Contact of Oil with Solid Surfaces in Aqueous Media Probed Using

Dec 28, 2011 - We have studied the interface between hexadecane droplets and sapphire substrates in water using infrared–visible sum frequency gener...
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Contact of Oil with Solid Surfaces in Aqueous Media Probed Using Sum Frequency Generation Spectroscopy Ping Yuan Hsu and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: We have studied the interface between hexadecane droplets and sapphire substrates in water using infrared−visible sum frequency generation spectroscopy (SFG). At high pH and above the isoelectric point of the sapphire substrate, the hexadecane drop is repelled due to electrostatic forces. The SFG measurements are consistent with the observation that a thick layer of water is present between the oil and the sapphire substrate. Below the isoelectric point of the sapphire substrate, the hexadecane drops stick to the sapphire surface. Surprisingly, the SFG results show the presence of a thin layer of water between hexadecane drop and the sapphire substrate. At this contact interface, we observe contributions to the SFG signal from both the hexadecane/water and water/sapphire interfaces. The reasons for the presence of a thin water layer with adhesive contact can be explained due to weaker repulsive double layer and the attractive van der Waals interactions.

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INTRODUCTION An oil drop in contact with a solid surface in aqueous environment is encountered in many practical applications such as oil recovery, coatings, filtration, and cosmetics. Scientifically, this is an interesting system to study hydrophobic, lipid−solid, and membrane−membrane interactions.1 Here, we have studied the contact of hexadecane oil droplets with sapphire substrates in water as a function of pH. Sapphire prisms used in this study are composed of α-alumina crystal, and the surface is hydrated when exposed to atmospheric moisture.2−4 The hydrated α-alumina surfaces are composed of Al3−OH, Al2− OH, and Al−OH (abbreviated as (Al)nOH) functionalities. When “n” changes from 3 to 1, the O−H bond is stronger and deprotonation becomes more difficult. In water, the surface hydroxyl groups of aluminum oxide surfaces have different reaction activity with H+ and OH− depending on the pH. Thus, the surface charge can be controlled by creating positive charged groups (Al2OH2+, Al3OH+, and AlOH2+0.5) or negative charged groups (Al2O−, AlO−1.5, and Al3O−0.5), by changing pH.5 The surface of the hexadecane droplet is also not neutral, and a negative pH-dependent zeta potential has been reported.6,7 Although the origin of the negative charges is not well understood, it has been suggested that hydroxide ions accumulate preferentially at the oil−water interface and the negative ζ-potential increases with increase in pH from 4 to 9. The possibility that both hexadecane and sapphire substrate can be negatively charged could lead to repulsive electrostatic forces that depend on pH and the IEP of the sapphire substrate. Direct force measurements using atomic force microscopy (AFM) between two droplets of tetradecane and tetradecane− mica interface show strong repulsive forces at pH of 9 and strong attractive forces at lower pH (3−5.6).8 The repulsion forces between two oil droplets and oil−solid contact interfaces can be explained by double layer forces in © 2011 American Chemical Society

conjunction with attractive van der Waals interactions, often referred as DLVO forces. At high pH, strong repulsive electrostatic forces are observed, and at low pH, the attractive van der Waals interactions dominate and the oil droplets stick to solid surfaces. Although the problem of oil droplets contacting the solid surface appears to be well understood, no direct measurements of structure of molecules at the interface have been reported. For example, force measurements using AFM at low pH do not have the sensitivity to determine if water layer is present in the adhesive contact. If water is present, the structure of this complex water interface is not understood. To address these questions, we have developed experimental setup to directly probe the contact interface between oil− sapphire interface in aqueous media using infrared−visible sum frequency generation spectroscopy (SFG) in total internal reflection geometry.9 SFG is a second-order nonlinear optical technique. Using the dipole approximation, the SFG generation is forbidden in the bulk of isotropic materials and is only allowed at surfaces and interfaces where the inversion symmetry is broken. The intrinsic interface sensitivity of SFG allows investigations of buried interfaces that are not easily accessible by other techniques.10,11 The SFG signals are enhanced when the scanning IR wavelengths overlap with the molecular stretching vibrations that are infrared and Raman active. The SFG intensity and peak position can be used to determine the orientation and chemical composition of the surface groups. The combination of SFG with total internal reflection geometry permits us to probe the contact interface between the oil−sapphire interface in the presence of aqueous media. Received: August 19, 2011 Revised: December 19, 2011 Published: December 28, 2011 2567

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polarization of the probe (IR and visible) beams and that of the SFGsignal beam (before it reaches the detector) is set in one of two ways: S-polarized, electric field parallel to the surface; or P-polarized, electric field in the incident plane. The combination of polarizations of all three beams (e.g., SSP) is given in the following sequence: polarization of the SFG beam, visible beam, and IR beam. Incident angles of 16° and 10° of the IR beam were used to collect SFG spectra for water/ sapphire interface and the liquid-oil/water/sapphire interface, respectively. The incident angles are with respect to the surface normal drawn outward from the face of the sapphire prism. The incident angle of the visible beam was ≈1.5° lower than the incident angle of the IR beam. For consistency, we have used the same surface of the prism for all the SFG measurements. The size of the oil droplet was ≈1.5 mm in diameter. An oil droplet was brought into contact with sapphire in aqueous solution at selected pH value and held for a half-hour to reach equilibrium before collecting the SFG spectra. The SFG spectra reported in this paper were normalized by the IR intensity to account for the variation in IR power as a function of wavenumber. The SFG signals in the D2O were weaker than the H2O region, and this is not explained by the differences in the IR intensity. We attribute the differences in the SFG signals due to the sensitivity of the SFG instrumentation in these two regions.13 To obtain quantitative information, the spectra were fitted using the Lorentzian equation

EXPERIMENTAL SECTION

Sample Preparation. Hexadecane (C16H34, Tm = 289.5 K, 99.9% purity) was purchased from TCI and was used as received. Ultrapure distilled water (18 MΩ·cm) was obtained after running deionized water through a Millipore filtration system. D2O was purchased from Cambridge Isotopes (D: 99.9%). Sapphire prism were purchased from Miller Optics with the c-axis parallel to the prism face. The three sides of the prism do not have the same crystallographic plane, and no attempt was made in this study to differentiate the results based on the differences in the crystallographic planes. The prisms are cleaned in a sonication bath using hexane, ethanol, and pure water for a total time of 60 min. The sapphire prisms were then mildly etched using a 10−15 mM solution of HNO3 under sonication for 30 min followed by rinsing thoroughly with deionized water (resistivity: 18.3 Ω·cm). The prism was then dried by blowing dry nitrogen gas. The pH solutions from 4 to 10 were prepared using a 1 mM solution of NaNO3 for all the measurements. pH values are adjusted by using HCl/H2O and DCl/D2O with NaOH and NaOD, respectively. The value of 0.45 was used for the correlation between the pH-meter reading in D2O and H2O solutions.12 Design of the Sample Cell. To perform the SFG measurement at the contact interface between fluidlike droplets and sapphire substrate in the total internal reflection geometry, a novel sample cell was built (Figure 1). The sample cell is composed of two parts: liquid cell and a

ISFG ∝ χeff,NR +

∑

iφ A qe q

ω IR − ωq − i Γq

2

(1)

where Aq, Γq, φs, and ωq are the strength, damping constant phase, and angular frequency of a single resonant vibration, respectively. χeff,NR is the nonresonant part of the signal.

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RESULTS AND DISCUSSION The results and discussion is divided into two sections. In the first section, we have determined the IEP for the sapphire prisms using SFG. In the second section, we present the SFG results at hexadecane/sapphire interface at low and high pH values. A. Isoelectric Point (IEP) for the Sapphire Prisms. The literature reported values of IEP of sapphire surfaces range from 3 to 9.5.3,4,14−16 The large ambiguity in the reported values of IEP is perhaps due to the differences in the crystallographic plane, experimental techniques, and the different surface treatments used in these studies. To avoid this ambiguity, we have directly measured IEP of sapphire prisms used in these experiments using SFG. Figure 2 shows SFG spectra for the sapphire/water interface collected using SSP polarization as a function of pH. The peaks from 3100 to 3700 cm−1 are assigned to water and the peaks between 3600−3800 cm−1 to the surface OH bands. The SFG spectra were analyzed using a Lorentzian equation and the phase information provided by Zhang et al.4 The SFG spectra were fitted using the amplitude strengths, peak positions, and nonresonance signal as variables. The peak at ≈3670 cm−1 was assigned to the hydroxy Aln(OH), and the peaks at 3200 and 3400 cm−1 were assigned to water bands. The surface OH peak only appears at low pH values and is deprotonated at high pH. To analyze the IEP, we have plotted the amplitude strengths for the surface OH (Figure 2B) and water peaks (sum of both 3200 and 3400 cm−1 3) (Figure 2C) as a function of pH. The amplitude of the surface OH signal increases near pH of 6. The IEP is defined when the surface has equal number of positive and negative charges or the net surface charge is neutral. The amplitude strength of the surface OH continues to increase at lower pH. We can define the isoelectric point as the pH value

Figure 1. (A) Schematic of sample cell where a needle is used to open an oil droplet in a water reservoir and a translation stage is move the oil droplet in contact with the sapphire prism. The SFG experiments are done in total internal reflection geometry. (B) Illustration of the contact interface of oil/sapphire interface with a layer of water in between the two surfaces. mobile syringe. For the liquid cell shown in Figure 1A, the sapphire prism is fixed on a small recessed well by threaded rods embedded into the machined surface of a stainless steel body. A rubber sheet is placed on the back of the cell. The soft rubber sheet enables a needle to go through the rubber into the well. Thus, the well is sealed with the rubber sheet, and it allows liquid droplets to be injected in a fluid environment. The sample cell can be connected to a chiller or a heater for controlling temperature. The syringe (or a needle) is fixed on a translation stage which is connected with a micrometer (Figure 1A) which allows controlled displacement of the needle to bring a oil droplet in contact with a sapphire prism. The angle of incident laser beam can be controlled by rotating the sample cell with respect to the laser beam. For measuring the contact interface, the oil droplet is moved forward to contact the prism surface. To ensure that the contact area is larger than the area of the laser beam, we have applied a small amount of pressure to deform the oil droplet and increase the contact area. SFG Spectroscopy. SFG spectra were acquired at ambient conditions using a picoseconds laser system (Spectra-Physics) with a tunable IR beam (2000−3800 cm−1, 1 ps pulse width, 1 kHz repetition rate, and a diameter of 100−200 μm) and a visible beam (800 nm, 1 ps pulse width, 1 kHz repetition rate, and a diameter of 1 mm). To collect SFG spectra over a wide range in IR wavelength, we have designed a computer-controlled delay stage that ensures that the IR and visible beams are synchronized in time while scanning the IR wavelength. The 2568

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Figure 2. (A) SFG spectra collected in SSP polarization for sapphire/water interface as a function of pH. The peaks between 3100 and 3500 cm−1 are assigned to water and the peak between 3600 and 3800 cm−1 to sapphire OH peak. (B) The amplitude strength for the 3670 cm−1 peak determined by fitting the spectra in (A) as a function of pH. (C) The amplitude strength of the water bands (both 3200 and 3400 cm−1 peaks) plotted as a function of pH.

where the 50% of the surface OH are deprotonated. Because we do not know the maximum amount of surface OH groups, we can only estimate the 50% decrease in signal lies in range of pH between 5 and 6. The interpretation of the amplitude strength of the water signals is more subtle (Figure 2C). When the sapphire surface is positively or negatively charged, there is a net increase in orientation of the water molecules, and this results in an increase in the SFG signal. From Figure 2c, we can conclude that the SFG signal is lowest when pH ≈ 6, and we can define that the IEP lies in the range from pH 5−6. Above and below this pH, the SFG signal strength increases because of increase in surface charge on the surface. Yeganeh et al.3 have reported IEP of 8 for sapphire prism measured using SFG. The sapphire prisms used in both these studies have different crystallographic orientation for the three faces of the prism, and the IEP differences may be due to the differences in the crystallographic plane of the sapphire prisms. B. Interface between Oil Droplets and the Sapphire Substrate. Figure 3 shows the optical picture of the hexadecane droplets in contact with sapphire surfaces in the presence of water. At high pH values the hexadecane droplets are repelled, and we anticipate the presence of thick layer of water between hexadecane and the sapphire substrate. For low pH, the hexadecane droplets stick to sapphire substrates. The transition from stick to nonstick state takes place at pH between 6 and 7. In Figure 3, we have plotted the results for six different sapphire prisms. The results are in general reproducible except the two prisms showing either high or low transition states. As emphasized before, these differences in the IEP could be due to the fact that the thee faces of the sapphire prism have three different crystallographic orientations. The transition from stick to nonstick state coincides with the IEP for the sapphire prisms measured using SFG. Above the IEP point the surfaces are negatively charged and we observe repulsive electrostatic forces, and below pH of 6, the attractive

Figure 3. (A) Wetting of hexadecane on six different sapphire prisms as a function of pH. The hexadecane droplet is formed using a needle in a chamber filled with water. The dark shaded region corresponds to the pH values where the hexadecane droplet stuck to the sapphire substrate. The lighter region corresponds to values of pH where we observed strong electrostatic repulsion. (B) Optical picture of the hexadecane droplet at high pH. (C) Optical picture of the hexadecane droplet at low pH where we observed adhesion.

van der Waals forces dominate and the droplets stick to sapphire substrates. For the SFG experiments reported in this paper we have used the sapphire prism labeled (i) in Figure 3. Before we discuss the SFG results of the contact interface, we need to understand the contribution to the SFG signal from the two interfaces (hexadecane/water and water/sapphire) that are involved in these measurements (Figure 1B). We have determined the conditions for total internal reflection by measuring the intensity of He−Ne laser beam as a function of incident angle at two different pH of 9 and 5 (Figure 4A). The critical angle at which we achieve total internal reflection is very different for these two pH values. The intensity data can be modeled using linear Fresnel factors for a 60° prism of refractive index of 1.76 and either a hexadecane layer of refractive index of 1.43 (solid line) or water layer of refractive index of 1.33 (dashed line) next to sapphire surface. For 2569

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the condition used in the SFG experiments. Figure 4B shows the predictions for sapphire−water (Aq = 1) and hexadecane− water (Aq = 1) interfaces without the interference effects (solid lines). The long dashed line shows the SFG intensity for Aq = 1 for sapphire−water and Aq = −1 for water−hexadecane interface. The short dashed line shows the SFG intensity for Aq = 1 for sapphire−water and Aq = 1 for water−hexadecane interface. All these cases are important for understanding the experimental results presented below. For example, the calculations shows that for thicker films we will observe only the signals from sapphire−water interface. When films are thinner than 500 nm, if there is no interference, for example in two different spectral regions, we will observe a steady increase in the SFG signals from the hexadecane−water interface. For water molecules, the SFG signals could be lower or higher as we decrease the thickness depending on the orientation of the water molecules at the sapphire−water and water−hexadecane interfaces. The model presented here assumes that the refractive index of water and hexadecane is a real quantity, and absorption in the IR wavelength is neglected. Calculations using complex refractive index (n + ik) indicate that the intensity of the SFG beam will also be a function of k. The relative differences between the hydrocarbon and water signals will not be affected by this assumption unless there is a significant differences in the k values for water and hydrocarbon molecules. With the understanding of linear reflectivity and model SFG analysis, we discuss the SFG experimental results next. Figure 5

Figure 4. (A) Intensity of He−Ne laser light reflected from the sapphire prism as a function of incident angles. The intensity reflected is normalized with the intensity of the incident beam (I0). The incident angles are measured with respect to the face of the sapphire prism. The data shown in circles are for pH ≈ 5 and squares are for pH ≈ 9. The linear Fresnel equations were used to model the data using a two-layer interface model. The solid lines fitting the data in circles are for hexadecane/sapphire interface and water/sapphire interfaces. The dashed line corresponds to a three-layer model consisting of hexadecane/water/sapphire geometry. (B) Predictions of the SFG signal in SSP polarization as a function of the thickness of the water layer between the sapphire prism and the oil droplet. The dashed lines correspond to the combined contributions to the SFG signal from both the interfaces (hexadecane/water and water/sapphire) when the amplitude strength from both these interfaces are in-phase (short dashes) or out-of-phase (long dashes). The red (hexadecane/water) and blue solid (water/sapphire) lines are the results of the calculations for SSP signals from these two interfaces separately.

hexadecane in contact with sapphire substrate a better fit is obtained if we use a three-layer model with a 50 nm thick layer of water. However, these measurements are not sensitive enough to draw conclusions about the thickness of the water layer, and the main inferences we can draw are that at high pH the hexadecane droplets are repelled with a thick layer of water between the two surfaces. At low pH, if water is present between the two surfaces, we anticipate the thickness to be less than 50 nm. Because there are two interfaces, sapphire−water and water− hexadecane, that can contribute to the SFG signal, we have determined the SFG signal in SSP polarization generated from both these interfaces using a mathematical model developed recently.17 This SFG model takes into account the interference effects from the SFG signals from both these interfaces as a function of film thickness. The interference effect between the water and hydrocarbon bands is not important because the resonant enhanced SFG signals are at two different wavelengths. However, for water molecules next to sapphire and oil interface, we anticipate interference of the SFG signals from these two interfaces as a function of film thickness. We have focused our analysis for incident angle of 10° because this was

Figure 5. SFG spectra for hexadecane/water/sapphire interface measured using SSP polarization for pH of 9.6 (squares) and pH of 5.6 (circles) as a function of IR wavenumbers. The left axis is for low pH and the right axis for high pH values.

shows the SFG results measured using SSP polarization for hexadecane/water/sapphire system at a pH of 9.6 (squares). At this pH, from linear reflectivity measurements we anticipate the hexadecane droplet is separated from the sapphire substrate with a thick layer of water. We have presented the spectral range from 2800 to 3800 cm−1 which covers the hydrocarbon and water region. These results are very similar to the SFG results for sapphire/water interface (no oil droplet). This is consistent with the linear reflectivity measurements which shows that the data could be modeled using only a two-layer model where sapphire is in contact with bulk water. The negative surface charges on the hexadecane and sapphire surfaces results in long-range repulsive forces that prevents the surfaces from approaching each other. The fitting of the SFG spectra was discussed in the previous section, and we have not made any attempts to interpret orientation and structure of 2570

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water molecules because the spectra are broad and due to considerable uncertainty in the assignments of the water peaks. In general, the peak at 3200 cm−1 is assigned to ice-like water and at 3400 cm−1 to liquid-like water. The results at lower pH are more intriguing. Figure 5 (circles) shows the results for hexadecane/water/sapphire interface at a pH of 5.6. At this low pH, the sapphire surface is below the IEP, and we have observed that the hexadecane droplets stick to the sapphire substrates. Interestingly, we observe now the hydrocarbon signals from hexadecane/water interface. In addition, we observed spectral peaks above 3000 cm−1 that are associated with water and surface OH groups. The interpretation and assignment of hydrocarbon peaks are much more easier if H2O is replaced with D2O because the D2O peaks are between 2300−2800 cm−1. Figure 6A shows the

Figure 7. SFG spectrum in SSP polarization for D2O/sapphire interface collected in the O−D region (left) and the O−H region (right). The solid line is a guide to the eye.

substrate. This indicates that the sapphire hydroxyl is in direct contact with D2O because it is shifted to 3620 cm−1. This is also directly confirmed by measuring the SFG spectrum for D2O in direct contact with the sapphire substrate (Figure 7). The peak at 3620 cm−1 is due to surface OH groups in contact with D2O. It is also possible that the surface OH peak could be a result of inaccessible surface OH peaks that do not come in contact with D2O. There are three published studies that discusses the surface OH peak on sapphire substrates. Braunschweig et al.14 concluded that the surface OH groups were inaccessible to the solvent. Zhang et al.,4 on the other hand, concluded that the surface OH groups are accessible, and they substitute very rapidly with D2O. In this study and our previous work,21 we have not observed exchange of the surface OH groups with D2O without the presence of a strong base such as NaOD. The reasons we conclude that the surface OH groups are accessible is because we have observed the intensity of the surface OH peak reduces with increase in pH 9 (Figure 2). Second, we have observed the surface hydroxyl peaks shift in contact with liquids and polymers based on the strength of acid−base interactions.22 In addition, the D2O peak is shifted in comparison to non-hydrogen-bonded hydroxyl groups at 3720 cm−1. Finally, we have observed that the surface OH exchanges to OD if we use a strong base such as NaOD. On the basis of these results for hexadecane/D2O/sapphire interface, we can now interpret the results for hexadecane/H2O/sapphire interface. The two peaks observed between 3600 and 3800 cm−1 are due to sapphire OH in contact with H2O and non-hydrogen bonded water O−H groups in contact with hexadecane molecules at hexadecane/water interface. The presence of water signals in Figures 5 and 6 shows that at pH of 5.6 and for conditions where the hexadecane droplet is sticking to the sapphire surface there is a thin layer of water trapped between the two interfaces. The quantitative determination of contribution of the SFG signals from both these interfaces is difficult because we are unable to determine the thickness of the water layer directly from these measurements. The results from the SFG interference model (Figure 4B) indicate that the SFG signals from water can increase or decrease in intensity as the thickness of the water layer reduces. The changes in the SFG intensity are related to the orientation of water molecules at both water/sapphire and water/ hexadecane interfaces. Because of this complexity, any quantitative conclusions of the relative signals of water with respect to the hydrocarbon peaks is limited. However, we

Figure 6. SFG spectrum in SSP polarization for hexadecane/sapphire interface in D2O at pH of 5.6. The solid line corresponds to a fit using the Lorentzian equation.

results for hexadecane/D2O/sapphire interface. The peaks in Figure 6A at 2850, 2920−2925, and 2965−2970 cm−1 are assigned to methylene symmetric, methylene asymmetric, and methyl asymmetric vibrations, respectively. The observations of methylene peaks and low methyl symmetric peaks are similar to the spectral features observed for hexadecane in contact with sapphire or glass substrate.18,19 Figure 6A shows the spectral features in the D2O region. The peaks at 2410 and 2510 cm−1 are assigned to ice-like and liquidlike water peaks using the convention proposed by Shen and co-workers.20 The O−D signals could originate from either D2O/oil or sapphire/D2O interfaces. A much sharper peak at 2690 cm−1 is assigned to dangling O−D vibration. Because we have not observed this peak for sapphire/D2O interface (Figure 7), we conclude that this peak originates from the O−D dangling signal from the D2O/hexadecane interface. The peak in Figure 6B at 3620 cm−1 is assigned to sapphire hydroxyl groups. The sapphire hydroxyl peak in contact with hydrophobic liquids is expected to be near 3690 cm−1 as measured for hexadecane in direct contact with sapphire 2571

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Figure 8. A model illustrating the repulsive and attractive interactions above and below the IEP of sapphire. At low pH, a thin layer of water is trapped between the two surfaces. We have observed OH surface peak in contact with water and also non-hydrogen-bonded OH peaks of water next to the hexadecane/water interface.

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definitely are observing contributions from both these interfaces as indicated by the O−D (or O−H) bands next to hexadecane and sapphire OH bands next to D2O and H2O. The observations of water trapped between the hexadecane/ sapphire contact at low pH is unambiguous. The time scale of the SFG experiments is hours, and the trapping of water cannot be explained based on the time required to drain the water from these narrow confined gaps, also referred to hydrodynamic effects.8,23 To explain these results, we have to consider the weakening of the repulsive double-layer forces near the IEP or attractive Coulombic forces at low pH. After further compression of the droplet, only results in flattening of the contact area. The additional repulsion with a layer of water in between the two surfaces could be due to repulsive hydration forces that have been discussed for mica−mica contact in salt solution.1

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ACKNOWLEDGMENTS We are grateful for the financial support from NSF (DMR0512156).

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REFERENCES

(1) Israelachvili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: San Diego, 1991. (2) Eng, P. J.; Trainor, T. P.; Brown, G. E.; Waychunas, G. A.; Newville, M.; Sutton, S. R.; Rivers, M. L. Science 2000, 288, 1029− 1033. (3) Yeganeh, M. S.; Dougal, S. M.; Pink, H. S. Phys. Rev. Lett. 1999, 83, 1179−1182. (4) Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. J. Am. Chem. Soc. 2008, 130, 7686−7694. (5) Fitts, J. P.; Shang, X.; Flynn, G. W.; Heinz, T. F.; Eisenthal, K. B. J. Phys. Chem. B 2005, 109, 7981−7986. (6) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045−2051. (7) Beattie, J. K.; Djerdjev, A. M. Angew. Chem., Int. Ed. 2004, 43, 3568−3571. (8) Clasohm, L. Y.; Vakarelski, I. U.; Dagastine, R. R.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Langmuir 2007, 23, 9335−9340. (9) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854−3857. (10) Shen, Y. R. Proc. Natl. Acad. Sci. U. S. A. 1996, 93, 12104− 12111. (11) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons: New York, 1984. (12) Krezel, A.; Bal, W. J. Inorg. Biochem. 2004, 98, 161−166. (13) Sovago, M.; Campen, R. K.; Wurpel, G. W. H.; Müller, M.; Bakker, H. J.; Bonn, M. Phys. Rev. Lett. 2008, 100, 173901. (14) Braunschweig, B.; Eissner, S.; Daum, W. J. Phys. Chem. C 2008, 112, 1751−1754. (15) Kershner, R. J.; Bullard, J. W.; Cima, M. J. Langmuir 2004, 20, 4101−4108. (16) Franks, G. V.; Gan, Y. J. Am. Ceram. Soc. 2007, 90, 3373−3388.

SUMMARY

In summary, we have developed a novel experimental geometry to study contact interfaces between an oil droplet and solid substrate using surface sensitive SFG spectroscopy. We have illustrated the useful of this setup to probe the contact interface between hexadecane liquid and a solid sapphire substrate. In the case of hexadecane/sapphire contact in aqueous solution, we observe a transition from an adhesive contact to repulsive contact above the isoelectric point of the sapphire substrate. This results can be explained by the presence of negative charges on hexadecane and the sapphire substrate above the IEP of sapphire. We have observed the presence of a thin layer of uniform water layer between hexadecane and sapphire substrate below the IEP. The presence of water layer is intriguing and can be explained by the reduction in the repulsive double-layer forces and attractive van der Waals forces. A simple model illustrating the results are shown in Figure 8 The observations of a thin layer of water between oil and hydrophilic substrates have important consequences in adhesion, wetting, and transport of oil droplets next to solid surfaces. 2572

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(17) Li, G.; Dhinojwala, A.; Yeganeh, M. S. J. Phys. Chem. C 2011, 115, 7554−7561. (18) Nanjundiah, K.; Dhinojwala, A. Phys. Rev. Lett. 2005, 95, 154301. (19) Sefler, G. A.; Du, Q.; Miranda, P. B.; Shen, Y. R. Chem. Phys. Lett. 1995, 235, 347−354. (20) Ji, N.; Ostroverkhov, V.; Tian, C. S.; Shen, Y. R. Phys. Rev. Lett. 2008, 100, 096102. (21) Nanjundiah, K.; Hsu, P. Y.; Dhinojwala, A. J. Chem. Phys. 2009, 130, 024702. (22) Kurian, A.; Prasad, S.; Dhinojwala, A. Langmuir 2010, 26, 17804−17807. (23) Dagastine, R. R.; Manica, R.; Carnie, S. L.; Chan, D. Y. C.; Stevens, G. W.; Grieser, F. Science 2006, 313, 210−213.

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