Accepted Manuscript
Synthesis of sulfur nanoparticles in aqueous surfactant solutions
Rajib Ghosh Chaudhuri, Santanu Paria
PII: S0021-9797(09)01539-2
DOI: 10.1016/j.jcis.2009.12.004
Reference: YJCIS 15460
To appear in: Journal of Colloid and Interface Science
Received Date: 30 September 2009
Accepted Date: 2 December 2009
Please cite this article as: R.G. Chaudhuri, S. Paria, Synthesis of sulfur nanoparticles in aqueous surfactant solutions,
Journal of Colloid and Interface Science (2009), doi: 10.1016/j.jcis.2009.12.004
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Synthesis of sulfur nanoparticles in aqueous surfactant solutions
Rajib Ghosh Chaudhuri and Santanu Paria* Department of Chemical Engineering, National Institute of Technology,
Rourkela769008, Orissa, India.
*Corresponding author. Email address: [email protected] or [email protected];
Fax: +91 661 246 2999
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Abstract
Sulfur is a widely used element in different applications such as fertilizers,
pharmaceuticals, rubber, fiber industries, bioleaching processes, anti microbial agents,
insecticides, and fumigants, etc. Nanosize sulfur particles are useful for pharmaceuticals,
modification of carbon nano tubes, and synthesis of nano composites for lithium
batteries. In this study we report a surfactant assisted route for the synthesis of sulfur
nanoparticles by an acid catalyzed precipitation of sodium thiosulphate. We use both the
inorganic and organic acids, and find that organic acid gives lower size sulfur particles.
The size of the particles also depends on the reactant concentration and acid to reactant
ratio. The effect of different surfactants (TX-100, CTAB, SDBS, and SDS) on particle
size shows that the surfactant can significantly reduce the particle size without changing
the shape. The size reducing ability is not same for all the surfactants, depending on the
type of surfactant. The anionic surfactant SDBS is more effective for controlling the
uniform size in both the acids media. Whereas, the lowest size (30 nm) particles were
obtained in a certain reactant concentration range using CTAB surfactant. The objective
of this study is to synthesize sulfur nanoparticles in aqueous media and also study the
effect of different surfactants on particle size.
Key words: sulfur nanoparticle, aqueous surfactant solution, interparticle exchange, film
flexibility, nucleation.
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1. Introduction
Elemental sulfur in nano, micro or bulk state is widely used for different industrial
applications such as production of sulfuric acid, nitrogenous fertilizers, phosphatic
fertilizers, plastics, enamels, antimicrobial agents, gun powders, petroleum refining, other
petrochemicals, ore leaching processes, pulp and paper industries, and in different other
agrochemical industries [1]. Nanosize sulfur particles also have many important
applications like in pharmaceuticals, synthesis of nano composites for lithium batteries
[2-5], modification of carbon nano tubes [6], synthesis of sulfur nano wires with carbon
to form hybrid materials with useful properties for gas sensor and catalytic applications
[7]. In agricultural field, sulfur is used as a fungicide against the apple scab disease under
colder conditions [8], in conventional culture of grapes, strawberry, many vegetables, and
several other crops. Sulfur is one of the oldest pesticides used in agriculture and it has a
good efficiency against a wide range of powdery mildew diseases as well as black spot.
Different methods were used for nanosize particle synthesis; among those,
microemulsion method is one of the very important methods to control the particle size.
But microemulsion itself is a very complicated system, composing of oil, surfactant, co-
surfactant and aqueous phases with the specific compositions. The main disadvantages of
microemulsion method are the difficulties in process scale up, separation and purification
of the particles from the microemulsion, and finally this method is consumed huge
amounts of surfactants.
Despite many exciting applications, there are only a few recent literatures
available on synthesis of sulfur nanoparticles by different investigators [9-12] in both
aqueous and micro emulsion phase by different routes. Deshpande et al. (2008) [9] have
synthesized sulfur nanoparticles from H2S gas by using biodegradable iron chelate
catalyst in reverse microemulsion technique. They found -sulfur or rhombic sulfur of
average particle size 10 nm with a particle size of 515 nm. They have also studied the
antimicrobial activity of sulfur nanoparticles and shows it is very much effective,
especially when the particle size is low. Guo et al. (2006) [10] have prepared sulfur
nanoparticles from sodium polysulfide by acid catalysis in reverse microemulsion
technique. They found monoclinic or -sulfur with an average particle size of around 20
nm. Xie et al. (2009) [12] have prepared nanosized sulfur particles from sublimed sulfur.
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They added aqueous cystine solution drop wise on a saturated alcoholic sulfur solution
with constant ultrasonic treatment and crystinenanosulfur sol was obtained.
Now, realizing the importance of sulfur nanoparticles in different applications, the
development of an easy synthesis method is highly essential for nanosize sulfur particles.
In the method of Deshpande et al. (2008) [9] H2S gas was used as a reactant, therefore the
arrangement of the heterogeneous phase reaction (gas-liquid) between gaseous H2S and
iron chelate solution is more complicated apart from the complicity of microemulsion.
The disadvantage of the method proposed by Guo et al. (2005, 2006) [10, 11] was the use
of polysulfide as a source of sulfur, where, micro sized sulfur particles were used as a raw
material for the synthesis of polysulfide. The method proposed by Xie et al. (2009) [12]
was also required small sized sulfur particle as a raw material for the synthesis
nanoparticles. In the present method, sulfur can be synthesized in aqueous media from
thiosulphate solution. Large amount of sulfur particles can be synthesized easily by using
a cheap reactant for agricultural and other applications where the consumption is more.
This reaction was studied by LaMer and coworkers [13-15] and proposed mechanism and
kinetics of particle formation in aqueous acidic media. This study is mainly concentrated
on effect of surfactants on particle size in aqueous media, which has not been studied
before. Moreover, from the basic understanding point of view it is also very important for
apply this route in microemulsion based synthesis.
In this study, orthorhombic or -sulfur with S8 structure have been synthesized in
an aqueous micellar solution. An attempted has been made to understand the basic
mechanism of particle formation in the presence of different inorganic and organic acids,
reactant to acid ratio, effect of reactant concentration, and the effect of different micellar
solutions (TX-100, SDBS, SDS, and CTAB). The studies on kinetics of particle
formation in different surfactant assisted media are also in progress and will be
communicated soon.
2. Materials and Method
2.1. Materials
The chemicals used for this study were taken from the following companies: Sodium
thiosulphate (Na2S2O3, 5H2O), Oxalic acid (H2C2O4, 2H2O) from Rankem (India), Triton
X-100 (TX-100) with 98% purity, Sodium dodecyl sulphate (SDS) with 99.5% purity,
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Cetyl trimethylammoniumbromide (CTAB) with 99% purity from Loba Chemie Pvt. Ltd.
(India), Sodium dodecyl benzene sulphonate from Sigma Aldrich (Germany) (Technical
grade, Cat no. 28995-7), and Hydrochloric acid (HCl), Sulfuric acid (H2SO4), Nitric acid
(HNO3) Merck (India). All the chemicals were used as those were received without any
further purification. Ultrapure water of 18.2 M.cm resistivity and pH 6.46.5 (Sartorius,
Germany) was double distilled again and used for all the experiments.
2.2. Particle Synthesis
Stock sodium thiosulphate was prepared by dissolving solid thiosulphate in double
distilled water and different acid solutions were prepared from the pure stock. Both the
reagents were filtered with 0.2 nylon 6, 6 membrane filter paper from Pall Life science,
USA. In an acidic solution, sodium thiosulphate undergoes through a disproportionation
reaction to sulfur and sulfonic acid according to
Na2S2O3 + 2HCl 2NaCl + SO2 + S + H2O
SO2 + H2O H2SO3
After mixing the reactants, 30 and 40 minutes equilibrium time was given for the
completion of reaction in the case of inorganic and organic acids respectively. After
equilibration, the sample was sonicated in a bath for 2 minutes and particle size was
measured by DLS method immediately. CMC values of CTAB (mM), SDBS (mM), and
TX-100 (mM) were taken from our previous study [16], and that of SDS (mM) was
measured by Wilhelmy plate technique with a surface tensiometer (DCAT-11EC, Data
Physics, Germany ). A constant temperature 28 + 0.5 C was maintained throughout the
experiments.
2.3. Particle Characterization
The structure of sulfur particles was characterized by X-ray diffraction (XRD) using
Philips (PW 1830 HT) X-ray diffractometer with scanning rate of 0.01/sec in the 2
range from 20 to 40. Particle size measurement was carried out by dynamic light
scattering (DLS) using Malvern Zeta Size analyzer, U.K. (Nano ZS) with the help of
cumulants fitting model and intensity distribution. The size and shape of particles were
observed under a scanning electron microscope (JEOL JSM-6480LV) and transmission
electron microscope (Tecnai S-twin).
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3. Results and Discussion
3.1. Effect of stoicheometry ratio on particle size
According to the stoicheometry of the reaction one mole of thiosulphate reacts with two
moles of mono-basic acid to precipitate one mole of sulfur. Firstly, we have studied the
effect of stoicheometry ratio of acid (H+) to thiosulphate concentration on particle size for
a particular reactant concentration (5 mM) and later we used the same ratio for that
particular acid. Fig. 1 shows that with increasing acid (H+) to thiosulphate molar ratio the
particle size increases, but the size is almost constant above the ratio of 2:1 for inorganic
mono-basic acid, 4:1 for inorganic di-basic acid, and 6:1 for organic di-basic acid. Hence,
all the experiments with inorganic mono-basic (HCl, HNO3) and di-basic (H2SO4) acids
were carried-out with the acid (H+) to thiosulphate ratio of 2:1 and 4:1 respectively.
However, in the case of organic di-basic acid (oxalic acid) we used that ratio of 6:1 for
the all experiments. As a general rule, larger the ionization constant values (Ka) stronger
the acid, to get an idea about the ionization constants of different acids we have presented
the values in Table 1. The acid requirement of H2SO4 is about twice than that of HCl,
probably due to first ionization constant value of H2SO4 is high but the second is low and
the requirement of oxalic acid is further more due to low values of both the ionization
constants.
Fig. 1 The effect of acid to thiosulphate ratio on the size of sulfur particles. Thiosulphate
concentration 5 mM.
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3.2. Effects of acid type and reactant concentration on particle size:
Fig. 2 shows the effects of acid types and thiosulphate concentration on particle size. Let
us first consider the effect of reactant concentration on particle size for a particular acid;
it is very clear from the Figure that the particle size increases with the increase in
thiosulphate concentration. The particle size is mainly influenced by two factors: (i)
nucleation and followed by (ii) particle growth and agglomeration. For this reaction
nucleation is an instantaneous process and completed within 2 minutes after mixing the
reactant with acid [21]. A nucleus is essentially a small group of atoms with a critical size
that have taken up arrangement in space, is stable and capable of further growth. Sub-
critical particles are called embryos, and of larger size are called nuclei. As well as, when
the particles are achieved a critical radius (nuclei), then the growth process predominates
over the nucleation. The rate of reaction also increases with the reactant concentration.
According to LaMer and Zaiser, (1948) [22] the rate of reaction depends on both the
concentration of thiosulphate and acid types. They proposed the rate of reaction = k [T]1.5
[A]0.5. where, k is the reaction rate constant, [T] and [A] are the thiosulphate and acid
concentrations respectively. Therefore, in higher reactant concentration media the density
of nuclei will be more, as a result, after the growth process the particle density will also
be high. When the particle density is more, the collision between the particles lead to
more agglomeration, that will increase the ultimate equilibrium particle size [22]. Furedi-
Milhofer et al. (1990) [23] have mentioned that for nucleation from a super saturated
solution growth process will start when the nuclei density is 1012 cm-3 for
heterogeneous and homogeneous nucleation respectively. Furthermore, when collision
between those new born particles increases because of higher particle density larger
particles are formed and they are stabilized by minimizing the overall energy of the
system, this is termed as Ostwald ripening or coarsening [24].
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Fig. 2 The effect of reactant concentration on the particle size in different acidic aqueous
media.
In addition, comparing the results in the presence of different acids it is clear that
the particle size also depends on the acid types. There is a distinct size difference between
the organic and inorganic acids at a higher reactant concentration; the organic acid shows
smaller size particles. The system with lower reactant concentration in the presence of
HCl shows lowest particle size among all the acids but the differences are less. Among
the inorganic acids particle size in H2SO4 is always higher than HNO3 but HCl is
showing a different behavior. At high reactant concentration HCl is showing the highest
particle size but at lower reactant concentration it is just reverse, shows lowest size. In
addition, we can clearly see from the Fig. 2 that at higher reactant concentration (10 mM)
the increasing order of particle size in the presence of different acids are: C2H2O4 <
HNO3 < H2SO4 < HCl which is exactly the same as the order of acid ionization constants,
even though, the differences between the inorganic acids are less. Because of all the
inorganic acids are strong and ionization is very fast, the reaction rate is also expected to
be fast in compare to organic acid. Apparently there is a large difference in Ka value of
HNO3 with HCl and H2SO4. Generally, acids with a pKa value of less than about 2 are
said to be strong acids and completely dissociated in aqueous solution. From the Table 1
we can see the pKa value of HNO3 is close to -2 and the literature also shows that it
almost completely (93% at 0.1 mol/L) ionizes in aqueous solution. In the presence of
inorganic acids as the reaction is expected to be very fast, formation of the nuclei also
will be very fast with more nuclei density, which leads to higher equilibrium particle size.
Whereas, oxalic acid catalyzed reaction may have slower rate of nucleation as well as
lower the particle density in compare to that of inorganic acids, which may leads to
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formation of smaller particle size. In the case of oxalic acid, another reason of lower
particle size is probably because of the adsorption of oxalate ion (C2O42-) on the particle
surface, which is clear from zeta potential value. Zeta potential of the particles in the
presence of oxalic acid is more negative (8.05 mV) than in hydrochloric acid (2.99
mV) at the same pH 2.8. The zeta potential was measured by DLS Malvern zeta size
analyzer using Smoluchowski model. The zeta potentials in the presence of all other
inorganic acids are very close. Higher zeta potential in the presence of oxalic acid may
reduce the agglomeration tendency of the particles and finally the size becomes small and
the particle size distribution is also sharp. The values of zeta potentials in the presence of
different acids are given in Table 2. From the Table 2, it is clear that the agglomeration
tendency of the particles in the presence of inorganic acids is high because zeta potential
values are low. Apart from the average particle size, the size distribution is also very
important parameter in particle synthesis. Fig. 3 shows the comparisons of particle size
distribution among four acids used in our study. Sulfuric and nitric acids have similar size
distribution, hydrochloric acid is having little sharp distribution than the other two acids
but the change is not very significant. The difference was found for the oxalic acid where
the distribution is significantly narrow.
Fig. 3 Particle size distribution in different acids media at 10 mM thiosulphate
concentration.
3.3. Particle size in the presence of surfactant solution
The effect of surfactants on sulfur particle size was tested in the presence of HCl and
oxalic acids catalyzed reactions with different reactant concentration. The surfactant
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concentration was kept three fold CMC of respective surfactants. Fig. 4 shows the effect
of surfactants on particle size by HCl catalyzed reaction. In aqueous solution the particle
size is continuously increased with the reactant concentration, and after 10 mM reactant
concentration the particles are settled down from the liquid phase because of larger size.
However, from the Fig. 4, it is clear that in the presence of surfactants there is a
significant change in particle size than that in the absence of surfactant at 10 mM
thiosulphate concentration. Furthermore, it is worthy to note that the effect is not same
for all the surfactants. In addition, lower thiosulphate concentration (0.5 mM) where the
particle size is small in aqueous media, the effect of surfactant is not very significant. At
that reactant concentration, SDBS is showing almost no changes in particle size but other
surfactants are showing little higher particle size. It is observed from the Figure that the
plot of particle size vs. reactant concentration in the presence of anionic and nonionic
surfactants pass through a maximum. The maximum particle size was observed at the
same reactant concentration (5 mM) in the presence of both anionic surfactants (SDBS
and SDS) and at higher concentration (10 mM) in the presence of nonionic surfactant
(TX-100). Comparisons of maximum particle size obtained in the presence of surfactants
show the following order: SDBS
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media the mechanism of particle formation is little bit different than that from the pure
aqueous system. In the presence of SDBS and TX-100 we can observe initially the
particle size increases with increasing the reactant concentration and after a critical
concentration again there is a decrease in particle size. The researchers have also reported
both the increasing [23, 25] and decreasing [26] trends in particle size with reactant
concentration for the separate systems. In those literatures the researchers have studied a
narrow reactant concentration range to show the increasing or decreasing trends in
particle size. A wide range of reactant concentration is studied here and both the trends
are observed in the same system. In the first regime as soon as the nuclei are formed,
surfactants are adsorbed through the tailgroup and a micelle like structure is formed with
the nuclei inside the core. Thus the hydrophobic sulfur nuclei are also stabilized in the
aqueous media. Then, similar to intermicellar exchange in reverse microemulsion [25],
due to the collision between the surfactant coated nuclei, the growth process started and
the particle size increases. Therefore, at high reactant concentration when the number of
nuclei is more, the probability of inter-particle collision is also more and that may be the
reason of increase in particle size. In the decreasing regime, above a limiting reactant
concentration, the nucleation rate is very fast, as a result the size of the nuclei also
increases rapidly. In this case inter-particle exchange is less important for the particle
growth similar to that in microemulsion [26, 27]. Since the nuclei concentration is very
high at that regime immediately after the formation, the nuclei reached a certain size as
well as stabilized by the adsorbed layer of surfactant molecules. There may be only
diffusion of small nuclei through the micelles like structure of adsorbed surfactant layer
is occur instead of exchange by collision to get final lower particle size.
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Fig. 4 Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by hydrochloric acid catalyzed reaction. Inset shows the particle
size distribution at 4 mM and 5 mM thiosulphate concentration in the presence of CTAB.
Inset of Fig. 4 clearly shows the size distribution in 5 mM reactant concentration
is very narrow whereas at 4 mM it is significantly wide with a large particle size. We
believe that the type of surfactant plays an important role for this type of behavior. The
reason of minimum particle size obtained in the presence of CTAB is not completely
understood but it may be due the change in mechanism depending on the reactant
concentration. This can be attributed as in the increasing particle size regime, the
inter-particle exchange may be the main mechanism, but after reaching a critical size
(nuclei) at a particular reactant concentration film stability of the adsorbed surfactant
layer is more important. As a result, lower particle size is obtained when inter-particle
exchange rate is low due to higher adsorbed surfactant film stability. Indeed, this may be
the similar phenomenon that the effect of surfactant film flexibility in intermicellar
exchange in reverse microemulsion system reported by Tojo and his coworkers [28-30].
The other portion of the curve in CTAB can be explained similarly that of SDBS and
TX-100.
For 5 mM thiosulphate concentration, we also studied the effect of surfactant
concentration on particle size. It shows with the increasing surfactant concentration the
particle size decreases and shows a minimum of 30 nm size particle at 1 mM (close to
CMC) surfactant concentration but after that again size of the particle increases and
becomes constant at 50-60 nm size (Fig. 8). Fig. 5 shows the particle size distribution at
10 mM thiosulphate solution in different surfactant media. From the Figure it is clear that
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the particle size distribution in the presence of SDBS is sharper than the other surfactant
solutions at a constant reactant concentration (10 mM). In addition to narrow size
distribution at a constant reactant concentration, for the total range of the reactant
concentration studied here the change in size with thiosulphate concentration is less as
the height of maxima is less in the presence of SDBS. Therefore, SDBS can be use as
better size controlling agent.
Fig. 5 Plot of particle size distribution in different media at 10 mM thiosulphate solution
catalyzed by HCl.
Fig. 6 shows the change in particle size by oxalic acid catalyzed reaction in the
presence of different surfactants. The main difference found in the presence of organic
acid is the absence of maximum in particle size with the thiosulphate concentration in
SDBS solution. Furthermore, the change in particle size with the increase in reactant
concentration is also less for SDBS similar to that in HCl catalyzed reaction. TX-100
shows higher particle size than other two ionic surfactants similar to that is observed in
the presence of HCl. The change in particle size with reactant concentration is also less in
the presence of TX-100 in contrast to that for HCl. By comparing the results in the
presence of SDBS and TX-100 we can conclude that the change in particle size with the
increase in reactant concentration is less, as well as overall lower particle size is obtained
by oxalic acid catalyzed reaction than HCl. In the presence of CTAB similar to HCl
catalyzed reaction minimum particle size was found between 4 mM to 6 mM thiosulphate
concentration range. Minimum particle size was obtained ~35 nm at 4 mM thiosulphate
concentration.
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Fig. 6 Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by oxalic acid catalyzed reaction.
The particle size distribution at 10 mM thiosulphate concentration (See Fig. 7)
also shows the distribution is narrow in oxalic acid and SDBS combination. Unlike the
aqueous media, in the presence of surfactants the particle size is not continuously
increased with the reactant concentration. In the presence of higher surfactant
concentration (thrice of CMC), the monomer surfactant molecules are adsorbed on the
sulfur particle surface through its tailgroup. As a result, after the adsorption of the ionic
surfactant molecules, a charge is developed on the particle surface. Therefore, even at
high reactant concentration agglomeration tendency of the particles are reduced due to
the electrostatic repulsion between the particles adsorbed by ionic surfactants. For
TX-100 solution, due to hydration of the headgroups there is also minimum tendency
towards agglomeration. This explanation can be supported by zeta potential values of
sulfur particles in aqueous and different surfactant solutions.
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Fig. 7 Plot of particle size distribution in different media for 10 mM thiosulphate solution
catalyzed by oxalic acid.
Zeta potentials values of sulfur particles in different media are given in Table 2.
From the Table, it is clear that sulfur is having low negative potential at aqueous media
and closed to zero in the presence of TX-100. Among the ionic surfactants, anionic
surfactants show higher zeta potential value than cationic, and between two anionic
surfactants (SDS and SDBS), SDBS shows more negative potential. So, lower particle
size and sharp distribution in the presence of SDBS can be explained in terms higher
negative zeta potential due to adsorption of surfactant, which prevent growth as well as
the agglomeration of particles. Moreover, we have observed that the dispersing ability of
the particles in the presence of both the anionic surfactants is more than TX-100 or
CTAB. When the concentration of thiosulphate is more than 50 mM particles are separate
very fast from the aqueous media except the presence of anionic surfactants.
3.4. Effect of surfactant concentration on particle size
From the previous discussion it is clear that using different surfactant we can control the
size of the sulfur particles. In all the above studies we have used the concentration of
surfactants thrice of their respective CMCs. Here, we have studied the effect of surfactant
concentration (CTAB) on particle size for a fixed reactant concentration (10 mM) and the
results are plotted in Fig. 8. Fig. 8 shows there is a sharp decrease in particle size till little
below the CMC, above CMC the size becomes almost constant.
Zeta potentials were also measured for different CTAB concentration for the same
reactant concentration. The Figure clearly indicates that the zeta potential values are
increasing with the increase in surfactant concentration and ultimately become constant
close to the CMC of the surfactant where particle size is also becomes constant. The
increase in zeta potential is due to the adsorption of surfactant molecules on sulfur
surface by its tailgroup and forming a complete saturated monolayer near to CMC, also
zeta potential values indirectly proofs there is no bi-layer (hemimicelle) formation in the
adsorption. The particle size in the presence of surfactant is minimum when surface
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charge is maximum and surface is saturated by the adsorbed monomer surfactant
molecules.
Fig. 8 Variation of particle size and zeta potential with CTAB concentration in the
presence of 5 mM and 10 mM reactant concentrations by HCl catalyzed reaction.
3.5. X-ray diffraction (XRD) analysis
Figs. 9 (a) and (b) show the XRD pattern of sulfur particles in different acidic solution
and in the presence of surfactants and HCl media respectively. The positions and
intensities of the diffraction peaks are in good agreement with the literature values for
orthorhombic or -phase sulfur with S8 structure (74-1465 from JCPDS PDF Number).
The phase obtained is also independent of acid media. Fig. 9 (b) also shows the similar
structure is formed, with the difference of sharp peak indicates highly crystalline nature
in the presence of surfactant system than aqueous media except SDBS. The XRD samples
are prepared by successive washing by fresh water, so we got only pure sulfur peak. The
sample was prepared without any washing shows sulfur peak mainly as well as it also
shows surfactant and Na2SO4 peaks (pattern is not shown here).
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Fig. 9 XRD pattern of sulfur nanoparticles: (a) in different acid media, (b) in the presence
of different surfactants by HCl catalyzed reaction.
3.6. Analysis by Electron Microscope (SEM and TEM)
Figs. 10 A, B, C, D shows the SEM images of sulfur particle synthesize by HCl (Fig.
10A) and oxalic acid (Fig. 10B) catalyzed media without any surfactant, by HCl
catalyzed in the presence of CTAB (Fig. 10C), and SDBS (Fig. 10D) surfactants for
thiosulphate concentration of 5 mM. From the Figures it is clear that the sulfur particles
are almost spherical shape and uniform size. Fig. 11 shows the TEM image of sulfur
nanoparticle formed at 5 mM thiosulphate concentration by inorganic acid catalyzed
reaction in CTAB media and the size of the particles obtained is close to 50-55 nm. This
again confirms the formation of lower size particle in CTAB media. We have also done
SEM (Figure not shown) of samples by HCl media in the presence of CTAB surfactant
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for 4, 9 mM thiosulphate concentration solution and found the particles sizes are higher
than that of 5 mM. Form both the SEM and TEM figures we can say sulfur has a
tendency towards agglomeration, so we are not getting separate discrete sulfur particle
image.
Fig. 10 SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration:
(A) HCl catalyzed, (B) oxalic acid catalyzed, (C) CTAB and HCl (D) SDBS and HCl.
Fig. 11 TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate
concentration inorganic acid catalyzed reaction in CTAB surfactant media.
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4. Conclusion
Mostly spherical shape orthorhombic or -sulfur with S8 structure is formed by acid
catalyze thiosulphate reaction. Sulfur particle size is depends on the ionization constant
value of the acids as well as acid (H+) to thiosulphate molar ratio. Inorganic acids
produce larger size particles than the organic acid. The particle size distribution is also
more uniform by organic acid catalyzed reaction, as zeta potential of the particles is high
in organic acid media to prevent agglomeration tendency. All the surfactants, anionic,
cationic, and nonionic surfactants play a major role in the lowering the particle size.
Compare to nonionic and cationic surfactants, in anionic surfactant media the particle
size is low and even in our working concentration range the change in particle size also
less, and among two anionic surfactants studied here SDBS is more effective for
controlling the size of the particles. Surfactant concentration is also one important factor
for the particle sizes and it seems that above CMC is required. In the presence of CTAB
within a certain reactant concentration range lower particle sizes are obtained in contrast
to the other surfactants if all other conditions are remain same. Sulfur nanoparticles of
approximately 30 nm particle size can be synthesized by organic acid catalyzed
precipitation of thiosulphate in the presence of aqueous CTAB media.
Acknowledgement:
The financial support from Department of Science and Technology (DST) under
Nanomission, New Delhi, India, Grant No. SR/S5/NM-04/2007, for this project is
gratefully acknowledged. We also acknowledge Saha Institute of Nuclear Physics
(SINP), Kolkata for giving the opportunity to access their TEM facility.
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Figure Captions:
Fig. 1: The effect of acid to thiosulphate ratio on the size of sulfur particles. Thiosulphate
concentration 5 mM.
Fig. 2: The effect of reactant concentration on the particle size in different acidic aqueous media. Fig. 3: Particle size distribution in different acid media at 10 mM thiosulphate
concentration.
Fig. 4: Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by hydrochloric acid catalyzed reaction. Inset shows the particle
size distribution at 4 mM and 5 mM thiosulphate concentration in the presence of CTAB.
Fig. 5: Plot of particle size distribution in different media at 10 mM thiosulphate solution
catalyzed by HCl.
Fig. 6: Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by oxalic acid catalyzed reaction.
Fig. 7: Plot of particle size distribution in different media for 10 mM thiosulphate
solution catalyzed by oxalic acid.
Fig. 8: Variation of particle size and zeta potential with CTAB concentration in the
presence of 5 mM and 10 mM reactant concentrations by HCl catalyzed reaction.
Fig. 9: XRD pattern of sulfur nanoparticles: (a) in different acid media, (b) in the
presence of different surfactants by HCl catalyzed reaction.
Fig. 10: SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration:
(A) HCl catalyzed, (B) oxalic acid catalyzed, (C) CTAB and HCl (D) SDBS and HCl.
Fig. 11: TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate
concentration by inorganic acid catalyzed reaction in CTAB surfactant media.
Table Caption:
Table 1. Ionization constants of different acids.
Table 2: Zeta potential of sulfur nanoparticles in different media, Thiosulphate
concentration 5 mM.
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Figures:
Fig. 1 The effect of acid to thiosulphate ratio on the size of sulfur particles. Thiosulphate
concentration 5 mM.
Fig. 2 The effect of reactant concentration on the particle size in different acidic aqueous media.
300
400
500
600
700
800
900
1000
0 1 2 3 4 5 6 7
H2SO
4
HCl(COOH)
2,2H
2O
Part
icle
Siz
e (n
m)
H+/ Thiosulphate
0
200
400
600
800
1000
1200
1400
0 2 4 6 8 10
HClH
2SO
4
(COOH)2, 2H
2O
HNO3
Part
icle
Siz
e (n
m)
Reactant Concentration (mM)
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Fig. 3 Particle size distribution in different acid media at 10 mM thiosulphate
concentration.
Fig. 4 Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by hydrochloric acid catalyzed reaction. Inset shows the particle
size distribution at 4 mM and 5 mM thiosulphate concentration in the presence of CTAB.
0
200
400
600
800
1000
1200
1400
0 10 20 30 40 50
WaterCTABTX-100SDSSDBS
Part
icle
Siz
e (n
m)
Reactant Concentration (mM)
0
5
10
15
20
0 200 400 600 800 1000
5 mM thio4 mM thio
Inte
nsity
(%)
Particle Size (d,nm)
0
5
10
15
20
25
30
35
400 800 1200 1600 2000 2400
HClH
2SO
4
HNO3
(COOH)2, 2H
2O
Inte
nsity
(%)
Particle Size (nm)
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Fig. 5 Plot of particle size distribution in different media at 10 mM thiosulphate solution
catalyzed by HCl.
Fig. 6 Variation of particle size with the thiosulphate concentration in the absence and
presence of surfactants by oxalic acid catalyzed reaction.
0
200
400
600
800
1000
0 2 4 6 8 10
WaterCTAB TX-100SDBS
Part
icle
Siz
e (n
m)
Reactant Concentration (mM)
0
5
10
15
20
25
30
0 500 1000 1500 2000
SDBSCTABWaterTX-100SDS
Inte
nsity
(%)
Particle Size (nm)
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Fig. 7 Plot of particle size distribution in different media for 10 mM thiosulphate solution
catalyzed by oxalic acid.
Fig. 8 Variation of particle size and zeta potential with CTAB concentration in the
presence of 5 mM and 10 mM reactant concentrations by HCl catalyzed reaction.
0
200
400
600
800
1000
1200
1400
5
10
15
20
25
0 0.5 1 1.5 2 2.5 3 3.5 4
Size at 10 mM thio solnSize at 5 mM thio solnZeta Potential (mV) at 10 mM thio soln
Part
icle
Siz
e (n
m)
Zeta
Pot
entia
l (m
V)
Surfactant Concentration (mM)
0
5
10
15
20
25
30
35
200 400 600 800 1000 1200
SDBSWaterCTABTX-100
Inte
nsity
(%)
Particle Size (nm)
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Fig. 9 XRD pattern of sulfur nanoparticles: (a) in different acid media, (b) in the presence
of different surfactants by HCl catalyzed reaction.
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Fig. 10 SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration:
(A) HCl catalyzed, (B) oxalic acid catalyzed, (C) CTAB and HCl (D) SDBS and HCl.
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Fig. 11 TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate
concentration by inorganic acid catalyzed reaction in CTAB surfactant media.
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Tables: Table-1. Ionization constants of different acids.
Acid Ionization constant (Ka) pKa HCl [17] 1.74 106 -6.24
HNO3 [18] 40 -1.60
(H2SO4) K1 2.4 106 -6.38
H2SO4 [19] (HSO4-1) K2 1.02 10-2 2.00
(H2C2O4) K1 5.6 10-2 2.75
H2C2O4 [20] (HC2O4-1) K2 5.1 10-5 5.71
Table 2: Zeta potential of sulfur nanoparticles in different media, Thiosulphate
concentration 5 mM.
Medium Acid Zeta Potential (mV) Water HCl -2.99 Water H2SO4 -1.85 Water HNO3 -2.17 Water (COOH)2 -8.05
TX-100 HCl -0.557
SDS HCl -76.3
SDBS HCl -85.0 CTAB HCl 23.8
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Formation of sulfur nanoparticles by acid catalyzed precipitation of thiosulphate in aqueous surfactant solutions: The smallest particle size is generated in the presence of CTAB.
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