Sweet surfactants: Packing parameter-invariant amphiphiles as emulsifiers and capping agents for morphology control of inorganic particles

Article abstract of DOI:10.1039/c8sm01091a

Surfactants are not only pivotal constituents in any biological organism in the form of phospholipids, they are also essential for numerous applications benefiting from a large, internal surface, such as in detergents, for emulsification purposes, phase transfer catalysis or even nanoparticle stabilization. A particularly interesting, green class of surfactants contains glycoside head groups. Considering the variability of glycosides, a large number of surfactant isomers become accessible. According to established models in surfactant science such as the packing parameter or the hydrophilic lipophilic balance (HLB), they do not differ from each other and should, thus, have similar properties. Here, we present the preparation of a systematic set of glycoside surfactants and in particular isomers. We investigate to which extent they differ in several key features such as critical aggregation concentration, thermodynamic parameters, etc. Analytical methods like isothermal titration calorimetry (ITC), tensiometry, dynamic light scattering (DLS), small angle-X-ray scattering (SAXS), transmission electron microscopy (TEM) and others were applied. It was found that glycosurfactant isomers vary in their emulsification properties by up to two orders of magnitude. Finally, we have investigated the role of the surfactants in a microemulsion-based technique for the generation of zinc oxide (ZnO) nanoparticles. We found that the choice of the carbohydrate head has a marked effect on the shape of the formed inorganic nanocrystals.

Full text of DOI:10.1039/c8sm01091a

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Sweet surfactants: packing parameter-invariant
amphiphiles as emulsifiers and capping agents for
morphology control of inorganic particles†
Cite this:DOI: 10.1039/c8sm01091a
Michael Voggel, Rebecca M. Meinusch, Vanessa Siewert, Marius Kunkel,
Valentin Wittmann * and Sebastian Polarz
*
Surfactants are not only pivotal constituents in any biological organism in the form of phospholipids, they are
also essential for numerous applications benefiting from a large, internal surface, such as in detergents, for
emulsification purposes, phase transfer catalysis or even nanoparticle stabilization. A particularly interesting,
green class of surfactants contains glycoside head groups. Considering the variability of glycosides, a large
number of surfactant isomers become accessible. According to established models in surfactant science such
as the packing parameter or the hydrophilic lipophilic balance (HLB), they do not differ from each other and
should, thus, have similar properties. Here, we present the preparation of a systematic set of glycoside
surfactants and in particular isomers. We investigate to which extent they differ in several key features such as
critical aggregation concentration, thermodynamic parameters, etc. Analytical methods like isothermal titration
calorimetry (ITC), tensiometry, dynamic light scattering (DLS), small angle-X-ray scattering (SAXS), transmission
electron microscopy (TEM) and others were applied. It was found that glycosurfactant isomers vary in their
emulsification properties by up to two orders of magnitude. Finally, we have investigated the role of the
surfactants in a microemulsion-based technique for the generation of zinc oxide (ZnO) nanoparticles. We
found that the choice of the carbohydrate head has a marked effect on the shape of the formed inorganic
nanocrystals.
Received 27th May 2018,
Accepted 13th July 2018
DOI: 10.1039/c8sm01091a
rsc.li/soft-matter-journal
cationic or anionic groups is relatively limited, a large variety of
non-ionic head groups exists, and their choice profoundly
1 Introduction
The generic term ‘amphiphile’ refers to substances, which are affects the properties of the surfactant.2,3 Numerous non-
compatible with media characterized by different polarity. ionic surfactants contain ethoxylates and oligo-ethylene glycol
Regarding their polarity, solvents are typically considered either derivatives, respectively.
hydrophilic (water, alcohols) or hydrophobic (oils). Because of
Surfactants with hydrophilic head groups cannot only be
their unique property, amphiphiles are used in numerous generated by de novo synthesis, they can also originate from the
applications involving media of different polarity, for example natural pool of organic compounds. For example, alkyl (poly)-
as detergents, emulsifiers, phase-transfer agents or as stabilizing glycosides are surfactants produced at a large scale (4300 000 t/a)
agents in nanoparticle synthesis. Their ability to undergo self- from sustainable resources, such as starch and fat.4 The obtained
assembly is another fascinating feature of amphiphiles. Classical mixture of substances can vary in the number of carbohydrates,
examples are the formation of micelles or liquid crystalline phases. their anomeric configuration, and the degree of alkylation.5
Surfactants are one subclass among amphiphiles. They possess The chemistry of these carbohydrate surfactants was already
a well-defined molecular architecture, in which a hydrophilic head established by E. Fischer in 1911,6 and a comprehensive over-
group is attached to a hydrophobic moiety, in most cases an alkyl view was given by Rybinski and Hill in 1998.7 The advantages of
chain. Surfactants can be further categorized according to their carbohydrate surfactants are manifold and include biodegrad-
head group to be either ionic or non-ionic.1 While the number of ability, chirality and low toxicity. Due to these properties, they
find numerous applications in detergents and cosmetics.
¨
University of Konstanz, Department of Chemistry, Universitatsstraße 10,
In more specialized applications they have been successfully
used for the extraction and crystallization of proteins or for
transfection.8 Carbohydrate surfactants can also undergo self-
assembly and form so-called niosomes, the synthetic analogues
of liposomes.9 Due to their enhanced stability, niosomes have
78457 Konstanz, Germany. E-mail: valentin.wittmann@uni-konstanz.de,
sebastian.polarz@uni-konstanz.de
† Electronic supplementary information (ESI) available: Additional information
on synthetic procedures and molecular characterization data; additional data on
surfactant self-assembly Fig. ESI-1–4. See DOI: 10.1039/c8sm01091a
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might be appropriate for certain applications e.g. as oil-in-water
emulsifier. A change of the surfactant structure normally results
in a significant change of the packing and HLB parameters.
Therefore, carbohydrate surfactants are ideal models for investi-
gating, how even small changes in the surfactant structure can
affect amphiphilic properties. Important work in this research
field was done by Vill and coworkers.16–18 Their studies mainly
focused on the determination of the thermotropic behavior
(clearing points)16 and lyotropic phase diagrams obtained from
polarization microscopy.17,18 In 2014, Lindhorst and coworkers
investigated the micellization behavior of a series of octyl glyco-
side surfactants by applying isothermal titration calorimetry (ITC)
and diffusion ordered NMR spectroscopy.19 They could show that
the critical micelle concentration (CMC) value is influenced by the
choice of carbohydrate and anomeric configuration.
Here, we present the investigation of a comprehensive,
multidimensional library of glycoside surfactants (Fig. 1). In a
systematic way, we studied the effect of different parameters,
such as the nature of the tail and the head group. We investigated
the effect of subtle variations of the head group including isomeric
(glucose, galactose, mannose) or similar carbohydrates (xylose) as
well as different anomeric configurations. Major effort has been
put into the synthesis and isolation of surfactants in pure form.
After thorough investigation of self-aggregation processes, we tested
the use of the surfactants as emulsifiers. In a proof-of-concept
experiment, we show that sugar-based surfactants can also function
as capping agents influencing the morphology of inorganic zinc
oxide nanoparticles grown in an emulsion-based method. This
method was developed by us in the past,20–23 however, glycoside
Fig. 1 (a) Possibilities for the generation of a systematic map of surfactants
with carbohydrate head groups exemplarily shown for ManbR. Black = variation
of carbohydrate head group; red = variation of the hydrophobic chain length
(arrow); grey arrow = different surfactant length; blue = different anomeric surfactants had not yet been used as emulsifiers.
configuration. Electrostatic potential map of a mannoside surfactant is also
shown. (b) Overview of the surfactant library used in this study. Gal = galactose;
Glc = glucose; Mal = maltose; Man = mannose, Xyl = xylose.
2 Experimental
been successfully tested as biocompatible drug-delivery systems.10
2.1 General methods
The application of glycoside surfactants in enantioselective phase N-Iodosuccinimide was purchased from abcr; Span 20 and
transfer catalysis is highly interesting, but is still less explored.11,12 Triton X-100 were purchased from TCI chemicals; octyl a-D-gluco-
Further, carbohydrate-based surfactants are highly interesting pyranoside (GlcaC8), hexyl and octyl b-D-glucopyranoside (GlcbC6,
from a fundamental perspective. The huge number of different GlcbC8), and hexyl b-D-maltoside (MalbC6) were purchased from
carbohydrates and in particular the accessibility of various Sigma Aldrich; octyl, decyl, and dodecyl b-D-maltoside (MalbC8,
isomers makes sugar-based amphiphiles special.13 In contrast MalbC10, MalbC12) and dodecyl a-D-maltoside (MalaC12) were
to other systems, one can apply subtle differences to the purchased from Carbosynth. Isopropyl b-D-1-thiogalactoside
structure of the surfactant head group region while maintaining (IPTG) was purchased from Carbolution Chemicals. All solvents
key surfactant parameters such as the packing parameter or the were distilled prior to use and if required, dried according to
HLB value (see also Fig. 1).
standard procedures. All reactions were monitored by TLC on
The packing parameter P is a widely used concept, which silica gel 60 F254 (Merck) on aluminum sheets with detection by
was introduced by Israelachvili.14 P describes the shape of a UV-light (l = 254 nm). Additionally, a solution of Ce(SO4)2ꢀ4H2O
surfactant taking into account the volume of the hydrophobic (25 mM) and (NH4)6Mo7O24ꢀ4H2O (16 mM) in H2SO4 (10 wt%)
chain (V0), the equilibrium area per molecule at the interface (Amin), followed by gentle heating was used for visualization. Preparative
and the length of the hydrophobic chain (l0); P = V0/(Aminꢀl0). flash column chromatography (FC) was performed with silica
Another concept, which is particularly useful for classifying 60 M (0.04–0.063 mm, Macherey-Nagel). NMR spectra were
non-ionic surfactants, is the hydrophilic–lipophilic balance recorded at room temperature on Avance III 400 and Avance III
(HLB) introduced by Griffin.15 The HLB value is proportional 600 instruments from Bruker. Chemical shifts are reported in
to the ratio of the molar mass of the hydrophilic moieties (Mh) of ppm relative to solvent signals (CDCl3: dH = 7.26, dC = 77.16;
the surfactant and its total molecular mass (M); HLB = 20ꢀ(Mh/M). DMSO-d6: dH = 2.50, dC = 39.52). Signals were assigned by
The HLB value can serve as first evidence, whether the surfactant first order analysis and two-dimensional 1H,1H, 1H,13C and
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1H,31P correlation spectroscopy (COSY, HSQC and HMBC). MeOH (22 mL mmolꢁ1). The mixture was vigorously stirred under a
High-resolution electrospray ionization mass spectra (ESI-HRMS) hydrogen atmosphere (1 atm) for two hours, filtered over celite, and
were recorded on a Bruker micrOTOF II instrument.
the solvent was evaporated yielding the alkyl galactopyranosides
(for details, yields, and product characterization see ESI†).
2.2 Synthesis
2.2.3 General procedure for the synthesis of saturated
xylosides. To prepare the xylopyranosides, a procedure from
Lopez and Fernandez-Mayoralas30 was modified as follows.
1,2,3,4-Tetra-O-acetylxylose (1 equiv.) and alkyl alcohol (1.2 equiv.)
were dissolved in dry CH2Cl2 (20 mL gꢁ1). BF3ꢀOEt2 (1 equiv.) was
added dropwise and the mixture was stirred overnight. The reac-
tion mixture was diluted with CH2Cl2, washed with saturated
NaHCO3 solution and water, and the organic phase was dried over
MgSO4 and the solvent evaporated. The crude product was purified
and the anomers separated by FC (petroleum ether/ethyl acetate)
yielding the acetylated alkyl a- and b-xylopyranosides. For deacetyl-
ation, the alkyl xylopyranosides were dissolved in dry MeOH
(19 mL gꢁ1), NaOMe (0.5 M in MeOH, 0.15 equiv.) was added
and the mixture stirred for 1 h. Then the mixture was neutralized
with Amberlite IR-120 ion exchange resin (H+-form), filtered over
celite and the solvent evaporated yielding the alkyl xylopyranosides.
2.2.4 Synthesis of (Z)-dodec-6-en-1-ol31 (3)
(6-Hydroxyhexyl)-triphenylphosphonium bromide. 6-Bromohexan-
1-ol32 (20.0 g, 110.4 mmol) was dissolved in dry acetonitrile
(250 mL) and triphenylphosphine (30.4 g, 116 mmol) was
added. The reaction mixture was heated to reflux for 24 h.
Afterwards, the mixture was concentrated under reduced pres-
sure and the residue redissolved in dry CH2Cl2, precipitated by
addition of diethyl ether and freezing to ꢁ196 1C (to achieve full
precipitation). After thawing, the supernatant was decanted.
This procedure was repeated two times to ensure removal of
2.2.1 General procedure for the synthesis of glycosurfactants
by anomeric O-alkylation
(a) Formation of the triflated alcohol. A solution of alkyl
alcohol (1.5 equiv.) and 2,6-lutidine (2.25 equiv.) in dry CH2Cl2
(15 mL gꢁ1 alcohol) was cooled to 0 1C and triflic anhydride
(2.1 equiv.) was added dropwise. After 15 min, n-hexane (30 mL gꢁ1
alcohol) was added and the reaction mixture was washed with ice
cold 0.1 M KHSO4 solution, dried over Na2SO4 and concentrated
under reduced pressure at 0 1C. The crude triflated alcohol was
then immediately used without further purification.
(b) Anomeric O-alkylation24. A solution of the corresponding,
previously prepared, 2,3,4,6-tetra-O-acetylhexose25,26 (1 equiv.)
in dry dimethoxyethane (10 mL gꢁ1) was cooled to 0 1C and
sodium hydride (1.1 equiv., 60 wt% dispersion in mineral oil)
was added portionwise. After gas evolution had ceased, the
reaction mixture was cooled to ꢁ50 1C and the triflated alcohol,
dissolved in dry dimethoxyethane (5 mL gꢁ1 carbohydrate), was
added and the reaction stirred overnight. The solvent was
removed under reduced pressure and the remaining syrup
was redissolved in ethyl acetate and washed with brine, dried
over Na2SO4 and the solvent was evaporated. The crude product
was purified by FC (petroleum ether/ethyl acetate) yielding the
acetylated alkyl a- and b-D-gluco- and -mannopyranosides.
27
´
(c) Deprotection (Zemplen-protocol) . The acetylated alkyl excess triphenylphosphine. Remaining solvent was removed
glucopyranoside or alkyl mannopyranoside was dissolved in under reduced pressure to yield the phosphonium salt as white
1
dry MeOH (19 mL gꢁ1) and NaOMe (0.2 equiv., 0.5 M solution in solid (43.9 g, 90%). H NMR (400 MHz, CDCl3): d = 7.81–7.66
MeOH) was added and stirred for 1 h. The reaction mixture was (m, 15 H, Ph), 3.68–3.61 (m, 2 H, H-1), 3.56 (t, J = 5.4 Hz, 2 H, H-6),
treated with Amberlite IR-120 ion exchange resin (H+-form), 2.60 (s, 1 H, OH), 1.64 (s, 4 H, H-2, H-3), 1.49–1.42 (m, 4 H, H-4,
filtered over celite and the solvent evaporated yielding the alkyl H-5); 31P NMR (162 MHz, CDCl3): d = 24.36 (s).
glucopyranoside or alkyl mannopyranoside (for details, yields,
and product characterization see ESI†).
(Z)-Dodec-6-en-1-ol (3). NaH (60 wt% dispersion in mineral
2.2.2 General procedure for the synthesis of glycosurfactants oil, 7.81 g, 195.5 mmol) was washed two times with dry
from IPTG. Isopropyl 2,3,4,5-tetra-O-benzyl-1-thio-b-D-galacto- n-hexane in order to remove the mineral oil. Then, dry DMSO
pyranoside (6) was prepared from commercially available isopropyl (72 mL) was added and carefully heated to 75 1C until gas-
1-thio-b-D-galactopyranoside (IPTG) according to a procedure from evolution had ceased. The reaction mixture was diluted with
Du et al.28 To prepare the alkyl galactosides, a procedure from dry THF (30 mL) and cooled to 0 1C. To the cooled mixture the
Ohlsson and Magnusson29 was modified as follows. Benzylated phosphonium salt (43.3 g, 97.8 mmol), dissolved in dry DMSO
thioglycoside (1 equiv.) and alkyl alcohol (1.1 equiv.) were dissolved (50 mL, 40 1C), was slowly added and stirred for 45 min.
in dry diethyl ether (50 mL gꢁ1 thiogalactoside) and cooled to 0 1C Subsequently, freshly distilled n-hexanal (12 mL, 97.7 mmol) was
(for high b-selectivity, the reaction is carried out in dry CH2Cl2 at added dropwise and stirred for 20 min. Afterwards, the reaction
ꢁ78 1C). NIS (1.2 equiv.) and TMSOTf (0.2 equiv.) were added. It mixture was poured on ice water and extracted with diethyl ether.
was stirred for 1.5 h and NEt3 (4.2 equiv.) was added. The reaction After removal of the solvents, the residue was extracted with n-
mixture was washed with Na2S2O3 solution (10 wt%), saturated hexane. The combined extracts were washed with water, dried over
NaHCO3 solution and water. The organic phase was dried over MgSO4 and the solvent evaporated. The crude product was purified
MgSO4 and solvent evaporated. The crude product was purified via FC (petroleum ether/ethyl acetate 30 : 1) to yield 7.5 g (42%) of
and the anomers separated by FC (petroleum ether/ethyl acetate) the alcohol (E:Z = 1 : 10) as colorless oil. Rf = 0.28 (petroleum ether/
yielding the benzylated alkyl a- and b-D-galactopyranosides. For ethyl acetate 5 : 1); 1H NMR of Z-product (400 MHz, CDCl3): d =
deprotection, hydrogenation catalyst (10% palladium on carbon) 5.40–5.31 (m, 2 H, H-6, H-7), 3.63 (t, J = 6.6 Hz, 2 H, H-1),
was added to a solution of benzylated alkyl galactopyranoside in 2.06–1.96 (m, 4 H, H-5, H-8), 1.61–1.54 (m, 2 H, H-2) 1.39–1.24
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(m, 10 H, H-4, H-9, H-3, H-11, H-10), 0.88 (t, J = 7.2 Hz, 3 H, H-12); 6.34 mmol) was deprotected following the general procedure
13C NMR (101 MHz, CDCl3): d = 130.3 (C-6/7), 129.7 (C-6/7), 63.2 described in Section 2.2.1 to yield ManaCuns as yellow solid
12
(C-1), 32.9 (C-2), 31.7 (C-10), 29.7 (C-9), 29.6 (C-4), 27.3 (C-5/8), 27.3 (2.16 g, 98%). Rf = 0.41 (CH2Cl2/MeOH 5 : 1); 1H NMR (400 MHz,
(C-5/8), 25.6 (C-3), 22.7 (C-11), 14.2 (C-12).
DMSO-d6): d = 5.33 (m, 2 H, H-12, H-13), 4.67 (d, J = 5.3 Hz, 1 H,
OH-4), 4.64 (d, J = 4.6 Hz, 1 H, OH-2), 4.57 (d, J = 1.4 Hz, 1 H, H-1),
2.2.5 Synthesis of ManaCuns and ManbCuns
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12
(Z)-Dodec-6-enyl 2,3,4,6-tetra-O-acetyl-a/b-D-mannopyranoside 4.50 (d, J = 5.9 Hz, 1 H, OH-3), 4.39 (d, J = 5.89 Hz, 1 H, OH-6),
(4a,4b). First, the triflated alcohol (21.82 mmol) was synthesized 3.67–3.60 (m, 1 H, H-6a), 3.60–3.54 (m, 2 H, H-2, H-7a), 3.47–3.40
as described in 2.2.1. 2,3,4,6-Tetra-O-acetyl-D-mannopyranose (m, 2 H, H-3, H-6), 3.40–3.34 (m, 1 H, H-4), 3.30–3.24 (m, 2 H, H-5,
(1, 5.06 g, 14.55 mmol) was dissolved in dry dimethoxyethane H-7b), 2.07–1.88 (m, 4 H, H-11, H-14), 1.55–1.43 (m, 2 H, H-8),
(80 mL) and cooled to 0 1C. NaH (60 wt% dispersion in mineral 1.38–1.19 (m, 10 H, H-9, H-10, H-15, H-16, H-17), 0.86 (t, J = 6.8 Hz,
oil, 615 mg, 16 mmol) was added and after 10 min the triflated 3 H, H-18); 13C NMR (101 MHz, DMSO-d6): d = 129.7 (C-12/13),
alcohol, dissolved in dimethoxyethane (20 mL), was added 129.5 (C-12/13), 99.7 (C-1), 73.9 (C-5), 71.0 (C-3), 70.4 (C-2), 67.0
dropwise and the reaction stirred overnight. After removing (C-4), 66.2 (C-7), 61.3 (C-6), 30.8 (C-16), 28.93 (C-9/10/15), 28.87
solvent under reduced pressure, the residue was dissolved in (C-9/10/15), 28.8 (C-8), 26.58 (C-11/14), 26.55 (C-11/14), 25.4
ethyl acetate, washed with water and dried over Na2SO4. The (C-9/10/15), 21.9 (C-17), 13.9 (C-18); ESI-HRMS: calcd for C18H34O6Na
solvent was evaporated and the crude product was purified by FC [M + Na]+, 369.2246; found, 369.2208.
(petroleum ether/ethyl acetate = 10: 1 to 6 : 1) to yield the pure
a-product 4a (3.26 g, 44%) and the b-product 4b (500 mg, 7%) as
(Z)-Dodec-6-enyl b-D-mannopyranoside (ManbCuns12). (Z)-Dodec-
slightly yellow solids. Rf(4a) = 0.49 (petroleum ether/ethyl acetate 6-enyl 2,3,4,6-tetra-O-acetyl-b-D-mannopyranoside (4b, 498 mg,
3 : 1); Rf(4b) = 0.38 (petroleum ether/ethyl acetate 3 : 1); 1H NMR 0.97 mmol) was deprotected following the general procedure
of 4a (400 MHz, CDCl3): d = 5.41–5.32 (m, 3 H, H-3, H-12, H-13), described in Section 2.2.1 to yield ManbCuns as yellow solid
12
5.27 (t, J = 10.0 Hz, 1 H, H-4), 5.23 (dd, J = 3.3 Hz, 2.5 Hz, 1 H, (322 mg, 96%). Rf = 0.43 (CH2Cl2/MeOH 5 : 1); 1H NMR (400 MHz,
H-2), 4.80 (d, J = 1.3 Hz, 1 H, H-1), 4.28 (dd, J = 12.3 Hz, 5.4 Hz, DMSO-d6): d [ppm] = 5.39–5.29 (m, 2 H, H-12, H-13), 4.69 (d, J =
1 H, H-6a), 4.10 (dd, J = 12.2 Hz, 2.3 Hz, 1 H, H-6b), 3.98 (ddd, J = 4.9 Hz, 1 H, OH-4), 4.49 (d, J = 5.9 Hz, 1 H, OH-3), 4.39 (t, J =
12.3 Hz, 5.3 Hz, 2.3 Hz, 1 H, H-5), 3.67 (dt, J = 9.6 Hz, 6.9 Hz, 1 H, 5.9 Hz, 1 H, OH-6), 4.33 (s, 1 H, H-1), 4.23 (d, J = 5.0 Hz, 1 H,
H-7a), 3.45 (dt, J = 9.6 Hz, 6.6 Hz, 1 H, H-7b), 2.15 (s, 3 H, Ac-2), OH-2), 3.75 (dt, J = 9.8 Hz, 7.2 Hz, 1 H, H-7a), 3.71–3.63 (m, 1 H,
2.10 (s, 3 H, Ac-6), 2.04 (s, 3 H, Ac-4), 2.03 (m, 4 H, H-11, H-14), H-6a), 3.63–3.58 (m, 1 H, H-2), 3.48–3.36 (m, 2 H, H-6b, H7-b),
1.99 (s, 3H, Ac-3), 1.61 (m, 2 H, H-8), 1.41–1.22 (m, 10 H, H-9, 3.29–3.19 (m, 2 H, H-4, H-3), 3.04–2.97 (m, 1 H, H-5), 2.07–1.88
H-10, H-15, H-16, H-17), 0.89 (t, J = 6.8 Hz, 3 H, H-18); 13C NMR (m, 4 H, H-11, H-14), 1.56–1.45 (m, 2 H, H-8), 1.37–1.39 (m, 10 H,
of 4a (101 MHz, CDCl3): d = 170.7 (Ac-6), 170.1 (Ac-2), 169.9 H-9, H-10, H-15, H-16, H-17), 0.86 (t, J = 6.8 Hz, 3 H, H-18);
(Ac-3), 169.8 (Ac-4), 130.3 (C-12/13), 129.4 (C-12/13), 97.6 (C-1), 13C NMR (101 MHz, DMSO-d6): d = 129.7 (C-12/13), 129.6
69.8 (C-2), 69.2 (C-3), 68.5 (C-5/7), 68.4 (C-5/7), 66.3 (C-4), 62.6 (C-12/13), 100.2 (C-1), 77.5 (C-5), 73.7 (C-3), 70.6 (C-2), 68.3
(C-6), 31.5 (C-16), 29.5 (C-9/10/15), 29.4 (C-9/10/15), 29.2 (C-8), 27.2 (C-7), 67.2 (C-4), 61.4 (C-6), 30.8 (C-16), 29.1 (C-9/10/15), 29.0
(C-11/14), 27.1 (C-11/14), 25.7 (C-9/10/15), 22.6 (C-17), 20.9 (C-Ac), (C-9/10/15), 28.8 (C-8), 26.61 (C-11/14), 26.56 (C-11/14), 25.2
20.8 (C-Ac), 20.71 (C-Ac), 20.70 (C-Ac), 14.1 (C-18); ESI-HRMS: calcd (C-9/10/15) 21.9 (C-17), 13.9 (C-18); ESI-HRMS: calcd for C18H34O6Na
for C26H42O10Na [M + Na]+, 537.2669; found, 537.2762.
[M + Na]+, 369.2246; found, 369.2307.
1H NMR of 4b (400 MHz, CDCl3): d = 5.47 (dd, J = 3.2 Hz, 0.8
Hz, 1 H, H-2), 5.34 (m, 2 H, H-12, H-13), 5.25 (t, J = 9.9 Hz, 1 H,
2.2.6 Synthesis of GalaCuns and GalbCuns
12
12
(Z)-Dodec-6-enyl 2,3,4,6-tetra-O-acetyl-a/b-D-galactopyranoside
H-4), 5.05 (dd, J = 9.9 Hz, 3.2 Hz, 1 H, H-3), 4.62 (d, J = 0.9 Hz, (5a,5b). First, the triflated alcohol (1.5 equiv.) was synthesized
1 H, H-1), 4.30 (dd, J = 12.2 Hz, 5.6 Hz, 1 H, H-6a), 4.15 as described in Section 2.2.1. 2,3,4,6-Tetra-O-acetyl-D-galacto-
(dd, J = 12.2 Hz, 4.2 Hz, 1 H, H-6b), 3.86 (dt, J = 9.4 Hz, 6.7 pyranose (2, 1.26 g, 3.62 mmol) was dissolved in dry dimethoxy-
Hz, 1 H, H-7a), 3.66 (ddd, J = 9.9 Hz, 5.6 Hz, 4.2 Hz, 1 H, H-5), ethane (20 mL) and cooled to ꢁ50 1C. NaH (60 wt% dispersion
3.49 (dt, J = 9.4 Hz, 6.9 Hz, 1 H, H-7b), 2.18 (s, 3 H, Ac-2), 2.09 in mineral oil, 159.3 mg, 3.98 mmol) was added. After 10 min,
(s, 3 H, Ac-6), 2.04 (s, 3 H, Ac-4), 2.01 (m, 4 H, H-11, H-14), 1.99 the triflated alcohol, dissolved in dimethoxyethane (20 mL),
(s, 3 H, Ac-3), 1.59 (m, 2 H, H-8), 1.38–1.23 (m, 10 H, H-9, H-10, was added dropwise and the reaction stirred overnight. After
H-15, H-16, H-17), 0.88 (t, J = 6.8 Hz, 1 H, H-18); 13C NMR of 4b removing the solvent under reduced pressure, the residue was
(101 MHz, CDCl3): d = 170.9 (Ac-6), 170.6 (Ac-2), 170.2 (Ac-3), dissolved in ethyl acetate, washed with water and dried over
169.7 (Ac-4), 130.3 (C-12/13), 129.6 (C-12/13), 98.9 (C-1), 72.6 Na2SO4. The solvent was evaporated and the crude product was
(C-5), 71.3 (C-3), 70.6 (C-7), 69.0 (C-2), 66.4 (C-4), 62.8 (C-6), 31.7 purified by FC (petroleum ether/ethyl acetate = 10 : 1 to 6 : 1) to
(C-16), 29.60 (C-9/10/15), 29.56 (C-9/10/15), 29.4 (C-8), 27.3 yield the pure a-product 5a (244 mg, 13%) and the b-product 5b
(C-11/14), 27.2 (C-11/14), 25.7 (C-9/10/15), 22.7 (C-17), 21.0 (C-Ac), (1.17 g, 63%) as white solids. Rf(5a) = 0.20 (petroleum ether/
20.9 (C-Ac), 20.85 (C-Ac), 20.7 (C-Ac), 14.2 (C-18); ESI-HRMS: calcd ethyl acetate 5 : 1); Rf(5b) = 0.14 (petroleum ether/ethyl acetate
for C26H42O10Na [M + Na]+, 537.2669; found, 537.2672.
5:1); 1H NMR of 5a (400 MHz, CDCl3): d = 5.46–5.45 (dd, J = 3.4 Hz,
1.2 Hz, 1 H, H-4), 5.40–5.30 (m, 3 H, H-2, H-12, H-13), 5.12–5.09 (m,
(Z)-Dodec-6-enyl a-D-mannopyranoside (ManaCuns12). (Z)-Dodec- 2 H, H-3, H-1), 4.22 (dt, J = 6.3 Hz, 1.2 Hz, 1 H, H-5), 4.09 (ddd,
6-enyl 2,3,4,6-tetra-O-acetyl-a-D-mannopyranoside (4a, 3.26 g, J = 6.3 Hz, 3.2 Hz, 2.3 Hz, 2 H, H-6), 3.68 (dt, J = 9.9 Hz, 6.5 Hz, 1 H,
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H-7a), 3.42 (dt, J = 9.9 Hz, 6.5 Hz, 1 H, H-7b), 2.14 (s, 3 H, Ac-4), 2.07 3.26–3.24 (m, 2 H, H-2, H-3), 2.01–1.96 (m, 4 H, H-11, H-14), 1.53–
(s, 3 H, Ac-3), 2.04 (s, 3 H, Ac-6), 2.02 (m, 4-H, H-11, H-14), 1.98 1.50 (m, 2 H, H-8), 1.32–1.23 (m, 12 H, H-9, H-10, H-15, H-16, H-17),
(s, 3 H, Ac-2), 1.61–1.57 (m, 2 H, H-8), 1.38–1.27 (m, 10 H, H-10, H-15, 0.86 (t, J = 6.6 Hz, 3 H, H-18); 13C NMR (101 MHz, DMSO-d6):
H-9, H-17, H-16), 0.88 (d, J = 13.2 Hz, 3 H, H-18); 13C NMR of 5a (101 d = 129.64 (C-12/13), 129.56 (C-12/13), 103.5 (C-1), 75.1 (C-5), 73.5
MHz, CDCl3): d = 170.6 (Ac-3), 170.5 (Ac-6), 170.4 (Ac-4), 170.2 (Ac-2), (C-2/3), 70.5 (C-2/3), 68.4 (C-7), 68.1 (C-4), 60.4 (C-6), 30.8 (C-16/17),
130.4 (C-12/13), 129.5 (C-12/13), 96.3 (C-1), 68.8 (C-7), 68.4 (C-3), 68.3 29.2 (C-9/10/15), 29.0 (C-9/10/15), 28.8 (C-8), 26.6 (C-11/14), 26.55
(C-4), 67.9 (C-2), 66.3 (C-5), 62.0 (C-6), 32.7 (C-16), 31.7 (C-9/10/15), (C-11/14), 25.2 (C-9/10/15), 21.9 (C-16/17), 13.9 (C-18); ESI-HRMS:
29.6 (C-9/10/15), 29.4 (C-8), 27.4 (C-11/14), 27.3 (C-11/14), 25.9 (C-9/ calcd for C18H34O6Na [M + Na]+, 369.2246; found, 369.2250.
10/15), 22.7 (C-17), 20.9 (Ac-3), 20.84 (Ac-6), 20.82 (Ac-4), 20.80 (Ac-2),
14.2 (C-18); ESI-HRMS: calcd for C26H42O10Na [M + Na]+, 537.2669; synthesis. The hetero cubane precursor [MeZnoiPr]4 was prepared
found, 537.2742.
according to a procedure by Lizandara-Pueyo.20 62.5 mg of the
2.2.7 General procedure for emulsion-based ZnO-particle
1H NMR of 5b (400 MHz, CDCl3): d = 5.38 (dd, J = 3.5 Hz, respective glycosurfactant were dissolved in 35 mL warm cyclo-
1.0 Hz, 1 H, H-4), 5.36–5.29 (m, 2 H, H-12, H-13), 5.20 (dd, J = hexane (40 1C), 1.5 mL water was added and an emulsion was
10.5 Hz, 8.0 Hz, 1 H, H-2), 5.01 (dd, J = 10.5 Hz, 3.5 Hz, 1 H, prepared by applying ultrasound for 10 minutes using a Bandelin-
H-3), 4.45 (d, J = 8.0 Hz, 1 H, H-1), 4.20–4.10 (m, 2 H, H-6), Sonoplus HD3100-MS73. The emulsion was heated to 55 1C and a
3.91–3.85 (m, 2 H, H-5, H-7a), 3.46 (dt, J = 9.7 Hz, 7.0 Hz, 1 H, solution of 1 g [MeZnOiPr]4 in 10 mL dry cyclohexane was added at a
H-7b), 2.14 (s, 3 H, Ac-4), 2.04 (m, 6 H, Ac-2, Ac-6), 2.01 (m, 4 H, rate of 0.55 mL minꢁ1. Ultrasound was applied during addition and
H-11, H-14), 1.98 (s, 3 H, Ac-3) 1.64–1.55 (m, 2 H, H-8), 1.35–1.25 for additional 8 h afterwards. The solvent was removed under
(m, 10 H, H-9, H-10, H-15, H-16, H-17), 0.88 (t, J = 6.9 Hz, 3 H, reduced pressure to obtain ZnO nanoparticles as a white powder.
H-18); 13C NMR of 5b (101 MHz, CDCl3): d = 170.5 (Ac-6), 170.4
2.2.8 General procedure for emulsion experiments. For a
(Ac-4), 170.3 (Ac-3), 169.5 (Ac-2), 130.4 (C-12/13), 129.6 (C-12/13), typical experiment, a mixture of 6 mL cyclohexane, 6 mL water
101.5 (C-1), 71.1 (C-3), 70.7 (C-5), 70.3 (C-7), 69.1 (C-2), 67.2 (C-4), and 53.4 mg surfactant (0.5 wt%) was ultrasonicated for one
61.4 (C-6), 31.7 (C-16), 29.6 (C-8), 29.55 (C-10), 29.48 (C-15), 27.3 min and the formed emulsion immediately transferred into
(C-11/14), 27.3 (C-11/14), 25.6 (C-9), 22.7 (C-17), 21.2 (Ac-4), 20.9 a 10 mL measuring cylinder with a 14/23 ground glass joint
(Ac-2), 20.8 (Ac-6), 20.7 (Ac-3), 14.2 (C-18); ESI-HRMS: calcd for (to prevent solvent evaporation). The volume of separated and
C26H42O10Na [M + Na]+, 537.2669; found, 537.2761.
emulsion phase was monitored over time.
2.2.9 Molecular structures and electrostatic potential mapping.
(Z)-Dodec-6-enyl a-D-galactopyranoside (GalaCuns12). (Z)-Dodec- Molecular structures were drawn in Avogadro and their geometry
6-enyl 2,3,4,6-tetra-O-acetyl-a-D-galactopyranoside (5a, 234 mg, optimized using MMFF94 force field optimization. Electrostatic
0.455 mmol) was deprotected following the general procedure potential maps of the optimized structures were calculated in
described in Section 2.2.1 to yield GalaCuns as white solid Maestro (Schrodinger).
¨
12
(194 mg, 94%). Rf = 0.28 (CH2Cl2/MeOH 10: 1); 1H NMR
2.3 Analytical techniques
(400 MHz, DMSO-d6): d = 5.39–5.29 (m, 2 H, H-12, H-13), 4.61
(d, J = 3.5 Hz, 1 H, H-1), 4.50–4.48 (m, 2 H, OH-6, OH-3), 4.35
(d, J = 6.3 Hz, 1 H, OH-2), 4.31 (d, J = 4.3 Hz, 1 H, OH-4), 3.69
(t, J = 3.1 Hz, 1 H, H-4), 3.59–3.47 (m, 5 H, H-7a, H-2, H-5, H-3,
H-6a), 3.45–3.39 (m, 1 H, H-6b), 3.29–3.27 (m, 1 H, H-7b), 1.99–
1.96 (m, 4 H, H-11, H-14), 1.53–1.50 (m, 2 H, H-8), 1.24–1.25 (m,
12 H, H-9, H-10, H-15, H-16, H-17), 0.86 (t, J = 6.7 Hz, 3 H, H-18);
13C NMR (101 MHz, DMSO-d6): d = 129.7 (C-12/13), 129.6 (C-12/
13), 98.8 (C-1), 71.1 (C-2/3/5), 69.6 (C-4), 68.9 (C-2/3/5), 68.4 (C-2/
3/5), 66.8 (C-7), 60.6 (C-6), 30.8 (C-16/17), 29.0 (C-8), 28.8 (C-9/10/
15), 26.6 (C-11/14), 26.55 (C-11/14), 25.4 (C-9/10/15), 21.9
(C-16/17), 13.9 (C-18); ESI-HRMS: calcd for C18H34O6Na [M + Na]+,
369.2246; found, 369.2226.
2.3.1 Isothermal titration calorimetry (ITC). Isothermal
titration calorimetry experiments were performed on a GE
Microcal iTC200 system at 20 1C. For a typical experiment
(CAC o 100 mM), a solution with a concentration of approx.
10-fold CAC was prepared in milliQ-water. This solution was
titrated into milliQ-water at 750 rpm stirring speed and with
120 s initial delay for equilibration. Depending on the amount
of heat generated, the reference power was adjusted. Usually,
39 injections of 1 mL each were performed, with a 0.4 mL injection
prior to the first injection. The data were processed using Origin 7
with the iTC Data analysis plugin, applying manual baseline
correction and manual integration if necessary. The integrated
data was transferred to Excel, correct concentration values were
recalculated, plotted in Origin 2018 and fitted with a sigmoidal
function. The free energy (DGꢂdemic), enthalpy (DHdꢂemic) and entropy
(DS1) of the demicellation process were calculated based on the
phase separation model using the following equations19,33 (with
molar fraction CAC0, observed reaction enthalpy DRH1 and the
concentration of injected surfactant solution csyringe):
(Z)-Dodec-6-enyl b-D-galactopyranoside (GalbCuns12). (Z)-Dodec-
6-enyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranoside (5b, 1.17 g,
2.28 mmol) was deprotected following the general procedure
described in Section 2.2.1 to yield GalbCuns12 as white solid (831
mg, 92%). Rf = 0.53 (CH2Cl2/MeOH 5 : 1); 1H NMR (400 MHz,
DMSO-d6): d = 5.39–5.29 (m, 2 H, H-12, H-13), 4.75 (d, J = 4.0 Hz,
1 H, OH-2), 4.64 (s, 1 H, OH-3), 4.52 (t, J = 5.6 Hz, 1 H, OH-6), 4.30
(d, J = 4.5 Hz, 1 H, OH-4), 4.04 (d, J = 7.3 Hz, 1 H, H-1), 3.71 (dt, J =
9.6 Hz, 6.8 Hz, 1 H, H-7a), 3.62 (s, 1 H, H-4), 3.55–3.44 (m, 2 H, H-6),
3.39 (dt, J = 9.5 Hz, 6.7 Hz, 1 H, H-7b), 3.30–3.28 (m, 1 H, H-5),
DGdꢂ emic ¼ ꢁRT ln CAC0 ¼ DHdꢂemic ꢁ TDSꢂ
À
Á
DHdꢂemic ¼ DRHdꢂemic ꢀ csyringe= csyringe ꢁ CAC
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For determination of CAC 4 100 mM, a diluted solution of the Beside surfactants with a saturated alkyl chain, we also included
surfactant (approx. 0.5-fold CAC) was placed in the sample cell, in novel galactoside and mannoside surfactants with an unsaturated
which and a concentrated surfactant solution (approx. 2.5–3-fold chain. This structural variation can also be regarded as almost P
CAC) was titrated. Due to the smaller concentration window, and HLB invariant.
multiple titration cycles were performed consecutively while
removing the supernatant solution after each cycle. Each data set
3.2 Synthesis of glycosurfactants
was processed as described before, plotted in Origin 2018, and the Except for some glycosurfactants that were commercially available,
combined titrations were fitted with a sigmoidal function.
the members of the library were synthesized either by anomeric
2.3.2 Dynamic light scattering (DLS). Dynamic light scatter- O-alkylation (Glca/bC12, Gala/bCuns12, Mana/bCn, Mana/bCuns12),
ing measurements were conducted on a Malvern Zetasizer mV at by NIS-promoted glycosylation of thiogalactoside 6 (Gala/bCn), or
901 scattering angle. For each surfactant, a series of concentra- by Lewis-acid catalyzed glycosylation of peracetylated xylose 7
tions (5-, 4-, 3-, 2-, 1-, 0.5- and 0.1-fold CAC) was evaluated. All (Xyla/bC12) as shown in Fig. 2. For the synthesis of Gala/bCuns
12
solutions were filtered using CHROMAFIL 0.45 mm PVDF syringe and Mana/bCuns by anomeric O-alkylation, (Z)-dodec-6-en-1-ol
12
adapter filters.
(3) was triflated using triflic anhydride and then reacted with
2.3.3 Tensiometry. Surface tension g measurements were 2,3,4,6-tetra-O-acetylated mannose 1 or galactose 2 and sodium
performed on a Kru¨ss K100 tensiometer at 20 1C with aqueous hydride to obtain the glycosides 4 and 5, respectively, as mixtures
surfactant solutions with a concentration c 4 CAC. For poorly of anomers (Fig. 2a). The anomers could be separated by flash
soluble surfactants, saturated solutions were used. CACs were chromatography to obtain 4a (44%), 4b (7%), 5a (13%), and 5b
´
determined by the intersection point of linear regressions of (63%). Subsequent deacetylation under Zemplen conditions gave
the curves at c 4 CAC and c o CAC. The slope of the curve at the unsaturated glycosurfactants in high yields. Being stereo-
c o CAC (dg/d log c) was used to calculate the surface excess isomers, all four compounds lead to identical high-resolution
concentration Gmax and the minimum molecular area occupied mass spectra (Fig. 2b).
Amin by34
For the synthesis of the saturated galactosides, we reacted
benzylated thiogalactoside 6, which is conveniently obtained
from commercially available IPTG in one step, with the respective
alcohol in presence of NIS. After separation of the anomers by
flash chromatography and debenzylation, pure glycosurfactants
were obtained. For the synthesis of Xyla/bC12, peracetylated
xylose 7 was activated with BF3 etherate and reacted with
dodecanol. Interestingly, this reaction gave high amounts of
the a-anomer despite the presence of the neighboring group-
Gmax = ꢁ(2.303RT)ꢁ1ꢀ(dg/d log c)
ꢁ1
Amin = (NAꢀGmax
)
2.3.4 Small angle- and powder X-ray diffraction (SAXS/PXRD).
SAXS diffractograms of surfactant samples were acquired on a
Bruker Nanostar system equipped with pinhole collimation and
CuKa radiation. All samples were equilibrated at air prior to
measurements. PXRD-diffractograms of ZnO nanoparticles
were recorded on a Bruker D8 system.
2.3.5 Polarization microscopy. Polarization microscopy
images were acquired on an Olympus CX41 microscope with
crossed polarization filters.
´
active acetyl group in the 2-position. Zemplen deacetylation gave
the xylose surfactants.
All new compounds were fully characterized by 1H and
13C NMR spectroscopy as well as ESI-HRMS. The anomeric
3
configuration was determined from the JH-1,H-2 coupling con-
stants for all glucosides, galactosides, and xylosides. For the
2.3.6 Transmission electron microscopy (TEM). TEM
micrographs were acquired on a JEOL JEM-2200FS.
1
mannosides, we relied on heteronuclear JC-1,H-1 coupling con-
stants that were obtained from non-decoupled 1H,13C HSQC
spectra. Exemplarily, the values for Mana/bCuns are 170 Hz
12
(a-anomer) and 155 Hz (b-anomer) which is in agreement with
3 Results and discussion
3.1 Library of glycosurfactants
reported values for mannosides.35
3.3 Surfactant properties
Fig. 1 gives an overview of the glycosurfactants used in our
studies. We included compounds with mono-hexose-type head To get a first impression of similarities and differences between
groups ranging from the well-known glucose surfactants over the surfactants, we generated optimized molecular structures
galactose and mannose to xylose derivatives. Compounds with with electrostatic potential mapping of the surfactants (cf. Fig. 1).
the same length of the saturated hydrophobic side chain are It is obvious that the packing parameter and the HLB value are
either true isomers (for glucose, galactose, and mannose) or at mainly influenced by two factors: the number of hexose units in
least very similar (xylose differs from glucose only by a missing the head group (HLBGlcbC = 10.3 - HLBMalbC = 13.3) and
12
12
hydroxymethyl group). Therefore, these amphiphiles formally the length of the hydrophobic chain (HLBGlcbC = 13.6 -
6
do not differ in their packing parameter P or HLB value. The HLBGlcbC = 11.2). Considering the situation for different iso-
10
same is true, if the anomeric configuration (a,b) is varied. A mers of the surfactants, the HLB value and also the packing
significant change in the head group region is realized for the parameter will remain similar. The incorporation of a double
surfactants containing the disaccharide maltose. A variation of bond should also only slightly influence these parameters.
the hydrocarbon chain length will obviously affect P and HLB. Therefore, we expected the surfactants with maltose head groups
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Fig. 2 (a) Synthesis of Mana/bCuns and Gala/bCuns by anomeric O-alkylation. (b) ESI-HRMS spectra of the newly synthesized carbohydrate
12
12
surfactants with unsaturated side chain. Experimental (black) and calculated spectra (red) of the Na+ adduct. (c) Synthesis of Gala/bCn from
thiogalactoside 6. (d) Synthesis of Xyla/bC12
.
to behave differently than the monosaccharide derivatives, in
3.3.1 Lyotropic liquid crystal formation. Surfactants form
agreement to literature.36 It seems that the anomeric configu- higher organized structures induced by a microphase separation of
ration also has a noticeable effect because at least for our hydrophobic and hydrophilic domains. At higher concentration,
calculations the molecular structures are different when compar- one can observe the occurrence of lyotropic liquid crystal (LLC)
ing the a- and the b-form (for an example see Fig. 1a). The latter phases in contact to water. Because many LLC phases are optically
expectation is also in line with the literature.19 To further study anisotropic, it is possible to use optical polarization microscopy
these hypotheses and findings, we systematically checked the (POLMIC) as a quick tool for checking for their presence. POLMIC
key properties of the surfactants of our library.
images of selected examples are shown in Fig. 3a–c.
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dried from aqueous solution. All samples show a distinct SAXS
pattern containing 1–3 signals. With increasing length of the
hydrocarbon chain the surfactant size increases. A shift of the
LLC lattice planes to larger distances and, according to Braggs
law, a shift of the diffraction signals to lower scattering vectors
q would be expected. This is exactly what we can extract as
qualitative result from Fig. 3d and e.
For the saturated surfactants the lattice distance corres-
ponding to the most intensive signal (dmax) linearly depends
on the length of the hydrocarbon chain. Comparing a surfac-
tant containing an unsaturated tail with the saturated counter-
part, it is seen that the dmax value for ManaC12 is 0.2 nm smaller
than dmax(ManaCuns12) = 2.7 nm. This effect can be explained
by the influence of the CQC bond, which hampers an ordered
arrangement of the carbohydrate chain and leads to an
increased lattice spacing. Furthermore, the latter value corre-
sponds well to the double of the molecular length of
ManaCuns12. A distance resembling twice the molecular length
is expected for almost any LLC-phase. In comparison, the
ManbCn compounds exhibit dmax values, which are approx.
0.2 nm larger, which can be attributed to the more linear
arrangement of head group and tail of the b-linked surfactants.
The head group has an influence as well, and it seems that the
LLC phases of galactosides are slightly larger than for the
mannosides. In combination, one can shift dmax(ManaC12) =
2.5 nm to dmax(ManbCuns12) = 3.3 nm (Fig. 3e), although P is
almost constant for these two surfactants.
The precise assignment of a supramolecular structure is difficult,
if only a limited number of diffraction signals is available as in the
current case, and one cannot exclude that mixtures of different
phases occur. However, a satisfactory agreement was found to the
pattern created by a so-called ordered bicontinuous double diamond
phase (OBDD, space group Pn3m) with a lattice constant of a =
3.831 nm for ManaC10 as shown in Fig. 3d. The OBDD structure
itself has been found rarely in general,37,38 and to the best of our
knowledge it has not been reported for a carbohydrate-based
surfactant. OBDD is a complex structure formed by the interpene-
tration of two diamondoid networks. It is interesting to note that
researchers recently have discussed the correlation between such
structures and chirality.39
3.3.2 Aggregate formation in water at low concentration.
There are several parameters of interest for the characterization of
surfactants. Information about intermolecular interactions can be
obtained from thermodynamic data (DGag, DHag, DSag). The
concentration at which a surfactant forms aggregates in aqueous
solution (DCAC) and the size of these aggregates (Dag) also
provides important information. Prior to aggregate formation, the
water–air interface gets occupied by the surfactants. The packing of
Fig. 3 POLMIC images of a- (left) and b-anomers (right) for ManC12 (a),
GalC12 (b) and GalCuns (c). (d) SAXS patterns of ManaC8, ManaC10
,
12
ManaC12 and ManaCuns12. The grey bars indicate the pattern expected
for a OBDD phase with a periodicity of 3.8 nm. (e) dmax-dependence on
chain length Cn of surfactant samples equilibrated at air. Gal (black), Man
(blue), a (squares), b (circles), Cn (solid symbols), Cuns (hollow symbols).
n
Although the assignment of a microstructure from POLMIC the surfactants is expected to influence the minimum surface
textures is often difficult, it becomes visible that the change of tension (gmin) and the minimum molecular area occupied at the
the head group Man-Gal (a,b), the anomeric configuration interface (Amin). For the determination of the mentioned features,
a-b (a,b,c), and also the shape of the tail C12-Cuns (b,c) we applied a combination of analytical techniques including
12
influence the structure of the LLCs. More information about isothermal titration calorimetry (ITC), concentration-dependent
the microstructure of the LLCs phases are obtained by small surface tension measurements and dynamic light scattering (DLS).
angle X-ray scattering (SAXS, see Fig. ESI-1, ESI†). For better
Fig. 4a exemplarily shows the integrated ITC curve for one
reproducibility, all samples were equilibrated in air after being measurement of the novel unsaturated surfactant GalbCuns
.
12
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Each ITC measurement was repeated several times yielding
consistent data. The ITC curves of other surfactants involved in
the current study are given in Fig. ESI-2 (ESI†). For GalbCuns
,
12
an aggregation point at a concentration of c = 1.12 ꢃ 0.08 mM
becomes visible, which corresponds to the CAC of this surfactant.
In an ITC experiment, concentrated solutions of surfactants were
titrated into water. With this technique, the heat that is produced
during titration is detected. From this, thermodynamic values for
the de-aggregation process upon dilution can be directly derived.
Assuming reversible processes, which is typically the case for
soft-matter interactions, the thermodynamic functions for
aggregation and de-aggregation are equitable, but with oppo-
site sign. For GalbCuns we obtained DGag = ꢁ26.35 kJ molꢁ1
12
and DHag = 2.40 kJ molꢁ1
.
Concentration-dependent surface tension (g) was also used
as additional technique to determine the CAC (see Fig. 4b
for GalbCuns and Fig. ESI-3, ESI† for the other surfactants
12
samples). The g–c curve of GalbCuns exhibits features, which
12
Fig. 5 (a) DLS data of GalbCuns12 in water at c = 0.5 ꢄ CAC (blue), c = CAC
(red) and c = 2 ꢄ CAC (black). (b) DLS data of GalbC8 in water at c = 0.1 ꢄ
CAC (blue), c = CAC (red), c = 2 ꢄ CAC (black) and c = 5 ꢄ CAC (orange).
Cryo-TEM micrographs of GalbCuns12 at c E CAC (c) and c E 3 ꢄ CAC (d).
are characteristic for surfactants. With increasing concen-
tration, more and more molecules get located at the water–air
interface. Consequently, g decreases until the interface is fully
occupied. At this point aggregation in solution takes place,
and the slope of the g–c changes. The CAC was determined as
1.05 mM from the tensiometry data (Fig. 4b), which is in good in diameter, can be observed. Upon raising the concentration
agreement with the ITC measurements. The value for surface the aggregate size increases (Dh = 60 nm). The latter findings
tension reached at this transition (gmin) is 28.3 mN mꢁ1, which can be confirmed by transmission electron microscopy under
is at the lowest end for surfactants with alkyl side chains and cryogenic conditions taken from GalbCuns dispersions at
12
non-ionic head groups.40,41
c = CAC and 3 ꢄ CAC (Fig. 5c and d). At c = CAC a large number
From DLS measurements at three different concentrations, of particles with a diameter of E25–30 nm is visible. The size
we obtained particle size distribution curves which are shown of the particles (100–300 nm) has increased significantly at
in Fig. 5a. Below the CAC, as expected, the scattering intensity c = 3 ꢄ CAC. The dimension of a conventional, spherical micelle
is very low and there are barely any aggregates observable. would be of the order of 2-times the diameter of the surfactant
At c = CAC, significant scattering, originating from objects 25 nm (2 ꢄ 1.8 nm E 3.6 nm). The observed aggregates are obviously
much larger. It has been reported in the literature,42 that sugar-
based surfactants have a high tendency for the formation of non-
equilibrium aggregates in water.
The same set of data was acquired for all other compounds
of the series shown in Fig. 1 (refer to the ESI†). As example
with a shorter chain, the data for surfactant GalbC8 is shown in
Fig. 4c, d and 5b. Because the length of the hydrocarbon chain
affects the packing parameter and the HLB value, a significant
change of the surfactant parameters is expected for GalbC8, and
this is exactly what we found. DLS measurements revealed
small aggregate sizes of 4.4 nm only slightly rising at higher
concentrations, which suggests the presence of micelles. In
addition, ITC and tensiometry data are in agreement. We could
confirm that the used methods resulted in reliable data about
the surfactants’ behavior, giving the possibility to analyze the
data for systematic correlations.
The dependencies on the hydrocarbon chain length is the
most obvious correlation, as indicated in the last paragraph.
The most influenced factor is the CAC as seen from Fig. 6a. For
aggregate formation of glycoside surfactants with Cn = 6, high
concentrations (4100 mM) are required. Doubling of the chain
length (Cn = 12) leads to a decrease of CAC by two orders of
magnitude. This effect found for our carbohydrate surfactants
Fig. 4 (a) Integrated ITC curve and (b) tensiometry curve of GalbCuns12 in
water. (c) Integrated ITC curve and (d) tensiometry curve of GalbC8 in
water.
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is stronger compared to classical ionic surfactants such as sodium
alkyl sulfates (e.g. sodium dodecyl sulphate, SDS), where an increase
of only one order of magnitude comparing C6 with C12 tailed
surfactants is found.43,44 Because the monosaccharide-surfactants
with longer alkyl chains showed reduced solubility in water, it was
not possible to determine the CAC for a chain-length 48 and 410,
respectively.42 However, the solubility increased for the novel surfac-
tants containing the unsaturated C12 carbon chain. We found that
their CAC values lie above the line for the extrapolation to n = 12 of
the respective saturated compounds. Again, as for the lyotropic
liquid crystals described before, the presence of the CQC double
bond has a remarkable effect. This may arise from an unfavorable
conformation of the hydrocarbon chain, which is beneficial for a
better solubility.
For the thermodynamic data, some distinct trends can be
identified. The free enthalpy of aggregation DGag depends almost
linearly on Cn (Fig. 6b). Considering the DHag values, which are
slightly positive, it was found that the aggregation of the surfactants
in water is predominantly entropy-driven. This effect was assigned in
surfactant chemistry to the so-called hydrophobic effect.45 Because
water molecules cannot solvate the hydrophobic chain of the
surfactant, they have to adopt an entropically unfavorable state for
a single surfactant molecule in solution. When aggregates like
micelles form, these water molecules are released gaining entropy.
Thus, it is expected that the influence of entropy increases with
longer alkyl chains, which is also in agreement with our data. The
decrease of DHag with increasing Cn indicates, that the overall
interaction becomes less repulsive, which can be explained by the
increasing van der Waals interaction between the extended
hydrocarbon chains. Comparing surfactants of the same chain
length (e.g. C8) but with different head groups reveals that the
DHag values differ from each other, which is a sign for the strong
influence of different head groups.
Regarding the aggregate size, a general trend exists as well.
Here, on can distinguish between the cases, where micellar aggre-
gates are formed, and those with formation of larger aggregates
(Fig. 6c). Because the diameter of a spherical micelle equals in good
approximation twice the length of a surfactant, the observed
increase with longer hydrophobic chain is expected. The size of
aggregates, which are much larger than micelles, qualitatively
Fig. 6 (a) CAC–Cn correlation (dashed lines represent linear extrapola-
tion). (b) DGag (circles) and DHag (squares)–Cn correlation. (c) Size of
aggregates–Cn correlation at c = 2 ꢄ CAC; dashed lines = transition
micellar to larger aggregates, arrows = difference between anomers, Gal
(black), Man (blue), Glc (red), Mal (orange), a (squares), b (circles), Cn (solid
symbols), Cuns (hollow symbols).
n
increases with longer alkyl chains as well. The transition from (in the case of xylose) in the cases presented in Fig. 7, the
micelles to larger aggregates is influenced by the head group. mannoside head groups occupy a significantly higher surface
Maltoside surfactants remain in the micellar state even for a long area than the xyloside head groups. It can be understood that
alkyl chain such as C12. In the case of galactoside- or mannoside the surface tension reached at c 4 CAC (gmin) is smaller for
surfactants, the transition occurs already for C8 and C10 chains, smaller values of Amin, because the alkyl chains can pack denser
respectively, as indicated by the dashed lines in Fig. 6c. It can also at the air–water interface.
be seen, that the anomeric configuration has a strong impact. For
The anomeric conformation should also influence the way
the galactose- as well as the glucose-based surfactants there is a the surfactants pack at the air–water interface (see also Fig. 1).
remarkable difference in aggregate size for the a-form compared to Therefore, it is not surprising that a- and b-forms differ, if all
the b-form indicated by the arrows in Fig. 6c.
other parameters are kept constant. This effect can arise from
The influence of the head group can be evaluated in further the ‘‘molecular kink’’ introduced in the a-form, which leads to
depth by taking a closer look at the tensiometry data. By an increase in Amin. However, it is not expected that the alkyl
calculating the maximum surface excess concentration Gexc
the minimum molecular area of the surfactant occupied at the
,
chains pack denser as indicated by the lower gmin values.
3.3.3 Emulsification properties. Another important appli-
air–water interface Amin can be determined. Although the cation of surfactants is their use as emulsifiers. According to
molecular mass of the head group is the same or very similar their HLB values (10.3–16.0; see Fig. ESI-4, ESI†) the surfactants
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for Span 20, and, after 250 h, still 63% of the entire volume
remains as emulsion (Fig. 8c). The short-chain maltoside
MalbC6 turned out to be not suitable as emulsifier at all. Within
minutes, the entire emulsion was separated. With increasing
alkyl chain length (C10), the surfactants stabilize the emulsion
even better than Span 20.
Fig. 8d shows the emulsification behavior of the novel
surfactants ManaCuns and GalaCuns in comparison to their
12
12
Fig. 7 Head group dependency of the minimum area occupied at the
air–water interface (a) and the minimum surface tension at saturation at
the interface (b). C12-Surfactants: Xyl (purple), Gal (black), Man (blue), Glc
(red), a (squares), b (circles).
saturated counterpart. It is noticeable that the saturated
and unsaturated surfactants of the same head group exhibit
nearly identical stabilization capabilities. This suggests that
the unsaturated surfactants are promising substitutes (higher
solubility, but similar emulsification behavior) for the saturated
surfactants. However, it is obvious that the choice of headgroup
greatly impacts the performance of the surfactant (see also
Fig. 8f). For better comparison, we seek to parameterize the time
dependent curves.
from our library should be suitable for preparing oil-in-water
(O/W) emulsions. Therefore, as test system, we emulsified
cyclohexane in water (for a detailed procedure see the Experi-
mental section). The stability of the emulsion was monitored
over time by recording the volume of the phases formed by
potential demixing (see Fig. 8a). Photographic images of a
representative series are shown in Fig. 8b. As reference com-
pounds, we used two commercially available emulsifiers:
sorbitane monolaureate (Span 20) and octylphenol ethoxylate
(Triton-X 100). After emulsification, demixing starts very slowly
One parameter obviously is the remaining volume of the
emulsion at t = 250 h. The demixing process was fitted by a
mono-exponential decay (Vem,rel p exp(ꢁt/k)), and k was used
as parameter for the description of the emulsions stability as
well. Thus, high k values characterize a stable emulsion, which
demixes slowly. Fig. 8e shows the two described parameters for
a series of surfactants differing in hydrocarbon chain length Cn.
Please note the logarithmic scale of the y-axis, indicating there
are huge differences in how fast the emulsions destabilize.
There is no uniform trend, however, a chain length of n 4 8
seems to be beneficial for the performance as emulsifier, which
is reasonable, because the HLB value decreases in this direc-
tion. In contrast to the properties discussed before, there are
only minor differences observed regarding a change in the
anomeric configuration or regarding the presence of an unsa-
turated side chain. The overall best emulsifiers from our library
were found to be ManaC10, ManaC12 and ManaCuns12, which
exhibited nearly identical emulsifier properties. Surfactants
with the same Cn have the same HLB value and should, thus,
also perform equally as emulsifiers. This statement is wrong as
seen from Fig. 8f for n = 12. Here, the head group has a huge
influence on the stability of the emulsion. However, giving a
universal explanation for these differences proves difficult, as
many factors have to be taken into account for such complex,
ternary systems (ratio of surfactant, water and hydrocarbon,
polarity of the latter). However, from our experiments, we can
deduce, that the rather small differences between our surfac-
tants, introduced by change of the anomeric configuration and/
or epimerisation of distinct OH-moieties in the carbohydrate
headgroup, can impact their performance to the same degree
as changing the size/polarity of the headgroup or changing the
length of the hydrophobic moiety.
Fig. 8 (a) Graphic representation of the method used for determining the
temporal stability of the formed emulsions. (b) Photographic images of
an emulsion experiment with GalaCuns at t = 0, 24, 48 and 72 h.
12
3.3.4 Glycoside surfactants as capping agents. Emulsions
can be used in very different ways. One very special applications
is in microemulsion-based techniques for the generation of
inorganic particles.46,47 In the past we have described a method
for the preparation of ZnO nanoparticles by a microemulsion-
based technique.20–23 In our approach, a water in oil emulsion
is prepared and an oil-soluble, organometallic ZnO precursor is
(c) Remaining, relative emulsion volume versus time curve for three
selected examples; grey line = reference span 20; circles = ManaC10
;
squares = MalbC6. (d) Comparison of relative emulsion volume versus time
of emulsions stabilized by saturated and novel surfactants. (e) Emulsion
stability parameters depending on hydrocarbon chain length. (f) Emulsion
stability parameters depending on head group. Gal (black), Man (blue), Glc
(red), Mal (orange), Xyl (purple), a (squares), b (circles), Cn (solid symbols),
Cuns (hollow symbols).
n
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added. Although based on the HLB concept (see above) Transmission electron microscopy (TEM) measurements show
our surfactants are expected to be more suitable for (O/W) that the samples are morphologically different as shown in
emulsions, it is important to note not only the temperature is Fig. 9c, d and Fig. ESI-5 (ESI†). The use of MalbC12 led to the
higher (T = 55 1C), which could lead to a change in the behavior formation of particles with a shape resembling a rugby ball. As
of the surfactant,48 but we also work under steady shearing interesting as this shape may seem, it actually represents
force resp. continuous application of ultrasound. This precursor nanoparticles in the so-called Wulff form. The Wulff form is
reacts in a defined way with water, resulting in crystalline ZnO.49 the shape a crystal adopts solely on the intrinsic stability of its
The emulsifying agent can interact with different facets of the facets.51 Therefore, it can be concluded, that the energy of
growing ZnO nanoparticle by stabilizing them.20,23,50 If certain surfaces terminating the crystal was not influenced by MalbC12
facets can be selectively stabilized, this will influence the during growth. This result indicates that the maltose head
morphology of the particle.51 Because of the demonstrated group does not specifically interact with the ZnO crystal. The
variability of the head group of carbohydrate-based surfactants, ZnO particle shape found for GalaC12 is much different from
it could be, thus, highly interesting to investigate their behavior the Wulff form. The particles have a rhombus-like shape with
as capping agents. Up to this point, we could show that the the edges forming angles of 1201 and 601, respectively (see
surfactants can be used to form emulsions. Although it is Fig. ESI-5, ESI†). Therefore, we assume the shape proposed in
beyond the scope of this paper to report about the ZnO Fig. 9a. This is in agreement with the good fit of the measured
nanoparticle synthesis in a comprehensive way, we demon- atomic plane distance (2.8 Å) to the literature value for the (100)
strate as proof-of-concept, that the glycoside-surfactants of this plane (2.81 Å). In addition to a restriction of growth in crystallo-
study are suitable for the generation of ZnO nanoparticles. graphic c-direction, GalaC12 seems to stabilize the (100) and the
With these experiments, we wanted to answer the question if (010) facets. Particles with crystallographic a,b-direction as
there is a morphological effect or not. Therefore, the experi- main directions of growth are not unusual.52,53 However, these
ment depicted in Fig. 9a was performed.
particles do typically represent hexagonal plates or variants of
Water in cyclohexane emulsions were prepared with either it. The more rod-like morphology in combination to this
MalbC12 or GalaC12. The ZnO precursor was then added particular crystallographic orientation is unusual to the best
(for detailed procedure refer to the experimental part). From of our knowledge. Based on these first promising findings we
the reaction, a white dispersion is obtained. After removal of will further investigate the potential of our surfactant library for
solvent, powder X-ray diffraction (PXRD) was conducted. The nanoparticle synthesis.
resulting patterns (Fig. 9b) clearly show that in both cases ZnO
with Wurtzite structure was obtained. Other crystalline phases
are not present, and there are no marked differences by
4 Conclusions
comparing the samples prepared using the two surfactants.
In this work, we presented a library of well-defined glyco-
surfactants. Besides known, saturated alkyl-glycosides, we
could successfully introduce a novel class of glycosurfactants
bearing an internally unsaturated alkyl chain. From our
library’s compounds, we could investigate the influence of alkyl
chain length/structure, carbohydrate moiety and anomeric
configuration on the properties of the surfactants. From our
data, we can conclude that the chain length influences the
parameters of the surfactants (dH, CAC, thermodynamic para-
meters) as expected. The novel unsaturated surfactants show a
differing behavior, which arises from a bent shape of the alkyl
chains. We could also find significant differences for changes
in the head group. The sugar moiety as well as the anomeric
configuration led to different CACs and also greatly impacted
the emulsification capabilities of the surfactants. The most
important take-home message of our study is, although invariant
in packing parameter and HLB value, surfactant isomers do vary
significantly in almost all key properties.
Two promising surfactants were then, as proof-of-principle,
Fig. 9 (a) Schematic representation of the emulsion-based method for
ZnO nanoparticle synthesis using carbohydrate-based surfactants. Two
resulting particle morphologies are shown (grey = Zn; red = O). (b) PXRD
patterns of the ZnO materials resulting from MalbC12 (grey) and GalaC12
(black) in comparison to the reference pattern of crystalline ZnO in
Wurtzite structure (black bars). TEM micrographs of ZnO particles obtained
using MalbC12 (c) and GalaC12 (d).
tested as capping agents for the generation of anisotropic ZnO
nanocrystals. We could show that subtle changes in the carbo-
hydrate head group can indeed induce changes in nanoparticle
growth. With these findings, we will now further explore the
potential of glycosurfactants for the synthesis of defined,
anisotropic nanoparticles.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
23 M. Gerigk, J. Bahner, T. Kollek, S. Helfrich, R. Rosenberg,
The project was supported by the Deutsche Forschungsge-
meinschaft (collaborative research center SFB 1214, project A1).
R. M. M. acknowledges a PhD fellowship from the Konstanz
Research School Chemical Biology. M. K. was funded by an ERC
consolidator grant (ISURF, project 614606). We thank A. Friemel
and U. Haunz for their assistance with NMR spectroscopy and
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