3 RESULTS AND DISCUSSION
3.2 Investigation of novel “multifunctional” surfactants
O O
COCl HO
O
O
+
nO
O O
O O
O
n triethylamine
toluol
“polymerizable hydrophobic part“ “hydrophilic tail“
O O
COCl HO
O
O
+
nO
O O
O O
O
n triethylamine
toluol
“polymerizable hydrophobic part“ “hydrophilic tail“
(1)
3.2 Investigation of novel “multifunctional” surfactants
In close cooperation with the Institute of Applied Synthetic Chemistry (IASC) two amphiphilic molecules were investigated with regard to their ability to form anisotropic LC phases. The molecules were designed with respect to a further possible alignment in magnetic fields and crosslinking of the organic phase.
3.2.1 Aqueous solutions of a benzoic acid derivative
The introduction of polymerizable groups to surfactant molecules allows an additional cross-linking in the hydrophobic region of the network. The thus stabilized organic phase in the mesopores should enhance the mechanical properties of the resulting material.
A polyethylene glycol derivative (compound (1), Figure 3.2.1 was developed at the IASC [146] and received as yellowish oil.
Figure 3.2.1. Synthesis path and molecular structure of the investigated benzoic acid derivative compound (1).
Mixtures of concentrations of 95%(w/w) to 25%(w/w) of compound (1) in water were produced by stirring in an ice bath. The resulting substances were filled in 2 mm-capillaries and further examined with SAXS at 21 °C. Down to surfactant concentrations of 40%(w/w), macroscopically homogeneous, transparent samples were obtained. At more dilute solutions (≤35%(w/w)) the systems developed 2 phases on a macroscopic scale.
For the homogeneous mixtures a change from transparent to turbid (white) was observed at temperatures higher than 30 °C. This effect was also visible under the light microscope
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
20 °C, before heating
25 °C 30 °C 35 °C
30 °C 32 °C 38 °C
32 °C 40 °C 40 °C
40 %(w/w) 60 %(w/w) 80 %(w/w)
200 μm
20 °C, before heating
25 °C 30 °C 35 °C
30 °C 32 °C 38 °C
32 °C 40 °C 40 °C
40 %(w/w) 60 %(w/w) 80 %(w/w)
200 μm
with crossed polarized filters for the 1 mm-diameter capillaries (change from dark to white). Since investigations on thin films did not show birefringence, the effect is most likely due to a phase separation starting with small droplets. Upon cooling below 30 °C the samples changed again to transparent. SAXS measurements did not show any liquid crystalline structure at 21 °C.
Figure 3.2.2. Microscopic phase separation during heating of 40, 60 and 80%(w/w) of C28H44O10 in water under the light microscope. No anisotropic liquid crystalline phase was observed at any temperature with crossed polarizing filters.
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
DSC-measurements for concentrations of 20, 40, 60 and 80%(w/w) did not show any phase transition. This result was confirmed by SAXS-measurements at the temperatures 40 °C and 60 °C, respectively. Microscopic images of the 20, 40, 60 and 80%(w/w) of the heated samples show that the mixtures undergo a phase separation into surfactant-rich and water-rich phases, in which both phases each consist of an isotropic liquid. Since no liquid crystalline region could be found by varying surfactant concentration and temperature, the molecule was not further investigated.
3.2.2 Aqueous solutions of a perylene derivative
The synthesis of an amphiphilic perylene derivative was performed mainly following the report of Arnaud et al. [147] and is described in detail elsewhere [146]. The molecule consists of a large aromatic core surrounded by four hydrophilic arms (see Figure 3.2.3).
This design should enforce a one-dimensional supramolecular assembly in water through intermolecular π-stacking and/or hydrophobic interactions (as was reported by [147] for n=2 and dilute concentration). The four hydrophilic arms hinder a possible aggregation of the assemblies and provide good water solubility. The perylene derivate with n=3 (Figure 3.2.3, compound (2)) was received as orange viscous liquid.
Figure 3.2.3. Synthesis path and molecular structure of the perylene derivate: compound (2) with n=3.
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
2 3 4 5 6 7
q / nm-1
intensity / a.u.
10 %(w/w) 40 %(w/w) 55 %(w/w) 60 %(w/w) 79 %(w/w) 83 %(w/w) 88 %(w/w)
Comp (2)/water/P123
=47/53/23 (w/w) Comp (2)/water/toluol
=39/9/61 (w/w) Comp (2)/water/butanol
=86/54/14 (w/w) Comp (2)/water/butanol
=56/44/14 (w/w)
2 3 4 5 6 7
q / nm-1
Concentrations of compound (2) of 10 up to 95%(w/w) in water were examined. The substance is highly soluble in water as well as in toluol and also butanol. POM investigations were performed by studying the phase behaviour depending on temperature and concentration, but did not show any anisotropic phases. SAXS measurements performed in a q-range of 1 to 8 nm-1 revealed only a broad peak characteristic for short-range order in solutions. The observed distances are depending on surfactant concentration and correspond to mean distances of 3.2 nm for 40%(w/w) to 1.6 nm for 88%(w/w) of compound (2) in water.
Figure 3.2.4. Short-range order of the amphiphilic perylene derivative in different concentrations.
Rheology experiments also could not confirm any stacking of the large hydrophobic aromatic core. Most likely, the four hydrophilic arms hinder the aggregation.
3.2.3 Aqueous solutions of an alkylpolyethylene oxide derivative
Non-ionic polyethylene oxide-based surfactants are commonly used in the templating of sol-gel derived mesoporous materials. By varying the length of the polyethylene glycol chain, properties such as pore diameter or lyotropic liquid crystal structure may be controlled to some extent. Investigations were carried out on the non-ionic surfactant Brij 97 (polyoxyethylene 10 oleyl ether) and a very similar newly designed molecule with up to three double bonds in the hydrophobic alkyl chain. Both molecules are depicted in Figure 3.2.5. The positioning of unsaturated fatty alcohols in the non-commercial molecule should allow oxidative as well as photochemical crosslinking in the hydrophobic region of the
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
Brij 97
modified
O OH
n=10
O
O n=10
(3)
surfactant
H2O
x 100
x 320
(b) (c)
(a)
surfactant micelles. The fabrication of the modified molecule is described in detail in [146]. Investigations with POM show at least two different liquid crystalline phases between the isotropic surfactant-rich and the water-rich phase. In a first experiment a drop of water and a drop of amphiphile were put on a slide side by side and allowed to mix. The result is shown in Figure 3.2.6. Hereby, the fan-like texture at higher water concentration suggests a hexagonal arrangement, whereas the Maltese crosses at high surfactant concentration indicate a lamellar ordering. This is in accordance with the phase diagram of Brij 97 as reported in [14] (see Figure 2.1.3).
Figure 3.2.5. Molecular structure of polyoxyethylene 10 oleyl ether (Brij 97) and the polymerizable alkylpolyethylene glycol(compound (3)) with up to three double bonds in the hydrophobic alkyl chain.
Figure 3.2.6. a) Schematic setup for a drop experiment. (b) and (c): Texture of the mixed region under crossed polars with fan-like structure characteristic for hexagonal arrangement and Maltese crosses at high surfactant concentration, characteristic for lamellar structure.
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
1 2 3
H2O 10-2 M HCl 10-1 M HCl 1 M HCl
intensity / a.u.
q / nm-1
1 2 3
q / nm-1
EGMS(13) / 20 °C
40 °C
1 2 3 4
EGMS(13) EGMS(17) EGMS(21) EGMS(25)
q / nm-1
2 3
(200)
log intensity / a.u.
1 M HCl / 20 °C
Figure 3.2.7. Small-angle X-ray scattering profiles for vacuum-dried gels prepared from EGMS, for starting composition of EGMS(13) at different HCl concentrations (left) and for increasing EGMS concentrations at an acid concentration of 1 M HCl (right).
Further POM investigations show that hexagonal LLC phases are existent between 38 to 60%(w/w) surfactant in water with anisotropic to isotropic phase transitions around 36 °C.
This temperature decreases with increasing amphiphile concentration. Above 60%(w/w) a lamellar structure is indicated by the POM investigations up to a concentration of 75%(w/w). The phase behaviour of the newly synthesized amphiphile is qualitatively similar to the commercially available Brij 97 with slightly narrower phase regions and lower thermal boundaries. Therefore, experiments on the commercially available Brij 97 were carried out to investigate the possibilities of the molecule as structure directing agent in the gel synthesis with glycol-modified silanes. Brij 97 has already been reported as structure directing agent in the fabrication of periodic mesoporous silica using conventional tetraalkoxysilanes [31, 148, 149], employed surfactant concentrations ranging from 4-10%(w/w) resulting in regions with highly ordered pores of hexagonal symmetry.
3.2.4 Brij 97 as structure directing agent
Small-angle X-ray scattering results for wet gels prepared from EGMS as silica source and Brij 97 (in a concentration of 30%(w/w) in aq. HCl) as templating surfactant are shown in Figure 3.2.7. Four concentrations of EGMS, ranging from 10 to 25 (w/w) SiO2-content, to lyotropic liquid crystalline phase (30/70 (w/w)) have been examined at different pH.
Therefore, the gels were prepared with H2O, 10-2 M, 10-1 M and 1 M hydrochloric acid.
The series of gels show that the long-range order improves at lower pH.
3.2 INVESTIGATION OF NOVEL “MULTIFUNCTIONAL” SURFACTANTS
0,5 1,0 1,5 2,0 2,5 3,0
(11)(20)
q / nm-1 (10)
(20) (10)
50 %(w/w) *)
40 %(w/w)
30 %(w/w)
0,00 0,25 0,50 0,75 1,00
0,00
0,25
0,50
0,75
1,00 0,00
0,25 0,50 0,75 1,00
Brij 97 1 M HCl
SiO2EGMS
Figure 3.2.8. SAXS profiles for gels mixed from SiO2(EGMS)/(Brij 97+1 M HCl) equal to 10/100 and changing Brij 97/1M HCl ratios of 30:70, 40:60 and 50:50 (w/w) (*SAXS curve for 50%(w/w) Brij 97 was measured with laboratory equipment, the other two curves were measured at the synchrotron facility Elettra).
Results for all prepared gels are depicted in the ternary phase diagram (right). The encircled points indicate the region with pronounced periodic mesostructure.
The SAXS profiles in Figure 3.2.7 have been standardised to depict the decreasing FWHM with increasing acid concentration and decreasing SiO2-content.The peaks are situated at q=1.1 nm-1 which corresponds to a periodic distance of 5.7 nm. From the SAXS patterns it can be concluded that the order becomes more pronounced with decreasing silica concentration. Aging of the gels was performed at 20 °C, 30 °C and at 40 °C with the 20
°C-aged samples leading to higher periodic ordering proved by SAXS measurements. As known from the phase diagram of Brij 97 in water (Figure 2.1.3), a hexagonal phase is found in a concentration range of ~33-63%(w/w) surfactant in water. Therefore, Brij 97 in concentrations of 40 and 50%(w/w) in aq. HCl was additionally investigated in the sol-gel process. In Figure 3.2.8 scattering curves for different starting concentrations of EGMS/surfactant/1 M HCl are depicted. Results from all gels have been summarized in the ternary phase diagram. From the measurements it is clearly visible that periodic mesostructure could only be achieved at relatively low SiO2 concentrations. Gelation times of gels prepared at 20 °C and with 1 M HCl amount to approximately 48 h, independent of surfactant/water ratio. At elevated temperatures gelation takes place about twice as fast.
Gels with ordered mesopores at low SiO2 content were very soft and collapsed into powders after ambient pressure drying. Samples were also dried with supercritical fluid extraction with MeOH as supercritical fluid, leading to monolithic material with less pronounced peaks in the SAXS curves and a periodic distance of approximately 5.3 nm.