Aqueous Interface

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy Shu-hei Urashima, Anton Myalitsin, Satoshi Nihonyanagi, and Tahei Tahara J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01650 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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The Journal of Physical Chemistry Letters

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy Shu-hei Urashima, †1 Anton Myalitsin,‡1 Satoshi Nihonyanagi,1,2 Tahei Tahara1,2* 1

Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

2

Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), 2-1

Hirosawa, Wako, Saitama 351-0198, Japan.

Present addresses †

Research Institute for Science & Technology, Tokyo University of Science, 2641 Yamazaki,

Noda, Chiba 278-8510, Japan. ‡

Function Analysis Department, NISSAN ARC Ltd., 1 Natsushima, Yokosuka, Kanagawa

237-0061, Japan. Corresponding Author Corresponding Author: [email protected]

ACS Paragon Plus Environment

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Abstract

Despite recent significant advance of interface-selective nonlinear spectroscopy, the topmost water structure at the charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with the second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ(2) spectrum. The obtained Imχ(2) spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ(2) spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 M to 2 M.


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Silica is one of the most fundamental and abundant minerals on the earth. It is known that the silica surface at aqueous interfaces is negatively charged under usual environmental conditions (pH > 2) 1-2 because of deprotonation of the surface silanol (Si-OH → Si-O- + H+). Therefore, the charged silica surface is of fundamental and industrial importance, and a number of studies have been conducted to elucidate the surface properties such as pKa of the surface deprotonation reaction, electric double layer (EDL) structure, and water structure in the EDL.1-17 However, our molecular-level understanding on the structure of the silica/aqueous interface is still insufficient, mainly because of experimental limitations. One of the reasons of the poor understanding of interfacial structure is the difficulty in probing interfacial molecules selectively. Even-order nonlinear spectroscopy, such as vibrational sum frequency generation (VSFG),12, 18 second harmonic generation (SHG),19 and fourth-order coherent Raman,20-21 can selectively monitor interfacial molecules, and they have been intensively applied to various interfaces. In particular, the heterodyne-detected (HD-) VSFG spectroscopy enables us to experimentally determine the complex second-order susceptibility () , whose imaginary part (Im () ) directly exhibits an absorptive line shape at the resonance as IR and Raman spectra in the bulk do.22-24 The HD-VSFG technique was initially applicable only to the gas/liquid and gas/solid interfaces, but it has successfully been applied to the solid/liquid interfaces very recently.25-27 In our previous study, we measured Im () spectra of silica/aqueous interfaces by changing pH of the solution phase, and revealed the orientation and structure of interfacial water at different pHs.25 Nevertheless, when the interface is charged at pH > 2, the () spectra include not only the contribution from the water molecules in the close vicinity of the interface but also that in a substantially far region because the charge at the interface creates the electric field and


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orients the water molecules in the EDL. This means that the experimentally observed () ()

()

spectra, , are the sum of the () components arising from the topmost interface ( ) ()

()

()

and the EDL ( ).1, 28-29 ( and can also be called the contributions from the Stern ()

layer and Gouy-Chapman layer, respectively.30) Therefore, it is desirable to separate the ()

component from the for clear discussion about the interfacial structure, particularly at high pH. Here, we note that the signal from the EDL (or Gouy-Chapman layer) is often represented as () and is termed “third-order” nonlinear susceptibility1, 4, 14-15, 29 because this component arises under the existence of three electric fields (i.e., the electric fields of the two incident lights and one static field generated by the surface charge). However, sum-frequency generation in the Gouy-Chapman layer predominantly arises from second-order optical process, and the role of the static field is to make water molecules in the EDL oriented as clearly shown in recent ()

()

theoretical studies.31-32 Therefore, we describe this component as , not , in this paper. ()

()

For obtaining the component, Wen et al. determined the spectrum from subtly charged fatty acid/water interfaces and then subtracted it from the spectra of charged monolayer/water interfaces using the factor estimated from the surface charge density.28 They ()

successfully obtained , but it is not easy to apply this method to other interfaces because it is difficult to estimate the static electric field in the EDL. A simple strategy for experimentally ()

obtaining the spectrum is to measure () spectra with changing the ionic strength of the solution. Because the thickness of the EDL (i.e., Debye length) is largely dependent on the ionic strength, comparison of the spectra measured at different ionic strengths enables us to separate ()

()

and . This strategy does not require any rigorous knowledge about the charge density at the interface, and hence it can be a very powerful method, in particular for amorphous silica


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surface. This is because evaluation of the surface charge density at the amorphous silica surface is model-dependent1,

7

and its surface structure is changed by pretreatment.33-34 In fact, this

strategy has been employed in previous homodyne-detected VSFG studies,13,

15

and it was

proposed that strongly hydrogen-bonded water species and bulk-like hydrogen-bonded species are present in the EDL and at the topmost layer, respectively. However, it is often difficult to correctly interpret conventional homodyne-detected VSFG spectra that correspond to ()

()

because of the spectral distortion. For example, in the case that two components ( and ()

) are overlapped and coexist in the same spectral region, it is obvious that the cross term make it very difficult to separate them: ()

()

()

()

()

() ()∗

χ = χ + χ = χ + χ + 2Reχ χ . Thus, it is necessary to obtain Imχ(2) spectra of charged silica/water interfaces using HD-VSFG ()

()

for separating the and components unambiguously and for elucidating the interfacial water structure based on the observed spectra. In this study, while systematically changing ionic strength of the aqueous phase, we measured Im () spectra of the charged silica/aqueous interface at pH 12 using both of pure ()

water (H2O) and isotopically diluted water (HOD in D2O) in the aqueous phase. The Im spectrum obtained by HD-VSFG measurements is significantly different from that estimated

from the () spectra obtained by conventional VSFG measurements,13, 15 and it reveals the existence of characteristic asymmetric hydrogen bonding of water molecules at the topmost layer. Figure 1a shows Im () spectra of the silica/H2O aqueous interfaces measured at pH 12 with various ionic strengths ranging from 0.01 to 5 M, which were prepared with addition of NaCl


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Figure 1. (a) Im () spectra of silica/H2O aqueous interfaces at pH 12 with ionic strengths from 0.01 to 5 M. (b) The difference spectra calculated from the spectra shown in (a). The difference spectra are area-normalized for comparison of the spectral shape.

into NaOH solution. At this pH, deprotonation of the silanol group is saturated,3 so that the surface charge density (i.e., the surface density of silanolate groups) is considered to be independent of the ionic strength of the solution. Note that the lowest ionic strength studied here (0.01 M) is due to NaOH that is added for setting pH at 12. The spectra at low ionic strengths appear positive in the entire OH stretch region, indicating that the interfacial water molecules are net-oriented with their hydrogen atoms pointing toward the silica (we hereafter call this orientation “H-up orientation” according to our experimental configuration where the silica


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substrate is placed on the upper side over the solution25). This observation is consistent with previous reports for low ionic strengths.25, 35 At a glance, the OH band intensity monotonically decreases with increasing the ionic strength, and the spectral change is almost saturated at the ionic strength of 2 M. Following the discussions in previous homodyne-VSFG studies, the ()

decrease in the OH band intensity is attributed to the decrease of the contribution owing to the screening of the electric field by the counter cation (Na+). The Im () spectra above 2 M, ()

where the contribution is sufficiently suppressed, are predominantly attributable to the ()

topmost water layer, .14, 36-38 It is noteworthy that the spectrum shows small but noticeable changes even above 2 M. This change is ascribable to the subtle modification of the water structure in the topmost region, which we discuss in detail later. For examining the observed spectral change more quantitatively, we calculated the difference spectrum for each increase of the ionic strength, which is shown after area-normalization in Figure 1b. The difference spectra shown in this figure are well overlapped with each other in the ionic strength range of 0.01 - 2 M. This indicates that the spectral change in this range is ascribable simply to the decrease of one spectral component. Obviously, this decreasing ()

component, i.e. the difference spectra, is attributable to Im because at lower ionic strengths, e.g., 0.01 - 0.105 M, only shrinkage of EDL is expected according to the calculated field gradient as well as counter ion distribution (see Figure S1 in the supporting information (SI)). Important ()

finding here is that such simple decrease of the component continues up to the ionic ()

strength as high as 2 M. This means that the spectral shape of , and hence the water structure at the topmost layer, is unchanged in such a wide range of ionic strengths from 0.01 to ()

2 M. The contribution of component is considered sufficiently suppressed at 2 M because


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the spectra in the 3000 - 3300 cm-1 region no longer change at the ionic strength higher than 2 M. ()

Thus, the () spectrum at 2 M can be regarded as the “ spectrum”, which is commonly included in all the spectra measured at the ionic strength less than 2 M. On the other hand, we ()

refer to the difference spectrum between the “0.01 M” and “1 M” spectra as “ spectrum”. (The 1 M spectrum is chosen simply because of the low noise level especially in the HOD spectrum which is to be compared in the following section.) At the ionic strengths higher than 2 M, the Imχ(2) spectra exhibit a subtle but noticeable change, in particular the growth of the negative OH band at around 3500 cm-1 (Figure 1a). This change is more readily recognized in the “2 M - 5 M” difference spectrum shown in Figure 1b as a narrow positive OH band at around this frequency. This spectral change in the high frequency region implies that the topmost water structure slightly changes at the ionic strength higher than 2 M, presumably due to the presence of the dense ions near the interface. ()

()

The Im and Im spectra obtained by the above-described spectral decomposition ()

()

are shown in Figure 2. In this figure, we also show the Im and Im spectra measured with isotopically diluted water (HOD-D2O; H2O : HOD : D2O = 1 : 8 : 16) for showing the effect of the inter- and intramolecular vibrational couplings which substantially affect H2O spectra.25, 39-41

(The ionic strength dependent spectra for HOD-D2O solutions are shown in Figure S2 in SI.) ()

As shown in Figure 2a, the Im spectrum of the HOD-D2O solution interface exhibits the OH band peaked at around 3450 cm-1, while the corresponding H2O spectrum shows a substantially broader and red-shifted OH band. This is a typical difference between the H2O and HOD-D2O vibrational spectra of water, which arises from the vibrational coupling existing in ()

H2O. The Im spectra are similar to vibrational spectra of the corresponding aqueous


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()

()

Figure 2. Im (a) and Im (b) spectra obtained with H2O (black) and HOD-D2O ()

(red) solutions. The spectra were obtained as the difference of the spectra measured ()

with ionic strength of 0.01 M and 1 M, whereas the spectra were as-observed spectra measured with ionic strength of 2 M.

solutions in the bulk,

42-43

indicating that the water structure in the EDL is bulk-like, as

previously reported for charged monolayer interfaces.28, 39 ()

()

In contrast to the Im spectra, the Im spectra of H2O and HOD-D2O are very similar to each other, and they exhibit a broad positive peak at around 3200 cm-1 (Figure 2b),


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()

which is markedly different from the bulk water spectra. The Im spectra provide new insights into the topmost water structure at the charged silica interface. First, the OH band is positive and substantially red-shifted. This means that the OH group giving rise to this band is H-up oriented and forms a strong hydrogen-bond with the silica surface at pH 12. Possible hydrogen-bond accepting sites of the silica surface are the oxygen of silanolate (Si-O-), siloxane bridge (Si-O-Si), and the remaining silanol (Si-OH). Among these three, only silanolate decreases its population at lower pH, and the positive peak around 3200 cm-1 was in fact absent in the spectrum measured at pH 2.25 Thus, the positive broad band around 3200 cm-1 is safely assignable to the stretch mode of the H-up oriented OH group that forms a strong hydrogen-bond ()

with a negatively charged silanolate of the silica surface. Second, the similarity between Im spectra of H2O and HOD-D2O solutions indicates that the stretching vibrations of the two OH groups in a H2O molecule are decoupled at the topmost layer. This indicates an asymmetric hydrogen-bonding environment for the topmost water, i.e., two OH groups of the H2O molecule donate their hydrogen to different types of hydrogen-bond accepting sites so that they vibrate with substantially different frequencies. It means that a H2O molecule at the topmost layer forms a very strong hydrogen bond with surface silanolate with one OH group and forms a substantially different hydrogen bond by another OH with something else, e.g., another water ()

molecule. The Im spectra also exhibit a small negative band at around 3500 cm-1 and a small positive peak around 3600 cm-1. Although these bands are very weak, they are not noise because similar negative-positive features are recognized in all the spectra measured in the ionic strength above 0.3 M (Figure 1a). The weak positive peak around 3600 cm-1 can be assigned to the H-up OH of the topmost water molecule that weakly interacts with oxygen atom of siloxane bridge (Si-O-Si) or remaining silanol (Si-OH), as previously concluded based on the


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pH-dependent change of the Im () spectrum.25 Then, the negative band around 3500 cm-1 is attributable to the stretch vibration of the H-down OH (H atom is pointing toward the bulk) of the water molecule whose another H-up OH interacts with silica surface, i.e., the oxygen of silanolate (Si-O-), siloxane bridge (Si-O-Si), or the remaining silanol (Si-OH). This H-down OH forms a hydrogen bond with other water molecules or chloride anions locating in a deeper region, making the hydrogen bonds of two OHs of the topmost water molecules highly asymmetric. We note that the vibrational decoupling of water has also been observed for positively charged surfactant/aqueous halide salt interfaces, and it was attributed to the symmetry breaking induced by a hydrogen bond with chloride counterion.44 For further discussion, we quantitatively examined the ionic strength dependence of the ()

spectral intensity. According to the literature, it is expected that the amplitude of the linearly correlates with the surface potential (see SI for the details),1, 28 i.e., ()

()

()

()

()

= + = + , ()

where

(1)

is the () response of the water in the EDL per unit surface potential . In ()

Figure 3, we plot the experimentally evaluated amplitude (the OH band intensity integrated from 3000 to 3300 cm-1) against the surface potential calculated by so-called modified Poisson-Boltzmann theory (the detail of the theory is given in SI).28, 45 This plot clearly ()

shows that the amplitude of is linear to the calculated surface potentials in the ionic strength range lower than 2 M. This excellent agreement between experiment and the (modified) Poisson-Boltzmann theory shows that the classical EDL picture is valid for a surprisingly wide ionic strength range, at least for the silica interfaces.


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()

Figure 3. Intensity of the OH stretch band ( ) plotted against the surface potential ( ). ()

was evaluated as the OH band intensity integrated from 3000 to 3300 cm-1, and was calculated with modified Poisson-Boltzmann theory. The black dotted line is an eye guide, which is extrapolated from the points of the ionic strength between 0.01 and 0.105 M.

()

The contribution of in the () spectra of the charged silica/aqueous interfaces has been discussed experimentally11,

14-15, 37, 46

and theoretically.31-32 Most of the previous

experimental studies on the ionic strength dependence were performed at pH ~6 with conventional homodyne-detected VSFG, and they were essentially consistent to each other: the ionic strength dependence of VSFG intensity looks saturated at ionic strength around 0.1 M.13-14, 47

This observation had been interpreted as the EDL is sufficiently compressed by the ionic

strength of 0.1 M. Thus, the previous reports appear to contradict the result of the present study ()

that clearly shows the continuous decrease of the component up to the ionic strength of 2 M. We think that this apparent inconsistency is likely attributable to the different pH employed in the experiments, as well as the difficulty of interpreting conventional homodyne-detected


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VSFG spectra. In our previous HD-VSFG study25, we observed negative and positive peaks in the OH stretch region of the Im () spectra measured at pH < 7, revealing that “H-down” and “H-up” oriented water coexist at the topmost layer at less deprotonated or neutral silica/water ()

interface. Because the component is positive in the entire OH stretch region as shown in ()

()

this work, destructive interference between the and components may give rise to the

ionic strength range where the () spectrum looks rather insensitive to the change of ionic strength although the suppression of the EDL (and hence the spectral change of Im () ) is still continuing. In fact, our preliminary experiments support that this is the case (not shown). We also note that some previous homodyne-detected VSFG studies proposed that water in the topmost layer and those in EDL show OH stretch bands at around 3400 cm-1 and 3200 cm-1, respectively, at neutral pH13 as well as a wide pH range of 2-12.15 However, direct measurements of Im () spectra in the present study show that this is not the case: the OH band of the topmost water is peaked around 3200 cm-1 whereas the OH stretch band of water in EDL is peaked at a higher frequency, i.e., 3200-3500 cm-1 for H2O and 3450 cm-1 for HOD-D2O. This highlights the importance of the direct measurement of Im () spectra, which is realized with the heterodyne-detection in VSFG measurements. Finally, we mention two observations that provide information about the counterion distribution in the topmost water layer at the silica/water interface. First, as already described, the obtained Im () spectra show that the topmost water structure is unchanged in the ionic strength range of 0.01-2 M. This implies that one of the followings is the case: The topmost water structure is insensitive to the presence of Na+, or the interfacial density of Na+ is saturated at 0.01 M and unchanged at higher ionic strength. The modified Poisson-Boltzmann theory indicates that the latter is more likely because the local concentration of the hydrated cation at


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the topmost layer is calculated to be nearly saturated even at the ionic strength as low as 0.01 M (Figure S1 in SI). Second, the Im () signal does not vanish even at a very high ionic strength such as 5 M. This indicates that Na+ does not displace the topmost water molecules even at the ionic strength of 5 M, as suggested previously.36 This means that the contact adsorption of Na+ does not increase with ionic strength above 0.01 M, i.e., it is already saturated below 0.01 M even if the contact adsorption exists. In fact, a good correlation is seen between the OH band intensity and the surface potential calculated by the theory in the ionic strength range of 0.01-2 M as shown in Figure 3, which is consistent with the absence of the noticeable change in the contact adsorption in this range. In summary, we measured the Im () spectra of the charged silica/aqueous interface at a wide range of the ionic strength with HD-VSFG, and demonstrated that the spectra can be experimentally decomposed into two components, i.e., the Im () spectrum of the topmost layer and that of the EDL, on the basis of the ionic strength dependence of the spectra. The isotopic dilution experiment revealed that two OH stretch vibrations of the topmost water molecule are decoupled, indicating that two OH bonds of the water form significantly different hydrogen bonds. The concluded water structure at the charged silica/aqueous interface at pH 12 is sketched in Figure 4: The dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group which is H-up oriented (H atom is pointing toward the silica) (A). There also exists a minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol using its one OH group with H-up orientation (B). The other OH group of these water molecules in the topmost layer is H-down oriented (H atom is pointing toward the bulk aqueous phase), forming hydrogen bonds with the surrounding water or Cl- located in slightly deeper aqueous phase (C). This water


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Figure 4. Proposed water structures at the silica/aqueous interface at bulk pH of 12 at the ionic strength of 0.01-2 M. Only the water molecules observable by HD-VSFG spectroscopy are depicted. The red dotted lines represent hydrogen-bonding. The letters A, B, and C represent the OH groups which interact with silanolate (A), siloxane bridge or silanol (B), and chloride anion or another water molecule (C).

structure in the topmost layer is unchanged in the ionic strength range between 0.01 M and 2 M. We did not find any experimental evidence suggesting contact adsorption of Na+ onto the charged silica surface in the ionic strength region examined in the present study (0.01 - 5 M). On the other hand, the water molecules in the EDL are net “H-up” orientated due to the negative charge at the silica surface, forming bulk-like hydrogen bonds with surrounding water molecules. The intensity of the Im () signal from the water in the EDL is almost perfectly correlated with the surface potential (which is calculated by modified Poisson-Boltzmann theory) up to 2 M.


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Lastly, we wish to clearly mention the significant improvement of our molecular-level picture on the water structure at the charged silica/water interface. In our previous study on pH dependence of water structure at the silica/water interface, we could only conclude that interfacial water is H-up oriented at pH 12.25 In the present study, on the other hand, the asymmetric hydrogen-bonding structure of the topmost water molecules was clarified for the first ()

()

time, thanks to the separation of and . The experimental approach employed in this study opens a new way for systematic elucidation of interfacial water structures at mineral interfaces using interface-selective nonlinear spectroscopy.

Supporting Information. (1) Experimental setup, (2) brief description for the derivation of eq. 1, (3) brief descriptions of the modified Poisson-Boltzmann theory and the EDL properties calculated by the theory, (4) Im () spectra of the silica / HOD-D2O solution interfaces, (5) details of the Fresnel analysis,

and (6) () spectra reconstructed from the complex () spectra.

AUTHOR INFORMATION Corresponding author *Email: [email protected]; Tel: +81-48-467-4592. Present addresses S.U.: Research Institute for Science & Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan.


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A. M.: Function Analysis Department, NISSAN ARC Ltd., 1 Natsushima, Yokosuka, Kanagawa 237-0061, Japan. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP25104005, JP18H03905, and 18H05265.

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Aqueous Interface

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