Photodegradation of methyl thioglycolate particles as a proxy for organosulphur containing droplets

Article abstract of DOI:10.1039/c7cp08658j

Understanding the formation and transformation of sulphur-rich particles is of prime importance since they contribute to the global atmospheric sulphur budget. In this work, we performed a series of experiments on a photoactive organosulphur compound namely, methyl thioglycolate, as a model of an organosulphur species of marine origin. By investigating the photoproducts within levitated droplets, we showed that elemental sulphur (α-S₈) and sulphate (SO₄^2-) can be photochemically generated at the gas-liquid interface by heterogeneous interaction with gaseous O₂ and H₂O. These results demonstrate that the surface of levitated droplets facilitate the oxidation of methyl thioglycolate in the dark, while illumination is necessary to produce the oxidation in bulk experiments.

Full text of DOI:10.1039/c7cp08658j

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Picone, Y. B. Bava, L. C. Juncal, M. Moreau, R. Ciuraru, C. George, R. M. Romano, S. Sobanska and Y. A.
Tobon, Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/C7CP08658J.
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Photodegradation of Methyl Thioglycolate Particles as a Proxy for
Organosulphur Containing Droplets
Samantha Seng,a A. Lorena Picone,b Yanina B. Bava,b Luciana C. Juncal,a,b Myriam Moreau,a Raluca
Ciuraru,c‡ Christian George,c Rosana M. Romano,b Sophie Sobanska,a† and Yeny A. Tobona*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Understanding the formation and transformation of sulphur-rich particles is of prime importance since they contribute to
the global atmospheric sulphur budget. In this work, we performed a series of experiments on a photoactive
www.rsc.org/
organosulphur compound namely, methyl thioglycolate, as a model of an organosulphur species of marine origin. By
2-
)
investigating the photoproducts within levitated droplets, we showed that elemental sulphur (α-S8) and sulphate (SO4
can be photochemically generated at the gas-liquid interface by heterogeneous interaction with gaseous O2 and H2O.
These results demonstrate that the surface of levitated droplets facilitate the oxidation of methyl thioglycolate in the dark,
while illumination is necessary to produce the oxidation in bulk experiments.
dimethylsulphoniopropionate ((CH3)2S+CH2CH2C(O)O-, DMSP), which
is naturally produced by algae and aquatic plants. However, other
1. Introduction
organosulphur
compounds
such
as
methanethiol,
3-
Sulphur is an abundant element in the environment and its cycle is
one of Earth's fundamental geochemical cycles. In the atmosphere,
sulphur is present in various oxidation states, in both gas and
condensed phases. The most abundant gaseous S-containing
species is sulphur dioxide (SO2) which arises from a variety of
natural and anthropogenic sources. The oxidation of SO2 in air leads
to the formation of sulphuric acid as well as sulphate-containing
particles through a combination of gas and aqueous processes, with
some contribution from heterogeneous chemistry.1 Sulphate-
containing particles contribute significantly to the global mass
fraction of atmospheric submicrometre aerosols2 and have
recognized impacts on human health, and the climate. Although the
combustion fossil fuels account for the major source of SO2 in urban
and industrial environments,3 the oxidation of reduced sulphur
compounds (i.e., OCS, H2S, CS2 and dimethyl disulphide –DMS-)
account for much of the SO2 observed in the remote troposphere
and stratosphere.4
methylmercaptopropionate and 3-mercaptopropionate6-9 are also
produced by related degradation pathways of DMSP, and have a
sufficiently low vapour pressure to likely contribute to the global
dissolved organic sulphur (DOS) budget in oceans.10 The emission of
primary marine particles containing DOS from biogenic sources has
been largely ignored. However, a recent work has demonstrated
the presence of DOS in primary marine aerosols.11 Then, DOS
species may be emitted into the atmosphere as part of sea salt
particles, through bubble bursting and wind action. Once in the
atmosphere, organosulphur droplets are subject to atmospheric
chemical and photochemical aging processes. Although the
(photo)reactivity of sulphur compounds in the atmosphere is
complex and has been widely explored in the gaseous phase,12,13
(photo)transformation
of
particles
containing
dissolved
organosulphur compounds has been neglected.14,15 Recently,
Gaston et al.11 measured primary marine particles containing
reduced sulphur, detected as elemental sulphur ions, in seven field
campaigns conducted in various marine environment. Strong
diurnal profile of these particles was explained by the marine
biogenic activity and the rapid destruction of the sulphur-containing
compounds during the daytime due to photolysis.
DMS is of a particular interest since it is the main reduced
sulphur-rich compound produced in oceanic areas.5 The major
source
of
DMS
in
marine
environment
is
In this work, we have studied the photoreactivity of pure
and aqueous methyl thioglycolate –MTG, CH3OC(O)CH2SH–, a
photoactive organosulphur compound16 used as a proxy for
DOS. We have reported its photoreactivity, on single droplets,
using an acoustic levitation system.17 Indeed, studying the
processes in aerosols, at the single particle scale, is an added
value to better understand the physicochemical mechanisms
intervening in complex heterogeneous atmospheric
processes.18-22 Complementary experiments in bulk and at the
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air-solution interface were conducted to highlight single to the decay of Raman bands of the initial compounds and to
particle-only process. We showed that elemental sulphur and the appearance and subsequent behaviour of any new
sulphate can be formed on the droplet surface by photolysis of features. In order to compare the laser and lamp experiments,
the organosulphur compound in the presence of gaseous photon flux density at 325 nm was calculated from the power
oxygen and water. In addition, the surface of the droplet source data. In the case of the broadband lamp,
a
played an important role in the reactivity of MTG and triggered bandpass/single-band SemrockM filter centred at 325 nm
its oxidation, which alters the progress of the reaction and the (FWHM = 1.2 nm) and corrected by the transmittance of the
final products.
filter (85%) was used to obtain the photon flux density in the
325-nm region. The photon flux was 7.1 106 µmol.m-2.s-1 and
9.8 103 µmol. m-2.s-1 for the 325 nm-laser beam and the lamp
in the 325-nm region, respectively.
2. Experimental methods
2.1. Chemical reagents
2.3. Bulk experiments
Commercial samples of methyl thyoglycolate –MTG,
CH3OC(O)CH2SH– (Alfa Aesar) were used without further
purification. Ultrapure deionized water (Milli-Q™, 18 MΩ) was
used to prepare aqueous solutions of MTG (0.3M).
Pure MTG and aqueous solutions of MTG (0.3M) were placed
into a quartz reactor under constant stirring. Samples were
exposed to broadband UV-vis light from a Hg-Xe arc lamp,
Hamamatsu, LC8 Lightningcure, 200 W (300 ≤ λ ≤ 800 nm). The
output of the lamp was limited by a water filter to absorb infrared
radiation and minimize any heating effect. Chemical changes were
2.2. Single levitated particle experiments
The levitation system was already described previously.17 It
consists in an acoustic levitator (APOS BA 10, Tec5, Germany)
followed by Raman microspectrometry
.
equipped with
a homemade environmental cell coupled
horizontally to a confocal Raman microspectrometer. The cell
has four optics accesses and transparent quartz windows
allowing exposure to UV-visible light and particle analysis. Two
inlet/outlet valves for gas supplies can modify the
environment inside the cell, including the relative humidity
(RH).
2.4. Air/aqueous interface experiments
Air/aqueous interface photochemistry was performed in a
quartz cell (2 cm diameter and 5 cm length) filled with 7 mL of
mixtures (aqueous solutions of MTG, 0.3 M). The solutions
were irradiated using a Xenon lamp, operating at 150 W (300 ≤
λ ≤ 800 nm) and 400 sccm of purified air was flowing through
the quartz cell. A water filter was used in order to absorb
infrared radiation and minimize any heating effects. The gas-
phase products were investigated by proton transfer reaction
mass spectrometry using a commercial SRI-PTR-ToF-MS 8000
(Selective Reagent Ionization Proton Transfer Reaction Time of
Flight Mass Spectrometer) instrument from Ionic on Analytik
GmbH (Innsbruck, Austria). The PTR-MS sampled continuously
50 sccm through 1.5 m of 6 mm (i.d.) polyether ether ketone
(PEEK) tubing. Measurements were performed in H3O+
ionization mode at a drift voltage of 600 V, drift temperature
of 60°C and a drift pressure of 2.25 mbar resulting in an E/N of
130 Td (1Td=10-17cm2V-1), the spectra being collected at a time
resolution of 2 min. The compounds concentrations have been
calculated according to Cappellin et al.23and the same value of
k = 2×10−9 cm3s-1was used for all masses.
Raman microspectrometry measurements of the levitated
particles were performed with
a
LabRAM confocal
spectrometer (Horiba Scientific, S.A). Spectra were recorded at
a resolution of 4 cm-1. The instrument was equipped with an
Olympus BX40 microscope using
a Nikon x50 objective
(NA = 0.45; WD= 13.8 mm), and a He-Ne laser (λ= 632.8nm).
The laser power on the droplet surface was limited to 0.6 mW
(10% of full laser power). The lateral resolution was estimated
~ 2 μm and the depth of the laser focus to 16 μm with a ꢀz
limit ≥ ± 3 μm. Thus, considering the volume probed by the
laser beam, the analysed surface corresponds to a layer of a
few microns. A high-speed high-resolution video camera CMOS
monochrome (Basler Ace NIR, 2048 x2048 pixels) was also
adapted to the Raman microscope allowing the visualisation of
morphological changes on the surface of the particle before,
during and after the irradiation of the particle with
resolution time of millisecond.
a
Microdroplets ranging from 35 to 80 µm were trapped by
direct injection of a drop into the acoustic cavity under
3. Results and discussion
environmental conditions (20°C, 40-50 %RH). After trapping 3.1. Single levitated particle experiments
and Raman analysis of the single levitated particle, droplets
Pure MTG. Levitated droplets of pure MTG, of around 80 µm of
were exposed to a broadband UV-vis light (300 ≤ λ ≤ 800 nm)
from a Hg-Xe arc lamp, Hamamatsu, LC8 Lightningcure, 200 W
(power on the droplet = 3.9 10-3 mW.µm-2 ) equipped with a
water filter to absorb infrared radiation and minimize any
heating effect, or to a monochromatic radiation of 325 nm
from a CW He-Cd laser (power on the droplet = 2.6 10-3
mW.µm-2). Raman spectra of the levitated particle were
recorded at different time of irradiation, with close attention
diameter, were obtained by direct injection into the acoustic
cavity. Raman spectra of the levitated droplets, collected
immediately after injection, mainly displayed the Raman bands
characteristic of liquid MTG24,25 and a very small peak around
510 cm-1 attributed to the S-S stretching mode of the
disulphide
derivative
DMTG−)
[CH3OC(O)CH2S]2
(Dimethyl
26
dithiodiglycolate
−
that was present in the bulk
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sample as an impurity since we did not purify the sample irradiation time (A) and the integrated area before irradiation (A0).
before experiments. As shown in the preliminary results,17 in Considering the signal-to-noise ratio, using this method, we
Figure 1, before irradiation, oxidation of MTG to its disulphide estimate an error of 10%. Consequently, by plotting the ln(A/A0) as
derivative was evidenced by the growth of the S-S stretching a function of the irradiation time, we have determined that
mode around 510 cm-1, accompanied by the disappearance of disulphide photodegradation corresponds to a first order kinetics
the band near to 2568 cm-1, belonging to the S-H stretching with a reaction rate coefficient (k1) near to 4.0 10-2 min-1 and a
mode of the MTG. The chemical transformation occurred
simultaneously with the decrease in the size of the droplet
from 70 µm to 38 µm, revealed by the optical images (See
lifetime (τ) of 25.0 min (See Figure S2 of the Supporting
Information).
ν
(S-S)
Figure S1 of the Supporting Information). Some evaporation of
510 cm-1
MTG from the droplet surely caused the size reduction of the
particle diameter (MTG vapour pressure = 10 mbar at 25 °C). As
evidenced by Raman spectroscopy, oxidation of MTG into
DMTG occurred within a few minutes after exposure to
ambient O2, and was assumed to be complete after 16 hours
(Equation 1). Indeed, after 16 hours, no MTG was detected in
the core of the droplet.
ν
(S-H)
2568 cm-1
t
air=16 h
tair=10 h
air=0 h
t
4 CH3OC(O)CH2SH + O2→ 2 [CH3OC(O)CH2S]2 + 2 H2O
(Eq.1)
The relative humidity inside the cell was observed to increase
from 45 to 55 %RH mainly due to external RH variation during the
experiment. Although some water is produced from the MTG
oxidation reaction as showed in Equation 1, the very small quantity
500
1000
1500
2000
2500
3000
W a v e n u m b e r s / c m-1
Fig. 1. Raman spectra of a levitated droplet initially containing methyl thioglycolate in
the region of 200-3200 cm-1 before irradiation, immediately after injection (0h), and
of water generated from the microdroplet would increase by less after 10 and 16 hours exposed to air.
than 0.1% the %RH inside the 20-ml environmental cell.
153 cm-1
Thus, single particle irradiation experiments were performed
after the complete oxidation of MTG into DMTG. In a first
experiment, the droplet was irradiated with broadband UV-vis light
(300 ≤ λ ≤ 800 nm) from a Hg-Xe lamp . A second experiment was
performed irradiating a droplet with a 325-nm monochromatic light
emitted by a CW He-Cd laser was carried out. Changes in the Raman
spectra were recorded as a function of irradiation time. The decay
of the DMTG bands was accompanied by the appearance and the
growth of new bands in the two experiments.
468 cm-1
475 cm-1
S8
S8
219 cm-1
S8
S8
thν = 90 min
thν = 30 min
thν = 10 min
thν = 0 min
Figure 2 shows new bands at 153, 219, 468 and 475 cm-1 that
appeared after 10 minutes of irradiation with the broadband UV-
light. These bands are unambiguously attributed to the vibrational
fundamental modes of elemental sulphur (α-S8) by comparison with
the well-known reported Raman spectra27,28. Only the bands of
elemental sulphur dominated the Raman spectrum with almost
total extinction of the disulphide bands after 90 minutes of
photoprocess accompanied with a size reduction of the particle of
about 22%. For longer exposure time, we did not observe any
further changes in the Raman spectrum. However, when the laser
beam was focalised deeper in the particle core, in the Z direction,
some disulphide bands still remained in the spectra. Therefore, we
hypothesized that phototransformation likely occurred on the
droplet surface and elemental sulphur passivated the surface,
preventing further any photooxidation.
200
400
600
800
1000
1200
W a v e n u m b e r s / c m-1
Fig 2. Raman spectra of a levitated droplet initially containing dimethyl
dithioglycolate (Disulphide) in the region of 120-1200 cm-1before and after 10,
30 and 90 minutes of irradiation with broadband UV-Vis light (300-800 nm)
In this experiment, the surface of the particle was mostly
homogeneously irradiated and elemental sulphur was found
regularly distributed on the particle surface. Evidently, other
products were formed during the photoprocess but they have not
been detected in the particle by Raman microspectroscopy. They
were probably emitted into the gas phase.
Alternatively, as it is reported in our previous work,17when a
particle is irradiated with
a 325-nm laser, a very quick
The photoreaction rate coefficient of DMTG was deduced from
the time resolved Raman spectra, by measuring the integrated area
of the S-S stretching mode. Normalisation was carried out by
performing the ratio between integrated area of the bands for each
transformation is observed on the Raman spectra, with the
2
simultaneous appearance of bands attributed to sulphate (SO4
centred at 618 and 976 cm-1,29 and bands attributed to elemental
)
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sulphur (219, 468 and 475 cm-1). DMTG photodegradation seemed been reported in anaerobic conditions and its production is
to be faster using laser irradiation than in the broadband promoted by microorganisms in natural environments.32 The
experiments, with an uncorrected k2’ at 0.28 min-1 that belongs to a present work shows that elemental sulphur can be formed by
lifetime (
and local photon flux density from a unique wavenumber than presence of oxygen with a general equation (2).
broadband UV-lamp, which produced local transformation on the We hypothesise that the disulphide derivative was broken down
τ’) of 3.6 min. The 325-nm laser provided higher energy photolysis of organic sulphur compound at λ> 300 nm, in the
droplet surface. As the laser photon flux density was higher than to yield elemental sulphur and secondary volatile species that were
that obtained with the lamp (in the 325-nm region), the laser quickly released into gas phase. Identification of volatile
reaction rate coefficient (k2’) was corrected by a factor calculated photoproducts would require further experimental investigations
from the laser/lamp photon flux ratio to get a corrected k2 of 3.9 10- for analysis of gaseous phase in single droplet environment.
4
min-1
(τ = 2564 min) (Figure S2). The large reaction rate constant
difference between both experiments (k1 ≅ 100 k2) suggested that
the reaction was not only due to the 325-nm light absorption and
that broadband light irradiation also played an important role in the
photodegradation of the disulphide derivative.
By recording the Raman spectra on various points on the
surface, perpendicularly to the laser beam irradiation, we probed
sulphate ions and elemental sulphur products distribution on the
particle surface (Figure 3a). The SO42-/S8 ratios were roughly
estimated for each focal point by measuring the integrated area of
the 976 and 219 cm-1 Raman signatures of SO42- and S8 respectively,
and reported on Figure 3b. Contrary to the expected Gaussian
distribution of the products around the point B (initial 325-nm laser
focalisation), sulphur and sulphate species were heterogeneously
and randomly distributed within the particle. Under photolysis with
the 325-nm laser, particle was not completely immobilized do to
the radial forces induced probably by the high photon flux density
of the laser beam which produce a rapid photodegradation of the
particle. The ꢀx and ꢀy are estimated to be ∼20 µm from the high-
speed video camera. The instability of the droplet under the laser
beam at 325-nm induced some inhomogeneous irradiation of the
droplet which would explained the random chemical heterogeneity.
On contrary, the particle is stable under the Raman probe laser (633
nm) which does not produce photodegradation and enables
spatially-resolved Raman analysis. Additionally, we cannot ensure
that the Z-focalisation is on the same position from one point to
another. Thus, the volume probed can vary and the SO42-/S8 ratio
may change. Finally, optical images revealed the size and the
appearance of the particle as a function of irradiation time. The
particle, initially transparent, turned opaque during the photolysis,
and its size was reduced of about 20%.
Fig.
3 Raman spectra of a levitated droplet initially containing dimethyl
dithiodiglycolate (Disulphide) recorded at different location on the irradiated surface
droplet. (a) before and after 4 minutes of irradiation with a 325-nm laser. Letters refer
to the position of focal points. (b) Focal points of the Raman excitation beam on the
particle surface and SO42-/S8 Raman band integrated intensity ratio on each focal
point.
4 CH3OC(O)CH2SSCH2C(O)OCH3 + hν→ S8 + Volatile species (Eq. 2)
In the atmosphere, sulphate is mainly formed from the
oxidation of SO2 by complex homogeneous and heterogeneous
chemical processes.33-35 Nonetheless, the importance of
photochemical reaction, in the SO2-to-SO42- conversion, has already
been pointed out by Cheng et al.36 when the conversion rates of
SO2 to particulate sulphate was studied.
As summarised, elemental sulphur was formed in both
irradiation experiments and no additional product was detected in
condensed phase when
a single droplet was irradiated. The
formation of sulphate ions occurred only when irradiating with a
325-nm laser beam i.e. when high energy was provided to the
particle.
Alternatively, aqueous oxidation of natural and synthetic
sulphides have been investigated due to the industrial
application.37,38 Sulphate may be produced through the general
equation 3. This reaction is greatly dependent on temperature and
oxygen partial pressure.
The formation mechanism of elemental sulphur within aerosols
is still not well described.11 Photochemical generation of S8 has
been previously attributed to the photolysis of sulphur gases (SO2,
SO, H2S) with UV light (λ
< 220 nm) in anoxic environments.30-31
S + 3/2 O2 +H2O → H2SO4
(Eq. 3)
Recently, we demonstrated that the UV–vis broadband photolysis
of MTG in gas phase leads to CH3OC(O)CH3 and S8 in absence of
O2.16 In the condensed phase, the formation of reduced sulphur has
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In our experiments, the formation of sulphate occurred mainly
through photolysis since long-term exposure of S8-containing
droplet to the humid atmosphere did not produce any sulphate
ions. Thus, we propose that sulphate was formed through the
photoexcitation (with the 325-nm laser) of sulphur to S(1D) state
(Equation 4) followed by a quick oxidation reaction series starting
from the reaction of S(1D) with oxygen to form SO2 (Equation 5).39
We hypothesize that all the reactions occur simultaneously on the
surface of the droplet, at the gas-liquid interface, within the laser
beam area penetration. The photochemical excitation of SO2, in the
300 to 400 nm range, has been previously reported,40 and is known
to produce the singlet and triplet states of SO2 (1SO2, 3SO2)
(Equation 6). It was also demonstrated that these electronic excited
states reacted in the presence of O2 and light to form SO3, which in
term formed H2SO4 in presence of water,41,42 Billing et al.43
suggested the formation of O3 from the reaction of SO2 with O2, but
it was not reported in other works. 44 Thus, in our experiments, SO2,
promoted by the 325-nm laser beam, is photooxidized in presence
of O2 to form SO3 (Equation 7) and finally SO3 reacts with water
present in ambient atmosphere of the cell, to form sulphate,
probably mediated by H2SO4 (Equation 8).
decreased by 22% as observed previously and the particle
turned opaque. A long exposure to UV-Vis light up to 90
minutes produced strong fluorescence background in the
Raman spectra that prevents the detection of products.
The aqueous-DMTG and pure DMTG photodegradation
processes appeared to be similar in primary irradiation time
with the formation of S8 in condensed phase. However, long
exposure time to the UV-light lead to the formation of
additional secondary species that have induced florescence
effect hindering the secondary products detection.
3.2. Bulk experiments
Pure MTG. 1 mL of pure MTG, placed into a closed quartz cell,
was first saturated with O2 by bubbling it into the sample for
4.5 hours. Raman spectra of the sample did not show any
change during exposition to O2 in bulk. A mixture of O2/H2O
was also bubbled for 2 hours without evidence of disulphide
derivative formation. Then, the sample was illuminated with a
broadband UV-Vis light (300 ≤ λ ≤ 800 nm) for 48 h under
constant stirring and under O2 atmosphere. DMTG was formed
from the first minutes of irradiation, identified by the S-S
stretching mode near to 510 cm-1. After two hours of the
irradiation processes, the band attributed to elemental sulphur
(S8) appeared on the spectra accompanied by two new bands
centred at 642 and 847 cm-1. These two signatures were
identified respectively as the OCO bending mode and the CC
stretching mode of methyl acetate, MA, (CH3OC(O)CH3).45 The
other characteristic bands of MA were overlapped by MTG
bands (FigureS3 of the Supporting Information).
1/8 S8 + hν→ S(1D)
(Eq. 4)
(Eq. 5)
(Eq. 6)
(Eq. 7)
(Eq. 8)
S(1D) + O2→SO2
*
SO2 + hν→SO2
*
SO2 + O2 →SO3 + O
SO3 + H2O → SO42- + 2 H+
We also performed the same experiments without
bubbling O2 into the sample. The three products namely
DMTG, MA and S8 were formed from the first minutes of
irradiation. Additionally, formation of DMTG and MA were
more than four times slower than experiments in presence of
O2 (see Figure S4 and Table S1 in the Supporting information
section). Obviously, O2 promotes the phototransformation of
MTG into DMTG, MA and S8 in bulk conditions (Table 1).
Differences between the observed products in both
irradiation experiments can be explained by the higher energy
and photon density provided by the laser source in comparison
to the lamp. However, we cannot discard the possibility that
sulphate may be formed with longer broadband light
exposition under ambient atmosphere conditions.
MTG aqueous droplets. In order to evaluate the process on
water-containing droplet, a fresh 0.3 M aqueous solution of
MTG was injected in the acoustic cavity. A droplet of around
35 µm of diameter was trapped and a constant relative
humidity (RH = 80%) kept in the levitation cell during the
whole experiment time. Before irradiation, Raman spectra
showed only the presence of DMTG. Disulphide was
subsequently formed in a very short time in levitation
compared to the pure MTG experiment.
The normalised Raman intensities of MA and DMTG were
plotted against the irradiation time on Figure 5. Unlike the
linear formation of MA with the irradiation time, DMTG
formation curve showed a multistep feature that could be
associated to different formation channels as well as a
simultaneous formation-degradation process of the disulphide.
Photodegradation of MTG corresponds to a first order kinetics.
Table 1 gives the reaction rate coefficients, the half-lives and
the lifetimes obtained in the photodegradation of MTG in bulk
conditions. It worth mentioning that the lifetime estimated
The droplet was illuminated by a Hg-Xe lamp (300 ≤ λ ≤ 800
nm) through a quartz windows. As shown in Figure 4, after 70
from the bulk experiments (
relevant.
τ > 12h) are not atmospherically
minutes of irradiation, only elemental sulphur (α-S8) was
evidenced in the Raman spectra As observed previously, S8
.
Accordingly, elemental sulphur and methyl acetate were
directly formed by desulphurization of MTG (Equation 9).
Concurrently, formation of disulphide in this experiment
seemed to be stimulated by light, contrary to the levitation
experiments where it is spontaneously formed by exposition to
ambient air in dark conditions. Thus, the surface of the droplet
could have played an important role in the reactivity of MTG,
seemed to be preferentially distributed on the particle surface.
This result agreed with the hygrophobic properties of S8 that
resulted in demixing phases. Sulphate ions were not observed
during these experiments. It appeared that water slowed
down the photodegradation of DMTG (reaction rate coefficient
k3 near to 3.6 10-3 min-1and and
τ= 277.7 min) but did not
prevent elemental sulphur formation. The size of the particle
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promoting its oxidation and formation of the disulphide MTG aqueous solution. A 0.3 M solution of MTG, placed into a
derivative. Controversially, methyl acetate was not detected closed quartz cell, was illuminated with a broadband UV-Vis
into the droplet while this was the main product detected into light by a Hg-Xe lamp (300 ≤ λ ≤ 800 nm) for 32 h. Raman
the bulk. We assumed that MA is preferentially formed from signatures of MTG were observed to disappear while new
MTG and not from DMTG that is the starting species in bands were growing with irradiation time. The same products
levitation.
as the previous pure MTG experiments i.e. DMTG, MA, and S8
Transformation of MTG into DMTG could proceed via two (see Figure S5 of the Supporting Information) were observed.
channels: one in presence of oxygen and stimulated by light The photodegradation was promoted by water (Table 1).
(Equation 10), and the other without participation of O2 Indeed, the formation of methyl acetate is twice faster in
(Equation 11). Our results did not distinguish between the two aqueous medium than pure MTG in presence of O2 and nine
channels.
times faster than in absence of O2 (see Table S1 in the
Supporting information section). DMTG and partially MA were
photodegraded after 32 h of irradiation. Only S8 and some MA
dominated the Raman spectrum after 32h of irradiation.
Table 1. Reaction rate coefficients (k), half-lives (t1/2) and lifetimes (τ) in the
photodegradation of MTG with broadband UV-Vis light (300-800 nm) in bulk.
Bulk experiment
MTG/H2O solution
Pure MTG + O2
Pure MTG
k (h-1)
t1/2 (h)
10.0
τ (h)
14.5
20.2
29.4
S8
475 cm-1
6.92 10-2
4.95 10-2
3.40 10-2
S8
219 cm-1
14.0
20.4
Accordingly, in bulk conditions, MTG was not oxidised in
the dark even when exposed to oxygen. However, exposure to
the light produced the disulphide derivative, methyl acetate
and S8. Water and oxygen promoted the photodegradation
process and increased the yields of the products. Methyl
acetate was especially favoured in aqueous solution, while
disulphide formation was increased with the presence of
oxygen.
thν = 70 min
thν = 0 min
200
400
600
800
1000
1200
W a v e n u m b e r s / cm-1
Fig. 4 Raman spectra of a levitated droplet initially containing aqueous solution of
dimethyl dithiodiglycolate (disulphide) in the region of 200-1200 cm-1 before and after
70 minutes of irradiation with broadband UV-Vis light (300-800 nm)
3.3. Air/aqueous interface experiments
Finally, complementary air aqueous interface experiments
were conducted and these results are presented in the
Supporting Information section.
0.06
Disulphide
The formation of MA in the gas phase from the interface
was ascertained confirming the bulk experiments. Additional,
product Methanethioic acid (CH2OS) was also detected and
would be the only secondary product originated from the
interface irradiation since it was not detected neither in gas
phase nor in condensed phase.
0.04
0.02
Methyl acetate
4. Conclusions
0.00
Photochemistry of pure and aqueous solutions of MTG was
carried out by means of complementary approaches: on
acoustically levitated single droplets, in bulk and air-aqueous
0
4
8
12
16
20
I r r a d i a t i o n t i m e / h
Fig. 5 Plots as a function of irradiation time of the Raman intensities of the bands interface conditions. Reactivity of MTG was significantly
assigned to dimethyl dithiodiglycolate (disulphide) and methyl acetate produced after
different in all experiments. In levitation, droplet surface
irradiation of methyl thioglycolate in bulk in absence of O2.
showed to play an important role in the activation of the
oxidation reaction of MTG to form the disulphide derivative –
CH3OC(O)CH2SH + hν→ CH3OC(O)CH3
+
S•
(Eq. 9)
DMTG–, which is produced without irradiation. This dark
oxidation is promoted in water solution. Generally, reactions
occurring on levitated droplets were faster than in bulk
experiments. In bulk conditions, oxidation of MTG was not
4 CH3OC(O)CH2SH + O2 + hν→ 2 [CH3OC(O)CH2S]2 + 2 H2O (Eq.10)
2 CH3OC(O)CH2SH + hν→ [CH3OC(O)CH2S]2 + H2
(Eq. 11)
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spontaneous and stimulation by light was needed. Hence, This work was supported by funds from the "Laboratoire
photochemistry on levitated droplets was triggered by DMTG, d'Excellence, Labex CaPPA “Chemical & Physical Properties of
whereas bulk photochemical experiments were initiated from the Atmosphere” (WP-2), IRENI program, ECOS-MinCyT
MTG, which produced an additional species, the methyl (N°A13E05) and the CPER research project CLIMIBIO. The
acetate that was not observed in the levitated droplets.
CaPPA project (Chemical and Physical Properties of the
Irradiation of levitated single droplets containing DMTG Atmosphere) is funded by the French National Research
produced mainly S8 distributed over the droplet surface when Agency (ANR) through the PIA (Programme d'Investissement
irradiating with a broadband UV-Vis light. Formation of S8 and d'Avenir) under contract "ANR-11-LABX-0005-01”
2-
SO4 distributed in the droplet occurred with 325-nm
monochromatic irradiation. Additional energy is thus required Conflicts of interest
for the formation of sulphate. We propose that sulphate is There are no conflicts of interest to declare
formed at the surface of the droplet, as a consequence of S(1D)
excited state atom generation, through photoexcitation of S8
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Photochemical generation of elemental sulphur and
sulphate at the gas-liquid interface by heterogeneous
interaction with gaseous O2 and H2O.