Characterization of Acid-Base Interactions Using Interface-Sensitive

Jul 8, 2019 - Interfacial interactions govern a number of macroscopic behaviors including adsorption and self-assembly. Acid-base interactions have be...
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Characterization of Acid−Base Interactions Using Interface-Sensitive Sum Frequency Generation Spectroscopy Michael C. Wilson,‡ Saranshu Singla,‡ Amanda J. Stefin, Sukhmanjot Kaur, Jared V. Brown, and Ali Dhinojwala* Department of Polymer Science, University of Akron, Akron, Ohio 44325, United States

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ABSTRACT: Interfacial interactions govern a number of macroscopic behaviors including adsorption and self-assembly. Acid−base interactions have previously been shown to comprise a significant portion of the interaction strength for polar liquids in contact with high-energy solids. Previous studies using interface-sensitive sum frequency generation spectroscopy connected the frequency shift of sapphire surface hydroxyl groups in contact with liquids to the independently determined Drago−Wayland acid−base parameters. However, limitations in liquid selection and in data analysis prevented broad application of the approach for characterizing interfacial acid−base interactions. In this work, we address these limitations through a more comprehensive liquid selection process and a more thorough analysis procedure, for example, by accounting for the van der Waals effect on frequency shift. The frequency shift of sapphire surface hydroxyl groups correlates with the Drago−Wayland acid−base parameters, highlighting the connection between interfacial interactions and interactions of molecules dissolved in solution. The modified approach allows for the wide application of frequency shift and Drago−Wayland acid−base parameters for the characterization of interfacial acid−base interactions, with implications for various fields including chemistry, biology, and engineering.

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INTRODUCTION

ΔH = m(Δν) + C

Interfaces are prevalent across numerous fields, ranging from geology to engineering. The importance of interfacial interactions cannot be understated, as they dictate phenomena including self-assembly, wetting, separation, and ice nucleation. Typically, the interfacial interactions of organic molecules have been distinguished as dispersion and polar;1 however, the acid−base component (sometimes denoted as hydrogen bonding) can be separated from the polar component.2 Acid−base interactions have been shown in the past to significantly affect interfacial phenomena.2−6 The characterization of acid−base interactions can help us better understand the aforementioned interfacial phenomena. In the 1930s, Badger and Bauer proposed a relationship between interactions and the spectroscopic shifts (Δν) of molecular vibrations, which is typically considered as a linear equation relating Δν and the enthalpy of interaction (ΔH) (eq 1).7−9 The m and C parameters in eq 1 are empirically determined constants for a particular functional group. Previous research has utilized infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies to measure the shift of molecular vibrational frequencies and chemical shifts, respectively, to understand the interactions between molecules.10−12 Frequency shifts and chemical shifts have been previously connected to the ΔH measured using calorimetry.10 © XXXX American Chemical Society

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Drago and Wayland proposed an alternate method to predict acid−base interactions, without having to measure the frequency shift of all acids with all bases.13 They defined two parameters (Ei and Ci) for any acid (a) or base (b) to describe its proclivity to undergo acid−base interactions. The parameter Ei describes the ability of a molecule to undergo electrostatic interactions, related to the dipole moment, while the parameter Ci describes the ability of a molecule to undergo covalent interactions, related to the polarizability.13 Equation 2 was developed to connect these acid−base parameters to the enthalpy of acid−base interactions (ΔH) measured by IR and other techniques. In the original work, iodine was assigned the reference values of Ea = 1 and Ca = 1, and the Eb and Cb values were determined for four amines by creating a system of equations.13 The Drago−Wayland acid−base parameters were further determined for a number of acids and bases using eq 2 by measuring the enthalpy of interactions of an acid (or base) with unknown parameters against bases (or acids) with known parameters.13,14 The enthalpy of acid−base interactions can then be found for any mixture of acids and bases with known parameters. Thus, the knowledge of Drago−Wayland paramReceived: July 1, 2019 Published: July 8, 2019 A

DOI: 10.1021/acs.jpcc.9b06266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

used in this study were all purchased from Sigma-Aldrich with the exceptions of acetonitrile and mesitylene, which were from Mallinckrodt and Acros Organic, respectively. All liquids were used as received. The purity for each chemical is listed in Table S1. Sapphire (Al2O3) equilateral prisms (15 mm × 15 mm × 15 mm × 10 mm, c-axis ±2° parallel to the prism face to avoid birefrigence) were purchased from Meller Optics Inc. The liquid cells were custom-built from stainless steel, with arms for liquid injection (Figure S1). Glass syringes and metal needles were used to extract the liquids stored with sure-seal caps. Cleaning Procedure. The sapphire prisms were first atmospherically plasma-treated (Harrick Plasma Cleaner, PDC-32G) for 5 min before being baked at 760 °C in a tube furnace for at least 3 h. After baking, the prisms underwent sequential solvent cleaning in toluene, chloroform, acetone, ethanol, and ultrapure water (Millipore filtration system, 18.2 MΩ·cm, pH 6−7) using sonication (Branson 1510 Ultrasonic Cleaner) for at least 1 h per solvent to remove any remaining nonpolar or polar contaminants. For the sapphire prisms used with diethyl sulfide, mesitylene, and triethylamine, the prisms were sonicated with only toluene and chloroform. All prisms were then dried with nitrogen gas and atmospherically plasma-treated for 5 min before assembling onto the liquid cells. No significant differences in the sapphire/ air spectra were observed for prisms cleaned with five solvents versus the ones cleaned with only two solvents (toluene and chloroform), suggesting that sonication with these two solvents was sufficient to obtain a clean sapphire surface. A clean sapphire prism surface presents ∼9 hydroxyl groups per square nm, as previously characterized by X-ray photoelecton spectroscopy.3 The liquid cells, brass caps, glass syringes, Teflon spacers, and metal needles were cleaned using the same sequential solvent-cleaning procedure as described for the prisms and, with the exception of the Teflon spacers, atmospherically plasma-treated for 5 min just before use. All other glassware were base-bath-cleaned and plasma-sterilized for 5 min. SFG Procedure. Prisms were mounted on top of the liquid cells with a Teflon spacer in between the two to ensure a proper seal (Figure S1). Prior to injecting liquids, a blank (sapphire/air) scan was collected for each prism to confirm that no hydrocarbon contamination was present and to locate the peak position of the free hydroxyl peak at the sapphire/air interface needed for data analysis (Figure S2). If no contamination was detected from the blank scan, then the assembled prism was placed in a vacuum at room temperature until liquid injection to prevent contamination and remove residual bound water. Glass syringes with metal needles were used to extract the liquids from their containers and placed into clean glass vials. Liquids were injected into the cells using clean glass pipettes, and the cells were sealed using brass caps. The assembly was left in the fume hood for at least 30 min to confirm that the liquid was not evaporating out of the cell due to improper sealing. SFG spectra were collected using a Spectra Physics Laser system. Details of the system can be found in previous literature.22 SFG probes a surface or interface by the temporal and spatial overlap of two beams: a fixed visible beam of 800 nm (∼70 μJ energy, 1 ps pulse width, 1 kHz repetition rate, 1 mm diameter) and a tunable IR beam (∼3.5 μJ energy, 1 ps pulse width, 1 kHz repetition rate, 100−200 μm diameter). A total internal reflection (TIR) geometry was used to probe the sapphire/liquid interface, where the incident angle of the IR

eters avoids the necessity of measuring the frequency shift for every combination of acids and bases to determine the strength of acid−base interactions. ΔH = EaE b + CaC b

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Subsequent research highlighted the importance of the Drago−Wayland parameters for understanding interfacial behavior,2 as long as the Ei and Ci parameters can be determined for each material. The enthalpy of interaction for some solid materials cannot be determined using the previously applied techniques. The experimental measurement of ΔH demands the use of spectroscopic or calorimetric techniques, which often rely on dilute solutions.8,15 The translation of these approaches to quantify acid−base interactions for solid−liquid pairs typically requires materials with sufficient surface area (porous solids or powders) to generate detectable signal, provided the solid cannot be dissolved.16,17 In contrast, interface-sensitive sum frequency generation (SFG) spectroscopy does not require high-surface-area solids. SFG, a second-order nonlinear optical technique, can selectively probe interfacial interactions in situ at solid/liquid and even solid/solid interfaces.18,19 Kurian et al.20 utilized SFG to measure the spectroscopic shift (Δν) of sapphire surface hydroxyl groups to examine the strength of interfacial acid− base interactions between inorganic sapphire and both small organic molecules and polymers. Additionally, they combined the spectroscopic shifts with the Drago−Wayland base parameters (Ei and Ci) of the small molecules in order to determine the Ea and Ca for sapphire. The direct correlation between the Badger−Bauer and Drago−Wayland approaches highlighted the potential application of SFG for understanding interfacial acid−base interactions. However, some doubt remains on the general applicability of SFG to the Drago−Wayland method to characterize interfacial acid−base interactions on account of two primary reasons: liquid selection and hydroxyl peak analysis. In Kurian et al.,20 the number of liquids chosen was limited; however, the accurate quantification of Ea and Ca requires a wide range of Cb/Eb ratios.21 Second, in Kurian et al., the hydroxyl region was fit with a single Lorentzian peak, but two peaks can be seen in some of the spectra upon close examination. In the present work, we expand the work done by Kurian et al. to more accurately determine the sapphire surface characteristics by investigating the acid−base interactions at the sapphire/liquid interface for 10 liquids (including two from Kurian et al.). A number of these liquids have not been previously examined using SFG at solid/liquid interfaces. In addition, the Eb and Cb values of these liquids cover the range required of Cb/Eb ratio for accurate determination of sapphire Ea and Ca values. Furthermore, we fit the hydroxyl region with multiple peaks where necessary. In addition, we calculate Ea and Ca values for two different polarizations to examine the nature of the surface hydroxyls oriented at different angles with respect to the surface normal. Upon detailed investigation of the Drago−Wayland method and the SFG method of Kurian et al., the importance of van der Waals interactions on the frequency shift seen in SFG is revealed.

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EXPERIMENTAL SECTION Materials. The high-purity liquids (acetone, acetonitrile, benzene, dibutyl ether, diethyl sulfide, dimethylformamide, dimethyl sulfoxide, dioxane, mesitylene, and triethylamine) B

DOI: 10.1021/acs.jpcc.9b06266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C beam (with respect to the sapphire surface normal) was adjusted to the critical angle for that particular liquid (Table S1). The incident angle of the visible beam was ∼1.5° lower than the IR beam. SFG spectra were collected in two different polarizations (PPP and SSP) for both the hydrocarbon region (2750−3100 cm−1) and hydroxyl region (3100−3800 cm−1) with a resolution of 5 and 10 cm−1, respectively. The combination of letters describes the polarization of output and input beams (e.g., PPP = P-polarized SFG, P-polarized visible, and P-polarized IR). Of the 27 component nonlinear susceptibility tensor (χ(2)), PPP polarization examines only the (2) (2) (2) four nonzero components (χ(2) zxx, χxzx, χxxz, and χzzz), while SSP 19 (2) selectively probes solely the χyyz component. The different polarizations could show different peaks depending upon the orientation of the functional groups. SFG intensity was normalized by the simultaneously measured IR intensity. At least three repeats for each liquid were performed to ensure the reproducibility of results. Two spectra were collected for each repeat in the hydroxyl region and then averaged to improve the signal-to-noise ratio. Analysis. SFG spectra were fit using Lorentzian peaks (eq 3).19 ISFG represents the SFG intensity, and χNR represents the nonresonant contribution which is independent of wavenumber (ωIR). Aq represents the amplitude, Γq represents the damping constant, and ωq represents the resonant frequency of the qth vibrational mode. ISFG ∝ χNR + Σ

Aq ωIR − ωq + i Γq

Figure 1. Chemical structures of the liquids used in the present study. The Eb and Cb parameters of the liquids have been given in Table S1.

electrons. To choose the liquids for the study, the values of m, C, Ea, and Ca parameters for sapphire were obtained from Kurian et al.,20 while the Eb and Cb values for the liquids were taken from Drago et al.14 in order to calculate the predicted frequency shifts to ensure sufficient variation. The hydrocarbon (2750−3100 cm−1) region spectra are depicted for each liquid in PPP (Figure 2A) and SSP (Figure

2

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SFG spectra in the hydroxyl region (attributed to the sapphire hydroxyl peak) were fit with either one, two, or three peaks as determined by the visual examination for troughs in the hydroxyl region consistently appearing across the three repeats, highlighting a diversity of interactions.7 Additionally, for some liquids, a water peak corresponding to around 3250 cm−1 was added but not considered in the calculation of frequency shift. The relative amplitude between shifted hydroxyl peaks is related to the fraction of hydroxyl groups interacting in a certain fashion. Thus, we considered the amplitude-weighted shifted peak position (ωq,avg, eq 4) and subtracted it from the sapphire free hydroxyl peak position (ωfree‑OH) to calculate an average frequency shift (Δν) of the liquid (eq 5). The average frequency shift was further corrected for frequency shift purely due to van der Waals interactions and subsequently used to calculate the acid−base interaction strength by the Badger−Bauer rule (eq 1). ωq ,avg =

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∑i ωq , i Aq , i ∑i Aq, i

Δν = ωfree‐OH − ωq ,avg

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Figure 2. Representative SFG spectra collected in PPP polarization for the liquids in contact with sapphire in the hydrocarbon (A) and hydroxyl (B) regions. The spectra have been fitted using the Lorentzian equation (eq 3) and are displayed with similar peak heights simply for comparison.

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RESULTS AND DISCUSSION The chemical structures of the liquids used in the present study are shown in Figure 1. The liquids are selected with large variety in their predicted shift calculated from the combined Badger−Bauer−Drago−Wayland equation used in Kurian et al.20 A variety of electron donors are examined, including aromatic π-clouds, nitrogen, oxygen, and sulfur. Additionally, the electron donors chosen experience different bonding structures, which can aid or hinder their ability to donate

3A) polarizations. The peaks observed in the hydrocarbon region are attributed to the C−H stretch vibrations. For example, acetonitrile shows a peak at ∼2940 assigned to the symmetric methyl stretch in both PPP and SSP spectra, as seen in previous studies at rutile,23 zirconia,24 and yttrium aluminum garnet surfaces.25 Benzene, a symmetric molecule, is expected not to be SFG active.26 However, previous C

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Table 1. Peak Assignments in the Hydrocarbon Region for the Different Liquids Used in the Present Study chemical name 20,28−30

acetone acetonitrile25,31 benzene27

dibutyl ether32

diethyl sulfide33 DMF34

dimethyl sulfoxide28 dioxane35 mesitylene27

Figure 3. Representative SFG spectra collected in SSP polarization for the liquids in contact with sapphire in the hydrocarbon (A) and hydroxyl (B) regions. The spectra have been fitted using the Lorentzian equation (eq 3) and are displayed with similar peak heights simply for comparison.

triethylamine36

peak position (cm−1)

peak assignment

∼2930 ∼2940 ∼3043 ∼3073 ∼3090 ∼2860 ∼2923 ∼2957 ∼2890 ∼2955 ∼2820 ∼2896 ∼2945 ∼2920 ∼2910 ∼2985 ∼2917 ∼2945 ∼2975 ∼3015 ∼2840 ∼2890 ∼2955

νs(CH3) νs(CH3) ν(CH)7a, ν(CH)7b ν(CH)20a, ν(CH)20b comb/overtone (CH2) bend νas(CH2) νas(CH3) ν(C−H) ν(C−H) 1664 + 1160 ν(C−H) νs(CH3)N νs(CH3) ν(C−H) ν(C−H) νs(CH3) νas(CH3) νas(CH3) ν(C−H) 20a ν(C−H) ν(C−H) ν(C−H)

interactions between the sapphire surface hydroxyl groups and the molecules in contact (Figures 2B and 3B). Multiple peaks are observed in the hydroxyl region for the liquids used in this study, with the exception of benzene, indicating a variety of interactions. One hypothesis for the variety of interactions is that the molecules adopt more than one type of configuration on the sapphire surface. Using molecular dynamics simulations, acetone in contact with sapphire was observed to adopt two primary configurations, with the carbonyl oxygen pointed toward and away from the sapphire.39 Another hypothesis could be related to the packing of molecules on the sapphire surface. Steric constraints may alter the way neighboring molecules interact with the sapphire surface hydroxyl groups leading to different energy states. Furthermore, if multiple hydroxyl groups interact with one functional group, then a distribution of interactions strengths could be possible. To account for the multiple interactions, as reflected in the multiple hydroxyl peaks, the weighted-average position of the shifted sapphire surface hydroxyl peak is calculated using eqs 4 and 5. The amplitude-weighted average considers the fraction of hydroxyl groups undergoing a particular type of interaction. Thus, the average frequency shift accounts for the general behavior of sapphire surface hydroxyl groups in contact with a liquid. Liquids such as acetone and DMF shift the sapphire surface hydroxyl peak more drastically than mesitylene and benzene (Table 2). The frequency shift is similar across PPP and SSP polarizations in most cases. However, for dibutyl ether, the frequency shift is much lower in SSP polarization compared to that in PPP polarization. One could typically rationalize the observed differences on account of the differences in the orientation of the sapphire surface hydroxyl groups. It could be that interactions with dibutyl ether molecules affect the orientation of hydroxyl groups, leading to a different frequency shift between PPP and SSP polarizations. However, dioxane, an analogue for dibutyl

literature has shown that benzene exhibits strong SFG signals at the benzene/air interface due to distortion of electron clouds at the air interface.27 A similar argument could explain the presence of aromatic C−H stretch peaks at the sapphire/ benzene interface in PPP and SSP polarizations. The relative ratio of the different peaks in benzene differs between PPP and SSP on account of the orientation distribution of benzene molecules next to sapphire. A similar behavior is observed for both mesitylene and dibutyl ether. However, for the majority of liquids, similar spectra are observed in both polarizations. The detailed peak assignments for all liquids are listed in Table 1 using previous literature. In the hydroxyl region (Figure 2B), the sapphire/air interface in PPP shows a peak on average at ∼3706 cm−1, attributed to the O−H stretching vibration of the sapphire surface hydroxyl groups not participating in any interactions (or referred to as free surface hydroxyl groups). The peak position of the free surface hydroxyl groups varies from 3691 to 3727 cm−1 across repeats (Figure S2) encompassing the value of 3720 cm−1 seen in the previous work by Kurian et al.20 The peak position is sensitive to the nature of the hydroxyl group, which depends on the coordination of both the hydroxyl group and the aluminum atom.37 The relative acidity and basicity of the different surface hydroxyls varies with the nature of bonding and results in an average isoelectric point of pH 5−7, as measured by Hsu et al.37,38 In SSP (Figure 3B), the free surface hydroxyl peak occurs in the same position as that in PPP (Figure S3). Since the SSP signals were relatively weaker compared to PPP signals, we used the free surface hydroxyl peak position measured for each repeat using PPP for calculating the frequency shift in contact with a liquid for that repeat. When liquids are brought in contact with the sapphire surface, the vibrational frequency of the free surface hydroxyl groups red-shifts from the peak position for sapphire/air due to D

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of van der Waals interactions on frequency shift and the effect of C in the Badger−Bauer rule. Kurian et al. plotted Eb/Cb as a function of Δν/Cb (eq 6) to determine the values of Ea (1.05 ± 0.08) and Ca (0.06 ± 0.01) for sapphire from the slope and intercept. The C term in the Badger−Bauer rule in their calculations was considered negligible. However, C/CbEa should be compared to Ca/Ea to examine the necessity of considering C in the calculation, and these values can be calculated by plugging in the parameters determined by Kurian et al.20 From this calculation, the term C/CbEa contributes only 10% of the intercept term for triethylamine and pyridine, but it contributes around 20% for acetone. The contribution of C/ CbEa is significant for some of the liquids used in the present study, for example, benzene (40% of the intercept term). Thus, the C term cannot be ignored for calculations in the present study.

Table 2. Average Total Frequency Shift (Δν) for the Different Liquids Using PPP and SSP SFG Spectra in the Hydroxyl Regiona chemical acetone acetonitrile benzene dibutyl ether diethyl sulfide dimethylformamide dimethyl sulfoxide dioxane mesitylene triethylamine

Δν(PPP) (cm−1) 110 70 51 98 132 115 107 103 77 90

± ± ± ± ± ± ± ± ± ±

5 3 5 3 5 5 2 11 7 5

Δν(SSP) (cm−1) 90 78 50 55 117 96 113 91 72 89

± ± ± ± ± ± ± ± ± ±

4 6 2 6 2 5 6 12 6 1

The error bars represent ± standard error.

a

ij E yz ΔHab = Eajjj b zzz + Ca j Cb z Cb k {

ether, shows a more reasonably consistent behavior across PPP and SSP polarizations. If interactions determine the orientation of surface hydroxyl groups, one would expect similar behavior across these liquids. Another possibility is that adsorbed liquidlike water (with a peak position ∼3450 cm−1) on the surface of sapphire is not completely removed.22 As water tends to show significantly higher intensity in PPP versus SSP, these changes could affect the shape of the shifted surface hydroxyl peak. The frequency shift of a free surface hydroxyl peak in contact with a liquid can be attributed to both van der Waals and acid−base interactions.11 For example, the free hydroxyl peak of phenol was observed at 3654 cm−1 in the vapor phase, whereas the same peak for phenol dissolved in a nonpolar solvent was observed at 3609 cm−1, suggesting a significant contribution of van der Waals interactions toward the frequency shift of hydroxyl groups.8,15,17 Previous analysis of the frequency shifts of sapphire surface hydroxyl groups in contact with Lewis bases by Kurian et al. attributed the entirety of frequency shift solely to acid−base interactions.20 However, sapphire in contact with pentadecane shows a shifted-hydroxyl peak at ∼3690 cm−1, highlighting the significance of van der Waals interactions in frequency shift, even at interfaces. As previous literature suggests a variation in the strength of van der Waals interactions for different molecules,11 we first calculated the van der Waals work of adhesion of liquids against sapphire using the Lifshitz theory.40 The values of van der Waals work of adhesion obtained for a subset of liquids used in the present study with the inclusion of a nonpolar liquid, hexadecane, vary by 20% (Table S2). Thus, we performed experiments with a nonpolar liquid, heptane, to measure the frequency shift (∼20−25 cm1) due to van der Waals interactions (Figure S4). Accounting for the variation, the work of adhesion should correspond to at most a variation of ∼5 cm−1 in the frequency shift. This shift was subtracted from the average total frequency shift for sapphire surface hydroxyls in contact with a liquid, assuming additivity of interactions,11 to calculate the frequency shift solely due to acid−base interactions.

where ΔHab = mΔνab + C = m(Δν − Δνvdw ) + C

(7)

Including the subtraction of the van der Waals component from the total frequency shift and the effect of C, the equation can be rearranged to plot ΔHab/Cb instead of Δν/Cb (eq 7). The data is then plotted according to eq 7 in Figure 4 for PPP

Figure 4. ΔHab/Cb plotted as a function of Eb/Cb for the different liquids in PPP (filled markers) and SSP (empty markers) polarizations. The calculated Ea and Ca values for sapphire using the slope and the intercept of eq 7 are displayed on the graph.

and SSP polarizations, respectively, with ΔHab/Cb as a function of Eb/Cb. Similar to Kurian et al.,20 linear correlation is seen, despite the simplicity of the model in accounting for interfacial interactions. The Ea calculated for sapphire in Kurian et al. (Ea = 1.05 ± 0.08) for PPP polarization differs from the current study (Ea = 0.60 ± 0.09), especially by accounting for the effect of van der Waals interactions (Ea = 0.89 ± 0.13 when the van der Waals component is not included). The Ca values remain consistent across the studies (0.06 ± 0.01 from Kurian et al. and 0.10 ± 0.04 in the current study) for the same polarization. For SSP polarization, Ea = 0.58 ± 0.09 and Ca = 0.05 ± 0.04. The Drago−Wayland parameters calculated for sapphire do not seem to depend on polarization for the cases of PPP and SSP, though the slight difference in Ca becomes significant when calculating the enthalpy of interaction using the Drago− Wayland equation (Tables S3 and S4). While the magnitude of Ea is calculated to be significantly higher than Ca, the low

Eb C zy m ji Δν zyz jij C = jjj − a zzz zz + jj j z j Cb Ea k C b { k C bEa Ea z{ (6) We connect acid−base interactions determined by the Badger−Bauer rule considering the frequency shift (eq 1) to the Drago−Wayland parameters (eq 2) using a method similar to Kurian et al.20 with two notable changes: including the effect

E

DOI: 10.1021/acs.jpcc.9b06266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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magnitude of the Ca of sapphire cannot be ignored. For instance, the enthalpy calculated from the Drago−Wayland equation using the Eb and Cb values from the literature and the Ea and Ca values calculated from the PPP graph show that for a high Cb liquid like triethylamine, the covalent component contributes around 65% of the total enthalpy, and for lower Cb liquids like acetonitrile, the covalent component still contributes 20% (Table S3).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Edward Laughlin for designing and building the SFG liquid cells. We also like to thank Dr. Selemon Bekele and Nathaniel Orndorf for reviewing the manuscript. S.S., A.J.S., S.K., J.V.B., and A.D. were supported by National Science Foundation (NSF DMR-1610483). M.C.W was funded by Lubrizol (under a biomimicry fellowship). A.J.S. was also funded by LORD Corporation and Goodyear.

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CONCLUSION In this work, we investigate a series of liquids in contact with sapphire using SFG, connecting two different methods of quantifying enthalpy of interactions. We find a good correlation between frequency shifts and Drago−Wayland parameters, connecting the measurements of solution-state liquid interactions with interfacial interactions. We calculate the Ea and Ca parameters for sapphire surface hydroxyl groups using the correlation from Kurian et al. but incorporating van der Waals effect on frequency shift and C in the Badger−Bauer rule. The value of Ea for sapphire surface hydroxyl groups differs from Kurian et al. The higher Ea relative to Ca shows the dominance of electrostatic acid−base interactions, but only for liquids for which Cb is no more than an order of magnitude greater than Eb. The correlation between frequency shift in SFG for liquids in contact with sapphire and Drago−Wayland parameters determined from the solution-state IR spectroscopy of liquids remains clear. Using a similar methodology, the Drago− Wayland acid−base parameters can be determined for other planar solids. The exhaustive list of parameters available in the literature allows the prediction of the interaction strength of many liquids in contact with well-characterized substrates. Additionally, the Drago−Wayland methodology provides the ability to predict interactions at solid−solid interfaces,11 which SFG has the ability to verify.3 SFG empowers the Drago− Wayland approach to examine a broad expanse of previously inaccessible interactions, providing further directed study into a number of research topics, including tribology, adsorption, and protein binding.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b06266.

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REFERENCES

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Components of liquid cells, histogram of sapphire freehydroxyl peak position, calculation of frequency shift due to van der Waals interactions, comparison of PPP and SSP SFG spectra for sapphire/air, SFG spectra for the sapphire/heptane interface, parameters for all the liquids used in the current study, calculation of van der Waals work of adhesion, and calculated enthalpies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Saranshu Singla: 0000-0002-0690-3740 Ali Dhinojwala: 0000-0002-3935-7467 Author Contributions ‡

M.C.W. and S.S. contributed equally to this work. F

DOI: 10.1021/acs.jpcc.9b06266 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.jpcc.9b06266 J. Phys. Chem. C XXXX, XXX, XXX−XXX