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TECHNICAL REPORTS

Organic Compounds in the Environment

Characterization of Cation– Interactions in Aqueous Solution Using Deuterium Nuclear Magnetic Resonance Spectroscopy

^a Department of Civil Engineering, Texas A&M University, College Station, TX 77840
^b Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843
^c Department of Agricultural and Biological Engineering, Clemson University, Clemson, SC 29634-0357
^d Department of Environmental Toxicology and the Clemson Institute of Environmental Toxicology, Clemson University, Pendleton, SC 29670
^e Connecticut Agricultural Experiment Station, 123 Huntington Street, New Haven, CT 06504

^* Corresponding author (Don.Zhu{at}po.state.ct.us).

Received for publication April 23, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Chemical interactions of aromatic organic contaminants control their fate, transport, and toxicity in the environment. Recent molecular modeling studies have suggested that strong interactions can occur between the electrons of aromatic molecules and metal cations in aqueous solutions and/or on mineral surfaces, and that such interactions may be important in some environmental systems. However, spectroscopic evidence for these so-called cation– interactions has been extremely limited to date. In this paper, cation– interactions in aqueous salt solutions were characterized via ²H nuclear magnetic resonance (NMR) spin–lattice relaxation times (T₁) and calculations of molecular correlation times (_c) for a series of perdeuterated (d₆–benzene) benzene–cation complexes. The T₁ values for d₆–benzene decreased with increasing concentrations of LiCl, NaCl, KCl, RbCl, CsCl, and AgNO₃, with the largest effects observed in the AgNO₃ and CsCl solutions. Upon normalizing _c values by solution viscosity effects, an overall affinity trend of Ag⁺ >> Cs⁺ > K⁺ > Rb⁺ > Na⁺ > Li⁺ was derived for the d₆–benzene–cation complexes. The ability of Ag⁺ to complex d₆–benzene was significantly reduced upon addition of NH₃, which strongly coordinates Ag⁺ at high pH. Results with d₆–benzene, d₈–naphthalene, d₂–dichloromethane, and d₁₂–cyclohexane in 0.1 M methanolic salt solutions confirmed that spin–lattice relaxation rates are characterizing cation– interactions. The relatively strong cation– bonding observed between Ag⁺ and aromatic hydrocarbons probably results from covalent interactions between the aromatic electrons and the d orbitals of Ag⁺, in addition to the normal electrostatic interaction.

Abbreviations: NMR, nuclear magnetic resonance • PAH, polycyclic aromatic hydrocarbon • T₁, spin–lattice relaxation time • _c, molecular correlation time

	INTRODUCTION

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POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) are a group of ubiquitous hydrophobic organic contaminants (HOCs) that continue to receive considerable attention because of their toxicity, persistence, and extensive distribution in the environment (e.g., Means et al., 1980; Kubicki et al., 1999; Schwarzenbach et al., 2003). In general, the chemical state in which PAHs are present in the environment determines their fate, transport, and overall environmental effects (e.g., Schnoor, 1996; Kubicki et al., 1999; Schwarzenbach et al., 2003). At many contaminated sites, a complex suite of organic chemicals and inorganic elements are commonly present together (United States Department of Energy, 1992; National Research Council, 1994; Spiro and Stigliani, 1996). For example, polluted biogeochemical systems often contain PAHs, other HOCs (e.g., chlorinated hydrocarbons, pesticides), and heavy metals in addition to all naturally occurring chemical species (e.g., alkali and alkaline earth metals, trace metals, anions, organic matter, mineral surfaces). Characterization of the complex interactions between PAHs and cations in aqueous solution will therefore aid our understanding of the environmental fate and risk posed by these ubiquitous contaminants.

Relatively strong noncovalent bonds probably form between benzene and base cations in aqueous solution due to electrostatic attractions between the permanent quadrupole of benzene and the positively charged cations (Kumpf and Dougherty, 1993). The importance of these and other so-called cation– interactions has been recognized for some time in biological processes, including bimolecular recognition, protein–ligand binding, and the selectivity of K⁺ within K⁺ channels in cell membranes (Miller, 1991; Dougherty, 1996; Ma and Dougherty, 1997; Gokel et al., 2001). The importance of cation– interactions from the perspective of environmental science and engineering, however, has gone largely unexplored. For example, it is reasonable to expect that cation– interactions may affect the distribution of PAHs between water and mineral surfaces in low organic carbon environments (Kubicki et al., 1999; Zhu et al., 2004). Cation– interactions may also influence the relative toxicities of PAHs and heavy metals that co-exist in aqueous solution, because their association with one another would probably alter their transport across cell membranes. However, few toxicity studies of co-contaminant mixtures of heavy metals and PAHs have been performed to date (e.g., van den Hurk et al., 1998a, 1998b, 2000; Babu et al., 2001), and none of these has tried to elucidate the potential importance of cation– effects on toxicity.

Spectroscopic evidence for cation– interactions in various organic solvents and in the gas or solid phase has been widely reported in the literature over the past several decades (e.g., Baddiel et al., 1965; Sunner et al., 1981; Munakata et al., 2000; Gokel et al., 2001 and references therein). However, spectroscopic evidence for cation– interactions in aqueous solution has been extremely limited. Instead, much of the supporting evidence for cation– interactions in aqueous solution has come from theoretical studies. For example, formation of aqueous-phase cation– complexes between benzene and base cations was first evaluated by Kumpf and Dougherty (1993) using an electrostatic model. Based on their calculations, the ordering K⁺ > Rb⁺ >> Na⁺, Li⁺ was predicted for the overall binding energies of cation–benzene complexes when the cation desolvation energies were considered. More recently, a molecular modeling study of benzene and selected PAHs in the aqueous and adsorbed states identified the cation– interaction as a potentially important environmental process (Kubicki et al., 1999). Microscopic characterization of weak solute interactions in aqueous solution has been technically challenging. Routine spectroscopic techniques such as ¹H-NMR and Fourier-transform infrared (FTIR) are not able to provide useful information on cation– interactions in aqueous solution because of the relative weakness of the interactions and the insensitivity of these two spectroscopies. Therefore, no significant NMR chemical shifts or appearance of new absorbance bands are observed for aqueous-phase cation– complexes using these two conventional spectroscopic techniques. The strongest spectroscopic evidence to date for cation– interactions in aqueous solution comes from circular dichroism studies of synthetic cyclophane hosts, which contained clustered benzene rings able to compete with water molecules for cations via formation of cation– bonds (Shepodd et al., 1986; Forman et al., 1995). However, complete interpretation of circular dichroism spectra is indirect and complicated when detecting chirooptical properties of such molecules. Furthermore, the method is not applicable to studies of compounds such as benzene, which have rigid molecular structures and no chirooptical properties.

Spin–lattice relaxation times (T₁) can be used to obtain detailed information on molecular interactions (Smith, 1983). Deuterium NMR has the advantage of being sensitive to relatively weak solute interactions, and is well suited for NMR relaxation studies because deuterium relaxation is dominated by the quadrupole relaxation mechanism (Smith, 1983). This allows for a relatively simple correlation between the relaxation rate and molecular correlation time (_c). For example, complexation of CD₃CN by various cations was elucidated based on ¹⁴N- and ²H-NMR T₁ values (Bopp, 1967). Noncovalent interactions between perdeuterated monoaromatic compounds and geosorbents have been characterized using measured T₁ and T₂ (spin–spin relaxation times) values (Nanny and Maza, 2001; Zhu et al., 2003, 2004). Because the cation– interaction affects the _c of benzene and other aromatic compounds, the interaction can be characterized by quantifying the T₁ of perdeuterated compounds, which is inversely related to _c.

It is widely assumed that entropic effects control the distribution of aromatic molecules in biogeochemical systems (Schwarzenbach et al., 2003). We hypothesize, however, that specific covalent and noncovalent interactions such as cation– interactions may be important in influencing aromatic molecule distributions and reactivities in the environment. The specific objectives of this investigation were to (i) test the utility of ²H-NMR relaxation measurements for probing the interactions between aromatic molecules and cations in solution and (ii) evaluate the influence of different cations on the formation of cation– interactions. We used ²H-NMR to measure T₁ values of d₆–benzene and other perdeuterated probes in a variety of aqueous and methanolic electrolyte solutions. We then calculated viscosity-normalized _c values from the T₁ values to separate the cation– interactions from normal bulk solution viscosity effects.

	MATERIALS AND METHODS

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Materials
The I A group monovalent cations (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) varying in softness (i.e., positively correlated to cation radius) and a representative soft transition metal (Ag⁺) were selected for studying their cation– interactions with benzene. Perdeuterated compounds including d₆–benzene (Cambridge Isotope Laboratories, Andover, MA), d₈–naphthalene (Aldrich Chemical, St. Louis, MO), d₂–dichloromethane (Aldrich Chemical), d₁₂–cyclohexane (Aldrich Chemical), d₄–methanol (Aldrich Chemical), and d₂–water (Aldrich Chemical) were used as received, as were LiCl (Fisher Scientific, Hampton, NH), NaCl (EM Science, Gibbstown, NJ), KCl (EM Science), RbCl (Aldrich Chemical), CsCl (Fisher Scientific), NaNO₃ (EM Science), AgNO₃ (Fisher Scientific), and 1.0 M NH₄OH (Fisher Scientific). All solutions were prepared using high performance liquid chromatography (HPLC)-grade methanol (Fisher Scientific) and/or double-distilled water.

Sample Preparation
Aqueous solutions of LiCl, NaCl, KCl, RbCl, CsCl, and AgNO₃ ranging from 0 to 3 M were prepared. Diammonium-silver solutions ranging from 0.1 to 0.5 M were prepared by mixing and diluting AgNO₃ and 1.0 M NH₄OH at a mole ratio of 1:2 (AgNO₃ and NH₄OH). Methanolic solutions of 0.1 M NaCl, CsCl, NaNO₃, and AgNO₃ were prepared by shaking the solutions vigorously. The pH of all aqueous solutions except those containing Ag⁺ was adjusted to 7 using 0.5 M HCl and/or 0.5 M NaOH unless otherwise indicated. After transferring 5 mL of the solution of interest to a 10-mm broadband, 17.8-cm (7-in) NMR tube (Wilmad, Buena, NJ), 5 µL (0.1% v/v, 0.011–0.025 M for different compounds) perdeuterated solute or 0.0136 g (0.020 M) d₈–naphthalene was added and then the tube was immediately sealed with silicone stoppers (Fisher Scientific). These samples were kept in the dark at ambient temperature and shaken overnight (8–12 h) to allow full equilibration before NMR analysis.

Deuterium Nuclear Magnetic Resonance Experiments
Most NMR spectra were recorded at ambient temperature (19.0 ± 0.5°C) using a Varian (Palo Alto, CA) XL-200 FT-NMR spectrometer operating at 30.7 MHz with a 10-mm broadband probe. A Varian 400 FT-NMR spectrometer was used to run RbCl samples at 61.3 MHz with a 5-mm switchable probe. After locking and shimming on a reference sample containing pure d₂–water, the spectrometer was run unlocked following a standard inversion-recovery pulse sequence after carefully shimming for the NMR signal of interest. A recycle delay of at least 5T₁ was applied, and at least 10 transients were used to give a good signal-to-noise ratio. Spectra were processed with an exponential multiplication corresponding to line broadening of 2 Hz. Chemical shifts were referenced to the natural abundance of deuterium in water or to d₄–methanol spiked into the methanolic salt samples. The value for T₁ was calculated by an exponential regression with an associated error reported by the instrument.

	RESULTS AND DISCUSSION

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Experimental Nuclear Magnetic Resonance Results
No detectable chemical shift (i.e., >0.05 ppm) was observed for any deuterated probe in the different aqueous or methanolic solutions. For mobile isotropic fluids in the extreme narrowing case, the spin–lattice relaxation rate (1/T₁) of deuterium can be expressed as (Abragam, 1961; Mantsch et al., 1977):

[1]

where _c is the molecular rotational correlation time, is the asymmetry parameter of the electric field gradient and is very close to zero for a C–²H fragment (Mantsch et al., 1977), and e²Qq/ℏ is the quadrupolar coupling constant in rad s^–1, equal to 196.5 ± 1.3 kHz for benzene (Diehl and Khetrapal, 1969), 160 ± 3 kHz for dichloromethane (Glasel, 1969), and 165 ± 2 kHz (assumed uncertainty) for methanol (Glasel, 1969). As shown in Eq. [1], an inverse relationship exists between _c and the spin–lattice relaxation time T₁. Figure 1a shows T₁ values of d₆–benzene for LiCl, NaCl, KCl, RbCl, CsCl, and AgNO₃ aqueous solutions. Viscosity can also affect the solute correlation time, which in turn affects the measured T₁ of that solute. Assuming the simplest case of spherical molecules in a viscous medium, the relationship between the dynamic viscosity () of a solution and _c is given by the Stokes–Einstein hydrodynamic formula (Glasel, 1969)

[2]

where f is a microviscosity factor that accounts for influence of a noncontinuous medium on the diffusion of molecules, k is the Boltzmann constant, and T is the absolute temperature. In general, the value of f is set equal to 1/6 or 1/12 (Mantsch et al., 1977). The molecular volume is calculated from pure liquid properties using 0.74 (M/N), assuming hexagonal close-packed spherical molecules where N is Avogadro's number, M is the molecular weight, and is the density. By comparing Eq. [1] and [2], it can be seen that T₁ and are inversely related. This inverse relationship is particularly evident when the nuclear spin relaxation is dominated by molecular rotation.

View larger version (21K): [in this window] [in a new window]   Fig. 1. The relaxation of organic solutes in different salt solutions. (a) Spin–lattice relaxation times (T1) for d6–benzene in aqueous solution as a function of salt concentration. Uncertainties in the values were generally less than 2% for a 70% confidence level, and would be approximately the same size as the symbols. (b) Normalized molecular correlation times (c) for d6–benzene in aqueous solution as a function of salt concentration. Uncertainties in the values were generally less than 3% based on the propagated errors of both the measured T1 values and the reported quadrupolar coupling constants. (c) Normalized c values for d2–dichloromethane in aqueous solution as a function of salt concentration.

Compared with the other cations tested, Ag⁺ exhibited a much stronger capability to decrease T₁ (Fig. 1a), and increase the normalized _c (Fig. 1b) of d₆–benzene in aqueous solution, indicating a relatively strong cation– interaction between Ag⁺ and benzene. A strong affinity between Ag⁺ and aromatic hydrocarbons was reported for many years (Andrews and Keefer, 1949) in aqueous solubility studies. The investigators observed that benzene and phenanthrene solubilities increased by about 4 and 10 times, respectively, in 1 M AgNO₃ versus KNO₃ solutions. Andrews and Keefer postulated the presence of two water-soluble complexes, AgAr⁺ and Ag₂Ar⁺, to explain their solubility data but offered no other independent evidence to corroborate this hypothesis. Recently, studies have shown that Ag⁺ has a very high affinity for some spherical hydrocarbons containing several benzene rings that form a cavity to host Ag⁺ (Gross et al., 1995; Munakata et al., 2000). However, no spectroscopic evidence of the strong interaction between Ag⁺ and benzene in aqueous solution has been reported previously.

Figure 1c shows that the normalized _c of d₂–dichloromethane increases with increasing concentrations of CsCl and AgNO₃, but not NaCl. The binding energy sequence Cs⁺ > Ag⁺ > Na⁺ can be derived for dichloromethane–cation interactions by comparing the normalized _c values. Considering the relatively small polarity of dichloromethane compared with water, dichloromethane–cation interactions should have much smaller interaction energies than the corresponding cation hydration energies. Thus, water can easily out-compete dichloromethane for cations in aqueous solution, and the strongest electrostatic interaction between cations and dichloromethane would be expected for the cation with the lowest hydration energy (i.e., –302 kJ mol^–1 for Cs⁺, versus –496 and –422 kJ mol^–1 for Ag⁺ and Na⁺, respectively; Burgess, 1988). Because both Cs⁺ and dichloromethane have relatively high polarizabilities (Israelachvili, 1991), non-electrostatic forces between Cs⁺ and dichloromethane due to mutual polarization can also be important. Thus, the dichloromethane–Cs⁺ complex has the highest binding energy. Similarly, Ag⁺ interacts with dichloromethane more strongly than Na⁺ because the former has a larger radius and is more polarizable, even though it has an overall higher energy of hydration than does Na⁺.

Figure 2 shows normalized _c values of perdeuterated benzene, dichloromethane, and methanol (–CD₃ group) in CsCl aqueous solutions. It can be seen that benzene interacts more strongly with Cs⁺ than does dichloromethane; a relationship is also observed with the other cations (Fig. 1). This is because the intensity of benzene–cation interactions is higher than that for dichloromethane–cation interactions. Figure 2 also shows that the interaction between methanol and Cs⁺ is very weak. One reason for this is that the "desolvation penalty" for breaking hydrogen bonds between methanol and water is very high. In addition, because methanol is much less polarizable than benzene and dichloromethane, the contribution from mutual polarization to methanol–Cs⁺ interactions is negligible. Therefore, little interaction between methanol and cations in aqueous solution would be expected.

View larger version (14K): [in this window] [in a new window]   Fig. 2. Normalized molecular correlation time (c) values for d6–benzene, d2–dichloromethane, and d4–methanol (–CD3 group) in aqueous solution as a function of CsCl concentration.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Relaxation of d6–benzene in aqueous solution as a function of Ag(I) concentration. (a) Spin–lattice relaxation times (T1). (b) Normalized molecular correlation times (c).

View larger version (12K): [in this window] [in a new window]   Fig. 4. Spin–lattice relaxation times (T1) of d6–benzene in 0.1 M NaCl aqueous solution as a function of pH.

View larger version (26K): [in this window] [in a new window]   Fig. 5. Spin–lattice relaxation times (T1) for perdeuterated solutes in 0.1 M methanolic salt solutions: (a) d6–benzene, (b) d8–naphthalene (white bars for peaks and black bars for ß peaks), (c) d12–cyclohexane, and (d) d2–dichloromethane.

View this table: [in this window] [in a new window]   Table 2. Ionic index and covalent index of cations.

[3]

where T_1F and T_1A are the relaxation times for the free and associated species, respectively, and T₁ is the observed value averaged on these two. Similarly, the fraction of benzene associated with cations can be calculated by assuming 1:1 M⁺–benzene complexation (where M is a cation). To remove viscosity effects from our calculation, normalized _c values should be used instead of relaxation times:

[4]

where _cF and _cA are the normalized _c values for the free and associated benzene, respectively. The term _cF can be calculated from the observed T₁ value for d₆–benzene in pure water, and _cA can be obtained from model fitting of the binding constant at different salt concentrations. Using this approach, the binding constant for a 1:1 Ag⁺–benzene complex was determined to be 2.26 ± 0.36 M^–1, which is very close to the value of 2.41 M^–1 determined by Andrews and Keefer in their solubility enhancement study (Andrews and Keefer, 1949). A 2:1 Ag⁺–benzene complex with a binding constant of 0.212 M^–2 was also reported by those investigators, but was neglected here because it is relatively less important.

Using Monte Carlo simulations based on statistical perturbation theory, Kumpf and Dougherty (1993) calculated the free energy (G) needed to exchange K⁺ with Li⁺, Na⁺, and Rb⁺ in M⁺–benzene complexes in aqueous solution. As shown in Fig. 6 , a very good correlation is observed between the difference in normalized _c values (_c) for the maximum salt concentrations minus no salt and Kumpf and Dougherty's G values for the different cations. This result indicates that the change in normalized _c values observed with increasing salt concentration is a direct measure of the cation– binding energy. It should be noted that higher binding constants would be expected for PAHs compared with benzene because their higher electron densities favor cation– interactions.

View larger version (18K): [in this window] [in a new window]   Fig. 6. Relative free energies (G) (kcal mol–1) (from Kumpf and Dougherty, 1993) as a function of difference in normalized molecular correlation times (c) between 3 M and water for different cations.

Development of "smart" materials that can efficiently and selectively bind the chemical species of interest is the current trend in modern system design. Cation– interactions have been used as important processes in some engineered treatment systems. For example, it has been reported that calix arenes can selectively remove certain cations such as Ag⁺ and Cs⁺ from aqueous solution through the formation of cation– complexes (Ikeda and Shinkais, 1994, 1997; Bocchi et al., 1995). The relatively strong cation– bonding capability of Ag⁺ and Cu⁺ has also been used to prepare cation-charged minerals as adsorbents for effective olefin separation (Takahashi et al., 2001; Padin and Yang, 2002). More recently, photooxidation of PAHs via the formation of cation–PAH complexes with Nb, V, and Ti in the gas phase using laser vaporization has been reported (Duncan et al., 2001; Foster et al., 2001).

Cation– interactions are also likely to be important in some environmental toxicity and bioavailability studies. For example, intracellular and extracellular cation concentrations are high enough (e.g., 0.14 M intracellular K⁺; Alberts et al., 1994) to allow for relatively intensive cation– interactions with PAH molecules, which may affect their transport across cell membranes and thereafter metabolism reactions. Despite the small thermodynamic gradient driving force, long-term evolutionary changes have enabled biological systems to selectively and efficiently extract cations from the aqueous phase (i.e., 1000:1 K⁺ over Na⁺ selectivity by K⁺ channels; Kumpf and Dougherty, 1993; Ma and Dougherty, 1997) using the underlying mechanism of cation– interactions. Because transition metals are expected to have even higher cation– interaction intensities compared with base cations (e.g., K⁺), they can be more efficiently extracted and enriched in biological systems by ion channels. However, further studies will be necessary to better understand the potential influence of cation– interactions on issues related to environmental toxicology and bioavailability.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Cation– interactions between benzene and cations in aqueous solution were characterized via ²H NMR relaxation times and calculations of molecular correlation times. Soft base cations and soft transition metals (e.g., Cs⁺, Ag⁺) have stronger cation– interactions compared with hard base cations (e.g., Li⁺, Na⁺). Results with d₆–benzene, d₈–naphthalene, d₂–dichloromethane, and d₁₂–cyclohexane in 0.1 M methanolic salt solutions confirmed that spin–lattice relaxation rates are characterizing cation– interactions. Overall findings from this study provide a methodology for investigating and quantifying cation– interactions and present preliminary experimental evidence suggesting that these interactions are likely to be important in some environmental systems.

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the constructive comments of Dr. Linda Lee and two other anonymous reviewers and their suggestions for improving this paper. The authors also gratefully acknowledge financial support from the National Science Foundation (CTS-0096053) and the gift of perdeuterated aromatic hydrocarbons from Cambridge Isotope Laboratories.

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