Science in China Series B: Chemistry
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FTIR–ATR in situ observation on the efflorescence and
deliquescence processes of Mg(NO3)2 aerosols
LI XiaoHong, DONG JinLing, XIAO HanShuang, LU PeiDong, HU YongAn & ZHANG YunHong†
The Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, China
The efflorescence and deliquescence processes of Mg(NO3)2 aerosol particles deposited on ZnSe substrate have been investigated through in situ Fourier transform infrared-attenuated total reflection
(FTIR-ATR) technique at the molecular level. At relative humidity (RH) of ~3%, Mg(NO3)2 particles existed as amorphous states. The amorphous Mg(NO3)2 particles were transformed into crystalline
Mg(NO3)2·nH2O (n ≤ 5) with slight increasing of RH. Thermodynamically stable Mg(NO3)2·6H2O crystals
were gradually formed on the particle surface and started to be dissolved at the saturation point (~53%
RH). At the same time, a continuous phase transition from Mg(NO3)2·nH2O (n≤5) to Mg(NO3)2·6H2O
occurred on the particle surface. This led the solid particles to completely deliquesce at 76% RH, which
was much higher than the saturation point of 53% RH. In the efflorescence process, Mg(NO3)2 droplets
entered into the supersaturated region due to the gradual evaporation of water. Finally, amorphous
particles were formed when RH decreased below 5%. In the FTIR-ATR spectra of the supersaturated
−
−
Mg(NO3)2 droplets, the absorbance of the symmetric stretching vibration of NO3 (v1- NO3) clearly became stronger. It resulted from the continuous formation of solvent share ion pairs (SIPs), and even the
−
contact ion pairs (CIPs) between Mg2+ and NO3.
FTIR-ATR, Mg(NO3)2 aerosol, phase transition, ion pairs
Mineral dust and sea salt are the most important sources
of the natural aerosols in the troposphere. It is estimated
that as many as (1―3) × 1012 kg of mineral dust aero―
sols[1 4] and 1012 kg of sea salt aerosols[5,6] enter into the
atmosphere every year. These aerosol particles can be
entrained in the air for days to weeks and have highly
reactive surfaces[7,8]. Thus, they usually react with nitrogen oxides such as NO2, NO3, HNO3, and N2O5,
when they are transported through polluted regions[9,10].
Since magnesium is one of the primary components both
of the mineral dust and of the sea salt, Mg(NO3)2 should
account for the main proportion both of the aged mineral
dust aerosols and of the aged sea salt aerosols. For example, the mole ratio of Mg to Na is about 0.114 in sea
salt aerosols. Especially, Langer et al. discovered that
Mg2+ played an important role in the reaction of sea salt
aerosols and NO2[11]. Hence, the surface properties of the
aged sea salt aerosols should be mainly determined by
Mg(NO3)2. Moreover, the physical and chemical properties of the particle surface are directly or indirectly related to a series of important environment problems,
such as the scattering of solar radiation and air visibility[12].
The hygroscopic properties of Mg(NO3)2 aerosols
were investigated by Ha and Chan using the electrodynamic balance (EDB) technique[13], which were in good
accordance with the predictions of the ZdanovskiiStokes-Robinson (ZSR) equation. The Mg(NO3)2 microdroplets levitated in the EDB were also found not to
effloresce even at fairly low relative humidity (RH)[13].
Received July 30 2006; accepted February 5, 2007
doi: 10.1007/s11426-007-0059-z
†
Corresponding author (email: [email protected])
Supported by the Trans-Century Program Foundation for the Talents by the Ministry
of Education of China, the National Natural Science Foundation of China (Grant Nos.
20073004, 20473012, and 20673010), the 111 Project (B07012), and the State Key
Laboratory of Physical Chemistry for Solid Surface of Xiamen University
Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
Using EDB-Raman technique, Zhang et al. further studied the effect of ion pairs on the hygroscopic behavior of
the supersaturated Mg(NO3)2 droplets at the molecular
level[14]. They found that amorphous Mg(NO3)2 particles
were occasionally formed in the ambient with RH of
3%[14]. Correspondingly, Gibson et al.[15] observed a
rapid increase for the growth factor of the Mg(NO3)2
particles as RH increased from 3% to 10% due to the
amorphous state of Mg(NO3)2 particles. Even though the
phase transition of Mg(NO3)2 aerosols is so complicated,
there has not been a detailed report about it.
Although EDB-Raman technique has been applied to
study the supersaturated Mg(NO3)2 aerosols[14], the minor spectral changes related to the formation of ion pairs
are not easy to identify due to the effect of Mie morphology resonances for the levitated spherical microdroplet. In the broad O—H stretching vibration envelope,
the disturbance of Mie morphology resonances is more
difficult to suppress and severely limit the analysis of
the water structures. Combined with aerosol flow tube
(AFT), the Fourier transform infrared (FTIR) technique
has been successfully used to study the hygroscopic
properties of aerosols[16,17]. However, the FTIR-AFT
spectra are strongly disturbed by the signal of water vapor in both the O—H stretching region and the O—H
bending region, which also makes it difficult to investigate the hydrogen bonding structures of water. On the
other hand, it has been proved that the structure of water
is extremely complicated in Mg(NO3)2 droplets. For
example, CCD spatial images[14] and fluorescence experiments[18] indicated that the ions in the outer, centric
and intermediate regions of the supersaturated
Mg(NO3)2 droplets have respective hydrated structures.
Besides, both EDB-Raman and FTIR-AFT techniques
are not very effective to research the metastable solid
particles resulting from the solidification of highly supersaturated Mg(NO3)2 droplets, especially the micro-physichemical structure of their surface.
Recently, we set up an in situ attenuated total reflectance (ATR) aerosol chamber. Cooperating ATR with
FTIR technique, the efflorescence and deliquescence
processes of NaClO4 aerosol particles deposited on ZnSe
substrate have been successfully studied[19]. The disturbance of water vapor on the FTIR spectra can be effectively suppressed since the penetration depth on the
ZnSe crystal surface is only about 4 microns. Therefore,
FTIR-ATR spectra with high signal-to-noise (S/N) ratio
can be obtained. They can provide important information on structural transition. In particular, the small
penetration depth ensures that the FTIR-ATR technique
would be very sensitive to the surface properties of
aerosol particles. Thus, it can be used to study the surface chemical properties and hygroscopic behaviors of
heterogeneous particles. In this paper, we attempt to
probe into the hygroscopic behaviors of metastable
Mg(NO3)2 particles, as well as the formation of solvent
share ion pairs (SIPs) and even contact ion pairs (CIPs)
in supersaturated Mg(NO3)2 droplets using FTIR-ATR
technique.
1 Experimental
Figure 1 shows the schematic diagram of the experimental setup for the FTIR-ATR in situ observation on
the efflorescence and deliquescence processes of
Mg(NO3)2 aerosols. 0.5 mol/L Mg(NO3)2 solution was
used to produce micro-sized aerosol droplets through the
ultrasonic humidifier. By use of a pump, these aerosol
droplets were inbreathed into the ATR aerosol room.
Some droplets were deposited on the substrate, i.e., a
baseline horizontal ZnSe crystal (refractive index: 2.4),
to be measured. Exposing these droplets to dry N2 for
about 30 min, the initial samples of Mg(NO3)2 aerosols
were prepared. The RH in the chamber was adjusted by
changing the flow rates of a stream of dry N2 and another of N2 saturated with water vapor. The spectra were
collected after 10 min at a given RH in order to ensure
that the samples in the chamber could equilibrate with
the ambient RH. The RHs were recorded on-line with
the RH meter (±2.5% RH, CENTER 313) placed near
Figure 1 Schematic diagram of the experimental setup for the in situ
observation on Mg(NO3)2 aerosols in the efflorescence and deliquescence
processes using the FTIR-ATR technique.
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
129
the exit. To increase the S/N ratio, the FTIR-ATR spectra were measured by a MCT (mercury cadmium telluride) detector cooled with liquid nitrogen, and 32 scans
were accumulated. The spectra from 650 to 4000 cm−1
have a resolution of 4 cm−1 and a repeatability of ±1
cm−1. Moreover, all the measurements were made in the
ambient with temperatures of 22―24℃.
ing RH. As shown in Figure 2, four spectral regions
correspond to the symmetric stretching vibration
(v1 -NO3− ),
the anti-symmetric stretching vibration
(v3 -NO3− )
and the out-of-plane bending vibration
(v2 -NO3− ) of NO3− , as well as the O—H stretching
band (v-H2O), respectively. It is notable that the v1 -NO3−
2 Results and discussion
mode becomes IR active in aqueous solution although it
2.1 FTIR-ATR spectra and hygroscopic curves in
the efflorescence and deliquescence processes
The reason is that the symmetry of NO3− is decreased
Since the total optical path of the infrared light passing
through the sample is only ~50 microns (12 reflections
each with penetration depth of ~4 microns), the disturbance of water vapor on the FTIR-ATR spectra is fairly
weak. Hence, the highly qualified spectra of the
Mg(NO3)2 aerosols in the efflorescence and deliquescence processes were obtained by continuously chang-
is IR forbidden for free NO3− with D3h point group[20].
by the adjoining solvent water molecules and cations. In
this study, the v1 -NO3− band of Mg(NO3)2 aerosols
were obtained with high S/N ratio. The continuous
changes of the v1 -NO3− band with the variation of RH
can provide more reliable information on the ionic interactions and phase transitions.
Figure 2 FTIR-ATR spectra of Mg(NO3)2 aerosols at various RHs in the deliquescence (a) and efflorescence (b) processes. The corresponding WSRs are included in Figure 2(b).
130
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
Figure 2(a) shows the FTIR-ATR spectra of
Mg(NO3)2 aerosols at various RHs in the deliquescence
process. According to the spectral features, the whole
process can be roughly divided into four regions as RH
increases. In the first region of RH ≈ 3%, the vibrations
of NO3− result in a relatively broad band at ~1051 cm−1
from the v1 mode, a band at ~819 cm−1 from the v2 mode,
and a band at ~1335 cm−1 with an obvious shoulder at
~1440 cm−1 from the v3 mode. In the O—H stretching
envelope, there is a main peak at ~3380 cm−1 with two
shoulders at ~3242 and ~3580 cm−1, respectively. In the
second region with RH of 8% ―44%, the v2 -NO3−
band blue shifts to ~821 cm−1 with a shoulder at ~830
cm−1. Correspondingly, a very weak but significant
−1
shoulder appears at ~1059 cm
in the
v1 -NO3−
region
and becomes more obvious at ~23% RH. Besides, the
v3 -NO3− vibration produces a main peak at ~1344 cm−1
and a shoulder at ~1397 cm−1, showing a relatively small
splitting. Compared with the main peak at ~3352 cm−1,
the shoulder at ~3242 cm−1 gradually becomes stronger
while the one at ~3580 cm−1 becomes weaker in the
O―H stretching envelope. In the third region of RH =
52%―67%, the ~830 cm−1 shoulder obviously strengthens even though the main peak remains almost immobile
in the v2 -NO3− region. Also, there are no evident
changes in other vibration regions. In the fourth region
with RH = 76%―90%, the v2 -NO3− band appears at
~828 cm−1, the v1 -NO3− band red shifts to ~1046 cm−1
(RH = 90%) accompanied by the disappearance of the
~1059 cm−1 shoulder, and the v3 -NO3− band moves to
~1338 cm−1 with an obvious shoulder at ~1397 cm−1. Also,
the intensity at ~3242 cm−1 becomes slightly larger than
that at ~3352 cm−1 in the O—H stretching region.
Slowly desiccating the Mg(NO3)2 droplets formed in
the deliquescence process, the efflorescence process was
observed. As shown in Figure 2(b), the FTIR-ATR spectral changes in the efflorescence process are relatively
simple, and the process can be approximately divided
into two RH regions. In the first region of RH = 90%―
5%, the FTIR-ATR spectra gradually change with RH
decreasing, showing no apparent transitions. Concretely,
the v1 -NO3− band slowly blue shifts from ~1046 to
~1051 cm−1 accompanied by the gradual increase of
the band intensity, the v3 -NO3− band occurs at ~1338
cm−1 with a gradually weakened shoulder at ~1397 cm−1,
and the v2 -NO3− band slowly moves from ~828 to
~821 cm−1. In the O—H stretching region, there are two
peaks at almost immovable positions, i.e., ~3242 and
~3352 cm−1, respectively. Compared with the latter, the
former is slightly stronger at RH≥73% and becomes
slightly weaker at RH < 73%. In the second region with
RH ≈ 3%, the v2 -NO3− band abruptly red shifts to ~819
cm−1, the v1 -NO3− band is obviously enhanced although
its position does not change visibly, and the shoulder of
the v3 -NO3− band blue shifts to ~1440 cm−1. Besides,
the main peak shifts to ~3380 cm−1 while the 3242 cm−1
shoulder is still obvious in O—H stretching envelope.
The relative intensity at higher frequency is enhanced,
leading to an observed shoulder at ~3580 cm−1. On the
whole, the FTIR-ATR spectrum at ~3% RH in the efflorescence process is similar to that of the initial samples.
In order to extract more information about the phase
transition, the hygroscopic curves of Mg(NO3)2 aerosols,
i.e., the curves of the area ratios of the v-H2O band to the
v3 -NO3− band in terms of RH, in the efflorescence and
deliquescence processes are shown in Figure 3. The data
in Figure 3 come from two independent deliquescence-efflorescence cycles. In two efflorescence processes, the hygroscopic curves accord with each other.
The area ratio of the v-H2O band to the v3 -NO3− band
decreases from 4.4 (92% RH) to 1.9 (5% RH), and finally to 1.3 (~3% RH). No obvious transition points are
observed even at the saturation point of Mg(NO3)2·6H2O,
i.e., 53% RH. However, the hygroscopic curves are quite
complicated in the deliquescence processes. They are
roughly divided into five regions depending on their
variations and whether or not they are overlapped with
the hygroscopic curves of the efflorescence processes. In
region I with RH < 8%, the hygroscopic curve is fairly
steep. The area ratio of the v-H2O band to the v3-NO3band increases rapidly from 1.5 (RH ≈ 3%) to 2.0 (RH =
8%). In region II with RH = 8%―23%, the area ratio
slowly increases to 2.3 as RH reaches 23%. In region III
(RH = 23%―53%) and IV (RH = 53%―76%), the area
ratio obtained in the deliquescence processes is always
less than that in the efflorescence processes at given RH.
Furthermore, it is not very sensitive to the increase of
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
131
RH in region III, changing from 2.3 (~23% RH) to 2.5
(~53% RH). However, it increases rapidly in region IV,
reaching 3.5 at ~76% RH. Besides, two hygroscopic
curves obtained from two experiments are slightly different from each other. On entering into region V (RH >
76%), the hygroscopic curves are almost overlapped
with each other in the deliquescence and efflorescence
processes, and the area ratio suddenly increases to 4.4 at
~92% RH.
the thin film of metastable amorphous Mg(NO3)2 produced on the interface. Besides, the O—H stretching
vibration of the amorphous Mg(NO3)2 thin film brought
forth an obvious shoulder at 3250 cm−1 and a relatively
strong absorbance in the higher frequency[21], which is
very similar to the O—H stretching band at ~3% RH in
Figure 2(a). Therefore, we deduce that the initial
Mg(NO3)2 aerosols mainly consist of amorphous hydrates which usually contain trace of adsorbed water on
the surface[12]. Similarly, Zhang[14] and Gibson[15] also
mentioned that the Mg(NO3)2 aerosol particles were
likely to be amorphous structure at 3% RH. As the RH
increased from 3% to 8%, an obvious augmentation was
observed in the area ratio of the v-H2O band to the
v3 -NO3− band, suggesting that the amorphous particles
rapidly absorbed water from the ambient environment. A
similar phenomenon was also observed by Gibson
et al.[15] and was attributed to the deliquescence of the
amorphous Mg(NO3)2. Correspondingly, the shoulder of
the v3 -NO3− band distinctly shifts to a lower frequency
(~1397 cm−1) and is significantly overlapped with the
main peak at 8% RH, leading the tiptop to occur at
~1344 cm−1 in the v3 -NO3− region. It has been discov-
Figure 3
Area ratios of the v-H2O band to the v3 -NO3− band and WSRs
of Mg(NO3)2 aerosols as a function of RH in the efflorescence and deliquescence processes. Solid and open symbols represent the efflorescence
and deliquescence process, respectively.
ered that the amorphous Mg(NO3)2 thin film was structurally rearranged to form crystalline Mg(NO3)2·nH2O
(4 < n ≤6) as RH increased from 2% to 7% due to the
effect of the surface adsorbed water[21]. Correspondingly,
the v3 -NO3− band appeared at 1382 cm−1 with no obvi-
2.2 Phase transitions in the efflorescence and deliquescence processes
The FTIR-ATR spectra and the hygroscopic curves of
Mg(NO3)2 aerosols provide useful information about the
phase transition at the molecular level, especially in the
complicated deliquescence process. In the spectrum of
the initial Mg(NO3)2 samples (~3% RH in Figure 2(a)),
the distinct splitting of the
v3 -NO3−
mode results in a
main peak and a shoulder at ~1335 and ~1440 cm−1,
respectively. This is well consistent with the observation
reported by Al-Abadleh et al. [21], that is to say, the
v3 -NO3− band showed a main peak at 1339 cm−1 and a
shoulder at ~1456 cm−1 in the transmission FTIR spectra
of nitric acid reacted MgO interfaces in the extremely
dry environment. They assigned this evident splitting to
132
ous splitting anymore in the transmission FTIR spectra[21]. On these bases, we ascribe the abrupt spectral
change at ~8% RH in Figure 2(a) to the phase transition
from amorphous states at ~3% RH to crystalline
Mg(NO3)2·nH2O (4<n≤6) induced by the trace of adsorbed water on the particle surface. In addition, the
v2 -NO3− band blue shifts to ~821 cm−1 at ~8% RH,
which likely implies that the Mg(NO3)2 aerosols are
completely transformed into crystals since no other
abrupt changes are observed before the entire deliquescence of the particles. This assumption is further supported by the first transition point (RH = 8%) in the hygroscopic curve. At the same time, the weak shoulder at
1059 cm−1 in the v1 -NO3− region also indicates the formation of crystalline Mg(NO3)2·6H2O according to
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
Chang et al. [22]. With further increasing of RH, the
Mg(NO3)2·nH2O (n ≤ 5) crystals keep on absorbing
water and changing into the thermodynamically stable
crystalline Mg(NO3)2·6H2O due to the continuous transition of the adsorbed water into coordinated water in
the lattice[21,23]. As a result, the Mg(NO3)2·6H2O crystals on the surface increase accompanied by the gradual
decrease of the internal Mg(NO3)2·nH2O (n ≤ 5) crystals. The shoulder at ~1059 cm−1 in the v1 -NO3− region
obviously becomes stronger when RH arrives at ~23%.
Upon the further increase of RH, the hygroscopic curve
becomes smoother. These suggest that the particle is
fully covered with a layer of crystalline Mg(NO3)2·6H2O,
which reduces the particle’s ability to absorb water.
When RH arrives at ~52%, near the saturation point of
Mg(NO3)2·6H2O, the shoulder at ~830 cm−1 clearly
strengthens, which indicates that the crystalline
Mg(NO3)2·6H2O starts to be partly dissolved to form a
liquid film. Similarly, Al-Abadleh et al.[21] have observed that the Mg(NO3)2 thin film deliquesced at
49%―55% RH. As RH further increases, the inner
Mg(NO3)2·nH2O (n≤5) crystals continuously absorb
water from the surface liquid film to immediately dissolve or to dissolve after changing into Mg(NO3)2·6H2O
crystals. The dissolution process enhances the hygroscopicity of the particles. At RH > 76%, Mg(NO3)2 particles completely deliquesce to solution droplets which
are continuously diluted with RH increasing, resulting in
the FTIR-ATR spectra being quite similar to those from
bulk solution[20]. It should be pointed out that the infrared absorbance of Mg(NO3)2·6H2O crystal at 819 cm−1
is far stronger than that of Mg(NO3)2·4H2O crystal at
815 cm−1 in the v2 -NO3− region[22]. However, the former at 1059 cm−1 is far weaker than the latter at 1045
cm−1 in the v1 -NO3− region[22]. Thus, it is reasonable
that the v2 -NO3− band (~821 cm−1) is close to the absorbance of Mg(NO3)2·6H2O crystal, while the main
peak of the v1 -NO3− band (~1050 cm−1) is close to the
absorbance of Mg(NO3)2·4H2O crystal at RH of 8%―
67%. In addition, the discrepancy between two hygroscopic curves in the region of 23%―76% should be attributed to the difference in the particle sizes in two experiments.
In the efflorescence processes of Mg(NO3)2 aerosol
particles, the changes of both the FTIR-ATR spectra and
the hydroscopic curves are relatively simple. At RH >
5%, the continuous spectral changes are probably only
related to the increase of the concentration due to the
evaporation. The weakening of the shoulder in the
v3 -NO3− region is in good agreement with the spectral
changes caused by the increased concentration of bulk
Mg(NO3)2 solution[20]. Judging from the FTIR-ATR
spectra, almost all the Mg(NO3)2 aerosol particles are
transformed into amorphous structure when RH approaches 3%. Correspondingly, no obvious transition
points are observed in the hygroscopic curve above 5%
RH, but the area ratio of the v-H2O band to the v3 -NO3−
band suddenly decreases at RH ≈ 3%. In addition, the
area ratio in the efflorescence process is larger than that
in the deliquescence process at a given RH in the region
of 23%―76% RH. It further confirms that the structures
of the particles are different in two processes, i.e., the
multi-phase mixed system in the deliquescence process
and the pure liquid state in the efflorescence process.
Figure 4 shows the schematic diagram of the phase
transitions of the Mg(NO3)2 aerosols in both deliquescence and efflorescence processes. Initially, the
Mg(NO3)2 aerosol particles are amorphous hydrates. As
RH slightly increases to ~8%, a transition from amorphous particles to Mg(NO3)2·nH2O (n≤5) crystals is
induced by adsorbed water. On the particle surface, the
thermodynamically stable crystalline Mg(NO3)2·6H2O
is even formed, resulting in a core-shell structure. The
Mg(NO3)2·6H2O crystal on the surface gradually increases accompanied by the reduction of Mg(NO3)2·
nH2O (n ≤ 5) in the inner of the particle with further
increasing RH. Furthermore, it begins to deliquesce to
form a liquid Mg(NO3)2 film when RH approaches the
saturation point (53% RH). Thus, a solid-solid-liquid
mixed system is developed. Upon the deliquescence of
on
the
surface,
the
the
Mg(NO3)2·6H2O
Mg(NO3)2·nH2O (n ≤ 5) in the inner are continuously transformed into Mg(NO3)2·6H2O and are dissolved. When RH increases to 76%, the aerosol particles
completely deliquesce to solution droplets. In the efflorescence process, there are no obvious changes above
5% RH. Finally, the Mg(NO3)2 droplets completely turn
into amorphous hydrates at ~3% RH, similar to the
components of the initial Mg(NO3)2 aerosols.
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
133
Figure 4 Schematic diagram of phase transition of Mg(NO3)2 aerosols in the efflorescence and deliquescence processes. Dash and solid
lines denote the efflorescence and deliquescence processes, respectively.
2.3 The IR activity of the v1 - NO 3− band and the
formation of CIPs
As shown in Figure 5(a), free NO3− has a planar structure with D3h symmetry, in which three N—O bond
lengths are equal and all three O—N—O bond angles
NO3−
are 120°. Therefore, free
has six normal vibra-
tional modes and four of them are pair-wise degenerated,
i.e., Γvib = A1′ + A2′′ + 2E′. The A1′ (v1, ~1045 cm−1)
and A2′′ (v2, ~830 cm−1) modes are the symmetric
stretching and out-of-plane deformation vibrations, respectively. Two E′ modes are the anti-symmetric
stretching (v3, ~1370 cm−1) and in-plane bending (v4,
~723 cm−1) vibrations, respectively. The v3 and v4 modes
are both Raman and IR active, whereas the v1 mode is
only Raman active and the v2 mode is only IR active. In
solutions, solvent molecules and cations surrounding
NO3− create asymmetric microenvironments which reduce the symmetry of NO3− . This makes the v1 -NO3−
mode IR active[20,24]. Since the magnitude of the infrared
absorbance of the
v1 -NO3−
vibration, i.e., the band in-
tensity, is dependent on the configuration and microenvironment of NO3− , it can reflect the combining modes
and ionic interactions to a certain extent. Hence, the
evolution of the ionic and molecular interactions in the
supersaturated Mg(NO3)2 droplets can be analyzed according to the intensity change of the v1 -NO3− band in
the efflorescence process.
134
Figure 6 displays the relative areas of the v1- NO3−
bands at various molar water-to-solute ratios (WSRs) in
the efflorescence processes. In order to connect two sets
of data, the spectra at the same WSR (14.1) obtained in
two experiments are selected as standards. The relative
area of the v1 -NO3− band at a given WSR refers to the
ratio of the area of the v1 -NO3− band to that at WSR =
14.1 in the same experiment. Two sets of data are in
good agreement with each other at all WSRs. According
to the component of the droplet, the efflorescence process can be divided into three regions, i.e., (I) WSR > 18,
(II) WSR = 18―6, and (III) WSR < 6. The relative area
of the v1- NO3− band slightly increases in region (I) but
rapidly increases in regions (II) and (III) with decreasing
WSR. For example, the relative areas are 0.66, 0.86,
1.64, and 2.1 at WSR = 44.0, 18.0, 6.0, and 4.9, respectively. For amorphous Mg(NO3)2 particles, the relative
area promptly increases above 3.0. It has been well established that there are 6 and 12 water molecules in the first
and second hydration layers of Mg2+, respectively[25]. At
WSR > 18, NO3− is hardly affected by Mg2+, and the
lowering of the NO3− symmetry is mainly caused by the
changes of the solvent water’s dipole moment[26]. Therefore, the infrared absorbance of the v1- NO3− mode is
very weak and is not sensitive to the change of WSR. In
the region of WSR = 18―6, however, Mg2+ and NO3−
form SIP since Mg2+ cannot retain intact second hydration shell. In SIPs, Mg2+ strongly destroys the mi-
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
Figure 5 The optimized configurations of NO3− obtained through quantum calculation. The corresponding peak position and infrared
intensity of the v1- NO3− vibration is calculated and displayed at the bottom of each configuration.
Mg2+ to form CIP, leading to an increase of IR activity
of the v1 -NO3− vibration. In Figure 5, the intensity
changes of the v1 -NO3− band with WSR further confirm
the investigations on the hydration numbers of Mg2+[25].
It was reported that the clusters of [NO3·H2O]− and
[NO3·(H2O)2]− do capture many of the features of the
experimental spectra although they were very simple[24].
With this consideration in mind, these two kinds of
clusters were calculated using the RHF/6-311++G**
basis set in the Gaussian 98 program[27] to obtain the
information on the symmetry of NO3− and the infrared
absorbance intensity of the v1 -NO3− band. The optimized configurations are shown in Figures 5(b)―(f).
The NO3− ions are planar in all of the configurations.
In two configurations, i.e., bidentate [NO3·H2O]− and
Figure 6
The relative areas of the v1 -NO3− bands as a function of WSR
bidentate-bidentate [NO3·(H2O)2]−, NO3− ions belong
in the efflorescence process.
to C2v point group, having two N—O bonds with equal
croenvironmental symmetry of NO3− through enhanc-
length. While the NO3− ions in other configurations
ing the polarity of the shared water molecules, which
have Cs symmetry. In addition, two typical structures of
SIP (Figure 5(g), [Mg(H2O)6NO3]+) and CIP (Figure
5(h), [Mg(H2O)5NO3]+) were calculated. In SIP, water
molecules in the first hydration shell of Mg2+ are severely polarized and form strong hydrogen bonds with O
increases the IR activity of the
v1 -NO3−
vibration. At
WSR < 6, Mg2+ directly affects the symmetry of NO3−
because NO3− enters into the first hydration shell of
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
135
in NO3− . As a result, the two N—O bond lengths near
Mg2+ (0.1263 and 0.1234 nm, respectively) are both larger than the rest (0.1170 nm). In CIP, Mg2+ interacts
directly with NO3− , leading the N—O bond near Mg2+
to increase to 0.1528 nm while the other two N—O bond
lengths are 0.1176 and 0.1228 nm, respectively. Besides,
NO3− ions in both SIP and CIP are no longer planar
structures and decrease their symmetry to C1. The calculation reveals that all the
−
v1 -NO3−
bands for
−
[NO3·H2O] and [NO3·(H2O)2] are fairly weak and the
largest intensity is only 11.1. For SIP and CIP, however,
the intensities of the v1 -NO3− bands reach 142.6 and
118.1, respectively. Obviously, the calculated results are
in good accordance with experimental results as shown
in Figure 6.
3 Conclusions
In this work, the phase transitions of Mg(NO3)2 aerosols
in the efflorescence and deliquescence processes were
observed using the in situ FTIR-ATR technique on the
molecular level. It was revealed that the phase transitions of Mg(NO3)2 particles are very complicated in the
1
2
Dentener F J, Carmichael G R, Zhang Y, Lelieveld J, Crutzen P J.
Role of mineral aerosol as a reactive surface in the global troposphere.
4
of Mg(NO3)2·6H2O are formed on the particle surfaces,
resulting in solid-solid mixed particles. When RH approaches the saturation point (53% RH) of
Mg(NO3)2·6H2O, the surfaces of the particles begin to
deliquesce, leading to liquid-solid-solid multiphase
mixed systems. Further increasing RH to 76%, the particles completely deliquesce to liquid phase. The discovery of core-shell structure and the gradual crystalline
transition and dissolution from outer to inner may help
to understand the heterogeneous gas-solid and gas-liquid
chemical reactions on the aerosol surface. The study of
efflorescence process revealed that Mg(NO3)2 droplets
gradually lose water to become supersaturated but are
not easy to effloresce upon reduction of RH. This may
be related to the difficulty in water evaporating due to
the formation of SIPs and even CIPs between Mg2+ and
NO3-. The calculation further confirmed that the formation of SIPs and CIPs can effectively increase the intensity of the v1 -NO3− band, showing a good accordance
with experiments.
10
J Geophys Res, 1996, 101: 22869―22889[DOI]
Tegen I, Fung I. Modeling of mineral dust in the atmosphere-sources,
transport, and optical-thickness. J Geophys Res, 1994, 99:
22897―22914[DOI]
3
deliquescence process. They are rapidly converted from
amorphous particles at extremely low RH into
Mg(NO3)2·nH2O (4<n≤6) crystals, and then thin films
sea salt particles and their components. J Phys Chem A, 2000, 104:
11463―11477[DOI]
11
Langer S, Pemberton R S, Finlayson-Pitts B J. Diffuse reflectance
infrared studies of the reaction of synthetic sea salt mixtures with NO2:
Ullerstam M, Johnson M S, Vogt R, Ljungström E. DRIFTS and
Knudsen cell study of the heterogeneous reactivity of SO2 and NO2 on
mineral dust. Atmos Chem Phys, 2003, 3: 2043―2051
Metzger S, Mihalopoulos N, Lelieveld J. Importance of mineral
cations and organics in gas-aerosol partitioning of reactive nitrogen
compounds: case study based on MINOS results. Atmos Chem Phys,
2006, 6: 2549―2567[DOI]
modulated by alkaline aerosol. J Atmos Chem, 2001, 40: 1―22[DOI]
Finlayson-Pitts B J, Hemminger J C. Physical chemistry of airborne
a key role for hydrates in the kinetics and mechanism. J Phys Chem A,
1997, 101: 1277―1286[DOI]
12
Martin S T. Phase transitions of aqueous atmospheric particles. Chem
Rev, 2000, 100: 3403―3453[DOI]
13
Ha Z Y, Chan C K. The water activities of MgCl2, Mg(NO3)2, MgSO4,
14
and their mixtures. Aerosol Sci Technol, 1999, 31: 154―169[DOI]
Zhang Y H, Choi M Y, Chan C K. Relating hygroscopic properties of
magnesium nitrate to the formation of contact ion pairs. J Phys Chem
A, 2004, 108: 1712―1718[DOI]
5
Blanchard D C. The oceanic production of atmospheric sea salt. J
Geophys Res, 1985, 90: 961―963
6
Tolocka M P, Saul T D, Johnston M V. Reactive uptake of nitric acid
into aqueous sodium chloride droplets using real-time single-particle
mass spectrometry. J Phys Chem A, 2004, 108: 2659―2665
15
Gibson E R, Hudson P K, Grassian V H. Physicochemical properties
of nitrate aerosols: implications for the atmosphere. J Phys Chem A,
2006, 110: 11785―11799[DOI]
7
Finlayson-Pitts B J. The tropospheric chemistry of sea salt: a molecular-level view of the chemistry of NaCl and NaBr. Chem Rev,
2003, 103: 4801―4822[DOI]
16
8
Usher C R, Michel A E, Grassian V H. Reactions on mineral dust.
Chem Rev, 2003, 103: 4883―4939[DOI]
Schlenker J C, Malinowski A, Martin S C, Hung H M, Rudich Y.
Crystals formed at 293 K by aqueous sulfate-nitrate-ammonium-proton aerosol particles. J Phys Chem A, 2004, 108:
9375―9383[DOI]
17
9
Song C H, Carmichael G R. Gas-particle partitioning of nitric acid
Braban C F, Carroll M F, Styler S A, Abbatt J P D. Phase transitions of
malonic and oxalic acid aerosols. J Phys Chem A, 2003, 107:
136
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
―
6594―6602[DOI]
18
19
20
Choi M Y, Chan C K. Investigation of efflorescence of inorganic
aerosols using fluorescence spectroscopy. J Phys Chem A, 2005, 109:
1042―1048[DOI]
Waterland M R, Stockwell D, Kelley A M. Symmetry breaking effects
in NO3− : Raman spectra of nitrate salts and ab initio resonance Ra-
Zhang Y H, Hu Y A, Ding F, Zhao L J. FTIR-ATR chamber for observation of efflorescence and deliquescence processes of NaClO4
man spectra of nitrate-water complexes. J Chem Phys, 2001, 114:
6249―6258[DOI]
22
23
24
aerosol particles on ZnSe substrate. Chin Sci Bull, 2005, 50(19):
2149―2152
25
Pye C C, Rudolph W W. Ab initio and Raman investigation of magnesium(II) hydration. J Phys Chem A, 1998, 102: 9933―9943[DOI]
Liu J H, Zhang Y H, Wang L Y, Wei Z F. Drawing out the structural
information of the first layer of hydrated ions: ATR-FTIR spectro-
26
Chang T G, Irish D E. Raman and infrared spectral study of magnesium nitrate-water systems. J Phys Chem, 1973, 77: 52―57
scopic studies on aqueous NH4NO3, NaNO3, and Mg(NO3)2 solutions.
27
Frisch M J, Trucks G W, Schlegel H B, Scuseria G E, Robb M A,
Cheeseman J R, Zarzewski V G, Montgomery Jr. J A, Stratman R E,
Spectrochim Acta Part A, 2005, 61: 893―899[DOI]
21
and nitric acid adsorption on MgO(100) and CaCO3(101 4). Langmuir,
2005, 21: 8793―88011[DOI]
Al-Abadleh H A, Grassian V H. Phase transitions in magnesium nitrate thin films: a transmission FT-IR study of the deliquescence and
Burant J C, Dapprich S, Millam J M, Daniels A D, Kudin K N, Strain
efflorescence of nitric acid reacted magnesium oxide interfaces. J
M C, Farkas O, Tomasi J, Barone V, Cossi M, Cammi R, Mennucci B,
Pomelli C, Adamo C, Clifford S, Ochterski J, Peterson J A, Ayala P Y,
Phys Chem B, 2003, 107: 10829―10839[DOI]
Cui Q, Morokuma K, Malick D K, Rabuck A D, Ragvachakari K,
Chang T G, Irish D E. Raman and infrared studies of hexa-, tetra-, and
dehydrates of crystalline magnesium nitrate. Can J Chem, 1973, 51:
Foresman J B, Cioslowski J, Ortiz J V, Baboul A G, Stefanov B B, Liu
G, Liashenko A, Piskorz P, Komaromi I, Gomperts R, Martin R L,
118―125
Krueger B J, Ross J L, Grassian V H. Formation of microcrystals,
Fox D J, Keith T, Al-Laham M A, Peng C Y, Nanayakkare A, Chal-
micropuddles, and other spatial inhomogenieties in surface reactions
under ambient conditions: an atomic force microscopy study of water
lacombe M, Gill P M W, Johnson B, Chen W, Wong M W, Andres J L,
Gonzales C, Head-Gordon M, Replodge R S, Pople J A. Gaussian 98,
Revision 5.4. Pittsburgh: Gaussian Inc. 1998
LI XiaoHong et al. Sci China Ser B-Chem | Feb. 2008 | vol. 51 | no. 2 | 128-137
137