Observation of Frost Heave of THF Clathrate Hydrate on Porous Glass Powder
Kunio Watanabe1, Kosuke Yokokawa 2 and Yoshiko Muto Author3
1

Department of Bioresources, Mie University, 1577 Kurima-machiya, Tsu 514-8507
Japan; email kunio@bio.mie-u.ac.jp
2
Nippon Tansan Co. Ltd, 1-10-15, Hkataeki-minami, Hakataku, Fukuoka 812-0016
Japan; email: k-y@mub.biglobe.ne.jp
3
Ddepartment of International Environmental & Agricultural Sciences, Tokyo
University of Agriculture & Technology, 3-5-8 Saiwaicho, Fuchu, Tokyo 183-8509
Japan; email: mutoy@cc.tuat.ac.jp

Abstract
The formation processes of clathrate hydrate in porous glass powder saturated with a
THF–water mixture were directly observed using directional cooling apparatus. As
the cooling rate decreased, the THF hydrate grew while excluding glass particles andformed a lens like layer of hydrate in the same manner as in the phenomenon of ice
lens formation in the freezing of soil. The growth rate of the hydrate lens was
proportional to the degree of supercooling of the growth surface and was about 1/20th
the growth rate of the hydrate layer in a THF–water mixture without glass particles.
When sodium chloride was added to the system, the growth of the hydrate lens wasinhibited depending on the salt concentration.

Introduction
When ground freezes, soil water forms ice in various shapes, including pore ice,
granular ice, and layered ice from which all soil particles have been expelled. The
ice layers, called ice lenses, sometimes cause severe frost heave. Although theconventional view treats frost heave and lens formation as unique attributes of waterand soil, similar phenomena can occur in other substances (Hiroi et al. 1989; Mutou
et al. 1998; Zhe et al. 2000). If soil is exposed to low temperatures or high pressure,
not only ice but also gas hydrate can form within it. Gas hydrates formed in poresor into granular or lens-like shapes are sometimes observed in permafrost andundersea sediments (Dallimore and Collett 1997). The effects of soil particles on
the thermal properties of tetrahydrofuran (THF) hydrate (Rueff and Sloan 1985) andthe static properties of methane hydrate in sand and clay (Chuvilin et al. 2000, 2002)

have been investigated. However, there have been few direct observations of theformation of clathrate hydrates in freezing soils.
Clathrate hydrates are ice like solids in which water molecules form a crystalline
lattice that is composed of many cavities occupied by guest molecules, such as certain
gasses or volatile liquids. From the perspective of recent environmental andindustrial concerns, the structures of various clathrate hydrates (Mak and McMullan
1965; Gough and Davidson 1971; Sloan 1990) and their thermal properties atequilibrium (Ross et al. 1981; Leaist et al. 1982; Handa et al. 1984) have beenclarified. Since the formation and dissociation of clathrate hydrate are greatly
affected by the solute, the dependence of the solute on the phase equilibrium
conditions has been investigated (de Roo et al. 1983). However, the soluteinfluences on the formation processes of clathrate hydrate have not been sufficiently
elucidated.
Since gas hydrate forms under relatively high pressures and cold temperatures, theformation process has seldom been observed directly, except under special conditions(Uchida et al. 1999). In contrast, THF hydrate forms at atmospheric pressure attemperatures below 4.4 °C. Therefore, THF can replace gas hydrate as a model
crystal for observations (Nagashima et al. 2000; Iida et al. 2001). Here, we present
the results of in situ observations of THF hydrate formation in a directionally freezing
porous medium consisting of glass microparticles saturated with different
concentrations of sodium chloride solution. The process of hydrate formation in a
porous medium and the effect of solute concentration are discussed.

Materials and Methods
Sample. The porous powder used in the experiment consisted of uniform glass
spheres 2.2 µm in diameter. Table 1 lists the characteristics of the powder. The
powder holds significant amounts of unfrozen water at subzero temperatures and has
frost susceptibility (Watanabe et al. 2001). Distilled, deionized, and degassed waterand tetrahydrofuran (THF) of 99.9% purity were used. THF is miscible in water andforms a structure II hydrate with water at a molar ratio of 1:17 (Gough and Davidson
1971). This hydrate melts at a temperature of 4.4 °C and 101.3 kPa, although themelting temperature varies between 0 and 4.4 °C depending on the THF
concentration (Hanley et al. 1989). Although mass transfer at the liquid–gas
interface controls the nucleation and crystal growth of gas hydrates, the mass transfer
effects are eliminated for THF hydrate (Devarakonda et al. 1999). The physicalproperties of solutions and THF hydrate are listed in Table 2. Glass particles haveno effect on the thermal properties of ice or THF hydrate (Rueff and Sloan 1985) andlower the phase equilibrium temperature of clathrate hydrate (Chuviline et al. 2000,
2002).

Table 1 Physical properties of the glass powder.
µm
nm
m3 kg-1
m2 kg-1
kg m-3
m s-1

Mean diameter
Mean pore size
Specific pore volume
Specific surface area
Density
Hydraulic cond.

2.2
3.3
0.3×10-3
129×1032120
1.9×10-8

Table 2 Physical properties of the solutions.

Thermal cond.
Specific heat
Density
Melting point
Latent heat
Diffusion coef.

1:17 sol.
Wm K
0.562a-1 -1
kJ kg K
4.21b
kg m-3997d°C
4.4e
kJ kg-1
260c
m2 s-1-1-1

Hydrate0.525a2.123c
997d

Water
0.561
4.217
1000

-

0
333
1.0×10-9 f

Ice
2.14
2.11
917

-

Values were obtained at 4.4 °C for THF-water solution at a molar ratio of 1:17 (1:17 sol.) and THF
hydrate (Hydrate), and at 0 °C for water and ice. aRoss and Anderson (1982), bIida et al. (2001),c
Leaist et al. (1982), dGough and Davidson (1971).eHanley et al. (1989), fGillen et al. (1972).

Table 3 NaCl concentration in solution and the ratio of solution to glass powder.
Name
S0
S003
S003w
S03
S3

Cint (%)
0
0.03
0.03
0.3
3

LL (%)
115
125
125
143
185

LL×1.1 w (%)
126.5
126.2
137.5
136.2
137.5
155.5
157.3
155.5
203.5
203.7

Directional freezing experiment. A THF–water solution at a molar ratio of 1:17
was prepared with different sodium chloride concentrations. The solution wascooled to 10 °C in a sealed chamber and then mixed with the glass powder. The
consistency of this sample changes markedly with the water content and soluteconcentration. Therefore, the initial water content was adjusted to be 1.1 times
greater than the liquid limit (LL). Table 3 shows the initial sodium chloride
concentration in the solution, the liquid limit, and the initial water content of thesample. Note that the water content used here was defined as the ratio of THF
solution to the porous powder by mass.

Thermocouples

-4℃

CCD

Insulator

10℃

The sample was packed into a cylindrical glass cell with an internal diameter of 7.55mm and a length of 150 mm. To prevent evaporation, the cell was sealed withparaffin film.
A temperature cell, which contained five copper–constantan
thermocouples, accurate to 0.03 °C, was also prepared using powder mixed with
ethylene glycol instead of THF solution. The bottoms of the sample andtemperature cells were soused in liquid nitrogen for a few seconds for seeding, andboth cells were then cooled simultaneously using the directional cooling apparatus(Fig. 1). The temperatures of the top and bottom of the cell were held at different
constant temperatures of 10 °C and –4 °C, respectively. An almost linear
temperature gradient was established after 120 min had elapsed and the final
temperature gradient was 0.14 ± 0.15 °C mm–1. A digital video camera and recorderwere placed by the cell and used to observe the formation of THF hydrate. The
captured images were divided into a 0.04-mm grid using a computer, and the locationand temperature of the crystallization front were analyzed.

Fig. 1[℃]
4.4

Cooling

Schematic illustration of the directional cooling experiment.

Results
Observation of THF hydrate formation in porous media. When different
temperatures were applied to either end of a sample cell, the sample froze
directionally beginning from the cold end. During sample cooling, intermittent
layers of crystals that expelled glass particles were observed. Figure 2 shows the
crystals observed in sample S0 (Cinit = 0%), which was cooled for 24 h. The scalebar indicates 1 mm. In Fig. 2, the bottom is colder and the top is warmer; the porous
medium appears white and the segregated crystals appear black or gray.
In the early stages of cooling, no crystal segregation was observed. When the rateof isotherm advance in the sample (the cooling rate) decreased below 1 µm s–1, thefirst crystal layer appeared and grew, while expelling glass powder. Once the crystal
had grown a few 100 µm, it stopped growing, and new crystal layer then appeared ina warmer location than that where the previous layer had grown (Fig. 2, from bottom).

Subsequently, a series of crystals formed intermittent layers in the direction
perpendicular to the heat flow. The layers that formed later were thicker and clearer.
After the cooling experiment, the warmest layer was taken from the cell and set on
fire. It burned readily in air, demonstrating that the segregated crystal was THF
hydrate. Here, we call such a layered crystal a hydrate lens due to its shape.

Fig. 2Crystallization in glass powder saturated with 1:17 THF solution.

When sodium chloride was added to the system, the concentration and water content
determined whether hydrate lenses formed in the sample (Fig. 3). The hydrate
lenses formed readily when the salt concentration was low and the initial watercontent was high. As the salt concentration increased and the water content
decreased, the shape of the hydrate lenses became unstable, and they no longer
maintained a lenslike shape, but became dendrite-like. Little or no lens or dendrite
formation occurred when the initial water content was lower than the liquid limit,
especially with a high solute concentration.

Lens like formation
DendriteNo segregation

Water content (g/g)

2

1.6

1.2

0.8

LL
0.01

0.1

1

10

Initial salt concentration (%)

Fig. 3

Diagram of the formation of a hydrate lens in glass powder.

Hydrate lens growth. Figures 4 and 5 show the growth of the warmest hydrate lensand estimated temperature at the growth surface, respectively. The elapsed time wascounted from the beginning of formation of the warmest hydrate lens. In eachsample, the warmest hydrate lens grew rapidly initially, and then grew more slowly,
and essentially stopped growing after 48 h. The hydrate lens started to form at atemperature a few degrees lower than its melting temperature (4.4 °C). The
temperature at the growth surface increased with its growth. In the salt-free sample(S0), the final temperature at the growth surface of the warmest hydrate lens was 4.13
°C, which was 0.27 °C lower than the melting temperature. This may have resulted
from lowering the phase equilibrium temperature of the THF hydrate due to thepresence of glass particles.

Growth amount (mm)

4
3
2
S0
S003
S03
S003w

1
0 0

Temperature ( ℃)

Fig. 460

Growth of the warmest hydrate lens.

4

2

S0
S003
S03
S3
S003w

0
0

Fig. 520
40
Elapsed time (hour)

20
40
Elapsed time (hour)60

Temperature at the growth surface of the warmest hydrate.

The hydrate lens became thinner with increasing initial sodium chloride concentration.
For 48 h, the warmest hydrate lens grew to 3.16, 2.51, and 2.03 mm thick in samples
S0, S003, and S03, respectively (Fig. 4). For sample S3, dendrite-like formationwas observed, not lenslike formation (Fig. 3). The temperature at the growthsurface of the hydrate lens and dendrite at an elapsed time of 48 h were 4.04, 2.73,
1.68, and –0.35 °C for samples S0, S003, S03, and S3, respectively. The
temperature decreased with increasing initial salt concentration (Fig. 5).
Sample S003w is the one in which the initial salt concentration equaled that of S003
(0.03%), but the initial water content was higher than in S003. Even if the sample
had the same initial salt concentration, the hydrate lens grew thicker at a higher
temperature in the water-rich environment (Figs. 4 and 5).

Growth rate (µm/sec)

Growth rate and supercooling at the growth surface.
Figure 6 shows therelationship between the growth rate and degree of supercooling, Tm – T, which was
calculated from Figs. 4 and 5. Here, we used Tm = 4.4 °C for THF hydrate. Figure
6 also shows the growth rate of ice in water, with the formation of an ice lens in thesame powder as used this experiment (Watanabe 2002), and THF hydrate in a 1:17
THF–water solution. The growth rate of the hydrate lens was linearly correlatedwith the degree of supercooling. Similar proportionality constants were observed insamples with different initial salt and water contents (0.033, 0.020, 0.024, and 0.032µm s–1° C–1 for samples S0, S003, S003w, and S03, respectively). Moreover, therespective x-intercepts shown in Fig. 6 were 0.27, 0.91, 1.47, and 2.75 °C. The final
temperatures for hydrate lens growth, which may correspond to the phase equilibrium
temperature, were estimated to be 4.13, 3.49, 2.39, and 1.65 °C, respectively.

(a)
0.03

S003
S003w

2

3

0.02
0.01
0

Growth rate (µm/sec)

S0
S03

0

1

(b)
0.03
0.02

Ice in waterIce lens in powder
THF hydrate in solution

0.01
0

0

0.1
Supercooling degree,

T(K)

0.2

Fig. 6 Growth rate of THF hydrate and the degree of supercooling at thegrowth surface. (a) THF hydrate in glass powder, (b) ice and THF hydrate in
the melt, and an ice lens in glass powder.

The growth rate of THF hydrate in the THF–water solution per degree ofsupercooling (0.5 µm s–1 °C–1) was about 1/25th the growth rate of ice in water (12.3µm s–1 °C–1). This may have arisen from the thermal property differences between
THF hydrate and water (Table 2). The growth rate of an ice lens in a powder
becomes markedly lower than that of ice in water due to the properties of unfrozen
water (structured water) near the ice lens (Watanabe 2002). The ratio of the growthrate of a hydrate lens (0.02 µm s–1 °C–1) to that of THF hydrate in a solution without
porous particles was 1:25, which was the same as the ratio of the growth rate of an ice
lens (0.43 µm s–1 °C–-1) to that of ice in water. The water structuring near THF
hydrate may play important roles in the formation of hydrate lenses.

Discussion
The molar depression of the melting temperature, Tm, of a bulk solution with themolar concentration of C is given by Tm = i KC, where i is the dissociation constantof the solute and K is the molar melting point constant of the solvent. If we denotethe depression of the melting temperature due to the existence of glass particles asTmp, the equilibrium temperature at the growth surface of the warmest hydrate lens,Tgs, would be approximated as Tgs = 4.4 – Tm – Tmp. Tgs can be obtained from thex-intercept in Fig. 6, and Tmp can be estimated from the experimental result for S0 as
0.27 °C. Assuming that the dissociation constant of sodium chloride in THF
solution is 2, and the molar depression of THF solution is the same as that of water(1.86 °C kg mol–1), we obtain sodium chloride concentrations of the THF solution of
1.9, 3.9, and 7.0% for samples S003, S03, and S3, respectively.
These
concentrations are 2.3–63 times greater than their initial concentrations. This showsthat the warmest hydrate lens grew while rejecting sodium chloride, which resulted in
the formation of a concentration gradient near the growth surface.
Using interferometry, Nagashima et al. (2000) observed a salt concentration field near
THF hydrate, which grew at a constant rate of 5 µm s–1 in sodium chloride solution
with an initial concentration of 3%, and indicated that the concentration increased
from 4.5 to 5.6% near the growth surface. Although the growth rate observed in our
experiment was slower than that in their experiment (Fig. 6: slower than 0.05 µm s–1),
a higher concentration of salt near the growth surface was observed. In addition, thetemperature at the growth surface of the hydrate lens in sample S003w, which had alower numerical density of glass particles, was higher than that in sample S003 (Fig.
5). It is thought that the particles inhibit the mobility of sodium or chloride ionsnear the hydrate lens.
A greater increase in the salt concentration near the growth surface (Csurface /Cinit )was prominent in the sample with the lower initial salt concentration. A similartendency has also been reported in the formation of an ice lens in the glass powder
media (Watanabe 2001), showing the similarity in the growth processes of hydrate
lenses and ice lenses.

Conclusions
Directional cooling experiments were conducted to clarify the fundamental
mechanism of the formation and frost heave of clathrate hydrate in porous media,
such as soil. The sequential formation of lenslike clathrate hydrate was observed inglass powder saturated with a THF–water solution. When sodium chloride wasadded to the solution, the growth of the clathrate hydrate was dendrite-like dependingon the sodium chloride concentration. The growth rate of the THF hydrate lens was
proportional to the degree of supercooling at the growth surface. The degree ofsupercooling decreased with increasing sodium chloride concentration, resulting in adecrease in the growth rate. These results are similar to ice lens formation in glass
powder and soil (frost heave). Considering the differences in thermal properties,
knowledge of frost heave should facilitate our understanding of the mechanism ofclathrate hydrate formation in soil.

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