Article pubs.acs.org/EF
Thermodynamic Properties of Inclusion Complexes between β‑Cyclodextrin and Naphthenic Acid Fraction Components Mohamed H. Mohamed,† Lee D. Wilson,*,† John V. Headley,‡ and Kerry M. Peru‡ †
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada Water Science and Technology Directorate, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan S7N 3H5, Canada
‡
ABSTRACT: The spectral displacement technique was used to determine the 1:1 equilibrium binding constants (K2) of the complexes formed between β-cyclodextrin and carboxylate anion guests, including either single components or mixtures in aqueous solution at variable temperatures (25−55 °C). A van’t Hoff analysis of the results afforded thermodyamic parameters (ΔH°, ΔS°, and ΔG°) of the single-component carboxylate anions and complex mixtures of carboxylic acid species, referred to as naphthenic acids (NAs). Three types of single-component examples of NAs with variable hydrogen deficiency (z) values were studied: n-octanoic (z = 0; S1), trans-4-pentylcyclohexanecarboxylic acid (z = −2; S2), and dicyclohexylacetic acid (z = −4; S3). The carboxylate anion mixtures were obtained from commercial suppliers and an industrial source obtained from oil sands process water. The estimated K2 values decrease with increasing temperature, and the standard Gibbs energy change (ΔG°) for complex formation is generally favorable (from −16 to −28 kJ/mol) and largely enthalpy-driven (from −12 to −30 kJ/mol). The change in entropy of complex formation (ΔS°) for S1, S2, and S3 varies (−23 and 44 J mol−1 K−1) depending on the guest size and relative lipophilicity. The positive correlation between complex stability, host−guest size-fit complementarity, and lipophilicity of the carboxylate anion reveals the importance of the hydrophobic effect, as evidenced by compensation phenomena for such host−guest complexes.
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INTRODUCTION The western Canadian oil sands industry in northern Alberta uses caustic warm water (i.e., the Clark caustic extraction process) to recover oil-laden bitumen during processing from the sand and clay fractions.1,2 The ensuing oil sands process water (OSPW) is saline and contains a complex mixture of organic compounds dominated by a class of naturally occurring naphthenic acids (NAs) shown in Scheme 1. The alkaline conditions enable solubilization of the carboxylate ions in aqueous solution. The conventional definition of NAs refers to these chemical species as carboxylic acids and may include one or more saturated ring structures. The definition has become more generally used to describe the range of organic acids found within oil-sands-process-affected waters.3 The broader definition includes oil sands acid-extractable organics with aromatic functional groups and nitrogen and sulfur atoms, along with unsaturated groups (chemical structures not shown).4−6 Classical NAs are a component of naphthenic acid fraction components (NAFCs) that are non-volatile and chemically stable and possess surface activity.7 The acid dissociation constants range between 10−5 and 10−6 M−1 and are similar to typical carboxylic acids (acetic acid, 10−4.7 M−1; propionic acid, 10−4.9 M−1; and palmitic acid, 10−8.7 M−1).8−10 The presence of NAs in petroleum has led to environmental, health, and industrial concerns because NAs are known to be toxic to aquatic organisms, algae, and mammals.8−11 The toxicology of the various single components and mixtures of NAs is poorly understood.12,13 Thus, the Government of Canada has issued a zero discharge policy because of environmental concerns and the limited understanding of the toxicity of NAs and other toxic © 2015 American Chemical Society
Scheme 1. Selected Molecular Structures of NAFCs in OSPWa
a R = alkyl group; X = COOH, R, OH, SOx, NOx, and SH; and Y = C, S, and N. Ring structures may not be fully saturated.
components found in OSPW.14−16 The long-term storage of OSPW in large on-site settling ponds is potentially problematic because of hydraulic fracturing and the possibility of accidental Received: February 5, 2015 Revised: May 7, 2015 Published: May 8, 2015 3591
DOI: 10.1021/acs.energyfuels.5b00289 Energy Fuels 2015, 29, 3591−3600
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Energy & Fuels release of the principal toxic components from oil sands tailings, such as NAs, into surface and groundwater supplies.17 Therefore, there is a great need to develop improved methods to remove leachates, such as NAs and the like, from aquatic environments to alleviate concerns of their deleterious health and environmental effects. Cyclodextrins (CDs) are of interest because of their ability to form inclusion complexes in aqueous solutions. CDs are wellknown to form relatively stable inclusion complexes with aliphatic and alicyclic carboxylic acids.18−21 CDs also possess strong binding affinity to NA molecules, as confirmed in a recent study.22 More recently, polymeric materials containing β-cyclodextrin (β-CD) were used to adsorb NAs from aqueous solutions with considerable success.23,24 Mohamed et al.24 demonstrated that urethane polymers containing β-CD display molecular selective sorption toward mixtures of NAs.25 The unique molecular recognition properties of such polymers are attributed primarily to the β-CD inclusion sites of the copolymer framework. However, there is a knowledge gap concerning the origin of the observed molecular selective uptake of NAs by such CD urethane polymers. Thus, an evaluation of the thermodynamic properties of the complexes formed between β-CD and NAs is highly relevant for addressing the origins of molecular selectivity reported for polymer sorbent materials that contain β-CD.26,27 This study outlines the use of the spectral displacement technique28 to determine the 1:1 binding constants for complexes formed between the inclusion sites of β-CD and alkyl carboxylate ions (single components and mixtures thereof) with variable molecular structure.19,22 Urethane copolymers containing β-CD were reported to display molecular selective fractionation of NAs in aqueous solution.25 To understand the thermodynamic contributions of the crosslinker framework and the β-CD adsorption sites of such polymer sorbents with NAs,26,27 an understanding of the relative binding contributions of the framework components29,30 is required. In turn, an understanding of the molecular recognition and sorption properties will lead to the rational design of polymer sorbent materials with enhanced uptake properties.31 This paper reports the determination of 1:1 binding constants and thermodynamic parameters (ΔH°, ΔS°, and ΔG°) for β-CD with single- and multi-component alkylcarboxylate anions in aqueous solution at alkaline pH. The singlecomponent guests were chosen as representative examples of specifc classes of NAs, according to their variable hydrogen deficiency (z values) and relative molecular weight. The single components include the following: n-octanoic acid (z = 0; S1), trans-4-pentylcyclohexanecarboxylic acid (z = −2; S2), and dicyclohexylacetic acid (z = −4; S3), as shown in Figure 1. The complex mixtures of NAs studied were commercially available (Acros and Fluka NAs) and industrial-sourced NAs (extracted from OSPW in Canada’s Athabasca region).
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Figure 1. Molecular structure of the carboxylic acids with variable z values, carbon number, and chemical structure: n-octanoic acid (z = 0 and C = 8; S1), trans-4-pentylcyclohexanecarboxylic acid (z = −2 and C = 12; S2), and dicyclohexylacetic acid (z = −4 and C = 14; S3). S3) were varied to exceed the 1:1 host−guest mole ratio. A stock solution of phenolphthalein (phth) in ethanol was made, and aliquots were used to prepare aqueous solutions of phth in buffer. The ethanol/ water (0.04%, v/v)27 solution was used to increase the solubility of phth. All aqueous solutions were freshly prepared and run within 1.5 h to ensure that the absorbance changes because of any instability of phth at pH 10.5 did not contribute to experimental artifacts. The relative compositions of Acros, Fluka, and industrial-sourced NAs were determined with electrospray ionization mass spectrometry (ESI−MS) using a Quattro Ultima mass spectrometer (Micromass, U.K.) equipped with an electrospray interface operating in the negative-ion mode configuration, as described previously.22 β-CD (VWR) was used as received, and the water content was determined using a thermogravimetric analyzer (Q5000, TA Instruments). The water contents were accounted for in the determination of Cβ‑CD, where the hydrate content varied between 11.0 and 11.5% (±0.3%) for different samples of β-CD hydrate. n-Octanoic acid (S1), trans-4-pentylcyclohexanecarboxylic acid (S2), dicyclohexylacetic acid (S3), Acros NAs, and Fluka NAs were obtained from Sigma-Aldrich. An industrial-sourced sample of NAs was extracted from Athabasca OSPW using a reported procedure.32 Phth (Aldrich) was used as received because no differences in the molar absorptivity between the purified and unpurified materials was observed.33 A nonlinear least-squares (NLLS) fitting procedure was used to determine the 1:1 binding constants, K1 (β-CD/phth) and K2 (β-CD/ inclusate), according to eqs 1−6.28 The method uses the Beer− Lambert law and the assumption that the molar absorptivity of the βCD/phth complex is zero.34 The calculation of binding constants assumes molar concentrations as activities; thus, the reported values of K1 and K2 herein are not dimensionless. Scheme 2 illustrates the
Scheme 2. Formation of a 1:1 Host−Guest Complex for βCD (Toroid) and a Chromophore Guest Species (Rectangle), where K1 Is the 1:1 Equilibrium Binding Constanta
a
The addition of an optically transparent inclusate (I) that displaces phth and the oval is included by β-CD as a 1:1 CD−I complex according to the 1:1 binding constant, K2.
relevant equilibria and mass balance relations for the spectral displacement method.
EXPERIMENTAL SECTION
Absorption measurements were obtained using a double-beam spectrophotometer (Varian CARY 600) at four temperatures (25, 35, 45, and 55 °C). All solutions were prepared by volume in a 0.1 M sodium hydrogen carbonate buffer adjusted to pH 10.5 with 6 M sodium hydroxide. The concentration of phenolphthalein (Cphth) was maintained at ∼2.0 × 10−5 M in all experiments. For the determination of the 1:1 CD/inclusate binding constants, K2, the concentration of β-CD (Cβ‑CD) was held constant at ∼2.6 × 10−4 M and the concentration of NAs or single-component NAs (S1, S2, or
[I]o = [I] + [CD−I] = [I](1 + K 2[CD])
(1)
[phth]o = [phth] + [CD−phth] = [phth](1 + K1[CD])
(2)
[CD]o = [CD] + [CD−I] + [CD−phth]
(3)
The terms [I]0, [I], and [CD−I] refer to the total, unbound, and bound inclusates, respectively. [CD−I] and [CD−phth] are related to [I]o, [phth]o, K2, and K1 as follows: 3592
DOI: 10.1021/acs.energyfuels.5b00289 Energy Fuels 2015, 29, 3591−3600
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Energy & Fuels ⎛ ⎞−1 1 [CD−I] = [I]o ⎜1 + ⎟ K 2[CD] ⎠ ⎝
(4)
−1 ⎛ 1 ⎞ [CD−phth] = [phth]o ⎜1 + ⎟ K1[CD] ⎠ ⎝
(5)
Upon substitution into eq 3 and rearrangement, the following cubic expression in terms of unbound β-CD ([CD]) is shown by eq 6.
⎛ K + K1 ⎞ + [phth]o + [I]o − [CD]o ⎟ [CD]3 + [CD]2 ⎜ 2 ⎝ K 2K1 ⎠ ⎛ 1 [I] [phth]o [CD]o [CD]o ⎞ + [CD]⎜ + o + − − ⎟ K1 K2 K1 K2 ⎠ ⎝ K 2K1 ⎛ [CD]o ⎞ −⎜ ⎟=0 ⎝ K 2K1 ⎠ (6)
Figure 2. Absorbance (abs; λ = 552 nm) versus concentration of β-CD (Cβ‑CD) in 0.1 M NaHCO3 at pH 10.5, with [phth] = 2.60 × 10−5 M at variable temperatures.
The real solution to the cubic root of eq 6 was obtained by application of the Newton−Raphson method35−37 with appropriate boundary values for [I]o, [phth]o, K2, and K1. [CD−I], [CD−phth], and [phth] were obtained using the Beer−Lambert law for phth and eqs 1−3. The criterion of best fit for the NLLS procedure required the minimization of the sums of the squares of the residuals (SSR), SSR = ∑i[(Acalc)i − (Aexpt)i]2, where Acalc and Aexpt are the calculated and experimental absorbance values, respectively. The lipophilic surface area (LSA), volume ratio, and solvation energies of the apolar fragments of the carboxylic acids were calculated using Spartan’08, version 1.2.0. The calculations were based on aqueous solution phase energy-minimized molecular structures. Equilibrium geometries were obtained in the ground state in aqueous solution with the Hartree−Fock 3-21G basis set.
absorbance values from eq 5 according to the NLLS fitting procedure. At 25 °C, the sharp decrease in absorbance as Cβ‑CD increases coincides with the formation of the β-CD/phth complex because the molar absorptivity of the β-CD/phth complex is nearly zero (εcomplex ≈ 0). With increasing temperature, the absorbance is less attenuated, which indicates a decrease in the fractional amount of bound phth ([β-CD/ phth]), in agreement with the lower K1 at higher temperatures. Benesi−Hildebrand40 plots were not used herein because the assumption that the bound species are much lower than [CD]o is generally not valid for strongly bound complexes at these conditions. In addition, double-reciprocal plots (1/abs versus 1/Cβ‑CD) often improperly weigh the data at lower absorbance values and higher values of Cβ‑CD. Thus, a NLLS fitting procedure was employed to obtain binding constants (K1 and K2). The estimated binding constant (K1 = 2.7 ± 0.3 × 104 M−1) for the 1:1 β-CD/phth system at 25 °C is in good agreement with other estimates.18,34,41,42 The K1 values at higher temperatures, 1.4 ± 0.2 × 104 M−1 (35 °C), 0.78 ± 0.07 × 104 M−1 (45 °C), and 0.44 ± 0.04 × 104 M−1 (55 °C) are in good agreement with those obtained by Zarzycki and Lamparczyk.43 The change in standard Gibbs free energy (ΔG°) of complex formation for the β-CD/phth system at variable temperatures was obtained, −25 ± 0.2 kJ/mol (25 °C), −24 ± 0.2 kJ/mol (35 °C), −24 ± 0.2 kJ/mol (45 °C), and −23 ± 0.1 kJ/mol (55 °C), using eq 7. The corresponding thermodynamic parameters (ΔH° and ΔS°) were calculated using the temperature dependence of K1 and the van’t Hoff relation (eq 8). The standard enthalpy (ΔH° = −49 ± 0.8 kJ/ mol) and entropy (ΔS° = −81 ± 1.7 J mol−1 K−1) were obtained, where the negative ΔS° value for the formation of the β-CD/phth complex was related to the optimal size-fit matching between host and guest, as described by Taguchi.44 The relative sign and magnitude of ΔS° reflects the overall change in hydration of the host, guest, and complex, respectively. Complex formation (ΔH° = −49 kJ/mol) is enthalpy-driven, in agreement with the attenuated binding affinity with increasing temperature.
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RESULTS AND DISCUSSION In a seminal study by Selvidge and Eftink,28 the authors outlined the general utility of the spectral displacement method that uses the decolorization of the phth dianion in its bound state as a measure of the 1:1 inclusion complex (cf. Scheme 2). The dramatic change in the photophysical properties of phth in its bound and unbound states affords quantitative assessment of the binding constant of competing optically transparent ligands, such as carboxylate anions (cf. Scheme 2).22,33 The spectral displacement method is suitable for determining relatively large 1:1 binding constants and is especially useful for singlecomponent carboxylate anions as well as their mixtures.22 Herein, the spectral displacement method was studied at variable temperatures to measure thermodynamic properties at equilibrium for complexes formed between β-CD and carboxylate anions to gain additional insight on the molecular level details of such host−guest systems. Figure 2 illustrates a typical plot of absorbance versus Cβ‑CD at a fixed concentration of phth. The concentration dependence of β-CD and the absorbance of phth provides an estimate of the 1:1 binding constant according to eq 5. To eliminate possible interferences because of competitive binding by ethanol and undue solvent effects, a dilute ethanol composition (0.04%, v/ v) was employed.33,37−39 The experimental conditions herein were similar to those employed by Selvidge et al.28 A pH value of 10.5 was chosen to optimize the optical density of phth (εphth) for the absorption measurements and to minimize the potential for deprotonation of the hydroxyl groups of β-CD (pKa ≈ 12). All absorption measurements were carried out at λ = 552 nm, and no change in the shape of the visible absorption band with Cβ‑CD was observed at these conditions. The solid line through the data points represents the calculated
ΔG° = −RT ln K i
ln K = − 3593
ΔH ° ΔS° + RT R
(7) (8) DOI: 10.1021/acs.energyfuels.5b00289 Energy Fuels 2015, 29, 3591−3600
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the concentration of free phth is much greater than that of bound phth. The concentration profile of the absorbance for a given inclusate is similar at various temperatures, but the absorbance values are offset because of enthalpic effects. As shown in Table 1, the ΔG° values range from −16 to −17 ± 0.2 kJ/mol, with negative values for the other parameters [ΔH° (−22 ± 0.4 kJ/mol) and ΔS° (−18 ± 0.5 J mol−1 K−1)]. The negative entropy contribution is attributed to inclusion of the apolar alkyl chain of the guest with a concomitant change in the hydration state of the guest upon formation of the host/guest complex. Figure 3b represents a strongly bound β-CD/inclusate system, where the absorbance changes sharply for S2 and the shape of the isotherm becomes increasingly sigmoidal, while the displacement of phth occurs over a relatively narrow range of the guest concentration. A maximum amount of phth is displaced when [S2]o reaches ∼1.6 mM, which is ∼5.3% relative to that for S1. Similar to S1, the absorbance of phth increases with increasing temperature. Values of ΔG° range from −27 to −28 ± 0.2 kJ/mol, while the corresponding enthalpic (ΔH° = −26 ± 0.5 kJ/mol) and entropic (ΔS° = 4.4 ± 0.2 J mol−1 K−1) values are given in Table 1. The positive entropy value for S2 is attributed to an overall net release of hydration to the bulk solvent because apolar guests with greater LSA often display an optimal size-fit relationship with the β-CD cavity interior.45 In Figure 3c, S3 displays an intermediate absorbance change when compared to S1 and S2, where the change in absorbance is more pronounced for S2. A maximum amount of phth is displaced when [S3]o reaches ∼5.0 mM; this concentration is 3-fold higher relative to S2 and 5-fold smaller than that for S1. The values of ΔG° for the various inclusates (S1, S2, and S3) range from −28 to −16 ± 0.2 kJ/mol with negative enthalpic (ΔH° from −26 to −18 kJ/mol) and variable entropic (ΔS° from −18 to 18 J mol−1 K−1) contributions. The relative ordering of ΔS° from positive to negative is given as S3 > S2 > S1. The trend is similar to the relative LSA of each inclusate: S1 (170 Å2), S2 (222 Å2), and S3 (233 Å2). There is a correlation between the standard entropy change and the LSA of the inclusate, where the ΔS° value becomes increasingly more positive as the LSA increases, in agreement with hydrophobically driven processes. Furthemore, the isotherms for S1, S2, and S3 illustrate differences in ΔG° (cf. Table 1) of the β-CD/inclusate systems as follows: K2 (S2) > K2 (S3) > K2 (S1), and range from 102 to 104 M−1. There are several factors that govern the formation of CD inclusion complexes,46 where hydrophobic hydration is a major driving force in the case of apolar and amphiphilic guests. The role of the hydrophobic effect is borne out by the correlation between the K2 values and bound LSA47,48 of the guest in the β-CD cavity. The approximate cavity length (8 Å)49 and diameter (6.6 Å) of β-CD can accommodate planar-shaped inclusates (∼106 Å2), while the cavity volume is ∼262 Å3, affords the inclusion of apolar groups, such as C12 alkyl chains.33 In contrast, α-CD has a smaller cavity diameter, which may partially include an octyl chain.48 Although the LSA values of the inclusates are obtained from approximate LSA calculations, the results provide semi-quantitative support for the stabilizing role of hydrophobic effects. The utility of LSA values finds support by Connors49 in his review of inclusion complexes between CDs and substituted carboxylic acids (cf. Table 1 in ref 44) for a series of cyclohexane carboxylic acids. The results for S1−S3 indicate that surface contributions and hydration phenomena stabilize such host−guest complexes with β-CD.33,50−52 This is illustrated by the Gibbs surface
Panels a−c of Figure 3 are representative plots of absorbance versus Cinclusate, which illustrate the formation of β-CD/inclusate
Figure 3. Absorbance (abs; λ = 552 nm) versus concentration: (a) S1, (b) S2, and (c) S3 in 0.1 M NaHCO3 at pH 10.5, with [phth] = 2.60 × 10−5 M and variable temperatures.
complexes at various temperatures. The inclusates represent the single-component NAs (S1, S2, and S3), and the absorbance values exhibit a smooth monotonic increase as Cinclusate increases. The trend corresponds to the displacement of phth from the β-CD cavity upon inclusion binding for the S1−S3 guests, where S1 displays the weakest binding (cf. Figure 3a) among the various single-component NAs in Figure 3. A maximum amount of phth is displaced when [S1]o reaches ∼30 mM. At higher temperatures, the absorbance increases because 3594
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Table 1. Thermodynamic Parameters of Complex Formation for Various β-CD/Guest Inclusion Complexes in Aqueous Solution at Variable Temperatures inclusion complex β-CD/S1
β-CD/S2
β-CD/S3
β-CD/Acros
β-CD/Fluka
β-CD/OSPW
temperature (°C)
K2 (×104, M−1)
ΔG (kJ/mol)
ΔH (kJ/mol)
ΔS (J mol−1 K−1)
25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55 25 35 45 55
0.070 0.064 0.046 0.031 6.2 4.8 3.3 2.4 1.3 0.91 0.79 0.65 3.3 2.3 1.9 1.5 2.8 2.0 1.8 1.7 1.3 0.8 0.5 0.4
−16 −17 −16 −16 −27 −28 −27 −28 −23 −23 −24 −24 −26 −26 −26 −26 −25 −25 −26 −27 −23 −23 −23 −23
−22 ± 0.4
−18 ± 0.5
−26 ± 0.5
4.4 ± 0.2
−18 ± 0.3
18 ± 0.5
−21 ± 0.4
18 ± 0.4
−12 ± 0.2
44 ± 0.8
−30 ± 0.6
−23 ± 0.5
these binary (guest + phth) systems and subtracted from the ternary (guest + phth + β-CD) system. The formation of 1:1 inclusion complexes was independently verified for these host− guest systems from a recent ESI−MS study.56 The reliability of the spectral displacement method requires that the inclusate fully displaces phth from the β-CD cavity to form a well-defined inclusion complex and not as a ternary outersphere complex, because the latter is known to affect the magnitude of K2.39 An additional practical consideration is that the value of K1 (β-CD/ phth) ≥ K2 (β-CD/inclusate)28 results in displacement of phth over a suitable concentration range to obtain measurable absorbance changes. Panels a−c of Figure 4 represent typical plots of absorbance versus Cinclusate for β-CD/inclusate systems. The commercially available and industrial-sourced NAs (Acros, Fluka, and OSPW) display profiles that are similar to that in Figure 3b for S2. Panels a−c of Figure 4 reveal behavior observed for that of a strongly bound β-CD/inclusate system, where the absorbance values change sharply over a narrow guest concentration. The isotherms are increasingly sigmoidal because the displacement of phth occurs over a narrower range of guest concentrations. In all cases, the absorbance increased as the temperature increased, in agreement with the negative enthapy contributions of the β-CD/phth system. Acros NAs are bound more strongly than either Fluka or Syncrude NAs, as observed by the more pronounced sigmoidal appearance for the Acros material (cf. panels a−c of Figure 4). In Figure 4a, the maximum amount of phth is displaced when [Acros]o ∼ 3.0 mM. The latter is approximately half the concentration range observed for S2. ΔG° (Acros) = −26 ± 0.2 kJ/mol for all temperatures, with a negative enthalpy (ΔH° = −21 ± 0.4 kJ/mol) and positive entropy (ΔS° = 18 ± 0.4 J/ mol−1 K−1) contribution (cf. Table 1). The Fluka NAs (cf. Figure 4b) display similar concentration dependence relative to
energy because of changes in molecular surface area as a result of hydration (ΔG° = γ dσ, where dσ refers to changes in the molecular surface area), in agreement with molecular modeling studies.53,54 The extensive rearrangement of the hydration shell about the guest, β-CD cavity, and complex contribute favorably to the value of ΔG° for such 1:1 CD−inclusate complexes55 ΔG° = ΔG°H/W + ΔG°I/W + ΔG°H/I/W + ΔG° W/W (9)
where I, H, and W represent the inclusate, host, and solvent (water), respectively. The LSA of the apolar guest has a large stabilizing effect, as anticipated for the inclusion complexes studied herein. The negative enthalpy values for the β-CD/ guest (guest = S1, S2, and S3) inclusion complexes are further evidenced by the reduced binding affinity as the temperature increases, in accordance with Le Chatêlier’s principle. Negative enthalpy values are characteristic for such processes driven by hydrophobic effects because of the net displacement of water molecules from the cavity of β-CD, hydrogen-bond formation, and favorable electrostatic interactions for the host/guest/ solvent assembly. The utility of the spectral displacement method for estimation of the 1:1 binding constants necessitates that certain conditions are met: (i) competing inclusate(s) must not absorb in the wavelength region of interest or interact with the chromophore, and (ii) the inclusate must bind with a welldefined 1:1 stoichiometry. The first condition was verified experimentally by measuring the absorbance change as a function of added inclusate. One exception to the latter condition is S2 (cf. Figure 3b), and the two commercial NAs (panels a and b of Figure 4) were shown by a small increase in absorbance because of secondary interactions between the unbound carboxylate anions and phth. To address this effect, appropriate corrections were applied to the absorbance data for 3595
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0), according to Figure 6. A comparison of OSPW NAs with the commercial NAs (Acros and Fluka) reveals that the mixtures differ markedly in their composition. OSPW contains low levels of saturated aliphatic carboxylates (z = 0) and may also contain alicyclic or aromatic rings with sulfur heteroatoms.57 In Figure 7, the ESI−MS profiles differ and the distribution of O and S species in the OSPW NAs is shown. By comparison, OSPW contains ∼61% O2 species, and the commercial samples (Acros and Fluka) have ∼99% O2 species. The relative binding affinity of these NAs with β-CD were also obtained using the spectral displacement technique. The binding constants reported for the mixtures of NAs are representative of the oil sands acids in the wider context but are not limited to the classical molecular structures shown in Scheme 1. Furthemore, the differences in composition provide additional support for the variation in the entropy values for the muti-component mixtures. Acros and Fluka NAs have positive entropy contributions to complex formation with β-CD, whereas a negative ΔS° was observed for OSPW NAs. Notwithstanding the variation in enthalpic and entropic contributions to ΔG°, there is a strong linear correlation observed between ΔS° versus ΔH° (results not shown), in agreement with the entropy−enthalpy compensation effect58 reported for such host−guest systems.59 The difference between commercial and OSPW NAs are shown by the variable LSA of NAs. Moreover, the estimated value of K2 for the β-CD/NAs system employs a numberaverage molecular weight according to ESI−MS results obtained herein. One limitation of this approach is that the types of specific congeners complexed with β-CD are not explicitly known because the absorbance changes represent an average as a result of a mixture of bound species. In the case of multi-component NAs, it is assumed that a random statistical distribution of the components form inclusion complexes with β-CD. However, it is anticipated that multi-component fractions with high-molecular-weight species and guests with greater LSA values are bound more strongly than the lower molecular weight components with lower LSA values.19,20 Despite the foregoing assumptions, the K2 value represents an “average” value for the 1:1 binding constant for the β-CD/NAs complexes contaning a multi-component guest mixture. In a previous study,24 the equilibrium constants derived from sorption isotherms were understood according to the relative contributions of each respective binding site and the corresponding adsorption constant (e.g., Kα and Kβ). The surface concentration (ΓA) of bound adsorbate (A) species for a monolayer-type isotherm with dual surface adsorption sites (α and β) is given by eq 10.60
Figure 4. Absorbance (abs; λ = 552 nm) versus concentration for various β-CD/guest systems: (a) Acros, (b) Fluka, and (c) OSPW NAs in 0.1 M NaHCO3 at pH 10.5, with [phth] = 2.60 × 10−5 M at variable temperatures.
the Acros NAs. The maximum amount of phth is displaced when [NAs]o (Fluka) reach ∼3.5 mM. The value of ΔG° (Fluka) ranges from −25 to −27 ± 0.2 kJ/mol, whereas the values differ compared to Acros (ΔH° = −12 ± 0.2 kJ/mol, and ΔS° = 44 ± 0.8 J mol−1 K−1), as seen in Table 1. The maximum amount of phth displaced for OSPW NAs (cf. Figure 4c) is similar for the two types of commercial NAs over a comparable concentration range. The K2 values in Table 1 and the observed differences for OSPW and commercial NAs are related to their composition (z value and carbon number). The differences in complex stability between Acros and Fluka NAs with β-CD are related to the variable composition and distribution of the individual components in such commercial mixtures. The distribution is revealed in Figure 5, where variations among the respective ESI−MS profiles are clearly observed, and Acros NAs are mainly comprised of n-alkyl carboxylic acids (z = 0). By comparison, Fluka NAs contain cyclic carboxylic acids (z
