Methanol Interface: Surface

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Nonpolar Adsorption at the Silica/Methanol Interface: Surface Mediated Polarity and Solvent Density across a Strongly Associating Solid/Liquid Boundary Debjani Roy,†,#,¶ Shule Liu,†,¶,□ B. Lauren Woods,‡ A. Renee Siler,§,∇ John T. Fourkas,*,†,§,∥,⊥ John D. Weeks,*,†,§,∥ and Robert A. Walker*,‡ †

Chemical Physics Program, University of Maryland, College Park, Maryland 20742, United States Department of Chemistry & Biochemistry, Montana State University, Bozeman, Montana 59715, United States § Department of Chemistry & Biochemistry, University of Maryland, College Park, Maryland 20742, United States ∥ Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, United States ⊥ Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, United States, and Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742, United States ‡

S Supporting Information *

ABSTRACT: Complementary experimental and theoretical studies presented in this work examine the structure, organization, and solvating properties of methanol at a silica/methanol, solid/liquid interface. Findings from these experiments illustrate how strong association between a silica substrate and methanol solvent creates a distinctly nonpolar solvation environment for adsorbed solutes. Resonance-enhanced second-harmonic spectra and timeresolved fluorescence emission in a total internal reflection geometry both show that adsorbed solutes sample an interfacial environment having properties resembling those of a nonpolar solvent. Molecular dynamics simulations identify the origin of this effect. Strong hydrogen bonding between the first layer of methanol and silica’s silanol groups creates what is effectively a methyl-terminated surface that leads to a second layer having significantly reduced density and hydrogen bonding compared to bulk solution. The calculated solvent reorientation times in these first two layers is significantly slower than in bulk, implying slow dielectric relaxation and supporting both second-harmonic and time-resolved fluorescence results. Collectively, these studies illustrate how surface-induced changes in solvent structure change the chemistry at strongly associating solid/liquid interfaces as compared to bulk solution limits.

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INTRODUCTION A common consideration linking chromatographic retention, heterogeneous catalysis, and the environmental persistence of pollutants is molecular adsorption to solid surfaces.1−7 While adsorption is driven by a system’s tendency to minimize its free energy, deciphering the molecular origins of adsorption can be difficult, especially at boundaries between two condensed phases. At a solid/liquid interface, for example, solute adsorption simply requires that a solute displace a solvent molecule. Quantitatively describing this phenomenon, however, requires knowing the affinity of a solute for the substrate, the solute’s bulk solution solvation energy, and the strength of interactions between the substrate and interfacial solvent. Furthermore, solutes approaching the solid/liquid interface may be subject to forces that differ significantly from bulk solution limits due to surface induced changes in solvent structure and organization. Formulating molecularly based descriptions of adsorption to buried interfaces is more than just an academic exercise. Adsorption mechanisms play critical roles in separation science and in the development of exposure estimates used for regulatory purposes. For example, functionalized surfaces © 2013 American Chemical Society

designed for improved HPLC performance show distinct differences in analyte retention and tailing depending on whether the eluting solvent is a water−acetonitrile mixture or a water−methanol mixture.1 Simulations have correlated these differences to a competition between solute adsorption to the solid/liquid interface and solute partitioning into a column’s hydrophobic layer,8 but the predictive capabilities of this model and others remain untested. In a related application, estimates of organic pollutant bioavailability depend on partitioning between two media such as soil and water or soil and air.9 Partitioning models, however, are often parametrized by linear free energy relationships that fail to distinguish between different types of interactions such as hydrogen bonding, dipole pairing, steric constraints, and hydrophobic effects.10 Two limiting cases can be used to describe adsorption and solvation at solid/liquid interfaces. An additive model of adsorption considers the idealized energetics of separate pairs of components−solute/substrate (with no solvent), solute/ Received: October 31, 2013 Revised: December 5, 2013 Published: December 5, 2013 27052

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near an idealized flat silica surface. Such a model system has already been used in our previous simulations of nitriles on a silica surface,38,45 and those simulation results are in good qualitative agreement with data from spectroscopic experiments. In the work presented below, resonance-enhanced secondharmonic generation (SHG) is used to probe adsorption and solvation of C151 at the silica/methanol solid/liquid interface. The excitation wavelength measured from adsorbed C151 implies that the interface formed between these two polar mediasilica and methanolis distinctly nonpolar. Furthermore, time-resolved fluorescence measurements carried out in a total internal reflection geometry show that the emission of adsorbed C151 is consistent with an environment that cannot stabilize the ICT state. The origin of this nonpolar region is identified explicitly in the MD simulations. Collectively, these findings present a detailed picture of how surfaces and solvents conspire to create distinctive environments having properties that differ from bulk solution limits. These boundaries require that solute behavior be described by a cooperative model of adsorption and solvation.

solvent (with no substrate), and solvent/substrate (with no solute). Based on this information alone, one tries to predict whether or not adsorption is favorable. This model represents an “averaged” description of adsorption and can be used to assess quickly if solutes are likely to adsorb spontaneously from solution.11−13 Furthermore, interfacial properties can be predicted simply by extrapolating from the corresponding properties of both bulk phases. Predictions using this approach can be quite accurate if the solvent and substrate do not have localized, directional interactions. Alternatively, a cooperative model of adsorption starts by acknowledging that a substrate alters the properties of the adjacent solvent creating an interfacial region that cannot be described by weighted averages of bulk properties. Extreme examples include silica/n-alcohol (n ≥ 3) solid/liquid interfaces whose properties are dominated by a high viscosity, alkane-like environment.14−19 This nonpolar boundary arises from strong hydrogen bonding between the alcohol and silica silanol groups and the van der Waals interactions among adjacent alkyl chains. The experiments and simulations described below examine differences between these additive and cooperative descriptions of adsorption and solvation by focusing specifically on adsorption of Coumarin 151 (C151) to the strongly associating, silica/methanol interface. C151 is a solute whose electronic structure is extremely sensitive to the local solvation environment. Solvatochromic effects shift C151’s excitation wavelength from ∼350 nm in alkane solvents to 410 nm in DMSO.20,21 Furthermore, in polar solvents, C151 fluorescence has a relatively high quantum yield (Φ ∼ 0.65) and long lifetime (∼5 ns), but in alkane solvents, C151 shows weaker fluorescence with a dominant, shorter emission lifetime (∼1 ns).20,21 These differences have been attributed to an excitedstate intramolecular charge transfer (ICT) conformer that is stabilized in polar media.20−24 (Figure 1).

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EXPERIMENTAL PROCEDURES AND SIMULATION CONDITIONS Sample Preparation. Laser-grade C151 was purchased from Exciton and used as received. All solutions were made using spectral-grade methanol (purity >99%). The bulk, steadystate absorption spectra were recorded using a Hitachi U-3010 UV/vis spectrophotometer, and steady-state fluorescence spectra were recorded using a JY Horiba Fluorolog 3. Steadystate fluorescence excitation and emission spectra for C151 in methanol are shown in Figure 2. Wavelength maxima are given in Table 1. Prior to all spectroscopic measurements, cells were cleaned with a 50:50 mixture (by volume) of concentrated sulfuric acid and fuming nitric acid. This treatment creates silanolterminated silica surfaces that are completely wetted by most solvents (including methanol).31 Given that solutions used in these studies were all nonaqueous, the silica surface was assumed to remain charge neutral (but polar). Surface-specific vibrational spectra from the silica/methanol solid/vapor interface confirmed the absence of covalently bonded methoxy groups that might result from acid−base reactions between methanol and the acidic silica surface.32 Resonance-Enhanced Second-Harmonic Generation (SHG). Resonance-enhanced SHG was used to probe the excitation wavelength of C151 adsorbed to the silica/methanol interface. SHG is a second-order nonlinear optical spectroscopy that is symmetry forbidden in isotropic media.33,34 SHG experiments detect the coherently scattered second-harmonic signal arising from an incident optical field having frequency ω. For molecules having large hyperpolarizabilities, the intensity of the signal at the second harmonic frequency, I(2ω), is enhanced significantly when it is resonant with allowed electronic excitations. In this way, scanning ω and monitoring I(2ω) results in an effective excitation spectrum of only those molecules subject to interfacial anisotropy.11,35−37 The relevant expressions describing the resonance-enhanced SHG response are shown in eqs 1 and 2:

Figure 1. Structures of C151 in ground electronic (a) and excited ICT (b) states.

While methanol can hydrogen bond to the silica substrate, this solvent can not exploit lateral, van der Waals interactions to form the Langmuir-film structure observed at silica/n-alcohol interfaces.14,18,19 Molecular dynamics (MD) and Monte Carlo simulations have been performed for systems with methanol in silica nanopores.25−30 In these simulations, methanol molecules are strongly correlated to the silica surface via hydrogen bonds with silanol groups, thus inducing slower orientational and translation dynamics at the methanol/silica interface. However, for methanol confined in silica nanopores, the liquid structure depends both on general properties of the silica surface and on details of the confined geometry. The relative importance of these contributions cannot be decoupled in a straightforward way. In the simulations reported here, we examined the simpler case of a slab of liquid methanol at liquid−vapor coexistence

I(2ω) = |χ (2) : E(ω)E(ω)|2

(1)

and 27053

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The SHG spectrometer used in these experiments has been described previously.38,39 Spectra are acquired by tuning a monochromator/photomultiplier tube assembly to the second harmonic wavelength being generated at the sample. Data are represented by the average of three or four 10 s acquisitions with photon counting electronics. Typical uncertainties for SHλmax values are ±2−3 nm. We note that the measured C151 SH line width is narrower than that of the corresponding fluorescence excitation and emission spectra. The narrow line width may be due to the quadratic dependence of the signal on incident power or reduced inhomogeneous broadening due to a more uniform interfacial solvation environment.40,41 Time-Correlated Single-Photon Counting (TCSPC). Experiments measuring fluorescence lifetimes used two separate TCSPC assemblies, one of which has been described previously.42,43 The second assembly is newly constructed and uses the frequency-doubled output of a Ti:sapphire oscillator (Coherent Chameleon) as the excitation source. The repetition rate is modulated by an electro-optic modulator (Conn Optics), and signal acquisition and processing are performed using a commercially available assembly (PicoQuant, Fluo Time 200). Additional details about the newer system will appear in a future paper. Measurements of C151 in the near surface region of the solid/methanol interface required the use of a total internal reflection (TIR) geometry. For bulk solution and TIR studies, the instrument response function (IRF) measured ∼200 ps (fwhm), and the data allowed for reliable measurements of lifetimes as short as 300 ps.42 Such constraints did not enable experiments to identify the fast, multiexponential solvent relaxation processes associated with methanol reorientation.31 Time-resolved fluorescence decays were analyzed assuming independent, single-exponential pathways following reconvolution with the IRF. The IRF was obtained from a dilute scattering solution. The single and biexponential nature of fluorescence decays were determined by minimizing the distribution of the weighted residuals. To within experimental uncertainty, both TCSPC systems provided identical results for C151 lifetimes in both bulk solution and in the TIR assembly. Simulation Details. MD simulations of liquid methanol at a planar hydrophilic silica surface have been performed using a model system based on previous work by Hu et al.44 The silica force field was constructed using a four-layer silica surface with an idealized β-cristobalite (C9) crystal structure.45,46 The silica surface was terminated with hydroxyl groups with a hydroxyl density 4.54/nm2 to make it hydrophilic. The system consisted of 1000 methanol molecules with the hydrophilic surface placed at z = 0 and a repulsive wall placed at z = 75 Å. We employed periodic boundary conditions with Lx = 45.60 Å, Ly = 43.88 Å, and Lz =150 Å. Slab-corrected Ewald 3D sums were used for treatment of electrostatic interactions in our system.47 We used the OPLS all-atom (OPLS-AA) model48 to describe the intra- and intermolecular interactions between methanol molecules. In the OPLS-AA model of methanol, “CT” represents the carbon atom, “HC” represents hydrogen atom in the methyl group, “O” represents the oxygen atom, and “H” represents the hydrogen atom in the OH group. For LennardJones interactions between methanol molecules, we used the combining rules for OPLS model: σij = (σiσj)1/2 and εij = (εiεj)1/2. For the Lennard-Jones interactions between methanol and atoms on the silica wall, Lorentz−Berthelot mixing rules were used: σij = (σi + σj)/2 and εij = (εiεj)1/2. MD simulations in the NVT ensemble were performed using the DL_POLY

Figure 2. (a) Flourescence excitation and emission spectra of C151 in a 2 mM solution of methanol. The sharp feature in the excitation spectrum near 400 nm is due to a vibrational Raman transition in the bulk solvent. (b) Time-resolved fluorescence from C151 in a 10 μM methanol solution. The black data are fit to a single-exponential decay (red). (c) Resonance-enhanced SHG spectrum of C152 adsorbed to a silica/methanol interface from a 10 μM solution. (d) Adsorption isotherm plotting the square root of the peak SH intensity as a function of bulk concentration.

Table 1. C151 Excitation and Emission Wavelengths λexca (nm)

λemissiona (nm)

380 350 364 ± 2

480 438

methanol n-hexane21 silica/methanol interface (SHG)

Uncertainties in reported excitation and emission wavelengths of ±3 nm unless otherwise noted.

a

(2) χ (2) = χnr(2) + χres = χnr(2) +

∑ j

Aj 2ω − ωres, j − i Γj

(2)

where E(ω) is the strength of the incident field having frequency ω and χ(2) is the second-order susceptibility with both resonant (res) and nonresonant (nr) contributions. The resonant part of χ(2) can be expressed as a sum over all contributing states, j, of Lorentzian line shapes having amplitudes (Aj) electronic resonance frequencies (ωres,j) and line widths (Γφ). When the second harmonic frequency at 2ω matches ωres, strong resonance enhancement is observed in I(2ω). 27054

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2.1849 package with a time step of 1 fs. After the system had been equilibrated for 300 ps at T = 298 K, configurations of the system were recorded every 15 fs. The total sampling lasted for 450 000 steps and gave a trajectory file with a total of 30 000 configurations for analysis.

other systems.50,51 (For comparison, the width of the bulk solution excitation spectrum in Figure 2 is ∼75 nm.) Given that the average orientation of C151 does not appear to change with surface coverage and that the SHG line shape and position do not change with bulk C151 concentration, the square root of I(2ω) is proportional to the C151 surface coverage. Figure 2 (bottom) shows adsorption data for C151 as a function of C151 bulk concentration. Fitting these data to a Langmuir isotherm results in a calculated adsorption energy of −38 ± 3 kJ/mol This value compares favorably with data for other aromatic species adsorbed to silica surfaces.52 From the adsorption isotherm, we observe that C151 forms a terminal monolayer with maximum surface coverage at bulk concentrations above ∼15 μM. TCSPC of C151 in Bulk Methanol and Adsorbed to the Silica/Methanol Interface. Shown in Figure 3 and reported in Table 2 are TCSPC data acquired in a TIR geometry from the silica/methanol interface for C151 solutions of 1 and 20 μM. Relative to the bulk methanol TCSPC data, TIR data show a distinct difference at early time, especially for the 1 μM solution. Specifically, the TIR data require that a second emission pathway with a lifetime of 1.1 ± 0.2 ns be included in the fit while the original 5.4 ns lifetime remains effectively unchanged. The shorter lifetime matches the average emission lifetimes of C151 in alkane solvents and has been ascribed to facile inversion (or “flip-flop”) about the sp3-hybridized amine with a correspondingly more rapid, nonradiative decay.21 With these considerations in mind, we assign the long-lived lifetime measured in the TIR experiments to C151 in a bulk solution environment excited by the evanescent wave of the incident light. The shorter lifetime is assigned to interfacial C151 constrained to keep sp3 hybridization about the amine following photoexcitation. This assignment is supported by the observation that the short lifetime is more pronounced at lower bulk concentrations (corresponding to lower surface coverage), whereas at a concentration slightly above the monolayer limit, the bulk solution lifetime becomes more dominant. We note that the amplitudes of the two lifetimes measured under TIR conditions cannot be correlated directly with relative surface coverage. The penetration depth of the evanescent wave used to excite the C151 depends sensitively on the angle of incidence, so small variations in optical alignment from experiment to experiment may disproportionately weight one lifetime contribution over the other. Furthermore, the fluorescence quantum yields of the two populations are likely to be different,21 meaning that the relative amplitudes reported in Table 2 will not correspond directly to relative concentrations. Nevertheless, the data reported in Figure 3 and Table 2 along with additional concentration-dependent measurements (shown in Supporting Information) provide consistent evidence that the silica/methanol interface is dominated by an environment that promotes fast radiative decay of adsorbed C151 solutes. Taken together, the SHG spectrum and the time-resolved TCSPC fluorescence data show that the silica/methanol interface creates an environment that is decidedly less polar than the bulk solution. Unresolved is the question of how a nonpolar environment arises. Two possible scenarios could explain these results: the solute in its excited state could retain its sp3 hybridization through strong, specific hydrogen bonding interactions with surface silanol groups (an “additive model”) or the nonplanar solute could be subject to a low-polarity region created by surface induced changes in long-range solvent

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RESULTS AND DISCUSSION SHG Studies of C151 at the Silica/Methanol Interface. In bulk methanol C151 has a maximum absorbance in its excitation spectrum of 380 nm (Figure 2, top). This value lies between the solvatochromic extremes bounded by alkanes (λexc = 348 nm) and DMSO (λexc = 386 nm). The second plot in Figure 2 shows the time-resolved fluorescence of a 10 μM solution of C151 in bulk methanol. The lifetime data fit to a single-exponential decay having a lifetime of 5.4 ± 0.2 ns (Figure 3 and Table 2). This result matches the value reported

Figure 3. TIR-TCSPC data from 1 μM (blue) and 20 μM (blue) methanol solutions in contact with a silica surface. The inset in the upper right shows an expanded view of the early time emission. Results from exponential fits are reported in Table 2.

by Nad et al.; the relatively long lifetime has been attributed to C151’s formation of a planar ICT state (Figure 1) in polar solvents.21 Table 2. C151 Fluorescence Amplitudes and Lifetimes system

A1a

τ1b

A2a

τ2b

methanol silica/methanol (TIR, 1 μM) silica/methanol (TIR, 20 μM) n-hexaned

1.00 0.18 0.06 0.75

5.4 1.1 1.2 0.68

0.82 0.94 0.25

5.3 5.4 1.35

χ2 c 1.1 1.2 1.2

Amplitudes normalized to unity with uncertainties of ±0.05. lifetimes in ns with uncertainties of ±0.2 ns. cχ2 test for quality of fit. dFrom ref 21.

a b

The SHG spectrum of C151 adsorbed to the silica/methanol interface from a 20 μM solution of C151 in methanol is shown in the third panel of Figure 2. Fitting the SHG data to eqs 1 and 2 results in an maximum in the effective excitation spectrum of 364 nm for adsorbed C151, a value closer to the nonpolar, alkane limit and consistent with a local effective dielectric constant of ∼2.3.21 The 11 nm (fwhm) line width is remarkably narrow, although similar results have recently been reported in 27055

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organization (a cooperative model). To determine whether the observed changes in solute properties result from an additive or cooperative model of surface adsorption, methanol structure at the silica/methanol interface was characterized using MD simulations. Simulation of Solvent Structure and Organization at the Silica/Methanol Interface. Density profiles with different molecular definitions, including the center of mass, the center of the O−H bond, and the carbon atom density, are plotted in Figure 4. Simulation data show that near the silica

Figure 4. Methanol density defined by the center of mass (cm, red line) and the center of the O−H bond (green line). The carbon atom density (blue line) is also given. The inset figure gives density profiles of the whole system (0−50 Å), where we can also see the transition from the liquid to vapor phase about 35 Å from the substrate. The density is normalized by the bulk density ρB.

Figure 5. (A) Center-of-mass density of liquid methanol at the silica surface (red) and liquid acetonitrile at the silica surface (green) from our previous work.38 (B) Mobile charge density for methanol/silica system (red) and acetonitrile/silica system (green).

surface all densities drop to their minimum at about 3.1 Å, indicating the existence of a tightly bound layer at the methanol−silica interface, which is defined as the first layer of liquid/solid (LS) interface. The O−H density (green line) inside this layer exhibit two peaks: a large peak near the silica surface, indicating strong hydrogen bonding between methanol and surface hydroxyl groups, and a small second peak, which presumably maintains the hydrogen bonding between methanol molecules in the surface layer and those in the bulk. Therefore, the O−H density profile indeed suggests that at the methanol− silica liquid−solid interface there exist sublayer structures. We may separate these two peaks at 1.5 Å, so as to study the sublayer structure. Sublayer structure has also been observed at acetonitrile− silica interfaces53 and propionitrile−silica54 interfaces. Comparison of the interfacial center-of-mass densities of methanol and acetonitrile is instructive53,55 (Figure 5A). At the silica surface, the methanol center-of-mass density is significantly higher than that of acetonitrile, indicating that more methanol molecules are tightly absorbed to the silica surface due to the strong hydrogen bonding. A less distinct feature of the methanol density is that it reaches its bulk value approximately 20 Å away from the surface while inhomogeneities in the acetonitrile density propagate for more than 25 Å. This result implies that surface methanol molecules screen the field from the silica surface more effectively than do surface acetonitrile molecules. To illustrate this point, we have plotted the mobile charge density of the methanol−silica and acetonitrile−silica systems in Figure 5B. Methanol molecules acting as both hydrogen bond donors and acceptors to the silica surface generate a large positive peak in the charge density near the surface, followed by a strongly negative peak (red curve) arising from contributions of the oxygen atoms. This positive−negative structure suggests the formation of a dipole layer that points toward the surface

and partially cancels the dipole moments from the surface that point nominally along the surface normal. A consequence of this opposing dipole structure is that the dipole field does not propagate far into the bulk. In contrast, at the silica−acetonitrile interface, acetonitrile molecules can only accept hydrogen bonds. Thus, the induced dipole moments point in the same direction as the surface dipoles, so that the surface dipoles are enhanced, leading to a greater influence of the surface dipole field on the acetonitrile liquid structure. To better understand liquid methanol structure and orientation at the silica surface, we have calculated the angular distribution of O−H and O−C vectors, shown in Figure 6. In the sublayer closest to the silica surface (z < 1.5 Å), the O−C vector orientation distribution has a very high intensity near cos θ = 1 (θ = 0° indicates pointing away from the interface), indicating most O−C vectors point perpendicularly away from the surface; this arrangement corresponds to the O−H orientation distribution centered at about cos θ = −0.3 (θ = 107.5°). In the second sublayer (1.5 Å < z < 3.1 Å), the O−C bond has a broad distribution centered at about cos θ = −0.25 (θ = 104.5°), and the orientation distribution of the O−H bond centers at about cos θ = −0.85 (θ = 148.2°). These results predict that O−H bonds in this sublayer tend to point toward methanol molecules in the first sublayer so as to form additional hydrogen bonds rather than exploiting van der Waals attractions between methyl groups in adjacent sublayers. One can visualize this orientational feature by means of a vector plot of bond orientations in a typical configuration, shown in Figure 7. From Figure 7 one observes that the strong hydrogen bonding of the methanol O−H to the silica surface results in the methyl group pointing upward and forming an effective hydrocarbon surface, as is shown by the green arrow layer in Figure 7. Molecules in the lower density, second sublayer can be recognized only by those few red arrows 27056

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vectors. Plots of orientational TCFs are given in Figure 8. Clearly both O−C and O−H orientational TCFs at the liquid/

Figure 6. (A) Orientational distribution of the O−C vector in two sublayers at the methanol/silica interface, where θ is the angle between the surface normal vector pointing from the silica into the liquid and the O−C vector. (B) Orientation distribution of the O−H vector in two sublayers at the methanol/silica interface, where θ is the angle between surface normal and O−H vector. P(cos θ) is the renormalized probability distribution of cos θ. In both figures the sublayers are defined by the position of O−H bond center, and P(cos θ) has been scaled by a constant such that it is unity for the bulk.

Figure 8. Orientational TCFs for the (A) O−C vector and (B) the O−H vector in the liquid/solid (LS) interface region (red) and the bulk region (green) of the methanol−silica system. Shown for reference is the orientational TCF for the CN vector of acetonitrile at the acetonitrile/silica interface (blue curve).

solid interface decay much more slowly than in the bulk. Here we have also compared the TCFs of methanol with those of acetonitrile. At the silica surface the O−H orientational TCFs decays much more slowly than the C−N orientational TCF of acetonitrile (Figure 8B), indicating that the interaction between methanol hydroxyl groups and surface silanol groups is stronger and more directional than that of acetonitrile. However, the O−C vector orientational dynamics of methanol at the liquid/ solid interface (Figure 8A) are not distinctly slower than those of the acetonitrile C−N vector reorientation. In the case of acetonitrile, the majority of surface orientational dynamics occurs without breaking of hydrogen bonds with silanol groups.56 The calculated TCFs shown in Figure 8 predict that the same argument should hold true for methanol hydrogen bonded to the silica surface. Slow reorientation of the interfacial methanol solvent would prevent the solvent molecules from responding rapidly to the change in electronic structure following C151 photoexcitation in the TCSPC TIR-fluorescence measurements. Consequently, the surface methanol species will be less able to stabilize an ICT state of photoexcited C151, and C151 should show more rapid nonradiative decay relative to a bulk methanol environment. This picture is consistent with the faster fluorescence lifetime (of ∼1.2 ns) assigned to the adsorbed C151 solutes. We have also performed a hydrogen bond analysis to quantify the hydrogen bond structure shown by snapshots in Figures 9A and 10. It has been known that methanol molecules can form hydrogen bond chains in bulk liquid57 (Figure 10B), with the average number of hydrogen bond per molecule roughly between 1.7 and 1.9, depending on specific force field

Figure 7. Arrow plot of O−C (green) and O−H (red) vectors in the methanol/silica interface region. The ends of both green and red arrows represent the oxygen atom, and heads of green and red arrows represent carbon and hydrogen atom, respectively. Vectors are projected onto the xz-plane, so in the y-direction they can appear to overlap.

embedded in the green arrow layers where the second sublayer inserts methyl groups into the hydrocarbon layer to enhance van der Waals interactions and points hydroxyl groups toward the surface to donate hydrogen bonds to molecules in the first sublayer. Time-correlation functions (TCFs) provide another powerful measure of the strong hydrogen bonding between methanol and the silica surface. We consider here orientational time correlation functions (TCFs), which are defined as C(t ) = ⟨v(0) ·v(t )⟩

(3)

where v is a unit bond vector whose orientation we wish to focus on. C(t) has been calculated for both O−C and O−H 27057

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Figure 9. (A) Snapshot of hydrogen bonding between methanol and surface silanols. Hydrogen bonds are represented by red dashed lines and atoms are represented by balls of different colors: yellow = silicon, white = hydrogen, red = oxygen, and cyan = carbon. (B) Number of hydrogen bond per molecule as a function as distance away from the silica surface.

and the definition of hydrogen bond. At the LS interface, the density profiles and bond orientations described previously suggest that its hydrogen bond structure should be very different from that of the bulk. We have calculated the average number of hydrogen bonds per molecule at a given distance away from the surface using as the definition of a hydrogen bond geometric criterion of previous work:26,27 r(O···O) is smaller than 3.4 Å, and the HO···O angle is smaller than 30°. The number of hydrogen bonds per molecule as a function of distance away from the surface is plotted in Figure 9B. The peak near the surface indicates that methanol molecules in the first sublayer participate in slightly more than two hydrogen bonds per molecule on average. This can be visualized by the snapshot of hydrogen bond chains in Figure 10A. In Figure 10A, we have shown all hydroxyl groups in the LS region, from both methanol and surface silanols, and hydrogen bonds between them (red dashed lines). It is obvious that hydroxyl groups together with silanol groups form hydrogen bond chains that are much longer than those formed in the bulk (Figure 10B). In other words, silanol groups serve as joints that connect hydrogen bond chains such that most methanol molecules can form two hydrogen bonds. A small portion of molecules can form more than two hydrogen bonds leading to an average number of hydrogen bonds per molecule in the first sublayer greater than two (and greater than the hydrogen bonding in bulk liquid). Our analysis also shows that for the hydrogen bonding between the first sublayer and silanol groups on the surface methanol molecules have equal chances to serve as a hydrogen bond donor or acceptor. However, in the second sublayer, the curve drops to a minimum less than 1, indicating that methanol molecules in the second sublayer are unable to form multiple hydrogen bonds with molecules in the first sublayer or in the bulk. Thus, the silica surface combined with tightly bound methanol molecules in the first sublayer is indeed nonpolar, and the interfacial solvent is restricted from interacting strongly with the rest of the system. In the bulk, since the length of hydrogen bond chains is shorter (Figure

Figure 10. (A) Snapshot of hydroxyl groups (white: hydrogen; red: oxygen) in the LS region, including hydroxyl groups from both the silica surface and methanol molecules. The hydrogen bonds are represented by red dashed lines. (B) Snapshot of hydroxyl groups (white: hydrogen; red: oxygen) in the bulk region (20 Å < z < 25 Å). The hydrogen bonds are represented by red dashed lines. The figure shows that on average methanol at the LS interface has longer fluctuating hydrogen bond chains than in the bulk. This arrangement results in a larger number of hydrogen bonds per molecule near the LS interface.

10B) and fewer molecules are participating with two hydrogen bond, the average number of hydrogen bonds per molecule is about 1.7, as evidenced by the plateau in Figure 9B. This detailed examination of the silica/methanol interface implies that unless an adsorbed solute enjoys strong local bonding to the silica surface it will not displace methanol from the first sublayer. Thus, the solute will be subject to a local environment that has lower solvent density, slower relaxation, and fewer hydrogen bonding opportunities relative to the bulk methanol solution. This environment significantly shifts C151’s electronic excitation energy to significantly shorter wavelengths. Furthermore, hindered motion of the interfacial solvent limits 27058

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Author Contributions

the ability of the interfacial environment to stabilize C151’s planar, ICT state resulting in faster radiative decay relative to what is observed in bulk solution. Collectively, these observations support a cooperative description of solute adsorption and solvation at a strongly associating silica/ methanol interface.



Both authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Collaborative Research in Chemistry program, grant CHE0628178. The MD simulations were carried out in part on the National Science Foundation TeraGrid network in (grant TGCHE070075N). B.L.W. acknowledges support from the National Science Foundation through grant CHE-1026870.

CONCLUSIONS Experiments presented in this work provide a detailed and comprehensive picture of how surfaces change the properties of an adjacent solvent from bulk solution limits. Two independent experimental methodsresonance-enhanced SHG and timeresolved emissionboth show that C151 adsorbed to the silica/methanol solid/liquid interface samples an environment that is decidedly less polar than bulk methanol. Molecular dynamics simulations show that this nonpolar region arises from surface induced organization of the interfacial solvent. This structure extends approximately two solvent layers into solution. Furthermore, solvent organization in these first two sublayers differs markedly from the organization observed at silica/acetonitrile interfaces, emphasizing differences between specific, hydrogen bonding association, and more general dipole−dipole interactions. These findings have consequences for models used to predict molecular adsorption and retention in separation applications. Mechanistic descriptions of solute behavior in liquid chromatography systems are relatively sparse, and most models assume that retention and elution are determined explicitly by a solute’s interaction with the stationary phase. The role of the solvent is considered only from the perspective of solute solubility. However, surface-induced changes in solvent organization can lead to solutes preferentially migrating to the interfacial region provided that the solute is stabilized by more favorable energetics regardless of the solute’s direct interactions with the substrate. Furthermore, these localized interfacial solvation environments can promote solute structure and reactivity not associated with bulk solution behavior. While such effects may have been inferred indirectly from previous studies, simulation data presented in this work show clearly that this cooperative picture of adsorption and interfacial solvation arises directly from the surface induced structure and organization of the interfacial methanol solvent.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing the dependence of TIR, time-resolved fluorescecne data as a function of bulk concentration. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.T.F.). *E-mail [email protected] (J.D.W.). *E-mail [email protected] (R.A.W.). Present Addresses

# D.R.: Naval Research Laboratory, 4555 Overlook Ave, Code 6126, Washington, DC 20375. ∇ A.R.S.: Coherent, Inc., Santa Clara, CA 95954. □ Department of Chemistry, University of Chicago, Chicago, IL 60637.

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