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Phase Transitions of Amorphous Solid Acetone in Confined
Geometry Investigated by Reflection Absorption Infrared
Spectroscopy
Sunghwan Shin,† Hani Kang,† Jun Soo Kim,*,‡ and Heon Kang*,†
†
Department of Chemistry, Seoul National University, 1 Gwanak-ro, Seoul 151-747, South Korea
Department of Chemistry and Nano Science, Ewha Womans University, 52 Ewhayeodae-gil, Seoul 120-750, South Korea
‡
ABSTRACT: We investigated the phase transformations of amorphous solid acetone
under confined geometry by preparing acetone films trapped in amorphous solid water
(ASW) or CCl4. Reflection absorption infrared spectroscopy (RAIRS) and temperatureprogrammed desorption (TPD) were used to monitor the phase changes of the acetone
sample with increasing temperature. An acetone film trapped in ASW shows an abrupt
change in the RAIRS features of the acetone vibrational bands during heating from 80 to
100 K, which indicates the transformation of amorphous solid acetone to a molecularly
aligned crystalline phase. Further heating of the sample to 140 K produces an isotropic solid
phase, and eventually a fluid phase near 157 K, at which the acetone sample is probably
trapped in a pressurized, superheated condition inside the ASW matrix. Inside a CCl4
matrix, amorphous solid acetone crystallizes into a different, isotropic structure at ca. 90 K.
We propose that the molecularly aligned crystalline phase formed in ASW is created by
heterogeneous nucleation at the acetone−water interface, with resultant crystal growth, whereas the isotropic crystalline phase in
CCl4 is formed by homogeneous crystal growth starting from the bulk region of the acetone sample.
I. INTRODUCTION
Phase transitions between liquid and solid phases and between
different solid phases are of critical importance in numerous
natural processes, such as ice formation on the Earth’s surface
and in the atmosphere,1,2 and mineralization of inorganic
substances in geology and biology,3 as well as in many chemical
applications, including crystallization of active pharmaceutical
ingredients4,5 and biological macromolecules.6 Although
acetone is one of the most widely used organic compounds
in chemical laboratories, the phase transitions of solid acetone
remain little understood. These phenomena are of great interest
in atmospheric and astrophysical sciences, because carbonyl
organic compounds are widely distributed in the Earth’s
atmosphere,7 as well as in interstellar clouds as ingredients of
ice mantles of dust particles.8
A limited number of studies have been reported on
investigations of the phase changes of solid acetone. In 1929,
Kelley9 reported an anomalous change of heat capacity near
130 K, which is well below the melting point (178 K) of bulk
acetone crystals, indicating a phase transition of the solid.
However, the physical origin of the transition remained unclear
for many years. In 1995, Ibberson et al.10 proposed that the
transition is not of the order−disorder type, based on the
observation of a small excess entropy value (2.04 J K−1 mol−1).
Later, Allan et al.11 examined acetone crystals prepared either
by solidification at room temperature under high pressure or by
cooling the liquid below the melting temperature at ambient
pressure. They reported stable and metastable structures of the
acetone crystals at various temperatures, and attributed the heat
capacity anomaly near 130 K to strengthening of the
© 2014 American Chemical Society
intermolecular electrostatic contacts of acetone molecules in
the crystals. Until now, however, many aspects of the phase
transitions of solid acetone have remained unclear.
In another line of research, the interactions of acetone with
the surfaces of amorphous solid water (ASW) or ice crystals
have been actively investigated by several research groups.
Schaff and Roberts12−14 reported extensive studies on the
adsorption, trapping, and desorption of acetone interacting with
ASW and crystalline ice films, using temperature-programmed
desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Souda15 examined the surface diffusion and
hydration of acetone molecules on porous and nonporous ASW
films, based on the results of time-of-flight secondary ion mass
spectrometry and TPD experiments. More recently, Lasne at
al.16,17 reported RAIRS studies of the interactions of acetone
and other oxygenated volatile organic compounds with the
surfaces of pure or nitric acid-containing ice films. Theoretical
studies of this subject include molecular dynamics simulations
by Picaud and Hoang,18,19 force-field method calculations by
Hammer and Schmidt,20 and quantum chemical calculations by
Marinelli and Allouche.21 In addition, flow-tube experiments
have been used to investigate the adsorption, trapping, and
dissolution behaviors of acetone in ice samples in the
temperature range of atmospheric relevance.22−27
Special Issue: Physics and Chemistry of Ice 2014
Received: April 24, 2014
Revised: May 31, 2014
Published: June 3, 2014
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was linearly p-polarized using a wire grid polarizer (Edmund
Optics). The beam path between the vacuum chamber and the
spectrometer was enclosed within a Plexiglas housing and was
purged with dry nitrogen gas. The absorbance spectrum of a
clean Ru substrate was used as a background reference. All
RAIR spectra were averaged 256 times to reduce the noise to
below 5 × 10−5 Abs at a spectral resolution of 4 cm−1.
In the present study, we investigated the phase transitions of
amorphous solid acetone samples, which were prepared by
acetone vapor deposition on a cold substrate and trapped inside
an ASW or CCl4 matrix. Although amorphous-to-crystalline
phase transitions have been observed and well established for
ASW,28,29 it remains an interesting question whether an
analogous process can occur for acetone. We explored this
phase transition in diverse environments, i.e., a wide temperature range, spatially confined geometry, and under interfacial
conditions with different materials. The occurrence of a phase
transition was monitored by measuring the changes in the
RAIRS acetone vibrational bands. This paper is organized as
follows. The experimental methods for sample preparation and
characterization are described in Section II. Section III-1
presents the results of RAIRS and TPD experiments with
acetone films trapped in ASW. Section III-2 highlights the
anomalous changes in the RAIRS intensities of acetone
vibrational bands in these samples, and compares them with
those observed for acetone films trapped in other environments. In Section III-3, we analyze the band shapes of ν(C
O) and other vibrations to examine the natures of different
solid phases of acetone. The experimental findings are
discussed in Section IV and summarized in Section V.
III. RESULTS
III-1. RAIRS and TPD Studies of Confined Acetone
Films in ASW. We prepared an amorphous solid acetone
sample confined in ASW as follows. First, a D2O-ASW film of
thickness ∼30 ML was prepared by D2O vapor deposition on a
Ru(0001) substrate at ∼80 K. An acetone film of thickness ∼14
ML was then grown on top of the ASW film at the same
temperature. Finally, a D2O-ASW layer (∼30 ML) was overlaid
on the acetone film at the same temperature, completing an
ASW-sandwiched acetone structure, in which an amorphous
acetone film is trapped between ASW films. The sample was
then heated slowly and the resulting phase changes of the
acetone film were monitored through RAIRS measurements of
the acetone vibrational bands. Figure 1 shows the RAIR spectra
II. EXPERIMENTAL METHODS
The experiments were carried out in an ultrahigh-vacuum
chamber equipped with instrumentation for RAIRS, TPD, and
other surface spectroscopic tools.30,31 The chamber background
pressure was maintained below 1 × 10−10 Torr during the
experiments. Acetone, water, and CCl4 samples were prepared
in the form of thin films grown on a Ru(0001) single-crystal
substrate. The substrate surface was cleaned using a standard
procedure of Ar+-sputtering at 2 kV (∼5 μA current and ∼20
min duration) and annealing at 1500 K; the surface cleanliness
was checked by examining the water TPD shape from a D2O
monolayer on Ru(0001).32 The substrate temperature was
controlled in the range 60−1500 K using a liquid-helium
cryostat and a resistive heater, and the temperature was
monitored using an N-type thermocouple wire connected to
the substrate. Acetone and CCl4 vapors were deposited on a
Ru(0001) substrate surface through a tube doser, at a
deposition rate slower than 0.15 monolayers per second (ML
s−1). A D2O film was prepared using a backfilling method, at a
deposition rate ≤0.1 ML s−1. Acetone, D2O, and CCl4 samples
were purified through freeze−vacuum−thaw cycles, and their
purities were checked using a quadrupole mass spectrometer
(Extrel) installed in the chamber. The thickness of the sample
film was estimated based on TPD measurements. For the D2O
and CCl4 films, the thickness was calculated by dividing the
TPD intensity of each film by that of the corresponding
monolayer on Ru(0001). For acetone, the TPD intensity was
converted to the total number of acetone molecules in the film,
and then to the film thickness using the unit cell volume of an
acetone crystal.11 In this paper, we express the film thickness in
units of ML, where we use 1 ML = 1.1 × 1015 molecules cm−2
for ASW and crystalline ice films, although this value is derived
from an ice Ih(0001) surface, 4.5 × 1014 molecules cm−2 for
CCl4, as estimated from the TPD intensity of CCl4 monolayer,
and 4.8 × 1014 molecules cm−2 for acetone, as estimated from
the crystal density. RAIRS was performed in a grazing angle
(84°) reflection geometry,33 using a commercial Fouriertransform IR instrument (PerkinElmer) equipped with a
mercury−cadmium telluride detector. The incident IR beam
Figure 1. RAIR spectra of acetone bands measured for ASWsandwiched acetone sample [D2O (∼30 ML)/acetone (∼14 ML)/
D2O (∼30 ML)/Ru(0001)]. Spectrum recorded (a) immediately after
sample preparation at 80 K, (b) after heating the sample at 120 K, and
(c) after heating to 140 K. (d) Transmission IR spectrum of liquid
acetone, shown as a band intensity reference for isotropic molecular
orientation.
of an ASW-sandwiched acetone film obtained immediately after
preparation of the sample at 80 K (spectrum a), after heating to
120 K (spectrum b), and subsequently heating to 140 K
(spectrum c). All these RAIR spectra were obtained at 80 K,
i.e., after cooling the sample back to the original temperature
(80 K) after each heating cycle; this procedure was used to
improve the quality of the difference spectrum by minimizing
thermal shifts in the background spectral intensity. In Figure 1,
it can be seen that the intensities of the acetone bands, as well
as their shapes, change with increasing temperature. The
intensity changes are as follows. The CO stretching [ν(C
O)] band decreases on heating to 120 K compared with its
original intensity at 80 K, but then it regains its original
intensity on further heating to 140 K. A similar intensity change
occurs for the asymmetric methyl deformation [ν(CH3)asym def]
band. In contrast, the asymmetric C−C−C stretching [ν(C−
C−C)asym] and symmetric methyl deformation [ν(CH3)sym def]
bands show the opposite behaviors, i.e., their intensities
increase from 80 to 120 K and then decrease again at 140 K.
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We interpret that the observed intensity changes arise from
reorientation of acetone molecules upon sample heating.
RAIRS preferentially detects the vibrations that induce dipole
changes in the direction of p-polarization of the light, or
perpendicular to the metal substrate surface. Accordingly, if the
reorientation of acetone molecules occurs collectively, it will
affect the RAIRS intensities of different bands in different ways.
The intensity changes are not due to loss of acetone molecules
from the sample, as will be shown shortly by TPD experiments.
Figure 1d shows the spectrum of liquid acetone, which serves as
a reference for isotropic molecular orientation. When compared
with this spectrum, the band intensities in spectra a−c indicate
that the average molecular orientation of acetone changes from
an isotropic distribution in an amorphous solid phase at 80 K
(spectrum a) to a molecularly aligned phase at 120 K
(spectrum b, called phase I hereafter), and then to an isotropic
phase at 140 K (spectrum c, called phase II). In this
interpretation, we assume that the acetone phase does not
change during sample cooling from 120−140 to 80 K, which
was used for the RAIR spectra shown in Figure 1. To check this
assumption, we recorded the RAIR spectra directly at 140 K,
without cooling the sample. The spectral features were basically
identical to those obtained after cooling to 80 K (Figure 1c),
which confirmed that sample cooling does not induce a phase
change.
Figure 2 shows diagrams of the normal modes of the four
vibrational bands of acetone shown in Figure 1. If we consider
Figure 3. TPD curves of acetone (lines) and water (shaded area) from
ASW-sandwiched acetone samples [D2O (30 ML)/acetone (d ML)/
D2O (30 ML)/Ru(0001)] with different acetone film thicknesses (d ≈
9, 14, and 64 ML). The TPD curves of acetone are shown after
normalization to the same height. The D2O TPD curve was measured
from the D2O (30 ML)/acetone (14 ML)/D2O (30 ML) sample. The
temperature ramping rate was 1 K s−1.
temperature is substantially below the melting temperature
(178 K) of bulk acetone, it is possible that the interfacial region
of the acetone film premelts into a fluid acetone layer. Also, it is
well-known35,36 that the melting temperature is significantly
lowered when the film thickness or particle size is reduced to
the nanometer scale.
An additional acetone desorption peak appeared at temperatures above the volcano peak, as shown for the 9 ML acetone
sample at 168−173 K in Figure 3. The intensity of this
desorption closely tracks the D2O desorption curve, and can
therefore be ascribed to acetone mixed with the D2O films.14
For a thick acetone film (∼62 ML), acetone desorption starts
at a much lower temperature (∼130 K; Figure 3). The
desorption rate of acetone increases with increasing temperature, in a similar way to the zeroth-order desorption kinetics of
a multilayer, and the volcanic desorption peak does not appear.
The TPD curve shows that the majority of trapped acetone
molecules desorb below the volcanic desorption temperature.
These features indicate that the ASW overlayer does not
effectively trap acetone molecules, and they migrate quite freely
to the sample surface and desorb from there. As will be shown
shortly, an amorphous solid acetone film crystallizes at 80−100
K. When a thick acetone film crystallizes, it may exert a strong
pressure against the upper ASW layer and generate cracks there.
In this case, acetone molecules can leak through these cracks
and cannot be pressurized as in the case of thin sandwiched
acetone films.
The RAIRS and TPD results presented in this section
illustrate that an amorphous acetone film sandwiched between
ASW layers changes its phase upon heating from 80 K to the
volcanic desorption temperature (157 K). The phase change
may occur in multiple stages; for example, amorphous solid
acetone with an isotropic distribution at 80 K changes to a
molecularly aligned structure (phase I) at 100−130 K and then
to an isotropic structure (phase II) at 140 K. Eventually, a fluid
phase may be formed before volcanic desorption at 157 K. In
the following sections, we investigate these different phases in
further detail by examination of differently prepared acetone
samples and by performing multiple-peak analysis of the
acetone vibrational bands.
Figure 2. Vibrational modes [ν(CO), ν(CH3)asym def, ν(CH3)sym def,
and ν(C−C−C)asym] of acetone and their displacement vectors.
the dipole change direction of the vibrations and p-polarization
direction of the light, which is perpendicular to the acetone−
water interface, we can deduce that phase I involves acetone
molecules that are oriented with the CO bond parallel to the
interface, and the C−C−C molecular plane perpendicular to
the interface.
Figure 3 shows the TPD results for ASW-sandwiched
acetone films of different thicknesses (9, 14, and 62 ML).
The most notable feature in TPD of acetone is a sharp
desorption threshold at 156−158 K, which appears for
relatively thin (9−14 ML) acetone films. Below this temperature, no desorption of acetone is observed. Schaff et al.14
previously observed this feature, and attributed it to a
“molecular volcano” phenomenon. Volcanic desorption can
occur as a result of the crystallization of ASW and
accompanying crack formation in the upper ice layer, through
which acetone molecules burst out.34 This observation indicates
that the acetone sample is fluid at ∼157 K. Although this
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III-2. RAIRS Vibrational Band Intensities: Molecular
Orientation of Acetone. We prepared acetone samples in
different trapping environments and compared their RAIR
spectra. The samples investigated were an acetone film
sandwiched between CCl4 films, a homogeneous acetone−
D2O mixture sample prepared by codeposition of two vapors,
and a pure acetone film grown directly on a Ru substrate, as
well as the acetone films of various thicknesses trapped in ASW
described in the previous section. These samples exhibit
characteristic RAIRS shapes for the acetone vibrational bands,
which can be differentiated from one another, and the ways in
which the band shapes and relative intensities change with
temperature are also different. In this section, we describe how
the band intensity changes with temperature; this gives
information on the acetone molecular orientations in the
samples.
Figure 4 shows the relative intensity variations of the ν(C
O), ν(CH3)asym def, ν(CH3)sym def, and ν(C−C−C)asym bands for
which the CO bond lies parallel to the plane of the acetone−
water interface, and the C−C−C molecular plane is
perpendicular to the interface. In the case of a thick (∼60
ML) acetone film in ASW, the decrease in the ν(CO)
intensity indicates alignment of the CO bond parallel to the
interfacial plane, as for the thin acetone film. However, the
ν(CH3)sym def and ν(C−C−C)asym intensities do not increase,
which indicate that the direction of the C−C−C molecular
plane is not ordered. Interestingly, a thin (∼15 ML) acetone
film grown directly on a bare Ru substrate is also ordered at
∼100 K, similar to that trapped between ASW layers. It was not
possible to heat the acetone film on Ru above 100 K, because of
rapid sublimation of the film. It appears that the only possible
interpretation of the formation of an ordered phase in these
samples is crystallization of amorphous solid acetone.
The other samples, including an acetone film sandwiched
between CCl4 layers and an acetone-D2O mixture film, do not
show noticeable changes in band intensity during heating. This
indicates that these samples do not form an ordered acetone
crystal as in phase I.
When a thin (∼14 ML) acetone film in ASW was further
heated to 140 K, beyond the temperature of crystallization to
phase I at 100 K, the intensities of the ν(CO) and
ν(CH3)asym def bands increased back to their original intensities
at 80 K, and the ν(CH3)sym def and ν(C−C−C)asym band
intensities decreased. These changes indicate that the
molecularly aligned crystals in phase I change into a new
isotropic phase (phase II). This phase appeared when an
acetone film was heated to 140 K, a condition that is achievable
only for relatively thin acetone films trapped in ASW.
III-3. Band Shape Analysis. This section focuses on the
vibrational band shapes of acetone samples. Because the ν(C
O) band shows more systematic evolution with changing
temperature than the other bands, the shape of the ν(CO)
band will be examined in detail through multiple-peak analysis.
Although the other bands also show noticeable shape changes
with changing temperature, they are difficult to interpret
systematically. The complex changes of these band shapes are
related to intermolecular vibrational couplings in the solid,
which are very difficult to analyze without information
regarding the packing structures of acetone solids. Figure 5
shows the ν(CO) band of acetone (left frame) and the
1120−1480 cm−1 region (right frame), which encompasses the
ν(CH3)asym def, ν(CH3)sym def, and ν(C−C−C)asym bands. The
multiple-peak analysis results of the ν(CO) band are also
shown.
In Figure 5a, an acetone film (∼14 ML) sandwiched between
ASW layers shows a broad, structureless ν(CO) band when
the sample is prepared at 80 K. The band shape does not
change when the sample is prepared at a lower temperature (60
K), as long as it is below the transition temperature to
crystalline phase I. These observations support that the
morphology of the acetone film is amorphous. When this
sample is heated to 120 K, the ν(CO) band shape changes
significantly, shown in Figure 5b. The broad band of
amorphous solid acetone changes to a shape that consists of
three peaks (1717, 1708, and 1699 cm−1), which is indicative of
formation of distinct solid structures. The splitting of the
ν(CO) band into three components is more clearly seen for
a thick acetone film in ASW, shown in Figure 5c. Here, the
1717 cm−1 peak becomes the largest among the three
components; this peak seems to be blue-shifted slightly (by
1−2 cm−1) for a thick film, although the shift size is comparable
Figure 4. Relative intensity changes of RAIRS bands for acetone (AC)
in different samples during temperature increase: AC (∼14 ML) in
D2O (●), AC (∼60 ML) in D2O (○), AC(∼100 ML) in CCl4 (Δ),
AC−D2O mixture (▼), and pure AC (∼15 ML) on Ru (★). (a)
ν(CO); (b) ν(CH3)asym def; (c) ν(CH3)sym def; (d) ν(C−C−C)asym.
The intensities are calculated from the integration of the band area,
and shown after normalization against those from the isotropic
samples prepared at 60−80 K (see text).
different samples with increasing temperature. The relative
band intensities are plotted after normalization with respect to
their intensities measured for the initially prepared samples at
60−80 K, which correspond to amorphous solid acetone or
homogeneous solid solution phases. In these samples, acetone
molecules should have an isotropic orientational distribution.
In the case of a thin (∼14 ML) acetone film sandwiched
between ASW layers, the ν(CO) intensity decreases upon
sample heating from 80 to 100 K (Figure 4a), with a
simultaneous decrease in the ν(CH3)asym def intensity (Figure
4b). Conversely, the ν(CH 3 ) sym def and ν(C−C−C) asym
intensities (Figures 4c and 4d) both increase simultaneously.
As mentioned in the previous section, these changes indicate
the formation of a molecularly ordered structure (phase I), in
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Figure 5. ν(CO) band (left frame) and 1180−1480 cm−1 spectral region (right frame) of various acetone samples. The results of multiple-peak
analysis for ν(CO) are shown by dotted curves. (a) Amorphous solid acetone film (∼14 ML) in D2O at T = 80 K, (b) thin (∼14 ML) acetone
film in D2O at 120 K (phase I), (c) thick (∼62 ML) acetone film in D2O at 120 K (phase I), (d) homogeneous mixture of acetone and D2O (1:8
ratio) at 120 K, (e) thick (∼100 ML) acetone film in CCl4 at 110 K (phase III), and (f) thin (∼14 ML) acetone film in D2O at 140 K (phase II). In
these figures, the vibrational bands are displayed so as to have similar heights for all samples. The actual intensities of these bands can be estimated
from the absorbance bars shown in the figure.
to the uncertainty of the fitting procedure. The 1717 and 1708
cm−1 components increase significantly as the thickness of the
acetone film increases, whereas the 1699 cm−1 component does
not change much with changing film thickness. We therefore
interpret that the 1717 and 1708 cm−1 peaks are derived from
bulk acetone, whereas the 1699 cm−1 peak is related to acetone
molecules in the interfacial region. Previously, the appearance
of a red-shifted ν(CO) band (1701−1708 cm−1) was
assigned to a H-bonded CO stretching mode in RAIRS studies
of acetone adsorption on ASW films.12−14 In IR studies of
acetone−water solutions,37 the 1698−1708 cm−1 peaks were
assigned to a H-bonded CO stretching mode. However, we
observed that acetone films grown on a bare Ru(0001)
substrate without water also have these spectral components
(not shown), similar to Figure 5c. This casts a doubt on the
assignment of the 1699 and 1708 cm−1 peaks uniquely to Hbonded acetone. Our observations indicate that these peaks
also appear for a pure acetone film, and the 1708 cm−1 peak
may be related to a bulk acetone state.
A homogeneous mixture of acetone and D2O (Figure 5d)
shows a ν(CO) band shape that is quite different from those
observed for amorphous solid acetone (Figure 5a) or crystalline
acetone (Figure 5b,c). The ν(CO) band of the acetone−
D2O mixture can be fitted to two broad peaks centered at 1708
and 1699 cm−1. By examining acetone−D2O mixture samples
with different acetone/D2O ratios, we verified that the 1708
cm−1 peak intensity increases relative to that of the 1699 cm−1
peak when the acetone/water ratio is increased (data not
shown). This again supports our interpretation that the 1699
cm−1 peak observed for ASW-sandwiched acetone samples
originates from acetone interacting with water in the acetone−
water interfacial region. Heating an acetone−D2O mixture to
120 K did not change the band shapes and intensities.
An acetone film sandwiched between CCl4 layers can be used
as a reference sample to separate the effects of an ASW surface
on acetone crystallization. A CCl4-sandwiched acetone film
prepared at 60 K shows the spectral features of amorphous
solid acetone, similar to those observed for an ASWsandwiched acetone sample in Figure 5a. However, as shown
in Figure 5e, thermal heating of this sample to 110 K changes
the band shapes significantly and very differently from the other
samples. Specifically, the ν(CO) band in CCl4 consists of a
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sharp component at 1712 cm−1 and an additional broad
component at 1717 cm−1, whereas the ν(CO) band in ASW
splits into three components. Note that the ν(CO) band in
CCl4 does not have the 1699 or 1708 cm−1 components that
appear in ASW. The other vibrations in CCl4 are also different
than in ASW; for example, the ν(CH3)sym def band (∼1370
cm−1) develops a new shoulder peak on the lower-frequency
side (∼1350 cm−1) on heating the sample in CCl4 to 110 K,
whereas this band sharpens in ASW. In addition, the ν(C−C−
C)asym band position (∼1237 cm−1) is not changed by heating
in CCl4, whereas this peak is slightly blue-shifted in ASW. All
these observed distinct differences between the samples in CCl4
and ASW suggest that the crystal phase formed in CCl4 at 110
K is different from the phase I formed in ASW. Regarding the
band intensities, the sample in CCl4 shows only marginal
variations in the band intensities on heating from 80 to 110 K
(Figure 4). This suggests that the crystals formed in CCl4 have
nearly isotropic orientational distributions. This new structure
is called phase III. The properties of this sample were not
examined above 110 K, because of sublimation of the CCl4 film.
Figure 5f shows the spectra of an acetone film in phase II,
obtained by heating an ASW-sandwiched acetone film (∼14
ML) at 140 K. The spectra were recorded at 80 K, as
mentioned above, but they are basically the same as those
recorded at 140 K. The ν(CO) band shape changes from the
three-component shape of phase I at 120 K to a broad shape
centered at 1708 cm−1 at 140 K. The ν(CO) band can be
fitted to a single broad Gaussian curve or, alternatively, it can be
deconvoluted into the three components (1717, 1708, and
1699 cm−1) of phase I, but with different relative intensities
from those in Figure 5b.
Figure 6. Structure of a stable orthorhombic acetone crystal
constructed by duplicating the unit cell by 2 × 2 × 1 in the x, y,
and z directions, using crystallographic information from ref 11. The
unit cell dimensions are a = 9.172(8), b = 7.761(8), and c = 21.66(2),
with the Pbca space group, at 110 K. (a) Orthographic and perspective
views of the crystal structure are visualized using VMD.38 CO bonds
are colored in red, and C−H bonds in white. (b) Two neighboring
layers perpendicular to the z-axis, where all CO bonds are aligned
perpendicular to the z-axis.
acetone films in ASW. We propose that the crystals in phase I
have a structure similar to those reported by Allan et al.11 If, for
example, these crystals are formed in a preferential orientation
such that the acetone molecules are aligned along the z-axis, as
shown in Figure 6a, then the observed spectral features of phase
I can be explained.
The question of how the crystals grow with such a
preferential orientation arises. At the moment, this question
lies beyond our experimental capabilities, but some hints are
available; for example, phase I crystals are formed in acetone
samples trapped in ASW or grown on Ru(0001), but not in
those in CCl4. These observations indicate that the interfacial
structure is critical for the formation of phase I crystals.
Previous molecular dynamics studies of acetone molecules
adsorbed on ice surfaces have shown that acetone molecules are
aligned with CO bonds slightly tilted from the interface near
the acetone−ice interface.18,19 By analogy, we suggest that the
ordering of acetone molecules starts from a specific interface
such as an acetone−water interface or Ru(0001) surface, and
propagates into growth of ordered crystals via heterogeneous
nucleation. On the other hand, the porous structure of ASW
may not be effective to align acetone molecules collectively to
form an ordered crystal phase.
B. Isotropic Phase III in CCl4 at 90−110 K. The
interactive force between acetone and CCl4 is weaker than that
between acetone and water, which involves H bonds.
Heterogeneous crystal growth from the interface is therefore
less likely to occur for acetone in CCl4 than for acetone in
ASW. If homogeneous crystal growth in the bulk prevails over
interfacial heterogeneous crystal growth in the case of acetone
in CCl4, then the isotropic nature of crystal phase III can be
explained by the formation of multiple crystals via homoge-
IV. DISCUSSION
The RAIRS study results indicate that amorphous solid acetone
trapped in ASW undergoes phase changes twice during
temperature increases from 60 to 140 K: first to an aligned
crystalline phase (phase I) at 80−100 K and then to an
isotropic phase (phase II) at 130−140 K. Also, we observed
that amorphous solid acetone in CCl4 crystallizes into an
isotropic phase (phase III), which has quite different RAIRS
features from those of phases I and II in ASW. In what follows,
we further discuss the nature of these different phases produced
by the thermal treatment of various acetone samples.
A. Molecularly aligned phase I in ASW at 100−130 K.
There is clear evidence that an acetone film in ASW forms an
aligned crystal phase upon heating to 100 K. The evidence is
that the ν(CO) band has a distinct shape (Figure 5c) and the
relative intensities of different bands indicate the formation of a
molecularly aligned state. The band shapes and relative
intensities do not change on further increasing the temperature
from 100 to 130 K (Figure 4), which is consistent with the
characteristics of a phase transition, indicating that the
crystallization has occurred abruptly and is completed somewhere between 80 and 100 K. We therefore conclude that
acetone undergoes an amorphous-to-crystalline phase transition
at this temperature.
Allan et al.11 reported the structures of stable and metastable
acetone crystals, where all the CO bonds are aligned along
the same plane. Figure 6 shows one of these structures for a
stable crystal. In the present samples, the crystallization
preferentially aligns CO bonds parallel to the acetone−
water interface. The C−C−C molecular plane is also ordered
perpendicular to the interface in the cases of thin (∼14 ML)
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Although current understanding of the underlying principles of
solid-phase transitions is insufficient to explain these phenomena clearly, there have been reported examples of experimentally observed crystal morphology changes, and these were
interpreted in terms of crystallization theories such as Ostwald’s
law of stages, independent nucleation, and cross nucleation.5,39
D. Fluid Phase near 157 K. Volcanic desorption of acetone
at ∼157 K indicates that acetone is fluid underneath the
crystallizing ASW film near this temperature. The fluid acetone
may be formed in the premelting surface region of an acetone
film, whose interior remains solid. However, the possibility that
the entire acetone film melts cannot be excluded, if thin films
have a substantially lower melting temperature than the bulk, as
mentioned above. Also, if phase II is acetone−ASW solution
rather than pure solid acetone, as we discussed above, then fluid
acetone may be produced by freezing out of the acetone−ASW
solution to pure ice crystals and acetone liquids near ∼157 K.
neous nucleation, which would result in orientational isotropy
on ensemble averaging, regardless of the crystal structure.
The RAIRS band shapes are quite different for phases I
(Figure 5c) and III (Figure 5e), which leads us to conclude that
these phases have different crystal structures. Although their
overall band shapes are distinctly different, multiple-peak
analysis of the ν(CO) band indicates that the 1717 cm−1
peak may exist commonly in phases I and III, although its
intensity in phase III is much weaker than that in phase I. If the
1717 cm−1 peak is uniquely assignable to phase I, then it is
possible that the sample in phase III contains a small
proportion of phase I crystals, oriented randomly inside the
sample. Further discussion of these possibilities will be deferred
until further research has been undertaken.
C. Isotropic Phase II in ASW at 140 K. The broad ν(C
O) band of phase II (Figure 5f) can be fitted either to a single
Gaussian curve (1708 cm−1) or deconvoluted into the three
components (1717, 1708, and 1699 cm−1) of phase I, but with
different relative intensities. The band shapes of phase II and III
are well distinguished, which suggests that they are independent
structures; for example, the 1708 and 1699 cm−1 components
appearing in phase II do not appear in phase III. Although they
are not yet definite, we suggest the following interpretations of
phase II.
When judged in terms of spectral resemblance, an acetone−
D2O mixture sample (Figure 5d) is closest to phase II. We
therefore propose that a large part of phase II consists of an
acetone−water solid solution, which is formed by dissolution of
an acetone film into ASW at 140 K. A very thin (<10 ML)
acetone film trapped in ASW at 120 K (not shown) shows
similar spectral features to those of phase II, which, if we
consider that a thinner film dissolves more easily into ASW,
supports the proposed solid solution formation in phase II. The
appearance of 1717, 1708, and 1699 cm−1 components in
phases I and II suggests the possibility that a sample in phase II
consists of, in addition to the solid solution, a certain amount of
phase I crystallites, which remain undissolved in ASW. An
observation that seemingly contradicts this interpretation of
solid solution formation, however, is the volcanic desorption of
acetone at 157 K observed in the TPD experiments (Figure 3).
If an acetone−water solution is formed by spontaneous mixing
at 140 K, then how can it be separated back to pure water and
acetone phases at higher temperatures? The answer to this
question may be that this phase separation is driven by
crystallization of the acetone−ASW solution at 157 K, which
freezes out ice crystals and expels acetone from the crystals.
This interpretation assumes that acetone has high solubility in
ASW, which is consistent with previous reports,12−14 and is
insoluble in ice crystals.
There is a certain resemblance between the RAIRS features
of phase II and amorphous solid acetone (Figure 5a). It is
therefore possible that acetone crystalline phase I could change
to an amorphous solid by heating to 140 K. To the best of our
knowledge, however, phase transition from a crystal to an
amorphous solid phase is very unusual under ordinary
conditions.
Finally, there is a possibility that phase II corresponds to a
new crystalline form, distinct from phases I and III. In this case,
we need to explain how one crystal structure formed at a low
temperature (phase I at 100 K) can be transformed into a
different crystal structure at a higher temperature (phase II at
140 K). We did not observe any evidence for melting of
acetone crystals or a glass transition between 100 and 140 K.
V. SUMMARY
RAIRS and TPD studies of acetone samples trapped in ASW
and CCl4 show that acetone can exist in various solid phases
under confined conditions. An amorphous solid acetone film
trapped in ASW crystallizes into a molecularly aligned phase I
upon heating to 100 K. Further heating of the crystal produces
an isotropic solid phase II at 140 K, and then eventually an
interfacial fluid phase near 157 K. In addition, amorphous solid
acetone changes into an isotropic crystalline phase III in CCl4
at 90 K. Analyses of the RAIRS band shapes and relative
intensities show that phase I consists of acetone crystals with
molecular ordering such that the CO bonds are aligned
parallel to the acetone−water interface and, in the case of a thin
acetone film, the C−C−C planes are perpendicular to the
interface. The ordered crystal phase may be formed by
heterogeneous nucleation at the acetone−water interface and
resultant crystal growth. In contrast, the isotropic phase III
observed in CCl4 may be formed by homogeneous crystal
growth from the bulk state of amorphous solid acetone; this
may be kinetically favored over interfacial crystal growth. Phase
II may correspond to an acetone−water solid solution formed
by dissolution of acetone in ASW, although other interpretations are also currently possible. These various acetone phases
are formed because high-temperature and high-pressure
conditions are produced for the acetone samples in a confined
geometry (a pressure-cooker effect). Similar conditions might
exist in nature, for example, in star-forming regions or comet
tails in space, where the ice mantle of dust particles experiences
thermal processing.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: surfi[email protected]; Phone: +822-875-7471 (H.K.).
*E-mail: [email protected] (J.S.K.)
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work was supported by a grant from the Samsung Science
and Technology Foundation (SSTF-BA1301-04, H.K.).
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