Temperature-dependent IR spectroscopic and structural study of 18-crown-6 chelating ligand in the complexation with sodium surfactant salts and potassium picrate

Article abstract of DOI:10.1016/j.saa.2013.12.092

18-crown-6 ether (18C6) complexes with the following anionic surfactants: sodium n-dodecylsulfate (18C6-NaDS), sodium 4-(1-pentylheptyl)benzenesulfonate (18C6-NaDBS); and potassium picrate (18C6-KP) were synthesized and studied in terms of their thermal and structural properties. Physico-chemical properties of new solid 1:1 coordination complexes were characterized by infrared (IR) spectroscopy, thermogravimetry and differential thermal analysis, differential scanning calorimetry, X-ray diffraction and microscopic observations. The strength of coordination between Na⁺ and oxygen atoms of 18C6 ligand does not depend on anionic part of the surfactant, as established by thermodynamical parameters obtained by temperature-dependent IR spectroscopy. Each of these complexes exhibit different kinds of endothermic transitions in heating scan. Diffraction maxima obtained by SAXS and WAXS, refer the behavior of the compounds 18C6-NaDS and 18C6-NaDBS as smectic liquid crystalline. Distortion of 18C6-NaDS and 18C6-KP complexes occurs in two steps. Temperature of the decomplexation of solid crystal complex 18C6-KP is considerably higher than of mesophase complexes, 18C6-NaDS, and 18C6-NaDBS. The structural and liquid crystalline properties of novel 18-crown-ether complexes are function of anionic molecule geometry, type of chosen cation (Na⁺, K ⁺), as well as architecture of self-organized aggregates. A good combination of crown ether unit and amphiphile may provide a possibility for preparing new functionalized materials, opening the research field of ion complexation and of host-guest type behavior.

Full text of DOI:10.1016/j.saa.2013.12.092

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Temperature-dependent IR spectroscopic and structural study
of 18-crown-6 chelating ligand in the complexation with sodium
surfactant salts and potassium picrate
Tea Mihelj a, , Vlasta Tomaši c´ , Nikola Biliškov , Feng Liu
⇑
a
b
c
a
b
c
-
Department of Physical Chemistry, Ruder Boškovi ´c Institute, POB 180, HR-10002 Zagreb, Croatia
-
Division of Organic Chemistry and Biochemistry, Ruder Boškovi ´c Institute, POB 180, HR-10002 Zagreb, Croatia
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis of novel 18C6 ether
coordination complexes with
different guests.
Baseline analysis and temperature-
dependent IR spectra obtained phase
transitions.
Temperature-dependent IR
spectroscopy gave thermodynamic
decomplexation parameters.
18C6-sodium 4-(1-
pentylheptyl)benzenesulfonate is a
compound with low melting point.
18C6-potassium picrate has a high
thermal stability with the two-step
distortion.
a r t i c l e i n f o
a b s t r a c t
Article history:
18-crown-6 ether (18C6) complexes with the following anionic surfactants: sodium n-dodecylsulfate
Received 2 September 2013
Received in revised form 9 December 2013
Accepted 15 December 2013
Available online 2 January 2014
(
(
18C6-NaDS), sodium 4-(1-pentylheptyl)benzenesulfonate (18C6-NaDBS); and potassium picrate
18C6-KP) were synthesized and studied in terms of their thermal and structural properties. Physico-
chemical properties of new solid 1:1 coordination complexes were characterized by infrared (IR) spectros-
copy, thermogravimetry and differential thermal analysis, differential scanning calorimetry, X-ray
diffraction and microscopic observations. The strength of coordination between Na and oxygen atoms
of 18C6 ligand does not depend on anionic part of the surfactant, as established by thermodynamical
parameters obtained by temperature-dependent IR spectroscopy. Each of these complexes exhibit
different kinds of endothermic transitions in heating scan. Diffraction maxima obtained by SAXS and
WAXS, refer the behavior of the compounds 18C6-NaDS and 18C6-NaDBS as smectic liquid crystalline.
Distortion of 18C6-NaDS and 18C6-KP complexes occurs in two steps. Temperature of the decomplexation
of solid crystal complex 18C6-KP is considerably higher than of mesophase complexes, 18C6-NaDS, and
+
Keywords:
1
8-crown-6 ether
IR spectroscopy
X-ray diffraction
Anionic surfactants
Thermal properties
Liquid crystals
1
8C6-NaDBS. The structural and liquid crystalline properties of novel 18-crown-ether complexes are func-
+
+
tion of anionic molecule geometry, type of chosen cation (Na , K ), as well as architecture of self-organized
aggregates. A good combination of crown ether unit and amphiphile may provide a possibility for prepar-
ing new functionalized materials, opening the research field of ion complexation and of host–guest type
behavior.
Ó 2013 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Address: Ru d- er Boškovi c´ Institute, Department of Physical Chemistry, Laboratory for synthesis and processes of self-assembling of organic
molecules, Bijeni cˇ ka c. 54, P.O. Box 180, HR-10002 Zagreb, Croatia. Tel.: +385 14571211; fax: +385 14680245.
E-mail address: tmihelj@irb.hr (T. Mihelj).
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.12.092

T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
13
Introduction
4 w
further purification. Sodium n-dodecylsulfate (C12H25SO Na, M /
g mol = 288.38) was obtained from Merck and recrystallized sev-
À1
Several molecular families have been widely examined for the
development of supramolecular chemistry. The group of macrocyclic
polyethers, known as crown ethers, have become valuable tools in or-
ganic synthesis due to their ability to solvate alkali, alkaline-earth,
transition-metal, and ammonium cations [1–4]. Their selective cation
binding makes them applicable for different environmental usage
eral times from ethanol. Sodium 4-(1-pentylheptyl)benzenesulfo-
À1
25 6 4 3 w
nate (C12H C H SO Na, M /g mol = 348.48) was analyzed and
determined previously [26]. Potassium picrate, i.e. potassium 2,4,6-
À1
trinitrofenolate (C
6
H
2
N
3
O
7
K, M
w
/g mol = 267.20) was prepared
and purified according the procedure described earlier [27,28].
18C6 ether complexes with different anionic constituent were
prepared by high temperature mixing of equimolar aqueous solu-
tions of both, 18C6 ether and sodium surfactant salt/potassium
[5,6], drug delivery [7,8], recovery or removal of specific species, mod-
els for biological receptors [9], reaction catalysts as well as active sites
in ion selective electrodes [10] or chromatographic agents [11]. Alkali
metal elements have indispensable role in many biological processes,
primarily to be as bulk electrolytes that stabilize surface charges on
proteins and nucleic acids [12], and also play unique structural roles
in biological systems [13,14]. Their complexes with crown ligands
are coordination compounds based on electrostatic interaction
through ion–dipole attractions [15], with the usage for simulations
of natural substances, their properties and behavior. Some new surfac-
tants derived from crown ethers are used as templates with a particu-
lar morphology in the preparation of siliceous mesoporous molecular
sieves [16].
+
À
picrate. The complex formation equilibrium is defined as M . X + -
L M ML . X , where M , X and L refer to metal ion (Na+ or K+),
counter anion (n-dodecylsulfate, 4-(1-pentylheptyl)benzenesulfo-
nate or picrate), and crown ether as neutral, endopolarophilic li-
gand. Samples were left aging for few days at room temperature,
during which water spontaneously evaporated. 18-crown-6 ether
complex with potassium picrate, formed yellow crystals that were
filtered and vacuum dried till constant mass was obtained, while
other two samples were waxy and after vacuum dried, glassy, col-
orless and transparent. The samples were stored protected from
moisture and light before use.
+
À
+
À
One of the most relevant crown ethers, 18-crown-6 (18C6) fea-
tures a flexible six-oxygen cyclic backbone and uncomplexed does
not exhibit any liquid crystalline behavior. However, these phe-
nomena are caused by one or more mesogenic groups attached to
the molecules containing crown ether, aza, thia crown ethers, or
crown ethers with several different heteroatoms [17–22]. Synthesis
and properties of cholesteryl moiety bearing 16-membered crown
ethers show cholesteric [23] and nematic [24] liquid crystalline
behavior. Thermochemical properties of 18C6 ether complexes
with aralkylammonium perchlorates show higher melting points
than of both the host and the guest compound, the decomposition
begins immediately after melting is completed, and each of the
examined complexes is characterized by its individual properties
Measurements
The complexes are shown in Scheme 1. Elemental analysis
(
Perkin–Elmer Analyzer PE 2400 Series 2) confirmed that the
complexes were 1:1 charge ratio adducts. 18C6-sodium n-
dodecylsulfate (compound 1, C24
found: C, 52.18; H, 9.00% (calc. C, 52.16; H, 8.94%). 18C6- sodium
À1
H49SO10Na, M
w
/g mol = 552.70)
4
M
-(1-pentylheptyl)benzenesulfonate (compound 2, C30
H
53SO
9
Na,
À1
w
/g mol = 612.80) found: C, 58.78%; H, 8.70 (calc. C, 58.80; H,
8
M
.72%). 18C6-potassium picrate (compound 3,
C
18
H
26
N
3
O13K,
À1
w
/g mol = 531.52) found: C, 40.60; H, 4.90; N, 7.82% (calc. C,
4
0.68; H, 4.93; N, 7.91%).
[
25]. The study of stable complexes formed between crown ether
TG and differential thermal analysis, DTA, were obtained on a Shi-
compounds and surfactants is less explored area, especially in
terms of thermochemical and structural studies. So far, most stud-
ies on metal ion–crown ether complexes were focused on the deter-
mination of relative affinities and stoichiometries of the complexes
in solution, rather than on their solid structures. Thus, in the pres-
ent study we report the formation of defined complexes between
matzu DTG-60H. Samples were heated from room temperature to
À1
5
5
73 K at the heating rate of 5 K min
0 mL min . Differential scanning calorimetry, DSC, was carried
in synthetic airflow of
À1
out with a Perkin Elmer Pyris Diamond DSC calorimeter in N
2
atmosphere equipped with a model Perkin Elmer 2P intra-cooler
1
8C6 and different guest constituent. Two amphiphiles are chosen;
one conventional known as sodium n-dodecylsulfate and one com-
mercial known as sodium 4-(1-pentylheptyl)benzenesulfonate. The
third chosen guest is potassium picrate that possesses hydrophilic-
hydrophobic balanced properties, but is not a real amphiphile. The
purpose of the present study is to provide an insight into thermal
and structural behavior of 18C6 chelating ligand in the complexa-
tion with sodium surfactant salts and potassium picrate. Tempera-
ture-dependent IR spectroscopy was used in order to detect and
characterize phase transitions at molecular level, as well as to
determine thermodynamic parameters of the decomplexation pro-
cess. The present study provides the relationship between molecu-
lar structure and physico-chemical properties by combining the
properties of complex formation and supramolecular arrangements
provided by liquid crystals. This ensures a guideline for further de-
sign and fine-tuning of the properties of new materials with a spe-
cific structure, allowed by an appropriate choice of cation and
crown ether size, and by varying the nature of anionic constituent.
Experimental
Materials and sample preparation
Scheme 1. The scheme of the examined complexes: 18C6-sodium n-dodecylsulfate
1
8-Crown-6 ether, i.e. 1,4,7,10,13,16-hexaoxacyclooctadecane
(
compound 1), 18C6-sodium 4-(1-pentylheptyl)benzenesulfonate (2), and 18C6-
À1
(
C
12
H
24 6 w
O , M /g mol = 264.32; Sigma–Aldrich) was used without
potassium picrate (3) complex.

1
4
T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
À1
in N
D
2
atmosphere, at the rate of 2 K min . The transition enthalpy,
À1
H/kJ mol , was determined from the peak area of the DSC ther-
mogram; and the corresponding entropy changes,
À1 À1
D
S/J mol
K ,
was calculated using the maximal transition temperature. Textures
were examined with Leica DMLS polarized optical light micro-
scope, equipped with a Mettler FP 82 hot stage, Sony digital color
video camera (SSC-DC58AP). Infrared transmission spectra of the so-
À1
lid samples were recorded at 4 cm resolution in KBr pellets on an
ABB Bomem MB102 single-beam FT-IR spectrometer with CsI op-
tics, DTGS detector. The KBr sample pellets were prepared by mix-
ing ꢁ2 mg of the individual sample with 100 mg of KBr with a
pestle and mortar made of agate. Specac 3000 Series high-stability
temperature controller with water cooled heating jacket was used
to measure the spectra within the temperature range from room
temperature up to 523 K under atmospheric conditions and at
À1
heating rate of 2 K min and 2 K steps. Each single-beam spec-
trum collected in a temperature run for individual sample was rat-
ioed to the single-beam spectrum of the sample-free setup (the
reference spectrum) recorded immediately before starting the
temperature-dependent measurements. Wide angle X-ray scatter-
ing (WAXS) results were obtained by automatic powder diffrac-
tometer, Philips PW 3710, with monochromatized Cu
Ka
radiation (k/Å = 1.54056) and proportional counter. The interlayer
spacing (dhkl) was calculated according to the Bragg’s law. In addi-
tion, small angle X-ray scattering (SAXS) measurements were made
using a MAR Research image plate camera with a rotating anode
generated beam doubly focused by multilayer mirrors. Samples
were held in Lindeman capillaries temperature controlled with
an Oxford Cryostream cryostat. Diffraction intensities were ob-
tained by azimuthal integration using the Fibrefix program
(
CCP13 suite) and were Lorentz and multiplicity corrected.
Results and discussion
Novel inclusion complexes, based on coordination and particu-
lar affinity for sodium or potassium cations inside six-oxygen cav-
ity, are presented in Scheme 1.
Thermogravimetric analysis (Fig. 1 and Table S1-Supplementary
data) obtained the most prominent decrease in mass for both of the
sodium-containing systems between 423 and 523 K, and for com-
pound 3 between 523 and 573 K. In the case of system 2, this is
attributable to decomplexation of the 18C6 crown from sodium.
In the case of complex 1 and 3, however, decrease in mass of
Fig. 1. Thermograms (straight line) and DTA results (dashed line) for compound
(a), 2(b) and 3(c).
1
of the sample, this decrease could be explained with dealkylation
of the DS anion, which leads to the formation of sodium sulfate,
confirmed by infrared spectrum of the sample after heating to
4
53 K. Above the temperature of 523 K, further decrease in mass
7
8% and 85%, respectively, is not explainable by decomplexation
is attributed to pyrolysis of the DS and DBS anion, respectively.
For compound 3, a negative peak occurs at 485 K, which suggests
decomplexation of the crown, followed with a very intensive
only, but also with simultaneous decomposition of the anion.
DTA curve of complex 1, shows two minima in the 473–523 K
region, one at 478 K and another at 495 K. Considering the formula
Table 1
À1
À1 À1
Transition temperatures, T/K, enthalpies,
D
H/kJ mol , and entropies,
D
S/J mol
K
of examined 1–3 compounds given by the DSC measurements, with thermodynamic
functions for decomplexation of 18C6 from synthesized compounds, given by the temperature-dependent IR measurements (bold).
Compound
Heating
T (K)
Cooling
À1
À1 À1
D
H (kJ mol
)
D
S (J mol
K )
1
382.12
2.55
9.14
93.30
6.67
22.96
236.10
Decomposition
3
98.30
4
01.00
2
3
241.19
1.94
0.55
0.39
7.85
2.12
1.46
No transitions detected
Partial decomposition
2
2
4
57.60
69.05
04.00
81.80
206.30
328.06
22.76
4.34
23.28
47.94
164.10
69.38
12.56
59.87
101.45
358.40
3
3
4
4
45.66
88.85
72.57
61.00

T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
15
Fig. 2. The micrographs of the characteristic textures of the examined samples 1–3 taken at different temperatures in heating (h) and cooling (c) cycle, as observed by the
optical microscope under crossed polarizers. The bar represents 250
indicated.
lm (1a, 1b, 1d, 2a, 3b and 3c), 100 lm (1c, 2b), and 50 lm (1e, 2c and 3a). The temperatures, T / K, are as
positive peak at 566 K, which might be explained with decomposi-
tion of picrate anion (not observed by infrared spectroscopy, IR).
potassium cations, the difference can also be noticed among anio-
nic constituents. The micrographs (Fig. 2) refer the behavior of
compounds 1 and 2 as Sm liquid crystalline; but 3 as crystalline.
Fig. 3 and Table 2 contain the diffractograms of the samples 1
and 2 recorded at different temperatures. Thermal changes on
the molecular level as well as thermodynamic parameters of
Table 1 shows transition temperatures, T/K, enthalpies,
D
H/
À1
À1 À1
kJ mol , and entropies,
D
S/J mol
K , during heating and cooling
cycles of examined compounds 1–3. Although two different types
of complexes are considered, first with sodium and second with

1
6
T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
Fig. 3. WAXS diffractograms recorded at room temperature (left) and SAXS diffractograms at lower temperatures (right) of the samples 1 (upper part) and 2 (bottom).
system is relative fragility of the S-O-C linkage in n-dodecylsulfate
anion. As previously considered by TGA, dodecyl moiety is ab-
stracted from the system simultaneously with crown ether. From
variable-temperature IR spectra (Fig. 4a) it is confirmed that
Table 2
Interplanar spacing, d, Miller indices, hkl, and relative intensities Irel, for compound 1
at room temperature.
4
53 K spectrum corresponds to spectrum of sodium sulfate [29].
d/Å
hkl
I
rel
001a
002
003
004
005
006
008
009
0,0,14
0,0,15
When heated from room temperature, sample 1 (Fig. 2, Fig. 1a
and b, Table 1) exhibits solid–liquid crystalline transformation at
382 K, characterized by a double refracting lancets and pseudoiso-
tropic regions, indicating most probably SmB mesophase that is
stable until melting accompanied with beginning of decomposi-
tion, confirmed by temperature dependence of the baseline
3
1
1
9
7
6
4
4
2
2
8.56
9.28
2.89
.68
.75
.46
.84
.35
.76
.58
>100
92.7
100
1.7
3.5
4.6
1.4
0.4
0.1
0.1
À1
absorption at 2500 cm (Fig. 4b). A sharp transition in the interval
393–413 K with temperature of 401 K characterizes diffusion of
the crown ether from the sample. The second transition is a milder
step at 425 K and is solved with the help of temperature behavior
a
Calculated.
of the spectral features due to the crown and dodecyl group
À1
decomplexation were explained with temperature-dependent IR
spectroscopy. Fig. S1 (Supplementary data) and Table 3 contain
the most important functional groups assigned to IR spectral fea-
tures at room temperature. Decomplexation enthalpy and entropy
changes were obtained from the absorbance measurement of com-
plexed and decomplexed sodium salts. The detailed calculations
are explained in Supplementary data S2 and shown in Fig. S2, while
parameters obtained by linear fitting of the data are given in
Table 1.
(Fig. 4c). The band at 1109 cm
is due to the m(C-O), which
+
directly binds Na [30], strongly overlapped with sulfate anion
stretching, and with the feature at 1195 cm that arises above
À1
À1
À1
403 K. 960 cm and 836 cm absorption that correspond to the
deformation vibrations of CH groups [30] also show dependence
characteristic for both processes. However, a relatively linear shape
of the first step, together with linearity of the absorbance over the
whole temperature range, indicates that crown decomplexation
and diffusion from the system occurs over the completely consid-
ered temperature range, while dodecyl group is detached in the
408–438 K range. More information given by the IR spectroscopy
is in Supplementary data S3 and Fig. S3.
System 1 is crystal at room temperature (Fig. 2, Fig. 1a) with a
bilayer-like arrangement and lamellar thickness of 38.56 Å
(Fig. 3, upper part, right, Table 2). The crystal smectic (Sm) layer
is composed of repetitive units of two crown ether layers with ex-
tended dodecyl chains. When untreated sample is chilled from
room temperature to 265 K, slightly striated, smooth fan-like tex-
tures are formed (Fig. 2, Fig. 1c), characteristic for SmA, and con-
firmed by the SAXS (Fig. 3, upper part, left). This phase is
composed of the layers with repetitive units of two crown layers
and one layer of interdigitated dodecyl chains. Specificity of this
The soft anhydrous sample 2 shows properties similar to those
of sample 1, but it is also very specific. Microscopic examinations
during heating detected only changes of textures at 318 K, seen
as Maltese crosses (Fig. 2 2a). Contrary to this, after the sample
was chilled for 24 h, the smooth fan-like textures characteristic
for SmA phase were obtained (Fig. 2 2b). Shorter time of cooling
or gentle temperature elevation results with changed and

T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
17
Table 3
Assignation of the infrared spectra of the complexes.
Compound 1
Compound 2
Compound 3
Assignation
3
3
463
402
3453
3450
Water
Crown
3
3
2952
100
081
Picrate
Picrate
Crown
m
m
(CH)
(CH)
2
955
917
2956
2
925
m(CH)
2
2909
Crown
Crown
Crown
Crown
2888
2
2
853
824
2857
2862
2831
1977
Picrate d(CH) overtone
(COO—H O)
m
2
1
1
1
1
1
635
613
566
557
512
1
1
1
472
456
432
1469
1457
1479
1457
1437
Crown
Crown
Crown
1365
1351
1351
1353
Crown
1
1
1
334
311
289
Crown
Crown
Crown
Alkyl
1280
1249
1227
1222
1284
1248
1272
1256
1218
C–S
1
193
NaHSO
Picrate m(C–O)
4
1
1
164
136
1
1
107
082
1108
1112
Crown; DS and DBS SO
Alkyl
4
group; picrate m(CO)
1074
1
1011
038
Benzene ring
C–S
1019
9
9
9
97
65
21
965
964
Crown
9
8
10
72
Picrate
Crown
837
764
721
635
590
837
837
787
741
708
600
537
Picrate
Picrate d(C–NO
Crown
Crown
2
)
584
5
4
28
68
530
470
526
515
Crown
Crown
disturbed pseudoisotropic spherulitic phase (Fig. 2 2c). Although
WAXS measurement evidences no order (Fig. 3, bottom, left), SAXS
measurement of the sample kept in freezer for several hours indi-
cates the most probably short-range, layer-like order (Fig. 3, bot-
tom, right), with the diffuse peak corresponding the lamellar
thickness of 29.7 Å. The similar values of lamellar thicknesses of
sample 1 and 2 indicate the minor role of the benzene ring, but
at the same time confirm disordering abilities of 4-(1-pentylhep-
tyl)benzenesulfonate anion. This is in accordance with sulfonate
hydrotropic properties; they are defined as amphiphilic molecules
that cannot form well organized structures, cause microstructural
changes, decrease membrane stability [31], disrupt lamellar
liquid–crystalline phase, alter macroscopic properties [32,33]. Based
on crystal structure analysis, these compounds form open-layer
assemblies, consisting of clustered non-polar regions, adjacent to
ionic polar regions, knitted together in a two-dimensional network,
where stacking of aromatic ring is not detected [1].
Thermodynamic parameters of compound 2 (Table 1) point out
relatively low transition temperatures during heating cycle. The
first is for solid–liquid crystal transition, followed by a weak endo-
thermic process, for which parameters may imply as Sm polymor-
phic transition, capacitance of possible moisture content or volume
changes. The third one, at 265 K is the liquid crystal-isotropic
liquid transition. Dependence of the baseline absorption at
À1
2500 cm shown in Fig. 5a presents the next transition tempera-
ture (404 K), determined by differentiation of the absorbance, and
fitting to fourth order polynomial. Again, the decomplexation of
the crown occurs with its consequent diffusion from the sample,
and the absorption due to the sulfate moiety remains constant over
the considered temperature range. Variable-temperature IR

1
8
T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
Fig. 4. Baseline corrected variable-temperature infrared spectra for compound 1 in the 373–453 K interval (a). Temperature dependence of the baseline absorption at
À1
2
8
500 cm . Inset shows the first derivative of the curve for the purposes of determination of the transition temperatures (b). Temperature dependence of the characteristic
À1
36 cm band (c).
spectra for system 2, together with detailed description of the ade-
quate bands for sulfate moiety and crown feature are in the Sup-
plementary data S4 and Fig. S4.
Conclusions
Complexes based on 18C6 ether were synthesized and studied
in terms of their thermal and structural properties. The present
data point out that starting from aqueous 18C6-anionic surfactant
solutions along with electrostatic interactions; hydrophobic inter-
actions assist in a favorable way the molecular recognition proper-
ties. A strong entrapment of the organic surfactant guests of
miscellaneous geometries into macrocyclic cation receptor, results
in solid compounds that self-assemble in aggregates with wide
spectrum of their physico-chemical characteristics. The properties
of novel 18C6 complexes are function of anionic molecule geome-
The crystal structure of compound 3 with triclinic space group
P1 was solved earlier by Barnes and Collard [34]. However, system-
ꢀ
atic thermochemical study of this complex has never been per-
formed. As seen from Table 2, it undergoes crystal-crystal
polymorphic phase transitions that are not visible with micro-
scope, until 473 K, when melting occurs (Fig. 2, Fig. 3b) and paral-
lel, partial decomposition is involved. TG suggests that crown
decomplexation occurs simultaneously with decomposition of pic-
rate. However, temperature-dependent infrared baseline absorp-
À1
+
+
tion at 2500 cm (Fig. 5b) clearly resolves two processes, with
try, type of chosen cation (Na , K ), as well as architecture of self-
organized structures. The strength of coordination between Na+
and oxygen atoms of 18C6 ligand does not depend on anionic part
of the surfactant.
transition temperatures of 445 K and 490 K, respectively. In order
to extract information on decomplexation, the absorption in
À1
3
000–2650 cm , which arises due to the
m
(CH), was considered.
In this region, a rather complex absorption occurs. Therefore, we
have fitted only the absorption at 2830 cm to Lorentzian profile
function, as shown in the inset of Fig. 5b. It is evident that decom-
plexation occurs in two steps. Since the boiling point of crown
Synthesized coordination complexes are specific due to their
different endothermic transitions. Enthalpy and entropy values of
thermal transitions point to mesomorphous behavior of 18C6-
NaDS and 18C6-NaDBS, confirmed with PXRD. Maltese crosses
and smooth fan-like textures, characteristic for smectic A phase,
and double refracting lancets i.e. smectic B mesophase, were ob-
tained for 18C6-NaDS, which is also crystal smectic at room tem-
perature. 18C6-NaDBS is characterized as compound of low
melting point, forming smectic A phase during heating cycle. These
results are valuable proofs that anionic surfactant molecule acts as
promoter of thermotropic mesomorphism. The similar values of
lamellar thicknesses of the 18C6-NaDS and 18C6-NaDBS indicate
the minor role of the benzene ring size, but at the same time con-
firm disordering abilities of 4-(1-pentylheptyl)benzenesulfonate
anion. Temperature of the decomplexation of 18C6-KP is consider-
ably higher than of 18C6-NaDS and 18C6-NaDBS, and distortion of
18C6-NaDS and 18C6-KP complexes occurs in two steps. A good
À1
1
8C6 is 389 K, both steps include decomplexation and diffusion
of free crown. Thus, we rather attribute the difference in behavior
to two phases, with a phase transition at 461 K. However, this
phase transition is not observed in temperature dependence of
baseline absorption. At least for this moment, this difference re-
mains unresolved. Melting point of potassium picrate is 520 K.
From this fact, it is evident that in the temperature range 483–
5
23 K, which is characterized by abrupt increase in baseline
À1
absorption, constant absorption at 2830 cm , and only a minor
decrease in mass for TG, large structural changes due to the phase
transformation occur before melting. Variable-temperature IR
spectra for 3 is shown in Fig. S5 in Supplementary data S5 with
clear explanation of thermal behavior.

T. Mihelj et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 12–20
19
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