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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Heterodyne-Detected Vibrational Sum Frequency Generation Study of Air-Water-Fluoroalcohol Interface: Fluorocarbon Group Induced Structural and Orientational Change of Interfacial Water Subhadip Roy, Biswajit Biswas, Jahur A. Mondal, and Prashant Chandra Singh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07949 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

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The Journal of Physical Chemistry

Heterodyne-Detected Vibrational Sum Frequency Generation Study of Air-WaterFluoroalcohol Interface: Fluorocarbon Group Induced Structural and Orientational Change of Interfacial Water Subhadip Roy+, Biswajit Biswas++, Jahur A. Mondal+*, Prashant Chandra Singh++* +Radiation

& Photochemistry Division, Bhabha Atomic Research Centre, Homi Bhabha National Institute, Trombay, Mumbai 400085, India ++School

of Chemical Sciences, Indian Association for the Cultivation of Science, Kolkata 700032, India *[email protected], [email protected].

Abstract: Presence of fluorocarbon group in alcohols provides them unique and important physicochemical properties. Fluoroalcohol containing aqueous interfaces are used in diverse fields, such as liquid-liquid phase separation, chemical reactions, and polymer industry, etc. However, the molecular level understanding of interfacial water in presence of fluorinated alcohol is sparse. In this paper, we have applied phase-sensitive and surface-specific vibrational spectroscopic technique, heterodyne-detected vibrational sum frequency generation (HD-VSFG), to understand the structure and orientation of interfacial water in presence of 1,1,1,3,3,3hexafluoroisopropanol (HFIP). To realize the role of fluorination, the results of the air-waterHFIP interface have been compared with the corresponding hydrogenated alcohol, air-waterisopropanol (IP) interface. It is observed that the net orientation (H-down) and hydrogen bonding of water at the air-water-IP interface are similar to the neat air-water interface. In contrast, water changes its net orientation (H-up) and becomes more inhomogeneous at the air-water-HFIP interface. On increasing the bulk pH (11), the interfacial water becomes increasingly ordered (Hup oriented) at the air-water-HFIP interface, whereas there is no significant change at the airwater-IP interface. At acidic pH (2.8), the interfacial water takes the same H-down orientation for both IP and HFIP, though the average H-bonding is stronger for HFIP. Thus, fluorination of isopropanol changes the preferred orientation and structure of interfacial water, which may be useful in understanding chemical and biochemical processes that occur at fluoroalcohol containing aqueous interfaces. 1 ACS Paragon Plus Environment

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Introduction Aliphatic alcohols and their fluorinated derivatives are amphiphilic molecules which have application in various fields, ranging from organic chemistry, polymer industry, biochemistry, and phase-separation.1-4 A fluorocarbon group is not only hydrophobic (like an aliphatic group), but also exhibits different properties from that of aliphatic groups.5-6 Due to high electronegativity of fluorine, the polarizability of a fluorocarbon is less than the corresponding hydrocarbon. The dipole moment of C−F bond is more than that of C−H and directed opposite to that of the latter, so that C−F bonds generate strong electrostatic drag while interacting with neighboring water molecules. As a result, the hydrophobicity of fluorocarbon is different from that of hydrocarbon and described as polar hydrophobicity.7 Moreover, the C−F bond is longer than C−H, increasing the molar volume of fluorinated molecules, which may lead to different hydration characteristics of fluorocarbon group. These unique features of C−F bond presumably make fluorinated alcohols useful in pharmaceutical and medical applications, as well as in water repellent surfaces.1-3,

5-6

Amphiphilic alcohols get adsorbed at aqueous interfaces and change

their structure, hydrogen-bonding, and other properties, which make these interfaces unique sites in different chemical, biological, and atmospheric processes.8-10 Despite its immense importance in several areas, the effect of fluoroalcohol on interfacial water is largely unknown. For the macroscopic aqueous interface (e.g. amphiphile monolayer-water interface), even-order nonlinear technique, such as second harmonic or vibrational sum frequency generation (SHG or VSFG) provides interface-specific response.11-15 Moreover, development of heterodyne-detected (HD-) VSFG (single channel as well as broadband detection) enabled direct recording of imaginary-(2) spectrum (Im (2); where (2) is the second order electric susceptibility).16-21 The sign of Im (2) spectrum is associated with the net orientation of interfacial molecules. Several groups22-26,

27-30

have used HD-VSFG technique to understand the hydrogen-bonding and

orientation of water at different interfaces. These studies have provided deeper insight into the structure and orientation of water at the air-water and surfactant (or lipid)-water interfaces. Recently, HD-VSFG study of alcohol-water interfaces31 showed that interfacial water changes its preferred orientation depending upon the length of the alkyl chain (hydrophobicity) of alcohol. Since fluorination changes the hydrophobicity/lipophobicity of alcohol, it is interesting to examine the effect of fluoroalcohol on interfacial water. 2 ACS Paragon Plus Environment

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In the present study, we investigated the effect of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and isopropanol (IP) on the structure and orientation of interfacial water at different bulk pHs, using HD-VSFG spectroscopy. We chose HFIP, as it is extensively used in several applications, including fibrillation and stabilization of protein, separation science, and polymer industry.32-33 Comparison of the Im (2) spectra (air-water-HFIP vs. air-water-IP interface) enables us to understand the role of fluorination in the structural and orientational transformations of interfacial water. Experimental Methods and Materials Details of the HD-VSFG spectrometer have been discussed in our previous publications.34-36 Briefly, a home-built HD-VSFG spectrometer, based on a Ti:Sapphire regenerative amplifier laser system (Trident M (V2), Amplitude Technology; average power 3.0W @ 800 nm, repetition rate 1 kHz, pulse width ~50 fs, frequency conversion units: TOPAS-C, NDFG, Light Conversion) has been used to measure the surface signal of the alcohol-water mixtures. The sample sum frequency, SF1 was generated by spatially and temporally overlapping the spectrally narrow visible pulse (ω1; centre wavelength 800 nm, fwhm ~ 16 cm-1, energy ~15 µJ/pulse at the sample) with a broadband IR pulse (ω2, fwhm ~ 300 cm-1 @ 3300 nm, 5 µJ/pulse) at the sample surface (SF1= ω1+ ω2). For the heterodyne detection, the reflected ω1 and ω2 beams were further refocused on GaAs(110) using a spherical concave mirror (CM; f = 100 mm) to generate another sum frequency (SF2), called local oscillator (LO). In order to create interference pattern (by dispersion of SF1 and SF2 in polychromator), the SF1 was delayed with respect to the SF2 by selectively passing SF1 through a 1.0 mm thick anti-reflection coated glass plate, located in between the sample and the concave mirror. Finally, the SF1 and SF2 pulses were collimated and focused into the slit of a polychromator (1200 grooves/mm) and detected by a thermo-electric and circulating water cooled (-90°C) charge coupled device (CCD; Model no DU920N-BR-DD, Andor Technology). Similar interference fringes were obtained by replacing the sample with reference quartz (z-cut). The imaginary- and real-(2) (Im(2) and Re(2)) spectra of the sample were obtained by normalizing the sample signal with the reference quartz signal following the procedure of Tahara and coworkers.16, 37 The SFG, ω1 and ω2 beams are s-, s-, p- polarized (SSP polarization), respectively. As the SF2 signal was generated by the reflected ω1 and ω2 from the sample (or quartz) surface, a reflectivity correction factor was introduced along with the 3 ACS Paragon Plus Environment


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normalization. The reflectivity correction factor was determined by separately measuring the LO signal with quartz and sample, respectively, while the SF1 was physically blocked before the GaAs. The HPLC grade IP (99.8%) and HFIP (99.5%) were purchased from Spectrochem, India. NaCl (99.5 %) was from Sigma Aldrich. A surface tension meter (Kibron, Finland) with Wilhelmy rod of diameter ~0.5mm was used to measure the surface tension of neat water and alcohol-water mixtures. The surface tension of neat water (72.3 mN/m at 23°C) was used for the calibration of the surface tension meter. The surface tension values of alcohol-water mixtures were maintained at 52 ± 4 mN/m by adding required amount of alcohol in water. Approximate bulk concentration of HFIP and IP are 150 mM and 650 mM, respectively. It is important to note that both IP and HFIP are quite soluble in water, and hence do not form well defined monolayer on water. However, because of the amphiphilic nature, they are preferentially adsorbed at water surface leading to the substantial decrease in surface tension. Because of higher surface activity (of HFIP) the required bulk concentration of HFIP is lower than that of IP. Milli Q water (18.2MΩ cm resistivity) was used for all the measurements. pH of the experimental solution was varied within 2.8 – 11, by adding either HCl or NaOH. Depending upon the pH, ionic strength of the solution varies in the range of 10-6 to 10-3 M. Variation of ionic strength at such low concentration range is not expected to significantly affect the electric double layer at the interface. All measurements were performed at room temperature (23.8 ± 0.3°C). Results and Discussions Figure 1 shows the Im(2) spectra of the air-water-HFIP, air-water-IP, and neat air-water interfaces (bulk pH 5.8). The spectrum of the neat air-water interface (black curve) shows a broad negative band with maximum at around 3450 cm-1 and a sharp positive band around 3700 cm-1 corresponding to the net H-down orientated interfacial water and dangling OH at the water surface, respectively. These observations are in good agreement with previous studies.38-39 In presence of HFIP (i.e. air-water-HFIP interface), the Im(2) spectrum in the CH stretch region (2800 - 3000 cm-1) shows a double-peaked negative band, assignable to the CH stretch of HFIP. Density functional calculation and IR study40 suggested the existence of different conformations (gauche and trans of CH-OH) of HFIP that exhibit double-peaked C-H stretch. Raman spectroscopy of deuterated isopropanol (CD3CHOHCD3)41 also shows two CH stretch bands, at 4 ACS Paragon Plus Environment

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2887 cm-1 and 2940 cm-1, corresponding to the gauche- and trans- conformers with dihedral angles (between CH and OH groups) ~60 and ~180, respectively. Thus, the appearance of a double-peaked C-H stretch at the air-water-HFIP interface is not unlikely. Moreover, because of higher surface activity of HFIP (than IP) and possible difference in orientational distribution, HFIP may exhibit quite a strong CH stretch signal, though IP has more number of C-H bonds per molecule. It is well known that a fluoroalkyl group is more hydrophobic than its hydrogenated counterpart and hence it is reasonable that the fluorinated isopropyl group of HFIP will be exposed to towards the air (i.e. away from the aqueous phase). The negative sign of the CH stretch band indeed shows that the fluorinated isopropyl group is pointed away from the aqueous phase (hyperpolarizability of CH stretch is opposite to that of the OH stretch). Unlike the neat air-water interface, the Im(2) spectrum in the OH stretch region becomes positive with apparent maximum at 3150 cm-1 and a dip-feature at around 3400 cm-1. Thus, the presence of HFIP at the air-water interface increases the preferred H-up orientation of interfacial water, which could be a manifestation of weak negative electric field at air-water-HFIP interface.

Fig 1. Im(2) spectra of the air-water-HFIP (blue) and air-water-IP (red) interfaces (bulk pH ~ 5.8). The spectrum of the neat air-water interface (black) is shown for reference. The effect of salt (0.8 M NaCl) on the air-water-HFIP interface is shown in the green line. Spectra are measured in SSP polarization combination. Surface tension of the alcohol-water mixtures were 52 ± 4 mN/m. Previous HD-VSFG study of the air-water-n-propanol (n-propanol has same no of carbon as that of HFIP) interface showed a net H-down orientation of interfacial water, similar to that of the 5 ACS Paragon Plus Environment


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neat air-water interafce.31 However, long chain alcohol (e.g. 1-octanol) changes the preferred orientation to H-up and increases the average H-bond strength.31 Thus, in presence of HFIP, the H-up orientation of interfacial water could be either due to fluorination of the alkyl group and/or due to the configurational change of the hydrophobic alkyl group (isopropyl vs n-propyl group). Examination of the Im(2) spectrum at the air-water-IP interface (red curve in Fig 1) shows a negative sign similar to the neat air-water interface. Thus, it is evident that the configurational change of the aklyl group (from ‘n-propanol’ to ‘isopropanol’) does not affect the structure and net orientation of interfacial water. In other words, the orientational change of water at the airwater-HFIP interface is due to the fluorination of isopropyl group. Similar amplitude of Im(2) signal (OH stretch) at the air-water and air-water-IP interfaces is an indication of negligible contribution of alcoholic OH in the Im(2) spectrum. Fluorination increases the ‘group electronegativity’ of an alkyl group, which affects the acidity of the alcoholic OH. In fact, higher group electronegativity of hexafluoroisopropyl group than that of the isopropyl group makes HFIP a stronger acid (pKa(HFIP) ~9.3) than IP (pKa(IP) ~17.1).42 Nevertheless, the pKa(HFIP) is still quite high, and is not expected to be deprotonated at pH 5.8. However, the experimental result (positive OH stretch signal, blue curve in Fig 1) suggests that the adsorbed HFIP at the interface is partially deprotonated, which could be due to higher ‘interfacial pH’ than that of the bulk pH (5.8).43 To examine this issue in more detail, we measured the air-water-HFIP interfaces at different bulk pHs (pH = 7.3, 11, and 2.8). Neutral pH (7.3): Figure 2A shows the Im(2) spectra of the air-water-HFIP, air-water-IP and the neat air-water interfaces at a slightly higher pH (7.3) than that in Fig 1 (pH = 5.8), but lower than the bulk pKa of HFIP (9.3). In the case of air-water and air-water-IP interfaces, the Im(2) spectra are similar to each other in the OH stretch region. There is almost no change on increasing the pH from 5.8 to 7.3 (Figure 1 and 2A). In the 2800 – 3000 cm-1 region, both IP and HFIP show negative CH stretch bands as observed at pH = 5.8. However, the positive Im(2) signal (OH stretch) at the air-water-HFIP interface is increased from that at pH 5.8 (compare blue curves in Fig 1 and 2A). Increased OH stretch (positive) signal with increasing pH is a reflection of further deprotonation of interfacial HFIP molecules.

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Basic pH (11): On increasing the pH to 11 (higher than the bulk pKa of HFIP), the positive Im(2) signal again increases to some extent for the air-water-HFIP interface (Fig 2B). In fact, the Im(2) spectrum (OH stretch) looks like a typical negatively charged surfactant-water interface.16 This is a clear evidence of deprotonation of HFIP at the interface. Surprisingly, the Im(2) spectra of the air-water and air-water-IP interfaces do not show any significant change even at this high pH. Acidic pH (2.8): At neutral and basic pHs, the Im(2) spectra at air-water-HFIP interface suggest the deprotonation of HFIP, which in turn makes the surface negatively charged and orient the interfacial water as H-up. Accordingly, at acidic pH (e.g. pH ~ 3) where both the alcohols exist in the neural form, we expect similar Im(2) spectra (OH stretch) at the air-waterHFIP and air-water-IP interfaces. Indeed, the sign and amplitude of the Im(2) spectra (OH stretch) are similar for both IP and HFIP at pH = 2.8 (Figure 2C). The similar amplitude of the Im(2) signal (OH stretch) at the air-water-HFIP (or air-water-IP) and the neat air-water interface is an indication that there is no preferential adsorption of hydronium ion (bulk pH = 2.8) at these interfaces. The OH stretch band position for the air-water-HFIP interface is red-shifted (maximum ~ 3200 cm-1) from that at the air-water-IP interface (~ 3450 cm-1), which reveals that the interfacial water associated with the alcoholic-OH (HFIP) is strongly H-bonded. This is presumably due to higher group electronegativity of hexafluoroisopropyl group which makes HFIP a stronger hydrogen bond donor than IP. It is important to note that for precise determination of the extent of red-shift and band shape of the OH stretch, it is necessary to measure the interface with isotopically diluted water that minimizes the OH stretch spectral deformation by vibrational decoupling. In the high frequency region, the positive band around 3700 cm-1 clearly shows the presence of dangling OH with increased prominence of the shoulder band at around 3590 cm-1, which is probably due to water molecules interacting with the fluoroalkyl group of HFIP.44

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Fig 2. Im(2) spectra of the air-water-HFIP (blue), air-water-IP (red), and neat air-water (black) interfaces at a bulk pH 7.3 (A), 11 (B), and 2.8 (C). The green curve in each panel is the Im(2) spectrum of the air-water-HFIP interface in presence of 0.8M NaCl. Spectra are measured in SSP polarization combination. Surface tension of the alcohol-water mixtures were 52 ± 4 mN/m. Effect of electrolyte at the air-water-HFIP interface: At the air-water-HFIP interface, pH-dependence of the Im(2) spectra suggests deprotonation of interfacial HFIP even at the neutral ‘bulk pH’. We further cross-examined the interfacial deprotonation by monitoring the water signal (positive OH stretch) in presence of electrolyte (0.8M NaCl) in the aqueous phase. The green curves in Figure 1 and 2 show the Im(2) spectra of the air-water-HFIP interface in presence of 0.8M NaCl at their respective bulk pHs. At pH 5.8 (Fig 1), the strong positive band 8 ACS Paragon Plus Environment

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at 3150 cm-1 almost disappeared in presence of 0.8M NaCl, but the signal above 3500 cm-1 is only weakly affected. Similarly, at pH 7.3 and 11 (Fig 2A and 2B), the positive signal below 3500 cm-1 decreases significantly in presence of 0.8M NaCl, and there is no appreciable change above 3500 cm-1. The decrease of positive Im(2) signal (OH stretch) is due to screening of the surface negative charge by the cation (Na+) of the added electrolyte. In other words, the airwater-HFIP interface is under the influence of a negative charge at neutral and alkaline bulk pHs. As the pH increases from 5.8 to 11, the magnitude of the positive OH stretch signal gradually becomes higher, even in the presence of the same concentration of NaCl (0.8M), (compare green curves in Fig 1, 2A, and 2B). This gradual increase of the signal is due to incomplete screening of the surface negative charge at higher pHs. Thus, it is quite evident that HFIP is deprotonated at the interface, even when the ‘bulk pH’ is lower than its ‘bulk pKa’. This can happen provided the pH at the air-water-HFIP interface is higher than the ‘bulk pH’, such that the onset of deprotonation of adsorbed HFIP starts at a bulk pH of ~5.8 (Fig 1). It is important to note that a quantitative estimation of the pH at the air-water-HFIP interface (or ‘interfacial pKa of HFIP) is beyond the scope of the present study. This is because pKa is related with the degree of dissociation of an acid, and hence it is necessary to measure the concentration of the acid and its conjugate base at the interface.43,

45

Here, we measured the oriented water signal which is

certainly related with protonation/deprotonation of HFIP, but not linearly proportional with the latter. At acidic pH (2.8), though there is no indication of preferential adsorption of hydronium ion (similar amplitude of the Im(2) signal (OH stretch) at the air-water-HFIP and neat air-water interfaces), presence of 0.8M NaCl decreases the negative OH stretch signal (compare blue and green curves in Fig 2C). This could be due to counter effect of adsorption of Cl- at the air-waterHFIP interface. The blue region (> 3600 cm-1) of the Im(2) spectra, which corresponds to weakly interacting topmost water molecules, are only weakly affected by the added NaCl (the signal in the dangling OH region is noisy because of the low IR power). Air-HOD-HFIP interface: Unlike the air-water-IP interface, the Im(2) spectra (OH stretch) at the air-water-HFIP interface change sign (become positive) at neutral and alkaline pHs (Fig 1, 2A, 2B). The positive Im(2) spectra show an apparent maximum at around 3200 cm-1. The OH stretch spectrum of water is heavily deformed around 3200 cm-1 due to Fermi resonance and intra- and intermolecular vibrational coupling. As a result, the apparent maximum at 3200 cm-1 9 ACS Paragon Plus Environment


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cannot be unambiguously assigned to the strengthening of H-bond interaction of water at the airwater-HFIP interface. Therefore, we measured the interface with isotopically diluted water (HOD; H2O/D2O = 1/3 (v/v)) which has reduced coupling effects. Figure 3a shows the Im(2) spectra of the neat air-HOD (gray curve) and air-HOD-HFIP (orange curve) interfaces. As expected, the air-HOD interface shows a negative band around 3400 cm-1 and a positive band around 3700 cm-1, corresponding to the H-down oriented interfacial HOD and dangling OH, respectively. At around 2700 cm-1, there is a positive band followed by a negative tail towards the extreme red region (

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