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Letter

Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with ThermoDynamics Odile Carrier , Ellen H.G. Backus, Noushine Shahidzadeh, Johannes Franz, Manfred Wagner, Yuki Nagata, Mischa Bonn, and Daniel Bonn J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02646 • Publication Date (Web): 16 Feb 2016 Downloaded from http://pubs.acs.org on February 18, 2016

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The Journal of Physical Chemistry Letters

Oppositely Charged Ions at Water-Air and Water-Oil Interfaces: Contrasting the Molecular Picture with ThermoDynamics

Odile Carrier1#, Ellen H.G. Backus2#, Noushine Shahidzadeh1, Johannes Franz2, M. Wagner2, Yuki Nagata2, Mischa Bonn2, Daniel Bonn1* 1

Van der Waals-Zeeman Institute, Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands 2

Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany #O.C. and E.H.G.B. contributed equally *Corresponding author: [email protected], T +31-20-5255887, F +31-20-5255887

1 ACS Paragon Plus Environment


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The surface-active ions tetraphenylarsonium (Ph4As+) and tetraphenylboron (Ph4B-) have a similar structure but opposite charge. At the solution-air interface, the two ions affect the surface tension in an identical manner, yet sum-frequency generation (SFG) spectra reveal an enhanced surface propensity for Ph4As+ compared with Ph4B-, in addition to opposite alignment of interfacial water molecules. At the water-oil interface, the interfacial tension is 7 mN/m lower for Ph4As+ than for Ph4B- salts, but this can be fully accounted for by the different bulk solubility of these ions into the hydrophobic phase, rather than inherently different surface activities. The different solubility can be accounted for by differences in electronic structure, as evidenced by quantum chemical calculations and NMR studies. Our results show that the surface propensity concluded from SFG spectroscopy does not necessarily correlate with interfacial adsorption concluded from thermodynamic measurements.

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The effects of ions on the molecular-level composition and organization of water and ions at aqueous interfaces are relevant for biology as well as atmospheric and solution chemistry. There is ample evidence from surface spectroscopy that the sign of the ionic charge is important 1-6, in spite of the fact that if water is viewed as a dielectric continuum, there is no reason to assume that positive and negative ions would have different effects. To investigate this asymmetry, simple monoatomic ions have been investigated in some detail both experimentally and theoretically 7-15. However, the simplest ions do not exhibit a strong surface propensity, making it difficult to study the effect of the charge at the interface. Therefore, to examine these effects in detail, the complex ions tetraphenylarsonium (Ph4As+) and tetraphenylborate (Ph4B-) have often been used. While carrying charges of opposite sign, they have a very similar molecular structure and volume. Due to their similar chemical structure, the bulk thermodynamic properties of the cation are very similar to those of the anion. These two ions have been therefore often used as a reference electrolyte for determining the bulk free energy and enthalpy of transfer 16-19. However from the point of view of the surface, the evidence that the hydration structure of the two ions is different

4, 16, 19

suggests that the sign of the charge does matter. Recently, Scheu and co-

workers4 explored the interactions between water molecules and Ph4As+ and Ph4B- ions at the water-oil interface. Sum-frequency scattering (SFS) spectra of the ions Ph4B- and Ph4As+ at the water-oil interface were found to be very different. Moreover, the positive ion reduced the secondharmonic scattering (SHS) signal dramatically, while the negative ion enhanced the SHS signal. Based on this observation, Scheu et al. concluded that the ordering of water at the water-oil interface is strongly reduced by the positive ion, while the negative ion enhances the ordering of water. This study provides a microscopic picture of these ions at the specific water-oil interface that is very different for the two ions. However this behavior cannot be extrapolated to the waterair interface and it is not clear what the implications of the reported changes in microscopic structure are for the surface thermodynamics of such aqueous interfaces. If the entropy of the



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water near the interface is reduced due to an enhanced ordering, one would expect this to show up in e.g., the value of the surface tension.

Table 1: United electrostatic potential (ESP) charges based on DFT calculations for the two ions Ph4B- and Ph4As+, in units of the elementary electronic charge e. ‘Centre part’ represents the As/B together with the four C atoms neighbouring to the As/B atom. ‘Benzene rings 1-4’ involve the charges for the remainder of the four benzene rings, reported for each ring. As

B

Centre part

0.18

0.38

Benzene ring 1

0.24

-0.34

Benzene ring 2

0.17

-0.36

Benzene ring 3

0.22

-0.24

Benzene ring 4

0.18

-0.43

Quantum-chemistry calculations demonstrate that the electronic structures of these two ions differ substantially. In this calculation, we optimized the molecular structures of the ions and calculated the electrostatic potential (ESP) charges. The details of the calculation can be found in the Supporting Information. The group charges, obtained by considering the core part (ion and neighbouring four carbon atoms) and the four benzene rings (excluding the carbon atom neighbouring the ion), are listed in Table 1. First of all, one can see that the charges on the four benzene rings are not identical, reflecting that the symmetry of the molecule is broken due to steric effects. Moreover, the sign of the charges on the phenyl rings are opposite for the Ph4B- and Ph4As+ ion and the charges on the benzene rings are appreciably larger for the Ph4B- ion compared to the Ph4As+ ion. This result agrees with previous quantum chemistry calculations using the 4 ACS Paragon Plus Environment

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Mulliken charges 20. The smaller charge of the Ph4As+ ion on the ring makes the H-π interactions weaker compared to Ph4B-. Therefore it is energetically less favourable for the Ph4As+ ion to stay in bulk water than it is for the Ph4B- ion, which is in agreement with experiments 4. This calculation is also in qualitative agreement with NMR data. Figure 1 shows the NMR spectra for both salts dissolved (10 mM) in water. The chemical shift for the protons on the benzene ring of Ph4B- is high field shifted due to the higher electron density in the aromatic ring system, while the protons of Ph4As+ are shifted to low field. Moreover, the shift for the para proton, with respect to benzene without any substituents, of Ph4B- is slightly larger than that of the para proton of Ph4As+, indicating the larger (absolute) charges for Ph4B-.

Figure 1: NMR spectra of benzene, 10 mM sodium tetraphenylborate, and 10 mM tetraphenylarsonium chloride in water at 25 °C.



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Thus, surface-sensitive spectroscopy, quantum chemistry calculations and NMR suggest that the two ions can behave differently close to a water surface. In this Letter, we elucidate the effects of the sign of the ionic charge on the thermodynamics of the water surface with surface-tension measurements in the presence of Ph4As+ and Ph4B- ions, and compare it to interface-specific sumfrequency generation (SFG) results. The surface tension measurements allow investigating the surface activity at the water-air interface. Recently, it was shown that the presence of the borate salt decreases the interfacial tension between oil and water and the surface tension at water-air interfaces

18

, contrary to a more usual salt such as NaCl, which increases both tensions. This

suggests that the bulky organic ion acts as a weak surfactant at both interfaces, and allows to demonstrate that the ion adsorbs at the interfaces 18. The variations of the surface tensions, measured using a Kruss® Easydrop apparatus, of the waterair interface upon changing the bulk ion concentration are plotted in Fig. 2. The surface tension is lowered with increasing Ph4As+ and Ph4B- ion concentrations. We have also measured the temperature dependence of the surface tension, which gives the surface entropy. We find that to within the experimental resolution, the decrease of the water air surface tension in the presence of the salts is identical to that of pure water (data not shown). Therefore this measurement does not yield any additional information on the thermodynamics of the solution-air interface. The Gibbs adsorption equation for dilute solutions dγ/dln(c/c0) =- RT Γ directly links the variation of the surface tension γ with salt concentration c (c0 being a reference concentration) to the adsorption Γ; here R is the gas constant and T the temperature. The observation that the surface tension decreases shows that both salts adsorb to the water-air interface, and more so if the bulk concentration increases; we find that Γ goes from 9.6 × 1016 m-2 for 1 mM concentration up to 1.2 × 1018 m-2 for the highest concentration probed here. The latter corresponds to a surface area of ~1 nm2/molecule, showing that the surface coverage is rather complete. Remarkably, the variations of the interfacial tensions with concentration are identical within the experimental error 6 ACS Paragon Plus Environment

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for the positive and negative ions, showing that the adsorption of the two ions is also identical: thermodynamically speaking it is impossible to distinguish between the effects of the two salts at the air-water interface.

Water/Air interfacial tension (mN/m)

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72

64

56 -3 10

-2

10

-1

10

Concentration (mol/l)

Figure 2: Air/water interfacial tension for the solutions of sodium tetraphenylborate (Ph4BNa, closed red symbols) and tetraphenylarsonium chloride (Ph4AsCl, open black symbols).

To gain molecular-level insight into the surface propensity of the ions, we measured SFG spectra in the frequency region from 2600 to 3400 cm-1. The SFG measurements were performed at the planar water-air interface. An infrared (~ 3 µm) and visible (800 nm) laser pulse were combined at the interface and the generated sum frequency light was detected (details of the set-up can be found in the Supporting Information and in Ref. 21). As this is a second-order nonlinear process, it is forbidden in centro-symmetric media such as bulk water, making it surface-specific and sensitive to the amount of order in the interfacial region.


1.2

a

b

Ph4AsCl

Ph4BNa

0.6

1.0

50 mM

50 mM

25 mM

0.8

25 mM 0.4

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c 1

8 6 4 2

0.1

Ph4AsCl Ph4BNa

8

10 mM

0.001

0.6

5 mM 0.4

5 mM 0.2

2.5 mM 2.5 mM

0.2

1 mM 0.0

1 mM 0.0

d 1

8 6 4 2

0.1

2600 2800 3000 3200 3400 3600 2600 2800 3000 3200 3400 3600 -1 -1

IR frequency (cm )

0.01

Concentration (mol/l)

10 mM

Amplitude CH (a.u.)

SFG intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Amplitude CH (a.u.)


IR frequency (cm )

Ph4AsCl Ph4BNa

8 8 9

10

17

2

3

4

-2

5

6 7 8 9

10

18

Γ (m )

Figure 3: Concentration-dependent SFG spectra of (a) Ph4AsCl and (b) Ph4BNa at the air-water interface. All spectra were recorded in ssp polarization combination. Note the ~2-fold difference in the scales of the ordinate axes between (a) and (b). The solid lines represent fits based on a Lorentzian model described in the text and the SI. Spectra are offset for clarity. Amplitude of the CH stretch peak (c) as a function of the bulk concentration and (d) as function of surface concentration.

The SFG spectra (Fig. 3a) for both salts at various concentrations are depicted in Fig. 3a. The spectra show a sharp peak (dip) at ~3060 (3040 cm-1) for the solutions of Ph4As+ (Ph4B-), together with a very broad feature at ~3200 cm-1. The spectral intensity increases with increasing salt concentration. The vibrational signatures arise, respectively, from the C-H stretch of the phenyl rings at the interface and the O-H stretch of interfacial water molecules. The 20 cm-1 difference in the C-H stretch frequency for the anion and cation is observed in bulk Raman spectra as well (see


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Supporting Information). The very weak band around 2950 cm-1 might, in analogy to Raman spectra for the benzene/air interface

22

, be assigned to a combination or overtone band. Another

potential explanation could be ion pairing 23. In contrast to Ref. 4, which reported SFG scattering experiments, no strong SFG signal for the Ph4BNa solution is observed here at ~2990 and 3060 cm-1. Interestingly, although the molecule is centrosymmetric, still an SFG signal for the C-H stretch is observed, which arises most likely from the broken symmetry of the vibrational response due to its half adsorption at the air-water interface. Indeed, also for liquid benzene an SFG signal has been observed due to symmetry breaking at the benzene-air interface and/or due to the presence of a quadrupole contribution to the signal

24-26

. The appearance of the C-H stretch as a

dip or a peak on the background of the O-H stretch vibrational intensity centred at 3200 cm-1, can be traced to interference between the C-H and O-H contributions to the susceptibility. When the transition dipole moments (or, equivalently, the C-H and O-H vector) are pointed in the same direction relative to the surface plane, the two signals interfere constructively, and the C-H response appears as a peak on top of the O-H response. This is the case for the Ph4As+-covered surface, for which the O-H groups of interfacial water are pointing down 27-28 into the bulk, as are the SFG-active C-H modes. For the Borate salt, the O-H groups point up

27-28

towards the

negatively charged ions at the surface, while the orientation of the C-H modes remains unaltered 24, 26

; in this case, destructive interference is observed. Thus, although on first thought one might

conclude that overlapping bands (CH and OH vibration) might be problematic, the interference gives additional information on the relative orientation of C-H and O-H groups, yet not on absolute orientation. Phase-resolved SFG experiments, which give information about the absolute orientation, confirm the opposite orientation of the water band for the two phenyl salts (see Supporting Information). In principle the SFG signals observed for the salt solutions could originate not only from the second-order signal from the interfacial water molecules, but also from bulk third-order 9 ACS Paragon Plus Environment


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contributions which can become significant due to the DC field caused by the charged molecules at the interface.29-30 In case the signal is fully determined by this third-order contribution, the opposite sign of the water band could originate from the opposite field and not from the different orientation of the water molecules. Note, however, that the shape of the SFG spectrum does not change from 1 to 50 mM (Fig. 3). Over this concentration range, the Debye length and thus the third-order contribution changes by roughly a factor of 10, and we therefore conclude that the second-order contribution must dominate the signal. The water molecules thus most likely have an opposite orientation for the oppositely charged ions. This conclusion is also consistent with recent simulations of the second-order contributions to the SFG signal for water underneath charged surfactants.31 Remarkably, the change of orientation of the water molecules has no repercussions on the thermodynamics of the air-water interface. Qualitatively, it is apparent that the signals, for both the C-H and the O-H stretch modes, are approximately twice as large for Ph4AsCl than for Ph4BNa solutions. To quantify the variations of the SFG amplitude with varying concentration, we describe the spectra using three Lorentzian resonances, where one Lorentzian represent the C-H stretch mode and two represent the lowerfrequency (~3200 cm-1) and higher-frequency (~3400 cm-1) O-H stretch modes

21, 32

. We use the

same frequency and width of the two OH bands for the positive and negative salt. The width of the CH vibration is also kept constant, while the frequency is allowed to vary in the fit as we observe a 20 cm-1 shift with Raman spectroscopy from the bulk solutions. Further details are given in the supporting information. Overall, as plotted in Fig. 3b, indeed an approximately two-fold lower amplitude is observed for the C-H stretch vibration of the borate salt compared to that of the arsenide salt, while for both salts the amplitude increases as function of bulk concentration. Moreover, up to 50 mM the total O-H amplitude also increases with ion concentration and is roughly twice lower for Ph4B-. The variation of the surface tension with concentration of Fig.2 allows us to transform the bulk concentration in the (thermodynamic) surface concentration Γ by 10 ACS Paragon Plus Environment

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taking the local slope of the surface tension vs. ln(c/c0) curve and compare the SFG signal directly to the measured adsorption. We then observe that the amplitudes of the C-H stretch SFG features increase with increasing adsorption for both the Ph4As- and Ph4B+ solutions (see the inset of Fig. 3b), although the C-H amplitude of the arsenide salt is always higher than the borate salt. Assuming a similar SFG activity for both ions rationalized by the same molecular structure, the different amplitudes for the C-H peak can be attributed to a different number of molecules that contribute to the SFG signal. Given the similar probing depth of ~5 Å for a wide variety of samples in SFG

33-35

and the similar diameter of the Ph4As- and Ph4B+ ion (~6 Å), we expect a

probing depth of ~5 Å for both ions. The larger amplitude of the Ph4As+ implies than that the SFG-active surface ion concentration is higher for Ph4As+ than for Ph4B-. The latter finding appears, at first glance, to disagree with the finding from the surface tension measurements that the adsorption of the two ions is identical. However the SFG measurements probe the concentration at the interface, whereas the adsorption obtained from surface tension is defined as 0

the spatial integral over the difference between bulk and surface concentration: Γ =

∫ dz(c( z) − c)

−∞

where c(z) gives the concentration profile, c is still the bulk concentration, and z=0 is set to the location of the Gibbs dividing surface. The conclusion must be that the concentration difference between the surface and the bulk extends over a larger length scale than is probed by the SFG measurements. The tendency of a molecule to be at the interface may in fact be used as a definition of the surface propensity: the propensity is then higher for the Ph4As+ ion, giving a higher SFG signal; however the integrated adsorption at the interface is identical to that of the Ph4B- ions. The free energy of adsorption is also the same for the two ions: We have estimated the Gibbs free energy of adsorption

∆ீ° ௞்

(with kT is the thermal energy following Ref. 36) as:

∆‫ܩ‬° 1 ݀ߨ = −݈݊ ൤ ൬ ൰ ൨ ݇ܶ ݇ܶܽ ݀ܿ ௖→଴ 11 ACS Paragon Plus Environment


where ܽ is a molecular size, c the bulk concentration and ߨ the surface pressure. The derivative on the right hand side can be readily obtained from the data in Fig.2; we then find

∆ீ° ௞்

≈ 10 for both

salts. This shows that the salts are poor surfactants, as expected; for normal surfactants typical values are around 40.

36

The difference between the salts and 'real' surfactants comes from the

larger affinity of the surfactants to go to the surface; this is due to the difference in entropy and interactions (enthalpy) between molecules at the surface and the same molecules in the bulk. 37

50 Interfacial tension (mN/m)

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40

30 Red: Ph4BNa Black: Ph4AsCl equilibrium Blue: Ph4AsCl non equilibrium

20 -4 10

-3

10

-2

-1

10 10 Concentration (mol/l)

0

10

Figure 4: Oil/water interfacial tension for different ionic solutions as a function of the concentration. (Triangles: heptane; Circles: hexadecane; Squares: undecane).

To see whether the oil-water interface behaves differently from the air-water interface, we now focus on the former. The variations of the interfacial tensions are plotted in Fig. 4, which shows that the behaviour of the two salts is no longer the same at water-oil interfaces, allowing for sufficient equilibration of the system. The surface tensions with the C7, C11, and C16 oils are the


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same within the experimental accuracy, indicating that the length of alkyl chains does not affect the surface activity of the ions and that there are no effects on the surface tension of having odd or even alkane carbon numbers. The lines for the two different ions are almost parallel, implying that the adsorption is roughly the same for both alkanes; we find Γ= 1.2 ±0.2 1018 m2 for the Borate salt, and Γ= 0.9±0.2 1018 m2 for the Arsenide salt. However, the interfacial tensions of the Ph4As+ and Ph4B- ions differ by ~7 mN/m. This difference can be accounted for by taking into consideration the different solubility of the two salts in alkanes. In fact, the Gibbs adsorption equation is derived by equating the chemical potentials of species at the surface and in the bulk, so that a change in bulk solubility can cause a shift of the surface tension. That this is indeed the case follows from a measurement of the interfacial tension of an aqueous solution of the Ph4As+ ion in contact with heptane (blue symbols in Fig.4). Before equilibration of the bulk phases, the interfacial tension is identical to that of the Ph4B- system. However upon equilibration, i.e. allowing the ion to dissolve into the hydrophobic phase, we observe a decrease of the interfacial tension of ~7 mN/m: the difference between the two results thus clearly originates from a difference in bulk solubility, whereas the surface excess is in fact very similar. In summary, the water-air tension is identical for the two salts, whereas the difference in oil-water tension change is due to a difference in bulk solubility; for both interfaces the surface excess of ions is identical for the two salts. The SFG spectra, however, show a different amount of SFGactivity for the Ph4As+ and Ph4B- ions at the water-air interface. The combination of these measurements demonstrates that the sign of the charge does not greatly affect the interfacial water properties, other than the water orientation. For the water-air interface the surface tension lowering is identical for the two salts. The difference in ion-induced change in surface tension at the wateroil interface is not due to a difference in adsorption, but rather due to a difference in bulk solubility in the oil. This effect is not present at the water-air interface, since the ions cannot go into the air. Thermodynamically, the difference in surface tension at the water-oil interface is then 13 ACS Paragon Plus Environment


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due to a difference in chemical potential in the bulk, as can be derived by equating the chemical potentials at the surface and in the bulk. The importance of ion dissolution into the hydrophobic phase is nicely illustrated by experiments for which the bulk oil phase is not equilibrated with the salt solution. In this case, the interfacial tension is again identical between the two salts, which demonstrates our point: the difference between the two salts in the interfacial tension with the oil is caused by a difference in bulk solubility. From the quantum chemical calculations, we concluded that the smaller charge on the ring in case of the Ph4As+ ion makes the H-π interactions weaker, and therefore it is energetically less favourable for the Ph4As+ ion to stay in the bulk water than it is for the Ph4B- ion. This can explain both (i) the larger solubility in the alkane phase, and also (ii) the increased propensity at the surface, i.e., our SFG observation of more intense C-H stretch intensity for the Ph4As+ solution at the air-water interface. In the experiments of Scheu et al. 4 the SHS signal is very small and not changed upon varying the Ph4As+ ion concentration, unlike for Ph4B-, for which a continuous increase is observed. These observations were attributed to different water organization at the solution-oil interface. Our data demonstrate that the interfacial water SFG signals for both solutions increases with the concentrations and the difference of the SFG signal arises not primarily from the destructive/constructive interferences between the H-π interactions and electrostatic interactions, as concluded from the SHS data 4, but from the different solubility of the ions due to the delocalized/localized nature of the electronic structures of the ions. The SHS data apparently not only report on the water organization, but likely also on the asymmetric hyperpolarizability of the tetraphenyl moiety induced by the asymmetric environment at the interface. Also, the organization of the ions and the water may be structurally different for the nanodroplet interfaces from those of the planar interfaces. In conclusion, surface-sensitive spectroscopy gives detailed information about the structure of the water interface at very small scales. However the thermodynamic adsorption can be the same for 14 ACS Paragon Plus Environment

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the two salts that have different spectroscopic signatures at the same interface. In general, from the spectroscopy measurements the surface propensity is deduced, and often interpreted to be equal to the surface density, i.e., the adsorption. Our combined thermodynamic and spectroscopic measurements show that propensity and adsorption can be different. This is an important conclusion, since discussions on effect of the surface propensity of different charges on the stability of the interface suggest these would lead to different surface tensions. Our thermodynamic measurements of the surface tension show that this effect, if present, is too small to be observable experimentally.

Acknowledgement We thank J. Hunger and S. Roke for useful discussions.

Supporting Information. Experimental details, additional NMR and SFG results

References (1)

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