Synthesis, Surface Properties and Effect of an Amino Acid Head Group of 11-(2-Methoxy-4-vinylphenoxy)undecanoicacid-Based Anionic Surfactants

Article abstract of DOI:10.1007/s11743-016-1869-9

Our present research describes the surface properties of three biobased anionic surfactant synthesized from vinylguaiacol and 11-bromo undecanoic acid. To further improve its hydrophobicity and bioavailability, amino acid head group incorporation was carried out. All these synthesized compounds were thoroughly characterized using NMR and mass spectroscopy. The performance properties such as foaming, wetting, emulsification value and calcium tolerance were evaluated. The studied surfactants possess excellent emulsion stability and moderate calcium tolerance as compared to commercially available surfactant sodium lauryl sulfate (SLS). The micelle formation and the thermodynamics involved at the air–water interface were estimated from surface tension measurements. These surfactants showed a higher tendency towards adsorption at the air–water interface than micellization. Dynamic light scattering and steady state fluorescence anisotropy study were carried out to shed light on the bulk micellization properties of the synthesized surfactant. Along with spherical micelles of?<5?nm size, larger aggregates (35–84?nm) were observed with higher anisotropy values. FESEM images further confirmed the larger spherical micelles formed by these surfactants. The surfactants formed chiral aggregates above the critical micelle concentration as indicated by circular dichroism spectra. These surfactants may be suitable candidates for additives to detergents to improve their calcium tolerance especially in the case of hard water. Furthermore, a low foaming ability along with high emulsion stability may find these surfactants to be better replacement of the conventional surfactant used as emulsifiers in many industrial applications.

Full text of DOI:10.1007/s11743-016-1869-9

J Surfact Deterg (2016) 19:1133–1142
DOI 10.1007/s11743-016-1869-9
ORIGINAL ARTICLE
Synthesis, Surface Properties and Effect of an Amino Acid Head
Group of 11-(2-Methoxy-4-vinylphenoxy)undecanoicacid-Based
Anionic Surfactants
1
1
Prabhakara Rao Darapureddi
•
Rati Ranjan Nayak
Received: 8 January 2016 / Accepted: 22 August 2016 / Published online: 2 September 2016
Ó AOCS 2016
Abstract Our present research describes the surface
properties of three biobased anionic surfactant synthesized
from vinylguaiacol and 11-bromo undecanoic acid. To
further improve its hydrophobicity and bioavailability,
amino acid head group incorporation was carried out. All
these synthesized compounds were thoroughly character-
ized using NMR and mass spectroscopy. The performance
properties such as foaming, wetting, emulsification value
and calcium tolerance were evaluated. The studied sur-
factants possess excellent emulsion stability and moderate
calcium tolerance as compared to commercially available
surfactant sodium lauryl sulfate (SLS). The micelle for-
mation and the thermodynamics involved at the air–water
interface were estimated from surface tension measure-
ments. These surfactants showed a higher tendency
towards adsorption at the air–water interface than micel-
lization. Dynamic light scattering and steady state fluo-
rescence anisotropy study were carried out to shed light on
the bulk micellization properties of the synthesized sur-
factant. Along with spherical micelles of \5 nm size, lar-
ger aggregates (35–84 nm) were observed with higher
anisotropy values. FESEM images further confirmed the
larger spherical micelles formed by these surfactants. The
surfactants formed chiral aggregates above the critical
micelle concentration as indicated by circular dichroism
spectra. These surfactants may be suitable candidates for
additives to detergents to improve their calcium tolerance
especially in the case of hard water. Furthermore, a low
foaming ability along with high emulsion stability may find
these surfactants to be better replacement of the conven-
tional surfactant used as emulsifiers in many industrial
applications.
Keywords Vinylguaiacol Á Undecanoic acid Á Amino
acid Á Surface activity Á Surface tension Á Micellization Á
Dynamic light scattering Á Anisotropy
Introduction
Surface-active agents or surfactants are widely used
chemicals consumed in large quantities for various appli-
cations ranging from household to industrial. Biocompati-
bility or biodegradability of these materials is highly
essential as these products create havoc in aquatic envi-
ronments after end use. Hence biodegradable surfactants
made from renewable resources are in high demand.
Renewable raw material such as oils, proteins and carbo-
hydrates are the alternatives to the petroleum resources for
the preparation of surfactants [1, 2].
Surfactants assemble into various aggregate structures in
aqueous solution such as micelles, vesicles, tubules and
rods etc. above a certain concentration called the critical
micelle concentration (CMC). The nature of these self-
assemblies largely depends on geometric factors such as its
hydrophobic head to hydrophobic tail ratio. Depending on
the chemical structure and/or the surface active properties,
surfactant molecule find application in industry or in
common household products [3]. Apart from the
biodegradability issue, surfactant should be stable and
efficient at low concentration, so that minimum amount of
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-016-1869-9) contains supplementary
material, which is available to authorized users.
&
Rati Ranjan Nayak
rrnayak@iict.res.in
1
Centre for Lipid Research, CSIR-Indian Institute of Chemical
Technology, Uppal Road, Hyderabad 500 007, India
1
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J Surfact Deterg (2016) 19:1133–1142
surfactant can be used for various applications there by
reducing the environment concern. One approach to sta-
bilized self-assembled structures is polymeric surfactants,
where the products require minimum concentration and are
highly stable to external stimuli and are concentration
independent [4–6]. Polymeric surfactants are used in all the
major industries such as agriculture, coatings, cosmetics,
biotechnology, medicine, water purification, electronic,
and enhanced oil recovery [7].
Glycine (99 %), L-alanine (99 %), sodium acetate (98 %),
calcium acetate, potassium carbonate anhydrous AR
(99.9 %), lithium hydroxide monohydrate (99 %) were
purchased from M/s SD Fine Chem. Mumbai, India. All
other common chemicals and solvents were purchased
locally and used after purification.
Analytical Methods
Our group has engaged in the design of new surfactant
molecules from renewable and cheap resources such as
vegetable oil and isolated proteins from waste biomass
Varian spectrometer (Model: INNOVA 400, Varian, Palo
1
13
Alto, USA) was used to record all H- and C-NMR
spectra. HRMS data were recorded on a Thermo Scientific
Exactive Orbitrap Mass spectrometer (Germany). ESI–MS
spectra were also recorded on a Waters LC–MS mass
spectrometer (Palo Alto, USA) in the EI mode and are
given in mass units (m/z). Melting points were determined
using a Barnstead Electro Thermal melting point apparatus.
Surface tension measurement was performed on a Kr u¨ ss
tensiometer (K100MK2 Tensiometer, Hamburg, Ger-
many). The surface tension (c) of the synthesized surfactant
molecule at different concentrations was measured at
27 °C. From the break point of the plot of c versus ln C
critical micelle concentration (CMC) was obtained.
As per the Indian Standard specification (BIS-1185,
Bureau of Indian Standards, New Delhi, 1957), the Draves-
Clarkson method was used for estimating the wetting time.
From unbleached grey carded Indian yarn of single 20s,
skeins of 34 cm circumference weighing 5.0 ± 0.1 g were
prepared. The sinking times of the skeins attached to the
hook weighing 4.5 g carrying a lead anchor weighing
27.1 g were determined on surfactant solutions taken in a
500-mL measuring cylinder.
[
8–11]. In order to coin the concept of polymeric surfactant
to our vegetable oil origin surfactant, we must design a
surfactant having a polymerizable group, such as vinyl
moiety, in its hydrophobic tail. Decarboxylated ferulic acid
is one molecule which can be coupled with any fatty acid
to generate a new class of polymerizable surfactants.
Ferulic acid (FA) is hydroxycinnamic acid, considered
cheap, abundant and easily available from vegetables,
wheat, maize, rice bran, fruits, coffee, and peanuts, etc.
[
12–14]. Biochemically or chemically degradation of FA
produces a volatile phenol named 4-vinylguaiacol (VG).
Used as a flavoring agent, VG is more economical than
ferulic acid [15, 16]. Further the phenolic group of VG can
be reacted with 11-bromo undecanoic acid to produce a
new kind of biocompatible surfactant.
Our current research focused on the synthesis and self-
assembly of purely biobased anionic surfactants with
polymerizable groups. Decarboxylated FA introduces the
polymerizable group, whereas the hydrophobic chain was
undecanoic acid, a derivative of castor oil. To further
enhance its performance along with biocompatibility;
amino acids were introduced as a polar head group. The
surface active properties, in particular foaming, wetting,
emulsification and calcium tolerance were studied. The
self-assembly properties such as critical micelle concen-
tration, thermodynamics of micellization were studied in
aqueous solution. Further insight was provided on the bulk
micelle properties by steady state fluorescence anisotropy
and light scattering.
A Ross-Miles foam apparatus was used to determine the
foaming power of the surfactant solutions at room temper-
ature. The apparatus consisted of a cylindrical column of
90 cm height and 5 cm internal diameter surrounded by a
jacket. The test solution was filled in a pipette (200 mL) with
an orifice of 3 mm, which was supported in the above col-
umn. To the 50 mL of solution placed well in advance in the
column, 200 mL solution from the pipette was poured into
the same column. The foam height at different time intervals
was measured using a scale attached to the column.
According to the method described by Prasad et al., the
emulsifying power was measured for the water/liquid
paraffin system [11]. To the 20 mL paraffin oil taken in a
100-mL stoppered graduated measuring cylinder, a sur-
factant solution of 20 mL was poured through the side wall
of the measuring cylinder. For 1 minute the measuring
cylinder was turned upside down 30 times. The emulsify-
ing power was noted for separation of the phase for 10 and
20 mL respectively.
Experimental
Chemicals
Ferulic acid (99 %), 11-bromoundecanoic acid (95 %) and
1
,6-diphenyl-1,3,5-hexatriene (DPH) were purchased from
Sigma-Aldrich, St. Louis, USA. 1-(3-Dimethylamino-
propyl)-3-ethyl carbodiimide hydrochloride (EDC.HCl)
(
99 %), 1-hydroxybenzotriazole (HOBt) (98 %) were pur-
A modified Hart’s method was used to determine cal-
cium tolerance of the surfactant solution. The point of
chased from Spectrochem Pvt. Ltd. Mumbai, India.
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J Surfact Deterg (2016) 19:1133–1142
1135
turbidity just occurred during the titration of 0.1 % sur-
factant solution (50 mL) against 1 % calcium acetate
solution was measured. Calcium tolerance was expressed
monitored by TLC, the reaction mixture was diluted with
ethyl acetate and washed with a saturated solution of NaCl,
5 % aq NaHCO , water and dried over anhydrous Na SO
4
3
2
2?
as Ca in ppm required to make 1 mL of the surfactant
solution turbid.
and concentrated under vacuum. The crude product was
purified by column chromatography using 2 % ethyl acet-
1
ate in hexane (19.2 g, 76 %). H NMR (300 MHz, CDCl )
The micelle size was determined by a dynamic light
scattering technique using a Malvern Zetasizer Instrument
3
d 6.98–6.84 (m, 3H), 6.64 (dd, J = 18.1, 11.3 Hz, 1H),
5.64 (s, 1H), 5.59 (d, J = 18.1 Hz, 1H), 5.13 (d,
?
J = 10.5 Hz, 1H), 3.92 (s, 3H). ESI MS: m/z 149 [M-H] .
(
Nano-ZS90, Malvern, U.K.) having a He–Ne laser. All the
surfactant solutions at a concentration of 5 times their
CMC were prepared in de-ionized water. Before mea-
surement, the solution was filtered through a 0.22-lm MF-
Millipore MCE Membrane. The measurement was carried
out at 25 °C with a scattering angle of 90°.
Synthesis of Methyl 11-(2-methoxy-4-
vinylphenoxy)undecanoate (2)
A fluorolog-3 spectrofluorometer (Horiba Jobin–Yvon,
U.K.) equipped with a polarization accessory which uses
the L-format instrumental configuration was used to mea-
sure the steady state fluorescence anisotropy. For the ani-
sotropy measurement all the surfactant solutions were kept
at the concentration 5 times of their respective CMC, and
the DPH probe at 5 lL concentration. All the samples were
kept in the dark overnight prior to their measurement. The
excitation and emission wavelengths were kept at 350 and
Methyl 11-bromoundecanoate (35 g, 126.6 mmol) and
2-methoxy-4-vinylphenol (19 g, 126.6 mmol) were dis-
solved in acetone (250 mL), K CO (35 g, 253 mmol) was
2
3
added to the reaction mixture and refluxed overnight. After
the completion of the reaction as monitored by TLC, the
solvent was removed under reduced pressure and the crude
product was diluted with ethyl acetate (250 mL) and the
organic layer was washed with saturated solution of NaCl
and dried over Na SO . The organic layer was concen-
2
4
4
50 nm respectively with a band pass of 5/5 nm.
trated to dryness. Then the resulting mixture was purified
by silica gel (eluent l % ethyl acetate in n-hexane) which
afforded methyl 11-(2-methoxy-4-vinylphenoxy)unde-
The microscopic characterization was performed in a
JEOL JSM-7610F, Field Emission Scanning Electron
Microscopy (FESEM), operated at 10 kV. For FESEM the
surfactant solutions were dropped onto small pieces of
thoroughly washed glass slide and the film allowed to dry
overnight in a vacuum desiccator. Prior to the measurement
the glass slides were placed on a copper tape supported on
a metal stub and sputter coated with gold to make it con-
ducting. In order to obtain the particle size distribution the
microscopic images were analyzed by a specialty image
processing software named the Fiji version of ImageJ [17].
ImageJ was developed and maintained by the National
Institute of Mental Health, Bethesda, MD.
1
canoate (35.26 g, 80 % yield) as a viscous liquid. H NMR
(300 MHz, CDCl ) d 7.00–6.89 (m, 2H), 6.82 (d,
3
J = 8.3 Hz, 1H), 6.65 (dd, J = 17.5, 10.7 Hz, 1H), 5.61
(d, J = 17.5 Hz, 1H), 5.14 (d, J = 10.7 Hz, 1H), 4.01 (t,
J = 6.9 Hz, 2H), 3.89 (s, 3H), 3.67 (s, 3H), 2.30 (t,
J = 7.5 Hz, 2H), 1.90–1.77 (m, 2H), 1.69–1.59 (m, 2H),
?
1.51–1.21 (m, 14H). ESI MS: m/z 349.12 [M ? 1] ,
?
371.12 [M ? Na] , 387.12 [M ? K] .
?
Synthesis of 11-(2-Methoxy-4-
vinylphenoxy)undecanoicacid (3)
The specific rotations were measured with an Anton
Paar, MCP-200 digital polarimeter. The circular dichroism
To a solution of methyl 11-(2-methoxy-4-vinylphenoxy)
(
CD) spectra were recorded with a Jasco J-850 spectropo-
undecanoate (35 g, 100.57 mmol) in THF: H O (7:3),
2
larimeter using a quartz cell with a path length of 10 mm.
LiOH.H O (1.07 g, 25.57 mmol) was added and the reac-
2
tion mixture was stirred at room temperature for 4 h. After
completion of reaction, the solvent was removed under
reduced pressure, diluted with ethyl acetate and neutralized
with 2 N HCl and the organic layer was separated and
washed with water, dried using anhydrous Na SO . The
Synthesis
A new class of vinylguaiacol-based anionic surfactant was
synthesized according to the synthetic scheme presented in
Fig. 1.
2
4
solvent was evaporated under reduced pressure and the
crude product was subjected for column chromatography
using silica-gel (10–30 % ethyl acetate: n-hexane) to
Synthesis of 2-Methoxy-4-vinylphenol (1)
1
obtain the pure compound (30.23 g, 90 %, m.p. 40 °C). H
To the ferulic acid (33 g, 170 mmol) dissolved in 250 mL
of DMF, sodium acetate (6.97 g, 85 mmol) was added (to
facilitate decarboxylation) and the reaction mixture was
refluxed for 1 h. After the completion of the reaction as
NMR (500 MHz, CDCl ) d 6.97 (d, J = 1.98 Hz, 1H),
3
6.92 (dd, J = 8.2, 1.9 Hz, 1H), 6.82 (d, J = 8.2 Hz 1H),
6.65 (dd, J = 17.7, 10.9 Hz, 1H), 5.61 (dd, J = 17.5,
0.7 Hz, 1H), 5.14 (dd, J = 10.8, 0.6 Hz, 1H), 4.01 (t,
1
23

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J Surfact Deterg (2016) 19:1133–1142
Fig. 1 Schematic
representation of synthesis of
the 11-(2-methoxy-4-
vinylphenoxy)undecanoic acid-
based surfactant
J = 7.01 Hz, 2H), 3.89 (s, 3H), 2.35 (t, J = 7.4 Hz, 2H),
Methyl2-(11-(2-methoxy-4-vinylphenoxy)
canamido)acetate (4a).Yield 82 %, (m.p. 82 °C)
unde-
1
1
1
1
2
.87–1.80 (m, 2H), 1.66–1.59 (m, 2H), 1.48–1.40 (m, 2H),
1
3
1
.39–1.24 (m, 12H). C NMR (126 MHz, CDCl ) d
H NMR (300 MHz, CDCl ) d 7.01–6.89 (m, 2H), 6.82
3
3
79.96, 149.39, 148.57, 136.56, 130.64, 119.48, 112.68,
(d, J = 8.1 Hz, 1H), 6.65 (dd, J = 17.5, 10.7 Hz, 1H),
5.94 (br, s, 1H), 5.61 (d, J = 17.5 Hz, 1H), 5.14 (d,
J = 10.7 Hz, 1H), 4.06 (d, J = 5.1 Hz, 2H), 4.01 (t,
J = 6.9 Hz, 2H), 3.89 (s, 3H), 3.77 (s, 3H), 2.24 (t,
11.70, 109.05, 69.06, 55.98, 34.04, 29.48, 29.37, 29.23,
?
9.16, 29.06, 25.95, 24.70. ESI MS: m/z 332.99 [M-H] .
?
HRMS (m/z) calculated for C H O , [M]
20 31 4
335.2216,
found 335.2212.
J = 7.3 Hz, 2H), 1.89–1.77 (m, 2H), 1.71–1.55 (m, 2H),
?
.51–1.21 (m, 14H). ESI MS: m/z 406.41 [M ? 1] ,
1
?
428.36 [M ? Na] .
General Procedure for the Synthesis of Peptide
Coupling Reactions (4)
Methyl2-(11-(2-methoxy-4-vinylphenoxy)
unde-
1
canamido)propanoate (4b), Yield 80 % (m.p. 66 °C) H
To a solution of 11-(2-methoxy-4-vinylphenoxy) unde-
NMR (300 MHz, CDCl ) d 6.97 (d, J = 1.8 Hz, 1H), 6.93
3
canoic acid (5 g, 15 mmol) in dry CH Cl (50 mL) at 0 °C,
(dd, J = 8.3, 2.0 Hz, 1H), 6.82 (d, J = 8.3 Hz, 1H), 6.65
(dd, J = 17.7, 10.7 Hz, 1H), 5.98 (d, J = 5.8 Hz, 1H),
5.61 (d, J = 17.7 Hz, 1H), 5.14 (d, J = 11.1 Hz, 1H),
4.67–4.55 (m, 1H), 4.01 (t, J = 6.7 Hz, 2H), 3.89 (s, 3H),
3.75 (s, 3H), 2.20 (t, J = 7.3 Hz, 2H), 1.89–1.77 (m, 2H),
1.69–1.53 (m, 2H), 1.51–1.24 (m, 17H). ESI MS: m/
2
2
EDCI (3.16 g, 16.47 mmol), HOBt (2.22 g, 16.47 mmol)
and DIPEA (2.87 mL, 16.47 mmol) were added at the
same temperature and the reaction mixture was stirred for
1
0 min. Then a solution of pre-neutralized amino acid
methyl ester (16.47 mmol) in dry CH Cl was added to the
2
2
?
?
?
reaction mixture at 0 °C and stirred. After completion of
the reaction as monitored by TLC, the reaction mixture was
diluted with CH Cl and washed with dil. HCl (100 mL
z 420.49 [M ? 1] , 442.44 [M ? Na] , 458.4 [M ? K] .
General Procedure for the Hydrolysis of Ester
to Acid (5)
2
2
9
2), 10 % Na CO (100 mL 92), water and saturated
2 3
solution of NaCl, dried over anhydrous Na SO and con-
2
4
centrated under vacuum. The crude product was subjected
to column chromatography using silica-gel (1–5 %
methanol: chloroform) to obtain the pure compound.
To a solution of compound 4 (5 g, 12.78 mmol) in THF:
H O (7:3), LiOH.H O (1.07 g, 25.57 mmol) was added and
2
2
the reaction mixture was stirred at room temperature for
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J Surfact Deterg (2016) 19:1133–1142
1137
4
h. After the completion of reaction, the solvent was
Results and Discussion
removed under reduced pressure, diluted with chloroform
and neutralized with 2 N HCl and the organic layer was
separated and washed with water, dried using anhydrous
Na SO . The solvent was evaporated under reduced pres-
A new class of biobased anionic surfactant was synthesized
taking vinylguaiacol (decarboxylated ferulic acid) and
11-bromoundecanoic acid as starting materials. To further
improve its solubility and bioavailability amino acid was
coupled to its head group. The synthesized surfactants were
2
4
sure and the crude product was subjected to column
chromatography using silica-gel (5–10 % methanol: chlo-
roform) to obtain the pure compound.
1
13
well characterized through H- and C-NMR, mass spec-
trometry (All the spectra are available as electronic sup-
plementary material).
2-(11-(2-Methoxy-4-vinylphenoxy)
unde-
canamido)acetic acid (5a) (VGGly) Yield 95 %, (m.p.
1
9
6
1
6 °C) H NMR (500 MHz, CDCl ) d 7.01–6.88 (m, 2H),
3
.82 (d, J = 8.1 Hz, 1H), 6.64 (dd, J = 17.5, 10.7 Hz,
Performance Properties
H), 6.39 (s, 1H), 5.61 (d, J = 17.5 Hz, 1H), 5.14 (d,
J = 10.7 Hz, 1H), 4.05 (d, J = 5.1 Hz, 1H), 4.00 (t,
J = 6.9 Hz, 2H), 3.88 (s, 3H), 2.26 (t, J = 7.3 Hz, 2H),
In order to check the industrial feasibility of these vinyl-
guaiacol-based anionic surfactants as replacements for
petroleum and/or other commercial surfactants, perfor-
mance properties such as wetting ability, emulsion stabil-
ity, foaming and calcium tolerance were evaluated and
these are presented in Table 1.
1
1
1
1
2
.90–1.74 (m, 2H), 1.71–1.54 (m, 2H), 1.50–1.21 (m,
3
1
3H). C NMR (126 MHz, CDCl ) d 174.65, 172.57,
3
49.33, 148.53, 136.53, 130.68, 119.53, 112.72, 111.76,
09.10, 69.10, 55.99, 41.54, 36.28, 29.42, 29.35, 29.23,
?
9.14, 25.94, 25.54. ESI MS: m/z 390.41 [M-H] . HRMS
One of the major criteria for choosing a surfactant to be
used in household or in industrial process is its wetting
ability. Hence we study the wetting properties of the syn-
thesized vinylguaiacol-based surfactants and compared
with commercially available sodium lauryl sulfate (SLS).
All of the vinylguaiacol-based surfactants show poor wet-
ting properties as compared with SLS. Among the three
surfactants, SVG showed better wetting ability. Introduc-
tion of amino acids head group further decreases its wetting
ability.
?
m/z) calculated for C H O N, [M] 392.2431, found
(
2
2 34 5
3
92.2428.
-(11-(2-Methoxy-4-vinylphenoxy)
canamido)propanoicacid (5b) (VGAla), Yield 80 %, (m.p.
2
unde-
1
8
1
1
6 °C) H NMR (500 MHz, CDCl ) d 6.97 (d, J = 1.9 Hz,
3
H), 6.93 (dd, J = 8.2, 1.8 Hz, 1H), 6.82 (d, J = 8.2 Hz,
H), 6.65 (dd, J = 17.5, 10.8 Hz, 1H), 6.11 (s, 1H), 5.61
(
dd, J = 17.5, 0.6 Hz, 1H), 5.14 (dd, J = 10.9, 0.6 Hz,
1
3
1
1
1
1
2
H), 4.58 (p, J = 7.1 Hz, 1H), 4.01 (t, J = 6.8 Hz, 2H),
.89 (s, 3H), 2.4 (t, J = 7.6 Hz, 2H), 1.89–1.79 (m, 2H),
.67–1.59 (m, 2H), 1.49–1.39 (m, 5H), 1.38–1.24 (m,
Superior emulsification of oil in water makes them
suitable candidates for the food/pharmaceutical/petroleum
industries. Better emulsion stability was observed com-
paring the emulsion stability of these synthesized surfac-
tants with SLS. For separation of 10 mL emulsions, SVG
took almost double the time taken for SLS. Amino acid
head group further strengthen the emulsion stability.
Increase in hydrophobic nature of the amino acid head
group further boosted the emulsion stability as was
observed in case of SVGAla (243 s for 20-mL separation).
The surfactants possess very low foam heights and these
are also unstable. Hence we could not measure any foam
height for SVG and SVGGly. The more hydrophobic
SVGAla shows little foaming, which is slightly stable. Low
foaming surfactants are really useful in emulsion poly-
merization, waste water treatment and even in industrial
cleaning [18].
1
3
1H). C NMR (75 MHz, CDCl ) d 175.58, 174.02,
3
49.34, 148.54, 136.54, 130.67, 119.51, 112.71, 111.74,
09.10, 69.09, 55.98, 48.29, 36.43, 29.40, 29.34, 29.22,
?
9.12, 25.93, 25.52, 18.01. ESI MS: m/z 404.53 [M-H] .
?
HRMS (m/z) calculated C H O N, [M] 404.2431, found
23 34 5
4
04.2432.
General Procedure for the Preparation of Sodium
Salts of Compounds
The compounds 3 and 5(a, b) were dissolved in ethanol and
equivalent ethanolic NaOH was added. The content was
stirred for overnight at room temperature and the solvent
was evaporated under vacuum to get corresponding sodium
salt: Sodium 11-(2-methoxy-4-vinylphenoxy) undecanoate
(
SVG), Sodium 2-(11-(2-methoxy-4-vinylphenoxy)unde-
Cleaning/washing in variable environments is related to
the resistance of the surfactant to the hardness of the water.
Hence the calcium tolerance of these surfactants was
studied. All these surfactants showed better calcium tol-
erance value as compared to the commercial surfactant. It
canamido)acetate (SVGGly), Sodium 2-(11-(2-methoxy-4-
vinylphenoxy)undecanamido)propanoate(SVGAla). The
specific rotation of the chiral SVGAla is found to be
[
a] = -0.12° (H O).
D
2
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Table 1 Surface active properties of the surfactants
Surfactant
Wetting time (s)
Emulsion stability time (s)
Foam height (cm)
Calcium tolerance (ppm)
10 min
1
0 mL
20 mL
0 min
5 min
SVG
69
293
191
14
99
97
181
230
243
130
–
–
–
18.98
18.98
82.3
SVGGly
SVGAla
SLS
–
–
–
111
53
9.5
13.5
6
5
13
12.5
11.39
is interesting to observe that the SVGAla showed much
higher calcium tolerance value.
better surface activity (cCMC = 38.45 mN/m and
pC20 = 3.48) than alanine (cCMC = 41.36 mN/m and
pC20 = 3.11). [20, 21].
Surface Properties
Using Gibb’s adsorption isotherm (Eq. 1) two important
thermodynamic parameter such as; the maximum surface
excess concentration (Cmax) and minimum surface area per
surfactant molecule (Amin) at the air–water interface was
calculated from the linear part of surface tension plot [22].
Monolayer formation at the air–water interface and micelle
formation in the bulk aqueous solution were determined by
surface-tension measurement. Figure 2 shows the plots of
surface tension (c) versus log (concentration) of the synthe-
sized surfactants. The critical micelle concentrations (CMC)
were obtained from thec versusthe ln C plotand are presented
in Table 2. SVG (7.56 mM) showed a similar CMC value to
the commercially available surfactant SLS (*8 mM)
whereas SVGGly (3.08 mM) showed lower CMC values.
As was observed from the performance property studies,
all the surfactants showed moderate surface activity. The
observed surface tension values at the CMC (cCMC) were in
the range of 38.45–43.79 mN/m, which was not highly
surface active in nature. Introduction of an amino acid head
group enhanced the surface activity as evidenced by their
higher ([3) pC20 value [19]. pC20 is defined as the negative
logarithm of the surfactant concentration required to reduce
the surface tension of water by 20 units. While comparing
the amino acid head group, the more polar glycine showed
Cmax ¼ Àð1=nRTÞðdc=d ln CÞ
Amin ¼ 1=N Á Cmax
ð1Þ
ð2Þ
where R is the gas constant (8.314 J/mol.K), T is the
absolute temperature, c is the surface tension, C is the
surfactant concentration and N is Avogadro’s number. As
there is one counter ion that is associated with one ionic
head group, the value of n is taken to be 2 [19]. The degree
of packing and the orientation of the surfactant molecule at
the air–water interface were obtained from the Cmax and
Amin values. SVGAla showed lower Cmax and higher Amin
whereas SVG showed higher Cmax and lower Amin at the
air–water interface area. This might be due to the bulkier
group (alanine) causing less order packing in the air–water
interface during the formation of Gibb’s monolayer.
The standard free energy of micellization (DG°mic = RT
ln CMC) in the bulk aqueous solution and the standard free
energy of adsorption (DG°ads = DG°mic - (PCMC/Umax) at
the air–water interface were also calculated [23, 24].
Where R is gas constant 8.314 J/mol K and T is absolute
temperature and the surface pressure at CMC; PCMC
(
PCMC = cwater - cCMC). The adsorptions at the air–water
interface as well as the micellization in the bulk aqueous
phase are a spontaneous process at 27 °C as observed by
the negative sign (Table 2). All these three surfactant
showed higher tendency towards adsorption than micel-
lization. The comparatively lower CMC surfactant,
SVGGly showed higher standard free energy of micel-
lization (DG°mic = -28.85 kJ/mol).
Dynamic Light Scattering
The surfactant in aqueous solution above the CMC forms
micelles of various sizes and shapes. The aggregated
Fig. 2 Surface tension versus concentration of surfactants in aqueous
solution
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J Surfact Deterg (2016) 19:1133–1142
1139
Table 2 Thermodynamic properties and anisotropy value of the surfactants
1
2 mol
Surfactant
CMC (mM)
c
CMC
pC20
C
mm
max 9 10
/
A
min
DG°mic
(kJ/mol)
DG°ads
(kJ/mol)
Anisotropy
(r)
2
2
(nm /molecule)
(
mN/m)
SVG
7.56
3.08
4.31
43.79
38.45
41.36
2.64
3.48
3.11
2.77
2.45
2.38
0.59
0.67
0.69
-24.37
-28.85
-27.17
-34.55
-42.54
-40.04
0.074
0.106
0.144
SVGGly
SVGAla
micelle size can be obtained through dynamic light scat-
tering (DLS) technique. In order to shed light on the var-
ious morphologies formed by the synthesized surfactant in
aqueous solution, concentration dependent aggregate size
measurement were carried out and presented in Table 3.
All the surfactants showed mostly 3 kinds of aggregates in
the solution. The first category; size B5 nm (around
In order to investigate the packing of the hydrophobic
chain of these surfactants steady state anisotropy mea-
surements were carried out using DPH as an anisotropy
probe. The fluorescence depolarization of a probe molecule
can be expressed in terms of anisotropy (r) as follows:
r ¼ ðIVVÀGIVHÞ = ðIVV þ 2GIVHÞ
ð3Þ
where IVV and IVH are the fluorescence intensities polar-
ized parallel and perpendicular to the excitation light, and
G is the instrumental correction factor (G = IVV/IVH). The
anisotropy values obtained for this anionic surfactant were
presented in Table 2. It has been observed that the rigid-
ness increases with the incorporation of the amino acid
head group, thus by increasing the anisotropy value. The
higher anisotropy value obtained for SVGAla (0.144) is
may be due to the compact packing of the hydrophobic
chain in a single micelle. The bulky head groups having
stereogenic centers may repeal each other and force the
hydrophobic tail to have compact packing.
1
0–25 % abundance) was found which corresponds to the
typical spherical micelle. The second category is the most
populated (63–84 %) aggregates around 35–84 nm in size,
which is slightly bigger than the conventional spherical
micelle. A similar kind of bigger spherical micelle of
5
0–200 nm size was reported for lauryl ester of L-pheny-
lalanine and lauryl ester of L-tyrosine in aqueous solution
25]. The third category is the ill aggregates formed by the
[
synthesized surfactant ([100 nm) in aqueous solution
whose population is *10 %. As expected, incorporation of
amino acid head group increases the size of the aggregates.
While comparing among the two amino acid head group
surfactant, SVGAla has lower aggregate size than SVGGly.
The size reduction of SVGGly to SVGAla may be due to
the bulkiness of the amino acid side chain and the spatial
orientation of the stereogenic centers, which affect the
extent of close packing of the surfactant molecule during
self-assembly formation. Similar size reduction was
observed from 151 nm to 135 nm for sodium N-[4-(n-do-
decyloxy)benzoyl]-L-leucinate to sodium N-[4-(n-dodecy-
loxy)benzoyl]-L-isoleucinate surfactant [26].
The morphology of the aggregates formed by the sur-
factant was observed by field emission electron micro-
scopy. The microscopic images along with the particle
distribution analysis were presented in Fig. 3. Spherical
micelle of little bigger size was observed for all the three
surfactant, which further confirms the findings of DLS
experiment. The average micelle size of SVGAla
(
(
22.9 nm) is comparatively smaller than the SVGGly
37.38 nm) micelle. The size reduction in the of case
Table 3 The size of the
surfactant micelles at various
concentrations
Sample
SVG
Concentration (mM)
15.0
Size (d, nm)
2.99 (15.7 %)
37.23 (67.3 %)
179.65 (17.0 %)
22.5
3.46 (19.9 %)
3.37 (25.3 %)
3.25 (23.0 %)
3.40 (19.7 %)
5.18 (16.4 %)
2.53 (17.5 %)
3.07 (24.8 %)
2.70 (15.6 %)
2.83 (9.6 %)
2.77 (16.3 %)
2.37 (17.5 %)
43.40 (72.1 %)
34.84 (64.0 %)
43.05 (63.1 %)
82.13 (63.5 %)
78.01 (66.1 %)
83.92 (65.9 %)
80.50 (75.2 %)
57.99 (84.4 %)
59.88 (82.4 %)
56.06 (83.7 %)
56.32 (82.5 %)
306.6 (8.0 %)
470.5 (10.7 %)
243.7 (13.9 %)
103.5 (16.8 %)
249.7 (17.5 %)
100.06 (16.6 %)
3
3
0.0
7.5
6
.0
SVGGly
SVGAla
9.0
2.0
5.0
1
1
8
.6
12.9
894.7 (8.0 %)
1
2
7.2
1.5
1
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J Surfact Deterg (2016) 19:1133–1142
SVGAla is correlated with the stereogenic center present in
the surfactant head group, which affect the aggregation
formation.
17.1 mM SVGGly, 21.9 mM SVGAla and 0.87 mM
SVGAla (below CMC) are presented in Fig. 4. We also
recorded the CD spectra for 1.7 mM SVG and 0.68 mM
SVGGly (both are below the CMC), but for clarity purpose
the spectra were excluded from the figure. Two distinct CD
bands were appeared at 218 nm and 236 nm for concen-
tration above the CMC of SVGAla. No such CD band
appeared for SVGAla, when the measurement was per-
formed below its CMC as shown in the figure. Hence it is
The effect of the stereogenic center present in the sur-
factant head group on the aggregation behavior in aqueous
solution was monitored by circular dichroism (CD) spectra
measurement. The CD spectra of the aqueous surfactant
solutions were recorded above and below its CMC. The CD
spectra plotted as [h] versus wavelength for 42.6 mM SVG,
0
0
Fig. 3 FESEM images and their corresponding ImageJ analyzed histogram plot for 42.6 mM SVG (A and A ), 17.1 mM SVGGly (B and B ),
0
2
1.9 mM SVGAla (C and C )
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J Surfact Deterg (2016) 19:1133–1142
1141
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Acknowledgments The authors are thankful to Dr. V. Jayathirtha
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acid-based surfactant work. PRD gratefully acknowledges the Council
of Scientific and Industrial Research (CSIR), Government of India for
a Senior Research Fellowship.
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Prabhakara Rao Darapureddi is a senior research fellow (SRF) at
the Centre for Lipid Research, CSIR-Indian Institute of Chemical
Technology, Hyderabad, India. His doctoral research work is on
‘‘Studies on surface and micellar properties of bio-based surfactants’’.
2
2
Rati Ranjan Nayak received his Ph.D. in organic chemistry from
Utkal University, Bhubaneswar, India. Presently he is working as a
scientist at the Centre for Lipid Research, CSIR-Indian Institute of
Chemical Technology, Hyderabad, India. His research interest is in
surfactants and nano-emulsions.
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