polymers.11,71 Adsorption of PNP at the noninclusion sites was found to increase as the level of cross-linking increased, in parallel with the results for the uptake of OA at the cross-linker domains of the MIMs adsorbent, especially at elevated cross-linker content. Dipolar interactions are more pronounced for the MIMs/ PFOA systems relative to the MIMs/OA systems, where

MIMs surface shows behavior that represents monolayer coverage, in agreement with the Langmuir model.61 Comparable isotherms are shown in Figure 1b for the adsorption of OA onto granular activated carbon (GAC). A surface coverage of unity implies that a single layer of OA is adsorbed onto the MIMs surface at higher concentration62 (Ce ≥ 10 mM; cf. Figure 1b). The probable binding of the OA anion onto the MIMs surface occurs via the apolar alkyl tail to lower the Gibbs surface energy since the carboxylate headgroup is well hydrated.15,63 In the case of the PFOA anion, there is evidence indicative of interactions between the carboxylate headgroup of PFOA and the dipolar functional groups (e.g., −NH and −OH groups) at the noninclusion domains of the MIMs surface (cf. Scheme 1). This “reversed” mode of binding where the carboxylate anion headgroup interacts with the MIMs surface is consistent with the reduced hydration of the PFOA headgroup due to inductive effects of the adjacent CF2 groups.64 Inductive effects on the headgroup are expected to lower the effective charge and hydration of the carboxylate anion, in agreement with the lower pKa of PFOA relative to OA (cf. Table 1).64 Furthermore, interactions between the carboxylate headgroup of PFOA and the polar surface domains of the adsorbent lead to the formation of multilayers due to headgroup binding with the MIMs surface, especially at elevated PFOA levels (Ce > 1.5 mM) in Figure 2.65 The inclusion sites of β-CD have greater site accessibility at lower vs higher cross-linking (MIM-1 vs. MIM-6), as illustrated in Scheme 2. Thus, inclusion complexes are more likely for OA Scheme 2. Schematic Presentation of the Sites of Substitution for the HDI Cross-Linker at the Primary (Narrow Rim) and Secondary (Wide Rim) Hydroxyl Groups in the Annular Region of β-CDa

The filled spheres represent covalently attached sites and illustrate low, medium, and high cross-linking, whereas the open spheres represent unreacted sites. Highly cross-linked polymers are susceptible to increased steric hindrance and reduced inclusion site accessibility. Redrawn from ref 66 with permission.

a

and PFOA with MIM-1; whereas surface interactions at the noninclusion sites are more likely for MIM-6. Table 1 lists the physicochemical properties of the various adsorbent materials (i.e., sorbent surface area, accessibility (%) of β-CD inclusion sites, and linker content) and properties of the adsorbates. 3.1.2. Sorption and Equilibrium Binding Parameters for PFOA and OA. The isotherm adsorption parameters for PFOA (Sips/BET) and OA (Sips) onto GAC/MIMs were used to provide a greater understanding of the nature of the interactions involved in these two systems (cf. Table 2). The Qm values enclosed in brackets for MIMs/PFOA system in Table 2 were estimated from the Sips model. It is worthwhile to note that the uptake of PFOA (Qm = 1.35 mmol/g) by the MIMs compares favorably with the uptake for GAC (Qm = 1.39 6557


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Table 2. BET and Sips (a) Isotherm Sorption Parameters for PFOA and OA with GAC and MIMs at 295 K and pH 4 and 8.5, respectively GAC PFOA

Sips

Qm (mmol/g) KSips (L/g) Keq (103 M−1) ns Ce,sat. (mM) mono/multilayer OA

1.39 2.39 ∼1 0.74 ∼0.5

Qm (mmol/g) KSips (L/g) Keq (M−1) ns Ce,sat. (mM) a

MIM-1

MIM-3

MIM-6

BET (Sips) 1.35 (2.63) 0.91 0.4 0.87 ∼1/>3.5

0.97 (1.38) 1.23 0.5 0.71 ∼1/>3.5

0.87 (1.28) 14.1 5.8 1.26 ∼0.5/>3

Sips 1.98 0.07 10 2.1 ∼10

NR -

0.84 0.01 1.4 3.1 ∼10

1.02 0.04 5.8 3.2 ∼8

The Qm values enclosed in brackets represent values estimated from the Sips model.

adsorption of OA is driven by interaction of the HC alkyl tail with the MIMs surface. By comparison PFOA interactions can occur via the headgroup or PFC alkyl chain depending on the site of interaction (inclusion vs noninclusion sites) onto the MIMs surface. Dipolar interactions (∼102 kJ/mol) are more prominent over weaker dispersion forces (99% for PFOS) at the conditions employed herein. The adsorption isotherms for PFOS with MIM-1 and -6 are shown in Figure 2a and b, while adsorption isotherms of PFOA with MIM-1 and -6 are shown as insets in Figure 2 for comparison. The isotherm parameters (Qm and Ki) for PFOS are listed in Table 4, and the corresponding data for PFOA are given in parentheses. The experimental results are well described by the BET model (R2 > 0.95). In Figure 2, the MIMs/PFOS system displays a Type III sorption isotherm according to the BET classification.74 An asymptotic rise for Qe is observed for Ce ∼ 3.0 mM with MIM-1 (cf. Figure 2a) and Ce ∼ 3.5 mM with MIM-6 (cf. Figure 2b and Table 4). The results for the MIMs/PFOA system differ according to the Type IV isotherm profile (see insets of Figure 2). PFOS displays a Type III isotherm that indicates weak adsorbate−adsorbent inter6558


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The Journal of Physical Chemistry C Table 3. Molecular Structures and Physicochemical Properties of PFOA and PFOS54,67

c

Vapor pressure value for PFOS corresponds to the potassium salt. Note that the conformations of the PFCs are not accurately depicted.

Table 4. BET Isotherm Sorption Parametersa for PFOS with MIM-1 and -6 at 295 K and pH ∼ 3 Qm (mmol/g) MIM-1 MIM-6

0.38 [1.35 0.69 [0.87

± ± ± ±

0.07 0.05] 0.07 0.03]

KBET (L/g)

Keq (103 M−1)

R2

εR (%/g)

Ce,sat (mM)

± ± ± ±

0.57 [5.2] 0.07 [15]

0.99 [0.98] 0.99 [0.96]

37

≥3.0

34

≥3.5

1.15 [12.6 0.14 [36.1

1.09 5.6] 0.04 18.7]

εR denotes the level of uptake of PFOS defined as (Co − Ce) × 100%/Co, where Co and Ce are the initial and residual adsorbate concentrations, respectively. All other parameters are defined in Table 2. Isotherm parameters for MIMs/PFOA system are given in parentheses. a

pronounced binding with the MIMs surface via ion−dipole interactions. Headgroup contributions are supported by the variable binding affinity (Keq values) for the MIMs/surfactant systems where PFOA exceeds PFOS, according to Table 4. The Qm values (mmol/g) for the monolayer sorption capacity for the MIMs with PFOS are listed as follows (Table 4): 0.38 (MIM-1) and 0.69 (MIM-6). The Qm values are lower than the corresponding values for PFOA listed in parentheses (Table 4). The results for PFOS show agreement with those reported by Yu and co-workers42 for adsorption of PFOS onto carbonaceous adsorbents at pH 5. The Qm values for PFOS are given for each adsorbent: AC (1.04 mmol/g), GAC (0.37 mmol/g), and an ionic resin AI400 (0.42 mmol/ g).42 The uptake of PFOS by the MIMs adsorbents reveal Qm values that are comparable to or exceed other types of organic adsorbents. It is worthwhile noting that the Qm values for PFC surfactants are dependent on the pH and concentration conditions, as anticipated according to Scheme 3. The lower Qm values for PFOS compared with PFOA are related to different modes of sorption in accordance with the amphiphilic properties of the adsorbate: (i) CMC value, (ii) hydrophobicity, (iii) molecular size, and (iv) the ionic character of the headgroup, as described previously.15 For example, the Qm values for the MIM-1/PFOA (1.35 mmol/g) system exceed the MIM-6/PFOA (0.87 mmol/g) system (cf. Table 4). An opposite trend was observed for PFOS where the Qm value for MIM-6/PFOS (0.69 mmol/g) exceeds that for the MIM-1/ PFOS (0.38 mmol/g) system. The Qm value of the MIM-1/ PFOS system was greatly attenuated relative to the MIM-1/ PFOA system. The greater charge density of the carboxylate anion of PFOA imparts greater hydrophilic character, as compared to the sulfonate anion described above. As such, PFOS is less hydrated with a reduced charge density relative to the PFOA anion in accordance with the relative differences in the cosmotropic nature of these anions.63 The charge density of the headgroup and hydrophobicity of PFOA and PFOS provide

actions, as supported by the adsorption results for the MIMs/ PFOS systems.75 In contrast, PFOA sorption is characterized by moderately strong interactions at low Ce as evidenced by the higher magnitude of Qe in the region where monolayer adsorption occurs as shown by the circled regions in Figure 2 (cf. Figure 1 in ref 15). Relatively weak interactions are evidenced for the MIMs/PFOS isotherms, according to the negligible uptake when Ce < 3 mM (cf. Figure 2), in agreement with the reduced Ki values in Table 4. These findings are consistent with the anticipated weaker ion−dipole interactions for the sulfonate headgroup of PFOS with the MIMs surface relative to the stronger ion−dipole interactions of the carboxylate headgroup of PFOA with the MIMs surface. The interaction of sulfonate vs carboxylate anions can be understood according to the relative cosmotropic nature of these anions as understood by their relative Lewis base character from the Hofmeister series.63,64 This approach provides an understanding of host−guest interactions according to the “law of matching water affinities”,63 especially at macromolecular surfaces.76 The attenuated ion−dipole interactions of PFOS also concur with the dissipation of ion charge over the larger headgroup of PFOS relative to PFOA64 (see Table 3), as described above. The dipolar domains on the MIMs surface consist of −NH and −OH functional groups (cf. Schemes 1 and 3) where these domains may serve as H-bond donor/acceptor sites. Ion− dipole interactions are likely between the MIMs surface and the headgroup of the surfactants of PFOA and PFOS. The hydration and ionic charge of the surfactant influence the electrostatic interactions at the MIMs/solution interface. The key differences between PFOA and PFOS are the headgroup size and variable charge distribution of each anion. The ion charge of PFOA is distributed over three atoms versus four atoms in the case of PFOS. Thus, each anion has variable polarizability and charge density, where greater charge resides on the carboxylate anion which contributes to more 6559


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Scheme 4. Schematic Illustration of Variable Adsorption of Surfactants onto the Surface of a CD Polymer: (a) MIMs/PFOA and (b) MIMs/PFOS Systemsa

a

PFOA confers stronger ion−dipole interactions with the dipolar regions of the MIMs surface (due to higher charge density) than PFOS which readily forms micelles. Adsorption of PFOS at the inclusion site (in the case of MIM-1) is attenuated due to surface-bound complexes. Note that the inclusion site accessibility for MIM-6 is negligible due to steric effects.

for the isotherm of MIM-6 at elevated adsorbate concentration (Ce,sat ≥ 3.5 mM), relative to MIM-1 (Ce,sat ≥ 3.0 mM). The inclusion site accessibility for MIM-6 is negligible66 (cf. Table 1). Therefore, binding at the cross-linker domains of the polymer is preferred over the inaccessible β-CD inclusion sites. The lower Keq value for MIM-6 suggests reduced dipolar interactions for PFOS relative to PFOA, in agreement with the lower charge density described above. It is concluded from the ongoing discussion that electrostatic and hydrophobic interactions contribute significantly to the sorption of PFC and HC surfactants onto the MIMs surface since hemimicelles and aggregates are formed near the CMC of the FC surfactants.65 Variable solid−solution interfacial interactions for the PFOA and OA anions with the MIMS surface are illustrated in Scheme 3. The observed trends correspond to the variable physicochemical properties (e.g., CMC, pKa, and hydration properties) of the surfactant species and the occurrence of two types of adsorption sites (inclusion vs noninclusion). The adsorption of PFOA onto the noninclusion sites of the MIMs surface is dominated by relatively strong dipolar interactions due to surface binding between the carboxylate headgroup of PFOA with the amide and hydroxyl surface functional groups. Aggregation via self-assembly is favored for PFOS over PFOA, due to the greater surface activity of PFOS. The cooperative formation of aggregates and multilayers occur as Ce nears the CMC of PFOS at the surface of the MIMs, especially when steric effects attenuate the inclusion binding contributions. The variable adsorption of PFOA and PFOS onto the MIMs surface is illustrated in Scheme 4. Further experimental evidence of the variable adsorption of PFC and HC surfactants onto the MIMs surface is provided in Section 3.2 according to spectroscopic (FT-IR and Raman), DSC, contact angle, and ITC. 3.2. Interaction of PFOA/OA with MIMs. Various spectroscopic techniques (FT-IR and Raman), DSC, ITC, and contact angle measurements were used to characterize the surface interactions involved in the binding of PFOA and OA anions onto MIMs surfaces. The preparation of surfactants (PFOA and OA) with the highly cross-linked MIM-6 as the model adsorbent system was described in section 2.2.2. Complementary experimental evidence is given in sections 3.2.1−3.2.3 to provide an improved understanding of the adsorption processes illustrated in Scheme 3. 3.2.1. DSC. DSC is a highly sensitive tool used for the study of the thermal and hydration properties of foods, polymers, and

an account for the observed differences in the adsorption properties of these surfactants with the MIMs adsorbents, especially the role of charged headgroup interactions with the MIMs surface.76 The MIMs adsorbents possess two types of binding sites: (i) β-CD inclusion sites and (ii) interstitial (noninclusion) domains of the cross-linker framework (cf. Scheme 3). The occurrence of dual-mode binding for cross-linked CD polymers was ́ accounted for in a thermodynamic model reported by GarciaZubiri et al.77 which describes two types of binding sites (inclusion and noninclusion) for such materials. Steric effects at the inclusion sites for the MIMs are more pronounced as the level of cross-linking increases (cf. Scheme 2), as follows: MIM6 > MIM-3 > MIM-1. The inclusion sites of MIM-1 have negligible steric effects (≈100% accessible) according to dyebased adsorption results listed in Table 1, originally reported by Mohamed et al.66 The MIM-1 adsorbent adopts a linear topology; whereas MIM-6 is a more highly branched network structure due to more extensive cross-linking (cf. Scheme 2). On the basis of the lower CMC value and the greater molecular size of the headgroup42,43 of PFOS compared with PFOA (cf. Table 3), hemimicelles and micelles are more likely to form for PFOS due to its lower CMC. The formation of surface-bound complexes via self-assembly lowers the overall level of dispersed surfactant species available for sorption. Thus, self-assembly may be favored over inclusion binding in cases where steric effects are more pronounced. The formation of surface-bound aggregates can attenuate the inclusion binding process, in agreement with a report by Tsianou et al.34 “Nonspecific” formation of micelles and hemimicelles was reported by Yu et al.42 to account for the reduced adsorption of PFOS over PFOA onto AI400 resin. A similar effect may occur for the adsorption of PFOS onto MIM-1, as evidenced by its reduced binding affinity with the MIMs (cf. Table 4). It is noteworthy that the equilibrium constant for the MIM-1/PFOS system (0.57 × 103 M−1) in Table 4 is about 10-fold lower than the corresponding value for the MIM-1/PFOA system (5.2 × 103 M−1), according to the BET model. The greater propensity of PFOS over PFOA to form molecular aggregates agrees with its lower CMC value. The MIM-6 adsorbent reveals greater adsorption of PFOS relative to the MIM-1/PFOS system, and this may correlate with preferred surface binding sites due to greater number of linker (noninclusion) domains for MIM-6. Adsorption at the interstitial surface domains concurs with the onset of saturation 6560


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The DSC region between 30 and 70 °C in Figure 3 is more revealing due to the “semisolid” phase transitions attributed to layered structures is shown for the MIMs/PFOA system (cf. Scheme 3a). A comparison of the PFOA loading from low (1 mM) to higher (7 mM) levels for the MIM-6/PFOA system coincides with the emergence of a melting endotherm for PFOA at ca. 60 °C. Despite the monotonic increase in the intensity of the endotherm event at ∼60 °C with greater PFOA loading, the event cannot be attributed to unbound PFOA since it appears at incrementally higher temperature upon going from free PFOA to the highest PFOA loading. The endothermic events at elevated temperature for the M6-PFOA-4 and M6PFOA-7 systems indicate a physical transition related to selfassembly due to its concentration dependence. The DSC results of M6-PFOA-1 (monolayer) reveal an endotherm similar to that for free PFOA that agree with the dehydration events at ∼98 and 105 °C. The lower-temperature endotherms relate to a form of PFOA with reduced hydration due to competitive adsorption of this PFC at the surface of MIM-6. In the case of the M6-PFOA-4 and M6-PFOA-7 systems, the lower temperature endotherm (∼100 °C) disappears. The broader transition reveals a unique structure for these systems compared to the free PFOA and the M6-PFOA-1 system, in accordance with Scheme 3a. Several studies of polyelectrolyte multilayers reveal that significant hydration can ingress multilayer structures84,85 and contribute to broad dehydration endotherms, as observed herein. The DSC results in Figure 3 reveal that the dehydration endotherms for the MIM-6/PFOA systems are shifted to lower temperature values relative to the free PFOA. These trends suggest a different microenvironment for PFOA in a bound state (MIMs/PFOA) relative to the unbound state. It is concluded that the PFOA anion is bound via dipolar interactions onto the surface of MIM-6 through the polar domains (−NH/−OH) of the MIMs framework. The formation of multilayer structures is possible via adsorption of the PFC alkyl tail; whereas, hydration may be loosely bound within the layered structures. The importance of dipolar interactions, such as H-bonding, for stabilizing multilayers and aggregates is well established.57 The onset of the melting endotherm for the adsorbed PFOA to higher temperature (ca. 62 °C) and the offset of the dehydration endotherm to lower temperature (103 °C) for the M6-PFOA-7 system, relative to free PFOA, and provide evidence for multilayer formation. The DSC results indicate that the monolayer and multilayer structures for the MIMs/PFOA system are stabilized by Hbonding and ion−dipole interactions, in agreement with the above isotherm results. The DSC results of the unbound OA (Figure 4) consist of a very broad dehydration transition at higher temperature (ca.140 °C) that is characteristic of fatty acids, in agreement with the strong ion−dipole interactions in water.86 On the other hand, the DSC results for the MIM-6/OA mixtures are characterized by at least two dehydration events at ∼94 and 105 °C characteristic of bulk and bound water. The thermal events at higher temperature display sharp transitions characteristic of monolayer formation with ordered hydration. A lower enthalpy of dehydration for MIM-6/OA mixtures relative to MIM-6/ PFOA mixtures is hypothesized due to the formation of monolayers containing reduced hydration due to self-assembly, according to Scheme 3b. In contrast, multilayer structures were reported to ingress greater amounts of water as characterized by

gels.78−80 Figures 3 and 4 show the DSC results of hydrated solids containing MIM-6/PFOA and MIM-6/OA systems. In

Figure 3. DSC thermogram results of the hydrated samples of PFOA and systems containing MIM-6 at different PFOA loadings.

Figure 4. DSC thermogram results of the hydrated samples of OA and its mixtures with MIM-6 at different loading levels of OA.

Figure 3, the DSC traces for PFOA in the bound and unbound states with MIM-6 are shown, where unbound PFOA reveals an endotherm at lower temperature (∼45 °C), and this is attributed to the melting transition (cf. Table 3). The melting endotherm of PFOA shifts to higher temperatures for the MIM-6/PFOA mixtures as the loading of PFOA increases, especially at 4 mM (∼58 °C) and 7 mM (∼62 °C). The dehydration transition of free PFOA is composed of two main contributions at ∼100 and 120 °C, in addition to a small broad peak ∼130 °C. The variable dehydration endotherms for the unbound PFOA reveal different levels of bound water along the length of the PFC chain. Knowlton and White81 used DSC endotherms to describe three types of water bound to zeolite: external (bulk), loosely bound, and tightly bound water. Similarly, different types of hydration are anticipated for the surfactant anions and the MIMs surface due to variation in the hydrophile−lipophile character of the adsorption sites (cross-linker vs β-CD inclusion sites),82 according to the isotherm results herein. As well, water bound near the carboxylate headgroup is likely to desorb at higher temperature relative to bulk or hydration adjacent to the apolar PFC chain.83 6561


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Figure 5. FT-IR spectra of hydrated solids: (a) MIM-6, (b) PFOA, and (c) M6-PFOA-1, (d) M6-PFOA-4, and (e) M6-PFOA-7 systems at 295 K. The numbered bands (1−3) show the CF2/CF3 stretching bands for bound PFOA.

Figure 6. FT-IR spectra of hydrated solids: (a) MIM-6, (b) OA, and (c) M6-OA-1, (d) M6-PFOA-4, and (e) M6/PFOA-18 systems at 295 K.

Table 5. FT-IR Absorption Bands of MIM-6, PFOA, and their Bound Complexes wavenumber (cm−1) assigned bands

MIM-6

PFOA

M6-PFOA-1

M6-PFOA-4

ν(OH) ν(N−H) νas(CH) + νs(CH) ν(CO) δ(NH) + ν(C−N) ν(C−N) νas(CF2) + νas(CF3) νas(CF2), νs(CF2)

3538 3367 2934, 2859 1720, 1653 1545 1250 -

Broad 1777 1205 1242, 1149

3534 3363 2934, 2862 1717, 1645 1533 1252 1269, 1167

3533 3351 2935, 1719, 1529 1246, 1228 1263,

broader transitions and increased enthalpy of dehydration,85 as observed for the MIM-6/PFOA system. 3.2.2. FT-IR Spectroscopy. FT-IR spectroscopy was used to characterize the dipolar interactions for the surfactant systems with MIM-6 since IR was reported for the study of H-bond formation in polyurethanes and related compounds.54,87 Furthermore, FT-IR was used to probe spectral variations

2863 1645 1180 1167

M6-PFOA-7 3530 3344 2947, 1787, 1527 1250, 1228 1257,

2868 1707, 1647 1190 1173

between self-assembled structures such as monolayers and multilayers57 since these structures are often stabilized by dipolar interactions. The FT-IR spectra for MIM-6/PFOA and MIM-6/OA systems are shown in Figures 5 and 6. The general features of the vibrational bands in Tables 5 and 6 are in agreement with a previous report.54 6562


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The Journal of Physical Chemistry C Table 6. Selected FT-IR Spectral Bands for MIM-6, OA, and MIM-6/OA Systems assigned bands

MIM-6

OA

M6-OA-1

M6-OA-4

M6-OA-18

υ(OH) υ(NH) νas(CH) + νs(CH) υ(CO) δ(NH) + ν(C−N)

3538 3386 2934, 2859 1720, 1653 1545

broad broad 2942, 2876 1748, 1715 -

3576 3337 2949, 2873 1724, 1661 1562

3582 3347 2942, 2873 1722, 1659 1564

3582 3347 2946, 2876 1722, 1655 1562

Table 5, the FT-IR shift and intensity changes of the guest at ∼1200 cm−1 (band 2; νasCF2 + νasCF3) and ca. 1250 cm−1 (band 3; νsCF2) and 1150 cm−1 (band 1; νsCF2) were observed. The intensity of the CF3 band 2 increases at the expense of the CF2 band 3. Viana et al.57 reported integrated IR band areas for the characterization of multilayer structures. In Figure 5, an increase in the relative intensity of the CF3 bands (2) at ∼1205 cm−1 for the M6-PFOA-7 system provides further support of self-assembly and multilayer adsorption processes, as depicted by Scheme 3a. The FT-IR results for the MIM-6/OA mixtures are shown in Figure 6 and Table 6. The spectra of the OA mixtures in Figure 6b−e are similar with minor shifts (ca. 2−4 cm−1) for the amide I and II bands. The −OH band (∼3400−2400 cm−1) for OA overlaps with the C−H bands (∼2900 cm−1) that are typical of fatty acids.93 The band shifts for the −CO and −NH groups range from 13 to 20 cm−1 or up to 60 cm−1 for the MIM-6/PFOA mixtures. There are minimal band shifts for the MIM-6/OA mixtures, in agreement with the formation of a monolayer (cf. Scheme 3b) and the isotherm results in Figure 1b. Furthermore, the relative intensities of the −NH band at ∼1550 cm−1 are not significantly affected by changes in OA concentration as compared with the MIM-6/PFOA system described above. The results suggest that H-bonding interactions involving the carboxylate headgroup are not apparent for the MIM-6/OA systems, in agreement with the formation of a typical monolayer onto the MIMs surface via adsorption of the alkyl chain of OA with hydration of the headgroups (cf. Scheme 3b). 3.2.3. Contact Angle. Contact angles (θ-values) were measured to further characterize the nature of the surface interactions between MIM-6 with PFOA and OA, respectively. Contact angle was used to successfully characterize the surface properties of films and coatings.94 As well, the modality of adsorption at the solid−solution interface can be inferred from the wetting behavior of films according to θ-values, especially when the chemical nature and surface roughness of films are controlled.95,96 Static θ-values of droplets containing aqueous solutions of surfactant were measured at ambient conditions. Thin films of MIM-6 were prepared by drop-casting onto silicon wafer substrates where the measured θ-values for various liquid samples are listed in Table 7. The static θ-value for water on the untreated silicon wafer substrate was close to 90°, while the corresponding value on the MIM-6 film was 82 ± 1°. The results indicate that the silicon wafer is more hydrophobic relative to the MIM-6 film.93 Arkales94 reports θ values for water on silicon substrates ranging between θ = 86 and 88°.

The FT-IR spectra for the MIM-6/PFOA systems (Figure 5c,d) are dominated by IR bands of MIM-6(a). The spectrum of unbound PFOA (b) is characterized by a broad −OH stretching band at ∼3300−2600 cm−1 which is typical of carboxylic acids and the formation of H-bonded dimers.88 The key vibrational bands for the MIM-6/PFOA systems are summarized in Table 5. In general, broad −OH (∼3400 cm−1) and −CO (∼1700 cm−1) bands are observed for the mixtures (c−e) relative to MIM-6 (a) and unbound PFOA (b), providing additional evidence of dipolar interactions between MIM-6 and PFOA.89 The −CO bands of the MIMs/PFOA systems are shifted by as much as 60 cm−1 (blue-shifted) and 13 cm−1 (red-shifted) relative to the free PFOA and MIM-6 adsorbent, respectively. The significant changes in the −CO stretching in the presence of MIM-6 reveal the important role of the carboxylate anion interaction for the adsorption at the solid−solution interface.54 The amide I region for MIM-6 (a) and the mixtures (c−e) in Figure 5 consistently show a band at ∼1710 cm−1 (urethane −CO vibrational band) and a shoulder at lower frequency (∼1650 cm−1), assigned to a H-bonded urethane carbonyl feature.88 An additional band at ∼1787 cm−1 was recorded for the M6-PFOA-7 system with blue shifts of approximately 10 and 67 cm−1, relative to the free acid and MIMs adsorbent, respectively. In its adsorbed state, PFOA resides at the MIMs− solution interface as a H-bonded complex with the −OH/−NH groups, in contrast to the carboxylate moieties of PFOA in subsequent layers (cf. Scheme 3a). The IR bands of the acidic forms of benzoic and salicylic acids in solution undergo a wavenumber shift (∼80 cm−1) from ∼1620 to ∼1700 cm−1 relative to the position of the respective unbound carboxylate anions.53 As well, the formation of weak H-bonds may result in shortened X−H bonds (where X = O, F, or N) with subsequent attenuation of the FT-IR bands and blue frequency shifts.90 The observed shifts in frequency and band intensity of the COO−/COOH bands for the MIM-6/PFOA systems, relative to free PFOA and MIM-6, provide support that dipolar interactions occur when PFOA is adsorbed onto the MIMs surface.91 These dipolar interactions play an important role in the formation and stabilization of multilayers, aggregates, and complexes. In addition to the carbonyl bands, frequency shifts and changes in band intensities of the amide II (∼1500 cm−1) and the guest CF2/CF3 (∼1200 cm−1) stretching bands (bands 1−3 in Figure 5) are noted. The amide II band (∼1500 cm−1) results from the N−H bending and C−N stretching and displays a decreased frequency of up to 20 cm−1 for the mixtures as compared to MIM-6 (cf. Table 5). These frequency shifts are accompanied by a reduced intensity of the −NH band at ∼1500 cm−1 for the mixtures (c−e), relative to unbound MIM-6 (a). This is particularly true for the mixed system with the highest PFOA loading (M6-PFOA-7; e) where increased Hbonding for amide substituents was reported for the amide II band with attenuated wavenumber position.92 In Figure 5 and

Table 7. Static Contact Angles (θ) for PFOA and OA on MIM-6 Polymer Film at Variable Loading Levels at 295 K

6563

sample

OA-1

OA-18

PFOA-1

PFOA-4

PFOA-7

contact angle (deg)

87

83

82

79

75


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Figure 7. Calorimetric titration of (a) β-CD/PFOA and (b) MIM-1/PFOA at pH ∼ 4 and 298 K. The ITC isotherms reveal 1:1 and 2:1 β-CD/ PFOA (a-ii) and 1:1 MIM-1/PFOA (b-ii).

Figure 8. Calorimetric titration of host/guest systems: (a) β-CD/OA and (b) MIM-1/OA systems at pH ∼ 8 and 298 K.

The variable θ-values in Table 7 for OA (1 and 18 mM) and PFOA (1 mM) solutions relate to the attractive−repulsive interactions with the MIM-6 surface film. Tavana et al.95 studied the variation of dipole moments for a series of naphthalene congeners as a function of their interaction with a

fluorinated acrylate polymer films using θ-values. A decrease in θ was observed for naphthalene derivatives that contain highly electronegative substituents such as fluorine, where the results indicate the role of adhesive interactions at the solid−liquid interface. The low θ-values for M6-PFOA-7 suggest an 6564


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Table 8. Binding Constants (Ki; i = 1:1 or 2:1) and Standard Enthalpy (ΔH°) and Entropy Changes (TΔS°) for the Inclusion Complexes for PFOA and OA with MIM-1 at pH ∼ 4 (PFOA) and pH ∼ 8.5 (OA, in Phosphate Buffer) at 298 K H/G system β-CD/PFOA

K1:1

mode

n

1:1 1.0(±0.1) 2:1 1.8(±0.2) MIM-1/PFOA 1:1 1.0(±0.1) β-CD/OA 1:1 1.0(±0.3) MIM-1/OA 1:1 1.2(±0.1) (L mol−1), K2:1 (L mol−1)2, R = 8.3145 J/mol K

ΔG° (kJ/mol) −28.8 −22.7 −25.2 −19.3 −25.8

K2:1 (L2/mol2)

K1:1 (L/mol) 1.1(±0.7) 2.6(±1.0) 2.4(±1.8) 3.4(±3.0)

× 10

5

9.7(±11.) × 103 × 104 × 103 × 104

ΔH° (kJ/mol) −39.7 −69.1 −51.1 −11.1 −28.2

± ± ± ± ±

4.9 16. 8.1 6.2 5.2

TΔS° (kJ/mol) −10.9 −46.4 −25.9 +8.20 −2.40

type” 2:1 host−guest structures due to the structural disorder of β-CD for this polymer. Thus, 1:1 complexes are favored for the MIM-1/PFOA system. The inflection point beyond the 1:1 ratio in Figure 7b-ii (marked with a circle ∼0.5 host/guest mole ratio) may be attributed to secondary binding of PFOA at the noninclusion sites of the MIM-1 polymer, as illustrated in Scheme 3. The formation of stable inclusion complexes for the MIM-1/PFOA system is consistent with the inclusion site accessibility and the notable inclusion binding affinity of β-CD. The binding constants for β-CD/PFOA (K1:1 ∼ 1.1 × 105, K2:1 ∼ 9.7 × 103)30,33 and β-CD/OA (2.4 × 103)98 complexes are listed in Table 8 and show good agreement with independent estimates.30 The standard change in Gibbs energy (ΔG°) of complex formation for PFOA is largely enthalpy (ΔH°) driven, while the value of ΔG° for the β-CD/OA system is entropy driven in most cases. The binding of OA to β-CD/ MIM-1 is generally characterized by an increased entropy, where |ΔH°| < |TΔS°| for these systems. The higher entropic contribution is attributed to desolvation of the complex and the cross-linker domains. The fact that the binding process for the β-CD/OA and MIM-1/OA systems is entropy-driven reveals that the configurational entropy of the host (β-CD or MIM-1), guest (OA), and solvent must be considered when assessing the enthalpy contributions that govern complex stability.99,100 In the case of β-CD/PFOA and MIM-1/PFOA systems, the decreased entropy suggests the presence of complementary size-f it consideration for the stability of this host−guest system and the important role of the hydrophobic effects.101 Furthermore, the favorable enthalpy of the MIM-1/PFOA system relative to the MIM-1/OA highlights the importance of dipolar interactions in the formation and stabilization of selfassembled multilayer structures for systems containing PFOA.

increased wetting on the MIM-6 surface due to favorable adhesive interactions between the MIMs surface and PFOA. These results support the formation of monolayer vs multilayer structures, where dipolar interactions are prominent for the latter. In spite of the variable surface activity of OA and PFOA, the comparable θ values for these systems preclude a detailed interpretation of the surface interactions at the MIMs interface. Furthermore, the MIM-6 films were drop-casted from a solution of DMSO which may influence surface roughness of the MIMs surface films, and the presence of trace solvent residues may also affect the θ-values since the nature of the dipolar interactions likely vary with trace levels of DMSO. The results for the θ-values herein show good general agreement with results for polymer films55 and provide further support for the isotherm, FT-IR spectra, and DSC results described above.

3.2.4. ISOTHERMAL TITRATION CALORIMETRY (ITC) ITC provides thermodynamic characterization of host−guest interactions for CD-based systems.96,97 The sensitivity of ITC to enthalpy contributions of strongly bound host−guest complexes and their accompanying hydration phenomena and thermodynamic variation provide a basis for understanding the nature of the noncovalent interactions,96 in a similar way that adsorption processes of surfactants with the MIMs interface are amenable to ITC studies. The ITC results for β-CD/PFOA and MIM-1/PFOA systems were compared at ambient temperature (cf. Figure 7a and b). The results for β-CD/OA and MIM-1/ OA systems are shown in Figure 8a and b, where the corresponding thermodynamic parameters are listed in Table 8. The low water solubility of MIM-6 precludes an ITC study in homogeneous solution. Therefore, MIM-1 was used as the host substrate for ITC measurements (cf. Section 2.2.7). The ITC results indicate two types of binding equilibria according to the formation of a 1:1 complex (β-CD/OA; Figure 8a) and a 1:1 plus 2:1 complex (β-CD/PFOA; Figure 7a) for these systems. It should be noted that β-CD forms 1:1 (n = 1) and 2:1 (n = 2) inclusion compounds with PFOA,26,30,72 where two equilibrium constants (K1:1 and K2:1) describe the binding of PFOA to βCD. In the case of MIM-1, the formation of a 1:1 β-CD/PFOA complex is anticipated as the major stoichiometry for this host− guest system. HC analogues such as OA generally form a 1:1 βCD/OA complex,29 but higher-order host−guest stoichiometry such as 2:1 was reported in a previous ITC study at lower pH.98 In Figure 7a-ii, the molar enthalpy for the 1:1 complex is greater (800 μJ) than the molar enthalpy for the 2:1 complex (400 μJ). This offset is consistent with the relative binding affinity (K1:1 > K2:1) in Table 8 and the partial inclusion geometry of the 2:1 complexes described previously.30 The MIM-1 polymer does not favor the formation of 2:1 complexes with PFOA due to the linear topology of the polymer and the configurational entropy of β-CD along the polymer chain. Thus, MIM-1 does not favor the formation of such “channel-

4. CONCLUSION A systematic study was carried out to investigate the molecular level details of the adsorption processes between several anionic surfactants (perfluorooctanoate, PFOA; perfluorooctanesulfonate, PFOS; and octanoate, OA) with macromolecular imprinted materials (MIMs) in aqueous solution. The MIMs adsorbents contain β-CD within a urethane framework which affords two types of binding sites: β-CD inclusion sites and hexamethylene cross-linker noninclusion sites with polar (−OH/ −NH) domains. The adsorption properties relate to the inclusion and noninclusion sites which were studied using FT-IR spectroscopy, DSC, contact angle, and ITC. The unique role of the headgroup interactions was concluded by systematically comparing the isotherms of PFC anions with carboxylate and sulfonate headgroups (PFOA and PFOS) with the MIMs adsorbents. Unique adsorption behavior was observed for PFC surfactant anions (PFOA and PFOS) that differ markedly from the HC anions (OA). PFOA and PFOS anions adopt selfassembled multilayer structures onto the MIMs surface by 6565



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binding via the headgroup with the polar domains (−OH, −NH) at the MIMs interface. Bilayers and multilayer structures (micelles and hemimicelles) form at higher concentration of PFC surfactant (cf. Scheme 3a), while OA anions are adsorbed as conventional monolayers via the n-octyl chain onto the MIMs interface (Scheme 3b). Adsorption of anionic surfactants by MIMs adsorbents with low levels of cross-linking are mediated by inclusion binding as the dominant uptake process. In the case of the highly crosslinked adsorbents such as MIM-6, steric effects attenuate inclusion binding, and the primary mode of adsorption occurs at the noninclusion (interstitial) sites. This study illustrates the unique and variable modes of adsorption for PFOA and PFOS (monolayer and multilayer) and OA (monolayer only) onto MIMs adsorbents using complementary experimental methods (FT-IR, contact angle, ITC, and DSC). This study is anticipated to contribute to the further development of “smart” polymer surfaces with controlled adsorption properties to facilitate supramolecular patterning and assembly at the polymer/solution interface for diverse molecular recognition and sensing applications. Further spectroscopic studies are underway to develop a more detailed understanding of the structure−activity relationship for such polymer−surfactant systems reported herein at variable conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. The authors have given approval to the final version of the manuscript and have contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the partial support provided by the University of Saskatchewan and Dr. Howard Wheater of the Global Institute of Water Security for partial support of this research (CERC subproject 21111). A.H.K. wishes to acknowledge Jason Maley and Kenneth Thoms (Saskatchewan Structural Sciences Centre) at the University of Saskatchewan for their expertise and technical assistance in obtaining the ITC and ESI-MS results. L.D.W. acknowledges the University of Saskatchewan for supporting this research and Computers for Schools (Saskatoon, SK) for the donation of a desktop computer.

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DEDICATION The corresponding author (Lee D. Wilson) dedicates this paper in honor of Ronald E. Verrall to acknowledge his many significant research contributions and dedicated mentorship to generations of students throughout his distinguished career as a Professor of Chemistry at the University of Saskatchewan.

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Article

ABBREVIATIONS

β-CD, β-cyclodextrin; HDI, hexamethylene diisocyanate; PFOA, perfluorooctanoic acid; OA, octanoic acid; PFOS, perfluorooctanesulfonate; MIMs, macromolecular imprinted materials; BET, Brunauer−Emmet−Teller 6566


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