Aggregation and electrochemical properties of 1-(4-chlorophenyl)-3-dodecanoylthiourea: A novel thiourea-based non-ionic surfactant

Article abstract of DOI:10.1007/s12039-015-0899-6

A novel thiourea-based non-ionic surfactant 1-(4-chlorophenyl)-3-dodecanoylthiourea (4CPDT) was synthesized from decanoyl chloride, potassium thiocyanate and 4-chloroanline in high yield. The structural chemistry of the compound was done by multiple nucle

Full text of DOI:10.1007/s12039-015-0899-6

c
J. Chem. Sci. Vol. 127, No. 8, August 2015, pp. 1361–1367. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0899-6
Aggregation and electrochemical properties of
1-(4-chlorophenyl)-3-dodecanoylthiourea: A novel thiourea-based
non-ionic surfactant
IMDAD ULLAHa,b,∗, AFZAL SHAHb, MUSHARAF KHANc, KHALIDA AKHTERd and
AMIN BADSHAHb
aSchool of Chemistry, Monash University, Clayton 3800, Melbourne, Australia
bDepartment of Chemistry, Quaid-i-Azam University, Islamabad 45300, Pakistan
cDepartment of Biological Science, Federal College, Mardan, 27000, KP Pakistan
dCenter of Excellence in Physical Chemistry, University of Peshawar, KP 27320, Pakistan
e-mail: mdadktk70@gmail.com
MS received 9 December 2014; revised 18 April 2015; accepted 24 April 2015
Abstract. A novel thiourea-based non-ionic surfactant 1-(4-chlorophenyl)-3-dodecanoylthiourea (4CPDT)
was synthesized from decanoyl chloride, potassium thiocyanate and 4-chloroanline in high yield. The struc-
tural chemistry of the compound was done by multiple nuclear NMR (1H, 13C) and FT-IR. UV-Visible spec-
trophotometry and pendant drop methods were used to evaluate their critical micelle concentration in ethanol
and hexane. This surfactant showed very low solubility in water, and interestingly low but well-defined, sub-
millimolar critical micelle concentration (CMC) in ethanol and hexane, demonstrating that this is moderately
amphiphobic. Its low value of critical micelle concentration indicates economical use for cleaning purposes
and environment-friendly applications. It was also characterized by cyclic and square wave voltammetry and
found to be electrochemically active giving sharp signal in different pH media.
Keywords. Synthesis; critical micelle concentration; non-ionic surfactant; spectroscopy; electrochemistry.
1. Introduction
harmful surfactants excessive use of which have affec-
ted the flora and fauna.11–13
Surfactants are organic substances which decrease the
interfacial tension of liquids. They have long chains of
hydrophobic hydrocarbons and hydrophilic polar heads.
In the past, surfactants were limited to use for cleaning
purposes but nowadays; they are the backbone of mod-
ern industrial uses.1–6 Non-ionic surfactants is second in
the industrial production of surfactants and they are not
ionizable in aqueous solution due to their hydrophilic
group which is non-dissociable by nature. These groups
may be amide, thio, alcohol, phenol, ether, etc. The non-
ionic surfactants are neutral and can be applied to many
different chemical research areas such as electrochem-
istry, where neutral surfactants are needed for positive
responses in direct potentiometric measurements of var-
ious ionic species in clinical, environmental and indus-
trial processes.7–10 They are used in ion-selective elec-
trodes and their association results in increased washing
capability and decreased the entrapment of air bubbles
in flow channels. New surfactants are being explored
to uncover their potential as replacement of known
This article discusses the chemistry of a new surfac-
tant with a thiourea-based group that is known to pos-
sess environmentally friendly properties. The problem
with this surfactant is its partial solubility in water and
further study is in progress to enhance this property.
2. Experimental
2.1 Chemicals, Instruments and Methods
Lauryl chloride (99.9%), potassium thiocyanate (99.9%),
different aliphatic and aromatic primary and secondary
amines (99.9%) were purchased from Sigma Aldrich
(USA). Fresh analytical grade dry acetone was used
as the solvent and dried before experiments. The prod-
uct was purified by thin layer chromatography. NMR
spectra were recorded on Bruker AC Spectrometers at
1
300.13 MHz for H and 75.47 MHz for 13C. Thermo
Nicolet-6700 FTIR spectrophotometer was also used
for IR characterization. A double beam Shimadzu UV-
1800 scan UV-Visible spectrophotometer and pendant
drop methods were used for CMC determination and
investigation of pH effect in acidic to basic media.
∗For correspondence
1361

1362
Imdad Ullah et al.
3
(2H, d, 2CH, J[1H-1H] = 7 Hz), 7.61–7.765(2H, d,
2.2 1-(4-chlorophenyl)-3-dodecanoylthiourea
2CH, J[1H-1H] = 7 Hz), 8.89 (1H, s, NH), 12.48
3
1
(1H, s, NH). 13C NMR (75.5 MHz CDCl3, δ-ppm ):
2
For the synthesis of 1-(4-chlorophenyl)-3-dodecanoy-
lthiourea, long chain hydrocarbon dodecanoyl chlorides
was treated with potassium thiocynate in 1:1 mole ratio
in 50 mL dry acetone as solvent and reflux for about
40 minutes at temperature 70◦C to form the respective
isothiocyanate. Long chain dodecanoyl isothiocyanate
solution (in dry acetone) was separated using cannula
filtration, leaving behind KCl precipitate, and the den-
sity was compared to the isothiocyanate gravimetri-
cally. Subsequently, dodecanol isothiocyanate was cou-
pled with 4-chloroanaline in dry distilled acetone under
an N2 atmosphere, and stirred for 12 h. The progress
of the reaction was monitored by TLC at regular inter-
vals. Finally, the product collected on condensing in
iced water was washed with doubly distilled water for
the removal of impurities. The pure product with more
than 84% yield was obtained as white solid powder. The
structure of the compound can be seen in scheme 1 and
the stepwise preparation protocol is given in scheme
2. The purity of the compound was ascertained by
1H NMR, 13C NMR and FTIR spectroscopy and the
spectral details are given below:
14.1(C17) , 22.7–37.4 (C7−17), 125.4–135.9 (C1,2,2,3,3,4
)
174.4 (C6), 178.4 (C5). IR (υ cm−1): 3240.3, 3193.5
(N-H), 3037.5 (Ar-H, sp2), 2916.9 2849.0 (sp3, C-H),
1686.3 (CO), 1567.3, 1489.2 (sp2, C=C).
3. Results and Discussion
3.1 Surface and aggregation properties
The tendency of surfactants to adsorb at interfaces and
aggregate in bulk systems is responsible for the ratio-
nal and key performances in the modification of liquid
properties. The adsorption and aggregation phenomena
of the surfactants are vital and can be studied by a vari-
ety of physicochemical methods such surface tension,
conductance, light scattering, viscosity, etc. The selec-
tion of a technique to evaluate their properties is con-
cerned with the structural and chemical formulation of
the surfactant. Due to the induction of thiourea group
into the new surfactant solubility of the compound in
aqueous medium was reduced. To overcome this and to
determine the critical micelle concentration, both UV-
visible spectroscopy and interfacial tension are demon-
strated as effective tools and these are used to explore
aggregation.
1H NMR (300 MHz, CDCl3, δ-ppm): 0.87–0.91(3H,
t, CH3, 3J[1H-1H] = 7 Hz), 1.28–1.34 (18H, m, 9CH2),
3
2.37–2.42 (2H, t, CH2, J[1H-1H] = 7 Hz), 7.28–7.39
Cl
O
C
S
3.2 UV/Vis method
C
N
N
H
H
The CMC of 4CPDT in ethanol was determined by
UV-visible spectroscopy. A double beam UV–Visible
spectrophotometer was used to record the absorbance
spectra of surfactant solutions as a function of con-
centration over a wavelength range of 200 to 800 nm.
1-(4-chlorophenyl)-3-dodecanoylthiourea
Scheme 1. Chemical structures and names of non-ionic
thiourea-based surfactants.
O
O
O
S
Acetone
C
C
KSCN +
NH2C6H4Cl
C
C
O
N
C
S
+
- KCl
Cl
N
C
11H23
C11H23
C11H23
C11H23
C
HN
Cl
HN
C
S
1-(4-chlorophenyl)-3-dodecanoylthiourea
Scheme 2. Synthetic method for the preparation of non-ionic thiourea-based surfactants.

Aggregation and electrochemical properties of surfactant
1363
It has been previously shown14,15 that a discontinuity
1.0
B
is seen in the plot of absorbance versus concentration
due to micellisation, thus providing an effective method
for locating the CMC. The UV-visible spectrum of
the compound in ethanol at room temperature is pre-
sented in figure 1. The CMC was determined by plot-
ting absorbance versus concentration of both peaks of
the spectrum. In both cases, the plots presented two
linear parts with different slopes and the CMC was
evaluated from their intersection point. This process
along with the CMC values were obtained is shown in
figure 2. The obtained value for the CMC is surprisingly
low, given the high solubility in ethanol (>50 mg/mL),
possibly indicating a significant level of solvophobicity
of the surfactant. By comparing the solubility of decane
and thiourea in ethanol and water however, the behav-
ior is readily rationalized. Decane is effectively insol-
uble in water, but highly soluble/miscible in ethanol.
Thiourea however is only moderately soluble in water
(mole fraction ∼0.04 at 25◦C), and even less so in
ethanol (mole fraction ∼0.02 at 25◦C).16 This indicates
that inverse micelles of the thiourea surfactant form in
ethanol, where the well-solvated decane tails would be
exposed to the solvent and the insoluble thiourea heads
would be shielded inside the aggregate.
0.8
0.6
0.4
CMC = 0.083 mM
0.2
1.6
A
1.2
0.8
0.4
CMC = 0.083 mM
0.08
0.09
0.10
0.11
Concentration (mM)
Figure 2. Absorbance vs concentration plots of 4CPDT at
250 and 302nm in ethanol.
The pendant drop technique also provides an effec-
tive method for determination of CMC in the oil phase
(hexane), as this surfactant is only minimally solu-
ble in water. A concentration range of 0.02–0.15 mM
was assessed, with the surfactant dissolved in the hex-
ane phase, kept in a standard 1 cm path-length quartz
cuvette. A pendant droplet of pure water was then dis-
pensed from a glass syringe equipped with a stainless
steel blunt cannula (0.72 mm outer diameter). With
increasing surfactant concentration, the hexane-water
interfacial tension rapidly dropped from its equilibrium
value to ∼16 mN/m (figure 3). When compared to con-
ventional surfactants such as AOT and its analogues,17
the decrease in interfacial tension is modest, indicat-
ing that these surfactants are not particularly effec-
tive at the hexane-water interface and would thus not
make good single-component emulsifiers. For the same
solubility arguments as with ethanol, we can assume
that inverse micelles are formed in hexane after the
CMC.
Of note is that the CMC of 4CPDT in hexane and
ethanol is surprisingly similar – particularly unusual
for surfactants, given the different polarities and chemi-
cal characteristics of ethanol and hexane. This indicates
that the thiourea moiety is controlling aggregation by
its poor solubility in the solvents of interest here (water,
ethanol and hexane), despite their widely ranging polar-
ity. Interestingly, this highlights the thiourea group as a
potential design feature in future surfactants, acting as
an amphiphobic moiety.
3.3 Pendant drop tensiometry
Besides bulk aggregation of the surfactant, the surface
adsorption properties are also of great interest. Mea-
surement of surface and interfacial tension is invalu-
able for determining the suitability of a surfactant for
a given system, in terms of effectiveness and efficiency
of adsorption. We explored the properties of 4CPDT
at the hexane-water interface in order to investigate its
usefulness as an oil-water emulsifier.
1.6
B
Concentration
1.2
0.8
0.4
0.0
0.15 mM
A
0.02 mM
240
280
320
360
400
440
Wavelength (nm)
Figure 1. UV-Vis spectrum of 4CPDT in ethanol.

1364
Imdad Ullah et al.
electronic absorption behavior in acidic, neutral and
3.4 pH effect
basic media. The compound absorbs at 250, 302 nm and
effect of pH on the spectrum is shown in figure 4. The
results indicate that absorbance of surfactant gets low-
ered with change in pH from 4 to 9 and the two peaks
merge into a single peak (at pH 11) and absorbance
increases with further rise in pH up to 12. In strongly
alkaline conditions of pH 11-12, the narrowness and
increase in peak intensity of surfactant is attributed
to the deprotonation of –NH group and consequent
increase in conjugation as shown in scheme 3. Fur-
ther pH study is in progress for getting useful insights
about the indefinite aspects of thiourea-based non-ionic
surfactants.
1-(4-chlorophenyl)-3-dodecanoylthiourea (4CPDT) was
also studied in different pH media to visualize their
40
35
30
CMC = 0.077 mM
25
20
15
10
3.5 Electrochemistry
0.0
0.1
0.2
0.3
0.4
0.5
Concentration (mM)
Electrochemical characterization of the synthesized
surfactant was carried out using the following tech-
niques to investigate its activity in different pH media.
Figure 3. Interfacial tension vs concentration plots of
4CPDT in hexane.
3.5a Cyclic voltammetry: Cyclic voltammogram of
1 mM 1-(4-chlorophenyl)-3-dodecanoylthiourea was
first witnessed in the potential range of 0 – +1.4 V
at a scan rate of 100 mV s−1 using supporting elec-
trolyte of pH 10.0 as the signal at the mentioned pH
was more prominent.18–20 An irreversible anodic peak
at +1.029 V related to the oxidation of the compound
was observed as portrayed in figure 5.
Cyclic voltammograms were recorded at different
scan rates figure 6A. The voltammograms revealed that
the anodic peak shifts to more positive potentials with
the increase in scan rate. By plotting anodic peak cur-
rent vs. square root of scan rate using Randles-Sevcik
equation, diffusion coefficient for the compound was
pH-5
pH-7
pH-11
pH-12
a
b
c
d
0.9
0.6
0.3
0.0
240
280
320
360
400
440
wavelength (nm)
Figure 4. UV–Vis spectra of 4CPDT in different pH
media.
O
S
HN
N
Cl
H2O
-OH
+
C
NH
C
+
S
C
C11H23
NH
C
C11H23
-O
Cl
Scheme 3. Demonstration of the increase in volume of chromophore of 4CPDT under
alkaline conditions leading to more intense peak in UV-Vis spectra.

Aggregation and electrochemical properties of surfactant
1365
evaluated aand presented in figure 6B. The value of
diffusion coefficient found is 3.04 × 10−5 cm2 s−1.
In order to investigate electro-reduction of the com-
pound, cyclic voltammograms were recorded in the
potential range 0 to −1.5 V using 1 mM solution of
the compound buffered in a medium of pH 4, 7 and
10 as shown in figure S1 (A-C). It was observed that
there is no reduction peak in the medium at pH 4; a
reduction peak appeared at pH 7 at −0.859 V, but after
thorough purging with nitrogen gas the peak intensity
dramatically got decreased accompanied with a shift
in peak potential to −0.815 V. Such a behavior might
be due to involvement of oxygen in the redox reaction
which is forcing the peak current to decrease so much
21
14
7
0
0.0
0.4
0.8
1.2
1.6
E
/ V vs.Ag/AgCl
Figure 5. CV of 1 mM compound (c) in oxidation region
recorded in pH=10 at a scan rate of 100 mVs−1
.
25
24
Equation
Pearson's r
Adj. R-Square 0.9832
Intercept
Slope
y = a + b*x
0.99156
A
B
-1
90 mV s
21
18
15
12
9
20
-1
80 mV s
-1
70 mV s
0
15
10
5
-1
-1
-1
-1
-1
60 mV s
50 mV s
40 mV s
30 mV s
20 mV s
63.185
D = 3.04e-5
o
-1
100 mV s
0
6
-5
0.12
0.16
0.20
0.24
0.28
0.32
0.0
0.4
0.8
1.2
1.6
E
/ V vs.Ag/AgCl
Figure 6. CVs of 1 mM compound (c) recorded in pH=10 at (A) different scan rates, (B)
plot of anodic peak currents vs. square root of scan rate using CV data of compound (c)
obtained in pH 10.
5
pH 6
pH 5
pH 4
pH 3
8
4
pH 12
pH 11
4
3
2
1
0
6
4
2
0
0
5
4
3
2
1
pH 10
pH 9
pH 8
pH 7
2
0
0
5
4
3
2
1
2
0
2
1
0
4
0
5
4
3
2
1
0
2
0
0.0
0.3
0.6
0.9
1.2
0.0
0.3
0.6
0.9
1.2
E
/ V vs.Ag/AgCl
E / V vs. Ag/AgCl
Figure 7. Square wave voltammograms showing oxidation of compound (c) at 100 mV s−1 in (A) pH=3-6, (B) pH 7-12.

1366
Imdad Ullah et al.
and also a shift in peak potential.21,22 At pH 10, with-
out purging with nitrogen gas a broad reduction peak
appeared at −1.15 V and, after thoroughly purging the
solution with nitrogen gas, this broad peak disappeared
and a reduction peak was observed at −0.628 V. In
figure S1 (D), CV scans of purged solution are shown
at pH 7 and 10 and shift in peak potential was found
to be 62.33 mV/pH, which was in close agreement to
the theoretical value 59 mV/pH. This indicates the par-
ticipation of equal number of electrons and protons in
the redox process. On the basis of these experimen-
Supplementary Information
All additional information pertaining to characteriza-
tion and electrochemical behaviour of the surfactant
(4CPDT) using NMR technique (figures S3 and S4),
cyclic voltammograms (figures S1(A-D), S2(A-C)) are
given in the supporting information which is available
at www.ias.ac.in/chemsci.
Acknowledgements
tal evidences, it was inferred that the compound under The authors gratefully acknowledge the financial
investigation undergoes electro-reduction in the given support of Higher Education Department, Khyber
potential range with strong involvement of oxygen in Pakhtunkhwa, Higher Education Commission, Islam-
the redox reactions.23
abad, Pakistan, and School of Chemistry, Monash Uni-
versity, Clayton VIC, Melbourne, Australia.
3.5b Square wave voltammetry (SWV)-Electro oxi-
dation of compound: SWVs of the compound were
recorded in a wide pH range to identify the mecha-
nism for the electro-oxidation. The peak potentials of
the anodic peaks were found to depend strongly on
the pH of the medium. In the pH range 3–6, a single
anodic peak was witnessed which shifts to more neg-
ative potentials accompanied with an increase in peak
intensity in the mentioned pH range as shown in figure 7
(A, B). However, the anodic signal started splitting into
two peaks from pH 7 which got prominent as the pH of
the medium increases.24–28 Under highly basic medium
of pH 11 and 12 more splitting was observed giving rise
to three peaks.
In order to check the reversibility of the anodic peaks,
forward and backward current components of SWVs at
pH 6, 8 and 12 were plotted along with total current
and shown in figure S2 (A-C). It was noted that the
anodic peaks are irreversible at all the mentioned pH
values.
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