Combined Sum Frequency Generation and Thin Liquid Film Study of

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

A Combined SFG and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on Interfacial Water Structure Afshin Asadzadeh Shahir, Khristo Khristov, Khoi Tan Nguyen, Anh V. Nguyen, and Elena D. Mileva Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00648 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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A Combined SFG and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on Interfacial Water Structure a

b

a,c,*

Afshin Asadzadeh Shahir , Khristo Khristov , Khoi Tan Nguyen b Mileva a

a, , Anh V. Nguyen *, Elena

School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

b

Institute of Physical Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 11, Sofia 1113, Bulgaria c

School of Biotechnology, International University, Vietnam National University, Ho Chi Minh City, Vietnam

Abstract Some salts have been recently shown to decrease the SFG (sum frequency generation spectroscopy) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here we report a similar trend for the chloride salts of monovalent cations, i.e., LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration-dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. While the former was dominant only at low concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broadband intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the 1 ACS Paragon Plus Environment

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interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye – Hückel lengths using the TFPB technique. Keywords: Ion Specificity, Sum Frequency Generation Spectroscopy, Ion Adsorption, Water SFG Spectrum, Ion-Water Interactions

* Corresponding author: [email protected] or [email protected]

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1. Introduction It is well known that both surfactants and inorganic salts can present at the surface of aqueous solutions and interact in a complicated manner microscopically. In the case of insoluble monolayers of alcohols, the recent studies show that sodium cation of its halide salts positively adsorbs specifically by the interaction with the dipole moment of the insoluble monolayer1. The case of soluble surfactants is more complex because the soluble surfactants can be desorbed, and both water molecules and salt ions can penetrate into the adsorption layer, thus making significant changes in the physicochemical properties of an air/water interface. Understanding these changes is of great importance in explaining many interfacial phenomena in the realm of industrial and atmospheric surface chemistry.2, 3 The first step towards the applications is to establish how ions interact with and are distributed across the air/water interface. The observation of water surface tension increment by the addition of inorganic salts4 has attracted great attention of the community to the surface structure of electrolyte solutions for over a century. According to the classical thermodynamic model of the interface, the depletion of ions from water surface could have been the main reason for the observed increase in water surface tension. However, the cause of ion depletion remained under debate until Onsager and Samaras came up with the idea of image forces5, which was based on Wagner’s theory of dielectric discontinuity at interfaces.6 According to the Wagner-Onsager-Samaras (WOS) theory, ions are depleted from the air/water interface under the influence of repulsive forces between ions and their images5,7,8 whose interaction potential, ∆Wi ( x ) , can be improved by taking into account the factor of permittivity to give qi2  ε − ε '  exp ( ai / λ )  2x  ∆Wi ( x ) =  ⋅ ⋅ exp  −   ' 1 a / 16 x ε ε λ πε ε + + ( i )    λ  0

(1)

where ε0, ε and ε ' are the vacuum permittivity and the dielectric constants of water and air, respectively, ai and qi are the radius and charge of ion “i”, x is the distance from the interface, λ is the Debye – Hückel length being defined as follows: 1/2





λ =  εε 0 kBT / ∑ ci qi2  

i

(2)



where kB and T are Boltzmann constant and temperature, respectively. ci is the concentration of ion “i”. Although this theory was believed to have successfully explained the surface


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tension increment of dilute salt solutions, later reports of few controversial observations revealed some fundamental shortcomings of the WOS theory. As implied by Eqs. (1) and (2), image forces should be progressively screened by increasing electrolyte concentration until being faded out at high ionic strengths. Therefore, the strongest ion depletion should happen at low electrolyte concentrations. Nevertheless, Jones and Ray reported an initial decrease in the surface tension of salt solutions at about 2 mM, which would suggest the propensity rather than depletion of ions at air/water interfaces.9, 10 1600

20

Li Na Cs Cl

1200

15

ΔWi/kBT

1400

1000

ΔWi/kBT

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

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10 5

800 0 5

600

6

7

8

9

10

x (Ǻ)

400 200 0 0

1

2

3

4

5

6

7

8

9

10

x (Ǻ) +

+

+

-

Figure 1. Interaction energies of Li , Na , Cs , and Cl ions with their images versus their distance x from the air/water interface as calculated using Eq. (1) for 0.1 M salt solutions.

Another paradoxical observation was the specific effect of ions on the water surface potential (and surface tension).11 Although Eq. (1) accounts for ion specificity through ionic radii, ai, the ion-image interaction potential is almost the same for all ions at low electrolyte concentrations where the Debye–Hückel length is considerably greater than the ionic radii. It +

+

+,

is demonstrated in Fig. 1 where the calculated potentials for the interaction of Li , Na , Cs -

and Cl with their image forces in 0.1 M solutions of the corresponding chloride salts are compared. Obviously, the WOS model failed to describe the ion specificity and even predicted no difference in the distribution of anions and cations at the water surface. Thus, neither was it able to explain the observed surface potential of electrolyte solutions. Volkov and Markin employed a modified model of ions with finite radii to show that various ions have different distributions at the water surface.12 Larger ions were found to be repelled less than smaller ions resulting in a charge separation. This charge separation was then discussed to be responsible for the observed surface electric field whose direction was determined by the sign of the larger ions. A similar explanation was also given by Randles who proposed a solute-free water layer of 3 – 5 Å in thickness which was not accessible by cations but by anions.13 He also assumed that some adsorptive forces must exist which act on anions and 4 ACS Paragon Plus Environment

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oppose the repulsive image forces at the water surface, even leading to the adsorption of some anions at the surface layer. An inevitable consequence of such a model is the spatial separation of anions and cations with anions lying closer to the water surface and giving rise to the reported negative surface potential of the corresponding salt solutions. Randles suspected that the proposed adsorptive forces were related to the water structure at surface and solvation shells of ions.13 The first clues to the nature of these attractive forces were given by Ninham et al.7 who believed that dispersion forces were responsible for ion specificity. The dispersion potential, U Disp ( x ) , is directly related to the ionic polarizability, and hence, are inherently ion-specific. It is described by the following equation:7

U Disp ( x ) =

hω B 2 , B ≈ ( nw2 − nair ⋅ α * ( 0) ⋅ i ) 3 x 8

(3)

where n is the refractive index of the water phase and α * ( 0 ) is defined as the static excess polarizability of ion in water. Ivanov et al. later employed the dispersion theory (on ionsurface attraction) to calculate the specific adsorption energies, u0 k BT , of ions on water -

-

surface (see also Table 2).14, 15 In general, anions of bigger radii such as I , Br , and to some -

-

extent Cl , have more affinity for the water surface than smaller anions like F and cations. At high concentrations of sufficiently large and polarizable ions, the dispersion forces are strong enough to dominate the screened electrostatic forces, resulting in the adsorption of these ions even at the so-called “depletion layer”. This model of ion adsorption also requires partial dehydration and deformation of the solvation shell, which might be tolerated by large polarizable anions because of their weaker solvation. Strongly-solvated smaller anions and cations would, however, not prefer dehydration.16 In addition, their smaller polarizability leads to weaker dispersion forces and their stronger depletion from the surface compared to the polarizable anions. The overall effect of these surface forces leads to an interfacial charge separation in which the polarizable ions stay closer to water surface with their smaller counterions lying beneath. This general picture was later confirmed to a large extent by the advanced computational and experimental surface-specific techniques. Among the most helpful and reliable MD simulations of sodium halide solutions are the works of Jungwirth et al.17, 18 who -

-

employed polarizable force fields and found a significant surface propensity for I , Br ions +

followed by a well-separated layer of Na ions right beneath as predicted by the theory of ion 5 ACS Paragon Plus Environment

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adsorption. The polarizable anions were observed even to penetrate the uppermost water -

layer, known as the “depletion layer”. In the case of Cl ions, their surface propensity for the so-called depletion layer was, however, moderate, while a depletion of both anion and cation was found for NaF solutions. The advent of non-linear optical techniques such as second harmonic generation and sum frequency generation (SFG) spectroscopy opened up new opportunities to scrutinize the effect of such a specific distribution of ions on the organization and hydrogen-bonding network of the interfacial water.19,

20

SFG is a relatively new vibrational spectroscopy

technique that is inherently capable of distinguishing the interfacial molecules from those within the bulk. According to the SFG-activity rules, SFG signal is not detected from the medium with centrosymmetry. Since the centrosymmetry of bulk solution is broken at the few topmost layers of fluid interfaces, the recorded SFG signal is believed to have the contribution from only the interfacial molecules. This makes SFG one of the most suitable techniques to investigate the architecture of interfaces. In the presence of intense electrical fields of two temporally and spatially overlapping laser beams, i.e., IR and visible, a dipole moment is created within interfacial molecules. The oscillation of this induced dipole gives rise to an SFG signal whose frequency is equal to sum of the frequencies of two incident laser beams, i.e., ωSFG = ω IR + ωVis , and its intensity, I SFG , is proportional to the incident intensities, I IR (ω IR ) and IVis ( ωVis ) :21 2

( ) ( ) I SFG (ωSFG ) ∝ χ eff , NR + χ eff , R I ' IR ( ωIR ) IVis ( ωVis ) 2

2

(4)

(2) where χ eff is the non-resonant part of susceptibility and is small for most of the fluid , NR (2) interfaces. The resonant susceptibility, χ eff , is defined as: ,R

χ eff( 2 ), R = ∑ q

Aq

(5)

ω IR − ωq + i ϒ q

where Aq and ω q are the amplitude and frequency of the vibrational mode “q”, respectively. (2) is convertible to the ϒq denotes the damping constant of the qth vibrational resonance. χ eff ,R

second-order non-linear susceptibility, χijk( 2) through transmission Fresnel factors of all laser beams at the interface. χ ijk( 2) , which is defined in the laboratory coordinates of i, j and k, is proportional to both number density of the oscillators that contribute to the SF signal, N, and


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the molecular hyper-polarizability averaged over all molecular orientations, β lmn , at the interface:

χ ijk( 2) = N ∑ µijk :lmn β lmn

(6)

lmn

where the angle brackets describe the orientationally-averaged Euler angle transformation from the lab coordinates to the molecular coordinates, i.e., l, m, and n. According to Eqs. (4), (5) and (6), the SF intensity has contributions from both the number and the orientation of interfacial molecules. A variation in either of these contributors will also change the signal intensity. In addition, when ωIR is equal to the frequency of one of the SFG-active vibrational modes of the molecule, an increase in the SF intensity will be observed. Therefore, a vibrational spectrum of the interfacial molecules can be obtained by simply scanning the IR frequency through a range, e.g., 3000 to 3800 cm-1 of the OH stretches. To our knowledge, the SFG spectrum of neat water was first reported by Shen’s group suggesting some degree of molecular ordering at the water surface.22 Water spectrum has two main features. One sharp and well-separated band around 3700 cm-1 and one broadband spanning from 3000 to 3600 cm-1. The former is attributed to the free OH stretch of the water molecules at the depletion layer, while the latter is attributed to the stretches of the hydrogenbonded water molecules. A detailed discussion of peak assignments of OH stretches is provided in Section 3.2. Briefly, strongly-bonded (ice-like) and weakly-bonded (liquid-like) water molecules are believed to be represented by two overlapping peaks centered around 3200 cm-1 and 3400 cm-1, respectively. Analyzing the SFG spectrum of neat water provides valuable information about the microscopic architecture of the water surface. Besides, the community has recently shown great interest in exploiting the water SFG signal as an indirect probe to track the specific adsorption of individual ions at air/water interfaces and its impacts on the interfacial hydrogen-bonding network. The SFG measurements performed by numerous groups show a consensus on the general picture of the electrolyte solution surfaces, despite some inconsistencies in the findings and interpretations, few examples of which are discussed below. Raymond and Richmond recorded SFG spectra for 0.9 M NaF and 1.7 M NaCl, NaBr and NaI solutions.23 The change of interfacial hydrogen bonding with the anion type indicated the presence of polarizable anions in the interfacial region. The vibrations attributed to the tetrahedrally hydrogen-bonded water molecules were found to lose intensity and -

-

-

redshift in the presence of F , but gain intensity and blueshift in the presence of Br and I . 7 ACS Paragon Plus Environment

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-

With Cl , the water spectrum changed only slightly. They interpreted this observation as a consequence of the kosmotropic-chaotropic effect of these anions on the surrounding water network. A similar intensity drop was also reported by Nguyen et al. for 1 - 10 mM solutions of NaF.24 Also, for the same concentrations of NaBr, the broadband was found to -

-

significantly lose its intensity, in spite of the fact that Br , unlike F , is a chaotropic ion. Their observation for NaCl solutions was even more surprising. For 1 – 10 mM NaCl solutions, the SFG intensity of the broadband decreased in a similar way, but as soon as the salt concentration increased to 2 M and 4 M, the intensity was recovered and became even slightly larger than that of neat water. The decrease in the broadband intensity of water SFG spectrum at low salt concentrations has also been reported by others.25, 26 Allen et al. recorded the SFG spectra for 0.8 M NaF solution and 0.8 M and 2 M solutions of NaCl, NaBr and NaI.19 Contrary to the other works, the SFG spectra did not vary much for 0.8 M solutions of NaF and NaCl. Referring to the previous MD simulation findings, they showed that the repulsion of both anions and cations from interface could be the reason for the unaffected hydrogen bonding network in the presence of NaF. Their reasoning for the negligible effect of NaCl was, however, quite uncommon, as they attributed this to a potential reduction in the polarizability of chloride ion within the interfacial region. In 0.8 M and 2 M solutions of both NaBr and NaI, the peak intensity around 3200 cm-1 decreased only slightly, while the band around 3400 cm-1 was enhanced with salt concentration, suggesting the significant disturbance of interfacial hydrogen bonding network by polarizable anions, as claimed by the authors. An increment in the interfacial depth, due to an increase in the number of water molecules contributing to the SFG signal, was also given as another reason for the observed -

-

intensity enhancement in the presence of more polarizable anions, i.e., Br and I . Allen et al. ruled out the possibility of an enhancement in the interfacial ordering of water molecules. They believed that solvation of ions would randomize the water molecules orientation causing a loss of SFG intensity at a constant interfacial depth. Their following measurements using phase-sensitive SFG spectroscopy revealed that the orientation of interfacial water molecules could be influenced by an electrical double layer (EDL) created by the adsorption of ions at the water surface.27 The formation of a surface electric field and its effect on the interfacial water structure has also been evidenced by other researchers who claimed its magnitude and direction depends on the charge separation originated from a difference in the relative polarizabilities of the component anions and cations.28


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Being influenced by the traditional, thermodynamic view that supposes the complete depletion of cations from the surface, the majority of researchers have focused on the specific effect of anions. The specific effect of cations on the interfacial water structure has become the topic of only a few recent works. Wang et al. recorded the SFG spectra for 0.2 M, 0.5 M and 0.94 M solutions of NaF as well as 0.2 M, 0.5 M, 2 M and 6 M solutions of KF.26 In the case of NaF, the intensity of broadband decreased considerably with salt concentration, implying a concentration dependency of the SFG signal. Very slight (or even no) difference from neat water spectrum was found for 0.2 M and 0.5 M solutions of KF. The spectrum completely changed as KF concentration was increased to 2 M and 6 M though. Despite the clear specific effect of cations on hydrogen–bonding network, its origin was not clarified appropriately. Allen’s group studied the cation-specific effect in 2 M solutions of LiCl, NaCl, KCl, and NH4Cl using two different SFG spectroscopy techniques.27 The conventional SFG spectra demonstrated a remarkable perturbation of interfacial water structure by LiCl and NH4Cl, but only a slight change in neat water spectrum by NaCl and KCl salts. Nevertheless, PS-SFG measurements revealed that interfacial water molecules oriented their transition dipole moments towards the air phase. They interpreted this as the influence of a surface electric field created by the adsorption of anions lying above cations. The magnitude of this electric field showed cation specificity of the order Li + ≈ Na + >NH 4+ >K + . Apart from the inconsistencies discussed above, the review of the broader literature evokes the idea that the SFG response of interfacial water molecules to the presence of ions should be interpreted in relation to not only the ion specific effect but also the concentration dependence. We hypothesize in the present work that interfacial water structure is influenced by two main factors, namely, the direct ion–water interactions in and beyond the solvation shell and water–EDL interactions caused by interfacial charge separation. Both factors can be significantly affected by electrolyte concentration. In addition, neither the origin of such concentration dependence nor the mechanism of cation-specific effect is well understood yet. In this regard, the aim of the present work is to investigate the specific adsorption of structureless cations at air/water interfaces and its impacts on the interfacial water structure as well as its dependence on the electrolyte concentration. First, we employed thin film pressure balance (TFPB) method to determine the electric potential at the surface of the thin liquid films stabilized by a low molecular weight alcohol, methyl isobutyl carbinol (MIBC). This alcohol forms a highly ordered layer of molecular dipoles which creates a moderate electric potential at the water surface.29 This enabled us to identify a remarkable specific screening 9 ACS Paragon Plus Environment

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effect of cations on the surface potential. In the second step, the SFG spectra for the solutions of LiCl, NaCl and CsCl with concentrations ranging from low (50 mM and 0.5 M) to medium (2 M) were recorded in the OH stretch region, i.e. 3000 – 3800 cm-1. The spectra were interpreted carefully in order to understand the specific and concentration–dependent effect of cations on interfacial water molecules that are involved in various hydrogen-bonding environment. We also discussed the possible causes of these effects on the microscopic scale. The surface ionic capacitance model, proposed here in order to explain the mechanism of cation specificity, requires the identification of ion density and charge separation at the interfacial region. The former can be identified by TFPB measurements of surface electrical potential and Debye-Hückel length. This is covered in Section 3.1. The latter can be deduced from the SFG spectral analysis of the OH stretch region, as discussed in Section 3.3.

2. Materials and Methods 2.1. Materials Lithium, sodium, potassium and cesium chloride with purities of greater than 99% were purchased from Sigma-Aldrich (Sydney, Australia). The reason for choosing these salts is available in the Supplementary Information. Methyl isobutyl carbinol (MIBC, purity > 99%) was purchased from ACROS Organics (Belgium) and used as received. Solutions were made using deionized (DI) water with a resistivity of 18.2 MΩ.cm prepared using an Ultrapure Milli-Q unit (Millipore, USA). Solution contamination was checked through recording the SFG signal in the C-H stretch region from 2800 to 3000 cm-1. In the case of organic contamination detected at the first time, salts were roasted prior to use at 550°C for 10 hours for further purification and the experiments were repeated until no contamination was detected by SFG. All glassware including the TFPB cell were decontaminated by immersing in an alkaline ethanol/water solution for 10 minutes and then rinsing with 1% hydrochloric acid solution and finally, flushing vigorously with DI water. All experiments were performed in a clean, temperature-controlled laboratory at 23±2 °C.

2.2. Thin Film Pressure Balance Measurements We measured the electric potential at 10 mM MIBC solution surface in the presence of different salts using a thin film pressure balance coupled with a micro-interferometer. This method has been developed and widely used to study the drainage kinetics and stability of liquid films in foams and emulsions.30,

31

In this method, an initial thin layer of liquid is 10

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created between two interfaces using a film holder. Because of the capillary pressure created at the menisci of the film surfaces, the film keeps draining until its thickness drops to about 200 – 300 nm.32 At this film thickness, both interfaces begin to interact through two major surface forces, according to the classic DLVO theory: the electrostatic repulsion and van der Waals attractions.33, 34 Non-DLVO forces are also thought to be effective at low surfactant coverage and small film thickness.35,

36, 37

Nonetheless, in the presence of high surfactant

concentration, the hydration surface force is weak for relatively thick films, and the overall force, which is called “disjoining pressure, Π”, is given by:

Π = Π el + Π vdw

(7)

If the electrostatic disjoining pressure is so strong that the overall disjoining pressure is greater than the capillary pressure, film drainage will stop at some equilibrium thickness. Otherwise, the film will rupture at the critical thickness of film rupture. For relatively thick planar films stabilized by non-ionic surfactants, the contribution of van der Waals forces is negligible and the electrostatic disjoining pressure can be calculated using the superposition approximation as follows:38

ln Π ≈ ln Π el = − ( h / λ ) + ln Π 0 (8) In Eq. (8), h is the film thickness and λ is the Debye – Hückel length. Π 0 is defined as:  eψ s  Π 0 = 64k BTCt tanh 2    4 k BT 

(9)

where e is the electronic charge, Ct is the total electrolyte concentration and ψ s is the surface potential. By experimental measurement of Π el using a TFPB and simultaneous microinterferometric measurement of h, a linear ln Π el - h plot can be obtained whose intercept will give the value of Π 0 , and thence, the value of surface potential, ψ s . The superposition approximation agrees well with numerical results (particularly under the condition of the constant surface charge interaction) for h / λ > 0.5 which is our condition. Fig. 2 depicts a schematic illustration of a TFPB setup. It is composed of a film holder within a closed, air-tight chamber. To measure the disjoining pressure in 0.1 mM salt solutions, a horizontally-positioned porous plate was used as the film holder. The film holder


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was connected to an external barometer. By manually changing the chamber pressure, i.e., Pg, the height of water in the side tube, hc, as well as the film thickness changed. Disjoining pressure was then calculated using the following equation: Π ≈ Π el = Pg − Patm + Pc − ∆ρ ghc

(10)

where Patm is the atmospheric pressure, ∆ ρ is the density difference between air and solution and g is the gravitational constant. Film thickness was calculated indirectly through microinterferometry. White light (I0) was shone onto the film and the reflected light intensity (I) was recorded in the form of interference patterns using an inverted microscope (Nikon EPIPHOT 200) coupled with a CCD camera (Canon PSA640). The recorded images were monochromated using the green filter (546 nm) and digitized by means of ImageJ in order to extract the relative values of the minimum ( I min ) and maximum ( I max ) intensities. Film thickness was then obtained from the following equation:39 h=

Λ 2π n f

 I − I min  lπ ± arcsin I max − I min 

  

(11)

where Λ and n f are the wavelength of the light and the refractive index of the solution, respectively. l= 0, 1, 2,... is the order of the interference which varies during film drainage. To measure the film drainage in 2 M salt solutions, a common “Sheludko – Exerova cell” was employed under atmospheric pressure instead of a porous plate. In all experiments, the chamber atmosphere was saturated with 3 mL of each sample solution and all connections were sealed very well in order to minimize the evaporation from the film surface. Initial films were left in the chamber for 30 minutes to equilibrate prior to each measurement.


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Figure 2. A schematic illustration of a thin film pressure balance (TFPB) apparatus

2.3. Sum Frequency Generation Spectroscopy In the SFG setup, the visible beam and the tuneable IR were overlapped spatially and temporally on the sample interface. The visible beam was generated by frequency-doubling the fundamental output pulses (1064 nm, 10 Hz) of 20 ps pulse-width from an EKSPLA (Vilnius, Estonia) solid state Nd:YAG laser. The tuneable IR beam was generated from an EKSPLA optical parametric generation/amplification and difference frequency system based on LBO and AgGaS2 crystals. The temperature and humidity of the lab were kept constant at 23±2°C and 66±2% to minimize the environmental errors. The geometry of the SFG set-up was the same for all measurements with the incident angles of the visible and IR beams equal to 60° and 54°, respectively. Before each measurement, laser alignment was carried out at 3200 cm-1, 3400 cm-1 and 3700 cm-1 in order to ensure an optimum SFG signal throughout the whole spectrum. All SFG spectra for water were recorded in OH region from 3000 to 3800 cm-1 under ssp polarization combination with the first, second and third letters showing the polarizations of SF, visible and IR laser beams, respectively. The samples were left in the sample holder for 5 minutes to equilibrate with the environment before starting the experiments. Prior to sample measurements, a water spectrum was recorded as a reference and compared to those recorded on different days of the experiments. Using a laser beam reflected from the interface, the solution level was constantly monitored and adjusted to the same height for all experiments. Fig. S1 of the supporting information shows some of the recorded spectra for neat water with excellent reproducibility. 5 – 10 spectra were gathered for each sample and the averaged spectrum was deconvoluted to its component peaks using Eqs. (4) and (5).

3. Results and Discussion 3.1. Specific adsorption of cations and its effect on surface electric potential Liquid films are unstable structures. Adsorption of surfactants at their interfaces can contribute to their stability by enhancing the repulsive disjoining pressure. Although MIBC molecules are non-ionic, they are still capable of creating a moderate potential difference between the gas phase and their adsorbed layer at the aqueous phase through their permanent 13 ACS Paragon Plus Environment

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dipole moments (Fig. 3). This electric potential has been measured to be about 150 – 200 mV,40 which is typical of the adsorbed layers of alcohols. When the potential is suppressed in the presence of electrolytes, the liquid film becomes unstable, as also demonstrated in Fig. (4a). Qualitatively, the repulsive disjoining pressure is reduced sharply but by different amounts upon adding various salts. Even for CsCl concentration as low as 0.1 mM, the specific suppression of disjoining pressure was so strong that no stable film was obtained and film rupture was immediate. The corresponding ln Π el - h plots and their linear fits to Eq. (8) are confirmed and depicted in Fig. (4b). With 2 M salt concentration, no measurement of the disjoining pressure was made because of the instability of the films in all solutions. Therefore, only the film lifetime, τ, and thickness of film rupture, hr, were measured in this case. The values of surface potential calculated by Eq. (9), extracted Debye length of the slope of ln Π el - h plots and the measured rupture time and thickness are listed in Table 1. The extracted Debye lengths are smaller than the Debye length of 31 nm for 0.1 mM monovalent salt solutions. Evidently, the difference in the Debye lenghts is due to the contribution of the polarizable MIBC molecules in the liquid phase and/or the trace impurities of the MIBC sample (with the purity > 99%). Therefore, the concentration Ct in Eq. (9) was obtained from the extracted Debye length and Eq. (2). The values for Ct (much smaller than 10 mM but larger than 0.1 mM) were then substibuted into Eq. (9) to calculate the surface potential ψ s from the extracted values for Π 0 . It is noted that the surface potential ψ s is defined relatively the bulk (Likewise, the potential in the Poisson-Boltzmann equation approaches zero in the bulk solution), and its magnitude should be smaller than the magnitude of the potential difference between the phases as measured by the vibrating plate technique or the ionizing electrode method40.


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Figure 3. The molecular structure and partial charge distribution of an MIBC molecule calculated using the MMFF94 force field in Chem3D software. Gray, blue and red colors represent C, H and O atoms, respectively. The dashed black arrow shows the magnitude and direction of the calculated molecular dipole moment. The dashed red line illustrates the potential hydrogen bonding between a chloride ion and the hydrogen atom of the hydroxyl group.

1600

a

10 mM MIBC 10 mM MIBC + 0.1 mM LiCl 10 mM MIBC + 0.1 mM NaCl 10 mM MIBC + 0.1 mM KCl

1400

Disjoining Pressure (Pa)

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

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1200 1000 800 600 400 200 0 20

30

40

50

60

70

Film Thickness, h, (nm)


80

90

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Figure 4. (a) Disjoining pressure isotherms obtained from the TFPB measurements for 10 mM MIBC solutions in the absence or presence of 0.1 mM LiCl, NaCl and KCl at 23°C. (b) Corresponding ln (Πel) – h plots and the best linear fits, as represented by the solid lines and their linear equations.

Table 1. Debye length, λ , surface potential, ψ s , film lifetime, τ, and film rupture thickness, hr, values obtained from the TFPB measurements for different MIBC and salt solutions (Fig. 4) Solutions

λ (nm)

ln Π0

−ψ s (mV)

τ (s)

hr (nm)

10 mM MIBC 10 mM MIBC + 0.1 mM LiCl

25.1 16.4

8.98 9.11

68.5 44.3

-

-

10 mM MIBC + 0.1 mM NaCl

16.9

8.75

37.6

-

-

10 mM MIBC + 0.1 mM KCl

13.7

7.67

17.1

-

-

-

-

-

48.6 ± 4.3

31.6 ± 2.7

10 mM MIBC + 2 M LiCl 10 mM MIBC + 2 M NaCl

-

-

-

36.3 ± 11.3

38.8 ± 11.5

10 mM MIBC + 2 M KCl

-

-

-

30.2 ± 7.6

39.3 ± 7.2

10 mM MIBC + 2 M CsCl

-

-

-

16.8 ± 3.1

44.3 ± 5.2

The extracted values for the surface potential in Table 1 agree generally with the magnitude of the experimental results for zeta potential obtained by the micro electrophoresis. Since the zeta potential of air bubbles in deionized water is about – 65 mV 41, the extracted value of -68.5 mV for the surface potential of the MIBC solution can be expected. At low electrolyte concentration of 0.1 mM, the magnitude of surface potential has obviously decreased from 68 mV to 44 mV in the case of LiCl, and then to 37 mV and 17 mV in the presence of NaCl and KCl, respectively, exhibiting a remarkably specific effect of cations. Similarly, Debye length is the smallest for KCl. Debye – Hückel theory does not account for such ion specificity since it treats ions as point charges without polarizability. Therefore, a thinner ionic atmosphere indicates the presence of more electrolyte ions at 16 ACS Paragon Plus Environment

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solution surfaces. Despite the traditional view of cation repulsion from the solution surface, the observed trend in surface potential and Debye length displays the larger tendency of more polarizable cations to populate the interfacial area. Even at 2 M salt concentration, where electrostatic interactions are expected to have been largely screened, liquid films become unstable in the same order LiClNaCl>CsCl, implying a stronger electric field for LiCl capacitance than the other two!


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It is worth mentioning here that the thicknesses of the thin films, as listed in Table 1, are over one order of magnitude larger than the interfacial depth, which is believed to be less than 1 nm. Therefore, the electric potential measured by TFPB is not identical to the electric potential difference between the ionic capacitance plates, but the screened overall electrical potential in the bulk that both surfaces of the thin films apply on each other. The strength of the electric field between the ionic capacitance plates indeed depends not only on the surface population of the ions but also on the spatial separation of anions and cations capacitance plates. This charge separation cannot be deduced from TFPB measurements but from the SFG spectra. The observed paradox was, thus, resolved by accounting for the charge separation, which is controlled by the difference in the relative polarizability of anions and cations. For the studied salts, the polarizability difference between the anion and cation, and consequently the charge separation, follows the order LiCl>NaCl>CsCl, which is the same +

order as the observed broadband intensity enhancement. Li ions are the hardest ions and are repelled strongly from the interface. According to Eq. (12), this charge separation strengthens the electrical potential difference between the capacitance plates. Therefore, more water molecules are orientated by the LiCl capacitance than NaCl capacitance and then than CsCl capacitance, despite the fact that more ions reside at the surface of CsCl solution than NaCl solution and then than LiCl solution. Along with cation specificity, the initial decrease of the broadband intensity at low concentrations followed by an intensity increase at medium concentrations suggested a strong dependence of interfacial water SFG response on salt concentration. We explained this concentration dependence by assuming that the SFG response of salts at low concentrations is dominantly determined by the direct interactions of ions with water molecules in, and probably, beyond their first solvation shells as well as the weaker re-orientational effect of the surface electric potential on tetrahedrally hydrogen-bonded water molecules. The former explanation was based on the observation of a good correlation between the kosmotropicchaotropic nature of ions and the magnitude by which the broadband signal dropped. The real mechanism of this phenomena is not known to us, at present. However, we suspect that it originates from the specific effect of ions on the reorientation dynamics of interfacial water within the hydrogen-bonding network. At medium/high concentration, the ion-water interactions lose their effectiveness to creating a strong surface electric field. The strength of this electric field showed cation specificity which could not be explained by the traditional thermodynamic view that supposes a cation-depleted surface. The proposed ionic capacitance 28 ACS Paragon Plus Environment

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model also helped us to explain this observation, as the strength of this electric potential depends on the density and separation of charges, i.e., anions and cations, at the interface. The interpretations presented in this work can also be used successfully to explain the similar concentration-dependent ion specificity observed by the other groups. We invite the community to assess our ideas further using a combination of suitable techniques such as PSSFG and time-domain spectroscopy.

Acknowledgments This research has been supported under Australian Research Council’s Projects funding schemes (project numbers LE0989675, DP140101089, and DP150100395). This research was undertaken in collaboration with the scientists from Bulgarian Academy of Sciences during the first author’s visit as a holder of a University of Queensland Graduate School International Travel Award. The University of Queensland is also acknowledged for the IPRS and Centenary postgraduate scholarships provided to the first author (AAS). The authors would also like to thank the Reviewers for providing useful feedbacks which significantly improved our interpretation.


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22. 23.

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Combined Sum Frequency Generation and Thin Liquid Film Study of

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