Synthesis, spectroscopic characterization and pH dependent electrochemical fate of two non-ionic surfactants

Article abstract of DOI:10.1149/2.0391414jes

Two new nonionic surfactants; 1-sec-butyl-3-dodecanoylthiourea (DTU) and 1-dodecanoyl-3-phenylthiourea (DPTU) were synthesized and characterized by ¹H NMR, ¹³C NMR and FTIR and UV-Vis spectroscopy. The detailed electrochemical fate of DTU and DPTU was investigated in a wide pH range of 2-12 by employing three electroanalytical techniques. The voltammetric signatures of the analytes showed a single irreversible anodic peak followed by two reversible peaks of the oxidation product. The irreversible behavior of the oxidation process was witnessed by the unequal components of total current in square wave voltammetry and scan rate based deviation of peak potential. The involvement of protons accompanying the electron transfer processes was ascertained from peak potential versus pH plots. Critical micelle concentration and hydrophilic-lipophilic balance of the synthesized surfactants were determined for the assessment of their cleaning, wetting and emulsifying properties.

Full text of DOI:10.1149/2.0391414jes

Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
H885
Synthesis, Spectroscopic Characterization and pH Dependent
Electrochemical Fate of Two Non-Ionic Surfactants
Azeema Munir,a Imdad Ullah,a Afzal Shah,a,b,z Usman Ali Rana,c Salah Ud-Din Khan,c
Bimalendu Adhikari,b Syed Mujtaba Shah,a Sher Bahadar Khan,d Heinz-Bernhard Kraatz,b
and Amin Badshaha
aDepartment of Chemistry, Quaid-i-Azam University, 45320 Islamabad, Pakistan
bDepartment of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto M1C 1A4, Canada
cDeanship of Scientific Research, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia
dChemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Two new nonionic surfactants; 1-sec-butyl-3-dodecanoylthiourea (DTU) and 1-dodecanoyl-3-phenylthiourea (DPTU) were synthe-
sized and characterized by 1H NMR, 13C NMR and FTIR and UV-Vis spectroscopy. The detailed electrochemical fate of DTU and
DPTU was investigated in a wide pH range of 2-12 by employing three electroanalytical techniques. The voltammetric signatures
of the analytes showed a single irreversible anodic peak followed by two reversible peaks of the oxidation product. The irreversible
behavior of the oxidation process was witnessed by the unequal components of total current in square wave voltammetry and scan
rate based deviation of peak potential. The involvement of protons accompanying the electron transfer processes was ascertained
from peak potential versus pH plots. Critical micelle concentration and hydrophilic-lipophilic balance of the synthesized surfactants
were determined for the assessment of their cleaning, wetting and emulsifying properties.
© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0391414jes] All rights reserved.
Manuscript submitted July 17, 2014; revised manuscript received September 15, 2014. Published September 27, 2014.
Some detergents and surfactants have been restricted and prohib-
ited in the developed countries due to adverse environmental conse-
quences. The manufacturers are now trying to synthesize environmen-
tally friendly surfactants according to the environmental protection
laws.1,2 With these facts in mind, we synthesized thiourea based sur-
factants which are expected to increase the fertility of the soil if their
washed water is directed toward agricultural fields.
tants, 1-sec-butyl-3-dodecanoylthiourea (DTU) and 1-dodecanoyl-3-
phenylthiourea (DPTU). The lack of cost effective protocols for the
synthesis of electroactive surfactants from commonly available chem-
icals, insufficient information about the fundamental electrochemistry
of thiourea based surfactants and dearth of literature about the electro-
chemical fate of these surfactants prompted us to investigate the redox
behavior of two electroactive members of this class of surfactants in
a wide pH range using three electroanalytical techniques.
Surfactants are classified as ionics, nonionics and zwitter-ionics.
The demand and consumption of ionic surfactants were greater than
nonionics in the mid of 19 century but now the market share of non-
ionic surfactants has reached to 40% of the total worldwide surfactants
production. Nonionic surfactants excel other types due to their neu-
tral behavior, compatibility with ionic surfactants and capability of
forming complex mixtures as found in many commercial products.3,4
Nonionic surfactants are used as excellent solubilizing agents due to
their very low critical micelle concentration.5,6 Such surfactants can
work even in hard water as they do not yield ions in aqueous solution
and hence, less or almost insensitive to electrolytes. Due to their neu-
tral behavior, nonionic surfactants are none or less toxic than ionic
surfactants and hence find use in cosmetics, pharmaceuticals and food
products.7–9 They can also be used as wetting agents, emulsifiers and
detergents.10,11 Nowadays nonionic surfactants are in constant use in
different varieties of domestic and industrial products.12,13 Prompted
by the peculiar characteristics of non-ionic surfactants such as very
low critical micelle concentration, insensitivity to hard water, no or low
toxicity, low cost and compatibility with ionic surfactants, we synthe-
sized and spectroscopically characterized more effective candidates
of this class in high yield using the protocol reported in our recent
article.14 Moreover, the oxygen, nitrogen and sulfur donor atoms of
thiourea derivatives provide a multitude of bonding possibilities.15
Hence, the excellent metal ions complexation ability of our synthe-
sized surfactants is expected to help the removal of toxic metals from
polluted water.
Experimental
Chemicals and general methods.— Lauryl chloride (98%), potas-
sium thiocynate (99%) and different aliphatic and aromatic primary
and secondary amines (98%) were purchased from Sigma Aldrich.
Fresh analytical grade dry acetone was used as solvent and dried be-
fore experiment. The products were purified by thin layer chromatog-
raphy. NMR spectra were recorded on Bruker AC Spectrometers at
300.13 MHz for H and 75.47 MHz for 13C. Thermo nicolet-6700
1
spectrophotometer was used for FTIR characterization.
Synthesis of the compounds.— For the synthesis of DTU, 0.6 g
potassium thiocyanate was dissolved in 50 mL dry acetone and intro-
duced into 250 mL round bottom flask followed by the addition of
1.5 mL lauryl chloride. After heating for about 40 minutes, 2-butyl
amine was added and kept the solution on stirring for 10-12 hrs. The
product collected on condensing ice was washed for the removal of
impurities with doubly distilled water. DPTU was synthesized by the
same method using aniline instead of 2-butyl amine. The synthetic
method can be seen in Scheme 1. The pure products of DTU and
DPTU in 80 and 83% yield were obtained as white powder with melt-
ing points of 70 and 93◦C respectively. The purity of the compounds
was ascertained by 1H NMR, 13C NMR and FTIR spectroscopy. The
spectral details of the compounds are given below:
Thiourea and its derivatives are used in a variety of industrial ap-
plications such as rubber vulcanization, electrodeposition and pro-
duction of industrial cleaning agents.16 The properties of surfac-
tants are envisioned to enhance by the incorporation of thiourea
functionality. Hence, stimulated by the broad range applications
of compounds containing thiourea functionality we synthesized
and characterized two novel non-ionic thiourea containing surfac-
(i) Spectral details of DTU
1H NMR (300 MHz, CDCl3 δ-ppm): 0.86-0.96 (6H, m, 2CH3,),
1.11-1.13 (3H, t, CH3 3J[1H-1H] = 6 Hz ), 1.26-2.34 (24H, m, 12CH2),
8.57 (1H, s, 1NH), 10.43(1H, s, 2NH) 13C NMR 75.47 MHz CDCl3
δ-ppm): 20.9 (C1,C4, C17), 22.7-37.2 (C2, C8-17 ), 53.0 (C3), 172.4
(C6), 173.9 (C5). IR (υ cm−1): 3296 (N-H), 2960.8, 2915.0, 2871.5,
2848.4 (sp3, C-H), 1690 (CO), 1540.7, 1472.4 (sp2, C=C).
(ii) Spectral details of DPTU
1H NMR (300 MHz, CDCl3 δ-ppm): 0.87-0.92 (3H, t, CH3, 3J[1H-
1H] = 7Hz), 1.12-1.34 (18H, m, 9CH2 ), 2.38-2.43 (2H, t, CH2.
zE-mail: afzal.shah@utoronto.ca; afzal_toru@yahoo.com
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H886
Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
Scheme 1. Synthesis of DTU and DPTU.
3J [1H,1H]=7Hz ), 6.94-6.98(2H, m, 2CH), 7.39-7.45(1H, d, CH,
3J[1H-1H] = 7Hz ), 7.64-7.66 (2H,d, 2CH, 3J[1H-1H] = 7Hz ), 8.94
(1H, s, 1NH), 12.41 (1H, s, 2NH). 13C NMR 75.47 MHz CDCl3
δ-ppm): 14.1(C17), 22.7-37.9 (C7-17), 119.7(C1), 126.9(C3,,3), 129.0
(C2.2), 137.4(C4), 174.3 (C6), 178.4 (C5). IR (υ cm−1): 3287.7, 3182
(N-H), 3025.8 (Ar-H, sp2). 2916.7, 2847.9 (sp3, CH), 1699.9 (CO),
1537.9, 1464.6 (sp2, C=C).
Materials and reagents.— For voltammetric analysis, 2 mM stock
solutions of the synthesized compounds were prepared in ethanol and
stored at 4◦C. Fresh working solutions were prepared in 50% ethanol
and 50% supporting electrolyte. Britton Robinson buffer was used
as supporting electrolyte. For pH measurements INOLAB pH meter
with Model no pH 720 was used. Microvolume measurements were
done by EP-10 and EP-100 Plus Motorized Microliter Pipettes (Rainin
Instrument Co. Inc., MA, USA).
Figure 1. 1st and 2nd scan CVs of 1 mM DTU (A) and DPTU (B) obtained in
pH 2 and 7.4 at 100 mV s−1
.
Equipments and conditions for electrochemical experiments.—
Voltammetric experiments were performed using μAutolab running
with GPES 4.9 software, Eco-Chemie, The Netherlands. A glassy
carbon (GC) was used as working electrode. Pt wire and Ag/AgCl (3
M KCl) were used as counter and reference electrodes. Before every
experiment, the surface of GCE was polished with diamond spray
(particle size 1 μm) followed by thorough rinsing with distilled wa-
ter. All electrochemical experiments were conducted in a high purity
set for capturing the overall electrochemical signature of the analytes.
Cyclic voltammograms shown in Fig. 1 indicate the oxidation of DTU
and DPTU at 0.81 and 0.46 V. In the reverse scan, two reduction peaks,
c1 and c2 can be seen in the CV of DTU and only one in case of DPTU.
The second scan voltammograms indicate the reduction peaks to be
of reversible nature. On the basis of redox peaks at a potential very
close to 0.0 V, the oxidation products of DTU and DPTU can be used
as redox probes for the designing of electrochemical biosensors,17,18
characterization of lyotropic liquid crystalline phases19 and for the
investigation of interactions between molecules/ions.20,21
Cyclic voltammograms of 1 mM solutions of DTU and DPTU
shown in Fig. 2 were recorded at various scan rates ranging from
10 - 500 mVs−1. A slight positive shift of peak a1, with increase in
scan rate indicates the irreversible nature of oxidation process.22 The
absence of corresponding peak of a1 in the reverse scan also witnesses
the irreversible nature of the oxidation process. No shift in rest of the
peaks offers evidence about their reversible nature. Ep-Ep/2 values of
about 60 mV further ensure their reversible character. The straight line
plots of peak current versus square root of scan rate (Fig. 3) with zero
intercept are consistent with the diffusion limited processes. The plots
of log Ipa vs. log ν with slopes of 0.61 and 0.63 close to the theoretical
value of 0.5 for a fully diffusion controlled process23 also support the
oxidation processes to be mainly controlled by diffusion. However,
the oxidation peaks of the products of DTU are controlled by partial
adsorption and partial diffusion as witnessed from the slopes of log Ip
vs. log ν plots (Fig. 4). The contribution of adsorption is also evident
from the comparatively sharp peaks of the oxidation products than the
oxidation signal of DTU. The sharpness of these peaks is related to the
fact that electron transfer occurs after the adsorption of analyte during
an adsorption-controlled process while the broadness of the oxidation
peaks of surfactants demonstrates the diffusion mode of transport.
nitrogen atmosphere at room temperature (25
1◦C). Differential
pulse voltammetric experiments were carried out at 5 mV s−1 scan
rate. In square wave voltammetry (SWV) an effective scan rate of 100
mV s−1 was used by setting the experimental conditions of 50 Hz
frequency and 2 mV potential increments. For obtaining reproducible
experimental results, the polished working electrode was used to place
in the desired solvent followed by recording various differential pulse
voltammograms until the achievement of a steady state baseline.
Results and Discussion
Structural characterization.— The formation of DTU and DPTU
was ensured by the presence of stretches corresponding to C=O, C=S,
N-H and sp 3 C-H in the FT-IR spectra. The presence of characteristic
stretching vibrations of phenyl ring in the FT-IR spectrum of DPTU
and its absence in DTU distinguished the structures of the synthesized
compounds. In the proton NMR spectrum, a triplet due to CH3 of the
long chain, a singlet due to NH and CH of aromatic ring and multiplet
due to CH2 peaks of the long chain further confirmed the formation of
the desired surfactants. The proton NMR spectra of both surfactants
were differentiated by the alkyl and aryl group of the amine attached to
surfactant head group. Moreover, the presence of expected resonance
for carbons in 13C-NMR spectra of DTU and DPTU also confirmed
the successful synthesis of the designed surfactants.
Cyclic voltammetry.— Cyclic voltammetry of 1 mM solution of
both surfactants was first carried out between the potential limits
of 1.5 V at a sweep rate of 100 mV s−1. During voltammetric
experiments a constant flux of N2 was kept over the solution surface
in order to avoid the mixing of atmospheric oxygen into the solution.
Based upon the initial experiments, a suitable potential window was
Differential pulse voltammetry.— Differential pulse voltammo-
grams (DPVs) of 1 mM surfactants in media of pH 2 - 12 were
recorded for the determination of number of electrons and protons
involved in their redox processes. Like CV, 1st and 2nd scan DPVs of
DTU were also obtained as shown in Fig. 5. The width at half peak
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Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
H887
Figure 2. (A) 2nd scan CVs of 1 mM (A) DTU and (B) DPTU at different
scan rates.
Figure 4. Plots of log Ipa vs. log υ using Fig. 2 data of DTU (A) and
DPTU (B).
height (W1/2) of peak a1 with a value of 155 mV much greater than
the theoretical value of 90.4 mV24 for one electron redox process can
be related to the possible overlapping of two, one electron transfer
oxidation peaks. This oxidation peak shifts to more negative poten-
tials with increase in pH of the medium. This behaviour is consistent
with the comparatively facile electron abstraction in basic conditions
as expected. In the 2nd scan of DTU obtained without cleaning the
electrode surface, two product peaks a2 and a3 were obtained with
W1/2 values corresponding to the abstraction of one electron in each
step. The shift in the location of these peaks with increasing pH of
the medium shows the involvement of protons during electron trans-
fer processes. Peak a2 was found to merge with a3 at pH higher than
9.0. Like anodic peaks, three cathodic peaks also appeared during the
reduction of DTU and peak c2 was found to combine with c3 at pH
higher than 6. Like DTU, DPTU also showed large W1/2 value of the
oxidation signal due to possible combination of two peaks. More-
over, the product signal was found to split in highly acidic conditions
(Fig. 6) thus supporting our attribution of peaks overlapping in alka-
line media.
Figure 5. (A) 1st and (B) 2nd scan DPVs showing oxidation of 1 mM DTU
at 5 mV s−1 in different supporting electrolytes of pH 2 to 12 using Britton
Robinson buffer (C) Plots of (ꢀ) Epa2 and (●) Epa3 vs. pH.
Figure 3. Plots of anodic peak currents of DTU (A) and DPTU (B) vs. υ1/2
using CV data of Fig. 2.
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H888
Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
Figure 7. (A, B) SWVs of 1 mM DTU and DPTU in pH 7.4 at 100 mV s−1(C,
D) 2nd scan SWVs of 1 mM DTU and DPTU in pH 7.4 at 100 mV s−1
.
Figure 6. (A) 1st and (B) 2nd scan DPVs of 1 mM DPTU obtained at 5 mV
s−1 in different supporting electrolytes of pH 2 to 12 (C) Plot of Epa2 as a
function of pH.
products of DTU and DPTU. The 2nd scan SWVs (Figs. 7C and 7D)
showing forward and backward current components of the total cur-
rent of DTU and DPTU were obtained to comment on the nature of
the redox processes. The same direction of peaks and unequal currents
of forward and reverse components of peak a1 justify the irreversible
nature of oxidation corresponding to this signal. While the reversible
nature of a2 and a3 is evident from the equal peak current intensities
of both components of the total current.
For the determination of number of protons involved in the redox
processes peak potentials were plotted as a function of pH. For sig-
nal a1, a Nernstian slope of 60 mV/ pH suggested the oxidation of
DTU to occur by the involvement of equal number of electrons and
protons. The same behaviour was also witnessed for the oxidation of
DPTU. Plots of Ep vs pH of all other peaks (Fig. 6.) showed slope
values in very good agreement with Nernstian assessment for equal
participation of electrons and protons.25,26 These results helped in
proposing the pH dependent redox mechanistic pathways of DTU and
DPTU.
Proposed redox mechanism.— The results of voltammetric exper-
iments revealed the oxidation of DTU to occur in two-steps each
involving the removal of 1e− and 1H+. The proton coupled electron
loss is suggested to occur from the amine attached to carbonyl group
owing to its greater electron donating effect and higher ability of
electron abstraction as compared to the other amine attached to sec-
ondary butyl group. The diamine radical is expected to stabilize by
dimerization into a six membered ring. The results reveal the oxida-
tion product to exhibit reversible two steps redox behavior. In the first
step, amine group attached to secondary butyl and in the second step
amine linked to carbonyl group are suggested to reduce as shown in
Scheme 2. The pH dependent redox mechanism of DPTU can be seen
in Scheme 3. These mechanistic pathways are expected to encourage
the synthetic organic chemists to seriously ponder over the use of elec-
trochemical techniques for solving synthetic problems and transfor-
mation of compounds which cannot be achieved by traditional organic
methods.
Square wave voltammetry (SWV).— In SWV, the current is
recorded in both positive and negative going pulses, so anodic and
cathodic peaks of the electroactive species can be recorded in the same
experiment. In comparison to CV and DPV, SWV is more advanta-
geous owing to its ability of quickly measuring forward, backward
and total current in one only scan. The reversible, quasi-reversible,
and irreversible nature of the redox processes can be easily verified
by using SWV. Successive SW voltammograms of 1 mM DTU and
DPTU recorded in pH 7.4 are presented in Figs. 7A and 7B. The
results complement well with the CV and DPV results. The absence
of peaks a2 and a3 in the first scan and their appearance in 2nd scan
indicates these signals to correspond to the oxidation of the reduction
O
O
N
O
N
C11H23
S
C11H23
C11H23
S
-H+ , -e-
-H+ , -e-
NH
S
HN
HN
N
Dimerization
Scheme 2. Proposed redox mechanism of DTU.
O
O
O
N
C11H23
S
C11H23
S
C
11H23
HN
+H+ , +e-
+H+ , +e-
N
N
NH
HN
S
S
S
S
N
N
O
N
O
N
N
N
O
C11H23
C11H23
C11H23
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Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
H889
O
O
O
C11H23
O
C11H23
C11H23
-H+ , -e-
NH
N
-H+ , -e-
N
N
O
O
HN
HN
Dimerization
O
O
O
C11H23
O
C11H23
O
C11H23
O
HN
N
N
N
+H+ , +e-
+H+ , +e-
HN
NH
N
O
O
O
N
O
N
N
N
O
N
O
C11H23
C11H23
C11H23
Scheme 3. Proposed Redox mechanism of DPTU.
UV-Vis spectroscopy.— UV-Vis spectroscopy was employed for
the determination of CMC of both surfactants. The electronic absorp-
tion spectra of DTU and DPTU have been presented in Fig. 8A and
8B. DTU shows a single peak at 270 nm due to π→π∗ transition of
carbonyl group while DPTU gives two peaks at 248 and 298 nm due
to π→π∗ transition of phenyl and carbonyl groups respectively as
reported for such functional groups in literature.27,28 A small peak at
221 nm also appears due to possible n→σ∗ transition of the carbonyl
group.
ear segment than the second can be related to the presence of only
monomers in the sample solution (at concentration lower than CMC)
whose absorption increases with increase in their concentration. At
concentration ≥ CMC, micelles form in the solution and only their
number gets increased with further increase in surfactant concentra-
tion. In micelles the chromophores of the surfactant molecules get en-
trapped and hence less exposed to light. Therefore, the absorbance due
to increase in micelles does not increase as rapidly as monomers.29–31
The CMC of DTU and DPTU with values 15.31 and 15.73 μM were
determined which are very close to the conductometrically and tensio-
metrically determined CMCs of nonionic surfactants1 Such low CMCs
are suggestive of high cleaning ability of these surfactants. Moreover,
The CMCs of both surfactants was determined from the intersec-
tion of the two linear segments of the plots of absorbance versus
concentration (Fig. 8C and 8D). The greater slope of the first lin-
Figure 8. (A, B) Absorption spectra of different concentrations of DTU and DPTU (C,D) Plots of absorbance vs. concentration for the evaluation of CMC of
DTU and DPTU at 270 and 298 nm respectively.
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Journal of The Electrochemical Society, 161 (14) H885-H890 (2014)
the hydrophilic-lipophilic balance (HLB) of DTU and DPTU with
values 10.13 and 10.72 indicate their good wetting and emulsifying
properties.32–34
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Two new non-ionic surfactants 1-sec-butyl-3-dodecanoylthiourea
(DTU) and 1-dodecanoyl-3-phenylthiourea (DPTU) were success-
fully synthesized in 80 and 83% yield by a facile method. Their pH
dependent redox mechanistic pathways were proposed on the basis
of results obtained from three electrochemical techniques. DPTU was
found to oxidize in two irreversible one electron transfer steps follow-
ing a cascade mechanism. The two steps oxidation occurred by the
loss of one proton and one electron as evidenced from the Nernstian
slope of Ep vs. pH plot. Similar electrochemical response was ob-
served for DTU, and the same number of electrons and protons were
found to involve in its two steps oxidation mechanism as witnessed by
the half peak width and slope value of Epa vs. pH plot. The oxidation
product of the DTU showed reversible redox mechanism as observed
by the appearance of two peaks in acidic media. The two peaks merged
into one as witnessed by the large boost of half peak width in basic
conditions. DPTU showed this merging up to slightly acidic condi-
tions just below neutral pH. The studied thiourea based surfactants
followed irreversible diffusion controlled oxidation processes. How-
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Acknowledgments
The authors gratefully acknowledge the financial support of Higher
Education Commission of Pakistan through project number 20-3070,
Quaid-i-Azam University, the University of Toronto Scarborough,
NSERC and Deanship of Scientific Research at King Saud University
through the research group project number RGP-VPP-255.
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