Influence of Pore Wall Hydrophobicity on Freezing and Melting of

Subscriber access provided by TULANE UNIVERSITY

Article

Influence of Pore Wall Hydrophobicity on Freezing and Melting of Confined Water Kunimitsu Morishige J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00538 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Influence of Pore Wall Hydrophobicity on Freezing and Melting of Confined Water

Kunimitsu Morishige Department of Chemistry, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: To examine the influence of pore-wall hydrophobicity on freezing and melting behavior of a confined water, we measured the X-ray diffraction pattern of water in the nearly cylindrical pores of ordered mesoporous carbons during cooling and heating processes. The pore walls of the ordered mesoporous carbons are crystalline and cannot form hydrogen bonds with water molecules. Capillary condensation of water in the hydrophobic mesopores takes place very slowly only close to the saturation pressure of a bulk water. Nevertheless, freezing and melting of water confined in the hydrophobic mesopores occurred almost in the same way as those in mesoporous silicas with similar pore sizes, the pore walls of which are amorphous and can form hydrogen bonds with water molecules. This clearly indicates that the pore-wall hydrophobicity does not affect appreciably the freezing and melting behavior of the confined water. The existence of a thin water layer between core ice and the pore walls is responsible for it.

2


Page 2 of 25

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Freezing and melting of water confined in mesopores is directly relevant to many phenomena such as freeze avoidance in plants,1 stone weathering,2 and cloud glaciation.3 Effects of pore size,4-7 pore geometry,8-11 pore filling,12-15 and nonfreezable layer16-19 adjacent to the pore walls on the freezing and melting behavior have been extensively examined using mesoporous silicas and general conclusions have been reached. On the other hand, experimental results concerning the influence of pore wall hydrophobicity are in disagreement with each other. Hirama et al. examined the freezing and melting of water adsorbed in porous Vycor glass weakly treated with hexamethyldisilazone by means of nuclear magnetic resonance (NMR) spectroscopy and reported that the melting point depression of the confined ice in the hydrophobic pores is smaller than for the hydrophilic pores of original glass.20 Liquid water could not penetrate the fully treated glass and thus the freezing and melting behavior of water in the strongly hydrophobic sample was not examined. Strictly speaking, surfaces are referred to as hydrophobic only when a water droplet forms a contact angle larger than 90° with the surfaces.21 In this instance, water cannot be adsorbed at all in the hydrophobic pores even at the saturation pressure of the bulk liquid (p0) so that high pressures externally applied are required to keep water in the mesopores during freezing and melting processes.22 Measurements of freezing/melting of pore water under high pressures are very difficult. In the present study, surfaces are referred to as hydrophobic when the surfaces are less hydrophilic as compared to bare silica surfaces. Deschamps et al. examined the freezing and melting of water confined in the hydrophobic pores of MCM-41 treated with trimethylchlorosilane by differential scanning calorimetry (DSC) and showed that the melting point depression of the confined ice is larger in the hydrophobic pores,23 as opposed to the 3



results of Hirama et al. In their study, water was filled in the hydrophobic pores under high pressures. Since a sample containing pore water was depressurized before introducing into the DSC cell, however, most of the pore water was expelled from the hydrophobic pores. Jelassi et al. examined the freezing and melting of water in silica gels treated with trimethylchlorosilane by means of DSC and NMR and showed that the melting point depression does not appreciably change with surface modification.24 Findenegg et al. decorated the pore walls of SBA-15 with propionic acid, phosphoric acid and sulfonic acid and showed that the melting point depression of ice in the more hydrophilic pores is somewhat smaller than for bare silica.7 These discrepancies in the melting point depression seem to be due to different degrees of surface modification and/or large differences in the pore filling. As a matter of fact, in most of these studies, the adsorption isotherms of water were not measured and thus the degrees of hydrophobicity were not assessed. Aso et al. reported that the melting point depression of ice confined in mesoporous organosilica is slightly smaller than for MCM-41 silica of similar pore size, although the surface hydrophilicity of the organosilica is estimated to be almost the same as that of MCM-41 from the positions of adsorption steps due to capillary condensation and evaporation of water vapor.25 Water does not form hydrogen bonds with carbon surfaces that are almost free of surface functional groups and thus the carbon surfaces are more hydrophobic than the bare silica surfaces. It was reported that ices confined in multi-wall carbon nanotubes having inner diameters of mesopore region exhibit smaller depressions in the melting point than for the mesopores of the bare silica.26,27 However, adsorption in the carbon nanotubes occurs simultaneously in various regions of the tube bundle, and therefore it is difficult to distinguish the contributions from each region of the ices formed. In a previous study,10 we concluded that 4


Page 4 of 25

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ices confined in ordered mesoporous carbons CMK-3, CMK-8, and CFA2 exhibit similar depressions in the melting point as compared to the bare silica pores of similar pore sizes, although the pore size of these carbons is difficult to be defined. The structures of these ordered mesoporous carbons are exactly the inverse replicas of the ordered mesoporous silicas, and thus the pores consist of the void space between regularly arranged carbon nanorods.28 In addition, the carbon walls possess micropores that result in micropore filling of water vapor and thus make the pore walls less hydrophobic. All these experimental studies suggest that the influence of hydrophobicity on the freezing and melting of the confined water still remains to be unsettled. Molecular dynamics simulations of ice-liquid coexistence in cylindrical mesopore have suggested that the melting point depression of confined ice is insensitive to the hydrophobicity of the pore walls.21,29 Very recently, we showed that ordered mesoporous carbon synthesized by a micelle-templated method possesses almost cylindrical pores consisting of graphitic walls.30 Rare gases adsorbed on the pore walls form two-dimensional crystals, which indicates the crystalline properties of the carbon walls.31 In the graphitic pores of the ordered mesoporous carbons that are free of micropores and almost free of surface functional groups, water does not form an adsorbed film on the pore walls as the pressure approaches p0 because water does not form hydrogen bonds with the carbon surfaces. Nevertheless, a large uptake of water vapor occurs very slowly close to p0 due to capillary condensation of water.32 To examine the influence of pore-wall properties on freezing and melting behavior of water in mesopores, we measure the X-ray diffraction (XRD) pattern of water confined in the nearly cylindrical pores of the ordered mesoporous carbons during cooling and heating processes. The pore walls of the mesoporous silicas that can form hydrogen bonds with water molecules are amorphous and 5



hydrophilic, while those of the ordered mesoporous carbons that cannot form hydrogen bonds with water molecules are crystalline and less hydrophilic. For the mesoporous carbons, hydrophobicity of the pore walls is weak so that measurements of freezing/melting behavior of the pore water are possible without high pressures externally applied.

EXPERIMENTAL SECTION Materials and Characterization. Three kinds of the ordered mesoporous carbons (OMC-1, -6, and -7) were synthesized by a soft template method using resorcinol-formaldehyde and Pluronic F127 surfactant according to the modified procedure of Wang, Liang, and Dai.33 After carbonization by heating at 673 K and 1123 K under nitrogen atmosphere, graphitization was further carried out in a high-temperature furnace under argon atmosphere at 2473 K for 1 h, in order to remove micropores inherent to mesoporous carbons. Adsorption isotherms of nitrogen at liquid nitrogen temperature were measured volumetrically on a BELSORP-mini II (Bel Japan). Measurements. The measurements of adsorption of water vapor were conducted gravimetrically using a magnetic balance Rubotherm (BEL Japan) in a closed system, which is composed of a cylindrical vessel of stainless steel and a quartz pan for a sample. The vessel was immersed in a thermally regulated bath of water. The carbon sample was outgassed at 673 K for 4 h using a turbo-molecular pump prior to the measurements. The apparatus and procedures have been described in detail elsewhere.32 In kinetic measurements of water uptake, the vapor pressure of water in the vessel was rapidly raised up to saturation within a short time of 10 min and then kept saturation. The valve connecting the adsorption vessel to water reservoir was opened during the first 100 min of the measurements so that the measurements of uptake 6


Page 6 of 25

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kinetics were conducted in the presence of a small amount of liquid water inside the vessel. The experimental apparatus of XRD for freezing/melting measurements has been also given in detail elsewhere.34 Most of the measurements were carried out with MoKα radiation in a symmetrical transmission geometry, unless otherwise specified. Water was adsorbed in the hydrophobic pores of the mesoporous carbons according to the procedure mentioned in greater detail in a subsequent section. The substrate was then cooled to a desired temperature between 200 and 280 K, and the diffraction pattern was measured. Similarly, the diffraction pattern was also measured during subsequent heating process. The diffraction pattern of the confined water was obtained by subtraction of data for charged and empty substrate after correction for gas attenuation. A few measurements were also conducted with CuKα radiation in a Bragg-Brentano geometry (reflection geometry), in order to examine fine features of the diffraction pattern.

RESULTS AND DISCUSSION Rapid Adsorption of Water at Supersaturation. The ordered mesoporous carbons possess hexagonally shaped pores consisting of walls of turbostratic carbon.30 The specific surface area, pore

size,

and

pore

volume

were

estimated

using the adsorption branches in

adsorption-desorption isotherms of nitrogen at liquid nitrogen temperature.35 The pore radius was obtained using the Barrett-Joyner-Halenda method36 based on the assumption of a cylindrical geometry as usual. The micropore volume and total pore volume were estimated by using the t-plot method.37 Table 1 summarizes the main physicochemical parameters of the mesoporous carbons used in the present study. The O/C ratio of OMC-1 by X-ray photoelectron spectroscopy analysis is lower than 0.05 and comparable to that of a highly graphitized carbon black.38 These mesoporous carbons have no micropores and almost no functional groups on the 7



pore walls. Capillary condensation of water in the hydrophobic mesopores takes place very slowly close to saturation, although an uptake remains very low as the pressure approaches p0.35 If a high supersaturation was attained in the experiments, however, rapid adsorption of water would occur inside the hydrophobic mesopores.39,40

Table 1. Physicochemical Parameters of Mesoporous Carbons sample

Surface

Micropore

Total pore

Mean pore

area

volume

volume

radius

(m2/g)

(cm3/g)

(cm3/g)

(nm)

OMC-1

210

0.01

0.28

3.2

OMC-6

291

0

0.64

4.8

OMC-7

176

0

0.60

8.2

Figure 1 shows three kinds of the kinetic curves of water uptake due to capillary condensation of water vapor in the graphitic mesopores of OMC-7 with the largest pore size. One was measured at a constant temperature of 293 K and other two were measured with sudden change in temperature during the measurements. As the uptake curve measured at the constant temperature shows, the rate of water uptake was exceedingly slow. The uptake continued to occur almost for 3 days at 293 K. With decreasing temperature, the rate of uptake started to decline rapidly below 278 K, and the amount of uptake did not increase at all with time at 273 K, except for the rapid increase due to adsorption in the initial stage.35 When the temperature of the water bath, in which the adsorption vessel is immersed, was suddenly changed from 293 K to 273 K, the amount of water uptake rapidly decreased and the rate of 8


Page 8 of 25

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Uptake curves of water vapor in OMC-7. The explanation is given in the text.

uptake also decreased. Then, when the temperature of the water bath was suddenly returned to 293 K, the amount of water uptake rapidly increased up to nearly its maximum. In the gravimetrical measurement, a sample is isolated from the adsorption vessel. Therefore, there is time lag in the temperature change between the adsorption vessel and the sample. When the temperature of the adsorption vessel is suddenly decreased, the vapor pressure of water inside the vessel simultaneously decreases. However, the temperature of the sample still remains high for a while. The relative pressure of water vapor on the sample is decreased below p0 for a while so that capillary evaporation of water from the hydrophobic mesopores occurs in this short period. Capillary evaporation of water from the mesoporous carbon takes place very fast as compared to the capillary condensation. When the temperature of the adsorption vessel is then suddenly raised to 293 K again, the vapor pressure of water inside the vessel simultaneously increases. The temperature of the sample still remains low for a while and thus the vapor pressure of water on the sample is increased beyond p0 in a short time. 9



Supersaturation is expected to attain ∼3.8 in its maximum from a comparison between the saturation pressures of water at 293 and 273 K. This supersaturation would result in formation of liquid water near the pore entrances of the mesoporous carbons maintained at lower temperature. Liquid infiltration in mesopores takes place very fast as compared to the transfer of water molecules from the external water vapor into the mesopores.41 In a previous study,35 we have suggested that in the initial stage of capillary condensation the number of mesopores with liquid plugs of water at the pore entrances increases with time and then meniscus growth can be controlled by the rate of supply to the nanoplug at the pore entrance from the external water vapor and/or the rate of transfer of water molecules across the liquid plug. When the temperature of the adsorption vessel was kept at 273 K from the beginning, however, a sudden increase in the temperature did not result in a rapid increase of the uptake. This strongly suggests that the liquid plugs of water are never formed at the pore entrances over a long period of time when the temperature of the adsorption vessel is maintained at 273 K from the beginning. On the other hand, when the temperature of the adsorption vessel was suddenly decreased from 293 K to 273 K during the measurement, the amount of uptake still continued to increase steadily after the rapid decrease. This suggests that the rate of transfer of water molecules across the liquid plug does not appreciably decrease even at 273 K. Freezing of Water. Because of the different setup, filling of water into the hydrophobic pores of the mesoporous carbons inside the cryostat of X-ray apparatus cannot be conducted in the same way as in the kinetic study under the saturation vapor pressure of a constant temperature. Water was filled into the hydrophobic mesopores using generation of supersaturation inside the sample cell of the cryostat. After the sample inside the adsorption cell kept at 280 K was exposed 10


Page 10 of 25

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


through narrow tube to the supersaturated vapor of water equilibrated with liquid water maintained at room temperature for 10 min, the adsorption cell was isolated from the external reservoir of liquid water and then the temperature of the cell was returned to room temperature. After one day, extra water inside the cell was evacuated slowly by monitoring the vapor pressure of water inside the cell, in order to obtain the sample having water in the mesopores slightly below a complete filling. Figures 2(a), (b), and (c) show the powder XRD patterns from the water confined in the hydrophobic pores of OMC-7, OMC-6, and OMC-1, respectively, when the temperature was successively lowered from 280 K to 200 K. When the substrate was cooled, there was a sudden change in the diffraction pattern of the pore water from a liquid to a solid form. The freezing temperatures of confined water in OMC-7, OMC-6, and OMC-1 are ∼261, 258, and 248 K, respectively. The diffraction patterns of the resulting solids resemble that of cubic ice (Ic)

Figure 2. Change of the X-ray diffraction pattern of water confined in (a) OMC-7, (b) OMC-6, and (c) OMC-1 slightly below complete filling upon cooling. 11



instead of ordinary hexagonal ice (Ih). The three peaks can be indexed to the (111), (220), and (311) reflections of Ic, respectively. The irregularities observed in the diffraction patterns from pore water of OMC-1 are due to incomplete subtraction of Be sheets used for retainment of the powdered sample in the sample holder. The diffraction patterns of confined ices in OMC-7, OMC-6, and OMC-1 are almost the same and a small bump can be always observed between the (220) and (311) reflections of Ic. This suggests that cubic ice formed in the hydrophobic mesopores of the mesoporous carbons does not take a cubic structure as envisaged by König.5,42 Figure 3 compares the diffraction pattern of the confined ice formed in OMC-7 with pore radius 8.2 nm with that in mesoporous silica with pore radius 9.5 nm,5 where the diffraction patterns of higher resolution were measured using CuKα radiation in a reflection geometry. The diffraction pattern of the confined ice in the hydrophobic mesopores of the carbon is almost the same as that in the hydrophilic mesopores of silica with similar pore sizes, although the diffraction peaks of confined ice for the mesoporous carbon are slightly broader. This broadness

Figure 3. Comparison of the X-ray diffraction pattern of ice formed in the graphitic pores of pore radius 8.2 nm at 250 K with that in the silica pores of pore radius 9.5 nm at 220 K. 12


Page 12 of 25

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


is due to the low absorption of carbon for X-ray.43 Several diffraction peaks that cannot be assigned to Ic are observed. In fact, all these diffraction peaks including the peaks that were assigned to the reflections of Ic can be assigned to the reflections of ordinary Ih, although the relative intensities and widths are different from those expected from the powder of pure Ih crystals. Such diffraction patterns are typical of a so-called stacking disordered ice.44 Our results agree with a recent study concerning structures of pore ices formed in mesoporous carbons CMK-3 and CMK-8, namely the inverse replicas of the ordered mesoporous silica SBA-15 and KIT-6, respectively.45 Melting Point Depression. Figures 4(a), (b), and (c) show the powder XRD patterns from the ice confined in the OMC-7, OMC-6, and OMC-1, respectively, when the temperature was successively increased from 200 K to 280 K. When the resulting ices were heated, the melting always took place at temperatures higher than the freezing temperatures and thus thermal

Figure 4. Change of the X-ray diffraction pattern of water confined in (a) OMC-7, (b) OMC-6, and (c) OMC-1 slightly below complete filling upon heating. 13



hysteresis appeared. The width of the thermal hysteresis observed for water/mesoporous carbons is comparable to or slightly smaller than that for mesoporous silicas of similar pore sizes.5,12 When the melting temperatures of the confined ices were approached, the main diffraction peak at 2θ=11° of the confined ice decreased in intensity, while a broad peak around 2θ=12° appeared. The broad diffraction pattern contains three contributions from a supercooled liquid, nonfreezable layer located between the ice core and the graphitic walls, and water-carbon cross-terms46 arising from correlations across the interface. The third contribution is small because the pore sizes of the present materials are relatively large. The broad diffraction pattern of a completely melted ice in the 2θ region of 7−25° was fitted with a linear combination of three broad peaks each having a Gaussian line shape and a sloping background. Then, we fixed the shape of the broad diffraction pattern in subsequent fitting procedure of the diffraction patterns of the partially melted ice. The diffraction patterns of the partially melted ices were fitted with a linear combination of four peaks (2θ =11.0, 18.0, 19.3, and 21.2°) from a solid component, the broad pattern due to an amorphous component, and a sloping background. The main peak at 2θ=11° of the solid component was asymmetrical because the peak consists of the (100), (002), and (101) reflections of ice Ih. Therefore, this peak was fitted using a combination of a Gaussian line shape in the 2θ region lower than the peak position and a Lorentzian one in the 2θ region higher than the peak position, while other peaks were fitted using Gaussian line shapes. Figure 5 shows the results of the least-squares fitting of the observed patterns to a linear combination of a crystalline and amorphous component. The fits were quite good over the whole range of temperatures examined. This suggests that the diffraction pattern of the nonfreezable water layer 14


Page 14 of 25

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. X-ray diffraction patterns of ice confined in OMC-6 slightly below complete filling at (a) 200 and (b) 260 K upon heating. Red and green curves represent a whole pattern fitted and an amorphous component due to supercooled liquid and/or nonfreezable layer, respectively.

resembles that of liquid water confined in the graphitic pores. An amorphous component is still visible at 200 K. Figure 6 shows the fraction of liquid as a function of temperature for ice confined in OMC-1, -6, and -7 slightly below complete filling upon heating, where the fraction of liquid in the pores was assessed from the amorphous component in the diffraction pattern of the pore water. The fraction of liquid increased rapidly in the vicinity of the melting

Figure 6. Fraction of liquid as a function of temperature for ice confined in OMC-1, -6, and -7 slightly below complete filling upon heating. 15



temperatures with increasing temperature. In addition, the fraction of liquid at 200 K increased with a decrease of pore size. This also suggests that the amorphous component at lower temperatures is the nonfreezable water layer between the ice core and the graphitic walls. Assuming that the amorphous component is solely due to the nonfreezable water layer, its thickness d in the cylindrical pores of the pore radius r is given by47 d = r [1−(1−x)1/2].

(1)

Here, x is the melted fraction in the pore. The thicknesses of the nonfreezable layers at 200 K for OMC-1, -6, and -7 were calculated to be 0.68, 0.71, and 0.98 nm, respectively. These values are slightly larger as compared to the reported ones for mesoporous silicas.4,6,7,12,17,19 Although the absolute values are unreliable because the diffraction pattern of the nonfreezable water layer is not the same as that of a supercooled water confined in the graphitic pores, the appearance of a thin water layer between the core ice and the graphitic walls is consistent with the occurrence of premelting of ice in exfoliated graphite.22 Similarly, increase of a broad component in the diffraction pattern of a confined ice with increasing temperature has been reported for the silica mesopores.14 A rapid increase of the amorphous component in the vicinity of the melting temperatures may be accounted for by the effect of the pore-size distribution on the melting-point-depression.6 It is indicated from the change of the liquid fraction with temperature that the melting temperatures of ice formed in OMC-7, OMC-6, and OMC-1 are ∼263, 260, and 250 K, respectively. On the basis of a balance of bulk and surface free energy terms between solid and liquid in a mesopore, it has been suggested that the shift in the equilibrium melting point of a confined solid is controlled by the ratio S/V of the pore with volume V and surface area S, irrespective of pore geometries.10,48,49 16


Page 16 of 25

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


∆ = − =

° (γ − γ ) ∆

(2)

Here, Vm is the molar volume of the material, ∆Hf is the latent heat of melting, γsw is the solid-wall interfacial energy, γlw is the liquid-wall interfacial energy, and Tm and T° are the melting temperatures of a pore solid and a bulk solid, respectively. The sign and magnitude of

∆Tm depend on the values of γsw and γlw. When a nonfreezable water layer appears at the interface of core ice and the pore walls and the properties of the layer can be approximated by those of a bulk water, γsw is reduced to the solid-liquid interfacial energy γsl and γlw disappears. For ice confined in the cylindrical pores of the radius r with the nonfreezable water layer of the thickness t, eq 2 is reduced to a modified Gibbs-Thomson equation

∆Tm =

γ ° − ∆

(3)

As long as a thin water layer is formed between core ice and the pore walls, before onset of melting, the melting point depression of the confined ice is not at all influenced by the pore wall hydrophobicity, except for a possible influence on t. Figure 7 compares the melting point depressions of the confined ices in the hydrophobic mesopores of the mesoporous carbons with those in the hydrophilic mesopores of the mesoporous silicas with cylindrical pores. The melting point depression of ice confined in the hydrophobic mesopores of the mesoporous carbons is comparable to that in the hydrophilic mesopores of the mesoporous silicas of similar pore sizes. In the figure, the modified Gibbs-Thomson relation that was calculated using the same parameters of the surface and bulk as those in the silica pores reported by Jaehnert et al.6 is also illustrated. The pore walls of the mesoporous silicas that can form hydrogen bonds with water molecules are amorphous and hydrophilic, whereas those of the ordered mesoporous carbons that cannot form hydrogen bonds with water molecules are crystalline and less hydrophilic. Contact angles of water nanodroplets 17



Figure 7. Melting-point depression against reciprocal diameter of the pores for ice confined to the less hydrophilic and crystalline carbon pores as compared to the hydrophilic and amorphous silica pores.9 A solid curve denotes the modified Gibbs-Thomson relationship.

with the silica surfaces and the graphitic walls of the ordered mesoporous carbons are 0° and ∼46°, respectively.21,35 Nevertheless, similar depressions for the melting point of the confined ice were observed for the two different types of mesopores, suggesting that the thickness of the nonfreezable water layer adjacent to the graphitic walls is similar to that in the silica walls. The present results are in good agreement with the simulations results.21,29

CONCLUSION The influence of pore-wall hydrophobicity on freezing and melting behavior of a confined water is very small because the confined ice is separated from the pore walls owing to the existence of a thin water layer.

AUTHOR INFORMATION 18


Page 18 of 25

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Corresponding Author *Phone: +81-86-254-3250. Fax: +81-86-254-3250. E-mail: [email protected]. ORCID Kunimitsu Morishige: 0000-0003-3874-5115 Notes The author declares no competing financial interest. ACKNOWLEDGMENTS We thank Y. Matsutani and K. Maeda for their technical assistance in the synthesis of OMC samples. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

REFERENCES (1) Wisniewski, M.; Gusta, L.; Neuner, G. Adaptive Mechanisms of Freeze Avoidance in Plants: A Brief Update. Environmental and Experimental Botany 2014, 99, 133-140. (2) Scherer, G. W. Crystallization in Pores. Cem. Concr. Res. 1999, 29, 1347-1358. (3) Marcolli, C. Deposition Nucleation Viewed as Homogeneous or Immersion Freezing in Pores and Cavities. Atmos. Chem. Phys. 2014, 14, 2071-2104. (4) Morishige, K.; Kawano, K. Freezing and Melting of Water in a Single Cylindrical Pore: The Pore-Size Dependence of Freezing and Melting Behavior. J. Chem. Phys. 1999, 110, 4867-4872. (5) Morishige, K.; Uematsu, H. The Proper Structure of Cubic Ice Confined in Mesopores. J. Chem. Phys. 2005, 122, 044711. (6) Jaehnert, S.; Vaca Chavez, F.; Schaumann, G. E.; Schreiber, A.; Schoenhoff, M.; Findenegg, 19



G. H. Melting and Freezing of Water in Cylindrical Silica Nanopores. Phys. Chem. Chem. Phys. 2008, 10, 6039-6051. (7) Findenegg, G. H.; Jaehnert, S.; Akcakaryiran, D.; Schreiber, A. Freezing and Melting of Water Confined in Silica Nanopores. ChemPhysChem. 2008, 9, 2651-2659. (8) Morishige, K.; Yasunaga, H.; Denoyel, R.; Wernert, V. Pore-Blocking-Controlled Freezing of Water in Cagelike Pores of KIT-5. J. Phys. Chem. C 2007, 111, 9488-9495. (9) Morishige, K.; Yasunaga, H.; Uematsu, H. Stability of Cubic Ice in Mesopores. J. Phys. Chem. C 2009, 113, 3056-3061. (10)

Morishige, K.; Yasunaga, H.; Matsutani, Y. Effect of Pore Shape on Freezing and

Melting Temperatures of Water. J. Phys. Chem. C 2010, 114, 4028-4035. (11)

Petrov, O.; Furó, I. A Study of Freezing-Melting Hysteresis of Water in Different

Porous Materials. Part II: Surfactant-Templated Silicas. Phys. Chem. Chem. Phys. 2011, 13, 16358-16365. (12)

Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Melting and Freezing of Water in

Ordered Mesoporous Silica Materials. Phys. Chem. Chem. Phys. 2001, 3, 1185-1195. (13)

Morishige, K.; Iwasaki, H. X-ray Study of Freezing and Melting of Water Confined

within SBA-15. Langmuir 2003, 19, 2808-2811. (14)

Liu, E.; Dore, J. C.; Webber, J. B. W.; Khushalani, D.; Jaehnert, S,; Findenegg, G. H.;

Hansen, T. Neutron Diffraction and NMR Relaxation Studies of Structural Variation and Phase Transformations for Water/Ice in SBA-15 Silica: I. The Over-Filled Case. J. Phys.: Condens. Matter 2006, 18, 10009-10028. (15)

Liu, X. X.; Wang, Q.; Huang, X. F.; Yang, S. H.; Li, C. X.; Niu, X. J.; Shi, Q. F.; Sun,

G.; Lu, K. Q. Liquid-Solid Transition of Confined Water in Silica-Based Mesopores. J. Phys. 20


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem. B 2010, 114, 4145-4150. (16)

Overloop, K.; Van Gerven, L. Freezing Phenomena in Adsorbed Water as Studied by

NMR. J. Magn. Reson. A 1993, 101, 179-187. (17)

Schmidt, R.; Hansen, E. W.; Stöcker, M.; Akporiaye, D.; Ellestad, O. H. Pore Size

Determination of MCM-41 Mesoporous Materials by means of 1H NMR Spectroscopy, N2 Adsorption, and HREM. A Preliminary Study. J. Am. Chem. Soc. 1995, 117, 4049-4056. (18)

Valiullin, R.; Furó, I. The Morphology of Coexisting Liquid and Frozen Phases in

Porous Materials as Revealed by Exchange of Nuclear Spin Magnetization Followed by 1H Nuclear Magnetic Resonance. J. Chem. Phys. 2002, 117, 2307-2316. (19)

Endo, A.; Yamamoto, T.; Inagi, Y.; Iwakabe, K.; Ohmori, T. Characterization of

Nonfreezable Pore Water in Mesoporous Silica by Thermoporometry. J. Phys. Chem. C 2008, 112, 9034-9039. (20)

Hirama, Y.; Takahashi, T.; Hino, M.; Sato, T. Studies of Water Adsorbed in Porous

Vycor Glass. J.Colloid Interface Sci. 1996, 184, 349-359. (21)

Moore, E. B.; Allen, J. T.; Molinero, V. Liquid-Ice Coexistence below the Melting

Temperature for Water Confined in Hydrophilic and Hydrophobic Nanopores. J. Phys. Chem. C 2012, 116, 7507-7514. (22)

Gay, J.-M.; Suzanne, J.; Dash, J. G.; Fu, H. Premelting of Ice in Exfoliated Graphite; A

Newtron Diffraction Study. J. Cryst. Growth 1992, 125, 33-41. (23)

Deschamps, J.; Audonnet, F.; Brodie-Linder, N.; Schoeffel, M.; Alba-Simionesco, C.

A Thermodynamic Limit of the Melting/Freezing Processes of Water under Strongly Hydrophobic Nanoscopic Confinement. Phys. Chem. Chem. Phys. 2010, 12, 1440-1443. (24)

Jelassi, J,.; Castricum, H. L.; Bellissent-Funel, M. –C.; Dore, J.; Webber, J. B. W.; 21



Sridi-Dorbez, R. Studies of Water and Ice in Hydrophilic and Hydrophobic Mesoporous Silicas: Pore Characterization and Phase Transformations. Phys. Chem. Chem. Phys. 2010, 12, 2838-2849. (25)

Aso, M.; Ito, K.; Sugino, H.; Yoshida, K.; Yamada, T.; Yamamuro, O.; Inagaki, S.;

Yamaguchi, T. Thermal Behavior, Structure, and Dynamics of Low-Temperature Water Confined in Mesoporous Organosilica by Differential Scanning Calorimetry, X-ray Diffraction, and Quasi-Elastic Neutron Scattering. Pure Appl. Chem. 2013, 85, 289-305. (26)

Sliwinska-Bartkowiak, M.; Jazdzewska, M.; Huang, L. L.; Gubbins, K. E. Melting

Behavior of Water in Cylindrical Pores: Carbon Nanotubes and Silica Glasses. Phys. Chem. Chem. Phys. 2008, 10, 4909-4919. (27)

Jazdzewska, M.; Sliwinska-Bartkowiak, M. M.; Beskrovnyy, A. I.; Vasilovskiy, S. G.;

Ting, S. –W.; Chan, K. Y.; Huang, L.; Gubbibs, K. E. Novel Ice Structures in Carbon Nanopores: Pressure Enhancement Effect of Confinement. Phys. Chem. Chem. Phys. 2011, 13, 9008-9013. (28)

Morishige, K.; Nakahara, R. Capillary Condensation in the Void Space between

Carbon Nanorods. J. Phys. Chem. C 2008, 112, 11881-11886. (29)

Solveyra, E. G.; dela Llave, E.; Scherlis, D. A.; Molinero, V. Melting and

Crystallization of Ice in Partially Filled Nanopores. J. Phys. Chem. B 2011, 115, 14196-14204. (30)

Morishige, K. Freezing and Melting of Kr in Hexagonally Shaped Pores of

Turbostratic Carbon: Lack of Hysteresis between Freezing and Melting. J. Phys. Chem. C 2011, 115, 2720-2726. (31)

Morishige, K. Monolayer Solids of Kr on Graphitized Carbon Black Surfaces and in 22


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphitic Hexagonal Pores. J. Phys. Chem. C 2013, 117, 10360-10365. (32)

Morishige, K.; Kawai, T.; Kittaka, S. Capillary Condensation of Water in Mesoporous

Carbon. J. Phys. Chem. C 2014, 118, 4664-4669. (33)

Wang, X.; Liang, C.; Dai, S. Facile Synthesis of Ordered Mesoporous Carbons with

High-Thermal Stability by Self-Assembly of Resorcinol-Formaldehyde and Block Copolymers under Highly Acidic Conditions. Langmuir 2008, 24, 7500-7505. (34)

Morishige, K.; Innoue, K.; Imai, K. X-ray Study of Kr, Xe, and N2 Monolayers on

Boron Nitride. Langmuir 1996, 12, 4889-4891. (35)

Morishige, K.; Kittaka, S. Kinetics of Capillary Condensation of Water in Mesoporous

Carbon: Nucleation and Meniscus Growth. J. Phys. Chem. C 2015, 119, 18287-18292. (36)

Barrett, E. P.; Joyner, L. G.; Halenda, P. P. The determination of pore volume and area

distributions in porous substances. I. Computations from nitrogen isotherms. J. Am. Chem. Soc. 1951, 73, 373-380. (37)

Lippens, B. C.; de Boer, J. H. Pore systems in catalysts. V. The t-method. J. Catal.

1965, 4, 319-323. (38)

Horikawa, T.; Muguruma, T.; Do, D. D.; Sotowa, K. –I.; Alcantara-Avila, J. R.

Scanning curves of water adsorption on graphitized thermal carbon black and ordered mesoporous carbon. Carbon 2015, 95, 137-143. (39)

Monson, P. A. Contact Angles, Pore Condensation, and Hysteresis: Insights from a

Simple Molecular Model. Langmuir 2008, 24, 12295-12302. (40)

Factorovich, M.; Solveyra, E. G.; Molinero, V.; Scherlis, D. A. Sorption Isotherms of

Water in Nanopores: Relationship between Hydrophobicity, Adsorption Pressure, and Hysteresis. J. Phys. Chem. C 2014, 118, 16290-16300. 23



(41)

Washburn, E. W. The Dynamics of Capillary Flow. Phys. Rev. 1921, 17, 273-283.

(42)

König, H. Cubic Modification of Ice. Z. Kristallogr. 1943, 105, 279-286.

(43)

Kluc, H. D.; Alexander, L. E. X-ray Diffraction Procedures: For Polycrystalline and

Amorphous Materials (Second Edition); John Wiley & Sons: New York, 1974, p.290. (44)

Malkin, T. M.; Murray, B. J.; Salzmann, C. G.; Molinero, V.; Pickering, S. J.; Whale, T.

F. Stacking Disorder in Ice I. Phys. Chem. Chem. Phys. 2015, 17, 60-76. (45)

Domin, K.; Chan, K.-Y.; Yung, H.; Gubbins, K. E.; Jarek, M.; Sterczynska, A.;

Sliwinska-Bartkowiak, M. Structure of Ice in Confinement: Water in Mesoporous Carbon. J. Chem. Eng. Data 2016, 61, 4252-4260. (46)

Morineau, D.; Alba-Simionesco, C. Liquids in Confined Geometry: How to Connect

Changes in the Structure Factor to Modifications of Local Order. J. Chem. Phys. 2003, 118, 9389-9400. (47)

Ishizaki, T.; Maruyama, M.; Furukawa, Y.; Dash, J. G. Premelting of Ice in Porous

Silica Glass. J. Cryst. Growth 1996, 163, 455-460. (48)

Evans, R. Fluids Adsorbed in Narrow Pores: Phase Equilibria and Structure. J. Phys.:

Condens. Matter 1990, 2, R8989-R9007. (49)

Christenson, H. K. Confinement Effects on Freezing and Melting. J. Phys.: Condens.

Matter 2001, 13, R95-R133

24


Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphics

25


Influence of Pore Wall Hydrophobicity on Freezing and Melting of

Recommend Documents