Synthesis of Alkyl Monoglucoside Citric Monoester and Properties of Its Sodium Salt

Article abstract of DOI:10.1007/s11743-016-1833-8

A series of disodium alkyl monoglucoside citric monoesters (AG-EC) were synthesized with an indirect method using glucosides, lauryl/decyl/octyl alcohol and citric acid. The structure and composition of the synthesized surfactants were defined by Fourier transform infrared spectroscopy (FTIR) and liquid chromatography–mass spectrometry (LC/MS). The surface properties, foaming ability and wetting ability were investigated. AG-EC surfactants exhibited excellent water solubility which eliminated the defect of long-chain alkyl glucoside. All three surfactants showed high surface activities. AG12-EC and AG10-EC showed remarkable foaming abilities in distilled water and hard water. Aqueous solutions of AG-EC surfactants spread slowly on a parafilm surface.

Full text of DOI:10.1007/s11743-016-1833-8

J Surfact Deterg (2016) 19:885–891
DOI 10.1007/s11743-016-1833-8
ORIGINAL ARTICLE
Synthesis of Alkyl Monoglucoside Citric Monoester
and Properties of Its Sodium Salt
1
1
1
1
Zhu Jin
•
Jun Zhang
•
Xiuquan Yang
•
Yuan Zhou
Received: 18 September 2015 / Accepted: 28 April 2016 / Published online: 19 May 2016
Ó AOCS 2016
Abstract A series of disodium alkyl monoglucoside citric
monoesters (AG-EC) were synthesized with an indirect
method using glucosides, lauryl/decyl/octyl alcohol and
citric acid. The structure and composition of the synthe-
sized surfactants were defined by Fourier transform infra-
red spectroscopy (FTIR) and liquid chromatography–mass
spectrometry (LC/MS). The surface properties, foaming
ability and wetting ability were investigated. AG-EC sur-
factants exhibited excellent water solubility which elimi-
nated the defect of long-chain alkyl glucoside. All three
surfactants showed high surface activities. AG12-EC and
AG10-EC showed remarkable foaming abilities in distilled
water and hard water. Aqueous solutions of AG-EC sur-
factants spread slowly on a parafilm surface.
acid with the hydroxyl groups of the APG molecule. On the
one hand, it not only retains the excellent properties of
APG but also overcomes many of its limitations such as
poor water-solubility. In addition, APG-EC has reduced
irritation to eyes and skin. Even if the mass fraction of
APG-EC reaches 30 % in an aqueous product, no irritation
of eyes is observed [5]. Because of its excellent water-
solubility, APG-EC aqueous solutions remain clear and
bright after prolonged storage near 100 °C [6]. APG-EC
shows many improved performance attributes which
includes excellent foaming, hard water resistance and
water-solubility. It can be applied to cosmetics, beauty care
products and detergents in general.
There are two main reaction schemes to synthesize
APG-EC surfactants. One is the direct esterification
method where APG is reacted with aqueous citric acid
under reduced pressure at 120 °C. During the esterification
progress, water must be removed by distillation to drive the
reaction to completion. At high reaction temperatures, a
nitrogen flow is necessary to protect the reactants from
oxidation by air [7]. The products after the direct esterifi-
cation method only contain about 60–80 % monoester of
citric acid [8]. Di- and tri-esters of citric acid exhibit almost
no surface activities. The indirect method uses anhydrous
APG to react with citric acid anhydride. Citric acid anhy-
dride is obtained by reacting anhydrous citric acid and
acetic acid anhydride in the presence of acetic acid. In the
indirect method, the esterification is carried out under
reduced pressure at 90 °C and the reaction products are
mostly monoesters of citric acid [8, 9]. Compared with the
direct method, the indirect method economizes on starting
materials and optimizes the synthesis of APG-EC.
Keywords Alkyl glucosides Á Citric monoester Á Anionic
surfactant Á Surface properties
Introduction
With the mass production and the reasonable price of alkyl
polyglucosides (APG), derivatives such as the sulfonate
and sulfosuccinate of APG have attracted researchers’
interests [1–4]. Disodium alkyl polyglucosides citrate
(
APG-EC) based on corn starch, citrus juice and natural oil,
is a type of sugar-based green anionic surfactant that is safe
and mild [3]. APG-EC is obtained by esterification of citric
&
Xiuquan Yang
apggroup@sina.com
1
The synthesis of APG 810-EC and APG 1214-EC using
the indirect method and their properties has been reported
previously [9]. However, the yields of the products were
China Research Institute of Daily Chemical Industry,
34# Wenyuan Str., Taiyuan 030001, Shanxi,
People’s Republic of China
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J Surfact Deterg (2016) 19:885–891
determined from the ester value of the reaction mixture and
the structures of the products were only characterized by
infrared spectroscopy. The relative reactions and the
properties involved with APG and APG-EC cannot be
studied clearly since APG and APG-EC both are complex
mixtures and it is hard to isolate the pure substance alkyl
glucoside (AG) from APG [3]. At the same time, it is
difficult to accurately calculate the yield of the reaction
from the ester value of reaction mixture since the by-pro-
duct also contains esters. In this work, a series of AG-EC
with different hydrophobic chain lengths were synthesized
by the indirect method from the natural materials glucose,
fatty alcohol and citric acid. The AG purified from APG
synthesized by the Fischer method, was reacted with citric
acid anhydride, instead of APG. The pathway is given in
Scheme 1. The yields of the products were determined by
HPLC method. The structures of the products were char-
acterized by infrared spectroscopy and liquid chromatog-
raphy–mass spectrometry and the surface properties of the
three products were investigated.
Reagent Co. Ltd. (Tianjin China). Acetic acid and acetic
acid anhydride were purchased from Tianjin Chemical
Reagent Factory (Tianjin China). Squalane was purchased
from Flower’s Song Fine Chemical Co. Ltd. (Guangzhou
China). All the chemicals were of analytical grade and used
without further purification. Deionized water was used in
all measurements.
Synthesis and Purification of AG
About 0.3 mol anhydrous glucose powder, 1.8 mol lauryl/
decyl/octyl alcohol and 0.5 g p-toluene sulfonic acid were
added in a four-neck flask equipped with a mechanical
stirrer, a thermometer and a vacuum distillation unit. Under
the condition that the stirring was about 300 rpm and the
pressure was 45 mmHg, the reaction finished in 3.5 h at
105 °C. A 30 % NaOH aqueous solution was added to
adjust the pH of product to 8.0 and the temperature was
slowly increased to 230 °C to remove the excess lauryl/
decyl/octyl alcohol by vacuum distillation. The content of
AG in the product was about 75 % (wt.).
The product was first dissolved in boiling water before
10 wt% acetone was added. The mixture was held at
Experiment Section
Materials
-
10 °C for a couple of hours until crystals appeared. The
crystals were then isolated by vacuum filtration and
washed three times with ice cold acetone. The resulting
white powder was held at 60 °C and 20 mmHg for 24 h.
The purification process increased the purity of AG to
95 %.
Anhydrous glucose, octanol, decanol, dodecanol, anhy-
drous ethanol, sodium hydroxide, acetone and anhydrous
citric acid were purchased from Tianjin Kermel Chemical
Scheme 1 Preparation of
AG-EC
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887
Synthesis of Citric Acid Anhydride
Surface Tension and Critical Micelle Concentration
CMC)
(
Anhydrous citric acid powder (0.1 mol) and acetic acid
anhydride (0.12 mol) reacted in the presence of 0.15 mol
acetic acid as solvent at 52 °C in a closed flask equipped
with a thermometer and a magnetic stir. After few hours,
the reaction mixture changed to colorless solution which
indicated that the reaction was finished. The yield of citric
acid anhydride was 90 %.
The surface tension was measured with a Kr u¨ ss K12 ten-
siometer at 25 ± 0.1 °C using a platinum plate at a series
of concentrations. An average value was recorded after
repeating each measurement three times. Before measure-
ment, the surface tension of de-ionized water was con-
firmed at 72.0 ± 0.3 mN/m. The accuracy of the surface
tension measurements was within 0.01 mN/m. The uncer-
tainty limits on the critical micelle concentration (CMC) of
AG12-EC, AG10-EC and AG08-EC are estimated to
be ± 2, ± 2 and ± 4 %, respectively.
Synthesis and Purification of AG-EC
The ester was prepared by reacting 0.095 mol of AG
(
dispersed in squalene) with citric acid anhydride dissolved
The uncertainties in the surface tension at the CMC
(cCMC) are estimated to be less than 1 %.
The maximum surface excess concentration (sMAX) and
the minimum surface area (Amin) per molecule were cal-
culated according to the equations derived from the Gibbs
adsorption isotherm [10].
in acetic acid at 90 °C for about 2 h with stirring. The
excess acetic acid and acetic acid anhydride were removed
by vacuum distillation. After the esterification step, the
alkyl monoglucoside citric monoester was extracted from
the mixture by adding 50 mL anhydrous ethanol. NaOH
ꢀ
ꢁ
(
50 % aqueous) was added to adjust the pH to 8 with
1
oc
sMAX ¼ À
ð1Þ
constantly stirring. The precipitated AG-EC was isolated
and dried at 55 °C and pressure of 10 mmHg. The yield of
AG12-EC, AG10-EC and AG08-EC determined by HPLC
was about 95, 96 and 96 % respectively.
2
:303nRT oLgC
T
1018
N s
2
Amin ¼
ðnm Þ
ð2Þ
A
MAX
R is the gas constant, T is the absolute temperature;
oc
oLgC T
ð
curve; N is Avogadro’s constant. The surfactant anion and
Þ is the slope of the surface tension versus the LgC
Measurements
A
counterion were both adsorbed at the surface with
Structure Characterizations
increasing surfactant concentration hence n equals 2 [10,
1
1]. The adsorption efficiency (pC ) was determined by
20
The mass spectrum of citric acid anhydride was measured
using a Shimadzu LCMS-2020 mass spectroscope. Infrared
spectroscopy (IR) measurements were performed using a
Bruker VERTEX 70 FTIR spectrometer. Citric acid anhy-
dride and AG-EC were mixed with anhydrous KBr powder
and tableted. The molecular mass and component of the
intermediate which contains mono-(alkyl monoglucoside)
citrate were determined by liquid chromatography connected
with mass spectrometry (LC–MS). The LC–MS analysis was
performed using a Shimadzu LCMS-2020 spectroscope.
The LC separation used an Eclipse Plus C18 column
the value of the negative logarithm of the surfactant con-
centration C20 at which the surface tension of water is
reduced by 20 mN/m.
Foaming Property
Foaming properties were measured by the Ross–Miles
method [12] at 50 °C. The foam volume at 30 s and
5
min was measured. The foam volume at 30 s represents
the foaming power of the sample; the ratio the volume of
0 s and 5 min was used to obtain the stability of the
3
(4.6 9 100 mm, 3.5 lm) and gradient elution with solvent A
(methyl alcohol) and solvent B (water containing 0.02 %
foam.
trifluoroacetic acid). The gradient composition was linearly
changed within 10 min from 35 % solvent A plus 65 % sol-
vent B to 60 % solvent A plus 40 % solvent B. This final
composition was held for a further 8 min. The flow was 1 mL/
min. Massscans weremeasuredfromm/z 50upto m/z 1000, at
Wetting Property
The contact angle of the products (aqueous, 1 %) on
parafilm were measured using a Kruss DSA25 Video
based contact angle measuring device. The measurement
was repeated five times and an average value was
determined.
3
50 °C interface temperature, 250 °C DL temperature,
4500 V interface voltage, neutral DL/Qarray. Mass spec-
trometry data was acquired in the negative ionization mode.
-
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J Surfact Deterg (2016) 19:885–891
Results and Discussion
Structure Characterization
Structures of AG-EC and citric acid anhydride were char-
acterized by FTIR. The structure of lauryl monoglucoside
citric monoester, the intermediate to synthesis AG -EC
1
2
was measured by LC–MS.
The FTIR spectra of AG-EC and citric acid anhydride
are shown in Fig. 1. The major absorption bands of AG12-
-
EC are: 3450 cm (-OH), 2924 and 2853 cm (C–H),
1
-1
-
1
-1
1
730 cm
(ester carbonyl), 1591 and 1392 cm
-
-COONa), and 1053 cm (C–O–C in ester);the major
1
(
Fig. 2 The mass spectrum of citric acid anhydride
-
absorption bands of AG10-EC are: 3442 cm (-OH), 2927
1
-
and 2853 cm (C–H), 1732 cm (ester carbonyl), 1581
1
-1
-
1
-1
and 1406 cm
(-COONa), and 1053 cm
(C–O–C in
1
unreacted citric acid anhydride and AG12, respectively.
Peaks b, d, f and g indicate lauryl monoglucoside citric
monoester and isomers which were obtained by citric acid
anhydride reacting with hydroxyl at different loci on the
AG12. In Fig. 3b, corresponding to the peak c in the LC
chromatogram, the mass spectrum of lauryl monoglucoside
citric monoester was characterized by a molecular ion [M–
-
ester);the major absorptions of AG08-EC are: 3435 cm
-
-OH), 2925 and 2856 cm
1
-1
(
(C–H), 1733 cm
-
(ester
1
carbonyl), 1580 and 1406 cm
(-COONa), and
-
1
053 cm (C–O–C in ester). The major absorptions of
1
-
citric acid anhydride are 3492 cm (-OH), 2945 cm
1
-1
-1
(
C–H), 1868 and 1768 cm
(anhydride carbonyl),
-1
697 cm (acid carbonyl), 1230 cm (C–O in cycle-an-
-1
-
1
H] at m/z = 521. In Fig. 3c, corresponding to the peak e
hydride). Compared with citric acid anhydride, the signals of
ester and -COONa in the FTIR spectra of AG-EC indicate
successful synthesis of AG12-EC, AG10-EC and AG08-EC.
The mass spectrum of citric acid anhydride is shown in
Fig. 2, the major peaks (m/z, negative) are 60.1 (McLaf-
in the LC chromatogram, the mass spectrum of lauryl
glucoside was characterized by the peaks (m/z, negative) at
-
347 [M–H] , 379 [M ? CH
-
OH-H] , 695 [2 M-H] , and
-
3
-
718 [2M ? Na–H] .
-
-
ferty rearrangement), 111.2 [M-60-H] , 173.2 [M-H] , and
-
91.2 [Mcitric acid –H] . Starting raw material citric acid
Surface Activity
1
and the by-product (acetate of citric acid) are seldom
observed in the mass spectrum.
The surface tension and the surface activity parameters of
AG-EC in water are shown in Fig. 4 and Table 1, respec-
tively. Table 1 shows that AG12-EC, AG10-EC and
AG08-EC reduced the surface tension of distilled water to
a range of 30–34 mN/m, which indicated that AG-EC
molecules could adsorb at the air–water interface strongly.
Similar to normal surfactants, the CMC decreased with
increasing length of the hydrophobic carbon chain [10].
The cCMC values of AG-EC first increased and then
decreased with increase in the hydrocarbon chain length.
The sMAX values of AG-EC first decreased and then
increased with increasing hydrocarbon chain length. The
In the LC chromatogram of lauryl monoglucoside citric
monoester shown in Fig. 3a, peaks a and e represent
lowest sMAX value of AG -EC could be explained by the
10
co-effect of the electrostatic repulsion between the large
head groups of AG -EC molecules and the attrac-
1
0
tive forces between the AG -EC molecules. The style of
1
0
AG10-EC packing at air–water surface is different from
AG12-EC and AG08-EC, which occupied more surface
area per surfactant headgroup. The Amin of AG10-EC
which shows the packing densities [13] is smallest among
the three surfactants demonstrated the assumption above.
The cCMC of AG08-EC is lower than AG12-EC which may
be due to the hydrophobic carbon chain of AG08-EC much
Fig. 1 The FTIR spectra of AG-EC and citric acid anhydride
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J Surfact Deterg (2016) 19:885–891
889
shorter than AG12-EC. The molecules of AG08-EC
packing are closer than AG12-EC molecules at water/air
interface so that AG -EC obtained a smaller Amin, a bigger
0
8
sMAX and a lower cCMC [14–16].
With the increase in the tail length, the pC20 values of
AG-EC increased. The efficiency of adsorption of a sur-
factant at the water surface is represented by the pC20
values. These surfactant molecules of a larger pC20 are
adsorbed at the water/air interface more efficiently and
reduce surface tension of water more efficiently [10, 17].
Foaming Ability
Due to poor water solubility, the foaming ability of AG is
also rather poor [18]. By introducing the –COONa head
group, the solubilities of AG-EC in water are greatly
improved. As seen in Fig. 5, AG12-EC and AG10-EC show
excellent foaming power. With the increasing carbon atom
number of hydrophobic groups, the foaming power of AG-
EC increased in both distilled water and hard water. The
longer the hydrophobic groups, the lower the CMC and the
more efficient the surfactant is as a foamer [10]. The foam
stabilities of AG-EC increased with increasing hydrocar-
bon chain length. On the one hand, the increased
hydrophobic chain length enhances the attraction between
molecules, which makes the surface film more stable [10,
19]. On the other hand, the viscosity of surface film increases
and the rate of liquid drainage slows down which improves
the foam stability [10, 20]. With regard to AG08-EC, a short
hydrocarbon chain causes poor foam stability and a high
CMC (2.6 g/L) beyond the concentration used in the foam-
ing test. The AG08-EC monomers cannot transfer to the new
air/water interface because of the insufficient micelle in bulk
water [21]. So the AG08-EC exhibited poor foam ability.
The foaming power and foaming stability of AG-EC were
Fig. 3 The LC–MS spectrum of lauryl monoglucoside citric
monoester
2
?
better in hard water than in distilled water. The Ca in the
hard water was adsorbed into the surface layer by the nega-
2
?
tive surfactant ions. As divalent counterions, Ca drove the
surfactant molecules to pack tighter which decreased the
cross-sectional area of surfactant monomers and the elec-
trostatic repulsion between charged head groups and
improved the strength of the surface film [10, 20–22].
Wetting Ability
As shown in Fig. 6, the wetting abilities of AG-EC are not
ideal. The contact angles of AG12-EC, AG10-EC and
AG08-EC solutions (1 %, wt.) on paraffin film are 73.5°,
70.9° and 62.3° respectively. The large electrostatic
repulsion between the head group yields a loosened
assembly at the solid–liquid interface [10]. Not in agree-
ment with normal surfactants, the contact angles of AG-EC
decrease with decreasing hydrocarbon chain length [10].
Fig. 4 Surface tension versus the logarithm of the aqueous molar
concentration of the AG-EC at 25 °C
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J Surfact Deterg (2016) 19:885–891
Table 1 Values of CMC, cCMC
,
2
2
Surfactant
CMC (mmol/L)
cCMC (mN/m)
s
MAX (lmol/m )
A
min (nm )
pC20
sMAX, Amin and pC20 for AG-EC
AG12-EC
AG10-EC
AG08-EC
0.23
1.04
5.10
31.94
33.60
29.72
1.74
1.59
1.80
0.95
1.05
0.92
4.64
4.01
3.39
characterized by FTIR and LC–MS. Moreover, the surface
properties of the products were also investigated. The
lowest surface tension values obtained with 0.23 mmol/L
AG12-EC, 1.04 mmol/L AG10-EC and 5.10 mmol/L
AG08-EC are 31.94, 33.60 and 29.72 mN/m respectively.
AG12-EC and AG10-EC show good foam ability in both
distilled water and hard water. The contact angles of these
1
% aqueous solutions of AG12-EC, AG10-EC and AG08-
EC on parafilm are 73.5°, 70.9° and 62.3°, respectively.
Because they show many excellent properties in surface
activity and nonirritating effects on humans, AG-EC may
be used in wide applications in cosmetics and personal care
products.
Fig. 5 The foaming properties of AG-EC
Acknowledgments The authors would like to thank the Chinese
National Science & Technology Pillar Program during the Twelfth
Five-year Plan Period (Grant No 2014BAE03B03) and the Youth
Fund of Shanxi Province (Grant No 2014021015-6) for financial
support.
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Jun Zhang is a chemical engineer at the China Research Institute of
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University in 2009. His current research interests are surfactant
analysis and formulation design.
8
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Xiuquan Yang obtained his M.Sc. in fine chemicals from Dalian
University of Technology in 1990. He is now the Vice-Chief Engineer
at the China Research Institute of Daily Chemical Industry. His
research focuses are on synthesis, application of natural-derived fine
chemicals and the transfer of technological achievements.
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