Synthesis and Surface Properties of a Novel Sodium 3-(3-Alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate at the Air-Water Interface

Article abstract of DOI:10.1007/s11743-015-1691-9

The present paper describes the synthesis and evaluation of surface properties of a novel series of anionic surfactant, namely sodium 3-(3-alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate with varying alkyl chain length (C8-C16). Synthesis involves initial formation of the 3-alkyloxy-3-oxopropyl acrylate along with fatty acrylate during the direct esterification of fatty alcohol with acrylic acid in the presence of 0.5 % NaHSO₄ at 110 C followed by sulfonation of the terminal double bond of the 3-alkyloxy-3-oxopropyl acrylate. Synthesized compounds were evaluated for surface and thermodynamic properties such as critical micelle concentration (CMC), surface tension at CMC (γ_cmc), efficiency of surface adsorption (pC₂₀), surface excess (Γ_max), minimum area per molecule at the air-water interface (A _min), free energy of adsorption (?G _ads), free energy of micellization (?G _mic), wetting time, emulsifying properties, foaming power and calcium tolerance. Effect of chain length on CMC follows the classic trend, i.e. decrease in CMC with the increase in alkyl chain length. High pC₂₀ (>3) value indicates higher hydrophobic character of the surfactant. These surfactants showed very poor wetting time and calcium tolerance, but exhibited good emulsion stability and excellent foamability. Foaming power and foam stability of C14-sulfonate were found to be the best among the studied compounds. Foam stability of C14-sulfonate was also studied at different concentrations over time and excellent foam stability was obtained at a concentration of 0.075 %. Thus this novel class of surfactant may find applications as foam boosters in combination with other suitable surfactants.

Full text of DOI:10.1007/s11743-015-1691-9

J Surfact Deterg (2015) 18:689–695
DOI 10.1007/s11743-015-1691-9
ORIGINAL ARTICLE
Synthesis and Surface Properties of a Novel Sodium 3-(3-
Alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate at the
Air–Water Interface
Pandari Phani Kumar1 Rati Ranjan Nayak1 Sanjit Kanjilal1
•
•
Received: 24 December 2014 / Accepted: 11 May 2015 / Published online: 2 June 2015
Ó AOCS 2015
Abstract The present paper describes the synthesis and
evaluation of surface properties of a novel series of anionic
surfactant, namely sodium 3-(3-alkyloxy-3-oxopropoxy)-3-
oxopropane-1-sulfonate with varying alkyl chain length
(C8–C16). Synthesis involves initial formation of the
3-alkyloxy-3-oxopropyl acrylate along with fatty acrylate
during the direct esterification of fatty alcohol with acrylic
acid in the presence of 0.5 % NaHSO4 at 110 °C followed
by sulfonation of the terminal double bond of the 3-alky-
loxy-3-oxopropyl acrylate. Synthesized compounds were
evaluated for surface and thermodynamic properties such
as critical micelle concentration (CMC), surface tension at
CMC (ccmc), efficiency of surface adsorption (pC20), sur-
face excess (Cmax), minimum area per molecule at the air–
water interface (Amin), free energy of adsorption (DG°ads),
free energy of micellization (DG°mic), wetting time,
emulsifying properties, foaming power and calcium toler-
ance. Effect of chain length on CMC follows the classic
trend, i.e. decrease in CMC with the increase in alkyl chain
length. High pC20 ([3) value indicates higher hydrophobic
character of the surfactant. These surfactants showed very
poor wetting time and calcium tolerance, but exhibited
good emulsion stability and excellent foamability. Foaming
power and foam stability of C14-sulfonate were found to
be the best among the studied compounds. Foam stability
of C14-sulfonate was also studied at different
concentrations over time and excellent foam stability was
obtained at a concentration of 0.075 %. Thus this novel
class of surfactant may find applications as foam boosters
in combination with other suitable surfactants.
Keywords Acrylic acid Á Fatty alcohol Á Surface tension Á
Emulsifying properties Á Foaming power Á Calcium
tolerance
Introduction
Fatty acid-derived acrylic esters are versatile monomers
having wide industrial applications [1]. There are several
methods reported in the literature for acrylation of fatty
acid moieties, e.g. through alcoholysis of methyl
methacrylate, acylation with acrylyl chloride, dehalo-
genation of alkyl a,b-dibromopropionate, dehydrohalo-
genation of b-halopropionic ester etc. [2]. Commercially,
n-alkyl acrylates are prepared by direct esterification of
fatty alcohol with acrylic acid in the presence of an es-
terification catalyst such as sulfuric acid, hydrofluoric
acid, p-toluene sulfonic acid etc. [2–4]. These acid cata-
lysts are toxic, corrosive and difficult to remove from the
product. These acidic catalysts can be replaced with var-
ious solid acid catalysts such as acidic cation exchange
resin, solid oxides, heteropolyacids supported onto suit-
able supports etc. [5–9].
Electronic supplementary material The online version of this
article (doi:10.1007/s11743-015-1691-9) contains supplementary
material, which is available to authorized users.
Direct esterification of fatty alcohol with acrylic acid
catalysed by all the reported acid catalysts in general re-
sults in about 90 % selectivity for fatty acrylate, but suffers
from very poor conversion [7]. This is primarily due to the
subsequent reactions of products occurring under the same
reaction conditions. The literature also reports the forma-
tion of a range of by-products along with fatty acrylate
& Sanjit Kanjilal
sanjit@iict.res.in
1
Centre for Lipid Research, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India
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J Surfact Deterg (2015) 18:689–695
during the direct esterification reaction; the extent of their
formation was found to be dependent on the type of cata-
lyst, chain length of fatty alcohol and the reaction condi-
tions [7]. The major by-product is the 3-(alkyloxy)-3-
oxopropyl acrylate, formed by the Michael addition of
oxygen functionality of unreacted acrylic acid with the
terminal double bond of alkyl acrylate; however, there is
no report in which more than 12 % formation of this by-
product was formed. In the present study, direct esterifi-
cation of n-octanol with acrylic acid at 2:1 molar ratio was
carried out in the presence of 0.5 % w/w of NaHSO4 at
110 °C for 2 h. Column chromatographic purification fol-
lowed by spectral characterization using several spectral
techniques indicated near equal formation of the expected
octyl acrylate along with 3-(octyloxy)-3-oxopropyl acrylate
as a major side product.
Experimental Procedures
Chemicals
Acrylic acid and calcium acetate were purchased from SD
Fine Chemicals Pvt. Ltd., Mumbai (India). Different long
chain fatty alcohols (1-octanol, 1-decanol, 1-dodecanol,
1-tetradecanol and 1-hexadecanol) and the fluorescence
probe N-phenyl-1-naphthylamine (NPN) were purchased
from Sigma Aldrich. All other solvents, anhydrous
NaHSO4 and paraffin liquid (light) were purchased from
SD Fine (Mumbai, India).
Synthesis
Synthesis of sodium 3-(3-alkyloxy-3-oxopropoxy)-3-oxo-
propane-1-sulfonate involves direct esterification of
acrylic acid with fatty alcohol to get a mixture of alkyl
acrylate and 3-(alkyloxy)-3-oxopropyl acrylate. Column
chromatographic purification of 3-(alkyloxy)-3-oxopropyl
acrylate and its subsequent sulfonation at the terminal
olefin produced the desired compound (Fig. 1).
Synthesis of structurally novel surfactants has attracted
organic and physical chemists to study their surface prop-
erties. The present reaction has given us an opportunity to
concentrate more on interesting functionalities of 3-(alky-
loxy)-3-oxopropyl acrylate rather than alkyl acrylate to
synthesize a new range of products either by functional-
ization of the active terminal olefin or by polymerization.
In the present work, sulfonation of the double bond was
carried out to synthesize five new sulfonates of varying
alkyl chain length. All the sulfonates were evaluated for
surface and thermodynamic properties such as critical mi-
celle concentration (CMC), surface tension at cmc (ccmc),
efficiency of surface adsorption (pC20), surface excess i.e.
the amount of surfactant adsorbed per unit are at the air–
water interface (Cmax), minimum area per molecule at the
air–water interface (Amin), free energy of adsorption
(DG°ads), free energy of micellization (DG°mic), wetting
time, foaming, emulsion stability and calcium tolerance
were evaluated.
General Procedure for the Synthesis of 3-(Alkyloxy)-3-
oxopropyl Acrylate
In a round-bottom flask, acrylic acid and fatty alcohol were
taken in a 1:2 molar ratio followed by the addition of 0.5 %
w/v of NaHSO4 (with respect to acrylic acid). The reaction
mixture was stirred at 110 °C and the progress of the re-
action was monitored by TLC as well as GC. Both fatty
acrylate and 3-(alkyloxy)-3-oxopropyl acrylate were found
to be formed nearly in equal proportion within 2 h, as
indicated by GC analysis. After 2 h, the reaction mixture
was solubilized in hexane and washed with water to
Fig. 1 Direct esterification of fatty alcohol with acrylic acid in the presence of 0.5 % NaHSO4 followed by sulfonation of 3-(alkyloxy)-3-
oxopropyl acrylate
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691
remove unreacted acrylic acid and NaHSO4, dried over
anhydrous Na2SO4 and concentrated using rotary
evaporator to get the crude product. Finally, the reaction
mixture was purified by column chromatography using
hexane and ethyl acetate as eluent. The fatty acrylate was
eluted with 0.3 % ethyl acetate in hexane and 3-(alkyloxy)-
3-oxopropyl acrylate was eluted with 0.5 % ethyl acetate in
hexane.
CO–O–CH2–), 165 (CH2=CH–CO–O–CH2–), 131
(CH2=CH–), 127 (CH2=CH–), 64.5 (CH2=CH–CO–O–
CH2–), 59.5 (–CH2–CO–O–CH2–), 22.5–33.8 [–O–CH2–
(CH2)8–CH3], 14 (–CH3); ESI MS: m/z 307 (M ? Na), 323
(M ? K), 127 [M - O(CH2)9–CH3].
3-(Dodecyloxy)-3-oxopropyl Acrylate
Acrylic acid (5.5 g, 0.076 M) was reacted with 1-dode-
canol (28.5 g, 0.153 M) in the presence of sodium bisulfate
(28.0 mg) at 110 °C. After 2 h, the reaction mixture was
worked up and purified by column chromatography to get
6.6 g of pure dodecyl acrylate (isolated yield 36 %) and
7.7 g of 3-(dodecyloxy)-3-oxopropyl acrylate (isolated
yield 40.5 %). The desired compound was characterized by
NMR (1H and 13C), IR and mass spectroscopy. IR (cm-1):
2928, 2958 (aliphatic C–H stretching), 1734 (carbonyl
stretching), 1466 (C–H scissoring), 1177 (O–C–O stretch-
ing), 721 (C–H rocking); 1H NMR (300 MHz, CDCl3,
ppm): d 6.4 (d, 1H; CH2=CH–; cis to carbonyl), 6.1 (dd,
1H; CH2=CH–), 5.8 (d, 1H; CH2=CH–; trans to carbonyl),
4.4 (t, 2H; CH2=CH–CO–O–CH2–), 4.1 (t, 2H; –CH2–CO–
O–CH2–), 2.65 (t, 2H; CO–O–CH2–CH2–CO–O), 1.6 (t,
2H; –O–CH2–CH2–), 1.3 [m, 18H; –(CH2)9–CH3], 0.9 (t,
3H; –CH3); 13C NMR (75 MHz, CDCl3, ppm): d 170
(–CH2–CO–O–CH2–), 165 (CH2=CH–CO–O–CH2–), 131
(CH2=CH–), 127 (CH2=CH–), 64.5 (CH2=CH–CO–O–
CH2–), 59.5 (–CH2–CO–O–CH2–), 22.5–33.8 [–(CH2)10–
CH3], 14 (–CH3). ESI MS: m/z 313 (M ? 1), 335
(M ? Na), 351(M ? K), 127 [M - O(CH2)11–CH3].
3-(Octyloxy)-3-oxopropyl Acrylate
Acrylic acid (16.5 g, 0.23 M) was reacted with 1-octanol
(60 g, 0.46 M) in the presence of sodium bisulfate
(82.5 mg) at 110 °C. After 2 h, the reaction mixture was
worked up and purified by column chromatography to get
20.3 g of pure octyl acrylated (isolated yield 48.2 %) and
28.0 g of 3-(octyloxy)-3-oxopropyl acrylate (isolated yield
47.7 %). The desired compound was characterized by
NMR (1H and 13C), IR and mass spectroscopy. IR (cm-1):
2928, 2958 (aliphatic C–H stretching), 1734 (carbonyl
stretching), 1466 (C–H scissoring), 1177 (O–C–O stretch-
ing), 721 (C–H rocking); 1H NMR (300 MHz, CDCl3,
ppm): d 6.4 (d, 1H; CH2=CH–; cis to carbonyl), 6.1 (dd,
1H; CH2=CH–), 5.8 (d, 1H; CH2=CH–; trans to carbonyl),
4.45 (t, 2H; CH2=CH–CO–O–CH2–), 4.1 (t, 2H; –CH2–
CO–O–CH2–), 2.7 (t, 2H; CO–O–CH2–CH2–CO–O), 1.65
(t, 2H; –O–CH2–CH2–), 1.3 [m, 10H; –(CH2)5–CH3], 0.9
(t, 3H; –CH3); 13C NMR (75 MHz, CDCl3, ppm): d 170
(–CH2–CO–O–CH2–), 165 (CH2=CH–CO–O–CH2–), 131
(CH2=CH–), 127.5 (CH2=CH–), 64.5 (CH2=CH–CO–O–
CH2–), 59.5 (–CH2–CO–O–CH2–), 22.5–33.8 [–O–CH2–
(CH2)6–CH3], 14 (CH3); ESI MS: m/z 279 (M ? Na), 295
(M ? K), 127 [M - O(CH2)7–CH3].
3-(Tetradecyloxy)-3-oxopropyl Acrylate
Acrylic acid (3.1 g, 0.043 M) was reacted with 1-tetrade-
canol (18.5 g, 0.086 M) in the presence of sodium bisulfate
(16.0 mg) at 110 °C. After 2 h, the reaction mixture was
worked up and purified by column chromatography to get
4.4 g of tetradecyl acrylate (isolated yield 39 %) and 6.2 g
of 3-(tetradecyloxy)-3-oxopropyl acrylate (isolated yield
42.0 %). The desired compound was characterized by
NMR (1H and 13C), IR and mass spectroscopy. IR (cm-1):
2928, 2958 (aliphatic C–H stretching), 1734 (carbonyl
stretching), 1466 (C–H scissoring), 1177 (O–C–O stretch-
ing), 721 (C–H rocking); 1H NMR: (300 MHz, CDCl3,
ppm): d 6.4 (d, 1H; CH2=CH–; cis to carbonyl), 6.1 (dd,
1H; CH2=CH–), 5.8 (d, 1H; CH2=CH–; trans to carbonyl),
4.4 (t, 2H; CH2=CH–CO–O–CH2–), 4.1 (t, 2H;–CH2–CO–
O–CH2–), 2.65 (t, 2H; CO–O–CH2–CH2–CO–O), 1.6 (t,
2H; –O–CH2–CH2–), 1.3 [m, 22H; –(CH2)11–CH3], 0.9 (t,
3H; –CH3); 13C NMR (75 MHz, CDCl3, ppm): d 170
(–CH2–CO–O–CH2–), 165 (CH2=CH–CO–O–CH2–), 131
(CH2=CH–), 127 (CH2=CH–), 64.5 (CH2=CH–CO–O–
CH2–), 59.5 (–CH2–CO–O–CH2–), 22.5–33.8 [–(CH2)12–
3-(Decyloxy)-3-oxopropyl Acrylate
Acrylic acid (5.0 g, 0.07 M) was reacted with 1-decanol
(22.0 g, 0.14 M) in the presence of sodium bisulfate
(25.0 mg) at 110 °C. After 2 h, the reaction mixture was
worked up and purified by column chromatography to get
5.2 g of pure decyl acrylate (isolated yield 35 %) and 7.7 g
of 3-(decyloxy)-3-oxopropyl acrylate (isolated yield
39 %). The desired compound was characterized by NMR
(1H and 13C), IR and mass spectroscopy. IR (cm-1): 2928,
2958 (aliphatic C–H stretching), 1734 (carbonyl stretch-
ing), 1466 (C–H scissoring), 1177 (O–C–O stretching), 721
1
(C–H rocking); H NMR (300 MHz, CDCl3, ppm): d 6.4
(d, 1H; CH2=CH–; cis to carbonyl), 6.1 (dd, 1H; CH2=CH–),
5.8 (d, 1H; CH2=CH–; trans to carbonyl), 4.4 (t, 2H;
CH2=CH–CO–O–CH2–), 4.1 (t, 2H; –CH2–CO–O–CH2–),
2.65 (t, 2H; CO–O–CH2–CH2–CO–O), 1.6 (t, 2H; –O–
CH2–CH2–), 1.3 [m, 14H; –(CH2)7–CH3], 0.9 (t, 3H;
–CH3); 13C NMR (75 MHz, CDCl3, ppm): d 170 (–CH2–
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CH3], 14 (–O–CH2–(CH2)12–CH3); ESI MS: m/z 363
(M ? Na), 379 (M ? K), 127 [M - O(CH2)13–CH3].
(m/z) calculated for C14H25O7S is 337.13155, found
337.13187; sodium 3-(3-decyloxy-3-oxopropoxy)-3-oxo-
propane-1-sulfonate (C10-sulfonate): melting point
131 °C; HRMS (m/z) calculated for C16H29O7S is
365.16285, found 365.16298; sodium 3-(3-dodecyloxy-3-
oxopropoxy)-3-oxopropane-1-sulfonate (C12-sulfonate):
melting point 152 °C; HRMS (m/z) calculated for C18H33
O7S is 393.19415, found 393.19443; sodium 3-(3-tetrade-
3-(Hexadecyloxy)-3-oxopropyl Acrylate
Acrylic acid (2.55 g, 0.0354 M) was reacted with 1-hex-
adecanol (17.15 g, 0.0708 M) in the presence of sodium
bisulfate (13.0 mg) at 110 °C. After 2 h, the reaction
mixture was worked up and purified by column chro-
matography to get 3.66 g of hexadecyl acrylate (isolated
yield 35.0 %) and 5.3 g of 3-(hexadecyloxy)-3-oxopropyl
acrylate (isolated yield 41.0 %). The desired compound
was characterized by NMR (1H and 13C), IR and mass
spectroscopy. IR (cm-1): 2928, 2958 (aliphatic C–H
stretching), 1734 (carbonyl stretching), 1466 (C–H scis-
cyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate
(C14-
sulfonate): melting point 177 °C; HRMS (m/z) calculated
for C20H37O7S is 421.22545, found 421.22621; sodium 3-
(3-hexadecyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate
(C16-sulfonate): melting point 182 °C; HRMS (m/z) cal-
culated for C22H41O7S is 449.25675, found 449.25691.
1
soring), 1177 (O–C–O stretching), 721 (C–H rocking); H
Analytical Methods
NMR (300 MHz, CDCl3, ppm): d 6.4 (d, 1H; CH2=CH–;
cis to carbonyl), 6.1 (dd, 1H; CH2=CH–), 5.8 (d, 1H;
CH2=CH–; trans to carbonyl), 4.4 (t, 2H; CH2=CH–CO–
O–CH2–), 4.1 (t, 2H; –CH2–CO–O–CH2–), 2.65 (t, 2H;
CO–O–CH2–CH2–CO–O), 1.6 (t, 2H; –O–CH2–CH2–), 1.3
[m, 26H; –(CH2)13–CH3], 0.9 (t, 3H; –CH3); 13C NMR
(75 MHz, CDCl3, ppm): d 170 (–CH2–CO–O–CH2–), 165
1
All H and 13C NMR spectra were recorded on 300 and
75 MHz (Varian, Palo Alto, USA) spectrometers, respec-
tively. HRMS data was recorded on a Thermo Scientific
Exactive Orbitrap Mass spectrometer (Germany) and are
given in mass units (m/z). 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 Branstead
Electro Thermal melting point apparatus. Synthesized
compounds were crystallized from ethanol before
evaluating their surface properties using surface tension.
The surface tension was measured using a Kruss K100
tensiometer (Kru¨ss GmbH, Hamburg, Germany) equipped
with a platinum ring having a mean circumference of 6 cm.
The surface tension (c) was measured at different con-
centrations by adding a subsequent volume of stock sur-
factant solution with a 765 Dosimat (Metrohm), connected
to the system. All surface tension measurements were
performed at 27 °C. Foaming properties were evaluated at
ambient temperature using a Ross-Miles pour foam appa-
ratus [11], which consists of a jacketed cylindrical column
of 90 cm height and 5 cm internal diameter. The studied
surfactant solution (0.025 %) was taken in a 200-mL
pipette with an orifice of 3 mm and fixed at the top of the
column containing the same test solution (50 mL). The
surfactant solution in the pipette was allowed to drop onto
the solution in the column. Foam height obtained initially
and after a regular interval of time was recorded on the
scale attached to the column. Emulsification properties of
the studied surfactant solutions (0.025 %) were determined
according to the method described in the literature [12].
Equal volumes (40 mL) of surfactant solution and paraffin
liquid (light) were taken in a 500-mL Erlenmeyer flask and
the mixture was given 10 downward strokes and transferred
immediately to a 100-mL measuring cylinder. The times
taken for the separation of 10 mL and 20 mL of the
(CH2=CH–CO–O–CH2–),
131
(CH2=CH–),
127
(CH2=CH–), 64.5 (CH2=CH–CO–O–CH2–), 59.5 (–CH2–
CO–O–CH2–), 22.5–33.8 [–O–CH2–(CH2)14–CH3], 14
(–CH3); ESI MS: m/z 391 (M ? Na), 407 (M ? K), 127
[M - O(CH2)15–CH3).
General Procedure for the Synthesis of Sodium 3-(3-
Alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate
Sulfonation of 3-(alkyloxy)-3-oxopropyl acrylate was car-
ried out according to the method reported in the literature
[10]. Briefly, a solution of 3-(alkyloxy)-3-oxopropyl acry-
late in isopropanol was added to an aqueous solution of
NaHSO3 (1.5 mole equivalent) and the resultant reaction
mixture was stirred at 50 °C for 28 h. After completion, the
organic solvent was evaporated using a rotary evaporator
and the dried mass was solubilized in 100 mL of methanol
and filtered through Whatman filter paper to remove un-
reacted NaHSO3. The filtrate was concentrated to obtain a
white powder, which was washed extensively with dry
ether to remove unreacted 3-(alkyloxy)-3-oxopropyl acry-
late. The residue was dried under vacuum to get sodium
3-(3-alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate as
a white amorphous powder.
Isolated yields of all the analogues are in the range of
75–78 %. The structures of final compounds were con-
firmed by high resolution mass spec (HRMS) data and are
given below along with melting points.
Sodium 3-(3-octyloxy-3-oxopropoxy)-3-oxopropane-1-
sulfonate (C8-sulfonate): melting point 119 °C; HRMS
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693
aqueous phase solution were determined. For estimating
the wetting time, the Draves–Clarkson method was em-
ployed [13]. Briefly, skeins of 34 cm circumference
weighing 5.0 0.05 g were prepared from unbleached
grey carded Indian yarn of single 20s. A hook weighing
4.5 g carrying a lead anchor weighing 27.1 g was attached
to the skeins and sinking times were determined on sur-
factant solution (0.025 %) taken in a 500-mL measuring
cylinder. Calcium tolerance of a surfactant solution is de-
fined as the amount of Ca2? ion required to make 1 mL of
surfactant solution turbid and was determined by a mod-
ified Hart’s method [14]. Surfactant solution (0.025 %,
50 mL) was taken in a conical flask and titrated against
1 % calcium acetate solution in water, taken in a 50-mL
burette. Titration was carried out till the turbidity of the
solution just obscured a strip of printed paper fastened to
one side of the beaker.
out efficiently at the active terminal olefin according to the
method reported in the literature [10]. The structures of the
sodium 3-(3-alkyloxy-3-oxopropoxy)-3-oxopropane-1-sul-
fonates were confirmed by HRMS data.
Surface Activity
Aqueous solutions of the studied surfactants were prepared
by dissolving appropriate amounts in Milli-Q water, and
surface tension and CMC were measured using a Kruss
K100 tensiometer. Variation of surface tension as a func-
tion of logarithm of surfactant concentration is shown in
Fig. 2. The CMC was determined from this plot of surface
tension vs log C, from the intersection of two lines after
fitting linearly. Three measurements were taken in total for
all the studied analogues and were averaged. The surface
properties of the synthesized surfactants measured by the
surface tension method are given in Table 1. The CMC of
the synthesized sulfonates decreases with the increase in
chain length. This is expected as the increase in chain
length is associated with increased hydrophobicity,
favouring aggregation in the aqueous environment. The
surface tensions at CMC of the synthesized surfactants are
in the range of 30–47 mN/m. The efficiency of a surfactant
can be assessed by the value of pC20, estimated as the
negative logarithm of concentration of surfactant required
to reduce the surface tension of water by 20 units. Higher
values of pC20 ([3) indicate higher hydrophobic character
of the surfactant, resulting in higher efficiency of reduction
of surface tension [16].
Results and Discussion
Synthesis and Characterization
Direct esterification of 1-octanol with acrylic acid catal-
ysed by 0.5 % NaHSO4 at 110 °C results not only in the
formation of the expected octyl acrylate but also a major
side product. Column chromatographic purification and
thorough characterization of the side product using NMR
(1H and 13C), IR and mass spectroscopy indicated the
compound to be 3-(octyloxy)-3-oxopropyl acrylate. Such a
side reaction during the esterification of acrylic acid is well
documented in the literature [7, 15]. The mechanism for the
generation of this side product is by Michael-type addition
through hydroxyl functionality of one acrylic acid to the
terminal double bond of the formed fatty acrylate. In the
present work, direct esterification of 1-octanol and acrylic
acid under the studied reaction conditions resulted in very
high conversion ([95 %) and near equal specificity to-
wards the formation of octyl acrylate (48.2 %) and
3-octyloxy-3-oxopropyl acrylate (47.7 %). This indicates
that both esterification and Michael-type addition of es-
terified product happened simultaneously. There is no lit-
erature wherein such a large formation of Michael adduct
was reported during direct esterification of fatty alcohol
and acrylic acid. Further variations in reaction conditions
did not improve much the specificity towards the formation
of 3-(octyloxy)-3-oxopropyl acrylate. Accordingly, a series
of fatty alcohols (1-decanol, 1-dodecanol, 1-tetradecanol
and 1-hexadecanol) were similarly esterified with acrylic
acid to get their corresponding Michael adducts. The
structures of all the esterified products were established by
NMR (1H and 13C), IR and mass spectral data. Finally, the
sulfonation of 3-alkyloxy-3-oxopropyl acrylate was carried
The surface excess, Cmax (mol/cm2), i.e. the amount of
surfactant adsorbed per unit area at the air–water interface
after complete monolayer formation, and minimum surface
area occupied by each surfactant molecule (Amin) were
calculated using Eqs. 1 and 2 from the slope of the linear
Fig. 2 Plot of surface tension (c) versus log C of 3-(alkyloxy-3-
oxopropyl)-3-oxopropane-1-sulfonates having varying chain lengths
(C8–C16)
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Table 1 Surface properties of sodium 3-(3-alkyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate of varying alkyl chain length (C8–C16)
Surfactant
CMC (mM)
ccmc (mN/
m)
Cmax 9 1010
pC20
Amin
(nm2/mol)
DGmic (kJ/mol)
DGads (kJ/mol)
(mol/cm2)
C8-sulfonate
C10-sulfonate
C12-sulfonate
C14-sulfonate
C16-sulfonate
5.3 0.4
1.6 0.12
0.88 0.04
0.17 0.04
0.01 0.002
30.3 0.3
31.5 0.9
30.7 0.7
35.4 1.0
47.1 1.6
1.69 0.3
3.11 0.2
2.38 0.1
1.36 0.3
1.79 0.3
3.24 0.16
3.35 0.07
3.78 0.12
4.64 0.21
5.37 0.06
0.98 0.18
0.53 0.1
0.7 0.05
1.22 0.13
0.93 0.07
-26.14 0.12
-32.11 0.41
-35.09 0.74
-43.30 0.58
-57.43 0.10
-50.46 1.6
-44.57 0.41
-52.07 0.75
-68.01 1.90
-70.98 0.89
CMC critical micelle concentration measured by surface tensiometer, ccmc surface tension at CMC, pC20 efficiency of surface adsorption, Cmax
surface excess at the air–water interface, Amin minimum area per molecule at the air–water interface, DGads free energy of adsorption, DGmic free
energy of micellization
part of the surface tension plot using the Gibbs adsorption
isotherm [17].
The synthesized surfactants were also evaluated for
their surface active properties such as wetting time, cal-
cium tolerance, emulsion stability and foaming charac-
teristics. As a result of the poor solubility of the higher
homologue (C16-sulfonate), all these properties were
evaluated by preparing 0.025 % aqueous solutions of
these surfactants. The studied surfactants exhibited poor
wetting time and moderate calcium tolerance but exhib-
ited excellent foamability and emulsion stability. Except
C8-sulfonate, all other studied surfactants exhibited su-
perior emulsion stability (35–94 s for 10 mL separation
and 78–200 s for 20 mL separation) compared to sodium
lauryl sulfate (SLS; 18 s for 10 mL separation and 46 s
for 20 mL separation). The emulsion stabilities of the
studied surfactants were found to be dependent on the
hydrophobicity, showing an increasing trend (C8-sul-
fonate, 10 s for 10 mL separation and 25 s for 20 mL
separation; C16-sulfonate, 94 s for 10 mL separation and
200 s for 20 mL separation) with the increase in alkyl
chain length.
Cmax ¼ À ð1=nRTÞ ðdc=dln CÞ
Amin ¼ 1=N:Cmax
ð1Þ
ð2Þ
where R is the gas constant (8.314 J mol-1 K-1), T is
absolute temperature, c is surface tension, C is surfactant
concentration, n = 2 for 1:1 type anionic surfactant and
N is Avogadro’s number. Cmax and Amin predict the packing
pattern and orientation of the surfactant molecule at the
air–water interface. Both parameters are found to be de-
pendent on alkyl chain length or hydrophobicity of the
surfactant. However, in this studied class of surfactant, no
specific trend was observed in the values of Cmax and Amin
indicating their independence on alkyl chain length.
,
The adsorption of a surfactant at the air–water interface
and its micellization in the bulk aqueous solution occur
simultaneously. The free energy of adsorption (DG°ads) and
free energy of micellization (DG°mic) of the synthesized
surfactants at the air–water interface were calculated using
Eqs. 3 and 4 [18, 19].
DGꢀmic ¼ nRT ln CMC
ð3Þ
where R is gas constant 8.314 J mol-1 K-1, n = 2 for 1:1
type anionic surfactant and T is absolute temperature.
DGꢀads ¼ DGꢀmicÀ ðPcmc=Cmax
Þ
ð4Þ
The negative values of DG°ads and DG°mic shown in
Table 1 indicate that both adsorption and micellization
occur spontaneously at 27 °C. Generally, a surfactant with
high DG°ads and low DG°mic will be more favourable for
adsorption rather than micellization and hence will exhibit
higher CMC and vice versa. This is exactly what is ob-
served in the case of sodium 3-(3-octyloxy-3-oxopropoxy)-
3-oxopropane-1-sulfonate having a CMC in the millimolar
range (5.3 mM), whereas that for sodium 3-(3-hexadecy-
loxy-3-oxopropoxy)-3-oxopropane-1-sulfonate is in the
micromolar range (0.01 mM). Thus, with the increase in
chain length, micellization is more favourable over
adsorption.
Fig. 3 Foaming properties of 0.025 % of 3-(alkyloxy-3-oxopropyl)-
3-oxopropane-1-sulfonates having varying chain lengths (C8–C16) at
0 min and after 5 min
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The foamability and foam stability of all the synthesized
sulfonates were evaluated by a simple foaming test. The
foamability is simply the ability to produce foam immedi-
ately after agitation and the foam stability is estimated by
studying the foam volume after some time. Figure 3 shows
the foaming power of the studied sulfonates at 0 min and
also after 5 min. An increase in foamability of the surfactant
was observed with the increase in alkyl chain length,
reaching maxima at C14-sulfonate and thereafter decreas-
ing. Thus among the studied surfactants, sodium 3-(3-te-
tradecyloxy-3-oxopropoxy)-3-oxopropane-1-sulfonate ex-
hibited maximum foamability. This prompted us to pick
C14-sulfonate and study the dependence of foam stability as
a function of concentration. Figure 4 shows such depen-
dence of foam stability over 20 min at four different con-
centrations of C14-sulfonate (0.0125, 0.025, 0.05 and
0.075 %). Although the influence of surfactant concentra-
tion on foam stability is not pronounced at the studied
concentration range, an increasing trend was observed.
Pandari Phani Kumar 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 some synthesized new
surfactants’’.
Acknowledgments A Senior Research Fellowship from the Council
of Scientific and Industrial Research (CSIR), Government of India to
PPK is gratefully acknowledged.
Rati Ranjan Nayak received his Ph.D. in organic chemistry from
Utkal University. He is a scientist at the Centre for Lipid Research,
CSIR-Indian Institute of Chemical Technology, Hyderabad, India and
his research interests are in surfactants and nano-emulsions.
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Sanjit Kanjilal received his Ph.D. in organic chemistry from
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123