Solution Interface by Deprotonation of Dangling

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Charging of the Ice/Solution Interface by Deprotonation of Dangling Bonds, Ion Adsorption, and Ion Uptake in an Ice Crystal As Revealed by Zeta Potential Determination Arinori Inagawa,*,† Makoto Harada,‡ and Tetsuo Okada*,‡ †

Graduate School of Engineering, Utsunomiya University, Utsunomiya, Tochigi 321-8585, Japan Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan

‡

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ABSTRACT: We reveal the charging mechanism and behavior of ions at the ice/solution interface through measurements of the zeta potential of ice. The zeta potential of ice, which is calculated from the migration of a probe in an ice channel under various conditions, is interpreted using the Stern double layer model. The zeta potential of ice is generated by the deprotonation of dangling OH bonds, the adsorption of ions on the ice surface, and ion uptake in the ice crystal lattice. The deprotonation of the dangling OH bonds on the surface of ice is enhanced compared to that in bulk liquid water; the pKa of the former is estimated to be ∼3. Interestingly, only 1.41% of the total dangling OH bonds on the ice surface are deprotonated, even at pH > 6, suggesting that the deprotonation of a dangling bond suppresses further dissociation of the nearby OH sites. This is caused by the facilitated reorientation of the water molecules in ice in the presence of L-defects. The ion adsorption constants indicate that the interaction of ions other than H+ with the ice surface is mainly driven by coordination of the dangling bonds to the ions. Therefore, smaller ions are adsorbed more readily on the ice surface than their larger counterparts. Additionally, uptake of F− in the ice crystal lattice is suggested. Elucidation of the ice/water interface properties will allow us not only to understand the unique properties found in nano- or micro-sized liquid phases confined in ice but also to develop novel separations and reactions using frozen aqueous media as platforms.

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INTRODUCTION Surface potential is an important physicochemical property of materials, affecting solute interactions at the solid/liquid interface, the stability and aggregation of colloid materials, and electrokinetic phenomena such as electrophoresis, electroosmosis, and streaming potential.1 Zeta potential, which is a convenient measure of the surface potential, has been used to discuss not only the stability of emulsions, colloids, and bubbles but also the interactions between cells and phagocytosis.2−6 The zeta potentials of nanoparticles,7 carbon nanotubes,8 metallic materials,9 and textiles10 have been used to evaluate their surface properties. In many cases, the zeta potential of a particulate material is determined from its electrophoretic mobility; most commercially available zeta potential analyzers also rely on this principle. However, if this approach is inapplicable, another appropriate method is required. Ice is a material to which the usual methods of zeta potential measurement cannot be applied directly because of the instability of the ice/liquid interface that is formed when an aqueous solution is frozen. When the temperature is lower than the eutectic point, the system involves pure ice crystals and the solid phases of the impurities. As the temperature rises above the eutectic point, the solutes are dissolved to form a © XXXX American Chemical Society

concentrated liquid phase, which is referred to as a freezeconcentrated solution (FCS).11−15 FCSs have been used as platforms for fabricating reaction and analytical systems. Chemical reactions in the FCS are of particular interest because of their involvement in processes of environmental, biological, and industrial importance. The acceleration reactions such as nitrite oxidation16 and the hydrolysis of fluorescein diacetate,17 as well as a 4 order of magnitude enhancement of crown ether complexation,18 have been reported in frozen systems. FCSs exist ubiquitously in natural environments such as ice sheets, snowpack, and clouds. Reactions in naturally formed FCSs play key roles in the global circulation of various compounds of environmental importance.19−24 Klán et al. found that the photochemical reaction of 4-chlorophenol in the FCS gives different products from the corresponding reaction in bulk solution.25 Kim et al. studied the photochemical oxidation of I− to gaseous I2 in frozen solution, which can be a pathway of iodine emission in the polar atmosphere.26 Biological reactions are also affected by freezing. Attwater et al. reported that the replication of RNA Received: December 26, 2018 Revised: February 14, 2019 Published: February 18, 2019 A

DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C by ribozymes is accelerated in the FCS.27 The acceleration of this biological reaction in the FCS suggests that freezing may have played a critical role in the origin of life. The acceleration of reactions in the FCS is attributed not only to the concentration of the solutes frozen in the FCS but also to the interaction of the solutes with the ice surface.28 However, a comprehensive explanation of these processes has not been reported because the properties of the FCS and ice/FCS interfaces are not fully understood. The interface between the FCS and ice provides chemical environments different from that in bulk solution, with high local concentration of the solutes, polarized dangling bonds on the ice surface, and ice/water phase transition, which occurs microscopically even at thermal equilibrium.29 However, only a few studies have focused on this interface because it is difficult to keep the interface stable, and appropriate approaches selective to the interface are limited. The electrostatic properties of the ice interface are an important factor affecting physicochemical processes occurring at the ice/FCS interface. Kallay et al. studied the surface potential of ice using an ice electrode, which revealed Nernstian behavior in acidic media. Although this method has the advantage of directly determining the surface potential of ice, the interactions of ice with ions other than H+ cannot be probed.30,31 Drzymala et al. determined the zeta potential of D2O ice particles at 3.8 °C using electrophoresis.32 Electrophoretic measurements of ice migration are not straightforward because ice, which has a lower density than liquid water, floats on the surface of water. In reality, the results reported in the literature have large deviations. Recently, we proposed a reliable method for the measurement of the zeta potential of solidified solvents such as ice.33 The zeta potential is determined from the migration velocities of probe particles in a microchannel (tunnel) fabricated within the ice. Precise zeta potential determination under various conditions using this method would allow us to understand the phenomena occurring at the ice/FCS interface from a microscopic viewpoint. In this paper, we discuss the behaviors of ions at the ice/FCS interface based on the zeta potential of ice, which is determined at varying pH and salt concentrations. Using the Stern model, we observe various interesting features in the processes of ice interface charging.

Precision Inc.). The EOF and probe migration were observed using a laser confocal microscope (FV1200, Olympus). The μp values were determined under various solution conditions (pH, type of salt, and concentration). A microchannel (tunnel) in bulk ice was fabricated in the same way as reported in our previous paper.33 Figure S1 shows a schematic illustration of the channel fabrication process and the fluorescent images of a fabricated ice microchannel. The running solution was 1.92 M aqueous glycerol containing a salt. For the pH dependence study, 0.1 mM phosphate buffer was added to the running solution for pH adjustment, and an ionic strength of 1 mM was achieved by adding NaCl. For the measurements at pH ≤ 3, the pH was adjusted with HCl; the ionic strength was higher than 1 mM in such cases. Two Ag/ AgCl electrodes were inserted into the microchannel. The distance between the two electrodes was set to 6.5 mm in all of the experiments. A voltage of 40 V was applied to the electrodes, and the migration of the probe particles in the ice microchannel was captured using a charge-coupled device camera installed on the fluorescence microscope.

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RESULTS AND DISCUSSION Salt Concentration Dependence of the Zeta Potential of Ice and Its Interpretation Assuming Ion Adsorption. The determination of the zeta potential from the migration velocities of the probe particles is briefly described here; details are given in our previous publication.33 The apparent velocity of the probe particles (vapp) in the ice microchannel is the sum of the electrophoretic migration rate (vp) and the EOF rate (vEOF) vapp = vp + vEOF = μp E + vEOF

(1)

where μp is the electrophoretic mobility of the particle and E is the electric field strength. Thus, vEOF can be estimated from μapp and μp. The latter was determined in advance using MCE. Some of the μ p values (pH dependence and NaCl concentration dependence) were determined in our previous work.33 Figures S2−S4 summarize the μp values, which are required for the subsequent discussion. The EOF rate is expressed as

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vEOF =

EXPERIMENTAL SECTION The method for zeta potential measurements was essentially the same as that used in our previous work33 and is briefly described here. All of the experiments were conducted at −4.0 °C. The electrophoretic mobility of the probe microparticles (μp) at −4.0 °C was determined using microchip electrophoresis (MCE). A glass microchip having a crossed channel with a width of 50 μm and a depth of 20 μm (Micronit Co.) was used. The lengths of the separation and injection channels were 35 and 10 mm, respectively. The microchip was placed on a Peltier unit (Takagi Manufacturing Co., Ltd.), which was driven by a Peltier controller (model TDC-2030R, Cell System). The microchannel was filled with 1.92 M aqueous glycerol as a running solution, which was in equilibrium with ice at −4.0 °C. Carboxyl-modified polystyrene (PS) particles with a diameter of 1 μm (Polyscience Inc.) were injected to the channel as the probe, with rhodamine B as the electroosmotic flow (EOF) marker. The PS particles were stained with yellow/green dye (λex = 441 nm; λem = 486 nm). A voltage of 3.0 kV was applied to both ends of the separation channel using a high-voltage power supply (HCZE-30P, Matsusada

ε0εζ E η

(2)

where η is the viscosity of the medium, ε0 is the vacuum permittivity, and ε is the relative permittivity of the medium. The relative permittivity follows the Curie−Weiss equation above the Curie temperature ε=

B T − TC

(3)

where B is a material-dependent constant and TC is the Curie temperature. The relative permittivity of 1.92 M aqueous glycerol at various temperatures is summarized in Table S1. The permittivity values at subzero temperatures can be extrapolated from data in the literature.34 On the basis of these data, an ε value of 86.1 was used for the calculations in this work. Figure 1 shows the dependence of the zeta potential of ice on the NaCl concentration. The potential changed from −43.6 to −12.7 mV when the NaCl concentration was increased from 1 to 20 mM. Although pH was not adjusted in these experiments (and was typically 5−6), the pH dependence of B

DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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OH bonds occurs on the ice surface, whereas the adsorption of ions takes place on the Stern layer, which is separated from the ice surface by the radii of the ions. Thus, the charge of the ice surface arises from the O− sites and adsorbed ions. If the ion adsorption is assumed to obey the Langmuir adsorption isotherm, the surface concentration of an ionic species i on ice, Γi, is given by Γi =

Γi ,maxK ici 1 + K ici

(4)

where Γi,max is the maximum adsorption capacity per unit area and Ki is the adsorption constant of species i. The value of Γi,max for an anionic species is equivalent to the total surface concentration of dangling OH bonds on the ice surface. For a cation, Γi,max is given by σs/F because electrostatic interaction with the O− site was assumed. According to the Stern electrostatic model, the charge density at the Stern layer, σst, is expressed by

Figure 1. Dependence of the ice zeta potential on the NaCl concentration. The zeta potential data are taken from ref 33.

the zeta potential reported in our previous paper suggests that the dissociation of the dangling OH bonds on the ice surface in this condition. The decrease in the zeta potential has two origins: the shrinking of the electric double layer at higher ionic strength and the adsorption of ions at the ice/liquid interface. The following model was utilized to interpret the dependence of the ice zeta potential on the salt concentration. Interactions were assumed to occur between the anions and the OH sites on the ice surface and between the cations and the O− sites, as depicted schematically in Figure 2A. Similar interactions were assumed for the interpretation of the behavior of ions at solution interfaces with metal oxides.35−37 This model assumes that the deprotonation of the dangling

i z FΨ y ∫a F∑ zici0 expjjjj− iRT st zzzzdx k

∞

σst =

i

{

(5)

where F is the Faraday constant, z is the charge of the ion, ci0 is the bulk concentration of ionic species i in the solution, Ψst is the surface potential, a is the closest approach of the ions, R is the gas constant, and T is the absolute temperature. For 1:1 salts, |z+| = |z−| = 1 and c0 = c0+ = c0. Thus, eq 5 can be simplified as i zF Ψst yz zz 8RTε0εc0 sinhjjjj− z k RT {

σst =

(6)

The surface charge density at the Stern layer is given by the summation of the charge of the ice surface and the charges of the adsorbed ions i |z|F Ψst yz zz 8RTε0εc0 sinhjjjj z k 2RT { = σs −

+

(

(ΓmaxF − σs)K −c − exp

(

1 + K −c − exp z+F Ψst RT

(

σsK+c+ exp

(

1 + K+c+ exp

z −F Ψst RT

z −F Ψst RT

)

)

)

z+F Ψst RT

)

(7)

where σs is the surface charge density of ice and K− and K+ are the adsorption constants of the anion and cation on ice, respectively. The zeta potential (ζ), which can be experimentally determined, is the electrostatic potential at the slipping plane and is different from the Stern layer potential. Thus, the estimation of the Stern layer potential is needed to discuss the ion adsorption on ice. As shown in Figure 2B, the electrical double layer at the surface was divided into three layers,38,39 and the entire system was regarded as two connected capacitors. The charge density at each plane can be described as Figure 2. Schematic illustrations of the electrical double layer at the ice surface in contact with an aqueous electrolyte. (A) Ion interactions on the ice surface. (B) Equivalent electrical circuit for the electrical double layer illustrated in part (A). C

σs = Cst(Ψs − Ψst)

(8)

σst = Cst(Ψst − Ψs) + Cd(Ψst − Ψd)

(9)

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Figure 3. Dependence of ice zeta potentials on the salt concentration (solid circles) and estimated Stern potentials (triangles). Electrolyte: (A) NaCl, (B) CsCl, (C) NaF, and (D) NaClO4. The blue curves represent the results of calculation using eq 7.

Table 1. Adsorption Constants of Ions Determined from the Zeta Potential of Ice in Contact with an Aqueous Solution Containing a Salt and Standard Gibbs Energies of the Hydration of the Ions ion +

Na Cs+ F− Cl− ClO4−

K+/10−1 M−1

K+,O/10−3 M−1

8.37 4.22

6.74 1.89

σd = Cd(Ψd − Ψst)

∂σst = ∂Ψ

i zF Ψst yz 2000z 2F 2ε0εrc zz coshjjjj z RT k 2RT {

Assuming Ψd = ζ, σd =

39,40

(11)

σd is given by

i zFζ zy zz 8RTε0εc sinhjjj− k 2RT {

ΔGhyd ° /kJ K−1 mol−1

6.18 5.94 4.78

−481 −258 −472 −347 −214

This suggests that the charge from adsorbed ions was 2 orders of magnitude smaller than that from the dissociated dangling bonds. Thus, the charging of the ice/FCS interface mainly arises from the dissociation of the dangling OH bonds. The dependence of the ice zeta potential on the salt concentration for CsCl, NaF, and NaClO4 is also shown in Figure 3B−D. The adsorption constants for these ions are summarized in Table 1. Equation 7 explained the Stern potential well for all these electrolytes, as shown in Figure 3. Although as noted above, ion adsorption plays a minor role in the charging of the ice surface, the zeta potential measurements indicated that different ions behaved in different fashions at the interface. Thus, the variation in the zeta potential with the salt concentration was attributed not only to the shrinkage of the electrical double layer but also to the adsorption of ions on the ice surface. The K− values increased in the order ClO4− < Cl− < F−, indicating that the smaller anions had greater affinity for the ice surface than their larger counterparts. Because we assumed that anions were adsorbed on the dangling OH bonds, K− is related to the proton acceptor ability of an anion or its basicity. Thus, an increase in the K− value correlates with the Gibbs energy of hydration of the anion because anion adsorption is similar to

(10)

where Cst and Cd are the capacitances of the Stern layer and the diffusion layer, respectively, and Ψd is the potential at the diffusion layer. According to the Gouy−Chapman model, Cd is given by Cd =

K−/10−1 M−1

(12)

Thus, Ψst can be estimated using eqs 10−12 and ζ. The triangles in Figure 3A represent the values of Ψst estimated from ζ. The blue curve represents the results of curve fitting by eq 7 with the fitting parameters KNa+ = 8.37 × 10−4 mol−1 m3, KCl− = 5.94 × 10−4 mol−1 m3, and σs = 6.50 × 10−3 C m−2. When ice was in contact with 1 mM aqueous NaCl, the surface concentration of the adsorbed Cl− was equivalent to a charge density of 3.80 × 10−5 C m−2 while that of the deprotonated dangling bonds was 6.50 × 10−3 C m−2. D

DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C hydration. Additionally, Na+ exhibited a higher affinity for the −O− sites than Cs+. This cannot be explained by the electrostatic interactions of these cations because Cs+ interacts more strongly with anionic sites than Na+ in aqueous media. Therefore, the higher affinity of Na+ for the ice surface should be explained by electron donation from dangling −O− bonds rather than by purely electrostatic interactions with the −O− sites. This interaction of a cation with the dangling −O− bond is also considered similar to its hydration from the molecular perspective. As discussed later in the article, the negative charge on the oxygen atom was distributed over a wide range and was not localized on a particular site. The adsorption constants of the cationic species on the dangling −O− bond, K+,O, were also estimated and are listed in Table 1. Because the population of dangling −O− bonds was much larger than that of −O− sites, K+,O was smaller than K+. In addition, K+,O was smaller than K−, suggesting greater adsorption of anions than cations on the ice surface. As shown in Table 2, the σs value of NaF was significantly larger than those of the other salts. This suggests that the

crystal and the ion interaction on the ice surface. Thus, the zeta potential of ice shows similar features to the WRFP. However, the WRFP no longer appears when the growth of ice crystals stops. Wilson and Haymet44 reported that the WRFP value linearly increases with an increase in the crystal growth rate. In the present case, the phase transition at an ice/solution interface reaches an equilibrium. Although the microscopic phase transition may occur even at a thermodynamic equilibrium as stated above, the rate of ice crystal growth should be negligibly low. Thus, the WRFP does not contribute to the zeta potential determined in the present study. pH Dependence of Zeta Potential of Ice. The above discussion suggested that the main source of electric charge on the ice surface was the deprotonation of the dangling OH bonds, which in turn should be a function of the pH of the FCS. Figure 4 shows the pH dependence of the zeta potential

Table 2. Surface Charge Density of Ice in Contact with an Aqueous Electrolyte electrolyte

−σs/10−3 C m−2

anionic radius/Å

NaF NaCl CsCl NaClO4

7.97 6.50 6.50 6.36

1.19a 1.67a 1.67a 2.40b

a

Chemical Society of Japan. Kagaku Binran (Chemical Index), 4th ed.; Maruzen: Tokyo, 1993. bRoobottom, H. K.; Thakur, K. P. Thermochemical Radii of Complex Ions. J. Chem. Educ. 1979, 56(9), 576−577.

Figure 4. pH dependence of the ice zeta potential (black circles), Ψst (red circles), and Ψs (blue diamonds). The zeta potential data were taken from ref 33.

surface charge density of ice in contact with aqueous NaF was enhanced by a mechanism other than ion adsorption. One possibility is that F− was entrapped in the ice lattice or defects therein. Although the ice/solution interface was macroscopically stable, continuous solid/liquid phase transition occurs on the microscopic level. During this process, ions can be entrapped in the ice crystal. Such phenomena were detected as freeze potential.28 Additionally, we detected an imbalance between the anion and cation distributions in the FCS and ice using precise pH measurements.41 F− can be accommodated in ice crystals because its ionic size corresponds to the size of a water molecule.34,42 The negative potential of the ice surface can be enhanced by this process. Hence, the zeta potential of ice in contact with an aqueous electrolyte arises from the deprotonation of dangling OH bonds, adsorption of ions, and ion uptake into the ice crystal. The electrostatic potential is generated when ice crystals grow in a diluted aqueous electrolyte, known as the Workman−Reynolds freezing potential (WRFP).43,44 The WRFP arises from an imbalance between the cation and anion uptake into ice crystals during the freezing process. Reported WRFP values vary more markedly with the type of anion rather than that with cation as far as halide anions and alkali cations are concerned. Also, larger WRFP occurs in an aqueous electrolyte containing a smaller ion, for example, F−, in halide anion. This tendency agrees with the present results. Because the interaction between an ion and water molecules has an electrostatic nature, the charge density of an ion is an important factor governing both the ion uptake in the ice

of ice. Numerical data were taken from our previous paper.33 As the pH was increased, the zeta potential became more negative and then remained almost constant at around −56 mV at pH > 5. This indicates that the dissociation of the dangling OH bonds had reached equilibrium in this pH range. The surface potential of ice, Ψs, which was estimated using the Stern model, is plotted in Figure 4. For this calculation, the capacitance of the Stern layer is given by a Cst = ε0ε (13) We assumed that a was equal to the radius of the adsorbed anion;45 in this case, a = 1.67 Å for Cl−.34 The degree of deprotonation of the dangling OH bonds, α, is given by α=

ΓO − Γd ‐ DB

(14)

where ΓO− is the surface concentration of the dissociated dangling bonds and Γd‑DB is the total surface concentration of the dissociable dangling bonds. The dissociation constant of the dangling bonds, Ka, can be described as Ka = E

[H+]s ΓO − [H+]s α = Γd ‐ DB − ΓO − 1−α

(15) DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C where [H+]s is the proton concentration at the surface. Because the proton concentration near the ice surface follows the Boltzmann distribution, Ka is given by

ice. Podeszwa and Buch used molecular dynamics simulations to demonstrate that the energetic barrier for defect migration is so low that the defects migrate on a time scale of picoseconds at T = 200−230 K.48 According to a study by Koga et al., Ldefects are trapped in preferential spots for a few tens of picoseconds.49 Their results also indicated that the lifetime of L-defects is about 1/5 of the trapping time. de Koning et al.50 calculated the electrical conductivity of ice using density functional theory and found that the migration of L-defects dominates the conductivity of ice, which was consistent with the results of Podeszwa and Buch. These findings suggest that the reorientation of water molecules in ice is accelerated by the presence of defects and that defects migrate rapidly in the ice crystal. Thus, the surface O− sites are quickly spread throughout a large volume of the ice crystal. The fast diffusion of the defects inhibits the further deprotonation of dangling OH bonds over a wide region surrounding a dissociated site. The interaction of cations with the O− sites was first assumed for the discussion of the ice charging by ion adsorption. However, the negative charge is spread over a wide region, thus indicating that the interaction of the cations should not be purely electrostatic but should also involve electron donation from the O sites, which are more widely distributed over the ice surface than O− sites. For this reason, we calculated the interaction constants between cations and the dangling −O− bonds as well. The acid dissociation constant of the dangling OH bonds was significantly larger than that of liquid water. This indicates that deprotonation is enhanced when water is frozen. Kato et al. reported that OH bonds are weakened when water is condensed on SrTiO3 (0 0 1).51 Johnson et al. reported that both OH− and H+ are stabilized in water condensed on MgO (0 0 0).52 These results suggest that the condensation of water molecules on a solid surface reduces the activation energy for water dissociation. Water molecules on the surface of ice are strongly connected to those inside the ice crystal by hydrogen bonding; this effect enhances the deprotonation of dangling OH bonds.

i FΨ y α K a = [H+]0 expjjjj− s zzzz· (16) k RT { 1 − α + where [H ]0 is the bulk concentration of protons. Figure 4 indicates that the zeta potential becomes constant at pH > 5. The zeta potential data are replotted in Figure 5 by taking α

Figure 5. Degree of deprotonation of the dangling bonds on the ice surface. The red curve shows the calculated degree of deprotonation using Ka and Γd‑DB as fitting parameters.

instead of the potential as the ordinate. The red curve in Figure 5 represents the results of the calculations using eq 16 with Ka and Γd‑DB as the fitting parameters; the optimized values were Ka = 9.68 × 10−4 and Γd‑DB = 1.14 × 10−7 mol m−2. Because the distance between adjacent oxygen atoms in the Ih ice crystal is 2.75 Å,46 the total surface density of the dangling OH bonds on the basal plane of the Ih ice crystal is 8.09 × 10−6 mol m−2. Thus, the Γd‑DB value of 1.14 × 10−7 mol m−2 determined above suggests that only 1.41% of the dangling OH bonds on the surface were involved in the proton dissociation, while the rest remained undissociated even at pH > 8. This indicates that the deprotonation of a particular dangling bond inhibits further dissociation at nearby sites. This effect arises from the fast reorientation of water molecules in ice, which is caused by the proton disorder of the Ih ice crystal.47 As shown in Figure 6, the orientation of water molecules in ice changes to eliminate the formed L-defect, in which there is no hydrogen atom between a pair of neighboring oxygen atoms. Several researchers have studied the effect of defects on the reorientation of water molecules in

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CONCLUSIONS The present work has discussed the charging mechanism of the ice surface through the zeta potential measurements. The charge on the ice surface originates from the dissociation of dangling OH bonds, ion adsorption, and ion uptake in the ice crystal. The dissociation of the dangling OH bonds plays the most important role. Interestingly, only 1.41% of the dangling OH bonds on the ice surface were deprotonated because the fast reorientation of water molecules facilitates the diffusion of the O− sites or L-defects over a wide range. The ice/water interface has characteristic features because both phases consist of the same molecules, that is, H2O molecules. This makes the interface ambiguous at the molecular scale because continuous freezing and melting cycles occur at the microscopic level. This often complicates the measurement of the physicochemical properties of this interface. We previously reported the viscosity of the FCS and suggested the possible existence of a low-density water phase near the ice/FCS interface.53 Our research group also observed enhanced dissolution of hydrophobic substances near the ice/ FCS interface by cyclic voltammetry.54 These results support the existence of low-density water at the ice/FCS interface. Frozen aqueous solutions are useful for designing separation and reaction systems.55−57 Separations using a frozen aqueous

Figure 6. Schematic illustration of the migration of an L-defect by reorientation of water molecules in the ice crystal. F

DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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phase rely strongly on the interaction of the solutes on the ice surface, and the separation process is influenced by the electrostatic nature of the ice surface. Thus, the results in the present paper allow us not only to facilitate the understanding of the fundamental aspects of the ice surface but also to help us to devise novel methodologies using the characteristics of the ice/water interface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b12435.

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Relative permittivity of the running solution, fabrication process of an ice microchannel, and electrophoretic mobility of the probe microspheres (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.I.). *E-mail: [email protected] (T.O.). ORCID

Arinori Inagawa: 0000-0003-0629-6060 Tetsuo Okada: 0000-0002-1976-7936 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research and by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science. A.I. also thanks the Japan Science Society for a Sasakawa Scientific Research Grant.

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b12435 J. Phys. Chem. C XXXX, XXX, XXX−XXX