Self-aggregation of surfactant ethane-1,2-diyl bis(N, N -dimethyl- N -hexadecylammoniumacetoxy) dichloride: Tensiometric, microscopic, and spectroscopic studies

Article abstract of DOI:10.1021/jp5097822

We have investigated the effect of salt additives (NaCl, Na₂SO₄, Na₃PO₄, NaTos, and NaAn) on the aggregation behavior of a cleavable biodegradable ester-bonded dicationic gemini surfactant, ethane-1,2-diyl bis(N,N-dimethyl-N-hexadecylammoniumacetoxy) dichloride (16-E2-16). A multitechnique approach employing tensiometry, fluorescence, proton magnetic resonance (¹H NMR), transmission electron microscopy (TEM), absorption spectrophotometry (UV), and Fourier transform infrared spectroscopy (FTIR) was utilized to probe physicochemical fluctuations. Appreciable changes were observed in various physicochemical parameters, viz., critical micelle concentration (CMC), surface excess concentration (Δ_max), minimum area per headgroup (A_min), free energy of micellization (ΔGmic°), free energy of adsorption (ΔGads°), and aggregation number (N_agg). Counter ions were found to affect through electrostatic and hydrophobic influence obeying the overall trend as NaAn > NaTos > Na₃PO₄ > Na₂SO₄ > NaCl. ¹H NMR, TEM, UV, and FTIR results reveal microstructure evolution and phase transitions. These results thus provide deeper insights in understanding of self-aggregation and microstructure evolution of biocompatible (green) aqueous systems of the gemini surfactant and their implications in the biomedical and pharmaceutical world, which could be helpful to improve their bioavailability and other biochemical aspects like drug delivery and gene transfection.

Full text of DOI:10.1021/jp5097822

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Article
Self-Aggregation of Surfactant Ethane-1,2-diyl bis(N,N-
dimethyl-N-hexadecylammoniumacetoxy) Dichloride:
Tensiometric, Microscopic and Spectroscopic Studies
Mohammad Akram, Imtiyaz Ahmad Bhat, and Kabir-ud-Din
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp5097822 • Publication Date (Web): 09 Jan 2015
Downloaded from http://pubs.acs.org on January 13, 2015
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SelfꢀAggregation of Surfactant Ethaneꢀ1,2ꢀdiyl bis(N,NꢀdimethylꢀNꢀ
hexadecylammoniumacetoxy) Dichloride: Tensiometric, Microscopic and
Spectroscopic Studies
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Mohd. Akram,,† Imtiyaz Ahmad Bhat,† KabirꢀudꢀDin*, ‡`,§
†Department of Chemistry, Aligarh Muslim University, Aligarhꢀ202002, India
‡Department of Applied Chemistry, Aligarh Muslim University, Aligarhꢀ202002, India
Corresponding author
*Eꢀmail: kabir7@rediffmail.com
Tel No: +91ꢀ9412517274
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ABSTRACT
We have investigated the effect of salt additives (NaCl, Na2SO4,Na3PO4, NaTos and NaAn ) on
the aggregation behavior of a cleavable biodegradable esterꢀbonded dicationic gemini
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surfactant, ethaneꢀ1,2ꢀdiyl bis(N,NꢀdimethylꢀNꢀhexadecylammoniumacetoxy) dichloride (16ꢀE2ꢀ
16). A multiꢀtechnique approach employing tensiometry, fluorescence, proton magnetic
resonance (1H NMR), transmission electron microscopy (TEM), absorption spectrophotometry
(UV) and Fourier transform infrared spectroscopy (FTIR) was utilized to probe physicochemical
fluctuations. Appreciable changes were observed in various physicochemical parameters viz.
critical micelle concentration (CMC), surface excess concentration (Γmax), minimum area per
head group (Aꢀꢁꢂ), free energy of micellization (∆Gꢀ° ꢁꢃ), free energy of adsorption (∆Gꢄ°ꢅꢆ) and
aggregation number (Nꢄꢇꢇ). Counter ions were found to affect through electrostatic and
hydrophobic influence obeying the overall trend as: NaAn >NaTos >Na3PO4 >Na2SO4 >NaCl.
1H NMR, TEM, UV and FTIR results reveal microstructure evolution and phase transitions.
These results thus provide deeper insights in understanding of selfꢀaggregation
and
microstructure evolution of biocompatible (green) aqueous systems of the gemini surfactant and
their implications in the biomedical and pharmaceutical world, which could be helpful to
improve their bioavailability and other biochemical aspects like drug delivery and gene
transfection.
Keywords: Esterꢀbonded gemini surfactant, Sodium anthranilate, Sodium tosylate, Critical
micelle concentration, 16ꢀE2ꢀ16 quencher
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1. INTRODUCTION
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Selfꢀaggregation of surfactants is an attractive, appreciative and extremely interesting theme. In
aqueous mixtures it is an outcome of two antagonistic events; constructive event, involving the
removal of nonꢀpolar chains from an aqueous environment and destructive event, arising from
repulsions among ionic head groups. Association of counterions (in ionic surfactants) to the
peripheral regions of aggregates appeases the latter event (destructive event). Physicochemical
calibrations between the two processes (construction and destruction of aggregates) along with
counterion binding govern the birth of aggregate entities. Usually addition of salts decreases the
CMC of ionic amphiphiles. Nature of the counterions has striking say on shape and size of micelles.
The inorganic additives may change cationic surfactant micelles from globular to rodꢀlike or wormꢀ
like micelles.1,2 Wormꢀlike aggregates are often observed with organic salts in ionic surfactant
solutions.3ꢀ6 Kosmotropic (hydrophilic) additives exert their influence through electrostatic
interaction which decreases repulsions among the head groups and hence favors aggregate growth
whileas chaotropic (hydrophobic) counterions register their presence via electrostatic as well as
hydrophobic interactions. This combined effect of organic counterions may finally lead to tight
packing and possible reduced curvature of surfactant aggregates. Cationꢀꢈ interaction between
positive charge of cationic micelle and ꢈꢀelectron ring of organic counter ions also exerts special
influence to the aggregate architecture. Moreover, these ions influence micellization (or micellar
growth) via modification of the water structure. Polar additives (hydrophilic) substantially dissolve
in water while as, presence of nonꢀpolar ones changes the water structure from hexagonal
arrangement to a pentagonal one, with slightly bent and strained hydrogen bonds. This phenomenon
end up with an interstitial accommodation of hydrophobic molecules and lowering of solvent
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dielectrics, which consequently allows the amphiphilic molecules to assemble at higher loadings,
inverse is true for polar ions as they interact via columbic interactions.
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Gemini surfactants consist of two hydrophobic chains and two hydrophilic head groups per
molecule linked by a spacer between the head groups. Compared to single head/single tail
surfactants, gemini surfactants are subject of interest in scientific community today due to their high
surface activity, lower CMC’s, better wetting properties, unusual aggregation morphologies and
wide applications in industry as well as in biomedical research like gene transfection, DNA
extraction, drug delivery agents, protein refolding, etc.7ꢀ11 They have been regularly reported as
future surfactants. However, cytotoxicity, environmental concerns and aquatic toxicity limit the
practical usage of oftꢀchosen ammoniumꢀionꢀbased cationic geminis. Therefore, it is meaningful to
develop surfactants which are biodegradable, ecoꢀfriendly and biocompatible in nature and study
their interaction behavior with inorganic and organic additives in aqueous solution in order to
explore their future utility in industrial and biomedical applications. Taking care of above features, a
biocompatible gemini surfactant12,13 (Scheme 1) containing cleavable diester group in the spacer part
has been synthesized (hereafter it will be referred to as mꢀE2ꢀm (where m= 16 is the number of
carbon atoms in the alkyl tail and E2 represents the diester group in the spacer part of the gemini))
and studied its aggregation behavior in presence of various inorganic and organic counterions. The
16ꢀE2ꢀ16 esterquat has special importance due to having two ester groups (E2). This new diesterꢀ
groupꢀcontaining 16ꢀE2ꢀ16 gemini surfactant has low CMC value (1000 times less than its
conventional homologue, CTAB, both having the same number of carbon atoms in the alkyl
chain), is of cleavable nature, and possesses low cytotoxicity, which can be utilized in several
technical areas including the biomedical application of drug delivery. Moreover, scarce literature
related to interaction of biocompatible ecofriendly cationic gemini surfactants with inorganic and
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organic additives motivated us to carry out this study. Tensiometry, fluorimetry, proton magnetic
resonance (1H NMR) , transmission electron microscopy (TEM), absorption spectrophotometry
(UV) and Fourier transform infrared spectroscopy (FTIR) results confirm formation of wormꢀlike
micelles, which are known for their potential applications such as fracturing fluids, thickeners of
personal care products, drag reducing agents14 and controlled drug release materials. Thus favorable
or synergistic interactions found in 16ꢀE2ꢀ16ꢀadditive mixtures make them even more attractive
and applicable.12, 13
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2. EXPERIMENTAL SECTION
2.1. Materials. Sodium chloride (99.5%, Fisher Scientific, India), sodium sulphate (98%, Merck,
India), sodium phosphate (98%, Sigma Aldrich, USA), sodium anthranilate (98%, CPC,
Philadelphia, Pennsylvania), sodium tosylate (70ꢀ80%, Fluka, Switzerland), hexadecylpyridinium
chloride (monohydrate) (98%,Merck, Germany), N,Nꢀdimethylhexadecylamine (95%, Sigma
Aldrich, USA), ethylene glycol (99%, Sigma Aldrich, USA) and chloroacetyl chloride (98%, Lobaꢀ
chemie, India) were used as received. The gemini surfactant ethaneꢀ1,2ꢀdiyl bis(N,NꢀdimethylꢀNꢀ
hexadecylammoniumacetoxy) dichloride (16ꢀE2ꢀ16 ) was synthesized as shown per Scheme 1. After
1
recrystallization, the surfactant was characterized by H NMR and FTꢀIR.12, 13 The data were in
agreement with the literature values. The purity was further ascertained on the basis of absence of
minima in the surface tension–concentration profiles.15
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OH
Cl
Cl
Cl
O
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50 oC , 8 h
O
O
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+ 2
O
N2
HO
O
Cl
C H
16 33
N+
Cl
O
O
O
O
O
Reflux, 10 h
C16H33
+ 2
N
Ethyl acetate
O
O
O
N+
Cl
C16H33
Scheme 1. Synthesis route of biocompatible gemini surfactant 16ꢀE2ꢀ16.
2.2. Methods. 2.2.1. Surface Tension Measurements. The PtꢀIr ring detachment method of SD
Hardson (Kolkata) tensiometer at 30 °C was employed for tensiometric measurements. Prior to every
measurement the PtꢀIr ring was thoroughly cleaned and dried (over alcohol flame). The tensiometer
was calibrated each time with twiceꢀdistilled water. Solutions of pure or mixed systems were added in
installments, using HamiltonꢀBonaduz, SCHWEIZ microsyringe and the surface tension values were
recorded after thorough mixing and temperature equilibration.
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2.2.2. Fluorimetric Measurements. The Shimadzu spectrofluorimeterꢀ5000 (Japan) was used to obtain
the fluorescence patterns of pure and mixed systems of 16ꢀE2ꢀ16. For the measurements, the
fluorimeter parameters were set as: slit width 3 nm, excitation wave length 337 nm and emission was
observed in the range of 350ꢀ450 nm. Pyrene and hexadecylpyridinium chloride (monohydrate) were
used as probe and quencher, respectively. The quencher concentration was varied but that of pyrene
concentration was kept constant at 3x10ꢀ6 M. Excitation wavelength was fixed at 336 nm and emission
spectra were recorded in the range of 350ꢀ450 nm. For the anthranilate fluorescence the range was
same (350ꢀ450 nm).
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1
2.2.3. Nuclear Magnetic Resonance (NMR) Measurements. The H NMR spectra of the
synthesized gemini surfactant 16ꢀE2ꢀ16 and its electrolyte combinations were recorded on 300 MHz
Bruker Avance NMR spectrometer (Central Drug Research Institute, Lucknow, India) in D2O with 1H
chemical shifts relative to internal standard tetramethylsilane (TMS). The stock solutions of gemini
(in the absence and presence of salts) were prepared in D2O. About 0.5 ml of each solution was
transferred to a 5 mm NMR tube and chemical shifts were recorded on the δ (ppm) scale at 303.15 K.
The line widths at half heights (lw) were measured from the obtained spectra.
2.2.4. TEM measurements. The TEM measurements were carried out on a JEMꢀ2100 (JEOL,
Japan) electron microscope at an accelerating voltage of 200 kV. The samples were prepared by
casting 1 drop of the pure 16ꢀE2ꢀ16 and mixed 16ꢀE2ꢀ16+additive solutions onto a copper grid on a
glass plate. The samples were then dried in air for 1 day.
2.2.5. UV and FTIR Studies. Perkin Elmer Lambda 25 UV/Visible spectrophotometer and Perkin
Elmer (FTIR) spectrometer were used, respectively, to probe the UV and FTIR spectral shifts. Pure
and mixed samples were loaded with requisite concentration and then respective spectra were
recorded (ranges: 3000ꢀ2800 cmꢀ1 (FTIR); 200ꢀ360 nm (UV)).
3. RESULTS AND DISCUSSION
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3.1. Micellization and Interfacial Adsorption. Surface tension profiles of pure 16ꢀE2ꢀ16 and its
combinations with various electrolytes are shown in Figure(s) 1 and S1 (Supporting Information).
From these plots the CMC values have been determined (Table 1) and are plotted against the salt
concentration (Figure 2). Pure 16ꢀE2ꢀ16 shows CMC value of 0.00120 mM, which agrees well with
the literature.13 From Figure 2 and Table 1 it is clear that increase in salt concentrations produces
CMC reducing effect. Further, the decrease in CMC value of surfactant depends upon the type and
nature of the added counterion: POꢉꢊꢋ ions bring much decrease than the SOꢌꢉꢋ and Clꢋ ions with the
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efficacy being POꢊꢉꢋ SOꢌꢉꢋ Clꢋ. The reason may be attributed to electrovalency of the counterion.
> >
Higher the charge density and elecrovalency of the counterion, greater will be the dispersal or
neutralization of positive charge on the amphiphile (16ꢀE2ꢀ16) and lower will be its CMC value. The
organic counter ions (Anꢋ, Tosꢋ) are found to influence the CMC of 16ꢀE2ꢀ16 much more than the
inorganic ones (POꢊꢉꢋ, SOꢉꢌꢋ, Clꢋ). Sodium anthranilate shows much more decrease than the sodium
tosylate. The reason is that sodium tosylate has sulphonate group which is less hydrophilic than
carboxylate group (Scheme 2 (a) and 2 (b)). Presence of a more hydrophilic group induces stronger
screening of electrostatic repulsions among head groups of 16ꢀE2ꢀ16 micelles, thus lowering CMC
readily than lessꢀhydrophilicꢀgroupꢀcontaining additives.16 Organic (aromatic) counterions affect the
CMC in two ways, via (a) hydrophobic interaction, and (b) electrostatic interaction. These
counterions intercalate and generate strong hydrophobic interaction between the alkyl chain of
surfactant and phenyl ring of additive. Besides these two interactions, the decrease in CMC due to
organic counterions may also be ascribed to cationꢀ
ꢈ interaction of organic additives with the
positive charge of gemini (Scheme 2(d)). The overall order of organic counterions to decrease CMC
of 16ꢀE2ꢀ16 gemini surfactant is: Anꢋ Tosꢋ . Also, presence of ꢀCH3 in Tos─ facilitates deeper
>
p
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penetration and hence interaction (a) is increased but, at the same time, the interaction (b) is
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decreased
−
consequently, it is less effective (visꢀ aꢀvis An─ ).
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[NaCl]
5 mM
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10 mM
20 mM
40 mM
-7.2
-6.8
-6.4
-6.0
-5.6
-6.8
-6.4
-6.0
-5.6
log [16ꢀE2ꢀ16] (mM)
log[16-E2-16] (mM)
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[Na3PO4]
mM
[Na2SO4]
5 mM
5
10 mM
20 mM
40 mM
10 mM
20 mM
40 mM
-6.8
-6.4
-6.0
-5.6
-6.8
-6.4
-6.0
-5.6
log[16-E2-16] (mM)
log[16-E2-16] (mM)
Figure
1
.
Surface tension profiles of 16ꢀE2ꢀ16 and its various electrolyte combinations.
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NaCl
Na2SO4
0.0012
0.0010
0.0008
0.0006
0.0004
Na3PO4
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NaAn
NaTos
0
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40
[Salt] (mM)
Figure 2. Variation of CMC of 16ꢀE2ꢀ16 gemini surfactant with concentration of added salts
(NaCl, Na2SO4, Na3PO4, NaAn, NaTos).
-
SO3 Na+
COO-Na+
a
NH2
a
a
f
b
c
f
b
c
f
b
c
e
e
e
d
d
d
Cationꢀ
interaction between 16ꢀE2ꢀ16 and organic additive
π
g CH3
(c)
(d)
(b)
(a)
Scheme 2. Structure and various interactions existing between the components of mixture: (a)
sodium anthranilate (NaAn) (b) sodium tosylate (NaTos) (c) 16ꢀE2ꢀ16 gemini surfactant (d) cationꢀ
ꢈ
interactions existing in the amphiphileꢀadditive mixtures.
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Surface excess concentration (Γꢀꢄꢍ) of the gemini surfactant molecules at the air/solution
interface was calculated by using Gibbs equation (1)17
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ꢎ
ꢔꢕ
Γꢀꢄꢍ = − ꢌ.ꢊꢏꢊꢐꢑꢒ ꢓꢔꢖꢗꢇꢘ
ꢙ
(1)
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where
R
is the gas constant (8.314 J molꢀ1 Kꢀ1) and
T
is the temperature in Kelvin. The factor
n
is the
number of species at the air/solution interface. For the pure divalent gemini,
n is taken as 3 (the
divalent amphiphile and two counterions) 18ꢀ19 and 4 for gemini+salt systems. As per equation(1), the
slope of the surface tension (γ) versus log[surfactant] plot was used to obtain Γꢀꢄꢍ and the values are
given in Table 1. From the Γꢀꢄꢍ value increase with increase in the concentration of additives we
can infer that at the air/solution interface more gemini surfactant molecules are adsorbed.
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Table 1. Various Thermodynamic Parameters (γcmc, Πcmc, Гmax, Amin, CMC, ꢁGomic, ꢁGoads) for 16ꢀE2ꢀ16+Salts Mixed Systems
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at 30 °C, Evaluated on the Basis of Surface Tension Measurements
∆Gꢀ° ꢁꢃ
∆Gꢄ°ꢅꢆ
[Salt]
(mM)
γCMC
Π CMC
Γ max
A min
CMC
(mM)
(mN mꢀ1)
(mN mꢀ1) (107molecule mꢀ2) (Å2 moleculeꢀ1)
(kJ molꢀ1)
(kJ molꢀ1)
9
NaCl
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47
48
49
0.00120 ± 1(10ꢀ4)
(0.00126a, 0.00128b)
0.00107 ± 6(10ꢀ5)
0.00095 ± 5(10ꢀ5)
0.00081 ± 5(10ꢀ5)
0.00072 ± 4(10ꢀ5)
0
5
10
20
40
42.6
38.2
40.1
40.3
38.2
28.7
32.8
30.9
30.8
32.8
11.0
11.3
11.6
12.7
13.0
146
142
135
133
130
ꢀ44.4 ± 0.05
ꢀ44.7 ± 0.15
ꢀ45.0 ± 0.10
ꢀ45.4 ± 0.15
ꢀ45.7 ± 0.15
ꢀ47.6 ± 0.1
ꢀ48.3 ± 0.20
ꢀ48.1 ± 0.10
ꢀ48.2 ± 0.15
ꢀ48.4 ± 0.12
Na2SO4
0.00120 ± 1(10ꢀ4)
(0.00126a, 0.00128b)
0.00087 ± 3(10ꢀ5)
0.00071 ± 1(10ꢀ5)
0.00065 ± 1(10ꢀ5)
0.00052 ± 1(10ꢀ5)
0
5
10
20
40
42.6
31.0
39.2
35.1
35.5
28.7
39.9
31.9
35.8
35.9
11.0
11.7
12.3
12.9
13.8
146
133
126
122
115
ꢀ44.4 ± 0.05
ꢀ45.4 ± 0.1
ꢀ45.8 ± 0.05
ꢀ46.1±0.05
ꢀ46.2±0.7
ꢀ47.6 ± 0.1
ꢀ49.6 ± 0.1
ꢀ49.1 ± 0.01
ꢀ49.3 ± 0.30
ꢀ49.1 ± 0.05
Na3PO4
0.00120 ± 2(10ꢀ4)
(0.00126a, 0.00128b)
0.00079 ± 6(10ꢀ5)
0.00067 ± 5(10ꢀ5)
0.00051 ± 5(10ꢀ5)
0.00045 ± 5(10ꢀ5)
0
5
10
20
40
42.6
36.0
39.1
40.6
43.1
28.7
34.9
31.9
30.8
27.9
11.0
12.2
12.6
13.9
14.7
146
132
123
119
113
ꢀ44.4±0.05
ꢀ45.5±0.2
46.1±0.4
ꢀ46.7±0.2
ꢀ46.9±0.2
ꢀ47.6 ± 0.10
ꢀ48.7 ± 0.60
ꢀ48.8 ± 0.10
ꢀ49.3 ± 0.20
ꢀ49.2 ± 0.60
NaAn
0.00120 ± 2 (10ꢀ4)
(0.00126a, 0.00128b)
0.00062 ± 3(10ꢀ5)
0.00061 ± 1(10ꢀ5)
0.00049 ± 2(10ꢀ5)
0
5
10
20
42.6
35.9
36.6
41.2
28.7
35.1
34.4
29.7
11.0
12.9
13.8
15.2
146
126
118
110
ꢀ44.4±0.05
ꢀ46.1±0.11
ꢀ46.3 ± 0.11
ꢀ46.9 ± 0.11
ꢀ47.6 ± 0.10
ꢀ49.4 ± 0.11
ꢀ49.3 ± 0.11
ꢀ49.1 ± 0.11
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NaTos
0.00039 ± 2(10ꢀ5)
43.0
27.9
17.1
99
ꢀ47.5 ± 011
ꢀ49.3 ± 0.11
0.00120 ± 2 (10ꢀ4)
(0.00126a, 0.00128b)
0.00063 ± 2(10ꢀ5)
0.00061 ± 3(10ꢀ5)
0.00050 ± 1(10ꢀ5)
0
5
10
20
42.6
38.2
43.5
42.0
28.7
37.5
37.6
39.0
11.0
12.6
13.4
14.9
146
129
120
112
ꢀ44.4±0.05
ꢀ46.0±0.05
46.2±0.05
ꢀ46.5±0.05
ꢀ47.6 ± 0.11
ꢀ49.2 ± 0.11
ꢀ49.3 ± 0.10
ꢀ49.5 ± 0.04
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turbid
turbid
turbid
turbid
turbid
turbid
turbid
The CMC values with superscripts a,b represent literature values (Ref.13, Ref.15)
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Aꢀꢁꢂ is the minimum area per head group. It suggests whether surfactant molecule is closely or
loosely packed at the air/solution interface. Its v
alue is evaluated by using the following equation20
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15
16
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22
23
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45
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ꢛꢜ
ꢎꢏ
(2)
ꢚꢀꢁꢂ
=
ꢝ ꢟ
ꢞ ꢠꢡꢢ
where ꢣꢤ is the Avogadro’s number (6.023x1023 molꢀ1). The Aꢀꢁꢂ values were found to decrease
with an increase in the salt concentration (Table 1). It means that the packing density of surfactant
monomers increases. The decrease is appreciable in 16ꢀE2ꢀ16 + organic salt systems than 16ꢀE2ꢀ16+
inorganic salt systems. Overall, order of packing density follows the trend as: 16ꢀE2ꢀ16+NaAn>16ꢀ
E16+NaTos>16ꢀEꢀ216+Na3PO4>16ꢀE2ꢀ16+Na2SO4 >16ꢀE2ꢀ16+NaCl.
Standard Gibbs energy of micellization (∆Gꢀ° ꢁꢃ is calculated by using equation (3)21
)
∆ꢥꢀ° ꢁꢃ = ꢦꢧlnXꢃꢀꢃ
(3)
Negative values of ∆Gꢀ° ꢁꢃ (Table 1) reflect that the micellization is spontaneous. Spontaneity in 16ꢀ
E2ꢀ16 + organic salt systems is significant than 16ꢀE2ꢀ16 + inorganic salt mixtures, following the
overall trend as 16ꢀE2ꢀ16 + NaAn >16ꢀE2ꢀ16 + NaTos> 16ꢀE2ꢀ16+Na3PO4>16ꢀE2ꢀ16+Na2SO4 >16ꢀ
E2ꢀ16 + NaCl.
Standard Gibbs energy of adsorption, ∆ꢥꢄ°ꢅꢆ , has been evaluated using equation(4)22
∆ꢥꢄ°ꢅꢆ = ∆ꢥꢀ° ꢁꢃ − (Пꢟ
)
(4)
ꢨꢠꢨ
ꢠꢡꢢ
where
Πcmc/Гmax implies work involved in transferring the surfactant molecule from monolayer at
zero surface pressure to micelles. The negative values of ∆ꢥꢄ°ꢅꢆ indicate spontaneous adsorption at
air/water interface. Average values of ∆ꢥꢄ°ꢅꢆ were found to obey the trend as: 16ꢀE2ꢀ16+NaAn>16ꢀ
E2ꢀ16+NaTos>16ꢀE2ꢀ16+Na3PO4>16ꢀE2ꢀ16+Na2SO4>16ꢀE2ꢀ16+NaCl. The magnitude of ∆Gꢄ°ꢅꢆ is
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greater than ∆Gꢀ° ꢁꢃ which implies that the micellization of 16ꢀE2ꢀ16 is secondary process and less
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6
7
23, 24
spontaneous than adsorption. Moreover, as per the Nagarajan’s model,
∆ꢥꢀ° ꢁꢃ can be split into
8
9
electrostatic as well as hydrophobic contributions. According to this model, there are four different
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°
∆ꢩ
ꢪ
contributions to the resultant standard free energy change
(
)
associated with the formation of
ꢫꢒ
micelle of surfactant from its infinitely dilute state to an aggregate size of g:
°
∆ꢩ
ꢪ
(a) contribution of surfactant tail transfer from solution to the aggregate core,
(
)
,
ꢬ
ꢫꢒ
°
∆ꢩ
ꢪ
(b) contribution of water molecule penetration to the aggregate core,
(
)
,
ꢭ
ꢫꢒ
°
∆ꢩ
ꢪ
(c) contribution of the repulsive interaction between the head groups at the aggregate,
(
)
, and
ꢮ
ꢫꢒ
°
ꢰ
∆ꢯ
(d) contribution of packing of monomer within the aggregate core, ( ꢱꢧ )P
.
The Gibbs free energy of micellization is thus
°
°
°
°
∆ꢥꢀ° ꢁꢃ=exp[(∆ꢫꢩꢒꢪ)ꢬ + (∆ꢩꢪ)ꢭ + (∆ꢩꢪ)ꢮ + (∆ꢩꢪ)ꢲ]× 55.55
(5)
ꢫꢒ
ꢫꢒ
ꢫꢒ
Detailed computations were made to obtain the individual contributions using literature values of the
required parameters.23ꢀ25. The results are summarized in Table 2 which correlates well with the
experimental data of Table 1
Table 2. The Various Contributions to Total Gibbs Free Energy Change (∆Gomic) as per
Nagarajan Model
∆ꢯꢳ°
ꢱꢧ
∆ꢯꢳ°
ꢱꢧ
∆ꢯꢳ°
ꢱꢧ
∆ꢯꢳ°
ꢱꢧ
∆ꢥꢀ° ꢁꢃ
16ꢀE2ꢀ16
(kJ molꢀ1)
−(
)
ꢬ
(
)
(
)
(
)
ꢲ
ꢭ
ꢮ
NaCl
0
5
46.60
46.60
46.60
0.40
0.42
0.40
0.20
0.42
0.40
0.036
0.039
0.044
ꢀ44.4
ꢀ44.7
ꢀ45.0
±
±
±
0.05
0.15
0.10
10
20
40
46.60
46.60
0.39
0.38
0.39
0.38
0.040
0.042
ꢀ45.4
ꢀ45.7
±
±
0.15
0.15
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Na2SO4
0
5
46.60
46.60
46.60
46.60
46.60
0.40
0.39
0.37
0.36
0.34
0.20
0.39
0.37
0.36
0.34
0.036
0.041
0.044
0.045
0.046
ꢀ44.4
ꢀ45.4
ꢀ45.8
ꢀ46.1
ꢀ46.2
±
±
±
±
±
0.05
0.1
0.05
0.05
0.70
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8
9
10
20
40
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60
Na3PO4
0
46.60
46.60
46.60
46.60
46.60
0.036
0.0042
0.0045
0.046
ꢀ44.4
ꢀ45.5
ꢀ46.1
ꢀ46.7
ꢀ46.9
±
±
±
±
±
0.05
0.20
0.40
0.20
0.20
5
10
20
40
NaAn
0
5
10
20
40
NaTos
0
5
10
20
40
0.39
0.36
0.35
0.33
0.39
0.36
0.35
0.33
0.049
46.60
46.60
46.60
46.60
46.60
0.40
0.37
0.35
0.32
0.29
0.20
0.37
0.35
0.32
0.29
0.036
0.044
0.047
0.050
0.056
ꢀ44.4
ꢀ46.1
ꢀ46.3
ꢀ46.9
ꢀ47.5
±
±
±
±
±
0.05
0.11
0.11
0.11
011
46.60
46.60
46.60
46.60
turbid
0.40
0.38
0.35
0.33
turbid
0.20
0.38
0.35
0.33
turbid
0.036
0.043
0.046
0.049
turbid
ꢀ44.4
ꢀ44.4
ꢀ46.0
ꢀ46.2
±
±
±
±
0.05
0.05
0.05
0.05
turbid
3.2. Fluorescence Studies (Miceller Aggregation Number, Nagg). Steadyꢀstate fluorescence
quenching was used to quantify micellar aggregation. The fluorophore pyrene and the quencher
hexadecylpyridinium chloride were assumed in the core of 16ꢀE2ꢀ16 micelles, obeying well the
Poissons distribution and inner filter effects were considered absent. The measurements were fitted
in equation (6) 26
ꢴꢵ ꢶꢷꢷꢜꢸ = ꢣꢄꢇꢇ
[ꢹ]
(6)
[ꢘ ꢋꢘꢻꢘ]
ꢺ
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and slopes of linear plots (Figure 3) obtained between ln[I0/I] versus concentration of
hexadecylpyridinium chloride [ꢼ] were used to calculate the aggregation number, Nagg.( number
6
7
8
9
of monomer units forming aggregate entity ), of the 16ꢀE2ꢀ16 micelles. From the Figure 4 it is
clear that increase in the concentration of additives (inorganic/organic) increases the aggregation;
which is more prominent with organic counter ions than the inorganic ones. Among inorganic
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counterions the trend to increase aggregation number follows the order: POꢊꢉꢋ SOꢉꢌꢋ Clꢋ. A
> >
higher value for POꢊꢉꢋ is attributed to higher electrovalency and charge density than SOꢌꢉꢋ and Clꢋ
ions. In other words, we can say that POꢊꢉꢋ exerts stronger electrostatic interaction (or screens
repulsions) among head groups of 16ꢀE2ꢀ16 entities, hence results in higher value of aggregation
number. Organic salt NaTos has been found to increase aggregation number much more than the
inorganic ones (Figure 4). It is due to the fact that Tos─ has additional hydrophobic interaction
besides electrostatic one. Due to the hydrophobic ring, Tos─ intercalates into the interior of micelle
and aligns sulfonate group in the outer hydrophilic shell of the micelle, resulting in a sort of mixed
micelle formation. This penetration results in charge neutralization in the head group region of the
micelle along with simultaneous increase in hydrophobic interactions between the surfactant tail
and the hydrophobic ring of Tos─. Both of these interactions favor micellization and hence result
in increase in the aggregation number (Nagg). Thus the overall trend for counterions to increase the
aggregation number of 16ꢀE2ꢀ16 surfactant was found to be Tosꢋ > ꢽꢾꢉꢊꢋ SOꢉꢌꢋ Clꢋ. On
> >
comparing these results with tensiometric results, both are in corroboration. Tensiometry results
reveal much lower CMC values at higher loadings of salt, which over here is reflected by higher
aggregation numbers. Thus on the whole both techniques support the view point that concentrated
1
mixtures are enriched with grown 16ꢀE2ꢀ16 aggregates. Furthermore, proceeding H NMR results
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are in total complement with above techniques. Merging and peak disappearances at higher
loadings evidence the bulky micelles and grown micelles.
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1.5
1.2
0.9
0.6
5mM NaCl
10mM NaCl
30mM NaCl
40mM NaCl
0.3
0.0
0.00 0.01 0.02 0.03 0.04 0.05
[Q] (mM)
Figure 3. Variation of ln(I0/I) versus quencher concentration (Q) for 16ꢀE2ꢀ16 (0.1 mM) at
different concentrations of NaCl
80
70
60
50
NaCl
Na2SO4
40
Na3PO4
30
NaTos
0
5 10 15 20 25 30 35 40 45
[additive] (mM)
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Figure 4. Variation of aggregation number (Ngg) of 16ꢀE2ꢀ16 gemini surfactant with concentration
4
5
6
of added salts (NaCl, Na2SO4, Na3PO4, NaTos).
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To reveal the interaction between 16ꢀE2ꢀ16 micelles and An─ counterions, fluorescence
emission peak of NaAn in the range of 350ꢀ450 nm was obtained (Figure 5). It is clear that the
fluorescence intensity decreases with an increase in concentration of gemini (16ꢀE2ꢀ16) micelles.
Hence 16ꢀE2ꢀ16 micelles act as quencher. The quenching pattern obtained can be attributed to
the formation of nonꢀfluorescent complex between NaAn and 16ꢀE2ꢀ16 gemini micelles or it
may be allocated to various molecular associations namely excited state reactions, energy
transfer, molecular rearrangements, ground state complex formation or collision quenching.
Thus, 16ꢀE2ꢀ16 gemini surfactant can substitute conventional quenchers as it is environmentally
green and ecoꢀfriendly. It is to be noted that it was not possible to calculate the aggregation
number of anthranilate, because of absence of characteristic spectra of pyrene (5 peaks).
Therefore, its interaction has been studied separately.
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a
9000
6000
I
0 mM 16-E2-16
0.121 mM
3000
0.238 mM
0.348mM
0.454mM
0.555mM
0.652mM
0
0.744mM
0.833mM
340 360 380 400 420 440
Wave length (mM)
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Figure 5. Fluorescence emission spectra of sodium anthranilate in presence of varying
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concentration of 16ꢀE2ꢀ16.
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1
3.3. H NMR Spectroscopy. NMR is a versatile technique for study of surfactants and their
additive systems. It provides information at molecular level that is not available by other
spectroscopic methods. Characterization of micellization process, hydration of micelles and
interaction of micelles with additives (inorganic, organic and biological) are studied by this
sophisticated technique, as it provides deeper insights of aggregate architecture. Aggregate
morphological transitions are sensed by peak broadening and line width fluctuations.27 Sphereꢀtoꢀ
rod phase trasitions are visualized by peak disappearances.
The 1H NMR spectrum of pure 16ꢀE2ꢀ16 (0.1 mM) in deuterated water (D2O) is shown in Figure
6. The concentration of gemini surfactant (16ꢀE2ꢀ16) is much higher (
~100 times) than its CMC
(0.00120 mM) value, so the chemical shifts observed can be of micellized form. Signal assignments
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Figure 6. 300 MHz 1H NMR spectrum of 16ꢀE2ꢀ16 (0.1 mM) in D2O at 30 °C.
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agree well with the literature.26 Downfield shift in delta (ꢿ) values was observed for almost all
protons of 16ꢀE2ꢀ16 upon addition of NaCl (Table S1). At higher concentrations, the downfield shift
becomes almost constant. This invariance at higher concentrations may be attributed to saturation of
double layer of micellar aggregates of 16ꢀE2ꢀ16, consequently more chloride (Clꢋ) ions reside in the
bulk solution as free ions and encourage deshielding and discourage shielding. The encouraged
effect lets the protons of gemini sense more magnetic influence, hence signal shifts to downfield.
The 1H NMR resonance values of 16ꢀE2ꢀ16 in presence of SOꢌꢉꢋ counterions shift downfield for
1
2
3
7
5
protons H, H, H,6H and H whereas protons 4H and H show upfield shift (see Figure 6 for
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assignments). Upfield shift of near head group protons (4H, 5H) have the same reasons of exertion of
electrostatic repulsion, shielding and screening contributions. Micellar core protons’ (1H,2H,3H)
downfield shifts are attributed to hydrophobic environment.
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The chemical shift values of Hꢀ5H protons of 16ꢀE2ꢀ16 in presence of POꢊꢉꢋ counterion are
6
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upfield but H and H protons show resonance disappearance. The upfield shift may be assigned to
charge neutralization on head groups of gemini aggregate entities, which reduces the deshielding and
increases shielding. Due to charge dispersal the electron withdrawing ability of nitrogen of head
groups mitigates and hence results in shielding and upfield shift. Signal disappearance of H and 7H
6
protons may be attributed to tight binding of counterions at higher loadings and restricted motions of
rodꢀshaped micelles and newer phase formations. Trend in shifting the NMR signal follows the
order POꢊꢉꢋ > SOꢌꢉꢋ > Clꢋ. Here too, higher electrovalence and charge density on the counterion
accounts for the observed order.
In regard to the inorganic ones, the organic counterions (due to their dual influenceꢀelectrostatic
as well as hydrophobic) produce quite appreciable effects on the NMR signals of microstructures.28
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With increase in concentration of sodium anthranilate, peaks of H, H, H and H protons of the
concerned gemini surfactant move towards lower side of
ꢿ scale, i.e, upfield shift (Table S1). In
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1
2
4
addition, signal disappearance is observed for protons H and H. The upfield shift for H, H, H
and 5H protons can be ascribed to combined space and compensation effects exerted by
aromatic ring currents of hydrophobic moieties on 16ꢀE2ꢀ16 esterquat or charge dispersal
phenomenon on head groups regions. As a result, tight packing of gemini monomers takes place in
presence of anthranilate and tosylate, which inspires continuous decrease. Signal disappearance for
−
N+CH3 and
ꢿ
6H and 7H and peak merging of 3H with 2H proton is due to formation of newer phases.
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With increase in concentration of sodium tosylate, 1Hꢀ7H protons, in general, move towards lower
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value of delta scale, i.e., upfield shift. Signals of H, H and H at higher loadings show signal
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disappearance due to the above discussed reasons.
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Further, H NMR signals of Nꢀmethyl protons were used to calculate line widths at halfꢀheights of
signals. Line widths were found to increase (although increase is marginal due to rigidity of micelles)
with an increase in the concentration of additive (Figure 7). Peak broadening is prominent in presence
of POꢉꢊꢋ than SOꢉꢌꢋand Clꢋ. Higher charge density and electrovalence of counter ions result in the
formation of large and bulky micelles, increasing endꢀoverꢀend tumbling motions and collisions27,
hence peak broadens and line width at halfꢀheight of resonance signal increases. Signal disappearance
at higher loadings of POꢊꢉꢋ is due to highly restricted motions of the aggregates . Line width increase
is more prominent in sodium anthranilate than sodium tosylate (Figure 7); an orthoꢀNH2 group in the
former may account for the same. It facilitates binding of COO─ with positively charged head group
of the surfactant through resonance of loneꢀpair with ─NH2 group, resulting in attenuation of head
group repulsions of 16ꢀE2ꢀ16 micelles, thus favoring micellar growth and evolution of nonꢀspheroid
(rod shaped) microstructures. The comparative line width increase follows the trend Anꢋ > ꢧꣀꣁꢋ >
POꢊꢉꢋ > SOꢌꢉꢋ > Clꢋ as shown in Figure 7.
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0
NaCl
Na2SO4
Na3PO4
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NaAn
NaTos
0
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20
30
40
[Additive] (mM)
1
Figure 7. Influence of additive concentration on variation in line width at halfꢀheight of the H
NMR signal corresponding to the
−
N+CH3 group of 16ꢀE2ꢀ16 (0.1 mM).
1
To get better perspective delta (ꢿ) values obtained from H NMR, the spectra of ring protons
b
of aromatic counterions were analyzed (Table 3). The H proton peaks due to –NH2 group of
anthranilate are not apparent. The nonappearance may be due to the labile nature and are supposed to
get exchanged with the resonances of D2O solvent peaks. In sodium anthranilate, the signal
e
d
f
resonances appear at
ꢿ
6.821, 7.255, 7.720 for (cH H), H and H ring protons, respectively (Table
3). It is clear from Table 3 that on increasing the concentration of salt (NaAn), the
ꢿ
value shifts
upfield for (cH eH), dH protons whileas fH protons show downfield shift. This infers that anthranilate
anion intercalates in to the palisade layer of 16ꢀE2ꢀ16 micelles; the reason for this behavior may be
attributed to nonpolar and polar environments experienced by cH,dH,eH and gH protons, respectively,
near palisade and headgroup regions. Generally, nonpolar environment enhances local magnetic
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effects in a direction opposite to applied magnetic influences, invigorates shielding; hence upfield
shift, the inverse is true for polar environment.
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Table 3. H NMR Chemical Shifts of Sodium Anthranilate (NaAn) and Sodium Tosylate
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(NaTos) in Presence of (0.1 mM) 16ꢀE2ꢀ16 Gemini Surfactant at 25 °C
[Salt] (mM)
[NaAn]
[16ꢀE2ꢀ16] (mM)
Chemical shift values (ꢿ, ppm)
c,e H
6.821
6.801
6.798
6.793
6.790
b,fH
7.689
7.690
7.692
7.696
7.702
dH
fH
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10
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40
0.0
0.1
0.1
0.1
0.1
7.255
7.250
7.244
7.243
7.241
c,eH
7.390
7.381
7.374
7.351
7.312
7.720
7.723
7.725
7.726
7.729
g H
[NaTos]
1
0.0
0.1
0.1
0.1
0.1
2.41
5
10
20
40
2.390
2.387
2.384
2.362
e
In sodium tosylate, (bHfH), (cH H) and (gH) protons resonate at ꢿ 7.689, 7.390, 2.41,
respectively (Table 3). From the Table it can be also inferred that, with increment in additive
e
g
concentration, the (cH H) and H protons shift upfield, confirming binding and intercalation of
f
tosylate ion into micellar interior, whileas (bH H ) protons show downfield shift (because of the
presence of SO3─ ). Moreover, the consistent upfield shift observed in trimethylammonium protons
signals confirms the fixation of counterions between the surfactant head groups (Table S1).
The above comparative insights reveal that, among inorganic counterions, higher electrovalent
ones are effective in bringing micellar growth and newer phase formation (sphericalꢀtoꢀrod
transitions). Organic counterions cause quite apparent growth, confirmed by clear merging,
disappearances and broadening of signals; the reason being more effective toward the interactions is
obviously due to the systems being affected by coulombic and hydrophobic interactions (the
inorganic anions interact only coulombically). Further sodium anthranilate was found more
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effective than sodium tosylate. Orientation of substituents and loneꢀpair effect may account for and
be responsible. Thus, our results can help researchers to design additives with proper substitution so
that ecoꢀfriendly and biocompatible microstructure/architecture can be tuned in accordance with
need of interest for biomedical research and drug delivery related phenomena.
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3.4. Transmission Electron Microscopy (TEM). To further ascertain the interaction between
16ꢀE2ꢀ16 and additives, transmission electron microscopy (TEM) was utilized which provides
information about morphological fluctuations in the aggregate entities or microstructures.29 TEM
micrographs of pure 16ꢀE2ꢀ16 micelles and their electrolytic combinations are shown in Figure 8(aꢀ
c). It is quite apparent from Figure 8 (a) that 16ꢀE2ꢀ16 micelles appear to be roughly spherical
structures (shown by hollow arrows) structures with a variable size in the range of 3 to 7 nm; The
combination of aromatic counterions (An─ and Tos─) results into morphological transition from
spherical to nonꢀspheroid structures, this is quite evident from micrographs 8(b) and 8(c) in which
shape as well as size of aggregate structures is different from the pure typical 16ꢀE2ꢀ16 micelles.
The nonꢀspheroid structures; were found to be in the size range of 30 to 60 nm; this dramatic
change in the shape as well as size may be attributed to the intercalation of aromatic counter ions.
This result complements our 1H NMR results (depiction of signal disappearance) at higher loadings
of additives in 16ꢀE2ꢀ16 gemini solution.
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(a)
(b)
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Figure 8. TEM micrographs (scale bar = 100 nm) of (a) 16ꢀE2ꢀ16 (0.1 mM), (b) 16ꢀE2ꢀ16 (0.1
mM) + NaAn (20 mM), and (c) 16ꢀE2ꢀ16 (0.1 mM) + NaTos (20 mM).
3.5. UV Studies. UV spectroscopy is a sensitive technique for studying the interaction of
organic hydrophobic solute molecules with surfactant micelles.30ꢀ32 Surfactantꢀadditive interaction
is reflected by changes in the absorption spectra of additive upon its placement in the micelle.
Depending on their polarity, organic solute entities are accommodated in different regions of of the
surfactant. 20 Blue shift (shift towards lower wave length) and red shift (shift to higher wave length)
along with fluctuations in molar extinction coefficients are implemented to confirm the extent of
penetration of additive into the micelle. Bathochromic and hypsochromic shifts are also utilized to
ascertain the dielectrics of medium around additive. Polar and nonpolar dielectrics of medium are
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sensed by low and high values of molar extinction coefficients. On the whole, the absorption
characteristics were utilized to evidence the interaction between the gemini and aromatic
counterions. Figure 9 shows the UV absorption spectra of two counter ions (An─, Tos─) in the
presence and absence of 16ꢀE2ꢀ16 micelles. Clearly, in presence of the gemini surfactant 16ꢀE2ꢀ
16), absorbance of the solution decreases. This decrease in absorbance confirms the intercalation of
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3.0
20 mM NaAn
0.1 mM 16-E2-16 + 20 mM NaAn
3.5
20 mM NaTos
0.1 mM 16-E2-16 + 20 mM NaTos
2.5
3.0
Red Shift
2.0
2.5
Red Shift
1.5
2.0
No Red Shift
Only decrease in absorbance
1.0
1.5
0.5
1.0
0.0
0.5
220 240 260 280 300 320 340 360
0.0
200
Wavelength (nm)
220
240
260
Wavelength (nm)
Figure 9. UV absorption spectra of (a) sodium anthranilate (NaAn) + 16ꢀE2ꢀ16, and (b) sodium
tosylate (NaTos) + 16ꢀE2ꢀ16.
aromatic counterions into the palisade layer of gemini surfactant micelles. The decrease in
absorbance is much prominent in the system with anthranilate than tosylate, confirming the fact
that anthranilate binds strongly to gemini micelles than tosylate. Moreover, to know the extent of
penetration of counterions into the interior of micelle, molar extinction coefficients (
ꣂ) were
calculated in the presence and absence of 16ꢀE2ꢀ16 micelles. For anthranilate, was found to
ꣂ
decrease from 242 molꢀ1cmꢀ1 to 204 molꢀ1 cmꢀ1 and, for tosylate, from 52 molꢀ1cmꢀ1 to 24 molꢀ1cmꢀ1.
This larger decrease (38 units) in case of anthranilate than tosylate (28 units) again supports the
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viewpoint that anthranilate ion penetrates more than tosylate ion. The reason for lower penetration
of Tos─ may also be attributed to a small bulky group present at para position; this group generates a
little steric hindrance among 16ꢀE2ꢀ16 micelles and disturbs their stability, hence exerts low
penetration in contrast to An─ having no such group. Moreover, bulky groups open up surfactant
micelles and increase water penetration into micelles, hence tosylate ions experience comparatively
more polar environment than ions of anthranilate. Further, geminiꢀanthranilate combination brings
shifting of wave length to higher wave length (red shift) in contrast to geminiꢀtosylate combination,
in which no visible red shift is observed (Figure 9); this evidences stronger binding of anthranilate
counterions to gemini micelles. The red shift may be attributed to lowering energy gap between the
electronic energy levels of the additive.
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3.6. FTIR Studies. To further reveal interaction between 16ꢀE2ꢀ16 micelles and hydrotope
combinations, Fourier transform infrared (FTIR) technique has been utilized. The FTIR technique is
considered to be an authentic and appropriate technique to probe structural fluctuations and
aggregation behavior of microstructures. Micellar growth induced by additives is monitored by
changes in frequency and shape of the methylene (ꢀCH2ꢀ) chain stretching and bending bands as
well as changes in the head group (ꢀCꢀCH3) region frequencies. 33,34 Figure 10 shows the IR spectra
of pure 16ꢀE2ꢀ16 (0.1 mM) and its combinations with sodium anthranilate/tosylate in the range
3000 cmꢀ1 to 2800 cmꢀ1. In this range three peaks are apparent at 2855 cmꢀ1, 2926 cmꢀ1 and 2959
cmꢀ1. The peaks visible at 2855 cmꢀ1 and 2926 cmꢀ1 were assigned to symmetric and asymmetric
stretching modes of methylene (ꢀCH2ꢀ) chain of 16ꢀE2ꢀ16. Peak at 2959 cmꢀ1 is assigned to
asymmetric stretching mode of methyl (ꢀCꢀCH3) head groups. Presence of sodium anthranilate shifts
the asymmetric peak of methylene (ꢀCH2) from 2926 cmꢀ1 to 2921 cmꢀ1and its symmetric stretching
peak from 2855 cmꢀ1 to 2850 cmꢀ1. Peaks in case of sodium tosylate combination shift from 2926
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