Accepted Manuscript -...

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

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

http://dx.doi.org/10.1016/j.jcis.2009.12.004http://dx.doi.org/10.1016/j.jcis.2009.12.004

ACCEPTED MANUSCRIPT

1

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

ACCEPTED MANUSCRIPT

2

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.

ACCEPTED MANUSCRIPT

3

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.

ACCEPTED MANUSCRIPT

4

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,

ACCEPTED MANUSCRIPT

5

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).

ACCEPTED MANUSCRIPT

6

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.

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

ACCEPTED MANUSCRIPT

7

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].

ACCEPTED MANUSCRIPT

8

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

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)

ACCEPTED MANUSCRIPT

9

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

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)

ACCEPTED MANUSCRIPT

10

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

ACCEPTED MANUSCRIPT

11

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.

ACCEPTED MANUSCRIPT

12

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

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50

WaterCTABTX-100SDSSDBS

Part

icle

Siz

e (n

m)


0

5

10

15

20

0 200 400 600 800 1000

5 mM thio4 mM thio

Inte

nsity

(%)

Particle Size (d,nm)

ACCEPTED MANUSCRIPT

13

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.

0

5

10

15

20

25

30

0 500 1000 1500 2000

SDBSCTABWaterTX-100SDS

Inte

nsity

(%)

Particle Size (nm)

ACCEPTED MANUSCRIPT

14


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.

0

200

400

600

800

1000

0 2 4 6 8 10

WaterCTAB TX-100SDBS

Part

icle

Siz

e (n

m)


0

5

10

15

20

25

30

35

200 400 600 800 1000 1200

SDBSWaterCTABTX-100

Inte

nsity

(%)

Particle Size (nm)

ACCEPTED MANUSCRIPT

15

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

ACCEPTED MANUSCRIPT

16

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).

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)

ACCEPTED MANUSCRIPT

17

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

ACCEPTED MANUSCRIPT

18

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.

ACCEPTED MANUSCRIPT

19

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.

ACCEPTED MANUSCRIPT

20

Reference:

(1) J. A. Ober, Materials flow of sulfur: U.S. Geological Survey Open file report. 2003

02-298, available online at http://pubs.usgs.gov/of/2002/of02-298/

(2) X. Yu, J. Xie, J. Yang, K. Wang, J. Power Sources. 132 (2004) 181.

(3) W. Zheng, Y. W. Liu, X. G. Hu, C. F. Zhang, Electrochim. Acta. 51 (2006) 1330.

(4) Z. Yong, Z. Wei, Z. Ping, W. Lizhen, X. Tongchi, H. Xinguo, Y. Zhenxing, J Wuhan

University of Technology - Materials Science Edition. 22 (2007) 234.

(5) T. Kobayashi, Y. Imade, D. Shishihara, K. Homma, M. Nagao, R. Watanabe, T.

Yokoi, A. Yamada, R. Kanno, T. Tatsumi, J. Power Sources, 182 (2008) 621.

(6) J. Barkauskas, R. Juskenas, V. Mileriene, V. Kubilius, V. Mater. Res. Bull. 42 (2007)

1732.

(7) P. Santiago, E. Carvajal, D. M. Mendoza, L. Rendon, Microsc. Microanal. 12 (2006)

690.

(8) M. A. Ellis, D. C. Ferree, R. C. Funt, L. V. Madden, Plant Disease. 82 (1998) 428.

(9) A. S. Deshpande, R. B. Khomane, B. K. Vaidya, R. M. Joshi, A. S. Harle, B. D.

Kulkarni, Nanoscale Res. Lett. 3 (2008) 221.

(10) Y. Guo, J. Zhao, S. Yang, K. Yu, Z. Wang, H. Zhang, Powder Technol. 162 (2006)

83.

(11) Y. Guo, Y. Deng, J. Zhao, Z. Wang, H. Zhang, Acta Chimica Sinica. 63 (2005) 337.

(In Chinese).

(12) X. Y. Xie, W. J. Zheng, Y. Bai, J. Liu, Mater. Lett. 63 (2009) 1374.

(13) V. K. LaMer, A. S. Kenyon, J. Colloid Sci. 2 (1947) 257.

(14) V. K. LaMer, R. H. Denegar, J. Am. Chem. Soc. 72 (1950) 4847.

(15) V. K. LaMer, Ind. Eng Chem. 44 (1952) 1270.

(16) R. G. Chaudhuri, S. Paria, J. Colloid Interface Sci. 337 (2009) 555.

(17) A. R. W. Marsh, W. McElroy, J. Atmospheric Environment. 19 (1985) 1075.

(18) G. Hill, J. Holman, Chemistry in context. 5th edition, Nelson thornes publication.

(19) Y. C. Wu, D. Feng, J. Solution Chem. 24 (1995) 133.

(20) W. Qin, Y. Cao, X. Luo, G. Liu, Y. Dai, Separation Purification Technol. 24 (2001)

419.

(21) E. M. Zaiser, V. K. LaMer, J. Colloid Sci. 3 (1948) 571.

ACCEPTED MANUSCRIPT

21

(22) Y. T. He, J. Wan, T. Tokunaga, J. Nanopart Res. 10 (2008) 321.

(23) H. Furedi-Milhofer, V. Babic-Ivancic, L. Brecevic, N. Filipovic-Vincekovic, D.

Kralj, L. Komunjer, M. Markovic, D. Skrtic, Colloids Surfaces, 48 (1990) 219.

(24) Z. Hu, J. H. Santos, G. Oskam, P. C. Searson, J. Colloid Interface Sci. 288 (2005)

313.

(25) W. Zhang, X. Qiao, J. Chen, Mat. Chem. Phys. 109 (2008) 411.

(26) M. M. Husein, E. Rodil, J. H. Vera, J. Colloid Interface Sci. 273 (2004) 426.

(27) M. M. Husein,N. N. Nassar, Current Nanoscience, 4 (2008) 370.

(28) C. Tojo, M. C. Blanco, M. A. Lopez-Quintela, Langmuir, 14 (1998) 6835.

(29) C. Tojo, F. Barroso, M. de Dios, J. Colloid Interface Sci. 296 (2006) 591.

(30) M. de Dios, F. Barroso, C. Tojo, M. A. Lopez-Quintela, J. Colloid Interface Sci. 333

(2009) 741.

ACCEPTED MANUSCRIPT

22

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



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


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


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:


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.

ACCEPTED MANUSCRIPT

23

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)


ACCEPTED MANUSCRIPT

24

Fig. 3 Particle size distribution in different acid media at 10 mM thiosulphate

concentration.




0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50

WaterCTABTX-100SDSSDBS

Part

icle

Siz

e (n

m)


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)

ACCEPTED MANUSCRIPT

25

Fig. 5 Plot of particle size distribution in different media at 10 mM thiosulphate solution

catalyzed by HCl.



0

200

400

600

800

1000

0 2 4 6 8 10

WaterCTAB TX-100SDBS

Part

icle

Siz

e (n

m)


0

5

10

15

20

25

30

0 500 1000 1500 2000

SDBSCTABWaterTX-100SDS

Inte

nsity

(%)

Particle Size (nm)

ACCEPTED MANUSCRIPT

26

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


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)

ACCEPTED MANUSCRIPT

27

Fig. 9 XRD pattern of sulfur nanoparticles: (a) in different acid media, (b) in the presence

of different surfactants by HCl catalyzed reaction.

ACCEPTED MANUSCRIPT

28

Fig. 10 SEM micrographs of sulfur nanoparticles from 5 mM thiosulphate concentration:


ACCEPTED MANUSCRIPT

29

Fig. 11 TEM micrographs of sulfur nanoparticle formed at 5 mM thiosulphate

concentration by inorganic acid catalyzed reaction in CTAB surfactant media.

ACCEPTED MANUSCRIPT

30

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

ACCEPTED MANUSCRIPT

31

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.

Accepted Manuscript -...

Documents

Transcript of Accepted Manuscript -...