High adsorptive properties of covalent triazine-based frameworks (CTFs) for surfactants from aqueous solution

Article abstract of DOI:10.1039/c4cc06393g

Carbonaceous covalent triazine-based frameworks from 1,4-dicyanobenzene (CTF-1) can adsorb up to about 4 g g^-1 of the surfactants alkyl polyglycolether, C₁₂EO₇ and sodium dodecylsulfate, SDS from aqueous solution (c = 8.0

Full text of DOI:10.1039/c4cc06393g

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High adsorptive properties of covalent
triazine-based frameworks (CTFs) for
surfactants from aqueous solution†
Cite this: Chem. Commun., 2015,
51, 484
Received 14th August 2014,
Accepted 6th November 2014
Asamanjoy Bhunia,a Subarna Dey,a Maria Bous,b Chenyang Zhang,b
Wolfgang von Rybinski*b and Christoph Janiak*a
DOI: 10.1039/c4cc06393g
www.rsc.org/chemcomm
Carbonaceous covalent triazine-based frameworks from 1,4-dicyano-
benzene (CTF-1) can adsorb up to about 4 g gꢀ1 of the surfactants
alkyl polyglycolether, C12EO7 and sodium dodecylsulfate, SDS from
aqueous solution (c = 8.0 mmol Lꢀ1) which is more than 20-times the
gravimetric uptake capacity of a carbon black (Printex L from Evonik).
This high uptake is due to CTF sheet exfoliation. Taking into account
the thermal and chemical stability of CTFs this first surfactant adsorp-
tion study with CTFs shows that CTFs are especially interesting for the
uptake of substances from solution.
The adsorptive properties of porous solid substances are of great
importance for many applications.1 Solids based on carbon black
structures and graphitic carbons which have a surface area between
100 and 500 m2 gꢀ1 are especially interesting as key sorbents,2 inter
alia for separation processes and gas storage.3,4
Covalent triazine-based frameworks (CTFs) were developed
by Kuhn, Antonietti and Thomas et al. as permanently porous
substances.5 They constitute a subclass of POMs (Porous Organic
Materials) and possess high chemical and thermal stability. Classes
of POMs are often differentiated according to their building units
Scheme 1 Idealized synthesis of CTF-1.
such as benzimidazole-linked polymers (BILPs),6 hyper-crosslinked reaction with ZnCl2 as in-between well-defined COFs and porous
polymers (HCPs),7 polymers of intrinsic microporosity (PIMs),8 carbon materials. So far, CTFs have been used in heterogeneous
porous aromatic frameworks (PAFs),9 conjugated microporous catalysis,12 as catalytic support in liquid phase reactions,13 for
polymers (CMPs)10 and covalent organic frameworks (COFs).11
CTFs are made by the idealized trimerization reaction of
gas storage and for separation.14
For applications of CTFs as adsorbents in liquids,15 the
carbonitriles to triazine rings (Scheme 1). The resulting sheets interaction with substances from solution is decisive.16 Important
of CTF-1 can form a hexagonal lattice with an eclipsed AAA substances from solution which strongly interact with surfaces are
structure and tubular channels of 1.2 nm diameter.5 CTFs are surfactants. Surfactant adsorption at solid–liquid or liquid–liquid
interesting because of cheap and readily available starting interfaces is crucial in industrial processes such as corrosion
materials, facile synthesis and certain hydrophilicity. From inhibition, dispersion stabilization, detergency, crude oil refining,
their elemental composition, we view CTFs from ionothermal purification and lubrication.17 Adsorption studies from solution
for many types of surfactants and numerous surfaces18 showed
a
¨
Institute for Inorganic and Structural Chemistry, University Dusseldorf,
that the adsorbed amounts and the structure within the adsorbed
layer depend on the properties of the surfaces, the type of the
surfactants and parameters like temperature and pH. Typical
gravimetric surfactant uptake capacities for porous graphitized
carbon black with a specific surface area of about 100 m2 gꢀ1 were
¨
D-40204 Dusseldorf, Germany. E-mail: janiak@uni-duesseldorf.de
b
¨
¨
Institute for Physical Chemistry, University Dusseldorf, D-40204 Dusseldorf,
Germany. E-mail: vonRybinski@uni-duesseldorf.de
† Electronic supplementary information (ESI) available: Synthesis, IR, powder
XRD, elemental analysis, TG analysis, SEM images, N2 and CO2 sorption and pore
size distribution, adsorption of surfactants. See DOI: 10.1039/c4cc06393g
in the range of 0.1 to 0.2 g gꢀ1 19
.
484 | Chem. Commun., 2015, 51, 484--486
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Fig. 1 SDS adsorption isotherms on CTF materials.
Fig. 2 Adsorption isotherms of SDS and C12EO7 on CTF-1(600) and SDS
on carbon black in the low concentration range.
Here we investigate for the first time the adsorptive properties
of a CTF member (CTF-1 synthesized from 1,4-dicyanobenzene,
Scheme 1; see ESI† for further details) towards a non-ionic and
an anionic surfactant (alkyl polyglycolether, C12EO7 and sodium
dodecylsulfate, SDS, respectively) from aqueous solution and
compare it with a typical carbon black with a specific surface
for the area per adsorbed molecule calculated according to
eqn (1) are given, assuming that the BET surface area of the
CTFs is accessible for the surfactant molecules.
1018
area of 150 m2 gꢀ1
The amount of surfactant adsorbed on CTFs was determined
.
as ¼
(1)
NG
from the surface tension of the solutions which correlates to where, as = area per molecule (nm2); N = Avogadro constant
the surfactant equilibrium concentration from a calibration (6.023 ꢁ 1023 molecules per mol). These as data in Table 1
curve (see Fig. S8 and details in ESI†). The CTF alone did not demonstrate that the adsorbed surfactant molecules form a
influence the surface tension of the aqueous solution.
monolayer if the values for the area per molecule are compared
The adsorption isotherms for SDS onto CTFs (Fig. 1) show with data for the molecules from Langmuir film balance
two regions. At low SDS concentrations, the adsorbed amount of measurement.20 The assumed arrangement of the molecules is
SDS increases gradually and then the isotherm reaches a plateau anywhere between horizontal and vertical (see Fig. S9, ESI†).
and is nearly constant up to an SDS equilibrium concentration of
Also the formation of hemimicelles on the surface can be
30 mmol Lꢀ1. At higher surfactant content, adsorption again possible which was shown by measurements with atomic force
increases nearly linearly up to high equilibrium concentrations. microscopy on graphite surfaces.21 If one considers the cross
The course of the adsorbed amounts in dependence on SDS section area of the surfactant with 0.2 nm2 for the alkyl chain
equilibrium concentration in the low concentration region corre- and about 0.5 nm2 for the ethylenoxide chain,22 both surfactants
sponds to the formation of a first adsorption layer of SDS on CTF. could be included into the channels of the solid (cross section
The almost linear increase of adsorbed SDS amounts at higher area about 1.4 nm2). On the other hand the fast and thorough
SDS concentrations in the second regime correlates with a second desorption of the long chain surfactants suggest no or little
adsorption process on the first SDS layer. It is interesting to see adsorption in the channels. Instead of channel filling the layer
that for both CTF-1 samples the adsorbed SDS amounts are the structure of CTF-1 can lead to interlayer surfactant insertion
same. Therefore the surface characteristics should be similar (similar to graphite) as evidenced by a shift of the (001) peak
despite the different surface area. In desorption experiments, we from 2y E 251 (3.6 Å) to 2y E 201 (4.4 Å) in the powder X-ray
have seen almost no hysteresis, so that the adsorption process is pattern (Fig. S2, ESI†). Exfoliation can explain the strong increase
reversible (data not shown). A more detailed insight into the of the adsorbed amounts at higher concentrations as multilayer
adsorption mechanism is given by the section of the adsorption formation or adsorption of micelles which has been claimed for
isotherm for low solution concentrations (Fig. 2). It shows more other surfaces23 becomes possible on the CTF sheet surfaces.
clearly the typical shape of an adsorption isotherm in the mono-
layer region during formation of a surfactant layer on a surface.16
Fig. 2 shows the adsorption of the non-ionic surfactant
12EO7 on CTF-1(600) in comparison to anionic SDS. The
C
If compared to the SDS adsorption on a typical carbon black, one adsorption of the non-ionic one is slightly higher than that of
can conclude that the adsorption behaviour on CTF at low the anionic surfactant which could be also shown for the
surfactant concentrations is similar to that of carbon black albeit adsorption on carbon black. The reason for the lower adsorption
with higher adsorbed amounts. Typical carbon black adsorbs of the anionic compared to the non-ionic surfactant is the
about 2 mmol(SDS) mꢀ2 in the plateau region (Fig. 2).
electrostatic repulsion of the polar group of the anionic surfactant
The adsorbed amounts G (in mol mꢀ2) represent the number which leads to lower packing density of the surfactant molecules
of adsorbed molecules per area on a solid. In Table 1 the values on the surface.
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Table 1 Data for adsorbed surfactant molecules from their adsorption isotherms on CTFs
Adsorbed amount Gb
Area per molecule,
Surfactanta
Solid [SBET/m2 gꢀ1
]
(mmol mꢀ2
)
(mg mꢀ2
)
(g gꢀ1
)
as (nm2)
c
SDS
SDS
SDS
C12EO7
C12EO7
CTF-1(600) [1390]
CTF-1(400) [970]
5.0
5.0
2.0
7.0
2.5
1.4
1.4
0.6
3.5
1.3
1.9
1.4
0.1
4.9
0.2
0.33
0.33
0.83
0.24
0.66
Carbon blackd [150]
CTF-1(600) [1390]
Carbon blackd [150]
a
b
c
SDS = sodium dodecylsulfate; C12EO7 = alkyl polyglycolether. Adsorbed amount G at c = 8.0 mmol Lꢀ1 from eqn (S1) (ESI). From the adsorption
isotherms, eqn (1). To be compared with a limiting area per molecule of 1.05 nm2 in the horizontal orientation and 0.2 nm2 in the vertical
orientation from Langmuir film balance measurements.20 d Printex L from Evonik.
´
7 J. Germain, J. Hradil, J. M. J. Frechet and F. Svec, Chem. Mater., 2006,
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From the gravimetric adsorption of the surfactants (Table 1)
related to surface area and mass of the solid adsorbent it
is evident that the gravimetric uptake capacity of CTFs is up
to 20-times the amount of carbon black by weight (g gꢀ1) and
about 2.5 times the amount by surface area (mg mꢀ2) for the
ionic SDS surfactant. For the non-ionic C12EO7 surfactant the
adsorbed amounts by weight (g gꢀ1) are even increased due to
the higher molecular mass of the non-ionic surfactant compared
to the anionic surfactant and the higher uptake of the non-ionic
surfactant by surface area (mol mꢀ2).
In summary, this comparative study demonstrates the
potential for unusually high adsorbed amounts of surfactants
on CTF solids due to their high surface area. Hence, CTFs are
not only interesting for gas adsorption but also show interesting
properties for the application as adsorbents from solutions,
taking into account their solvothermal chemical stability. SEM
images of before and after SDS adsorption show no evident
change in microscopic CTF morphology (Fig. S5, ESI†).
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486 | Chem. Commun., 2015, 51, 484--486
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