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Discrete Partitioning of Solvent Permittivity at Liquid-Solid Interfaces Xiaoyi Zhang† and Robert A. Walker*,†,‡ Department of Chemistry and Biochemistry, University of MarylandsCollege Park, College Park, Maryland 20742 Received March 29, 2001. In Final Form: May 31, 2001 Electronic excitation energies of chromophores adsorbed to strongly associating, alcohol-hydrophilic quartz, liquid-solid interfaces are measured using resonant second harmonic generation (SHG). Shifts in the SHG spectra relative to excitation spectra in various bulk solvents are used to infer the dielectric character of the interfacial environment. At interfaces formed between n-alcohol solvents and hydrophilic quartz surfaces, the local dielectric environment is less polar than bulk solution. This trend becomes more pronounced with longer chain n-alcohol solvents. The interfacial region formed between a branched alcohol (2-propanol) and hydrophilic quartz is more polar than bulk solution. These results are interpreted in terms of a solvent’s ability to order at the interface and cancel dipolar contributions from surface hydroxyl groups to the local solvation environment experienced by the chromophore solute.
Introduction Characterizing the properties of solutions near surfaces remains a challenging issue in the field of surface science. Surfaces alter solvent structure as well as solvent orientation, leading to changes in solvent density, viscosity, and refractive index from bulk solution limits.1-3 This surface-induced anisotropy presumably alters a solvent’s solvating properties. One property that figures prominently in solvent-solute interactions is solvent permittivity. A solvent’s dielectric character plays an important role in solution phase chemistry by controlling solute solubility, conformation, and reactivity.4 Arising from averaged corrections to the polarization of a solute due to an oscillating external field, a solvent’s static dielectric constant depends on long-range forces and can be described in terms of dipolar interactions between the solute and the total electric moment of the surrounding medium.5 At surfaces, however, short-range forces dominate solvent structure and orientation. Models of solvent permittivity frequently use dielectric continuum models to describe dipolar properties within liquids.6-9 While these models do enjoy some success, they necessarily oversimplify local intermolecular interactions that are expected to dominate solvent behavior near surfaces. In particular, dielectric continuum models cannot account for any dependence of solvent permittivity on specific solvent identity. A recent model of interfacial polarity at liquid-liquid boundaries suggests that dielectric properties within the interfacial environment can be described as an arithmetic mean of the two polarities corresponding to the adjacent, †
Chemical Physics Program, University of Maryland, College Park. ‡ Department of Chemistry and Biochemistry, University of Maryland, College Park. (1) Benjamin, I. Annu. Rev. Phys. Chem. 1997, 48, 407-451. (2) Lu, J. R.; Thomas, R. K. J. Chem. Soc., Faraday Trans. 1998, 94, 995-1018. (3) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992, 43, 627-661. (4) Murrell, J. N.; Jenkins, A. D. Properties of liquids and solutions, 2 ed.; John Wiley and Sons: New York, 1994. (5) Bo¨ttcher, C. J. F. Theory of Electric Polarization, 2 ed.; Elsevier: 1973; Vol. 1. (6) Liu, X. Y. J. Chem. Phys. 1993, 98, 8154-8159. (7) Benjamin, I. J. Phys. Chem. A 1998, 102, 9500-9506. (8) Marcus, R. A. J. Phys. Chem. 1990, 94, 1050-1054. (9) Pratt, L. R. J. Phys. Chem. 1992, 96, 25-33.
bulk solvents.10-12 This model is based upon observed changes in the electronic transition energies of adsorbed solutes and implies that long-range dipole interactions between the solute and surrounding solvent media are responsible for interfacial permittivity. While this result is intriguing given anisotropy inherent to interfaces, the organic solvents used in combination with an aqueous subphase were all aprotic and therefore incapable of hydrogen bonding. These interfaces may be characterized as weakly interacting in contrast to immiscible liquidliquid systems such as 1-octanol/water where interfacial interactions are dominated by hydrogen bonding between adjacent phases.1 Experiments discussed below investigate polarity at strongly interacting liquid-solid boundaries. Using solvatochromic shifts of chromophores adsorbed to these interfaces, we observe that the local dielectric environment sampled by the solute within the interfacial region varies in a nonadditive fashion. Specifically, second harmonic generation (SHG) experiments show that linear n-alcohol solvents adjacent to hydrophilic surfaces create regions of reduced permittivity relative to bulk solution limits. The disparity between these interfacial and bulk dielectric environments increases with increasing n-alcohol chain length. This trend reverses itself for a branched alcoholhydrophilic, liquid-solid interface, indicating that solvent structure must be considered when assessing how surfaces and solvents conspire to create chemically distinct interfacial regions. Experimental Methods Solvent dependent shifts in a solute’s electronic transition energies have long been used as experimental measures of solvent-solute interactions.13,14 Numerous empirical scales of solvent polarity quantify the long-range dipolar forces responsible for solvation energies by comparing the excitation spectrum of a solute in the solvent of interest to similar data acquired from (10) Wang, H.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. A 1997, 101, 713-718. (11) Wang, H.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 4927-4932. (12) Michael, D.; Benjamin, I. J. Phys. Chem. B 1998, 102, 51455151. (13) Suppan, P. J. Photochem. Photobiol. A 1990, 50, 293-330. (14) Suppan, P.; Ghoneim, N. Solvatochromism; Royal Society of Chemistry: Cambridge, U.K., 1997.
10.1021/la0104828 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
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the solute in several reference solvents.10,15 While such measures can be useful for comparative studies, the connection between observed spectroscopic shifts and specific solvent-solute interactions remains ambiguous. Measurements of interfacial polarity in this Letter characterize the local dielectric environment using the Onsager function, f(D),14,16
f(D) )
2(D - 1) 2D + 1
(1)
where D is a bulk solvent’s static dielectric constant and f(D) has limiting values of ∼0.4 for alkanes and ∼1 for polar liquids such as water ()0.98).14 Unlike empirical scales of solvent polarity, f(D) can be related to the nonspecific solvent-solute interactions and can be modified to describe the solvation of solute chromophrores having point dipoles.16 To study solvent permittivity at interfaces, we employ second harmonic generation (SHG), a surface specific spectroscopy that is sensitive to the energetics and orientation of electronic transition moments.17,18 In a typical SHG experiment, a single coherent optical field with frequency ω is focused on the interface under study and a nonlinear polarization with frequency 2ω is detected.19 The intensity of the 2ω field is proportional to the square of the second-order susceptibility, χ(2)
I(2ω) ∝ |χ(2)|(2)I2(ω)
(2)
where I(ω) is the intensity of the incident field and χ(2) is a thirdrank tensor that under the electric dipole approximation is zero in isotropic environments.17,20 The χ(2) tensor is responsible for the technique’s inherent surface specificity and contains both nonresonant and resonant contributions:10,21 (2) χ(2) ) χ(2) NR + χR
(3)
Typically, the resonant term is several orders of magnitude larger than the nonresonant contribution and can be related to microscropic hyperpolarizability:
χ(2) R )
∑(ω k,e
gk
µgkµkeµeg - ω - iΓ)(ωeg - 2ω + iΓ)
(4)
where µij is the transition matrix element between state i and state j (where g stands for ground state, k for an intermediate, virtual state, and e for contributing excited states). When 2ω is resonant with ωeg, χ(2) becomes large, leading to a strong resonance enhancement in the observed intensity at 2ω.10,21,22 Measuring the scaled intensity [I(2ω)/I2(ω)] as a function of 2ω records an effective excitation spectrum of solutes adsorbed to the solidliquid interface. For the solid-liquid systems discussed below, the nonresonant signal measured from silica surface-neat solution systems was always g10× smaller than that for the solid-liquid systems containing the adsorbed chromophores. The experiments described below measure the solvatochromic shifts of 4-aminobenzophenone (4ABP) adsorbed to different hydrophilic-n-alcohol interfaces. The π-π* transition of 4ABP is best represented by a charge transfer (CT) between the carbonyl (-) and amino (+) groups.14,23 The difference in dipole moments between the ground and CT state is ∼11.5 D, leading to a strong (15) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J. M.; Notario, R. J. Chem. Phys. 1994, 98, 5807-5816. (16) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991, 113, 4776-4782. (17) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107-125. (18) Shen, Y. R. Nature 1989, 337, 519-525. (19) Luca, A. A. T.; He´bert, P.; Brevet, P. F.; Girault, H. H. J. Chem. Soc., Faraday Trans. 1995, 91, 1763-1768. (20) Dick, B.; Gierulski, A.; Marowsky, G.; Reider, G. A. Appl. Phys. B 1985, 38, 107-116. (21) Miranda, P. B.; Pflumio, V.; Saijo, H.; Shen, Y. R. J. Am. Chem. Soc. 1998, 120, 12092-12099. (22) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley and Sons: New York, 1984. (23) Ghoneim, N.; Monbelli, A.; Pilloud, D.; Suppan, P. J. Photochem. Photobiol. A 1996, 94, 145-148.
Figure 1. Solvatochromism of 4-aminobenzophenone (4ABP) in various bulk solvents. Data show the UV-vis absorption maxima as a function of solvent polarity. Solvents include (1) isooctane, (2) cyclohexane, (3) carbon tetrachloride, (4) diethyl ether, (5) ethyl acetate, (6) 1-decanol, (7) 1-octanol, and (8) 2-propanol, 1-butanol, and 1-propanol. solvent dependent excitation energy that monotonically red-shifts by ∼30 nm as solvent polarity increases from an f(D) value of 0.41 (cyclohexane) to 0.98 (water) (Figure 1). Comparing the excitation energies of 4ABP solutes adsorbed to different solidliquid interfaces with bulk solution transition energies allows us to observe how surface-induced anisotropy changes the local dielectric environment and infer how interfacial solvent-solute interactions differ from their bulk solution limits. Solid-liquid interfaces are created by placing a fused silica prism (Edmund Scientific) hypotenuse side down onto a Teflon cell containing a reservoir of solution. Prisms are cleaned in an acid bath composed of a 50:50 mixture (by volume) of concentrated nitric and sulfuric acids. Prepared surfaces show a zero degree contact angle with water. 4ABP was purchased from Aldrich (98% purity) and used as received. All solutions are saturated. Experiments are carried out using a visible optical parametric amplifier (OPA, Clark-MXR) pumped with the output from a Ti:sapphire regeneratively amplified system (CPA2001). The output from the OPA is manually tuned from 560 to 700 nm and used as the fundamental frequency, ω. The visible bandwidth is 2.5 ( 0.5 nm, and the incident power varies between 0.07 and 1.0 µJ. The polarizations of the incident and second harmonic fields are selected with Glan-Taylor polarizers and half-wave plates. Filters and a monochromator separate the 2ω signal from the remaining residual fundamental light. The quadratic dependence of I(2ω) on I(ω) is checked regularly. Typical signal levels average 0.01-0.1 photons per shot. Spectra are recorded under conditions of PinPout, where P denotes plane polarization parallel to the plane of incidence. This combination simultaneously samples several different elements of the χ(2) tensor.20 Spectra recorded using other polarization combinations that sample individual χ(2) elements show identical resonance band profiles, indicating that the local dielectric properties sampled by the solute do not partition into distinct in-plane and out-ofplane environments.
Results and Discussion Figure 2 shows the PinPout SHG spectra of 4ABP adsorbed to hydrophilic substrates from solutions of 1-propanol and 1-octanol. Fits to SHG spectra were generated according to eqs 2 and 4. Compared to their excitation spectra in bulk 1-propanol and 1-octanol, spectra of 4ABP adsorbed to propanol-hydrophilic and octanolhydrophilic interfaces shift to shorter wavelengths by 4 and 8 nm, respectively. We have carried out similar experiments with 1-butanol and 1-decanol. The n-alcohol results show a consistent trend; namely, the π-π*
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Figure 2. PinPout SHG spectra of 1-propanol (open circles) and 1-octanol (filled circles). The vertical solid line is the λmax for UV spectra (1-propanol, 337 nm; 1-octanol, 336 nm). The two vertical dashed lines are the maxima of 4ABP SHG spectra of 1-propanol (333 nm) and 1-octanol (328 nm).
Figure 3. Molecular arrangement at the (A) hydrophilic quartz-1-propanol and (B) hydrophilic quartz-1-decanol interfaces.
transition energy of 4ABP shifts to shorter wavelengths relative to bulk solution limits. Consequently, we conclude that the n-alcohol solvents, regardless of the chain length, create an interfacial region that is less polar than bulk solution. The nonpolar environment becomes more pronounced as chain length increases, leading us to believe that substrate induced polar ordering of the solvent is partitioning the interface into regions having discrete dielectric properties that vary on molecular length scales. For the n-alcohol-hydrophilic substrate systems described in this Letter, surface induced ordering within the first several solvent layers may create a bilayer-like structure (Figure 3). Analogous structures within n-alcohol solvents have been observed by X-ray scattering at airliquid boundaries.24 If solvent-substrate hydrogen bonding is strong enough to inhibit dipolar solvent-solute (and (24) Gang, O.; Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Deutsch, M. Phys. Rev. E 1998, 58, 6086-6100.
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Figure 4. (Upper panel) UV-vis spectrum (dashed line) and SHG spectrum (open circles) of 4ABP in 1-propanol. (Bottom panel) UV-vis spectrum (dashed line) and SHG spectrum (filled circles) of 4ABP in 2-propanol.
substrate-solute) interactions, the interfacial environment will exhibit decidedly lower permittivity relative to that of bulk solution where solvent molecules can freely reorient. Size considerations show that 4ABP can actually span a bilayer of 1-propanol, meaning that the effects of bilayer formation on local dielectric properties should be smaller for short chain alcohols, consistent with the observed data (Figure 3A). In contrast, a bilayer consisting of 1-decanol can span ∼3 nm, meaning that 4ABP in the interfacial region can be completely surrounded by alkyl chains (Figure 3B). If interfacial permittivity at these strongly interacting surfaces depends on long-range order within the first several solvent layers, altering solvent structure to inhibit solvent ordering should disrupt this dielectric partitioning effect. The isomers 1-propanol and 2-propanol have similar dielectric constants at 25 °C (20.3 and 19.9, respectively),14 associated f(D) values (0.928 and 0.926, respectively), and UV absorption maxima that differ by only 1 nm. Figure 4 shows 4ABP adsorbed to 1-propanol-quartz and 2-propanol-quartz interfaces. We observe the previously discussed 4 nm blue-shift from 4ABP in the 1-propanolhydrophilic system. The spectrum from 4ABP adsorbed to the 2-propanol-hydrophilic interface, however, shows a 4 nm red-shift. Again using the solvatochromic behavior of 4ABP in Figure 1, we infer that while the interfacial environment formed at the 1-propanol-quartz boundary is less polar than bulk solution, the interfacial dielectric environment of the 2-propanol-quartz system is more polar than that in bulk solution. The enhanced permittivity observed at the hydrophilic-2-propanol interface exceeds effects predicted from simple solvation models with a projected f(D) value greater that unity. Physical considerations necessitate a second cause for the observed shift in transition energy, such as a field induced Stark shift. Figure 5 and Table 1 summarize the data discussed above, showing absorption maxima of UV-vis spectra and the SHG spectra of four linear and one branched alcohol solutions. The data clearly show the dependence of the
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Figure 5. Absorption maxima of UV-vis spectra (filled circles) and SHG spectra (filled squares) plotted versus bulk solvent polarity, f(D). Solvents include (1) 1-propanol, (2) 1-butanol, (3) 1-octanol, (4) 1-decanol, and (5) 2-propanol. Uncertainties in the SHG data represent a convolution of uncertainty in the bandwidth and results from fitting multiple spectra as described in the text. Table 1. UV-vis and SHG Absorption Maxima of 4ABP in Different Solventsa solvent
f(D)
λmax(UV) (nm)
λmax(SHG) (nm)
∆λ (nm)
1-propanol 1-butanol 1-octanol 1-decanol 2-propanol
0.93 0.92 0.86 0.83 0.92
337 337 336 335 336
333 ( 1 331 ( 1 328 ( 1 325 ( 2 340 ( 1
-4 -6 -8 -10 +4
a Uncertainties in UV maxima are < 1.0 nm. Reported uncertainties in SHG maxima result from fitting multiple spectra using procedures described in the text.
4ABP π-π* transition energy on both n-alcohol chain length and solvent structure at strongly interacting solidliquid surfaces. We suspect that these effects arise from both the substrate’s ability to inhibit solvent-solute interactions and a solvent’s ability to screen substrate dipolar forces. Fully hydroxylated silica substrates have
surface OH concentrations of 4.6 OH groups/100 Å2 or ∼21 Å2/OH group.25 The cross-sectional area of an alltrans alkyl chain is 22 Å2, meaning that the n-alcohol solvents can pack at the interface in a 1:1 ratio with surface silanol groups.26 If 2-propanol orients to optimize hydrogen bonding interactions with a hydrophilic surface, the crosssectional area of this solvent molecule is ∼50% greater than that of the 1-propanol isomer. Consequently, the 2-propanol-hydrophilic boundary presents a more polar environment due to the unscreened surface silanol groups, thus enhancing the local dielectric environment experienced by solutes. We believe that the findings presented above are significant in two respects. First, results show that an accurate treatment of interfacial solvation at strongly interacting surfaces should consider the molecular identity of a solvent as well as potential cooperative effects between a solvent and the adjacent phase. Second, the nonadditive effects observed in the second harmonic spectra of 4-ABP adsorbed to different alcohol-hydrophilic, liquid-solid boundaries suggest that these experiments are sensitive to electric field gradients across the interfacial region. The common picture of surface potentials at liquid-solid boundaries has the surface potential smoothly converging to zero as a function of distance from the interface. Our results, however, show that nonaqueous, strongly interacting systems create anisotropic regions having dielectric properties that are not simple convolutions between the polarizing properties of the surface and those of the bulk solvent. Experiments seeking to develop a stronger connection between cooperative solvent-substrate partitioning and interfacial solvent permittivity are currently underway. Acknowledgment. This work was supported in part by the NSF-UMD MRSEC under Grant DMR 9632521. The authors gratefully acknowledge financial support from the University of Maryland Graduate Research Board and equipment donations from Melles-Griot under the “Optics for Research in Higher Education” initiative. LA0104828 (25) Gee, M. L.; Healy, T. W.; White, L. R. J. Colloid Interface Sci. 1990, 140, 450-465. (26) Stanners, C. D.; Du, Q.; Chin, R. P.; Cremer, P.; Somojai, G. A.; Shen, Y. R. Chem. Phys. Lett. 1995, 232, 407-413.
