Nanobubble Skin Supersolidity - Langmuir (ACS Publications)

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Nanobubble Skin Supersolidity Xi Zhang,† Xinjuan Liu,‡ Yuan Zhong,§ Zhaofeng Zhou,§ Yongli Huang,§ and Chang Q. Sun*,∥ †

Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, Institute of Nanosurface Science and Engineering, Shenzhen University, Shenzhen 518060, China ‡ College of Materials Science and Engineering, Institute of Coordination Bond Metrology and Engineering, China Jiliang University, Hangzhou 310018, China § Key Laboratory of Low-Dimensional Materials and Application Technology (MOE) and School of Materials Science and Engineering, Xiangtan University, Xiangtan 411105, China ∥ NOVITAS, School of EEE, Nanyang Technological University, Singapore 639798, Singapore ABSTRACT: Water nanobubbles manifest fascinatingly higher mechanical strength, higher thermal stability, and longer lifetime than macroscopic bubbles; thus, they provide an important impact in applications in the biomedical and chemical industries. However, a detailed understanding of the mechanism behind these mysteries of nanobubbles remains a challenge. Consistency between quantum computations and Raman spectrometric measurements confirmed our predictions that a nanobubble skin shares the same supersolidity with molecular clusters, skins of bulk water, and water droplets because of molecular undercoordination (fewer than four nearest molecular neighbors). Molecular undercoordination (coordination number Zcluster < Zsurface < Zbubble < Zbulk = 4) shortens/ extends the H−O/O:H bond and stiffens/softens its corresponding stretching phonons, whose frequency shift is proportional to the square root of the cohesive energy and inversely proportional to the segmental length. The strongly polarized O:H−O bond slows the molecular dynamics and increases the viscosity. The freezing temperature is lowered by the softened O:H bond, and the melting temperature is enhanced by the stiffened H−O bond. Therefore, the supersolid skin makes the nanobubbles thermally more stable, less dense, and stiffer and slows the dynamics of their molecular motion. nanobubbles was thought to be due to “surface misbehavior”. A consensus theory for bubble stability is beyond the description of classical thermodynamics.20 Numerous schemes have been developed to explain the mechanism for the attributes of nanobubbles, such as the skin model,22 armored bubbles model,23 particle crevice model,24 electrostatic negative pressure at the surface,25 and many-body effect.26 However, it remains mysterious how nanobubbles are long-lived and mechanically stiff. It is clear that the molecular coordination number is an important parameter responsible for the subject area of undercoordination physics.27−29 Undercoordinated water molecules are referred to as those with fewer than four nearest neighbors in the bulk. If the coordination number (CN, Z) of a flat surface is denoted Zflat, which should be less than the full CN of bulk (Zbulk), then a positive curvature K > 0 (nanodroplet) makes Zcurve < Zflat the effective CN. The larger the K, the smaller the Zcurve obtained. Conventionally, in metal nanoparticles, Zcurve ∝ 1/K. For a negative curvature K < 0 (nanobubble), however, the effective CN should be larger than

1. INTRODUCTION Water nanobubbles have attracted increasing attention in recent years owing to their extraordinary properties including longer lifetime, high gas solubility, high thermal stability, and high mechanical strength. Nanobubbles have been applied in many fields such as biomedicine delivery, water treatment, nontoxic treatment, surface cleaning, and defouling membranes.1−6 Compared with macroscopic bubbles, nanobubbles remain for months without bursting.7−10 Nanobubbles at a surface are dynamically stable, as revealed by atomic force microscopy (AFM) investigations.11,12 Monolayer water can make surface nanobubbles stable.13 The stability of nanobubbles is considered to stem from the low diffusion of gas through water or from the pinning of the water surface contact line of the nanobubbles, as suggested by theoretical investigations. Nanobubbles have a slow dissolution rate of hours up to days, compared with the microsecond lifetime of macroscopic bubbles.14 Nanobubbles have a very large contact angle and high surface tension.15−19 Such nanobubbles should not exist at all based on classical theory of an air−water interface.20 According to classical theory, the small radius of a nanobubble implies that the Laplace pressure inside of the nanobubble is high. Gas diffusion across the interface can be driven, leading to the instant dissolution of the nanobubbles.21 This unexpected stability of © 2016 American Chemical Society

Special Issue: Nanobubbles Received: May 4, 2016 Revised: August 4, 2016 Published: August 5, 2016 11321

DOI: 10.1021/acs.langmuir.6b01660 Langmuir 2016, 32, 11321−11327

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Langmuir that of the flat surface because molecules arrange themselves more closely at the bubble surface but do not attain the bulk value, Zflat < Zcurve < Zbulk. Thus, the order of the Z values for clusters, droplets, flat surfaces, and the bulk is 2 ≤ Zcluster < Zdroplet < Zflat < Zbubble < Zbulk = 4. The Z value for a molecular dimer is 1. Molecular undercoordination of water exists ubiquitously in bubbles, nanodroplets, skins of water and ice bulk, ion hydration, and (H2O)N clusters.30−35 Molecular undercoordination makes bubble skin molecules fascinating, but it was often overlooked earlier.27,30 The objective of this paper is to show that hydrogen-bonding (HB) cooperative relaxation in water has enabled us to resolve this mystery both theoretically and experimentally. Molecular undercoordination shortens the H−O bond and extends the O:H nonbond in association with nonbonding electron polarization. The consequences of this reconcile the observed performance associated with molecular clusters,36,37 water droplets,38 water and ice skins,36,39,40 and nanobubbles. H−O bond contraction raises the vibrational frequency of nanobubbles, and O:H elongation lowers their phonon frequency. Undercoordinated molecules at the surface of nanobubbles demonstrate their supersolid nature, that is, they are mechanically stronger, viscous, and less dense and have a longer lifetime and slower molecular dynamics.28,38

the relaxation of the stiffness as a function of the segmental length dx and cohesive energy Ex30 ⎧ Δω(d , E , μ ) ∝ E /μ /d ∝ x x x x x x ⎪ ⎪ ⎨ ∝ kx + k C / μx ⎪ ⎪T ∝ E ⎩ m H

Yxdx /μx

(1)

The O:H nonbond is characterized by the stretching vibration frequency at ∼200 cm−1, and the H−O bond is characterized by the phonon frequency of ∼3200 cm−1 in bulk water. kx denotes the force constants or second differentials of the potential of H and L, and kC denotes the force constant of O−O repulsive potential. μx is the reduced mass of the H and L oscillators. In fact, the phonon shift is proportional to the square root of the stiffness Yxdx of H and L. Yx refers to the elastic modulii, which are proportional to the local energy density Ex/dx.3 Mechanical and thermal stabilities refer to Young’s modulus and the melting temperature (T m ), respectively. Tm is proportional to EH.42 Differential phonon spectrometrics (DPS) are obtained by comparing both Raman spectra of the bubbles and the reference spectrum from water upon background-correcting and normalizing the peak area. 2.3. Supersolidity of Negatively Curved Nanobubble Skin. For comparison purposes, we briefly describe the skin supersolidity of bubbles and their O:H−O bond length− frequency entity. According to bond relaxation theory,27 molecular undercoordination at the nanobubble surface shortens and stiffens the H−O bond, whereas O−O repulsion extends and softens the O:H nonbond in association with nonbonding electron polarization, which has been confirmed by molecular dynamics calculations and microjet ultraviolet photoelectron spectroscopy.36,37,39,40 Nonbonding electron polarization indicates that molecular undercoordination polarizes nonbonding electrons in two rounds by the densely entrapped H−O bonding electrons. The extent of polarization is proportional to the ωH blueshift.30 Most strikingly, the cooperative relaxation of O:H and H−O creates a supersolid skin phase that is less dense, thermally stable, and mechanically stronger, with slower molecular dynamics and a longer lifetime.30,39 The cooperative relaxation of the O:H−O bond is the key that controls the performance of the bubbles. The unusual mechanical stability of the interface and surface of bubbles is determined by the supersolidity of the skin.38,39 Because of its longer ωH phonon lifetime or slow molecular dynamics,43 the skin’s supersolidity reconciles the slipperiness of ice44 and the toughness of liquid water.28 In our previous work, we investigated water clusters (H2O)N (N = 2−20)42 and the flat skin of water and ice.45 Following our previous achievements, this work focuses on negatively curved nanobubble skin. We assume that the performance of a nanobubble should be dominated by the negative-curvature skin of water, which should be similar to the undercoordination effect of (H2O)N clusters and flat skin, and we use density functional tight-binding (DFTB) calculations and DPS to evaluate our assumption.

2. PRINCIPLES 2.1. O:H−O Bond Cooperativity. The structure of water is more highly ordered than one would expect.41 Water prefers a tetrahedrally coordinated, two-phase structure in a bulk−skin or core−shell order of different O:H−O bond lengths.30 Figure 1a illustrates the ideal structural unit cell of water and ice, which

Figure 1. (a) Structural unit cell. Four identical O:H−O bonds, with the blue dots representing the O lone pairs, are included. (b) Possible ways of the cooperative relaxation of the O:H−O bond under external excitation that approximates an oscillator pair with asymmetrical, short-range interactions and O−O repulsion.30

consists of two H2O molecules and four identical O:H−O bonds, with the blue dots representing the O lone pairs.41 The O:H−O bond has a weaker O:H bond (∼0.1 eV), a stronger H−O bond (∼4.0 eV), and a short-range repulsion between the bonding electrons and the nonbonding pairs.30 Figure 1b shows the cooperative relaxation of O:H−O bond. There are only two ways that the O:H−O bond can relax: elongation or contraction. The asymmetrical HB cooperative relaxation arises intrinsically from the O−O repulsion and the segmental strength disparity (EL/EH ≈ 0.1/4.0 eV). Molecular undercoordination lengthens the O:H−O bond associated with polarization. 2.2. Differential Phonon Spectrometrics (DPS). L and H denote the O:H nonbond and the H−O bond, respectively. The Raman phonon frequency shift Δωx (x = L or H) probes

3. COMPUTATIONAL AND EXPERIMENTAL APPROACHES 3.1. DFTB Length and Vibrational Calculation. DFTB calculations were performed using DFTB+ code46 to gain spatially resolved information on the O:H−O bond’s segmental length and vibrational frequency in nanobubbles. Figure 2a illustrates the unit cell 11322

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Figure 2. Atomic structures of cubic water without (a) and with (b) cavity bubbles. The yellow molecules in (a) were removed to form hollow bubbles in (b). The dangling O (in orange) and H atoms (yellow) are indicated in (b). There are two types of spatially resolved H−O bonds associated with dangling H2O molecules: the dangling H−O bond and the H−O bond directed into water. of cubic structure as the basic unit, and a 2 × 2 × 2 supercell containing 64 water molecules approximates highly ordered water. Three water molecules in the bulk supercell were removed to form a hollow bubble or cavity structure (Figure 2b), leaving the surrounding water molecules undercoordinated. The CHNO Slater−Koster parameter47 was adopted to describe the overlap integrals in the Hamiltonian matrix. Dispersion force correction was adopted to describe the van der Waals interactions of the hydrogen bond. The energy threshold for self-consistent charges is 10−8 eV. The tolerances for geometry optimization of energy, force, stress, and displacement are 0.0008 eV, 0.004 eV/Å, 0.05 GPa, and 0.001 Å, respectively. The vibrational spectrum was calculated from the Hessian matrix obtained with geometry optimization. 3.2. Raman Measurements. Raman measurements of 150 μL of bubble solution injected into a silica stage were conducted using a confocal Raman spectrometer (Renishaw inVia), with a 532 nm He− Ne laser as the light source at 298 K. To avoid evaporation, the stage was covered with a thin silica glass lid. The phonon frequency range was set at 50−4000 cm−1. Four scans were averaged for each spectrum, and each scan took 30 s. A 50× long-working-distance objective lens (Leica) was used to focus the laser light onto the sample and collect the scattered light. Deionized water (18.2 MΩ·cm resistivity) produced by a HiTech Laboratory water purification system was used for all measurements. The main gas used to produce bubbles was air. Bubbles were prepared by a microbubble generator (XZCP-K, Yunnan Xia Zhichun Environmental Technology Co. Ltd., China) in the flotation system. The bubbles were characterized with a Zetasizer Nano ZS 90 (Malvern), as shown in Figure 3. The results indicate that the sizes of the bubbles are in the range of 100−200 nm, and the volume percentage of bubbles with a size of 150 nm was about 40%.

Table 1. DFTB-Derived Relaxation of O:H and H−O Lengths (in Å)a

H−O bulk H−O water skin (DFT)39 dangling H−O H−O of undercoordinated O O:H bulk O:H water skin (DFT)39 O:H of undercoordinated O O:H in dangling H−O water

⟨dx⟩

Δε (%)

0.9848 0.9501

0 −3.53

0.9653 0.9787

−1.97 −0.61

1.843

1.768 1.901 1.829

0 7.52 6.12

1.839 1.820

1.823

3.09

dx1

dx2

dx3

0.9849

0.9847

0.9652 0.9787

0.9655 0.9772

0.9653 0.9802

0.9801 1.764

0.9773 1.772

0.9787

1.802

1.841

1.805 1.809

1.840 1.840

a Δε (%) is the strain with respect to bulk values. ⟨dx⟩ averages the derivatives from bonds at different positions marked in Figure 2b.

are three dangling H−O bonds (black numbers 1, 2, 3 in Figure 2b) at the inner skin of the bubbles. Their average length is 0.9653 Å, which is 1.97% lower than that of the bulk value. There are six H−O bonds of three undercoordinated O atoms (green numbers 1, 2, 3 in Figure 2b) at the surface. They also contract by 0.61%. Meanwhile, the length of the six O:H bonds of the three undercoordinated O atoms at the surface expands by 6.12% on average when compared with that of the bulk value. The O:H nonbonds of the dangling H−O bonded water are also elongated by 3.09% on average. These observations verify that the H−O bonds at the bubble surface are indeed shorter and that the O:H nonbonds become longer. Compared with the DFT calculation results for a water skin,39 the negatively curved bubble skin has the same trend as that of H− O and O:H cooperative relaxation but to a lesser extent. 4.2. DFTB-Derived ωH and ωL Relaxation. Figures 4 and 5 compare the DFTB-derived vibrational spectra of H−O (ωH) and O:H (ωL). The ωH peaks of the bubbles are located at wavenumbers around 3440, 3520, 3760, and 3900 cm−1, shifting from the bulk values to higher frequencies. Figure 2c−f shows the corresponding eigenvectors of the four featuring modes. From the position and direction of the green arrows, it can be seen that the peaks at 3900 and 3760 cm−1 correspond to the asymmetrical vibration mode of the surface H−O bonds, and the peaks at 3520 and 3440 cm−1 correspond to their

4. RESULTS AND DISCUSSION 4.1. O:H and H−O Length Relaxation. Table 1 shows the DFTB-derived, spatially resolved relaxation of the O:H and H− O bonds of bubbles compared with that of bulk water. There

Figure 3. Bubble size distribution. 11323

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Figure 4. (a) ωH (3200−4000 cm−1) of bulk water (black) and nanobubbles (red). The spectrum can be attributed to symmetrical and asymmetrical H atom vibrations. (b) Spectral difference between the bubbles and bulk water shows the phonon frequency and population relaxation. The vibrational modes and vectors of the featured peaks are illustrated in (c) 3900, (d) 3760, (e) 3520, and (f) 3400 cm−1. Green arrows indicate the eigenvectors of the vibrational mode.

Figure 5. (a) Low vibrational frequency spectra (150−600 cm−1) of bulk water (black) and bubbles (red). The features correspond to the O:H stretching and ∠O:H:O bending mode. (b) Differential spectra indicating undercoordination-induced O:H softening (arrows). The vibrational modes and vectors of the featured peaks are illustrated in (c) 210 and (d) 420 cm−1 (green arrows).

coordination softens the ωL cooperatively because of the O− O repulsivity. 4.3. Raman-Derived Blue Shift of ωH and Red Shift of ωL. To verify the DFTB derivatives, we recorded Raman spectra of the bubbles and bulk water under ambient conditions. The spectra in Figure 6 exhibit four regions representing the O:H stretching vibration (ωL ≈ 200 cm−1 for the bulk and ωL ≈ 75 cm−1 for the skin) and H−O stretching vibration (ωH ≈ 3200 cm−1 for the ordinary bulk and ωH ≈

symmetrical vibration. These results verify that molecular undercoordination stiffens the ωH of the bubble surface. Figure 5a,b compares the ωL spectra of bubbles and bulk water; two ωL features occur around 210 and 420 cm−1. The peaks are red-shifted. Figure 4c,d shows the corresponding eigenvectors of the two modes. The peak at 210 cm−1 corresponds to the O:H stretching mode of the bubble surface, and the peak at 420 cm−1 corresponds to the ∠O:H−O bending mode. The results verify that molecular under11324

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Figure 6. (a) O:H and (b) H−O segmental DPS for bubbles under ambient conditions. Insets show normalized O:H and H−O spectra.

Figure 7. Dependence of (a) O:H and (b) H−O segmental DPS on time for bubbles at room temperature. Insets show normalized O:H and H−O spectra.

3450 cm−1 for the water skin).48−50 The ωL and ωH values of the bubbles change insignificantly compared with those of bulk water because Zsurface < Zbubble < Zbulk. To analyze the phonon frequency shift, we differentiated the spectra of water in the presence and absence of bubbles to obtain the DPS. The phonon population is unambiguously proportional to the relaxation because of the bubble skin. As shown in Figure 5b,c, the ωL of bubbles shifts from 170 to 65 cm−1, and the ωH displays a blue shift from 3250 to 3500 cm−1 with respect to that of pure water. The segmental DPS consistently confirms the cooperative relaxation of ωx, as derived from DFTB calculations. Figure 7 shows the segmental DPS of ωL and ωH (b and c) as the bubble ages. It can be observed that the ωx cooperative relaxation can remain relatively stable for 49 days. Higher ωH peak frequency indicates longer H−O phonon lifetimes or slower molecular dynamics.30,51 4.4. Discussion. As shown above, molecular undercoordination at the negatively curved skin of nanobubbles shortens the H−O bond and lengthens the O:H nonbond. It stiffens the H−O phonon frequency and softens the O:H phonon, which offset the respective Debye temperatures of the O:H and H−O segments.28 A superposition of the segmental specific heat ηx(T/ΘDx) curves creates two intersecting temperatures that define the boundaries of the quasi-solid phase between ice and liquid water.38 Thus, nanobubble formation with negatively curved skin results in a higher melting temperature and a lower freezing point, being identical in nature but to a slightly lesser extent than that with positively curved skin, because Tm ∝ EH, which is the O−H bond energy that is increased by molecular undercoordination. Because the effective molecular coordination number of the positively curved skin is less than that of the negatively curved skin, EH is stronger in confined water nanoclusters with positively curved skin.36,37,39,40 A reduction in the number of water molecule

layers increases the viscosity. Viscosity can be related to the stress tensor as ηs =

V kT

∫0

∞

⟨σαβ(0)σαβ(t )⟩ dt

(2)

where σαβ denotes the three equivalent off-diagonal elements of the stress tensors. At the surface, the surface stress tensor will change when compared with the bulk value. The extent of O:H polarization determines the surface tension, as verified in the acid−salt−base solutions.52,53 This effect will manifest itself through the modulation of the ultrashort interactions within the O:H−O bond. The relaxation of the local O:H−O bond and its corresponding cohesive energy increase the local stress tensor and then affect the viscosity. We have reported that O:H−O cooperative relaxation at a water skin45 and the associated entrapment and polarization enhance the stress tensors, reaching a value of 73.6 for five layers and approaching the measured 72 dyn/cm surface stress at 25 °C. Hence, the entire O:H−O bond was extended by molecular undercoordination at the nanobubble skin, making the supersolid skin less dense. The increase in EH leads to thermal stability. The stiffened H−O phonon results in slower molecular dynamics with a longer lifetime, as confirmed by excitation frequency measurements.43 Reducing the number of intermolecular bonds at the water− gas interface results in the generation of surface tension forces, and these forces decrease with the radius of a bubble in liquid.54 The extent of O:H polarization determines the surface tension, as verified in acid−salt−base solutions.52,53 Other external factors, which may affect the properties of nanobubbles, can be considered as stimuli of the O:H−O segmental potentials. The effect of this will manifest through the modulation of the ultrashort interactions within the O:H−O bond. For example, a hydrophobic or hydrophilic substrate can be considered an external factor. In the interaction between a hydrophobic 11325

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(7) Oh, S. H.; Han, J. G.; Kim, J.-M. Long-term stability of hydrogen nanobubble fuel. Fuel 2015, 158, 399. (8) Takahashi, M. Base and technological application of micro-bubble and nano-bubble. Mater. Integr. 2009, 22, 2. (9) Chan, C. U.; Chen, L.; Arora, M.; Ohl, C.-D. Collapse of Surface Nanobubbles. Phys. Rev. Lett. 2015, 114, 114505. (10) Liu, Y.; Zhang, X. A unified mechanism for the stability of surface nanobubbles: Contact line pinning and supersaturation. J. Chem. Phys. 2014, 141, 134702. (11) Zhang, L.; Wang, C.; Tai, R.; Hu, J.; Fang, H. The Morphology and Stability of Nanoscopic Gas States at Water/Solid Interfaces. ChemPhysChem 2012, 13, 2188. (12) Zhang, X.; Lhuissier, H.; Sun, C.; Lohse, D. Surface Nanobubbles Nucleate Microdroplets. Phys. Rev. Lett. 2014, 112, 144503. (13) Wang, S.; Liu, M.; Dong, Y. Understanding the stability of surface nanobubbles. J. Phys.: Condens. Matter 2013, 25, 184007. (14) Weijs, J. H.; Lohse, D. Why Surface Nanobubbles Live for Hours. Phys. Rev. Lett. 2013, 110, 054501. (15) Stocco, A.; Möhwald, H. The Influence of Long-Range Surface Forces on the Contact Angle of Nanometric Droplets and Bubbles. Langmuir 2015, 31, 11835. (16) Chan, C. U.; Arora, M.; Ohl, C.-D. Coalescence, Growth, and Stability of Surface-Attached Nanobubbles. Langmuir 2015, 31, 7041. (17) Nishiyama, T.; Yamada, Y.; Ikuta, T.; Takahashi, K.; Takata, Y. Metastable Nanobubbles at the Solid−Liquid Interface Due to Contact Angle Hysteresis. Langmuir 2015, 31, 982. (18) Ushikubo, F. Y.; Furukawa, T.; Nakagawa, R.; Enari, M.; Makino, Y.; Kawagoe, Y.; Shiina, T.; Oshita, S. Evidence of the existence and the stability of nano-bubbles in water. Colloids Surf., A 2010, 361, 31. (19) Hampton, M. A.; Nguyen, A. V. Nanobubbles and the nanobubble bridging capillary force. Adv. Colloid Interface Sci. 2010, 154, 30. (20) Ball, P. Nanobubbles are not a superficial matter. ChemPhysChem 2012, 13, 2173. (21) Craig, V. S. J. Very small bubbles at surfacesthe nanobubble puzzle. Soft Matter 2011, 7, 40. (22) Yount, D. E. Skins of varying permeability: A stabilization mechanism for gas cavitation nuclei. J. Acoust. Soc. Am. 1979, 65, 1429. (23) Azmin, M.; Mohamedi, G.; Edirisinghe, M.; Stride, E. P. Dissolution of coated microbubbles: The effect of nanoparticles and surfactant concentration. Mater. Sci. Eng., C 2012, 32, 2654. (24) Strasberg, M. Onset of Ultrasonic Cavitation in Tap Water. J. Acoust. Soc. Am. 1959, 31, 163. (25) Duval, E.; Adichtchev, S.; Sirotkin, S.; Mermet, A. Long-lived submicrometric bubbles in very diluted alkali halide water solutions. Phys. Chem. Chem. Phys. 2012, 14, 4125. (26) Weijs, J. H.; Seddon, J. R. T.; Lohse, D. Diffusive Shielding Stabilizes Bulk Nanobubble Clusters. ChemPhysChem 2012, 13, 2197. (27) Sun, C. Q. Relaxation of the Chemical Bond; Springer-Verlag: Heidelberg, 2014; Vol. 108. (28) Huang, Y.; Zhang, X.; Ma, Z.; Zhou, Y.; Zheng, W.; Zhou, J.; Sun, C. Q. Hydrogen-bond relaxation dynamics: Resolving mysteries of water ice. Coord. Chem. Rev. 2015, 285, 109. (29) Liu, X.; Zhang, X.; Bo, M.; Li, L.; Tian, H.; Nie, Y.; Sun, Y.; Xu, S.; Wang, Y.; Zheng, W.; Sun, C. Q. Coordination-resolved electron spectrometrics. Chem. Rev. 2015, 115, 6746. (30) Huang, Y.; Zhang, X.; Ma, Z.; Zhou, Y.; Zheng, W.; Zhou, J.; Sun, C. Q. Hydrogen-bond relaxation dynamics: Resolving mysteries of water ice. Coord. Chem. Rev. 2015, 285, 109. (31) Kühne, T. D.; Khaliullin, R. Z. Electronic signature of the instantaneous asymmetry in the first coordination shell of liquid water. Nat. Commun. 2013, 4, 1450. (32) Petkov, V.; Ren, Y.; Suchomel, M. Molecular arrangement in water: Random but not quite. J. Phys.: Condens. Matter 2012, 24, 155102.

substrate and O:H−O, the hydrophobic substrate can easily shorten the O−H bond and polarize the O:H−O bond, leading to the supersolidity of the skin of a water nanobubble.

5. CONCLUSIONS Without any assumptions or approximations, incorporating the known structure of water and O:H−O bond relaxation dynamics has enabled us to clarify the unusual mechanical and thermal behavior of nanobubbles. Consistency between predictions and experimental observations makes clear the following: (1) Molecular undercoordination at the nanobubble surface shortens the O−H bond but extends the O:H nonbond because of the repulsive force between bonding electrons and nonbonding pairs on oxygen anions. (2) O−H vibrational frequency increases and O:H frequency decreases accordingly in nanobubbles. (3) O:H−O phonon and energy relaxation determines the mechanical stability, viscosity, thermal stability, and lifetime of nanobubbles. (4) The effect of molecular undercoordination on the nanobubble skin is weaker than water clusters, nanobubbles, and water skin because of the negative curvature. (5) Although it is hardly resolved experimentally, the undercoordination effect is significant with respect to the physical properties of nanobubbles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation (No. 21401180, 21273191, and 11502223) of China, Guangdong Innovation Youth Fund (No. 2015KQNCX144), Natural Science Foundation of Guangdong (No. 2016A030310060) and SZU (No. 827000131, No. 201528), and Shenzhen foundation fund (No. JCYJ20160427105015701) is gratefully acknowledged.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01660 Langmuir 2016, 32, 11321−11327

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DOI: 10.1021/acs.langmuir.6b01660 Langmuir 2016, 32, 11321−11327