Counterion Effect on Vibrational Relaxation and the Rotational

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

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment Dexia Zhou, Qianshun Wei, Shuyan Wang, Xiaoqian Li, and Hongtao Bian J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03389 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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The Journal of Physical Chemistry Letters

Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment Dexia Zhou, Qianshun Wei, Shuyan Wang, Xiaoqian Li, and Hongtao Bian* Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, 710119, China * To whom correspondence should be addressed. Email: [email protected] (H. B)

Abstract Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic head group in the RMs formed by Aerosol-OT (AOT) from Na+ to K+, Cs+ and Ca2+, the specific counterion effects on the rotational dynamics of water molecules was determined. The orientational relaxation time constant of water decreases in the order Ca2+ > Na+ > K+ > Cs+.

The SCN- anionic

probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.

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Aqueous electrolyte solutions are ubiquitous in nature, and they are crucial for sustaining all life. The structural dynamics of the water molecule in the presence of cations and anions is crucial to understand its peculiar physical and chemical properties.1-3 Much effort has been devoted to unravel the structure and dynamics of aqueous electrolyte solutions in bulk systems.

4-10

However, less work has been

performed to investigate the structure and dynamics of aqueous electrolyte solutions in a confined environment. In most biological related living systems, water molecules are surrounded by biomolecules and separated in the confined environment.11-22 Water molecules confined in a nanometer-scale region are expected to have different structural and dynamics properties to bulk water.23-26 The direct and indirect anion and cation effects on interfacial water at flat charged interfaces have been investigated by vibrational sum frequency generation (VSFG) spectroscopy,27-29

which is

expected to reveal the mechanism that gives rise to the Hofmeister series. Understanding the structural dynamics of interfacial water molecules in the presence of the cation and anions in confined environments could provide insights into the behavior of ions in biological systems at the molecular level. Reverse micelles (RMs) are one of the model confined environments for investigating the structure and dynamics of interfacial water molecules. RMs are small aggregates of surfactant in a nonpolar solvent, and water molecules are encapsulated inside the polar cavity.30 By changing the molar ratio of water to the surfactant (w0 = [H2O]/[surfactant]),31 the size of the water pool can be tuned and the water dynamics in the interfacial and the core regions can be investigated. A classical surfactant that has been extensively studied is sodium bis(2-ethylhexyl) sulfosuccinate (Na-AOT), which can form mono-dispersed and spherical RMs.30 Many experimental techniques have been used to investigate the structure and dynamics of water molecules in this RM confinement system, including NMR spectroscopy,32,33 fluorescence spectroscopy,34-37 neutron scattering,38 dielectric relaxation,39 and ultrafast infrared (IR) spectroscopy.23,24,40-48 Fayer and co-workers23 systematically investigated vibrational relaxation dynamics of water molecules in AOT RMs by two-dimensional (2D) IR spectroscopy. They proposed a core/shell model to describe



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the vibrational relaxation and orientational dynamics of water molecules inside the RMs.43 Water molecules in the interfacial region of the RMs have much slower rotational

dynamics

than

bulk

water,

which

can

be

explained

by

the

wobbling-in-a-cone model.49 The governing factors that control the dynamics of water in confined AOT RMs have also been investigated.48 It has been found that the confinement is the main factor that affects the dynamics of water, rather than the nature of the interface. However, a systematic study of the specific counterion effect on the structure and dynamics of water molecules in RMs by ultrafast IR spectroscopy has not been performed. The nature of the interface, especially the counterion of the hydrophilic head group of the RMs, is expected to play an important role in the water dynamics in the interfacial region. Eastoe et al50-53 found that the hydrated cation radius of the counteraction strongly affects the structure and shape of the surfactant aggregates based on the small-angle neutron scattering measurements. Ladanyi and co-workers54-56 investigated the effect of counterion type on the structure, dynamics, and properties of the interior region in the AOT RMs by molecular dynamic (MD) simulations. They found that the interfacial mobility of water molecules is strongly related to the nature of the counterion. The water molecules have slower mobility and a larger orientational time constant in the presence of a smaller radius cation. Mudzhikova et al57 investigated the counterion effect on the structure of AOT RMs by MD simulations and calculated the electric potential to understand the counterion effect. Although the counterion effect on the dynamics of interfacial water has been addressed by theoretical investigations, there are still some unanswered questions regarding the specific water countercation interaction and the heterogeneity at the interface. A detailed model that can take into account the counterion effect is necessary to completely describe the interfacial water dynamics, which relies on the direct experimental measurement. To answer the abovementioned questions, we investigated the specific counterion effect on vibrational relaxation and the rotational dynamics of water molecules by ultrafast IR spectroscopy and 2D IR spectroscopy. By changing the


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counterion of the hydrophilic head group in the AOT RMs from Na+ to K+, Cs+ and Ca2+ (for the experimental details, see the Supporting information (SI) and Figure S1), the counterion effect on the rotational dynamics of water molecules was investigated. We also used the SCN- anion as a vibrational probe and investigated the specific counterion effect on the vibrational relaxation dynamics of the SCN- anion. The nature of the countercation has a significant influence on vibrational population decay of SCN- in the RMs following the reverse order of Hofmeister series, which suggests the formation of ion pairs in the interfacial region of the RMs.

Figure 1. (A) FTIR spectra of the OD stretch of 6% HOD in H2O in KAOT RMs (w0=2, 3, 4, 5 and 6) and bulk water. (B) Central frequencies of the OD stretch as a function of w0 in XAOT (X=K, Na, Cs, and Ca) RMs. The frequency of the OD stretch in bulk water (2509 cm-1) is given as a reference. (C) Central frequencies of SCN anti-symmetric stretch as a function of w0 in XAOT (X=K, Na, Cs, and Ca) RMs. The peak position of the SCN- stretch in bulk water (2065 cm-1, 0.2 mol/kg NaSCN) is also plotted as a reference.

The Fourier transform IR (FTIR) spectra of the OD stretch of 6% HOD in H2O in KAOT RMs with different w0 (w0=2, 3, 4, 5 and 6) are shown in Figure 1(A). The gradual blue shift of the OD stretching mode with decreasing w0 indicates disruption of the hydrogen bond network of water molecules inside the RMs. Dynamic light scattering measurements show that the diameter of the water pool in the RMs of KAOT is in the range 1.5 nm to 3 nm when w0 is varied from 2 to 6. The blue shift of the OD stretch is mainly because the water molecule interaction with the head group of KAOT is weaker than the water-water interaction in the bulk. From Figure 1(B), there is not much difference in the OD stretching frequency between KAOT and NaAOT RMs by varying the w0. However, for the RMs with Cs+ and Ca2+



counterions, the blue shifts of the OD stretching frequencies are slightly smaller than those in the KAOT and NaAOT RMs at the same w0. This can be qualitatively explained by the larger sizes of the water pool in the CsAOT and Ca(AOT)2 RMs, which is confirmed by dynamic light scattering measurement(See Figure S2). When w0 reaches 8, there is not much difference in the OD stretching frequencies of water in the RMs with different counterions. This suggests that the counterion effect on the OD stretching frequency of water molecules became negligible with increasing size of the water pool, even though their FTIR spectra do not resemble that of bulk water. To investigate the structural heterogeneity in the interfacial region of the RMs, we used the SCN- anion as a vibrational probe to monitor the structural dynamics in the confined region, because there have been a few studies that show that small anions can be used to reveal the structural dynamics in confined environments.46,58-60 The antisymmetric stretch of the SCN- anions in RMs with different counterions is shown in Figure 1(C). As the size of the RMs decreases, the SCN- stretch shows a small red shift for all the RMs with different counterions. The red shift is mainly caused by the weak polarity and disrupted hydrogen bond network of the water molecules inside the RMs. In CsAOT, the SCN- stretch shows a slightly larger red shift than in the RMs with Na, K and Ca2+ counterions. This is probably because of the frequency differences of their molten salts.61 The bandwidth of the SCN- stretch is almost the same in the RMs with different counterions. The concentration of the SCN- anion is 0.2 mol/kg, and it is expected that there would be an average of about one SCNanions encapsulated in each water pool of the RMs. The number of water molecules is about 300 when the diameter of the AOT RMs is 2.3 nm.43 Therefore, vibrational relaxation and the rotational dynamics of both the water and anionic probe molecules can provide a comprehensive picture of structure and dynamics inside the confined RMs.


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Figure 2. (A) Normalized vibrational population decay of the OD stretch in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs. (B) Normalized anisotropies of the OD stretch as a function of the waiting time in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs. The vibrational population dynamics and anisotropy decay of OD stretch in bulk water (HOD solution) are also shown for comparison.

First, we will report the vibrational relaxation dynamics of water molecules in the RMs. For w0=5, the diameter of the water pool is about 2.6 nm for the NaAOT, KAOT and CsAOT RMs. For the Ca(AOT)2 RMs, the diameter is about 4.0 nm.(See Figure S2) Because of the small size of the water pool, the water molecules confined in the RMs cannot form a bulk-like hydrogen bond network.23 It is expected that the water molecules interact with the hydrophilic head group and counterions, and the relaxation and orientational dynamics should reflect the local environment change. Here, the vibrational relaxation dynamics of OD stretch in the dilute HOD solution was measured to represent the dynamics of water molecules at the interfacial region of RMs. Measurements of OD stretch can ensure the elimination of the resonance energy transfer between the water molecules that contributes to the spectral diffusion and anisotropy decay in pure water.23 The vibrational relaxation dynamics of the OD stretch in w0 = 5 XAOT RMs is much slower than in the 6% HOD solution, shown in Figure 2 (A). The vibrational lifetime of bulk water in pure HOD solution is 1.7±0.2 ps, which is consistent with previous studies. For the water molecules confined in the RMs, the vibrational relaxation dynamics can be described by a bi-exponential decay function. Regardless the nature of the counterion, fitting of the vibrational relaxation



dynamics of the water molecules confined in the RMs gives the same results with time constants of 0.5±0.2 ps and 3.7±0.2 ps. The heat effects were subtracted according to a previously reported procedure.62 The origin of the fast component is not clear, but it is related to intramolecular vibrational energy transfer of the confined water molecules. The slow component is caused by the population decay of the OD stretch. No clear specific counterion effect is observed for vibrational population decay of the OD stretch in the RMs. Anisotropy decay of the OD stretch in w0 = 5 RMs shows a strong specific counterion effect, shown in Figure 2(B). For the pure HOD solution, the rotational dynamics of the OD stretch can be described by a single exponential decay with a time constant of 2.6±0.2 ps. For water molecules confined in the RMs, the rotational dynamic data show fast decay and slow decay, which is well described by a bi-exponential function. The fast decay is in the range 1.0±0.2 ps, which is much faster than that in bulk water. Such fast decay is also observed in previous reports, and it is attributed to angular fluctuation of the intact hydrogen bond network (wobbling in a cone).23,49 The slow decay is much slower than that in bulk water, and it depends on the nature of the counterions in the RMs. For NaAOT at w0=5, the rotational time constant for the slow component is 9.6±0.5 ps. When the counterions are K+ and Cs+, the rotational time constants are 8.9±0.5 and 6.6±0.5 ps, respectively. For Ca(AOT)2 at w0 = 5, the rotational time constant is 31±4 ps, which is much larger than that in bulk water. The specific counterion effect on the rotational dynamics of the OD stretch might originate from the hydrated nature of the counter-cation. A smaller counter-cation results in slower rotational dynamics. The order of the rotational dynamics is Ca2+ > Na+ > K+ > Cs+, which is consistent with the order of the Hofmeister series. As mentioned above, the size of the water pool in the Ca(AOT)2 RMs is larger than those in the NaAOT, KAOT and CsAOT RMs. The slower rotational dynamics of the water molecules in the Ca(AOT)2 RMs indicates that the water molecules are mostly located in the interfacial region and affected by the countercations. Previous studies have shown that the presence of a geometric interface plays an important role in the rotational dynamics of water molecules, while the


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nature of the chemical structure of the interface does not affect the interfacial hydrogen bond dynamics.48 The results here show that the countercation of the surfactant forming the RMs strongly affects the rotational dynamics of the encapsulated water molecules.

Figure 3. (A) Normalized vibrational population decay of the SCN- stretch in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs. (B) Normalized anisotropies of the SCN- stretch as a function of the waiting time in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs. The concentration of SCN- is 0.2 mol/kg.

To reveal the underlying mechanism of the specific counterion effect on the interfacial water molecules, we also measured the structural dynamics of the vibrational probe SCN- in the interfacial region of the RMs. The control experiments show that the vibrational relaxation dynamics of the SCN- anion (0.2 mol/kg XSCN, X=Na+, K+, Cs+, and Ca2+) in bulk water is the same within experimental error, regardless of the nature of the cations (see Figure S3 in the SI). However, there is a strong counterion effect on the vibrational relaxation dynamics of the SCN- anion in the water pools of the RMs. The vibrational relaxation dynamics of the SCN- anion in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs is shown in Figure 3 (A). For the NaAOT RMs at w0 = 5, the vibrational relaxation dynamics of SCN- shows bi-exponential decay with a fast time constant of 1.5±0.2 ps and a slow time constant of 19±1 ps. The fast decay is suggested to be related to the intramolecular vibrational energy transfer in the SCN- anion.63 The slow decay is caused by the vibrational population of the SCN- anion, which is sensitive to the structure of the local environment. For the



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KAOT and CsAOT RMs, the slow components of SCN- vibrational relaxation decay are significantly enhanced to 28±1 ps and 33±1 ps, respectively. In the Ca(AOT)2 RMs at w0 = 5, the slow component of SCN- vibrational relaxation decay is much faster with a time constant of 18±1 ps. The counterion effect on the vibrational population decay of SCN- in the RMs follows the reverse order of the Hofmeister series: Ca2+ > Na+ > K+ > Cs+. For 0.2 mol/kg XSCN (X=Na+, K+, Cs+, and Ca2+) aqueous solutions, the SCNand metal ions are fully solvated by the water molecules. It is reasonable that the cation should have a small effect on the vibrational relaxation dynamics of the anions, because the cations and anions are well separated. However, when the SCN- anion is confined in the water pools of the RMs, it is suggested that the cation and SCN- anion cannot be fully solvated by water molecules because of the weak and disrupted hydrogen bond network. The results suggested that the SCN- anions may form ion pairs with the metal cations in the water pool in the interfacial region. Therefore, the size and charge density of the cation should have a profound effect on the vibrational relaxation dynamics of the SCN- anion. The smaller the cation and the higher charge density, the faster the population decay of SCN- owing to formation of contact ion pairs, which may facilitate vibrational energy transfer. This can qualitatively explain the trend of the counterion effect on the vibrational relaxation dynamics of SCN-. We also varied the sizes of the water pool of the NaAOT RMs from w0 = 3 to 15. The vibrational relaxation dynamics of SCN- is the same within experimental error. (See Figure S4) The results indicate that the SCN- anions should be close to the interfacial region of the RMs, rather than in the center of the water pool of the RMs. Further rotational dynamics measurement of SCN- in the RMs confirmed this assumption. The rotational dynamics of SCN- in the XAOT (X=Na, K, Cs and Ca) RMs

at

w0 = 5 is shown in Figure 3(B). The rotational dynamics of SCN- also shows bi-exponential decay behavior, similar to the rotational dynamics of the OD stretch in the RMs. The rotational time constants of the fast component are in the range 3.5±0.2 ps for all of the AOT RMs with different counterions, whereas the time constants of


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the slow component are quite different and strongly depend on the nature of the counterions. For the CsAOT RMs at w0=5, the rotational time constant of SCNanions is 43±4 ps. The rotational time constants of the SCN- anions are 52±5 and 68±8 ps for the KAOT and NaAOT RMs, respectively. For the Ca(AOT)2 RMs at w0=5, the rotational dynamics of SCN- (131±20 ps) is much slower than in the other RMs. The counterion effect on the rotational dynamics of the SCN- anion is consistent with the rotational dynamics of water molecules confined in the RMs, with the order Ca2+ > Na+ > K+ > Cs+. Here, we will discuss the possible reasons for the specific counterion effect on both the rotational dynamics of the water molecules and SCN- anions. For water molecules confined in the RMs, the geometric effect owing to the presence of the curved interface formed by the surfactants is critical for the structure and dynamics of the water molecules. With different counter-cations present in the vicinity of the interfacial region of the RMs, it is reasonable that a smaller cation with a higher charge density will have a stronger influence on the structural dynamics of water molecules, which can qualitatively explain the counterion effect observed in Figure 2(B). Analysis of the dynamics of the SCN- anions confined in the RMs is complicated. The rotational time constant of SCN- in aqueous solution is around 3.5±0.2 ps,64 regardless the nature of the counterions in the bulk solution. The much slower rotational dynamics of the SCN- anion in the water pool of the RMs could have two causes. One is the interfacial effect owing to the geometry constraint by the RM. The other is formation of ion pairs in the interfacial region, which is revealed from the vibrational relaxation dynamics.



Figure 4. Waiting time dependent 2D IR spectra of SCN- in the w0 = 5 of (A) KAOT and (B) Ca(AOT)2 RMs. The waiting time dependent 2D IR spectra of SCN- in NaAOT and CsAOT RMs with w0 =5 are plotted in the supporting information Figure S6. (C) Typical 2D IR spectra of the SCN- in Ca(AOT)2 RMs with w0 =5 at waiting time of 0.1 ps. The 0-1 (red peak) and 1-2 (blue peak) peaks are elongated along the diagonal. The dashed line shows at the center of SCN- stretching peak where the dynamical linewidth can be obtained by the cross-sectional cut of the red peak at a particular waiting time. (D) The normalized waiting time dependent dynamic linewidth determined from (A), (B) and Figure S6.

The interfacial effect or formation of ion pairs cannot solely explain the much slower rotational dynamics of SCN- in the Ca(AOT)2 RMs. We propose that the electric double layer formed in the RMs may also play an important role. The counterions and negatively charged head group of AOT can form an electric double layer in the interfacial region. The surface potential generated by the electric double layer can strongly affect the mobility of the SCN- anions present in the interface region, where the spectra diffusion of the SCN- anions should be very different.65 The typical waiting time dependent 2D IR spectra of SCN- in the KAOT and Ca(AOT)2 RMs are shown in Figure 4 (A) and (B). It is clear that the line shape of the diagonal peak evolves from the elongated along the diagonal to round with the increase of time


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due to the spectral diffusion. At an initial glance, the spectra diffusion of SCN- anion in Ca(AOT)2 RMs is slowed down significantly and is in the range of tens of picoseconds, which is much slower than that in the KAOT RMs. In order to quantitative analyze the spectra diffusion, the dynamic linewidth method is utilized following the reported procedure.62,66 The change of the dynamical linewidth can be determined by examining at a cross sectional cut of the red peak at different waiting time points (Figure 4 (C) ). The normalized time dependent dynamic linewidth of SCN- in w0 = 5 XAOT (X=K, Na, Cs and Ca) RMs is shown Figure 4 (D). For the CsAOT RMs at w0=5, the spectra diffusion of SCN- anions is 3.3±0.2 ps. The spectra diffusion time constants of the SCN- anions are 1.8±0.2 and 2.4±0.2 ps for the KAOT and NaAOT RMs, respectively. For the CsAOT RMs at w0=5, the spectra diffusion time constant of SCN- is much faster than in the other RMs, which is 1.1±0.2 ps. Kubarych and co-workers59 also reported the spectra diffusion of SCN- anion in the cationic and negative charged RMs, and the spectra diffusion time constant is in the range 1.2 ps with Na+ as the counterion. The counterion effect on the spectra diffusion of the SCN- anion is also following the order Ca2+ > Na+ > K+ > Cs+. This is the first measurement of the specific counterion effect on interfacial water molecules confined in RMs. The cooperative effects observed here all contribute to the specific counterion effect on the rotational dynamics of SCN- anions in the RMs. The specific counterion effect on the rotational dynamics of water molecules and SCN- anions is also observed in RMs with different water pool sizes ( Figure S5 and S6). In summary, the counterion effect on the structural dynamics of water molecules encapsulated in the AOT RMs has been investigated by ultrafast IR spectroscopy and 2D IR spectroscopy. By changing the counterion of the hydrophilic head group in the RMs from Na+ to K+, Cs+ and Ca2+, the specific counterion effect on the rotational dynamics of water molecules is clearly observed. For w0 = 5, the orientational relaxation time constant of water molecule is slowed down significantly, and it follows the order of the Hofmeister series: Ca2+ > Na+ > K+ > Cs+. This indicates that the nature of the counter-cations present in the interfacial region plays an important role in the dynamics of the interfacial water molecules. The vibrational relaxation



dynamics of the anionic probe SCN- shows confinement in the vicinity of the interfacial region, where heterogeneity at the charged interface of the RMs is revealed. In addition, the synergistic effect is an important phenomenon and needs to be considered to describe the structure and dynamics of water molecules in confined environments.

Supporting Information Details of ultrafast IR spectroscopy and 2D IR measurements, synthesis of XAOT surfactants (X=K, Cs, Ca), XPS and dynamic light scattering measurements, and vibrational relaxation dynamics measurements.

Acknowledgment. HTB acknowledges the support from the Natural Science Foundation of China (NSFC, No. 21603137), Fundamental Research Funds for the Central Universities (GK201701004) and startup fund support from Shaanxi Normal University (No. 1110010767).

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Captions Figure 1. (A) FTIR spectra of the OD stretch of 6% HOD in H2O in KAOT RMs (w0=2, 3, 4, 5 and 6) and bulk water. (B) Central frequencies of the OD stretch as a function of w0in XAOT (X=K, Na, Cs, and Ca) RMs. The frequency of the OD stretch in bulk water (2509 cm-1) is given as a reference. (C) Central frequencies of SCN anti-symmetric stretch as a function of w0in XAOT (X=K, Na, Cs, and Ca) RMs. The peak position of the SCN- stretch in bulk water (2065 cm-1, 0.2 mol/kg NaSCN) is also plotted as a reference.






Figures

Figure 1. (A) FTIR spectra of the OD stretch of 6% HOD in H2O in KAOT RMs (w0 =2, 3, 4, 5 and 6) and bulk water. (B) Central frequencies of the OD stretch as a function of w0 in XAOT (X=K, Na, Cs, and Ca) RMs. The frequency of the OD stretch in bulk water (2509 cm-1) is given as a reference. (C) Central frequencies of SCN anti-symmetric stretch as a function of w0 in XAOT (X=K, Na, Cs, and Ca) RMs. The peak position of the SCN- stretch in bulk water (2065 cm-1, 0.2 mol/kg NaSCN) is also plotted as a reference.


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Counterion Effect on Vibrational Relaxation and the Rotational

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