Synthesis, characterization and surface activity of new eco-friendly schiff bases vanillin derived cationic surfactants

Article abstract of DOI:10.1007/s11743-011-1249-4

Three eco-friendly cationic surface active agents were synthesized from the chemical modification of vanillin. The chemical structures of these surfactants were confirmed using elemental analysis, IR and NMR spectra. The surface activity measurements showed their high tendency towards adsorption and micellization and their good surface tension reduction, low interfacial tension. The emulsion stability measurements showed acceptable efficiency as emulsifying agents for short term emulsions. The biodegradability tests revealed that these compounds are eco-friendly and had completely degraded in 30 days. AOCS 2011.

Full text of DOI:10.1007/s11743-011-1249-4

J Surfact Deterg (2011) 14:325–331
DOI 10.1007/s11743-011-1249-4
ORIGINAL ARTICLE
Synthesis, Characterization and Surface Activity
of New Eco-friendly Schiff Bases Vanillin
Derived Cationic Surfactants
•
Nabel A. Negm Nadia G. Kandile
•
Mohamad A. Mohamad
Received: 30 October 2010 / Accepted: 16 January 2011 / Published online: 11 February 2011
Ó AOCS 2011
Abstract Three eco-friendly cationic surface active
agents were synthesized from the chemical modification of
vanillin. The chemical structures of these surfactants were
confirmed using elemental analysis, IR and NMR spectra.
The surface activity measurements showed their high ten-
dency towards adsorption and micellization and their
good surface tension reduction, low interfacial tension. The
emulsion stability measurements showed acceptable effi-
ciency as emulsifying agents for short term emulsions. The
biodegradability tests revealed that these compounds are
eco-friendly and had completely degraded in 30 days.
vanilla bean. It is also found in roasted coffee and the
Chinese red pine. Several studies were done on the vanillin
and its application as pesticide, green corrosion inhibitors
for corrosion of different metals. The urge of modification
of natural compounds in different applications is due to the
continuous accumulation of the chemical compounds in the
environment and the difficulty of their degradation. Then,
eco-friendly compounds were the solution of the environ-
mental pollution [1–6]. In this study, naturally occurring
vanillin was chemically modified into cationic surface
active agents containing hydrocarbon chains with different
lengths. The surface activity and some surface properties of
the produced cationic vanillin derivatives were determined.
In addition, the biodegradability of the new synthesized
surfactants was measured.
Keywords Vanillin ꢀ Eco-friendly ꢀ Surface activity ꢀ
Schiff base ꢀ Biodegradation
Introduction
Experimental
Natural occurring compounds are those compounds which
occur in different organisms, animals, plants or microor-
ganisms. These compounds are different and widely spread
in the plant and animal kingdoms. One of the most wide-
spread compounds in the plant kingdom is vanillin. Van-
illin is white or lightly yellow solid extracted from the
Chemicals
Vanillin (4-Hydroxy-3-methoxybenzaldehyde), 3-amino-
pyridine, bromoacetic acid and fatty alcohols were pur-
chased from Aldrich, Germany.
Synthesis
N. A. Negm (&)
Applied Surfactant Lab. Petrochemicals Department,
Egyptian Petroleum Research Institute,
Nasr city, Cairo, Egypt
Synthesis of Alkyl Bromoacetate (OB, DB, HB)
Bromoacetic acid (0.1 mol) and octyl, dodecyl and hexa-
decyl alcohol (0.1 mol) were esterified, respectively in
xylene in the presence of 0.1% p-toluene sulfonic acid (a
catalyst), until the azeotropic amount of water (1.8 mL)
was removed. After removal of the solvent under vacuum
using a rotary evaporator, the catalyst was extracted from
e-mail: nabelnegm@hotmail.com
N. G. Kandile
Faculty of Women, Ain Shams University, Cairo, Egypt
M. A. Mohamad
El Nasr Company for Intermediate Chemicals, Cairo, Egypt
123

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J Surfact Deterg (2011) 14:325–331
Measurements
the reaction medium using petroleum ether. Subsequent
purification was done by extracting the unreacted material
with petroleum ether [7] to afford the different fatty alcohol
esters of bromoacetic acid which are designated as octyl
bromoacetate (OB), dodecyl bromoacetate (DB) and
hexadecyl bromoacetate (HB).
Compound LB: IR: 2,940 cm-1 (CH3), 2,850 cm-1
(CH2), 1,736 cm-1 (C=O), 554 cm-1 (C–Br). 1H NMR
(CDCl3): 0.9 ppm (t, 3H, –CH3), 1.36 ppm (m, 20H,
–CH2–), 3.35 ppm (–O–CH2–), 4.17 ppm (Br–CH2–).
Surface Tension (c) and Interfacial Tension (cIT)
Measurements
Freshly prepared aqueous cationic surfactants solutions
with a concentration range of 0.01–0.000005 ML-1 were
poured into a clean 25-mL Teflon holder and allowed to
equilibrate for 2 h. Then, the platinum ring was adjusted
at the air–water interface of the surfactant solution. The
reading was recorded when the ring detached itself from
the solution surface. The platinum ring was removed
after each reading, washed with diluted HCl followed by
distilled water. Apparent surface tension values were
taken as the average of three replicates at 25.0 0.5 °C
[10].
Synthesis of 3-Aminopyridine-Vanillin Schiff Base (VSB)
Vanillin and 3-amino pyridine were mixed together in
equimolar ratio in a 250-mL round-bottom flask in the
presence of 50 mL ethanol as a solvent and 0.1% p-toluene
sulfonic acid as a dehydrating agent. The mixture was
refluxed under stirring for 4 h and then left to precipitate.
The product was filtered off and recrystallized twice from
ethanol to obtain the desired vanillin Schiff base (VSB) [8].
VSB: IR: 3,400 cm-1 (O–H), 2,850 cm-1 (CH3),
1,628 cm-1 (–C=N–, azomethine), 1,051 cm-1 (O–CH3),
Interfacial tension was measured between two layers
of surfactant solution and light paraffin oil at 25.0
0.5 °C. The surfactant solution (15 mL of 0.1% wt) was
added firstly into the Teflon holder and the platinum ring
was adjusted to touch the solution surface. Light paraffin
(15 mL) oil was then added smoothly onto the surface
and the system was left to equilibrate for 2 h. The
readings were recorded at the point which the ring
detached itself from the aqueous to the organic layer
(paraffin oil). The ring was removed and washed with
acetone followed by distilled water and the measurements
repeated three times for each surfactant. The interfacial
tension values were taken as the mean values of three
readings [11].
1
870 cm-1 (aromatic protons of phenyl group). H NMR
(CDCl3): 3.73 ppm (s, 3H, –OCH3), 7.26 ppm (t, 2H,
–CH=CH– benzylidenimine), 8.1 ppm (s, 1H, –N = CH–
azomethine).
Synthesis of the Cationic Schiff Base Derivatives
of Vanillin (VSBO, VSBD, VSBH)
Equimolar amounts of the vanillin Schiff base (VSB) and
alkyl bromoacetate esters (OB, DB, HB) were refluxed indi-
vidually in acetone (100 mL) as a solvent for 4 h and then left
to cool. The products were filtered off and recrystallized
twice from ethanol to remove the unreactants and the resid-
uals, and then dried under a vacuum at 50 °C for 6 h to yield
the desired cationic quaternary compounds which were des-
ignated as VSBO for octyl derivative, VSBD for dodecyl
derivative and VSBH for hexadecyl derivative [9].
Emulsification Power
A 20-mL amount of a different surfactant solution (0.1%)
was individually placed in a 100-mL cylinder and then
20 mL of the paraffin oil was added. The cylinder was
shaken vigorously for 10 min and then allowed to settle.
The time required to separate 18 mL of pure surfactant
solution was recorded and the experiment was repeated
three times for each surfactant. The average separation
time of the three experiments was taken as an indication of
the emulsification power of each surfactant [11].
Compound VSBD. IR: 1,628 cm-1 (–C=N–, azome-
thine), 870 cm-1 (aromatic protons of phenyl group),
2,920 cm-1 (CH2), 2,850 cm-1 (CH3), 3,040, 1,490 cm-1
1
(N?), 1,051 cm-1 (O–CH3). H NMR (CDCl3): 0.96 ppm
Biodegradation Test
(t, 3H, –CH3), 1.3 ppm (m, 20H, –CH2–[CH2]10–CH3),
3.73 ppm (s, 3H, –OCH3), 4.19 ppm (t, 2H, –COO–CH2),
7.26 ppm (t, 2H, =CH–CH=benzylidenimine), 7.33 ppm,
8.1 ppm (s, 1H, –N=CH– azomethine). 13C NMR (CDCl3):
14.1 ppm (–CH3), 22.8 ppm (–CH2CH3), 29.7 ppm
(CH2[CH2]9CH2CH3), 55.9 ppm (–OCH3), 63.2 ppm
(OCH2), 118.3 ppm (=CH–CH=CH–CH=benzylideni-
mine), 160.2 ppm (–N = CH– azomethine), 161 ppm
(–COOCH2).
The Modified Screening Test [12] described in the Orga-
nization for the Economic Cooperation and Development
(OECD) Guidelines was applied to determine the biodeg-
radation. A mixture of equal volumes of secondary effluent
from a wastewater treatment plant and garden soil aqueous
suspension was used as inoculums for the biodegradation
test.
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J Surfact Deterg (2011) 14:325–331
Results and Discussion
Chemical Structures
327
from the water phase. Increasing the amount of surfactants
added to the solution decreases the surface tension values
of the water gradually. The decrease in the surface tension
values indicates the tendency of surfactant molecules
towards adsorption at the liquid interface. At a certain
concentration, surfactant molecules cover the water inter-
face completely. In this case, surfactant molecules are
aggregated in the bulk of the solution to form the
micelles [14].
The chemical structures of the synthesized vanillin derived
cationic surfactants and all of their intermediates were
confirmed by elemental analysis, IR and NMR data pro-
vided in the synthesis section and illustrated in Scheme 1.
Surface Activity
The surface tension versus -log concentration profiles
of the vanillin derived cationic surfactant solutions (VSBO,
VSBD, VSBH) showed two characteristic regions (Fig. 1).
The first at low concentrations and characterized by con-
tinuous decrease of the surface tension values due to the
adsorption of surfactant molecules at the interface. The
second region is located at higher surfactant concentrations
where the surface tension values are almost stable. The
intercept of these two regions gave the critical micelle
concentration values, CMC [14] Table 1.
The hydrogen bonds formed between water molecules
make the water surface curved. Contaminants associate
with the water surface and break down the hydrogen bonds
(depending on the quantity of these contaminants at the
interface) and the surface tension of the water surface
deviates from the absolute value of the pure water (71.8
mNm-1) [13].
The amphipathic character of surfactant molecules is
due to the presence of two characteristic parts: the hydro-
phobic (organic) and the hydrophilic part (polar). When
surfactant molecules are dissolved in the aqueous medium,
they organize in the solution in which the hydrophile is
directed to the water phase. While, the hydrophobe is
located on the interface to reduce the repulsion generated
Commonly, the surface tension depression of the natu-
rally obtained surfactants is relatively high. While in case of
vanillin derived cationic surfactants, the surface tension
reduction is comparatively low. Table 2 reveals that
the surface tension values at the CMC range between 30
and 42 mNm-1 for octyl and hexadecyl derivatives,
Scheme 1 The synthetic route
of vanillin derived cationic
surfactants
H
O
C
NH2
N
+
CH3
O
OH
reflux in ethanol
0.1% p-toluene sulfonic acid
CH3 (CH2) OH
+
Br CH2 COOH
-H2O
Esterfication (100mL Xylene)
0.1% p-toluene sulfonic acid
-H2O
H
HO
C
N
O
CH3 (CH2)
O
CO CH2 Br
N
CH3
Reflux for 4 hrs
100 mL acetone
H
C
+
HO
N
-
Br
O
CH3
N
CH2COOCH2(CH2)nCH3
123

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J Surfact Deterg (2011) 14:325–331
71
66
61
56
51
46
41
36
31
26
[14]. The lowest CMC value was obtained at 0.56 mM
corresponding to the VSBH derivative.
VSBO
VSBD
VSBH
The decrease in the CMC values reveal that the mole-
cules tend to form micelles at relatively low concentrations
and that is due to the mutual repulsion between the polar
water molecules and the nonpolar hydrophobic chains.
Higher repulsion occurred between the longer hydrophobic
chain (SBVH) and the water phase, as a result, the CMC
value decreased considerably [14].
The CMC values varied linearly with the variation of the
hydrophobic chain length indicating that the relation
between them is that in Eq. 1 [14]:
-5.5
-4.5
-3.5
-2.5
-1.5
-1
Log C, ML
Log CMC ¼ A þ BN
ð1Þ
where, A and B are constants reflecting the free energy
changes involved in transferring the hydrophilic group and
a methylene unit of the hydrophobic group from the
aqueous phase to the micellar phase.
Fig. 1 Surface tension versus log C profile of the synthesized
°
vanillin-derived cationic surfactants at 25 C
respectively. On the other hand, the efficiency as a measur-
able parameter to compare the surface activity of different
surfactants showed that the octyl derivative has the lowest
value, i.e., low concentration of vanillin derived cationic
surfactant containing the octyl chain has the ability to
depress the surface tension of its solution to 52 mNm-1 than
the derivatives containing dodecyl or hexadecyl chains.
It is clear that the critical micelle concentration values of
the synthesized surfactants obtained from Fig. 1 have a
descending trend by increasing the hydrophobic chain
length. That shows the ability of these surfactants to mic-
ellized at relatively low concentrations. The formation of
micelles is a reflection for the repulsion occurring between
the different hydrophobic chains and the aqueous medium
Figure 2 shows the effect of the hydrophobic chain
lengths of the synthesized cationic surfactants at 25 °C on
their critical micelle concentration values. The gradual
increase in the hydrophobic chain length of the synthesized
cationic surfactants gradually decreases their critical micelle
concentration as listed in Table 2. The hexadecyl derivative
(VSBH) had the lowest CMC values (0.56 mmol L-1) at
25 °C compared to the other homologues.
The accumulation of surfactant molecules at the air–
water interface is calculated according to the dc/dlogC term
(surface pressure) which describes the change in the sur-
face tension by finite change of the surfactant concentra-
tion. Increasing the surface pressure of the system indicates
Table 1 Properties of the synthesized compounds
Compound
Mol. Formula
M. Wt. (gm/mole)
Yield (%)
C%
H%
N%
Br%
Calc.
Calc.
Calc.
Found
Calc.
Found
Calc.
Found
VSB
OB
C13H12N2O2
228.25
251.16
307.27
363.38
479.42
535.52
591.63
92
95
95
95
90
91
91
68.41
47.82
54.72
59.50
57.60
60.51
62.91
67.32
47.49
54.34
59.08
57.2
5.30
7.63
8.86
9.71
6.52
7.40
8.00
5.06
7.57
8.80
9.64
6.47
7.21
7.89
12.27
–
12.19
–
–
–
C
C
C
C
C
C
10H19O2Br
31.81
26.00
21.99
16.67
14.90
13.51
31.53
25.82
21.84
16.51
14.72
13.40
DB
14H27O2Br
–
–
HB
18H35O2Br
–
–
VSBO
VSBD
VSBH
23H31O4N2Br
27H39O4N2Br
31H47O4N2Br
5.84
5.23
4.77
5.79
5.11
4.68
60.10
62.42
o
Table 2 Surface and thermodynamic properties of the vanillin derived surfactants at 25 C
Compound CMC, mM cIT, mNm-1
p C
cmc, mN m-1 Pc20, mM dc/dlogC max, mol m-2 Amin, A2 DGmic, kJ mol-1 DGads, kJ mol-1
VSBO
VSBD
VSBH
2.50
1.51
0.56
4
3
2
30.5
34.0
42.0
0.026
0.067
0.135
-11.02 0.96
-13.61 1.18
-15.52 1.34
173.9
140.8
123.6
-29.97
-32.91
-36.89
-33.05
-36.19
-40.04
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J Surfact Deterg (2011) 14:325–331
329
3
2.5
2
In principle, the stronger the adsorption tendency is the
better the surfactant. The degree of surfactant concentra-
tion at a boundary depends on several factors including:
chemical structure of surfactants, nature of phases, con-
centration of the surfactant, chemical structure of the
nonpolar phase and temperature. The interfacial tension
values of the vanillin derived cationic surfactants were
measured between aqueous surfactant solution (0.1% wt)
and light paraffin oil at 25 °C, Table 2. Inspecting the
interfacial tension data leads to two points. First, the octyl
chain derivative (VSBO) has a high interfacial tension
compared to the long chain derivatives. Second, the
dodecyl and hexadecyl derivatives (VSBD, VSBH) showed
low interfacial tension values at 3 and 2 mNm-1, which is
lower than the conventional cationic ammonium surfactant.
This materializes in their applicability to interfacial appli-
cations including: corrosion inhibition, antimicrobial
agents and phase transfer processes.
1.5
1
0.5
0
6
8
10
12
14
16
18
Carbon chain length, m
Fig. 2 Relation between CMC values and the carbon chain length of
the vanillin-derived cationic surfactants
the high accumulation of surfactant molecules at the
interface, Table 2. The maximum surface excess values
(Cmax) of the surfactant solutions were calculated (Table 2)
according to Gibb’s adsorption equation (Eq. 2) [15].
Thermodynamics of Adsorption and Micellization
The thermodynamic characteristics of vanillin cationic
surfactants were studied using the calculated values of
Gmax ¼ ꢁð1=nRTÞðdc=d ln CÞ
Where, dc/dln C is the maximum slope and R, T, C are
the gas constant, temperature and concentration.
ð2Þ
adsorption and micellization free energies DGads, DGmic
These data were calculated using the thermodynamic
.
Eqs. 3, 4, [10, 14] Table 2.
It is clear that the maximum surface excess values are
increased gradually by increasing the hydrophobic chain
length. This indicates the high accumulation of the hexa-
decyl derivative at the interface, which indicates the
maximum compactness of these molecules. As a result, the
adsorbed monolayer formed is expected to be very dense
[16]. The compacted and highly dense adsorbed monolayer
plays an important role in several interfacial applications of
surfactant solutions including emulsification, solubiliza-
tion, corrosion inhibition and biological activity.
DGmic ¼ 2:303 RT logðCMCÞ
DGads ¼ DGmic ꢁ ð0:006 Xpcmc X Amin
The micellization free energies of the targeted surfactants
in their aqueous solutions showed negative sign, thus
ð3Þ
ð4Þ
Þ
indicating
a spontaneous micellization process. The
driving force of micelle formation is the repulsion
occurring between the hydrophobic chains and the polar
medium. The stabilization of the micelles formed occurred
through the attraction forces between the positively charged
head groups and the water molecules. The association of the
counter ions (Br-) in the attracted system also increases
the stability of the micelles formed. Obviously, increasing
the hydrophobic chain length increases the negativity of the
DGmic values. This enforces the idea of increasing the
micellization tendency by increasing the number of methyl
groups along the hydrophobic chains. The gradual increase
in the negativity of DGmic agrees with the trend of the critical
micelle concentration values listed in Table 2. On the other
hand, the adsorption free energy change of the studied
surfactants showed an exact response as DGmic by increasing
the hydrophobic chain lengths.
The compacted adsorbed layer restricts the motion of the
surfactant molecules at the interface and forces them to be
located in the minimum area, i.e. the molecules are located
perpendicular to the interface. This suggests the decrease of
the minimum surface area is caused by increasing the
hydrophobic chain length, Table 2.
Interfacial Tension
The adsorption of surfactant molecules at the boundaries
between two phases is to reduce the free energy at these
boundaries. This lowering is expressed or reflected on the
values of the interfacial energy. The term interfacial ten-
sion (cIT) is used instead of interfacial free energy per unit
area. The tendency of molecules towards accumulation at
the interfaces is a fundamental property of surfactant
molecules.
The negativity of both DGads and DGmic indicates that
the two processes occurred spontaneously. The two values
of DGmic and DGads are comparable and refer to the equi-
librium between the two phases of the surfactant molecules
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J Surfact Deterg (2011) 14:325–331
(adsorbed and micellized phase). Also, the slight increase
in DGads values (more negative) may be ascribed to the
tendency of the molecules to adsorb at the air–water
interface until complete surface coverage. Beyond this, the
molecules diffuse to the bulk of their solution to form
micelles. Hence, the micellization and adsorption processes
are governed by the thermodynamic aspects. The chemical
structure of these molecules is the main factor influencing
their thermodynamic aspects.
global environments. It may be defined as the destruction
of chemical compounds by the biological action of living
organisms. The biodegradability of the derived surfac-
tants was evaluated by applying the Modified Screening
Test [12]. In this test, the final biodegradation or min-
eralization of the surfactants (i.e., the microbial trans-
formation of the parent chemical into final products of
the degradation process, such as carbon dioxide and
water) was evaluated. In the course of the biodegradation
test, the DOC (dissolved organic carbon) concentrations
were determined at the beginning and at regular time
intervals for a 28 day period. Biodegradation was stated
as the percentage of DOC removal within 28 days
(Fig. 4).
Emulsification
The tendency to enhance oil-in-water emulsion formation
is one of the main characteristic applications of surface
active agents. Within the different types of surfactants,
anionic surfactant are generally the most useful emulsify-
ing agents. In spite of this, some cationic surfactants play
this role, especially for the formation of short-term emul-
sions. The emulsifying power of the synthesized surfac-
tants was expressed as the time required for the separation
of 90% of the water in the oil–water emulsion. Figure 3
reveals that there is a gradual increase in the emulsification
power of the synthesized surfactants with an increase in the
hydrophobic chain length. The maximum emulsification
power at 1,920 s. is obtained in the presence of hexadecyl
vanillin derived cationic surfactant (VSBH). The results
were strongly related to the surface concentration (Cmax) of
these surfactants at the interface. The high surface con-
centration indicates a high surfactant population at the
boundaries between the oil and water; this consequently
provides more stability to the emulsions formed.
Biodegradation percentages of approximately 81%
after day 28 are exceeded; the specified biodegradation
passes the level in this test (70%), allowing us to classify
the synthesized cationic surfactants as readily biodegrad-
able compounds, i.e., eco-friendly compounds.
These good results contrast with the low biodegradation
level of the classic cationic surfactants of quaternary
ammonium salts [17]. In fact, the chemical structures of the
synthesized cationic surfactants have been designed from a
naturally occurring compound (vanillin) which has the
ability to degrade by the action of the environmental
microorganisms. The simplest pathway of the degradation
considered in case of our compounds is that the microor-
ganisms attach themselves to the vanillin moiety in the
chemical structure and then the fatty alkyl chains follow
the pathway of chain-shortening through the b-oxidation
pathway [18] and finally, the microorganisms completely
degrade the hydrocarbon chain. The alkyl chain length does
not affect the biodegradation level; even a surfactant with
16 carbon atoms in the molecule (which gives it a high
hydrophobic character) exceeds the biodegradation level in
21 days.
Biodegradability Assessment
Biodegradation is the most important characteristic for
the irreversible removal of chemicals from aquatic and
2000
1800
1600
1400
1200
1000
800
100
90
80
70
60
50
40
30
20
10
0
VSBO
VSBD
VSBH
600
400
200
0
8
12
16
0
5
10
15
20
25
30
Hydrophobic chain length
Time (Day)
Fig. 3 Effect of hydrophobic chain length on the emulsification
power of the vanillin-derived cationic surfactants
Fig. 4 Biodegradation of the synthesized cationic surfactants
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331
14. Rosen MJ (1992) Surfactants and interfacial phenomena, 2nd
edn. Wiley, New York
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factants: synthesis, surface activity and antimicrobial applications
against pathogenic and sulfur reducing bacteria in oil fields.
J Adsorp Sci Technol (Accepted manuscript, in press)
12. OECD Chemical group (1993) Ready biodegradability: modified
screening test, method 301 D, OECD revised guidelines for ready
biodegradability. OECD Paris, France
Nadia G. Kandile received her Ph.D. in 1975 from Ain Shams
University in Cairo, Egypt, where she is currently a professor of
applied and environmental chemistry, after being a professor of
organic chemistry. Her research and development activities deal with
designing new chemical compounds using simple technology, envi-
ronmental problems in the petroleum sector, industrial waste water
treatment and drug agents such as those which are antimicrobial-
antifungal and anticancer. She is a visiting Professor for the
Surfactant Institute in Norman OK (USA) on a project on the
prevention of oil pollution by fluorinated surfactants. She is a Titular
Member of the IUPAC Chemistry & Environment division.
Mohamad Atwa Mohamad is an M.Sc. student working as a chemist
in the El-Nasr Company for Chemical Intermediates. His interests are
in synthesis and applications of environmentally friendly compounds
and their uses in industry.
13. Bentiss F, Bouanis M, Mernari B, Traisnel M, Vezin H, Lagrenee
M (2007) Understanding the adsorption of 4H–1,2,4-triazole
derivatives on mild steel surface in molar hydrochloric acid. Appl
Surf Sci 253:3696–3703
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