Synthesis and property of alkyl dioxyethyl α-D-xyloside

Article abstract of DOI:10.1016/j.molliq.2020.113770

Due to the inherent defects of the long alkyl chain in the related hydrophilicity and water solubility, alkyl α-D-xylosides (7) had hardly the practical application as sugar-based surfactants and should be reconstructed to obtain alkyl dioxyethyl α-D-xylosides (5) with dioxyethylene fragment (?(OCH₂CH₂)₂-)) as the hydrophilic spacer to increase the related TPSA value. With D-xylose as the raw material, 1,2-cis alkyl dioxyethyl α-D-xylosides (5a-5f, n = 6–12) were stereoselectively synthesized. Their physicochemical properties including water solubility, surface tension, foamability, emulsification, thermotropic liquid crystal, and hygroscopicity had been investigated. Their water solubility was found to decrease gradually whereas their calculated HLB numbers were 14.72 → 11.67 (n = 6 → 12) with increasing alkyl chain length (n). Dodecyl dioxyethyl α-D-xyloside (5f) had not water solubility because the HLB number was low. Furthermore, their CMC values decreased with increasing the alkyl chain length, and the CMC value of decyl dioxyethyl α-D-xyloside (5e) was as low as 9.21 × 10^?5 mol·L^?1. Octyl dioxyethyl α-D-xyloside (5c) had the lowest surface tension (27.25 mN·m^?1) at the CMC. Both of nonyl and decyl dioxyethyl α-D-xylosides (5d & 5e) possessed good foaming power and foam stability. Decyl dioxyethyl α-D-xyloside (5e) had the strongest emulsifying property either in the toluene/water system or in the octane/water system. Nonyl dioxyethyl α-D-xylosides (5d) had the most stylish S_A texture. Hexyl dioxyethyl α-D-xyloside (5a) possessed the strongest hygroscopicity. Therefore, the alkyl dioxyethyl α-D-xylosides as a class of novel sugar-based surfactants will be widely considered as promising candidates for various practical applications.

Full text of DOI:10.1016/j.molliq.2020.113770

Journal of Molecular Liquids 315 (2020) 113770
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and property of alkyl dioxyethyl α-D-xyloside
Xiubing Wu, Na Kuang, Langqiu Chen ⁎, Yulin Fan, Fang Fu, Jiping Li, Jing Zhang
College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, Xiangtan University, Xiangtan City, 411105, Hunan, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2020
Received in revised form 1 July 2020
Accepted 6 July 2020
Available online 08 July 2020
Due to the inherent defects of the long alkyl chain in the related hydrophilicity and water solubility, alkyl α-D-
xylosides (7) had hardly the practical application as sugar-based surfactants and should be reconstructed to ob-
tain alkyl dioxyethyl α-D-xylosides (5) with dioxyethylene fragment (−(OCH
to increase the related TPSA value. With D-xylose as the raw material, 1,2-cis alkyl dioxyethyl α-D-xylosides (5a-
f, n = 6–12) were stereoselectively synthesized. Their physicochemical properties including water solubility,
2 2 2
CH ) -)) as the hydrophilic spacer
5
surface tension, foamability, emulsification, thermotropic liquid crystal, and hygroscopicity had been investi-
gated. Their water solubility was found to decrease gradually whereas their calculated HLB numbers were
14.72 → 11.67 (n = 6 → 12) with increasing alkyl chain length (n). Dodecyl dioxyethyl α-D-xyloside (5f) had
not water solubility because the HLB number was low. Furthermore, their CMC values decreased with increasing
the alkyl chain length, and the CMC value of decyl dioxyethyl α-D-xyloside (5e) was as low as
Keywords:
Sugar-based surfactants
Alkyl dioxyethyl α-D-xyloside
Synthesis
Water solubility
Surface activity
.21 × 10− mol·L . Octyl dioxyethyl α-D-xyloside (5c) had the lowest surface tension (27.25 mN·m ) at
5
−1
−1
9
the CMC. Both of nonyl and decyl dioxyethyl α-D-xylosides (5d & 5e) possessed good foaming power and
foam stability. Decyl dioxyethyl α-D-xyloside (5e) had the strongest emulsifying property either in the tolu-
ene/water system or in the octane/water system. Nonyl dioxyethyl α-D-xylosides (5d) had the most stylish S
A
texture. Hexyl dioxyethyl α-D-xyloside (5a) possessed the strongest hygroscopicity. Therefore, the alkyl
dioxyethyl α-D-xylosides as a class of novel sugar-based surfactants will be widely considered as promising can-
didates for various practical applications.
©
2020 Published by Elsevier B.V.
1
. Introduction
renewable resources to relieve/delay the energy crisis/depletion to certain
content. Therefore, to avoid such an obstacle, much attention should be
paid to the efficient synthesis of the related surfactants rather than
petroleum-based surfactants.
For the fatty alcohol polyoxyethylene ethers (AEO) and the related de-
rivatives, such as fatty alcohol polyoxyethylene ether carboxylate (AEC),
sulfonate and sulfate, these surfactants have good biodegradability,
water-solubility, salt tolerance, resistance to hard water, wetting ability,
emulsifying capacity, foaming ability, detergency performance, lubrica-
tion, and flotation behavior, so they should have application value to a cer-
tain extent [1–4]. For example, AEO with a rather flexible hydrophilic part,
as the conventional poly(ethylene oxide) (PEO)-based surfactants, can
adsorb strongly on silica/water and air/water interfaces with the hydro-
gen bonding as the main driving force for the adsorption [3,4]. However,
one may perceive an issue that the raw material ethylene oxide (EO)
and its derivatives could be prepared from petroleum oil [5]. Due to a
lot of unexpected events including the subprime mortgage crisis in 2007
and the current COVID-19 (the Corona Virus Disease 2019) pandemic,
the relevant global economy should be seriously dragged down, resulting
in a short-term downward tendency on oil prices [6,7]. Nevertheless, the
petroleum oil, as a scarce fuel and non-renewable chemical raw materials,
should be replaced to afford other valuable surfactants by some
On the contrary, carbohydrates should be attractive bioorganic ma-
terials on the earth because they are natural sources, environmentally
friendly, and highly functionalized, thus promoting green and sustain-
able development in chemistry, life science, and functional materials
[3,8]. Besides, sugar-based surfactants would indeed be available for
various potential applications such as the chemical cleaning [9], the flo-
tation [10], the oil recovery [11,12], the oil spill treatment [13], the tex-
tile printing and dyeing [14], the pesticides [15], the liquid crystal
materials [16–19], the nanotechnology [20], the emulsion [21], the
membrane protein research [22–25], the chemical catalysis [26], the
pharmacology [27–31], the bacterial ecology [32], the toxicology [33],
the antibacterial activity [34,35], the cosmetics [36], and biotechnology
[37] due to their excellent surface activity, fine and stable foam, good
biocompatibility, low/non-toxic, seldom irritation, easy biodegradation,
and strong detergency. Also, such sugar-based surfactants would rather
stay in the aqueous phase than adsorb on silica [3,4].
n n
Besides polyethyleneglycol monoalkyl ether (C E ) [38], alkyl glyco-
sides such as alkyl xylosides [32], alkyl polyglucosides (APG) [39], alkyl
⁎
Corresponding author.
E-mail address: chengood2003@263.net (L. Chen).
β-D-maltoside [38], and NBD-alkyl lactosides [40], are classic nonionic
https://doi.org/10.1016/j.molliq.2020.113770
167-7322/© 2020 Published by Elsevier B.V.
0

2
X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
surfactants. Because of the unique function of hydrophilic glycosyl group,
the aqueous solutions of the sugar-based surfactants are less
temperature-sensitive than those of alkyl polyoxyethylene ether solutions
although the relevant water solubility decreases continuously with the
extension of hydrophobic chain length, and such results should hint that
the corresponding glycosides would be more advantageous than the cor-
responding non-ionic alkyl polyoxyethylene ethers in certain fields of ap-
plication. Unlike alkyl polyoxyethylene ethers and polysorbates
surfactants, alkyl glycosides surfactants such as dodecyl maltoside can
be prepared as pure single-chemical species, so that it should be escaped
from the serious issue of the mixtures with the various activity and more
or less side products and even residual starting material. Furthermore,
alkyl glycosides should be difficult to subject to oxidative degradation so
that the related harmful immune response would be not elicited. There-
fore, they can offer a potential alternative in the related therapeutic for-
mulations, whether oral, nasal, anal and external use or injection [41].
However, for both complex APG and anomerically pure alkyl glyco-
sides with long hydrophobic chains, their water solubility should be
poor since their hydrophilicity should be low [42,43]. In this context,
their applications would also be restricted to some content because of
their low or no water solubility. Therefore, it would be eager to develop
new surfactants with proper water solubility and excellent surface activity
with long hydrophobic chains.
To the best of our knowledge, D-xylose should be the second most
abundant monosaccharide with a reasonable low price after glucose be-
cause its natural polymer xylan is a renewable heterogeneous polysaccha-
ride existing in the plant cell wall, accounting for about 15–35% of the dry
weight of plant cells. As a major component of plant hemicellulose and a
complex polypentacarbose, D-xylose can be easily obtained from the agri-
cultural wastes and other biomass such as corn spike, bagasse wheat bran,
and straw [37,43–45]. Thereof, we should make full of xylan,
xylooligosaccharides and xylose to convert various value-added materials
including xylitol, succinic acid, furfural, calorie-free sweetener, baked
products, functional food (for the growth of probiotic Bifidobacterium),
high-grade soy sauce color, pharmaceutical excipients, drug, chiral mole-
cules, alkyl xylosides, and other products [32,43,46–50].
2 2 2
to introduce dioxyethylene fragment (−(OCH CH ) -)) as a hydrophilic
spacer to prepare alkyl dioxyethyl α-D-xylosides (5) with D-xylose as
raw material [42,51–53]. Their properties including HLB number,
water-solubility, surface tension, foamability, emulsification, thermo-
tropic liquid crystal, and hygroscopicity would be investigated as well,
and the relationship between their structure and properties would be
further disclosed. The data should provide a theoretical basis for their
practical application value and market potential as the nonionic sugar-
based surfactants.
2. Experimental
2.1. Material and instrument
Surface tension was measured with a BZY-2 full-automatic surface/
interfacial tensiometer employing the Wilhelmy plate method (Shang-
hai Hengping Instrument and Meter Factory, China). Differential scan-
ning calorimetry (DSC, TA-Q10, TA Instruments) was used for tracking
the thermal transitions of the samples through a heating and cooling
−1
procedure at a scanning rate of 5 °C·min with nitrogen as sweeping
fluid. Liquid crystal texture was carried out by polarized optical micros-
copy (POM, Leica DM-LM-P) equipped with a Leica heating stage
−1
(FP82HT), the samples were heated and cooled at 5 °C·min . All chem-
ical reagents were analytically pure or chemically pure.
2.2. Synthesis
As shown in Fig. 1, alkyl dioxyethyl α-D-xylosides (5a-5f) were readily
prepared by the Helferich method through three-step reactions. First of
all, with D-xylose (1) as raw material, acetylation was taken to afford
1,2,3,4-tetra-O-acetyl-D-xylopyranose (2) at the catalysis of anhydrous
NaOAc. Next, alkyl dioxyethyl 2,3,4-tri-O-acetyl-α-D-xyloside (4a-4f)
was stereoselectively synthesized by the coupling reaction of 2 and
4
diethylene glycol monoether (3a-3f) with SnCl as the Lewis acid catalyst.
Finally, alkyl dioxyethyl α-D-xyloside (5a-5f) was prepared through the
deacetylated process of 4a-4f. The detailed preparation and structural
characterization were documented in the Support Information (SI).
However, compared with other alkyl monoglycosides, alkyl
xylosides should not be suitable for hydrophilic surfactants since they
were not water solubility with longer alkyl chain (n) due to their
small hydrophilic sugar portion. For the hexoses such as glucose and ga-
lactose, they have a strong hydrophilic hydroxymethyl group, but xy-
lose lacks such group so that xylose's derivatives alkyl xylosides had
the low water solubility and were even insoluble in water for the slight
long alkyl chain [32,43]. Thus, the application of alkyl xylosides as sur-
factants would be severely limited. To make full use of the renewable
and economical D-xylose from the agricultural waste, how to solve the
water solubility barrier of its derivatives alkyl xylosides as a valuable
surfactant is likely to be a key issue in the sustainable and environmen-
tally friendly development process.
3. Property
3.1. Hydrophilic-lipophilic balance (HLB) number and octanol-water parti-
tion coefficients (LogP) value
The HLB numbers of alkyl dioxyethyl α-D-xylosides (5a-5f) were
calculated from Eq. (1) by the Griffin method [53–55].
HLB ¼ 2
0X
W
ð1Þ
As shown in Fig. 1, in order to improve further the corresponding hy-
drophilicity and water solubility of alkyl α-D-xylosides (7), we attempt
where X and W were the molecular mass of the hydrophilic portion and
the whole molecular mass, respectively.
HO(CH CH O) (CH ) CH3
O
2
2
3
2
2 n-1
O
HO
HO
NaOAc AcO
OH
AcO
OAc
OH
Ac O
2
OAc
1
2
SnCl4
O
MeONa
MeOH
O
AcO
AcO
HO
HO
OAc
OH
O(CH CH O) (CH ) CH3
2
2
2
2 n-1
O(CH CH O) (CH ) CH3
4
5
2
2
2
2 n-1
n = 6 7 8 9 10 12
a b c d e
f
Fig. 1. Synthesis of alkyl dioxyethyl α-D-xylosides (5a-5f).

X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
3
The LogP value of the synthetic alkyl dioxyethyl α-D-xyloside (5a-
f) was calculated by using ChemBioDraw (Ultra 14.0) to estimate
their lipophilic character [53].
determined at 25 °C. An aqueous solution of alkyl dioxyethyl α-D-
xylosides (0.25% (w/w)) 10 mL was obtained and transferred to a
100-mL graduated cylinder with a plug. The initial volume of foam
5
(
V
0
) was recorded just after 60 s of intense shock, which evaluating
the foaming performance according to the initial volume of foam. Just
after the solution kept in a static state for 5 min, the foam volume (V
3
.2. Solubility
5
)
The solubility of alkyl dioxyethyl α-D-xyloside (5a-5f) in water or
was recorded, the foam stability was evaluated by Eq. (8) according to
ethanol was determined at room temperature (25 °C) with the de-
the foam disappearance rate (υ).
scribed methods [52].
V0−V5 À
Á
3
−1
υ ¼
cm s
ð8Þ
3
.3. Surface tension
t
The surface tensions were measured at room temperature (25 °C) by
using the BZY-2 full-automatic surface/interfacial tensiometer that was
calibrated by double-distilled water. The aqueous solutions of alkyl
dioxyethyl α-D-xylosides (5a-5e) were freshly prepared as a stock solu-
tion and then diluted to the desired concentration for each measurement.
The Wilhelmy plate was rinsed with water, then flamed, whereas the
glassware was rinsed with water and washed once with the sample solu-
tion to be tested before testing. The surface tension was measured three
times for each concentration to reduce the measurement error.
3.5. Emulsifying property
According to the literature [58], the emulsifying property of alkyl
dioxyethyl α-D-xylosides (5a-5e) was measured at 25 °C. A series of
aqueous surfactant solutions (0.25% (w/w)) were prepared. At first, a
20 mL aqueous solution of alkyl dioxyethyl α-D-xyloside was put into
a 100-mL cylinder with a plug, and n-octane of 20 mL as the oil phase
was added. The related mixture was strongly shocked for 60 s and
kept in a static state for 1 h, and the volumes of the water layer, the
emulsion layer, and the oil layer were recorded respectively. The emul-
sifying ability was determined according to the volume of the emulsion
layer.
The critical micelle concentration (CMC) and the surface tension at
the CMC were determined from the breakpoint of the plot of the surface
tension concerning the logarithm of the concentration. The relevant sur-
factant surface excess concentration at the air-solution interface (Γmax
)
was calculated out by using the Gibbs adsorption isotherm equation
(
Eq. (2)) [52,56].
1
3.6. Thermotropic phase behavior
dγ
The thermal phase transition behavior of alkyl dioxyethyl α-D-
xylosides was measured by polarizing optical microscopy (POM) during
the heating and cooling process, and the rate of temperature change
Γmax ¼ −
ꢀ
:303 nRT d logC
ð2Þ
2
where γ was the surface tension, C was the surfactant concentration, T
was the thermodynamic temperature (K), R represents the gas constant
(
−
1
was controlled at 5 °C∙min . The phase transition temperature of
alkyl dioxyethyl α-D-xylosides was observed by DSC [53].
−
1
−1
8.314 J·mol ·K ), herein, n = 1 because alkyl dioxyethyl α-D-
xylosides should be classified as a nonionic surfactant, and dγ/dlogC
was the slope below CMC in the surface tension plots. The area occupied
by the surfactant molecule at the air-solution interface (Amin) was calcu-
lated by Eq. (3).
3
.7. Hygroscopicity
Approximately 1.00 g of each dried sample was placed in hermetic
pots in the surround of the relative humidity (RH = 81% or 43%), the
1
sample was weighed again after 2 h, 4 h, 6 h, 8 h, 12 h, 24 h, 48 h,
Amin
¼
ð3Þ
7
2 h, respectively. The hermetic pots were kept in an incubator with
NAΓmax
controlled temperature (25 °C). The hygroscopicity (M) of alkyl
dioxyethyl α-D-xylosides was expressed by Eq. (9) [55].
where N
A
is Avogadro's number (6.02 × 1023). Besides, the effectiveness
of reducing surface tension (πCMC) was obtained by Eq. (4). The adsorp-
tion efficiency of reducing surface tension (pC20) was calculated by
Eq. (5) [52].
Mt−M0
M0
M ¼
ꢁ 100%
ð9Þ
πCMC ¼ γ −γ
ð4Þ
ð5Þ
0
CMC
where M
samples after the test time.
0
was the initial quality of samples, and M
t
was the quality of
pC20 ¼ − lgC20
4. Results and discussion
Herein, γ
0
represented the surface tension of the distilled water,
γ
CMC was the surface tension of the aqueous solution at the CMC. C20
4
.1. Preparation and structural characterization
was the surfactant concentration required to reduce the surface tension
by 20 mN·m
The standard free energy of micellization (ΔGmic) and surface ad-
sorption free energy (ΔGads) for the nonionic surfactants were calcu-
lated using the relevant equations (Eqs. (6) and (7)) [52,57].
−1
.
In Fig. 1, with D-xylose (1) was used as the starting material, alkyl
dioxyethyl α-D-xylosides (5a-5f) were readily prepared by a three-
step reaction involving acetylation, stereoselective coupling with
diethylene glycol monoether (3a-3f) and deacetylation. Herein, the
Helfrich procedure was concisely used to get stereoselectively a series
of protected α-D-xylosides (4a-4f). The corresponding chemical shift
ΔGmic ¼ RT lnðCMCÞ
ð6Þ
ð7Þ
ΔGads ¼ ΔGmic−πCMC=Γmax
(δ) (coupling constant) of the anomeric hydrogen (H-1) in CDCl
5
3
was
.06 ppm (J1,2 = 3.6 Hz) (4a). The data disclosed that the coupled com-
pound was 1,2-cis α-anomer [51,53]. The protected α-D-xylosides (4a-
4f) were deacetylated to obtain alkyl dioxyethyl α-D-xylosides (5a-5f)
which were 1,2-cis α-anomers by the NMR spectroscopy [51,59]. The
preparation process and the NMR data were compiled in Supporting in-
formation (SI) in detail. Table 1 also showed that the chemical shifts (δ)
3
.4. Foaming property
According to the literature [52,53], the foaming property of the
aqueous solution of alkyl dioxyethyl α-D-xylosides (5a-5e) was

4
X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
Table 1
molecules involving the molecular transport properties, particularly in-
testinal absorption, and blood-brain barrier (BBB) penetration. How-
ever, in the virtual sugar-based surfactant library, they should also be
helpful in the respect of fast and effective screening of water solubility,
surface activity, bioavailability, and other functionality.
Chemical shift (δ, ppm) (J1,2, Hz) of alkyl dioxyethyl β-D-xyloside (5a-5f) in solvent.
Glycoside (n)
Chemical shift
a (6)
b (7)
c (8)
d (9)
e (10)
f (12)
4.85
(3.7)
4.88
(3.6)
4.86
(3.6)
4.85
(3.6)
4.61
(3.7)
4.61 (3.6)
(J
1,2
)
solvent
D
2
O
D
2
O
D
2
O
D
2
O
DMSO‑d
6
DMSO‑d
6
/D
2
O
The solubility of alkyl dioxyethyl α-D-xylosides (5a-5f) in water and
ethanol at 25 °C was shown in Fig. 3. Their solubility in water and in eth-
anol decreased gradually with increasing alkyl chain length (n). The sol-
ubility in ethanol was bigger than that in water. Shorter alkyl chain α-D-
xylosides (5a-5e, n = 6–10) were observed to have better water solu-
bility. Among them, decyl dioxyethyl α-D-xyloside (5e) could still be
dissolved in water with the water solubility of 41.8 g, and the related
HLB number was 12.54, but dodecyl dioxyethyl α-D-xyloside (5f) was
insoluble in water, and the related HLB number was 11.67, which was
smaller than 12.00. Therefore, alkyl dioxyethyl α-D-xylosides were ob-
served to have not water solubility with n ≥ 12, HLB ≤ 11.67, and LogP
≥2.79. For octyl/decyl/dodecyl α-D-glucopyranoside (8c/8e/8f), their
HLB numbers, logP values, TPSA values and water-solubilities were
(
the related coupling constants) of H-1 for the prepared 1,2-cis α-D-
xylosides (5a-5f) were 4.61–4.88 ppm (J1,2 = 3.6–3.7 Hz).
4
.2. Hydrophilic characteristics
The HLB number is introduced to be convenient for the explanation
of the functional properties of various surfactants. The high HLB number
should be ascribed to the hydrophilic surfactants, which would be hand-
ily used to construct spontaneously the oil-in-water (O/W) micellar mi-
crostructure by supramolecular self-assembly. Whereas the lipophilic
surfactants would be considered to possess low HLB number, they
would be applied to stabilize spontaneously water-in-oil (W/O) emul-
sions. In this paper, the related HLB numbers of alkyl dioxyethyl α-D-
xylosides (5a-5f) were calculated by the Griffin method, and the results
were listed in Table 2 [53–55].
2
12.3/11.2/10.3, 0.90/1.73/2.57, 99.38/99.38/99.38 Å , and 0.51 g/
0.06 g/insoluble, in respective. Whereas, for octyloxyethyl/
decyloxyethyl/dodecyloxyethyl α-D-glucopyranoside (9c/9e/9f), their
HLB numbers, LogP values, TPSA values and water solubilities were
2
13.3/12.2/11.4, 0.74/1.58/2.41, 108.61/108.61/108.61 Å , and 54.76/
From Table 2, the HLB numbers of alkyl dioxyethyl α-D-xylosides (5)
were from 14.72 (n = 6) to 11.67 (n = 12) with increasing alkyl chain
length (n). By making a sharp contrast with alkyl α-D-xylosides (7), the
HLB number of alkyl dioxyethyl α-D-xyloside (5a-5f) was bigger than
the HLB number of alkyl α-D-xyloside (7a-7f) with the same alkyl
chain length [53].
22.59/3.02 g, in respective. Hence, dodecyl dioxyethyl α-D-xylosides
(5f) and dodecyl α-D-glucopyranoside (8f) had similar TPST values
(97.61 and 99.38 Å ), so that they were generally considered as insolu-
2
bility in water.
As compared to the water solubility of alkyl α-D-xylosides (7), the
water solubility of alkyl dioxyethyl α-D-xyloside (5) was much higher
than alkyl α-D-xylosides (7) with the same alkyl chain length (n). The
α-D-xylosides (7) with n ≥ 8 showed very low water solubility. Indeed,
decyl α-D-xyloside (7e) was insoluble in water so that such alkyl α-D-
xylosides (7) should be hardly found to have the practical application
as hydrophilic sugar-based surfactants. Hence, it should be considered
how to reconstruct the structure? It should be said that a rational meth-
odology was used to improve the water solubility, i.e. dioxyethene frag-
The logarithm of the partition coefficient between octanol-water
(
LogP) is often used to represent the molecular lipophilicity, which is
an important parameter in many areas, including modeling pharmaco-
logical and toxicological properties, metabolism of molecules, the envi-
ronmental fate of chemicals, aggregation of surfactants, detergency and
coagulation. The LogP values of alkyl dioxyethyl α-D-xylosides (5a-5f)
were calculated by ChemBioDraw (Ultra 14.0) to estimate their hydro-
phobicity [58].
2 2 2
ment (−(CH CH O) -) as the hydrophilic spacer was inserted into
From Table 2, the LogP values of alkyl dioxyethyl α-D-xylosides (5)
were smaller than the LogP values of alkyl α-D-xylosides (7) with the
same alkyl chain length. The data should be considered as reasonable
and rather valuable since the dioxyethylene fragment as a hydrophilic
between the xylosyl and alkyl group to enrich the hydrophilic partition
and increase their HLB numbers and TPSA values. A series of novel com-
pounds alkyl dioxyethyl α-D-xyloside (5) were prepared to solve suc-
cessfully the issue. The experimental result has made an active reply.
The results revealed that the dioxyethylene segment indeed
strengthened hydrophilicity and improved water solubility by enriching
the intermolecular hydrogen-bond network. Besides, alkyl dioxyethyl
α-D-xyloside with such hydrophilic fragment can broaden the applica-
tion scope due to increasing the TPSA values (79.15 Å2 → 97.61 Å )
and improving the water solubility.
group was introduced to enlarge from the xylosyloxy group (C
5
H
9
O
5
-)
-)
of 7 to the xylosyloxyethyloxyethyloxy group (C (CH CH O)
5
H
9
O
5
2
2 2
of 5 for the related hydrophilic partion, just as described in Fig. 2. Mean-
while, their topological polar surface areas (TPSA) were estimated as
2
2
9
7.61 Å by ChemBioDraw (Ultra 14.0), obviously higher than the re-
2
lated TPSA (79.15 Å ) of 7 or its anomer alkyl β-D-xylosides (6) [8,43].
Hence, their difference (97.61–79.15 = 18.46 Å ) between 7 and 5 in-
2
deed responded to the hydrophilic impact due to the insertion of the hy-
drophilic heteroatoms (two oxygen atoms) with the hydrophilic spacer
4.3. Interfacial property
(
−(CH
2
CH
2
O)
2
-). At first, the related data of the HLB number, LogP
The surface tension of different concentration of aqueous alkyl
dioxyethyl α-D-xylosides (5a-5e) solution was measured by the BZY-2
full-automatic surface/interfacial tensiometer. However, the surface
tension of dodecyl dioxyethyl α-D-xyloside (5f) was not investigated
because of its insolubility in water.
value, and TPSA have used bioavailability screening and molecular mod-
ification of the huge druglike molecules, prodrugs, and other active
Table 2
Fig. 4 presented the relationship between the surface tension (γ)
and the concentration (C). Increasing the alkyl chain length resulted in
decreasing the critical micelle concentration (CMC). The surface tension
value (γCMC) firstly decreased and then increased with increasing the
alkyl chain length at the CMC although their difference was not very ob-
vious, and octyl dioxyethyl α-D-xyloside (5c) possessed the lowest sur-
face tension (27.26 mN∙m ) at its CMC. Compared with some
fundamental interfacial adsorption parameters of the traditional alkyl
β-D-xylosides, the CMC values and γCMC values of the synthesized
alkyl dioxyethyl α-D-xylosides were found to be less than that of alkyl
D-xylosides [43]. The results indicated that introducing the
HLB numbers and LogP values of alkyl α-D-xylosides and alkyl dioxyethyl α-D-xylosides.
Alkyl
LogP HLB
Water
Alkyl
LogP HLB
Water
α-D-xyloside
solubility dioxyethyl
solubility
(n)
α-D-xyloside
(n)
7
7
7
7
7
7
a (6)
b (7)
c (8)
d (9)
e (10)
f (12)
0.60 12.73 Soluble
1.02 12.01 Soluble
1.43 11.37 Low
1.85 10.79 Low
2.27 10.27 Insoluble 5e (10)
3.10 9.37 Insoluble 5f (12)
5a (6)
5b (7)
5c (8)
5d (9)
0.29 14.72 High
0.70 14.10 High
1.12 13.54 High
1.54 13.02 Soluble
1.96 12.54 Soluble
2.79 11.67 Insoluble
−
1

X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
5
Fig. 2. The hydrophilic fragment is introduced to form alkyl dioxyethyl xyloside.
example, hexyl dioxyethyl α-D-xyloside (5a), more free energy was re-
quired to form micelles and thereby resulting in a higher CMC value
−
4
−1
(
8.31 × 10
would stretch out and had in contact with water to some content. Fur-
thermore, the values tended to increase from
mol·L ) due to its small hydrophobic group which
Γ
max
−2
−6
−6
−2
4
1
.93 × 10 mol·m (5a, n = 6) to 6.64 × 10 mol·m (5e, n =
0) with increasing the alkyl chain length. On the contrast with the
Γ
max values, there can be no doubt that the Amin value showed a decreas-
2
2
ing trend from 33.67 Å (5a, n = 6) to 25.01 Å (5e, n = 10) with in-
creasing the alkyl chain length, which was just similar to the changing
trend of the CMC value. Both of the Γmax and the Amin values were a
sign of the packing densities of the absorbed molecules at the air-
solution interface. The larger Γmax value and the smaller Amin value
should correspond to the denser arrangement at the air-solution inter-
face. It was disclosed that the molecular arrangement in longer alkyl
chain α-D-xyloside was much denser than that in shorter alkyl chain
α-D-xyloside. Because there should be the repulsive force between hy-
drophobic alkyl chain and water, the longer alkyl chain α-D-xyloside
would readily adsorb onto the air-solution interface, such surfactant
molecule would have the better ability to maintain less contact with
water via the strong hydrophobic interaction between the long hydro-
carbon chains [60,61].
Fig. 3. The solubility of alkyl dioxyethyl α-D-xylosides in water or ethanol.
dioxyethylene fragment indeed improved the surface-active properties
of glycosides to some extent.
The obtained pC20 values showed an increasing trend with increas-
ing the alkyl chain length, which indicated that decyl dioxyethyl α-D-
xyloside (5e) had the highest adsorption efficiency, and the related ten-
dency to adsorb at the air-water interface was the greatest. But, the ob-
tained πCMC values revealed that first increased rapidly, then decreased
Table 3 listed the CMC, Γmax, and Αmin values. First of all, the CMC
−
4
−1
values decreased gradually from 8.31 × 10 mol·L (5a, n = 6) to
−5
−1
9
.21 × 10 mol·L (5e, n = 10). However, the γCMC values first de-
creased to the lowest, then increased with increasing the alkyl chain
−
1
−1
−1
length, i.e. from 28.06 mN·m
(5a, n = 6) → 27.25 mN·m
(5c,
with increasing the alkyl chain length, i.e. from 43.91 mN·m (5a, n =
n = 8) → 28.86 mN·m− (5e, n = 10). For the short alkyl chain, for
1
6) → 44.72 mN·m (5c, n = 8) → 43.11 mN·m (5e, n = 10).
The standard free energy of micellization (ΔGmic) was negative for
the prepared sugar-based nonionic surfactants (5a-5e), suggesting
that the micellization process was spontaneous. Whereas surface ad-
sorption free energy (ΔGads) implied that the surfactants 5a-5e had a
great ability to adsorb at the air-water interface. With the same alkyl
chain length, their negative ΔGmic values were deduced to be smaller
than their negative ΔGads values, indicating that the adsorption was pro-
moted more than the micellization. Besides, both of negative ΔGmic
values and negative ΔGads values increased with increasing alkyl chain
−1
−1
Table 3
Interfacial property of alkyl dioxyethyl α-D-xyloside.
Alky dioxyl α-D-xylosides
CMC (×10− mol·L−1)
5a
83.1
28.06
4.93
33.67
3.93
43.91
−17.58
−26.49
5b
53.1
27.89
5.18
32.08
4.09
44.08
−18.69
−27.21
5c
37.3
27.25
5.61
29.61
4.20
44.72
−19.57
27.54
5d
25.2
28.70
6.08
27.31
4.27
43.27
−20.54
−27.66
5e
5
9.21
−
1
γ
CMC (mN·m
)
28.86
6.64
25.01
4.65
43.11
−23.04
−29.53
−
6
mol·m−2)
Γ
max (×10
2
Α
min (Å )
pC20
1
π
CMC (mN·m−
)
ΔGmic (kJ·mol 1)
−
ΔGads (kJ·mol−
1
)
Fig. 4. The surface tension of alkyl dioxyethyl α-D-xyloside (5a-5e).

6
X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
length, suggesting that the driving force of the micellization and/or the
adsorption was derived from the hydrophobic alkyl moieties due to the
hydrophobic interaction between alkyl chains, those results were pow-
erfully supported by the pC20 values, HLB numbers, and LogP values as
described above [62].
xyloside itself migrated more easily away from the bulk solution and
went onto the interface to form readily a stronger interfacial film,
preventing the discontinuous liquid particles of the dispersed phase in
the emulsion layer from reaggregation [63]. However, the emulsifying
ability would not keep strengthening with increasing the alkyl chain
length because not only the appropriate HLB number, and LogP value
but also the rational TPAS value, and water solubility should be
demanded to certain content. Excessive long hydrophobic alkyl chain
made the related HLB number too small and LogP value too big with
the same TPAS value, which would destroy the hydrophilic and hydro-
phobic balance in the O/W emulsion due to violating of the Bancroft
rule [64–66].
4
.4. Foaming property and emulsifying property
0
Generally speaking, the bigger the foaming volume (V ), the better
the foaming performance; the slower the foam disappearing rate (ν),
the better the foam stability. In addition, the foaming ability and the
foam stability probably were closely related to each other since the sur-
factant molecules would adsorb on the liquid film to form strong or
weak adsorption film to stabilize the various foam.
Fig. 5 plotted the foaming property of alkyl dioxyethyl α-D-xylosides
4.5. Thermotropic liquid crystal property
0
(5a-5e). The results showed that their foam volume (V ) first increased
significantly from the bottom to a high platform (62 mL, 5d (n = 9)),
and then decreased slightly with increasing the alkyl chain length.
Among these glycosides, 5d had the strongest foaming performance.
Their foam disappearing rate (ν) gradually decreased from the high
point to almost zero (5a → 5d and 5e) with increasing the alkyl chain
length. Although the foam of 5a vanished the fastest, 5d, and 5e were
The thermotropic liquid crystalline properties of the homologous se-
ries of alkyl dioxyethyl α-D-xyloside (5a-5f) with different alkyl chain
lengths were investigated by the DSC and the POM, respectively.
Fig. 7 showed the DSC thermograms of decyl dioxyethyl α-D-
xyloside (5e). Through the first cooling and the second heating pro-
cesses at 5 °C·min , there were two-phase transition peaks, the first
phase transition was the melting point (solid → liquid crystalline), the
second phase transition was the clearing point (liquid crystalline → liq-
uid). Also, the peak area of the melting point was much larger than that
of the clearing point, so the enthalpy required from anisotropic liquid
crystal phase to the isotropic liquid phase was rather low. The DSC ther-
mograms of dodecyl dioxyethyl α-D-xylosides (5f) were similar to decyl
dioxyethyl α-D-xyloside (5e). For nonyl dioxyethyl α-D-xylosides (5d),
its peak of the melting point did not appear, and the peak of the clearing
point was not obvious, but its liquid crystalline property was observed
by the POM and the clear point temperature was measured as 38.9 °C.
The specific data were listed in Table 4.
−1
observed to have the minimum disappearance rate (υ
=
−
1
0
.01 mL·s ). Therefore, both of 5d and 5e were considered to have
good foaming property and foam stability in such homologous series.
For an emulsion system, the hydrophilicity and hydrophobicity of an
emulsifier would have an important impact on the type of the emulsi-
fied layer. In general, when the HLB number of a used emulsifier is
3
.5–6, the related emulsified layer should be the W/O type; however,
the related emulsified layer should be the O/W type when the HLB num-
ber of a used emulsifier is 8–18. For alkyl dioxyethyl α-D-xylosides
(
5a → 5e), their calculated HLB numbers were 14.72 → 12.54 and their
LogP values were 0.29 → 1.96 with increasing the alkyl chain length
n = 6 → 10). Therefore, their emulsified layer should reasonably be
(
In Table 4, for decyl dioxyethyl α-D-xyloside (5e), its melt point
(Mp) and clearing point (Cp) were −19.2 °C and 30.5 °C, respectively.
The length of the hydrophobic tail should directly have an impact on
oil-in-water (O/W) either in the toluene/water system or in the n-
octane/water system with the related concentration of 0.25%.
As shown in Fig. 6, the emulsifying ability in the n-octane/water sys-
tem was, on the whole, stronger than that in the toluene/water system
although both systems had rather near emulsion layer for 5a (n = 6)
with the short hydrophobic alkyl chain length. Their emulsion layers
thickened monotonously with increasing the alkyl chain length, 5e
p p
both of M and C [67]. Nevertheless, the phase transition temperature
of alkyl dioxyethyl α-D-xyloside (5f) was found to have its data. Indeed,
the stronger the intermolecular hydrogen bonds network and the
weaker the van der Waals interactions, in general. Therefore, both inter-
actions would just probably jointly determine the related phase transi-
tion temperature. Finally, nonyl dioxyethyl α-D-xyloside (5d) had only
one transition peak due to its shorter hydrophobic alkyl chain [67,68].
The POM was used to investigate the thermotropic liquid crystal be-
havior. For alkyl dioxyethyl α-D-xylosides (5a-5f), their liquid crystal-
line properties began to appear in the heating process as its alkyl
chain length n ≥ 9. From Fig. 8, alkyl dioxyethyl α-D-xylosides (5d-5f)
were observed to have birefringent structures that were considered as
(
n = 10) had the best emulsifying ability in both systems. A shorter hy-
drophobic chain in such sugar-based surfactants would severely hinder
the hydrophobic interactions between the surfactant and the oil phase
at the interface at all, which further weakened the adsorption at the in-
terface. On the contrary, a longer hydrophobic chain made the α-D-
c
the characteristic focal-conic fan schlieren textures below T during
the cooling process [52,58]. Although 5f had overall ambiguous focal-
conic domains, whereas the focal-conic domains of 5e were not big
and their edges seemed very indistinct and even slight blurring to
some content. In comparison, 5d displayed the biggest and rather
sharp focal-conic and fan-shaped schlieren domains, suggesting that
such texture might be attributed as the lamellar phase and/or smectic
phase which was similar to the stated texture in the previous studies
[68,69]. The formed focal-conic and fan-shaped schlieren texture was
characterized by hyperbolic and elliptical lines of optical discontinuity.
Such ordered liquid crystal structures could perhaps be arranged in se-
quence by self-assembly and self-organizing formations due to the driv-
ing forces involving strong intermolecular hydrogen bonding network
and weak intermolecular dispersion force. The phenomenon would per-
haps lead to open a new channel for the research and development of
liquid crystal light valve and light-addressable sugar-based material
with favorable biodegradability and biocompatibility.
Fig. 5. Foam property of alkyl dioxyethyl α-D-xyloside (5a-5e).

X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
7
Fig. 6. Emulsifying property of alkyl dioxyethyl α-D-xylosides (5a-5e).
Compared with the different RH, it could also be seen that the ab-
sorption rate of alkyl dioxyethyl α-D-xylosides for RH = 81% was
greater than that for RH = 43%. Such α-D-xylosides had the hygroscopic
properties, the main reason was that both of glycosyl group and
dioxythylene fragment interacted with water by the intermolecular hy-
drogen bond network, so they could absorb efficiently the water from
the environment. However, longer alkyl chain α-D-xylosides should
have a low moisture absorption rate because of their smaller HLB num-
ber and bigger LogP value.
5. Conclusions
With D-xylose as the raw material, a series of 1,2-cis alkyl dioxyethyl
α-D-xylosides (5a-5f) were stereoselectively synthesized via three-step
reactions including the acetylation, the coupling reaction, and the
deacetylation.
With the same TPSA value, their HLB number and water solubility de-
creased gradually with increasing alkyl chain length. Dodecyl
dioxyethyl α-D-xyloside (5f) had not water solubility at all whereas
hexyl dioxyethyl α-D-xyloside (5a) had strong water solubility. Also,
compared with alkyl α-D-xyloside with the same alkyl chain length,
alkyl dioxyethyl α-D-xylosides indeed improved the related water solu-
bility due to the introduction of dioxyethylene fragment as the hydro-
Fig. 7. Heat flow curve of decyl dioxyethyl α-D-xyloside (5e).
4
.6. Hygroscopicity
The hygroscopicity of alkyl dioxyethyl α-D-xylosides (5a-5f) was
determined with the relative humidity (RH) of 43% and 81%. The exper-
imental results were shown in Fig. 9.
As seen in Fig. 9, their hygroscopicity weakened gradually with the
extension of alkyl chain length. On the whole, the moisture absorption
capacity of the samples was close to the highest level after 48 h. The hy-
groscopicity of 5a (n = 6) had the same hygroscopic trend on the RH =
3% condition and on the RH = 81% condition. In the two RH systems,
the hygroscopic ability of 5a (n = 6) was the strongest. On the compar-
ison, the hygroscopic ability of 5f (n = 12) was the weakest.
2
2
philic spacer to increase their TPAS value from 79.15 Å (7) to 97.61 Å
(5). Their CMC values decreased with increasing the alkyl chain length.
The CMC value of decyl dioxyethyl α-D-xyloside (5e) was as low as
−
5
−1
9.21 × 10
mol·L . Among them (5a-5e), octyl dioxyethyl α-D-
−1
xyloside (5c) had the lowest surface tension (27.25 mN·m ) at the
CMC. Nonyl and decyl dioxyethyl α-D-xyloside (5d & 5e) had good
foaming power and foam stability. Decyl dioxyethyl α-D-xyloside (5e)
had the strongest emulsifying property either in the toluene/water sys-
tem or in the octane/water system. Moreover, glycosides (5d) had the
most stylish focal-conic fan schlieren texture. The hygroscopicity of
alkyl dioxyethyl α-D-xylosides (5a-5f) decreased gradually with in-
creasing alkyl chain length although hexyl dioxyethyl α-D-xyloside
4
(5a) possessed the strongest hygroscopicity.
In a word, such interesting and charming alkyl dioxyethyl α-D-
Table 4
xyloside indeed improved the water solubility and surface activity
with the hygroscopicity and the thermotropic liquid crystalline behav-
ior, In order to make full of the renewable and sustainable sugar re-
sources and activate their practical application as novel sugar-based
surfactants, it is suggested that a lot of the related technology [37], sur-
face activity [70,71], synergistic effect, phase behavior [72], functional
property [16,17,73], tolerance performance, toxicity [36], biodegrad-
ability, biocompatibility [29,33], structural modification [26,68], and
Thermotropic transition temperature and enthalpy under the second heating process with
DSC scan.
Alkyl dioxyethyl
α-D-xylosides
M
(°C)
p
C
(°C)
p
ΔT
(°C)
ΔH
m
ΔH
c
(kJ·mol−1)
(kJ·mol−1)
5d (n = 9)
5e (n = 10)
5f (n = 12)
–
38.9
30.5
50.0
–
49.7
39.7
–
12.1
15.9
–
2.2 × 10−
1
−19.2
10.3
2.6 × 10−
4

8
X. Wu et al. / Journal of Molecular Liquids 315 (2020) 113770
Fig. 8. Liquid crystal texture of alkyl dioxyethyl α-D-xylosides (5d-5f).
Fig. 9. Hygroscopicity of glycosides (5a-5f) (a) RH = 43%, (b) RH = 81%.
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