Alkyl Chain Length Dependent Structural and Orientational

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Alkyl Chain Length Dependent Structural and Orientational Transformations of Water at Alcohol-Water Interfaces and Its Relevance to Atmospheric Aerosols Jahur A. Mondal, Vinu Namboodiri, Mathi Pandiyathuray, and Ajay K Singh J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00324 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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The Journal of Physical Chemistry Letters

Alkyl Chain Length Dependent Structural and Orientational Transformations of Water at Alcohol-Water Interfaces and Its Relevance to Atmospheric Aerosols Jahur A. Mondal*, V. Namboodiri, P. Mathi, and Ajay K Singh Radiation & Photochemistry Division, Bhabha Atomic Research Centre, HBNI, Trombay, Mumbai400085, India ABSTRACT: Although the hydrophobic size of an amphiphile plays a key role in various chemical, biological, and atmospheric processes, its effect at macroscopic aqueous interfaces (e.g. air-water, oil-water, cell membrane-water, etc.) which are ubiquitous in nature is not well understood. Here we report the hydrophobic alkyl chain length dependent structural and orientational transformations of water at alcohol (CnH2n+1OH, n = 1-12)-water interfaces using interface-selective heterodyne-detected vibrational sum frequency generation (HD-VSFG) and Raman multivariate curve resolution (Raman-MCR) spectroscopic techniques. The HDVSFG results reveal that short chain alcohols (CnH2n+1OH, n < 4 i.e., up to 1-propanol) do not affect the structure (H-bonding) and orientation of water at the air-water interface: the OH stretch band maximum appears at ~ 3470 cm-1 and the water H-atoms are pointed towards the bulk water i.e., “H-down” oriented. In contrast, long chain alcohols (CnH2n+1OH, n > 4 i.e., beyond 1-butanol) make the interfacial water more strongly H-bonded and reversely orientated: the OH stretch band maximum appears at ~ 3200 cm-1 and the H-atoms are pointed away from the bulk water i.e., “H-up” oriented. Interestingly, for the alcohol of intermediate chain length (CnH2n+1OH, n = 4 i.e, 1-butanol), the interface is quite unstable even after hours of its formation and the time-averaged result is qualitatively similar to that of the long chain alcohols, indicating a structural/orientational crossover of interfacial water at the 1-butanol-water interface. pH dependent HD-VSFG measurements (with H2O as well as isotopically diluted water, HOD) suggest that the structural/orientational transformation of water at long chain alcohol-water interface is associated with the adsorption of OH- anion at the interface. Vibrational mapping of water structure in the hydration shell of OH- anion (obtained by Raman-MCR spectroscopy of NaOH in HOD) clearly show that the water becomes strongly H-bonded (OH stretch max. ~ 3200 cm-1) while hydrating OH- anion. Altogether, it is conceivable that alcohols of different hydrophobic chain lengths that are present in troposphere will differently affect the interfacial electrostatics and associated chemical processes of aerosol droplets, which are critical for cloud formation, global radiation budget, and climate change. TOC

Depending upon the size of a hydrophobe/hydrophobic molecular group, water undergoes distinct structural transformations in its hydration shell (hydrophobic molecular interface). For example, a small hydrophobic group (e.g. the alkyl group of methanol; dia. < 1 nm) promotes entropically

restricted tetrahedral water structure in its hydration shell. Whereas, a large hydrophobic group (e.g. the alkyl group of 1heptanol; dia. > 1 nm) favours disordered water structure of weaker H-bonding (enthalpy rich water) in its vicinity.1-5 The perturbed water structure around hydrophobes (either entropy

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restricted i.e., ∆S0) increases the free energy of the system, which in turn tend to minimize the hydrophobic surface area (of the system). As a result, amphiphiles are preferentially adsorbed at aqueous interfaces (air-water or oil-water) and/or undergo hydrophobic assembly, forming aggregates, such as micelle, vesicle, or even folding of proteins. These hydrophobic assemblies in return, create macroscopic aqueous interface where the structure and properties of water are different from that in bulk water, providing a unique environment to interfacial processes pertaining to atmospheric, chemical, and biological systems.6-9 In fact, the interfacial water is so crucial in biochemical processes that it is considered, not as a “solvent”, but as a “biomolecule”. In atmosphere, oxygenated organic compounds, especially alcohols, carbonyls, and organic nitrates affect the interfacial properties of aerosol droplets. Hence, the interfacial chemistry (on aerosol surface) and subsequent cloud formation, as well as global radiation budget (the balance between the incoming energy from the Sun and the outgoing energy from the Earth), are affected, creating the stage for climate change.7,10-12 Therefore, molecular level understanding of amphiphile containing macroscopic aqueous interface is indispensable for better elucidating interfacial biochemical processes and for the modeling atmospheric changes. Extensive research13-21 has been carried out to understand the properties of interfacial water and their dependence on the nature of the amphiphiles and vice-versa. Especially, with the advent of interface-selective spectroscopic techniques such as vibrational sum frequency generation (VSFG) and its phasesensitive variant, heterodyne-detected VSFG (HD-VSFG), valuable insights have been obtained about the interfacial electric field, binding of ions/zwitterions, orientational ordering, H-bonding, and dynamics of interfacial water.22-35 Nevertheless, these studies were largely focused to address the effect of amphiphile “headgroups” (cationic vs. anionic vs. zwitterionic, and the uncharged dipolar headgroups to some extent) on interfacial water. In other words, the effect of the “hydrophobic part” of an amphiphile (on interfacial water) has hardly been investigated, though the hydrophobic hydration is the driving force to the formation of an aqueous interface. Because of the lack of investigation, it is not known whether two amphiphiles, say ethanol and octanol, which have the same headgroup but different hydrophobic sizes will affect the interfacial water in the same way or differently. Presumably, the reason of such limited knowledge is the belief that hydrophobic alkyl chains, because of their weak interaction with water, do not affect interfacial water significantly. Contrary to this belief, Raman multivariate curve resolution (Raman-MCR) spectroscopy36 of aqueous surfactant solutions provided compelling evidence that the interfacial water penetrates much deeper into the hydrophobic core of a micelle than the previously thought first few carbons adjacent to the headgroup. More interestingly, it was observed that the penetration depth increases with the alkyl chain length of surfactants. At oil-water interface, amphiphiles of different hydrophobic sizes affect the interface differently, due to their varying degree of penetration into the oil phase.37 Recently, we observed to our surprise that a small amphiphile e.g. tertbutanol, though preferentially orients (as “methyl up”) at the air-water interface, does not affect the net “H-down” orientation and H-bonding of the interfacial water.38 Subsequent HD-VSFG studies from ours39 as well as Tian’s40

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groups, by contrast, showed that interfacial water takes “H-up” orientation (i.e. opposite to that at the tert-butanol-water or the air-water interface) for alcohols with larger hydrophobic groups (cholesterol and hexadecanol). Also, HD-VSFG study of bitumen (a mixture of hydrocarbons)-water interface by Chou’s group41 and the sum frequency scattering study of hexadecane droplet-water interface by Roke’s group42 have revealed that these hydrophobe-water interfaces are negatively charged and the interfacial water is “H-up” oriented. These observations motivated us to examine the effect of hydrophobic size of an amphiphile at a macroscopic aqueous interface. To successfully detect the hydrophobic size effect on interfacial water, we chose linear alcohol-water interfaces where the alkyl chain length was systematically varied keeping the alcoholic OH headgroup fixed (CnH2n+1OH with n = 1, 2, 3, 4, 5, 8, and 12). The HD-VSFG measurement of the alcohol-water interfaces (surface tension, γ = 50 ± 3 mN/m) provided the imaginary-χ(2) spectra (Imχ(2); χ(2) is the second order electric susceptibility) of the interfaces. The sign, amplitude, and the frequency maximum of the Imχ(2) spectra (2800 – 3600 cm-1) reveal the preferential orientation, degree of orientational order, and the H-bonding of interfacial water, respectively.43,44 The measured Imχ(2) spectra show that the short chain alcohols (CnH2n+1OH, n < 4) do not affect the structure (H-bonding) and net “H-down” orientation of water at the air-water interface, though the alcohol molecules are preferentially oriented as “methyl-up”. By contrast, long chain alcohols (CnH2n+1OH, n > 4), not only orient their alky chains as “methyl-up”, but also the interfacial water as “H-up”, and make the latter strongly H-bonded. pH-dependent HD-VSFG measurement shows that the distinct structure and orientation of water at the long chain alcohol (CnH2n+1OH, n > 4)-water interfaces are due to preferential adsorption of OH- ion at the interface, triggered by the hydrophobic size of the alcohols. By the vibrational mapping of water structure in the hydration shell of OH-, which is obtained by the Raman-MCR spectroscopy of aqueous NaOH solution, we reconfirm that the increased H-bonding of interfacial water is due to the hydration of the adsorbed OH- ions at the interface. Thus, HDVSFG in combination with the Raman-MCR shows that the hydrophobic size plays important roles in structural and orientational (interfacial electric field) transformation of water at uncharged alcohol-water interfaces. Materials and Methods. Linear alcohols, such as methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-octanol, and 1dodecanol (HPLC grade) were purchased from Spectrochem, India. Dodecylsulfate sodium salt (DDS-Na salt; AR grade) and D2O (> 99.9 atom%) were from Sigma Aldrich. Milli-Q water (18.2 MΩ cm resistivity) was used for all the measurements. pH of the water was adjusted to 3.0 and 12.0 by adding diluted HCl/NaOH. To minimize the counterion effect at the interface, care was taken not to add excess HCl/NaOH. To measure the OH stretch region with isotopically diluted water, H2O and D2O were mixed in 1:3 ratio (v/v) such that H2O/HOD/D2O = 1/6/9. For simplicity, isotopically diluted water is denoted as HOD, henceforth. Details of the HD-VSFG and Raman-MCR techniques, which have been described in our previous publications,38,39,45 are briefly discussed in the Supporting Information (SI). All the alcohol-water interfaces were measured (by HD-VSFG spectroscopy) at a surface tension (γ) of 50 ± 3 mN/m. Details


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of surface tension measurement has been described in the SI. All the alcohol-water interfaces were measured (by HD-VSFG technique) at a surface tension (γ = 50 ± 3 mN/m) considerably lower than that of the neat water (72.3 mN/m at 23°C). Also, the γ values of all the alcohol-water mixtures were kept quite close to each other (∆γ = ± 3 mN/m) during the HD-VSFG measurements. γ ≈ 50 mN/m corresponds to a substantial coverage of the water surface by alcohols (on the basis of reported literature on the surface tension of alcoholwater mixtures,46,47 the surface excess of the short chain alcohols were calculated using Gibbs adsorption equation, which indicated that a surface coverage of ~80% for γ ≈ 50 mN/m; it is important to note that for alcohol-water mixtures, the surface tension decrease with increasing alcohol concentration, which is not always the case with charged surfactants on48). Preferential Orientation and H-bonding of Water at Shortand Long Chain Alcohol-Water Interfaces. Similar γ values (50 ± 3 mN/m) of all the alcohol-water mixtures implies that they release/require the same amount of energy for their macroscopic contraction/expansion of equal extent. This is because, ∆W = π × ∆area (where ∆W is the work done by/on the system for the contraction/expansion of the surface area by ∆area units), i.e. ∆W depends on the value of π, but not on the type of molecules used to achieve the π-value. Thus, the macroscopic contraction/expansion of all the alcohol-water interfaces (with same surface tension) will be equally ease/difficult, irrespective of the length of their alkyl chains. However, that is not the case at the molecular level, as can be seen from the Imχ(2) spectra of the alcohol-water interfaces. Figure 1 shows that the sign and frequency position of the Imχ(2) spectra (markers of preferential orientation and H-bond

Figure 1. Imχ(2) spectra of different linear alcohol-water interfaces (panels ‘b’ – ‘h’) in 2800 - 3600 cm-1 regions (bulk pH ~ 5.8 without added NaOH or HCl). The magenta curve in panel ‘e’ is the average of successively measured Imχ(2) spectra which fall within the grey colored shaded region. The spectrum of the air-water interface (panel ‘a’) is shown for the reference.



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strength of interfacial water, respectively) vary to a great extent with the chain length of alcohols. For short chain alcohols (e.g. methanol, ethanol, and 1-propanol), the Imχ(2) spectra show a negative band in 3000 - 3600 cm-1 region with a maximum at ~ 3470 cm-1 (panel ‘b’ – ‘d’ in Figure 1). Compared to the neat air-water interface (Figure 1a), the amplitude of the negative band marginally decreases in presence of alcohol, and on increasing the chain length from methanol to 1-propanol. Thus, except a slight decrease in orientational order (as revealed by reduced amplitude of Imχ(2)), the net “H-down” orientation and average H-bond strength of interfacial water are almost unaffected by the presence of short chain alcohols (CnH2n+1OH; n < 4) on water surface (interpretation of the negative sign of Imχ(2) spectrum as “H-down” oriented water is based on the hyperpolarizibilty analysis of isolated water molecule,44 despite the collective nature of the OH stretch vibration due to vibrational coupling). In stark contrast to the short chain alcohols, the Imχ(2) spectra at the long chain alcohol (CnH2n+1OH; n > 4 i.e., 1-pentanol, 1octanol, and 1-dodecanol)-water interfaces are very different from that at the air-water interface (compare panel ‘f’-‘h’ with panel ‘a’ in Figure 1). The Imχ(2) spectra (OH stretch) show a positive sign with a largely red-shifted band maximum (~ 3200 cm-1) from that of the air-water interface (~ 3470 cm-1). The positive sign (Imχ(2)) implies net “H-up” orientation of water; and the red-shift “apparently” suggests stronger Hbonding of interfacial water in presence of long chain alcohols (CnH2n+1OH, n > 4). The increased H-bonding has been discussed in more detail latter. Interestingly, for the alcohol of intermediate chain length (CnH2n+1OH with n = 4 i.e., 1-butanol), the sign, amplitude, as well as the band maximum (OH stretch) fluctuate to a great extent among the successively recorded Imχ(2) spectra, measured within a span of one hour after mixing 1-butanol with water. The fluctuating Imχ(2) spectra fall within the gray shaded region in Figure 1e (the magenta curve is the average of all the measurements). This result suggests that the 1butanol-water interface is quite unstable compared to those of the shorter or longer chain (CnH2n+1OH, 4 < n < 4 i.e., n ≠ 4) containing alcohol-water interfaces; and there is a crossover of interfacial water structure and orientation (increased Hbonding and “H-down” to “H-up” conversion). The Imχ(2) spectra in the CH stretch region (~2800 – 3000 cm1 ), which are grossly representing the CH3 symmetric stretch and the Fermi resonance between CH3 stretch and its bend overtone, show a negative sign for all the alcohol-water interfaces, suggesting a “methyl-up” orientation (pointed away from the aqueous phase) of their alkyl chains.44 It is to be noted that since the methyls are point away from the aqueous phase, the alcoholic OHs, especially those of the long chain alcohols are preferentially pointed (OH vector) towards the aqueous phase and may contribute to the Imχ(2) signal (with a negative sign) in the OH stretch region. However, the observed net positive Imχ(2) signal (OH stretch) of long chain alcohol-water interfaces (Figure 1f - 1h) suggests that the relative contribution of alcoholic OH is lower than that of interfacial water-OH. Together, the results in Figure 1 clearly show that, though all the alcohols are preferentially oriented (“methyl-up”) at the interface, it is only the long-chain alcohols beyond 1-butanol, which affect the H-bonding and reverse the orientation of interfacial water. We note that these

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observations are valid even at lower surface tensions (γ ~ 40 mN/m) of the alcohol-water mixtures. Effects of pH and Electrolyte at the Short and Long Chain Alcohol-Water Interfaces. Alcoholic OHs are present at the interface irrespective of the chain length of alcohols (γ ≈ 50 mN/m)). Therefore, the structural/orientational change of interfacial water, observed at alcohol-water interfaces beyond 1-butanol (Figure 1), cannot be due to the alcoholic OHs. Recently, HD-VSFG studies of cholesterol-water39 and hexadecanol-water40 interfaces suggested the preferential adsorption of OH- ion at these interfaces (cholesterol is a secondary alcohol with a large hydrophobic group). Therefore, the alkyl chain length dependent water structure change could be due to the interfacial adsorption of OH- ions, provided the adsorption (OH-) occurs only at the long chain alcohol-water interfaces. To examine this hypothesis, we have measured the short and long chain alcohol-water interfaces at varying concentration of OH- ions in the bulk aqueous phase (i.e., at varying bulk pH), and also in presence of electrolytes (e.g. NaCl). Figure 2a shows the Imχ(2) spectra of a representative short chain alcohol (ethanol)-water interface with acidic (pH = 3.0) and basic (pH = 12.0) aqueous phases. The Imχ(2) spectra at the acidic and basic pHs are almost identical to each other, and very close that of the neat air-water interface (at pH 12, the

Figure 2. Imχ(2) spectra of (a) ethanol-water and (b) 1-octanolwater interfaces at pH = 3.0 and 12.0 with and without NaCl (0.2 M) in the aqueous phase. The spectrum of the neat air-water interface (black dotted curve) is shown for the reference in each panel.


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The Journal of Physical Chemistry Letters the H-bond interaction of water with its neighbors. This is because apart from H-bonding, intra- and inter-molecular vibrational couplings of OH chromophore strongly affect the band shape and frequency position of the OH stretch spectrum. This problem has been overcome by measuring aqueous interfaces with isotopically diluted water, HOD.50 The OH stretch response of HOD is largely due to vibrationally decoupled OH oscillators, and hence, free from the intra- and intermolecular vibrational coupling.

Figure 3. Schematic representations of preferential orientation of water at the air-water (a) and short and long chain alcohol-water interfaces (b and c). At the short chain alcohol (CnH2n+1OH, n < 4)-water interfaces, the average H-bond strength and preferential orientation of interfacial water are similar to that of the neat airwater interface; moderate variation of pH (3.0 - 12.0) has little or no effect on the interfacial water. Whereas, at long chain alcohol (CnH2n+1OH, n > 4)-water interfaces, the interfacial water exhibits “H-up” orientation and strong H-bonding at basic and even neutral pHs, which is mediated by the adsorption of OH- ion at this interface. The dashed circle around the interfacial OH- anion is a schematic represent of the hydration shell where the water molecules are strongly H-bonded. At the acidic pH, the interfacial water at long chain alcohol-water interface is similar to that of the air-water interface (scheme not shown in the Figure).

sharp band around 3600 cm-1 is a noise). Addition of electrolyte (0.2 M NaCl) into the aqueous phase hardly affect the amplitude of Imχ(2) signal (OH stretch), which reveals the absence of appreciable electric field at the interface. On the other hand, the Imχ(2) spectrum at the 1-octanol (representative long chain alcohol)-water interface shows strong pH dependence: at low pH (3.0), the OH stretch band is negative and similar to that of the ethanol-water (or air-water) interface; but at high pH (12.0), the OH stretch band is positive in sign with a large amplitude, which is similar to that of a negatively charged surfactant (or lipid)-water interface.23,44 Addition of electrolyte (0.2 M NaCl) into the aqueous phase decreases the amplitude of the Imχ(2) signal at high pH (12.0), but not so at low pH (3.0). This shows that long chain alcohol-water interfaces exhibit appreciable interfacial electric field (negative charge) and counterion effect49 at basic pH (12.0), but not so at the acidic pH (3.0). Moreover, at basic pH (12.0), the frequency position (maximum) of the OH stretch band is red-shifted by~250 cm-1 compared to that at the acidic pH (3.0), revealing a pH-dependent variation of H-bond strength of interfacial water. Thus, two alcohols, ethanol and 1-octanol, which differ from each other only by the length of their hydrophobic alkyl chain, dramatically affect interfacial electric field and the associated water at the interface. These observations are schematically illustrated in Figure 3. Origin of Increased H-bonding of Water at Long Chain Alcohol-Water Interface. Spectral shift of a vibrational band is a sensitive marker of the interaction of the vibrational chromophore with its surrounding. For example, a polar covalent bond, A−H (where A is an electronegative atom e.g. N, O, F, etc.) becomes weaker on H-bond interaction with another electronegative atom B (A−H---B; where B = N, O, F) and the frequency position of the parent A−H stretch band shifts toward lower frequency (red-shift). However, in the case of liquid water (OH stretch), such a straightforward interpretation of spectral shift is inadequate to precisely mirror

Imχ χ(2) Spectra at Long Chain Alcohol-HOD Interface. The Imχ(2) spectra of 1-octanol (a typical long-chain alcohol)-HOD interface at acidic and basic bulk pHs (pH = 3.0 and 12.0) are shown in Figure 4. It is observed that, at low pH (3.0), the sign and frequency position of the Imχ(2) spectrum (OH stretch) are close to that of the air-HOD interface (compare red and black curves in Figure 4). However, at high pH (12.0), the Imχ(2) spectrum changes its sign (becomes positive, green curve). It is to be noted that the spectrum at the neat air-water interface does not change within the pH range (3.0 – 12.0) studied.39 Given the pKa value of alcoholic OH (pKa ~ 16), it is expected that the alcoholic OH does not undergo acid-base reaction (protonation/deprotonation) within the measured pH range, and hence the chemical nature of the alcohol remains unchanged. Nevertheless, the strong positive signal at high pH (12.0) shows that the surface is effectively negatively charged. Apart from charging, the frequency position of the OH stretch band is red-shifted by ~ 200 cm-1 (3420 – 3220 = 200 cm-1) on increasing the bulk pH (from 3.0 to 12.0). Since the measurements were carried out in HOD, the large red-shift indeed shows that the interfacial water is strongly H-bonded at the long chain alcohol-water interface. As discussed in above, the increased H-bonding is observed only at long chain alcohol (CnH2n+1OH, n > 4)-water interfaces at basic and even neutral pHs. Therefore, we

Figure 4. Imχ(2) spectra of air-HOD (black, pH ~ 5.8), 1-octanolHOD (red, pH = 3.0; green, pH = 12.0), and DDS-HOD (blue, pH ~ 5.8) interfaces. The solid curves are visual guide to eye.

hypothesize that the pH dependent variation of the H-bond strength and orientation (of water) are mediated through the adsorbed OH- ions at the long chain alcohol-water interface. Intuitively, one can expect that the interfacial electric field created by the adsorbed OH- ion may induce stronger Hbonding at the interface, as has been suggested by the pH dependent VSFG measurement at the silica-water interface.51 In addition, the interfacial water may exhibit stronger H-



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bonding in presence of OH- ions, provided the H-bond of water in the hydration shell of OH- is stronger than that at the neat air-water interface. We have examined these two possibilities in the following Sections. Imχ χ(2) Spectrum at Anionic Surfactant-HOD Interface. Effect of interfacial negative electric field on the H-bond strength of interfacial water can be examined by recording the Imχ(2) spectrum of water at a negatively charged surfactant (e.g., dodecylsulfate, DDS)-HOD interface (blue spectrum at the right Y-axis of Figure 4) As expected, the Imχ(2) spectrum shows a positive sign (“H-up” orientation) and large amplitude due to the negative interfacial electric filed. However, the maximum of the OH stretch band is close to that of the neat air-water interface (~ 3420 cm-1). These observations suggest that interfacial electric field, though increases the orientational order of interfacial water, does not increase the H-bond strength of the oriented water, which is in conformity with previous HD-VSFG reports of charged lipid (or surfactant)water interfaces.23,52 Therefore, the stronger H-bonding of interfacial water is not due the interfacial electric field created by the adsorbed OH- ions at the long chain alcohol water interface. OH Stretch Spectrum of HOD in the Hydration Shell of OH- Ion. To examine the second possibility, i.e., the stronger H-bonding of interfacial water due to the hydration of adsorbed OH- ions, we have extracted the OH stretch spectrum of HOD pertinent to the hydration shell of OH- ion (green curve in Figure 5a), using Raman-MCR spectroscopy. The Raman spectrum of neat HOD (black curve) and 1.0 M

Figure 5. OH stretch (Raman) spectra of HOD in the hydration shell of (a) OH- and (b) SO42- anions (green curves). The Raman spectra of the neat HOD (black curve), NaOH, and Na2SO4 (1.0M each) solutions (blue dashed curve) are shown for references in the corresponding panels.

NaOH in HOD (blue dashed curve) are also shown in Figure 5a to compare with the spectrum of the hydration water. The Raman-MCR-extracted OH stretch spectrum shows a largely red-shifted broad band around 3200 cm-1 and a sharp band around 3600 cm-1. On the basis of vibrational predissociation

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spectroscopy of size-selected OH-(H2O)n clusters53 and Raman-MCR54 studies, the sharp band is assigned to the OH stretch band of OH- anion and the broad band (around 3200 cm-1) to the OH stretch of water hydrating the OH- anion. Since, the large red-shift (3420 - 3200 = 220 cm-1) is observed in isotopically diluted water, it is evident that the water in the hydration shell of OH- has stronger H-bonding than that of bulk water. Accordingly, the long-chain alcohol-water interfaces, which preferentially adsorb OH- anion, are expected to exhibit stronger H-bonding of interfacial water due to the hydration of the adsorbed OH- ions. We have crossexamined the above assignment, by comparing the Imχ(2) spectrum (OH stretch) of the anionic DDS-water interface with its corresponding hydration water spectrum. The maximum of the Imχ(2) spectrum at the anionic DDS-HOD interface (blue curve in Figure 4) appears at ~ 3420 cm-1 which is very close to the maximum of Raman spectrum of bulk HOD (solid black curve in Figure 5b). Meaning, the Hbond strength of water at the DDS-water interface is similar to that of bulk water. Accordingly, we expect that the H-bond strength of water in the hydration shell of DDS-headgroup will also be similar to that of the bulk water. The Raman-MCR extracted spectrum (OH stretch) of sulfate anion, an analogue of DDS headgroup, indeed shows that the H-bond strength of sulfate hydrating water molecule are comparable to that of the bulk water (compare green and black curves in Figure 5b). Thus, HD-VSFG measurements in combination with the Raman-MCR results provide direct evidence that the stronger H-bonding of interfacial water at the long chain alcohol-water interface is mediated by the adsorbed OH- ions at the interface (pictorially represented as hydrated OH- in Figure 3c). Interfacial Structural Implications. It is intriguing that though the hydrophobic chains of alcohols do not have “direct strong interaction” either with water or with the ions (e.g. OH-, Na+ or Cl-), the alcohol-water interfaces exhibit hydrophobic chain length dependent modification of interfacial properties (e.g. the interfacial field, counterion effect, orientation and Hbonding of water). For the hydrophobic hydration in bulk water, it is believed that water accommodates a small hydrophobe (dia. < 1 nm) without compromising much its structure (entropically restricted tetrahedral water structure). However, accommodating a large hydrophobe (dia. > 1 nm) is energetically costly affair and that the water in the vicinity of a large hydrophobe undergoes structural transformation, making it weakly H-bonded. Thus, in the bulk aqueous phase, the crossover of water structure in the vicinity of a hydrophobe occurs at an intermediate size of the hydrophobe (dia. ~ 1 nm).1-5 In the present study, we observe that different hydrophobic groups (alcohols with varying chain length) affect the water at the alcohol-water interface quite differently: for alcohols with small hydrophobic chains (methanol to 1propanol), the interfacial water is almost unaffected, and for alcohols with large hydrophobic chains (1-pentanol and above), the interfacial water is structurally and orientationally different from that of the air-water interface. For the alcohol of intermediate alkyl chain length (i.e. 1-butanol, hydrophobic chain length ~ 1 nm), the interfacial water undergoes a structural and orientational crossover, at room temperature. It is very interesting to see this remarkable similarity between the hydrophobic size dependent crossover of water structure at the macroscopic interface (observed in the present study) and in bulk water as discussed above. We believe that the alkyl chain length dependent response of interfacial water has its


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bearing with the size dependent hydration of alkyl groups (hydrophobic hydration) at the interface: In the case of methanol, ethanol, and 1-propanol, the alkyl groups are small enough to be accommodated (hydrated by) by the interfacial water without appreciably affecting its structure, and hence at the molecular level, these small alcohols experience substantial orientational freedom at the air-water interface and exhibit broad angular distribution with respect to surface normal. For larger alcohols, beyond 1-butanol, the alkyl groups are too large to be accommodated without affecting the water structure at interface. In response, these large hydrophobic alkyl chains are forced out (by water) at the airwater interface. So, for a long chain alcohol, the hydrophobic alkyl chain, as well as the alcoholic OH, experiences limited orientational freedom (narrow angular distribution) at the airwater interface, creating an ordered arrangement of H-down oriented alcoholic OH on water surface. In fact, such an alkyl chain length dependent orientational difference of alcohols at the air-water interface has been reported by X-ray synchrotron,55 X-ray photoelectron,56 as well as conventionalVSFG57,58 spectroscopic studies. The H-down oriented ordered arrangement of alcoholic OHs (on water surface), which occurs predominantly for the long chain alcohols (CnH2n+1OH, n > 4), favours the adsorption of OH- at these interfaces. The adsorbed OH- ions in turn, lead to the preferential “H-up” orientation and stronger H-bonding of the interfacial water, as has been discussed above. Alcohols of different hydrophobic sizes (i.e., chain length) and shapes (i.e., chain branching) are present in the troposphere and are believed to influence key atmospheric processes associated with the cloud formation and Earth’s radiation budget.7,59,60 The hydrophobic size-dependent flip-flop of interfacial water (“H-up” vs “H-down”) is a reflection of differential electrostatics at the ‘so-called uncharged’ alcoholwater interfaces which is capable of affecting the adsorption of ions/molecule and subsequent interfacial reactions. We envisage that alcohols of different hydrophobic sizes that are present in troposphere will differently affect the physicochemical processes at surface of aerosol droplets, which are critical for cloud formation and Earth’s radiation budget as well as for climate change. It will be interesting to compare the present results with those of the branched alkyl chain containing alcohols (structural isomers) whose arrangement and ordering at the interface are expected to be different from that of the linear ones, and with other uncharged amphiphiles, such as carbonyls,22 and organic nitrates.

Hydrophobicity induced structural and orientational transformations of water at alcohol-water interfaces have been investigated using surface tension guided HD-VSFG and Raman-MCR techniques. It is observed that at short chain alcohol-water interfaces (e.g. methanol, ethanol, and 1propanol), the interfacial water is very similar to that at the airwater interface. For the alcohol of intermediate chain length (i.e., 1-butanol), the interface is quite unstable even after hours of its formation. At long chain alcohol-water interfaces (i.e., beyond 1-butanol), the interfacial water becomes distinctly different from that at the air-water interface, exhibiting an “Hup” orientation and increased H-bonding. pH dependent variation of interfacial water structure/orientation suggests that the OH- ions are preferentially adsorbed only at the long chain alcohol-water interfaces. Then the adsorbed OH- anion, besides orienting the interfacial water as “H-up”, increases the average H-bond strength of interfacial water while getting hydrated by the latter. This has been confirmed by the RamanMCR spectroscopy of NaOH in isotopically diluted water, which shows largely red-shifted OH stretch (maximum ~ 3200 cm-1) for water associated with the hydration of OH- anion. Thus, the distinct structural (i.e., varying H-bond strength) and orientational (“H-up” vs. “H-down”) transformation of water at alcohol-water interfaces are triggered by the hydrophobic size of alcohols, and the crossover of the water structure/orientation occurs at the 1-butanol-water interface.

ASSOCIATED CONTENT Details of surface tension measurement; description of HD-VSFG and Raman-MCR spectroscopic techniques.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Funding Sources Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The authors gratefully acknowledge the support from Dr. D. K. Palit, Head, RPCD, BARC.

ABBREVIATIONS VSFG, vibrational sum frequency generation; HD-VSFG, heterodyne-detected VSFG; MCR, multivariate curve resolution; HOD, isotopically diluted water; DDS, dodecyl sulfate anion; OPA, optical parametric amplifier; DFG, difference frequency generator; CCD, charge coupled device.



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Alkyl Chain Length Dependent Structural and Orientational

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