Relative Average Mass Transport Properties of Mixture Components in

Article pubs.acs.org/JPCC

Relative Average Mass Transport Properties of Mixture Components in the Presence of a Porous Silica Gel Depend on the Solution/Solid Phase Ratio Caroline Carrara† and Stefano Caldarelli*,†,‡ †

Ism2 UMR 7313, Aix Marseille Université, Campus de Saint Jérôme, 13397 Marseille Cedex 20, France UPR 2301 ICSN CNRS Antenne Ecole Polytechnique 91120 Palaiseau, France

‡

ABSTRACT: In this work, we followed by pulsed-field gradient (PFG) NMR the evolution of the value of the average diffusion coefficient for molecules in a mixture in the presence of silica gel of chromatographic grade for variable solution/solid ratios. This analysis revealed a dramatic change, along the series of experiments, in the differences of the apparent diffusion coefficients of the mixture components, which determines the facility of separation. Notable consequences of this variation is that an effective separation is predicted to be possible in conditions that are specific to a mixture/solid phase combination and that agreement with column LC can be achieved only under certain conditions.

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INTRODUCTION Liquid chromatography provides one of the richest collections of experimental evidence on mass transport in porous materials. On the other hand, the microscopic characterizations of the interactions ruling the molecular displacement on a silica gel, functionalized or not, remains scarce and thus LC retention models are most often phenomenological. Pulsed-field gradient (PFG) NMR spectroscopy is a method capable of following the amplitude of mean square molecular displacements, over several hundreds of milliseconds.1 This technique can provide useful insight into mass transport in porous materials involving adsorption, considering that immobilized molecules do not contribute to the average displacement. When the solid is a chromatographic stationary phase, a mimicking of the chromatographic process may be achieved, by monitoring the spread of the spectral signatures of a mixture of components induced by the solid phase in a bidimensional experiment that can measure the molecular mobility.2,3 This ″chromatographicNMR″ (Chrom-NMR) is a version of PFG diffusometry originally introduced for improving the capability of the experiment at separating the spectra of mixtures of bioorganic molecules into the ones of the pure compounds, capitalizing on the vast experience in stationary phase design of chromatography. In the experiment, a mixture is analyzed in the presence of a solid chromatographic material, and the average mobility of the components is altered proportionally to their affinity for the solid, whence the enhancement of the resolution power of the technique. In a larger perspective, the method belongs to the class of experiments that has been dubbed ″matrix-assisted″ DOSY, the addition to a mixture of a carefully selected external agent to enhance the resolution of the experiment.4−13 ChromNMR shares a thermodynamics basis with LC, as the strength of the tracer/solid (or immobilized) phase interaction is reflected by a corresponding reduction of the average measured mobility, and thus it is natural to investigate how this technique can be used as an alternative tool for predicting chromato© 2012 American Chemical Society

graphic processes. However, a surprising difference between Chrom-NMR and LC, in which this latter was outperformed, was observed in the case of the separation of the spectral signatures of a series of homologous aromatic molecules (benzene, naphthalene, and anthracene).14 In the cited experiment, LC and Chrom-NMR produced equivalent degrees of separation using reverse phase liquid chromatography (RPLC) conditions: octadecylsilyl (ODS) bonded-silica as the stationary phase and acetonitrile/water (90/10 v/v) as the mobile phase/solvent, but when the stationary phase was replaced with bare silica gel (hidrophilic interaction chromatographyHILIC conditions), Chrom-NMR still produced a separation of the same quality, while the LC outcome degraded severely.14 This unexpected discrepancy pointed out to the necessity of better understanding the evolution of differential mass transport in mixtures in porous silica. Due to its adsorption properties, silica is a traditional and popular chromatographic support.15,16 Even if bare silica has been mostly superseded by bonded-phases, which represent nowadays the most employed supports for liquid chromatography, a resurgence in the utility of this material has recently been observed.17−19 In LC, adsorption is considered the main separation mechanism on bare silica but additional interactions between solutes and solvents and solvent and stationary support respectively, must also be considered to explain the totality of the separation process.20,21 In spite of these considerations, the explanation of the entire separation process still remains a source of debate. Here, we focus on the effect on the separation properties of the phase ratio, that is the quotient of the volume of the solution to the one of the solid phase.22 As a precursor to this study, the evolution of the transport of benzene, pure and in solution, in porous silica as a function of Received: May 4, 2012 Revised: August 7, 2012 Published: August 14, 2012 20030

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the solution-to-solid phase ratio was described.23 The diffusional behavior of the pure liquid could be quantitatively modeled, showing a complex contribution to diffusion coming from the access to inter- and intraparticle void of vapor and bulk molecules. Particularly, for low solution loadings, NMRmeasured transport was shown to be dominated by vapor phase benzene moving in the extraparticle space, but with a definite contribution from bulk molecules. A similar trend was observed for a solution of benzene in chloroform, but with obvious deviations from Raoult’s law.23 For more complex mixtures, variations of the mobility of each of the component are not necessarily going to follow a same trend. This can have an impact for the general chromatographic process. The relevance of phase ratio for mass transport in porous materials has been highlighted in several studies, including mesoporous silica and silicates,24,23,25 and glass beads,26−32 but its impact on the variations of molecular mobility in a mixture, and thus on separation processes, is analyzed here for the first time. Thus, in the following, we provide experimental evidence, via PFG-NMR, of the evolution of the relative mass transport for two test mixtures on bare silica and compare the results with LC, performed in similar conditions, so that the NMR analysis will provide at least partial insight on the chromatographic mechanism.

with the remaining part of the silica gel. The pulse sequence used was based on the stimulated echo and incorporated bipolar gradient pulses and an Eddy current delay (BPPLED).33 The shape of all gradient pulses was sinusoidal and the LED was held constant at 5 ms. The results of the PFG experiment were checked for deviations from the simple relation 1, which is indicative of the absence of fast-exchange. Diffusion measurements were realized with two diffusion times of 80 and 400 ms, to check for the presence of restricted (anomalous) diffusion,34 and gradient pulses of 600 and 2000 μs. The gradient strength, g, was linearly incremented in 16 steps from 2% to 95% of its maximum value and 16 or 32 scans were recorded for each sample. For Fourier transformation and baseline correction, the diffusion dimension was processed by means of the DOSY options of the Bruker TOPSPIN software package (version 2.1). The error was estimated on selected experiments performed in triplicate, being of the order of about ±5%. 2.3. Liquid Chromatography Experiments. Measurements were performed on an UltiMate 3000 series from Dionex GmbH (Idstein, Germany) equipped with a SD pump, a TCC compartment column, a DAD detector, and a data acquisition station (Chromeleon 7 software). The injection volume was 20 μL, the flow rate, 1 mL·min−1, and the temperature of analysis, 30 °C.

2. EXPERIMENTAL SECTION 2.1. Samples and Materials. The chromatographic phase used was LiChrospher Si100 (5 μm), obtained from Merck (Darmsdat, Germany), and its principal physicochemical properties are listed in Table 1. It was packed in a column of

3. THEORY Characterization via PFG-NMR of molecular displacement in porous inorganic materials has a long tradition.34 We shall thus summarize in the following only the most relevant points of its theoretical background. The PFG-NMR experiment focuses on the determination of an effective diffusion coefficient, D, defined as a function of the mean square displacement

Table 1. Physicochemical Properties of LiChrospher Si100

D = x 2 /(2Δ)

LiChrospher Si100 Characteristics −1

specific surface area (m ·g ) particle diameter (μm) pore diameter (Å) porous volume (mL·g−1) 2

400 5 100 1.25

(1)

This is obtained performing a series of spin−echo (or stimulated echo) experiments and by fitting its results to the Stejskal−Tanner equation35

250 mm of length and 4 mm of diameter for HPLC analysis and used in bulk for NMR experiments. All chemicals were purchased from Sigma-Aldrich (St-Louis, MO, USA) and used as received. Deuterated chloroform was obtained from Eurisotop (Saint-Aubin, France) and HPLC-grade chloroform from VWR International (Fontenay sous-bois, France). For chromatographic-NMR experiments, the two test mixtures were prepared at a solute concentration of 1 mg·mL−1 in CDCl3, while for HPLC analysis they were prepared as: benzene (0.1 mg·mL−1), naphthalene (0.1 mg·mL−1) and anthracene (0.1 mg·mL−1) (BNA mixture), and naphthalene (0.1 mg.mL−1), aniline (0.2 mg.mL−1) and phenol (0.6 mg.mL−1) (NAP mixture) in CHCl3. The flow was set to 1 mL·min−1. 2.2. NMR Measurements. All experiments were performed on a Bruker Avance III spectrometer operating at 400 MHz and equipped with a 1H HRMAS probe head producing magicangle gradients with a maximum strength of 60 G·cm−1. Spectra were recorded at a spinning rate of 4 kHz and at 30 °C, using 4 mm o.d. zirconia rotors of 50 μL of available volume. For samples preparation, 16 mg of silica were used with a variable volume of solution (6 to 50 μL). Roughly half of the chromatographic phase was placed in the rotor, then the solution was added with a micropipet and the rotor was filled

I = I0 exp( −D(Gδγ )2 (Δ − ε))

(2a)

log I /I0 = ( −D(Gδγ )2 (Δ − ε))

(2b)

or

where G is the strength of the gradient pulse, δ its duration, γ the gyromagnetic ratio, Δ the time allowed for diffusion, and ε a correction factor that depends on the shape of the gradient pulse. The series is recorded typically varying the gradient strength, so that I0 incorporates relaxation effects.35 Although eq 1 corresponds to a real diffusion coefficient only for a homogeneous fluid, this representation is still useful in more complex samples, as it provides a synthetic overview of the deviation of the mass transport from a reference fluid. The easiest case, but not an uncommon one, to treat for heterogeneous samples is when fast-exchange is present among all compartments.30 In this case, the EDC is the weighed average Δ=

∑ xiDi i

(3)

A classic signature of this regime is a purely Gaussian PFG decay of the ratio described in eq 2b as a function of the 20031

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Figure 1. (a) Logarithm of the normalized diffusion coefficient of benzene (●), naphthalene (■), and anthracene (▲) in CDCl3 at 30 °C and 4 kHz spinning speed as a function of the phase ratio (Φ) in the presence of LiChrospher Si100 (5 μm). (b) Chromatogram of BNA mixture in CHCl3 at 30 °C in presence of LiChrospher Si100 (5 μm).

gradient strength, a situation that has been validated for several systems.24

Although a full theoretical description of the evolution of the reduced EDC as a function of Φ is beyond the scope of this work and makes the object of studies in progress, some qualitative remarks can be drawn. The mass transport of benzene over this same porous silica gel has been the object of previous work.23 The high values of the EDC observed for this molecule for low Φ could be explained with a fast equilibration between molecules adsorbed, in solution and in the vapor phase. This latter predominate for low loadings, when the molecules in the vapor phase have the largest contribution, providing the observed growth of the EDC, which in some cases exceeds the value for pure solution. It is noteworthy that while the vapor is occupying intra- and interparticle voids, only molecules in the latter are providing measurable displacements in the PFG-NMR experiment, due to collision with the walls in the nanometric pores quenching any contribution to mass transport with the parametrization used here. Conversely, molecules in the liquid phase contribute to translational displacements. The change observed in the D vs Φ trend for benzene in Figure 1 at Φ of about 2.2 was previously demonstrated to correspond to the point were the solution volume corresponds exactly to the intraparticle void volume.23 Since for larger amounts of liquid, the interparticle void starts to reduce, and thus the vapor phase contribution to mass transport as well. The evolution of the EDC of naphthalene and anthracene interaction presents a change in trend at the same position of benzene, suggesting a difference of intra- and interparticle regions to of the mass transport measured via PFG-NMR also in this case. However, while interaction with the solid could be neglected as a first approximation for benzene, this is not case for the two heavier aromatic homologues. Consequently, a strong reduction in the reduced EDC appears for Φ values smaller than 2.2. For low loadings (Φ < 1.5), upon reduction of the phase ratio naphthalene, which has a non-negligible vapor pressure, shows an increase in the EDC, while anthracene reduces its mobility strongly, probably due to approaching monolayer coverage. For the sake of separation capabilities, a strong difference is thus observed if the solution is localized inside the pores or rather spread in both void regions. Specifically for the BNA mixture, enhanced separation is observed for loadings lower than the intraparticle porous volume (here Φ = 2.2) and agreement between the resolution achieved by PFG-NMR and LC is found for Φ > 5.

4. RESULTS AND DISCUSSIONS In order to stress the parallelism with chromatography, the results where presented in in the form of a reduced effective diffusion coefficient (EDC), that is D divided by the solution value, D0. This quantity represents best the relative changes in mass transport due to introduction of the solid, and it summarizes the delaying due to adsorption and the tortuosity effects, that describe the reduction in free space due to the solid. Figure 1a shows the evolution of this reduced EDC for a homologous set of aromatic molecules, benzene, naphthalene, and anthracene, dissolved in deuterated chloroform in the presence of silica, for various values of the solution/solid volume ratio, Φ. The choice of chloroform as a solvent, a less common LC mobile phase, in the present case was justified by its convenience for NMR, particularly in its deuterated form. Previous studies demonstrated that deuteration of the solvent does not particularly affect solute/solid interactions36 and that the BNA mixture could be separated by Chrom-NMR using deuterated chloroform as a solvent as effectively as in acetonitrile/water.37 Although the measurements were performed on single components, the separation among the dots at a given Φ value in Figure 1a provides a visual metric of the facility of resolution of the three components in the corresponding DOSY experiment. Thus, two regions can be singled out in this figure, one for which separation via NMR is straightforward (Φ < 2.2), while the three compounds possess very similar reduced mobilities for Φ > 5. The corresponding LC result is shown in Figure 1b, showing coelution at 2.513 min. This result reproduces well the incapability of bare silica to provide an adequate separation stationary phase previously observed using acetonitrile/water as the mobile phase.14 Agreement between Chrom-NMR and LC can thus be expected for large values of Φ. For the current setup, the phase ratio of the LC column could be estimated using the column geometry, the flow rate, and the retention time of a nonretained compound, to be between 4.1 and 7.5. The previously observed discrepancy between Chrom-NMR and LC on the BNA mixture can be now understood as the DOSY experiment reported in that work had been performed with a value of Φ of about 2.14 20032

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Figure 2. (a) Logarithm of the normalized apparent diffusion coefficient of naphthalene (▲), aniline (●), and phenol (■) in CDCl3 at 30 °C and 4 kHz spinning speed as a function of the phase ratio (Φ) in the presence of LiChrospher Si100 (5 μm). (b) Chromatogram of NAP mixture in CHCl3 at 30 °C in presence of LiChrospher Si100 (5 μm).

already under investigation. Furthermore, it can be noted that, according to the results presented here, the phase ratio can acquire a stronger role as a parameter in solid-induced separations, as demonstrated in previous chromatographicNMR experiments outperforming their LC counterpart.

The second example reported is a naphthalene, aniline, and phenol (NAP) mixture diluted in chloroform (Figure 2). Here, although trend variations in Chrom-NMR are observed around Φ = 2.2 consistently with the previous case (Figure 2a), the spectral separation is only possible for large solution excess (Φ ≅ 6). At this phase ratio, Figure 2a shows that naphthalene presents the highest reduced mobility, phenol the lowest one, aniline being intermediate. For Φ < 1.5, naphthalene and aniline show the typical behavior of volatile molecules, but the difference in their relative mobility is much reduced compared to Φ = 6. For low loadings (Φ < 1.5), the reduced EDC of phenol, in agreement with the limited vapor pressure of this molecule, follows a trend similar to the one observed for anthracene in the BNA mixture. All three compounds are well discriminated in LC (Figure 2b), with napthalene eluted first, with a retention time of 2.527 min, then aniline (4.633 min) and at last phenol (5.707 min). This second set of results confirms the possibility of using ChromNMR to predict LC outcomes using realtively large phase ratios. Interestingly, the comparison of the results of Figures 1 and 2 points out that, due to the molecule-specific evolution of the transport as a function of the phase ratio, there is no general optimal value for this parameter to perform a chromatographic separation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Spectropôle (Fédération des Sciences Chimiques FR 1739, Aix-Marseille Université) for privileged access to the NMR facility. Financial support from ANR (ANR-08-BLAN-273) and Region PACA (APO-G 2009) is acknowledged. This work is dedicated to Sir Paul Callaghan (1947-2012).

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REFERENCES

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4. CONCLUSION The chromatographic-NMR analysis presented here showed that, for the molecular mobility of mixture components in the presence of bare silica gel, the presence of two regimes of behavior for the mass transport has been qualitatively revealed: one when the solution is confined to the intraparticle porous volume and a second when it has access to interparticle void as well. Dramatic differences in the potential separation induced by the solid are possible in these two cases. The predictive ability of chromatographic-NMR with respect to LC has been demonstrated for this particular solid (bare silica) for a large excess of the liquid solution volume compared to the stationary support, the typical situation encountered in chromatographic setups. While the validity and limit of this prediction property remains to be assessed, it is noteworthy that, due to fast analysis implementation and low overall cost of chromatographic-NMR, this technique seems to present interesting attributes to be used as a predictive tool for LC experiments. A test of the extension of the validity of this comparison to ODS-bonded phases is 20033

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