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Viscosity of Freeze-Concentrated Solution Confined in Micro/Nanospace Surrounded by Ice Arinori Inagawa, Tomoki Ishikawa, Takuma Kusunoki, Shoji Ishizaka, Makoto Harada, Takuhiro Otsuka, and Tetsuo Okada J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 24, 2017

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The Journal of Physical Chemistry

Viscosity of Freeze-Concentrated Solution Confined in Micro/Nanospace Surrounded by Ice

Arinori Inagawa,1 Tomoki Ishikawa,2 Takuma Kusunoki,2 Shoji Ishizaka,2 Makoto Harada,1 Takuhiro Otsuka,1 and Tetsuo Okada1*

1

Department of Chemistry, Tokyo Institute of Technology, Meguro-ku,

Tokyo 15-8551, Japan 2

Department of Chemistry, Hiroshima University, Kagamiyama,

Higashi-Hiroshima 739-8526, Japan

Email, [email protected] Phone and Fax, +81-3-5734-2612

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Abstract An aqueous solution separates into ice and a freeze-concentrated solution (FCS) when frozen at temperatures above the eutectic point. The FCS acts as important reaction media in natural environment and industrial processes. The viscosities of the FCS in frozen glycerol/water solutions are evaluated by two spectrometric methods with different principles: (1) the reaction rate of the diffusion-controlled fluorescence quenching and (2) fluorescence correlation spectroscopy. Thermodynamics indicates that the concentration of glycerol in the FCS is constant at a constant temperature regardless of the glycerol ini ). However, the viscosity of the concentration in the original solution before freezing ( cgly

ini FCS measured at a given temperature increases with decreasing cgly , and this trend

becomes more pronounced with decreasing measurement temperature. Further, the viscosity of the FCS in a rapidly frozen solution is higher than that in a slowly frozen solution. These results suggest that the viscosity of the FCS depends on the size of the space, in which the FCS is confined, and is enhanced in smaller spaces. This result agrees well with several reports of anomalous phenomena in a microspace confined in ice. These phenomena should originate from the fluctuation of the ice/FCS interface, which is macroscopically stable but microscopically dynamic and undergoes continuous freezing and thawing. Thus, the FCS near the interface has ice-like physicochemical properties and structures, giving higher viscosity than the corresponding bulk solution. 2 ACS Paragon Plus Environment

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Introduction Developments in material science and nanotechnology have allowed us to study chemistry in nanospaces of well-defined dimensions. A liquid confined in a nanospace exhibits different features from its bulk phase. In particular, water in micro- or nanospaces has received considerable attention because such spaces are often found not only in organisms and natural environments but also in industrial materials, including frozen food, air-conditioning systems, hydrates, and micro- or nanofluidic devices. In addition, studies of confined water provide a clue to poorly understood phenomena in water chemistry. For these reasons, a number of experimental and theoretical studies have been conducted to investigate the intrinsic nature of water and its roles in organisms and natural environments.1–4 The freezing temperature of water decreases below the homogeneous nucleation temperature of bulk water (235 K) in nanopores,2 and the decrease caused by confinement in nanospaces is larger than that predicted for the Gibbs–Thomson effect.5 Thus, because the supercooled liquid phase is kept stable in nanospaces, confinement of water has often been used to study the low-temperature behavior of liquid water.1,2,6–8 The characteristics of confined water are often influenced by the properties of the surrounding walls. Simulation studies have indicated that bilayer ice is formed when water is confined between hydrophobic surfaces.9,10 A recent study indicated that bilayer ice is formed even on a hydrophobic surface, and confinement in a nanogap is not necessarily a critical factor.11 The hydrogen bonds between water molecules and the hydrophobic surface 3 ACS Paragon Plus Environment


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are so weak that the intermolecular hydrogen bonding of water is maximized under this condition. In contrast, on a silica surface, silanol groups interact with water molecules by hydrogen bonding, and the water structuring becomes stronger near the surface than in the bulk.2 Kurihara and her coworkers12,13 studied the effects of the surface properties on the fluidic nature of water sandwiched between silica plates or hydrolyzed silica plates. The viscosity of water increased as the gap between silica plates of either type became narrower. Interestingly, this effect was less pronounced for hydrolyzed silica plates, which enhanced the water structure near the wall, as revealed by sum frequency generation spectroscopy. Although the relationship between this structural behavior and the viscosity of water confined in the gap was not clearly explained, this study suggested that the wall properties clearly affect the nature of the confined water. NMR studies have revealed that when water is confined in a so-called extended nanospace with a dimension of Na+ > Li+.33 Because the distribution coefficient from the FCS to ice was estimated to be in the order of 10-3, this causes only a small removal of ions from the FCS. To study this effect on the determination 15 ACS Paragon Plus Environment


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of the FCS viscosities, LiCl instead of KCl was used for adjusting the ionic strength, because effects of the ionic distribution to ice are smaller for LiCl than for KCl. Figure S8 compares the viscosities determined using KCl with those using LiCl. The viscosities agree well at any temperature, strongly indicating that the removal of a salt from the FCS is not a critical factor in the viscosity measurements by the present approach. Consequently, the difference in the viscosity between the FCS and bulk becomes ini , and at lower Tfr. The phase diagram indicates more obvious at lower Tmeas, at lower cgly

ini that the total volume of the FCS becomes smaller as either Tmeas or cgly decreases,

suggesting that the FCS viscosity increases when its total volume decreases. The effect of Tfr is also interpreted in terms of the size of the FCS space. As shown in Figure 2, Tfr affects the dispersity of the FCS in a frozen sample. The FCS frozen at −7.0 °C forms channels with large volumes, whereas dispersed liquid inclusions are formed when the FCS is frozen at liquid nitrogen temperature. The FCS is confined in smaller spaces when it is frozen at lower temperature. Thus, all of the results suggest that the viscosity of the FCS increases when the FCS is confined in smaller spaces. The viscosity enhancement of the FCS caused by ice confinement is attributed to water structuring around the ice/FCS interface. Although the ice/FCS interface is macroscopically stable, freezing and thawing events occur repeatedly.44 The interfacial liquid water molecules are affected by the ice wall and should have the properties of 16 ACS Paragon Plus Environment

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low-density water (LDW).45 The viscosity of LDW is reported to be greater than that of normal water.46 When the FCS pools are large, the relative interfacial area having LDW-like nature is so small that only bulk properties arise. Thus, the viscosity is similar to that of the unfrozen solution. On the other hand, when an FCS pool is small, the ice interface occupies a large sector of the entire FCS, and the LDW-like properties become detectable. Thus, the higher FCS viscosities are ascribed mainly to the ice confinement.

Viscosity estimated by fluorescence correlation spectroscopy FCor is a method of measuring the translational motion of fluorescent molecules. The fluorescence intensity fluctuates owing to the passage of fluorescent molecules through the effective focal space of a laser beam. The fluctuation is a function of the size of the molecule, its diffusion constant, and the reaction constant of molecules in the targeted area. The inverse Fourier transform of the fluctuation gives the time-resolved autocorrelation function G, which is expressed as

G (τ ) = 1 +

1 τ τ (1 + ) −1 (1 + 2 ) − 2 N τD s τD

(8)

where τ is the delay time; N is the number of molecules in the effective focal volume; s is the structure parameter, which is the ratio of the axial length of the effective focal space to its radial length; and τD is the diffusion time. The viscosity of a medium can be evaluated from the Einstein–Stokes equation. 17 ACS Paragon Plus Environment


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τD =

6πw2ηr 4k BT

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(9)

where w is the radial length of the effective focal space, and r is the radius of the fluorescent probe. We measured the autocorrelation in unfrozen aqueous glycerol with a concentration FCS at Tmeas. The results are summarized in Figure 6. The G(τ) curve is shifted to equal to c gly

FCS the right side (toward longer τ) with decreasing Tmeas (also increasing c gly ), indicating that

τD becomes larger. This shift of τD is attributed to the increase in the viscosity of he medium. The viscosity values of the unfrozen solution follow the curve predicted by Eq. (7) and agree with the values determined from the rate constants of the quenching reaction discussed above. The laser beam was focused on the grain boundary for the FCor measurements of the FCS viscosities. Typical autocorrelation functions for the FCS are shown in Figure S9. The viscosities of the FCS determined from the autocorrelation functions are summarized in Figure 7. These values are larger than those of unfrozen aqueous glycerol. For example, the ini viscosities of the FCS for cgly = 500 mM are 1.37–1.68 times larger than that of the

ini unfrozen bulk solution; similarly, they are 1.48–1.88 times larger for cgly = 100 mM and

ini 1.72–1.90 times larger for cgly = 50 mM. Thus, the viscosity of the FCS obviously

ini increases with decreasing cgly , i.e., with confinement in ice.

Figure S10 compares the viscosities obtained from the emission intensities of 18 ACS Paragon Plus Environment

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[Ru(bpy)3]2+ and FCor measurements. The viscosities obtained from FCor measurements are larger than those obtained from the emission intensities of [Ru(bpy)3]2+ under any conditions. This difference is attributed to several factors. One possible factor involves the approximations used in the theories supporting these two methods, for example, the assumption of spherical molecules, the Debye–Hückel approximation, and the limited applicability of the Einstein–Stokes theory to molecules. However, because the viscosity values for unfrozen solutions obtained from these methods agree well, theoretical approximations do not cause intrinsic difference in determined value. Other possibility comes from the difference in actual freezing temperature. The cell structures are slightly different for the two spectrometry methods owing to instrumental requirements. Thus, the thermal conduction differs, and eventually the FCS channels or pools differ in size, even under the same conditions. Although discussions of these effects are left for future work, we have confirmed that ice confinement affects the liquid viscosities and the ice/FCS interface plays an important role.

Conclusion We successfully measured the viscosity of the FCS in the aqueous glycerol system. Two different methods indicate that the viscosity of the FCS is enhanced by ice confinement, and the enhancement becomes more pronounced with decreasing FCS space in the ice. This 19 ACS Paragon Plus Environment


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clearly suggests that the ice interface strongly affects the physicochemical properties of the FCS. Although the viscosity is a macroscopic parameter, we can discuss its molecular origins because the microscopic structure of a liquid affects its viscosity. A number of studies have shown that a liquid confined in nanospaces is more viscous than the bulk.12,13,16 In the present case, however, a confinement effect is found even when the size of the FCS is in the micrometer or submicrometer range. This behavior should originate from the fluctuation of the ice/FCS interface, at which the phase transition occurs continuously. The fluctuation range in this interface should be much larger than the thickness of typical rigid interfaces.47–49 Thus, the ice/FCS interface is dynamic and affects the structure and dynamics of the liquid far from the interface. Thus, it should be useful to discuss how other physical parameters, such as the hydrogen bonding strength, solubility, molecular distribution, and acidity, are affected by ice confinement. The size of the focal point of the laser beam in FCor measurement is on the submicrometer scale, which is smaller than the grain boundary. Although we tried to measure local viscosities by focusing the laser beam on the ice-interfacial region, measurements were not successful because of strong interference from ice. Local viscosity measurements should be possible if the laser beam is focused on a much smaller size. In this case, the time change in the FCor signal may involve the fluctuation of the ice/FCS 20 ACS Paragon Plus Environment

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interface, and is expected to reveal the dynamic nature of this interface. We believe that this is an important future task.

Supporting Information ୧୬୧ Details of sample preparation; kq obtained for ܿ୥୪୷ = 500 mM frozen in liquid N2;

phase diagram of the glycerol/water; Stern-Volmer plots obtained under various conditions; the temperature dependence of the FCS viscosities using KCl or LiCl as ionic strength adjuster; autocorrelation functions for FCS; comparison of viscosities obtained by emission quenching of [Ru(bpy)3]2+ and FCor; Table S1; Figures S1-S10.

Acknowledgments We thank Professor Kazuteru Shinozaki, Yokohama City University, for helping measure the lifetime of [Ru(bpy)3]2+ in the FCS. This work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science and by a Sasakawa Scientific Research Grant from the Japan Science Society.

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Table 1. Lifetime of [Ru(bpy)3]2+ in the FCS


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Figure 1. Optical setup for fluorescence correlation spectroscopy.



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Figure 2. FCSs formed on the ice surface. Before freezing, the solutions contained 1.0 µm fluorescein disodium. The green and black areas represent the FCS and ice, respectively. ୧୬୧ Aqueous glycerol was frozen by liquid N2 (A–C) or on the Peltier array at −7.0 °C. ܿ୥୪୷ = ୧୬୧ ୧୬୧ 500 mM for (A) and (D); ܿ୥୪୷ = 100 mM for (B) and (E); ܿ୥୪୷ = 50 mM for (C) and (F).


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Figure 3. Quenching of emission of [Ru(bpy)3]2+ in the FCS by [Fe(CN)6]3−. The original ୧୬୧ solutions contained glycerol at ܿ୥୪୷ = 500 mM, 125 µM of [Ru(bpy)3]Cl2, KCl, and

K3[Fe(CN)6], and 10 µM of 2-naphthol before they were frozen. The samples were frozen in liquid N2 and measured at −12.0 °C.



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୧୬୧ Figure 4. Stern–Volmer plots of glycerol at ܿ୥୪୷ = 500 mM frozen in liquid N2. Original

aqueous solutions contained 125 mM of [Ru(bpy)3]Cl2, K3[Fe(CN)6, and KCl and 10 mM of 2-naphthol. Tmeas = (A) −12.0 °C, (B) −10.0 °C, (C) −8.0 °C, (D) −6.0 °C, and (E) −4.0 °C.


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୧୬୧ Figure 5. Temperature dependence of the viscosity of the FCS for various ܿ୥୪୷ values. (A)

Samples frozen at −7.0 °C, (B) samples frozen in liquid N2.



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Figure 6. Autocorrelation functions for unfrozen aqueous glycerol at various concentrations. The glycerol concentrations were adjusted to model the FCS at Tmeas. R6G (0.1 nM) was added as a fluorescent probe.


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୧୬୧ values as Figure 7. Temperature dependence of the viscosity of the FCS for various ܿ୥୪୷

obtained from FCor measurement.



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TOC


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Viscosity of Freeze-Concentrated Solution ... - ACS Publications

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