Interaction between Langmuir and Langmuir-Blodgett films of two calix[4]arenes with aqueous copper and lithium ions

Article abstract of DOI:10.1021/la100808r

The binding interactions between aqueous copper (Cu²⁺) and lithium (Li⁺) ions and Langmuir monolayers and Langmuir-Blodgett (LB) multilayers have been investigated by studying surface pressure-area (π-A) isotherms and surface potential-area (ΔV-A) behavior in order to find the effective dipole moment, μ_⊥, of the calixarene molecules in the uncomplexed and complexed states. The orientation of both calix[4]arenes, namely, 5,11,17,23-tetra-tert-butyl-25,27-diethoxycarbonyl methyleneoxy-26,28- dihydroxycalix[4]arene and 5,17-(9H-fluoren-2-yl)methyleneamino)-11,23-di-tert- butyl-25,27-diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene, is such that the plane of the calix ring is parallel with the plane of the water surface regardless of the ion content of the subphase. The Gibbs equation was used to interpret the adsorption of ions with both calix[4]arenes as a function of the concentration. Effective dipole moments have been calculated from surface potential values using the Helmholtz equation. In this work, new LB films have been prepared employing two novel amphiphilic calix[4]arene derivatives bearing different upper rim substituents. Thus, the effect of modifiying the upper rim has been observed. The results have shown that these calixarenes may be useful components of ion sensors.

Full text of DOI:10.1021/la100808r

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© 2010 American Chemical Society
Interaction between Langmuir and Langmuir-Blodgett Films of Two
Calix[4]arenes with Aqueous Copper and Lithium Ions
Faridah L. Supian,† Tim H. Richardson,*,† Mary Deasy,‡ Fintan Kelleher,‡
James P. Ward,‡ and Vickie McKee§
†Department of Physics & Astronomy, University of Sheffield, Hounsfield Road, Sheffield S37RH,
United Kingdom, ‡Department of Science, Institute of Technology Tallaght (ITT Dublin), Dublin 24,
Ireland, and §Department of Chemistry, Loughborough University, Loughborough, Leicestershire LE113TU,
United Kingdom
Received February 25, 2010. Revised Manuscript Received April 6, 2010
The binding interactions between aqueous copper (Cu2þ) and lithium (Liþ) ions and Langmuir monolayers and
Langmuir-Blodgett (LB) multilayers have been investigated by studying surface pressure-area (Π-A) isotherms and
surface potential-area (ΔV-A) behavior in order to find the effective dipole moment, μ^, of the calixarene molecules in
the uncomplexed and complexed states. The orientation of both calix[4]arenes, namely, 5,11,17,23-tetra-tert-butyl-
25,27-diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene and 5,17-(9H-fluoren-2-yl)methyleneamino)-
11,23-di-tert-butyl-25,27-diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene, is such that the plane of the
calix ring is parallel with the plane of the water surface regardless of the ion content of the subphase. The Gibbs equation
was used to interpret the adsorption of ions with both calix[4]arenes as a function of the concentration. Effective dipole
moments have been calculated from surface potential values using the Helmholtz equation. In this work, new LB films
have been prepared employing two novel amphiphilic calix[4]arene derivatives bearing different upper rim substituents.
Thus, the effect of modifiying the upper rim has been observed. The results have shown that these calixarenes may be
useful components of ion sensors.
1. Introduction
a calix[4]resorcinarene in the form of an LB monolayer using
surface potential, ΔV, and found that the binding depends on the
orientation of the calixarene in the monolayer.8 Research carried
out by Nabok et al. revealed the behavior of calix[4]resorcinol-
arene using surface pressure-area (Π-A) isotherms and surface
potential-area (ΔV-A) data in order to investigate the adsorption
mechanism of four different upper rims of calix[4]resorcinolarene.9
Derivatization of calixarenes is possible at either the upper or
lower rim as in Figure 1, and usually has the added effect of
locking the calixarene into one conformation. Some of the earliest
work in this area involved converting the phenolic oxygens of the
parent calixarenes to their acetates.10 McKervey et al. produced a
group of calixaryl esters and ketones and investigated their
binding properties. The group devised simple, yet highly effective,
synthetic routes for this group of compounds, on which many
subsequent derivatives were based.11
Calix[4]- and calix[8]arenes, appropriately substituted on their
upper and lower rims, are known to form highly uniform
Langmuir monolayer films on a water subphase and also well-
ordered transferred Langmuir-Blodgett (LB) multilayers on
various solid substrates. Interest in such films has developed in
the areas of gas sensing, ion binding,1 and pyroelectric heat
detection.2 Calixarenes are unusual among small organic mole-
cules in that they exhibit relatively high thermal stability with
melting points typically around 250-280 ꢀC.3 Furthermore, this
is particularly exceptional among LB film forming materials
which commonly consist of more traditional amphiphiles such
as long-chain carboxylic acids or phospholipids possessing melt-
ing points closer to the range 60-80 ꢀC.4 For these combined
reasons, calixarenes represent a family of materials attracting
increasing interest in the nanoscience community.
The derivatization of calixarenes has been reported to influence
strongly on the complexation affinity for metal ion guests.
Selectivity for alkali metal cations is known to be altered by
changing the p-substituents.12 In recent years, there has been an
increase in the level of interest in the generation of ultrathin
organic films, encapsulating ionophoric macromolecules, using
techniques such as LB deposition. In 2009, Wang et al. reported
Previous research in this area has included the interaction
between Langmuir films of calix[n]arenes and various cations,
anions, and amines.1,5,6 Turshatov et al. studied the behavior of a
calix[4]resorcinarene with copper ions, although this work did not
include LB films or compare differently substituted (upper rim)
molecules.7 Weis et al. reported the interaction of dopamine with
*To whom correspondence should be addressed. Telephone: þ44114 22
23508. Fax: þ44114 22 24280. E-mail: t.richardson@sheffield.ac.uk.
(1) Hassan, A. K.; Nabok, A. V.; Ray, A. K.; Davis, F.; Stirling, C. J. M. Thin
Solid Films 1998, 686, 327.
(2) McCartney, C. M.; Richardson, T.; Greenwood, M. B.; Cowlam, N.
Supramol. Sci. 1997, 4, 385.
(8) Weis, M.; Janicek, R.; Cirak, J.; Hianik, T. J. Phys. Chem. B 2007, 111,
10626.
(9) Nabok, A. V.; Lavrik, N. V.; Kazantseva, Z. I.; Nesterenko, B. A.;
Markovskiy, L. N.; Kalchenko, V. I.; Shivaniuk, A. N. Thin Solid Films 1995,
259, 244.
(3) Boehmer, V.; Marschollek, F.; Zetta, L. Org. Chem. 1987, 52, 3200.
(4) Roberts, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990.
(5) Pathak, R. K.; Ibrahim, S. M.; Chebrolu, P. Tetrahedron Lett. 2009, 50, 2730.
(6) Joseph, R.; Ramanujam, B.; Acharya, A.; Rao, C. P. Tetrahedron Lett. 2009,
50, 2735.
(7) Turshatov, A. A.; Melnikova, N. B.; Semchikov, Y. D.; Ryzhkina, I. S.;
Pashirova, T. N.; Mobius, D.; Zaitsev, S. Y. Colloids Surf., A 2004, 240, 101.
(10) Bocchi, V.; Foina, D.; Pochini, A.; Ungaro, R.; Andreetti, G. D. Tetra-
hedron Lett. 1982, 38, 371.
(11) Neu, F. A.; Collins, E. M.; Deasy, M.; Ferguson, G.; Harris, S. J.; Kaitner,
B.; Lough, A. J.; McKervey, M. A.; Marques, E.; Ruhl, B. L.; Schwing, M. J.;
Seward, E. M. J. Am. Chem. Soc. 1989, 111, 8681.
(12) Calixarenes 2001; Asfari, Z., Bohmer, V., Harrowfield, J., Vicens, J., Eds.;
Kluwer Academic: Dordrecht, 2001; p 683.
10906 DOI: 10.1021/la100808r
Published on Web 04/23/2010
Langmuir 2010, 26(13), 10906–10912

Supian et al.
Article
Figure 1. Calix[4]arene with three distinct regions.
that a p-tert-butylthiacalix[4]arene LB film modified electrode
was found to greatly improve the measuring sensitivity of the Agþ
ion over traditional glassy electrode systems.13
In this investigation, the two calix[4]arenes are different only in
that one material contains a mixed upper rim system, consistingof
two alkyl groups and two highly conjugated groups, whereas the
other contains only alkyl groups in the upper rim positions. A
preliminary comparison of the shift in the UV-visible absorption
spectrum of these calixarene as a result of their complexation with
various metal salts indicated that their interaction with Cu2þ and
Liþ would be of great interest in the context of identifying ion-
sensing materials. Therefore, the two ions were chosen in this
study to observe their interaction with these calixarenes at
different ion concentrations in terms of ΔV and effective dipole
moment, μ^, properties of Langmuir monolayer and deposited
LB films to verify that these calixarenes are suitable candidates for
ion sensing.
Figure 2. Calixarenes A and B: 5,11,17,23-tetra-tert-butyl-25,27-
diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene and
5,17-(9H-fluoren-2-yl)methyleneamino)-11,23-di-tert-butyl-25,27-
diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene.
synthesized another novel calix Schiff base compound for incor-
poration into an LB film.
The upper rim derived Calix-Schiff B was synthesized by
the synthetic route shown in Figure 3. Nitro-dealkyation
was conducted in accordance with the literature procedure
of Verboom et al.28 Reduction of the nitro calix[4]arene to the
calix[4]arene diamine was achieved by hydrogenation, at 2 atm of
pressure, in ethanol in the presence of Raney-Nickel in almost
quantitative yield (99%), as published by our group previously.
The calixarene diamine was heated to reflux temperature in
ethanol with a 5 molar excess of fluorenecarboxaldehyde. A pink
red colored solid was recrystallized from chloroform/methanol in
a 63% yield.
2. Materials and Methods
2.1. Materials, Synthesis, and Compound Data. InFigure2,
calixarenes A and B are shown, and their syntheses are discussed
in the next section.
The synthesis of distal derived calixarenes with ionophoric
ligating groups, such as compound A, is well-known. Hence, A
was prepared in three steps as follows. p-tert-Butylcalix[8]arene
(cyclic octamer) was first prepared by the modified Munch
procedure14,15 from p-tert-butylphenol and paraformaldehyde
in the presence of KOH and was isolated as a white powder, in
70% yield. This was then subjected to molecular mitosis16,17 to
form p-tert-butylcalix[4]arene (cyclic tetramer). Sodium hydro-
xide was the base of choice due to the template effect between the
cavity size of the tetramer and the size of the Naþ cation, which
induced formation of the tetramer in a 71% yield. The reaction of
p-tert-butylcalix[4]arene with ethyl bromoacetate under conven-
tional conditions for alkylation (K2CO3 as base in acetonitrile as
solvent) resulted in the expected distal-dialkyl derivative A with
terminal ester groups.11 Pathak et al. have very recently reported
the synthesis of a series of lower rim 1,3-diderivatived calix-
[4]arene compounds possessing Schiff base cores with binding
The 1H NMR spectrumofB showeda sharp singlet at8.43ppm
for the newly formed imine proton. The remaining protons on the
fluorene component resonated between 8.10 and 7.30 ppm as
multiplets, resulting from coupling between the protons on each
aromatic ring, while the phenolic OH protons appeared as a
singlet at 7.65 ppm. The calixarene aromatic protons were
observed as a multiplet at 7.02-7.00 ppm. The bridging CH2 of
the fluorene resonated as a singlet at 3.94 ppm, while the tert-butyl
protons appeared as a singlet at 1.12 ppm. The imine CH
appeared at 158.2 ppm in the 13C NMR spectrum of B, while
the ester CdO resonated at 169.8 pm. A total of 10 signals,
between 151.8 and 128.7 ppm, were observed for the calixarene
and fluorene ArCq atoms. The ArCH carbons of the fluorene
moiety resonated between 144.1 and 126.5 ppm, while those of the
calixarene moiety were observed at 125.6 and 125.2 ppm. The
bridging CH2 of the fluorene appeared at 37.2 ppm, while the
calixarene bridging CH2 resonated at 34.5 ppm. The tert-butyl
CH3 appeared at 31.7 ppm. An absorbance at 3442 cm-1 was
observed in the IR spectrum of B for the phenolic OH, while the
ester CdO absorbed at 1752 cm-1 and the imine CdN absorbed
to Zn2þ ion taking place at the Schiff base binding sites.5
A
diderivatized calix[4]arene based fluorescent sensor has also
recently been reported by Rao et al.6 The sensor was found to
be highly selective for the Cu2þ ion over a range of metal ions
tested.
at 1626 cm-1
.
2.1.1. Preparation of 5,11,17,23-Tetra-tert-butyl-25,27-
diethoxycarbonyl Methyleneoxy-26,28-dihydroxycalix[4]arene
[A]. The calixarene diester derivative (A) was prepared in 81%
yieldfollowing aprocedure similartothatdescribedbyMcKervey
et al.11 mp: 172-174 ꢀC (lit.19 182-184 ꢀC). Rf: 0.21 (70% DCM/
petroleum ether). υmax/cm-1 (KBr): 3427 (polymeric OH str.),
2962, 2902, 2864 (C-H str.), 1751 (CdO str). δH/ppm (300 MHz,
CDCl3): 7.07 (2H, s, phenolic OH), 7.02 (4H, s, ArCH), 6.82
(4H, s, ArCH), 4.72(4H, s, -O-CH2-CO2CH3), 4.45 (4H, d, Ar-
CH2-Ar, J = 12.0 Hz), 4.30 (4H, q, CH2CH3, J = 7.0 Hz), 3.32
(4H, d, methylene, J = 12.0 Hz), 1.33 (6H, t, CH2CH3, J =
7.0 Hz), 1.30 and 1.26 (2 ꢀ 18H, s, tert-butyl). δC/ppm (75 MHz,
CDCl3): 169.2 (ester CdO), 150.7 (ArCq-OH), 150.3 (ArCq-OR),
We recently published the synthesis of a new upper rim derived
calixarene compound with Schiff base ligating groups which
showed good affinity for Fe(III).18,19 In this study, we have
(13) Wang, F.; Liu, Q.; Wu, Y.; Ye, B. Electroanal. Chem. 2009, 630, 49.
(14) Gutsche, C. D.; Dhawan, B.; No, K. H.; Muthukrishman, R. J. Am. Chem.
Soc. 1981, 103, 3782.
(15) Munch, J. H.; Gutsche, C. D. Org. Synth. 1990, 68, 233.
(16) Gutsche, C. D.; Levine, J. A.; Sujeeth, P. K. J. Org. Chem. 1985, 50, 5802.
(17) Schmitt, P.; Beer, P. D.; Drew, M. G. B.; Sheen, P. D. Angew. Chem., Int.
Ed. Engl. 1997, 36, 1840.
(18) Supian, F. L.; Richardson, T. H.; Deasy, M.; Kelleher, F.; Ward, J. P.;
McKee, V. Sains Malays. 2009, 39, 423.
(19) Ward, J. P. Ph.D. Thesis, Institute of Technology Tallaght, 2009.
Langmuir 2010, 26(13), 10906–10912
DOI: 10.1021/la100808r 10907

Article
Supian et al.
to leave compound IBas a pink solid (1.97 g, 99%). mp: 175-177 ꢀC.
Rf = 0.75 (DCM/pet. ether). υmax/cm-1 (KBr): 3310 (phenolic
OH and NH), 1746 (CdO). δH/ppm (300 MHz, CDCl3): 7.12
(s, 2H, phenolic OH), 7.01 (s, 4H, ArCH), 6.95 (s, 4H, ArCH),
6.30 (s, 4H, NH2), 4.82 (s, 4H, -OCH2CO), 4.52 (d, 4H, Ar-
CH2-Ar), 4.29 (q, 4H, -CH2CH3), 3.21 (d, 4H, Ar-CH2-Ar),
1.32 (t, 6H, -CH2CH3), 1.24 (s, 18H, tert-butyl). δC/ppm (75
MHz, CDCl3): 169.9 (CdO), 151.7 (CqAr-OH), 147.2-
(ArCq-OR), 149.3 (CqAr-tert-butyl), 141.8 (CqAr-NO2), 133.5
(ArCq), 130.7 (ArCq), 126.0, 116.2 (ArCH), 72.0 (-OCH2CO),
61.2 (ester CH2), 34.2 (tert-butyl CH3), 32.3 (Ar-CH2-Ar), 29.7
(tert-butyl CH3), 14.2 (ester CH3).
2.1.4. Preparationof5,17-(9H-Fluoren-2-yl)methyleneamino)-
11,23-ditert-butyl-25,27-diethoxycarbonyl Methyleneoxy-26,28-
dihydroxycalix[4]arene [B]. To a solution of diamine IB (0.7 g,
0.95 mmol) in ethanol (60 mL) was added fluorene carboxalde-
hyde (0.97 g, 5 mmol), and the solution was heated at reflux
temperature for 16 h and cooled. The solid residue was recove-
red by vacuum filtration and recrystallized from chloroform/
methanol to give a red solid (0.66 g, 63%). mp: 271-273 ꢀC. Rf:
0.41 (30% EtOAc/pet. ether); C72H70N2O8 H2O requires C,
3
77.95%, H, 6.54%, N, 2.53%; found C, 77.62% H, 6.46% N,
2.52%. ESþ for C72H70N2O8, expected [MþH]: 1010.5; observed
[Mþ1]: 1010.5. υmax/cm-1 (KBr): 3442 (phenolic OH), 2960; 2927
(aliphatic CH), 1626 (CdN), 1752 (CdO). δH/ppm (300 MHz,
CDCl3): 8.43 (s, 2H, a), 8.10 (s, 2H, b), 7.84-7.76 (m, 6H, c, d, e),
7.65 (s, 2H, phenolic OH), 7.57-7.54 (m, 2H, f), 7.41-7.30 (m,
4H, g, h), 7.02-7.00 (m, 8H, ArCH (calix)), 4.83 (s, ArCq-O-
CH2-O-), 4.58-4.54 (d, 4H, Ar-CH2-Ar (calix)), 4.37-4.30
(q, 4H, ester CH2), 3.94 (s, 4H, Ar-CH2-Ar (fluorene)), 3.45-
3.41 (d, 4H, Ar-CH2-Ar (calix)), 1.38-1.34 (t, 6H, ester CH3),
1.12 (s, 18H, tert-butyl). δC/ppm (75 MHz, CDCl3): 169.8 (CdO),
158.2 (imine CH), 151.8 (ArCq-OH), 148.2 (ArCq-OR), 144.5
(CqAr-tert-butyl), 144.1, 141.4, 139.8, 138.2, 126.5 (CqAr
(fluorene)), 132.9, 128.7 (CqAr (calix)), 129.7, 127.7, 127.3,
124.7, 121.5, 120.8, 120.3 (ArCH (fluorene)), 125.6, 125.2
(ArCH(calix)), 72.6 (ArCq-O-CH2-O-), 61.7 (ester CH2),
37.2 (Ar-CH2-Ar (fluorene)), 34.5 (Ar-CH2-Ar (calix)),
34.0 (Cq-tert-butyl), 31.7 (tert-butyl CH3), 14.6 (ester CH3).
2.2. X-ray Diffraction Study. Crystals of Calix-Schiff
Figure 3. Reaction conditions: (i) HCHO, KOH, xylene; (ii)
NaOH, Ph2O; (iii) BrCH2COOEt, K2CO3,, CH3CN; (iv) HNO3,
CH3COOH, CH2Cl2; (v) H2 (2 atm), Ra/Ni, EtOH; (vi) fluorene-
carboxaldehyde, EtOH.
147.1, 141.5 (ArCq-tert-butyl), 132.5, 128.0 (ArCq), 125.7, 125.1
(ArCH), 72.4 (Ar-O-CH2), 61.2 (ester -CH2CH3), 33.9, 33.7 (Cq
tert-butyl), 31.8 (Ar-CH2-Ar), 31.6, 31.0 (tert-butyl CH3), 14.2
(ester -CH2CH3).
B
1/2CHCl3, suitable for an X-ray diffraction study, were ob-
3
tained from chloroform/methanol. Diffraction data were col-
lected at 150(2) K on a Bruker Apex II CCD diffractometer.
The structure was solved by direct methods and refined on F2
using all the reflections.20 All the non-hydrogen atoms were
refined using anisotropic atomic displacement parameters, and
hydrogen atoms were inserted at calculated positions using a
riding model. There is disorder of the one of the upper rim
substituents, modeled as 50% occupancy of two sites related by
180ꢀ rotation. There is also a 70:30 disorder of one of the terminal
ethyl groups on the lower rim. The chloroform solvate molecule
was refined with 50% occupancy.
2.1.2. Preparation of 5,17-Dinitro-11,23-ditert-butyl-25,27-
diethoxycarbonyl Methyleneoxy-26,28-dihydroxycalix[4]arene
[IA]. Nitro dealkylation of the diethyl ester calixarene I was
achieved following the literature procedure of McGinley et al.29
using nitric acid in a mixture of acetic acid and dichloromethane.
Recrystallization from methanol afforded a yellow solid (4.32 g,
21%). mp: 197-199 ꢀC (lit.20 198-200 ꢀC). Rf: 0.42 (70% DCM/
pet. ether);. υmax/cm-1 (KBr): 3289 (phenolic OH), 2967; 2868
(aliphatic CH), 1727 (CdO), 1510; 1336 (NO2). δH/ppm (300
MHz, CDCl3): 8.94 (s, 2H, phenolic OH), 7.96 (s, 4H, ArCH),
7.03 (s, 4H, ArCH), 4.72 (s, 4H, -OCH2CO), 4.51 (d, 4H, Ar-
CH2-Ar), 4.32 (q, 4H, ester CH2), 3.44 (d, 4H, Ar-CH2-Ar),
1.36(t, 6H, ester CH3), 1.09 (s, 18H, tert-butyl). δC/ppm(75MHz,
CDCl3): 168.6 (CdO), 158.8 (CqAr-OH), 151.8 (ArCq-OR),
144.8 (CqAr-tert-butyl), 141.8 (CqAr-NO2), 131.1 (ArCq), 128.9
(ArCq), 126.8, 125.4 (ArCH), 73.5 (-OCH2CO), 60.8(ester CH2),
33.9 (Cq-tert-butyl), 31.7 (Ar-CH2-Ar), 29.9 (tert-butyl CH3),
14.0 (ester CH3).
C
72.50H70.50Cl1.50N2O8, triclinic, P1, a = 13.6175(10), b =
˚
13.7993(10), c = 19.4909(14) A, R = 95.490(1), β = 90.115(1),
3
˚
˚
γ = 119.018(1)ꢀ, V = 3183.2(4) A , T = 150(2) K, λ = 0.71073 A,
Z = 2, 27 813 reflections measured, 12 471 unique (Rint = 0.0416),
wR2 = 0.2669 (all data), R1 = 0.0749 (I > 2σ(Ι). Crystallographic
data have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC 766050. (Copies
of the data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. (fax, þ44-(0)1223-
336033 or e-mail, deposit@ccdc.cam.ac.uk.)
2.1.3. Preparation of 5,17-Diamino-11,23-ditert-butyl-25,27-
diethoxycarbonyl Methyleneoxy-26,28-dihydroxycalix[4]arene
[IB]. To a suspension of dinitro calixarene IA (2 g, 2.4 mmol) in
ethanol (200 mL) was added a catalytic amount of Raney-Nickel.
The mixture was placed on a hydrogenator for 2 h (2 atm) then
filtered through a bed of Celite. The solvent was removed invacuo
2.3. Langmuir-Blodgett (LB) Studies. Solutions of A and
B were prepared using chloroform as the solvent, and both
concentrations were 0.2 mg/mL. Chromatography paper was
used as the Wilhelmy plate in the trough for the measurement
of ΔV. The deposition process followed the Y-type LB method.
A NIMA Langmuir trough (model 611) was used with a water sub-
phase, namely, pure water (Elga Purelab water system, >15 MΩcm),
(20) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
10908 DOI: 10.1021/la100808r
Langmuir 2010, 26(13), 10906–10912

Supian et al.
Article
Table 1. Hydrogen Bonds for B 1/2CHCl3
3
D-H
A
d(D-H)
d(H A)
3 3 3
d(D A)
3 3 3
<(DHA)
3 3 3
O1-H1 O7
0.84
0.84
0.84
0.84
1.00
2.09
2.45
2.10
2.41
2.18
2.853(3)
3.024(3)
2.908(3)
2.987(3)
3.160(7)
151.3
126.5
160.4
126.8
165.9
3 3 3
O1-H1 O6
3 3 3
O5-H5 O2
3 3 3
O5-H5 O3
3 3 3
C1S-H1S N1
3 3 3
methylene carbons by 78.49(7), 49.35(6), 71.07(7), and 47.51(8)ꢀ
for the groups containing O1, O2, O5, and O6, respectively.
The two fluorene groups on the upper rim are orientated with
their planes approximately parallel. They are intercalated with the
fluorine groups of a second molecule to form centrosymmetric
π-stacked dimers as in Figure 5. The mean interplanar distances
˚
˚
for the stacked fluorenes are 3.41 A for the outer pair and 3.47 A
for the inner pair; there are further π-π interactions with
neighboring dimers in the lattice.
3.2. Langmuir-Blodgett (LB) Studies: Π-A Isotherms.
As an example of the isotherm behavior, Figure 6 shows the Π-A
isotherms of B using three different Liþ concentrations. Upon
compression, this calixarene undergoes two principal phase
transitions: the first is a gas-liquid transition that begins at
∼3.2 nm2/molecule (for the pure water subphase), and the second,
beginning at ∼2.0 nm2/molecule, can be thought of as a
liquid-liquid or liquid-quasi solid rather than a liquid-solid
transition since the compressibility of the high pressure phase is
lower than is normally expected for a two-dimensional solid
phase.
The well-defined isotherm is due to the highly amphiphilic
nature of these calixarenes which are known to yield well-ordered
Langmuir films at the air/water interface. The Π-A isotherms are
similar in terms of the phase structure, but increasing values of
area per molecule, obtained via extrapolation of the solid phase
region of the Π-A isotherm to zero surface pressure, can be seen
as the Liþ concentration increases. The value on pure water is
∼2.0 nm2, rising to ∼2.2 and ∼2.3 nm2 as the concentration
increases to 0.0125 and 0.0250 mM, respectively. Similarly, the
take-off areas also increase in proportion to the Liþ concentration
in the region ∼3.2-3.6 nm2. Furthermore, the isotherms cross
over at ∼16 mN/m; that is, the compressibility inthe highpressure
phase decreases with the Liþ ion concentration. These results
show that the inclusion of the bound Liþ ion leads to a larger area
per molecule and to a lower compressibility within the high
pressure phase, indicating that the Langmuir layer properties
are highly sensitive to the complexation interaction.
The area per molecule value for a pure water subphase is
consistent with the formation of a calixarene monolayer as
predicted using space-filling models (CPK molecular modeling)
of B in which the orientation of the plane of the calixarene ring
itself on the water surface is parallel to the plane of the water
surface. The CPK molecular modeling, which is useful in predict-
ing conformation and orientation, is a powerful method for
comparison with the experimental results, and therefore, it was
used widely here.21 The two ethoxycarbonylmethyleneoxy and
the two hydroxyl groups in the lower rim of both calixarenes are
highly polar and therefore are expected to anchor the molecule to
the water surface in this orientation, that is, with the plane of
calixarene ring parallel to the plane of the water surface. The
increase of the average area per molecule observed on the Liþ
subphase indicates that the Liþ ion interacts with the monolayer.
These observations are seen in both cases for A and B.
Figure 4. X-ray structure of Calix-Schiff B 1/2CHCl3. Hydrogen
bonds shown as dashed lines. Hydrogen atoms and the minor
components of disorder have been omitted for clarity.
3
modified by the addition of copper (Cu2þ) and lithium (Liþ) salts.
The Langmuir film material (A or B) was dispensed dropwise,
using a Hamilton microsyringe, onto the surface of the subphase
using volumes of 50 and 70 μL for A and B, respectively. A period
of ∼5 min was allowed for the solvent to evaporate. The barrier
was then closed in order to record Π-A isotherms and ΔV-A
characteristics; the average compression speed was 100 cm2 min-1
(the maximum area of the trough was 580 cm2). Seven sets of data
for different ion concentrations ranging from 0.0125 mM to
0.0750 mM were collected in order to observe the influence of
the presence of Cu2þ and Liþ ions on the Langmuir film behavior.
In order to prepare the solution of Cu2þ ions with the required
concentration, specific quantities of copper(II) perchlorate
(Cu(ClO4)2) and pure water were mixed thoroughly in a beaker.
Then the solution was poured into the trough. The same method
was repeated using lithium perchlorate (LiClO4) to obtain the Liþ
ion solution.
For the purpose of measuring ΔV on transferred LB films,
aluminum-coated glass substrates (Al thickness 210 nm) were
used as substrates. Samples of 1 and 5 layers of LB films were
deposited on the Al substrates using Y-type deposition with a
chosen subphase concentration of 0.0375 mM.
2.4. Surface Potential, ΔV, Measurements. ΔV values
were measured using a NIMA S-POT surface potential sensor
(withanaccuracyof(2 mV) attachedtothetrough. The vibrating
plate was aligned very near to the surface of the subphase while a
counter electrode plate was placed in the bottom of the trough
within the subphase. All experiments were performed at room
temperature (21 ( 0.2 ꢀC).
3. Results and Discussion
3.1. X-ray Diffraction Study. A perspective view of Calix-
Schiff B 1/2CHCl3 is shown in Figure 4. The partial-occupancy
3
chloroform solvate is hydrogen-bonded to one of the imine
nitrogen atoms (Table 1) but is outside the calixarene cavity
(the H-bond is tangential to the calixarene ring). At the lower rim,
the unsubstituted phenols each make bifurcated hydrogen bonds
with neighboring phenylether and carbonyl oxygen atoms (table 1).
The geometry of the calixarene ring is conventional, with the
phenol groups tilted with respect to the mean plane of the four
(21) Nostro, P. L.; Casnati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf.,
A 1996, 116, 203.
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Figure 5. Intercalation of two molecules of Calix-Schiff B; the molecule on the left was generated by symmetry operation -x, 1 - y, -z.
Figure 6. Pressure-area (Π-A) isotherm of B with three different
Figure 7. Dependence of Alim for A and B on concentrations of
concentrations of Liþ ions in the subphase. Inset diagram showing
Cu2þ and Liþ ions.
the ΔΠc = ΠHc O-Πicons where it shows the difference of the
2
collapse pressure of water and Liþ ion.
Gibbs-Shishkovsky equation.7 Then, by plotting the change in
collapse pressure (ΠHc O - Πcions) against ln(Cions), the gradient, m
2
Figure 7 shows that the limiting area per molecule, Alim
,
increases as the concentration of ions in the subphase increases.
Specifically, Alim increases most rapidly for B-Liþ and lowest by
A-Cu2þ. For A-Cu2þ, it appears that very little increase in Alim
occurs, while for A-Liþ, the increase only occurs over a con-
centration range from 0 to 0.0125 mM. Generally, calixarene B
shows a much stronger interaction compared to A.
It can be seen that Alim for the monolayer increases as the salt
concentration within the subphase increases. All the orientations
of the calixarene rings of A and B are parallel ( ) to the water/air
interface, as revealed by comparing the theoretical cross-sectional
area values found using CPK modeling with the values of Alim
following the method described elsewhere.22 Also, the -OH
groups at the lower position of the calixarenes can form H-bonds
with the water subphase, hence stabilizing the orientation at the
air/water interface.23
of the graph is given by
m = ΓimonaxsRT
ð2Þ
ions
where Γ
is the maximum adsorption of the ions, R is the
max
universal gas constant, and T is temperature.
Figure 8 shows the corresponding graph of (ΠHc O - Πcions
)
2
versus ln(Cions). The y- axis values are found by deducting the Πc
of the calixarene monolayeronthe subphaseof aqueous ions from
Πc of the calixarene on the pure water subphase (Figure 6 inset).
Each pair (calixarene-ion) yields a straight line graph whose
gradients can be used to estimate the number of ions interacting
with calixarene per unit area using eq 2. The gradients of those
lines indicate the extent of the interaction behavior of each
calixarene and each ion.
The Πc value in the monolayer of calixarene A is only subtly
affected by the presence of either Cu2þ or Liþ ions. The gradients
have opposite sign, since the Liþ ions cause a reduction in the Πc
value and the Cu2þ cause an increase in its value. The resulting
gradients are correspondingly very small, and from calculation
using eq 2 they have values of -1.045 ꢀ 10-4 and 2.373 ꢀ 10-5
mol m-2 for Cu2þ and Liþ, respectively.
3.3. Discriminating Cu2þ and Liþ Ions within A and B.
Liquid-vapor adsorption can be determined using interfacial
tensions as a function of adsorbate concentration. In this case,
the adsorbate is the Cu2þ or Liþ ion which is adsorbed by the
calixarene monolayer. Adsorption of Cu2þ and Liþ can be
calculated using the Gibbs equation:
Dγ ¼ - ΓionsRTd ln aions
ð1Þ
Calixarene B interacts much more strongly with both Cu2þ and
Liþ than calixarene A. The gradients have opposite sign, since the
Cu2þ ions cause a reduction in the Πc value measured from the
isotherm, whereas the Liþ ions cause an increase in the Πc value.
The interpretation of this is not straightforward but may be a
reflection of the particular location where the binding of the
cation takes place. It could be hypothesized that binding within
where ions here refer to Cu2þ or Liþ, activity aions = Cions (dilute
concentration in solution), and Γions is near Γmionaxs, forming the
(22) Lonetti, B.; Nostro, P. L.; Ninham, B. W.; Baglioni, P. Langmuir 2005, 21,
2242.
(23) Maheswari, R.; Parthasarathi, R.; Dhathathreyan, A. Colloid Interface Sci.
2004, 271, 419.
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Supian et al.
Article
Figure 8. Plot of Γimonaxs monolayer of A and B with different ions.
Figure 9. Relation between Π-A (solid line) and ΔV-A (dotted
line), while inset diagram shows ΔV-A (dotted line) and μ^-A
(solid line) of B-Cu2þ at 1.25 ꢀ 10-2 mM.
the calix[4]arene cavity or outside the cavity may lead to opposing
effects on the collapse pressure. The gradient values signify the degree
of interaction between the monolayer and the ions and are 3.802 ꢀ
10-4 mol m-2 for Cu2þ and -4.316 ꢀ 10-4 mol m-2 for Liþ.
3.4. Surface Potential, ΔV, and Effective Dipole Moment,
μ^, Measurements on Langmuir Monolayer. ΔV is defined as
the potential difference within the clean water surface and the
coated surface in the subphase.24 The Helmholtz equation states
the relation between ΔV and μ^ for a Langmuir monolayer:
μ^ ¼ ε0εAΔV
ð3Þ
where μ^ is the average dipole moment normal to the monolayer
plane, ε0 is the vacuum permittivity (8.854 ꢀ 10-12 C2 N-1 m-2), ε
is the relative permittivity of the material between the electrodes,
A is the area occupied by the molecule, and ΔV is the measured
surface potential. Most researchers assume a value of ε = 1 for
ultrathin films, since the thickness of the air gap between the
monolayer and the vibrating electrode is very large compared to
the thickness of the monolayer itself.
Figure 10. DependenceofΔVmax ofAandBinteractingwithCu2þ
and Liþ ions, respectively. Each point represents a maximum value
for the corresponding concentration.
As an example, a graph of Π-A, ΔV-A, and μ^ (inset
diagram) for B-Cu2þ at the concentration of 1.25 ꢀ 10-2 mM
is displayed in Figure 9. Here, the ΔV values were obtained from
experiment, while μ^ values were derived from eq 3. Referring to
the Π-A graph (Figure 6), in the gaseous state at large area per
molecule (in the range 3.3-3.8 nm2), ΔV does not increase above
zero since the domains within the uncompressed Langmuir film
are very sparsely distributed. Upon further compression, at ∼3.1
nm2, ΔV and μ^ values begin to rise rapidly until they reach
maximum due to the change in the orientation of the molecules.
At this point (∼1.8 nm2 for ΔV and ∼2.3 nm2 for μ^), the
optimum alignment of polar units within the calixarene layer
has been achieved.
Figure 11. Dependence of μ^max (D) for A and B.
Referring to Figure 9, the Π-A isotherm can be observed
∼2.3 nm2, where the effective dipole moment μ^max has reached
its maximum value, ∼1.9 D corresponding to the onset of the
gas-liquid phase transition. This can be explained by the fact,
that during compression within the gaseous phase, the calixarene
molecules undergo their greatest reordering/alignment. Upon
reaching the gas-liquid transition, the molecules are virtually
aligned as well as possible and little further improvement in
ordering, which would lead to an increase in dipole moment,
can be achieved throughout the remainder of the compression
isotherm. Such behavior has beenhighlighted byprevious authors
for other calix[4]arenes.8 Interestingly, a phase transition is
observed at an area per molecule of ∼2.8 nm2 in ΔV and μ^ inset
plot which is not identifiable (or very slightly change) in the
ΔV-A isotherm. The data sets of maximum values of ΔV and μ^
for both calixarenes were obtained over the concentration range
0-7.50 ꢀ 10-2 mM and are shown in Figures 10 and 11.
Figure 10 shows the overall behavior of calixarenes A and B for
the full range of subphase ion concentrations used for both Liþ
and Cu2þ ions. The most abrupt increase for the ΔVmax can be
observed for A-Cu2þ and least with B-Liþ. When the salt is
introduced into the subphase, the ions begin to interact with A
and B until stabilization (equilibrium) is achieved, indicating the
greatest number of ions that can bind with A and B has been
reached. Thus, ion sensing of Cu2þ and Liþ ions using these
calixarenes can be achieved over the ranges 0-0.0250 mM for
B-Cu2þ and 0-0.0375 mM for A-Cu2þ and A-Liþ. The ΔVmax
values measured for B are significantly higher than the corre-
sponding values for A. This is due to the much larger molecular
(24) Taylor, D. M.; Oliveira, O. N.; Morgan, H. Surface potential of mono-
molecular films. IEEE Xplore, 1990.
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Supian et al.
Table 2. ΔVmax of A and B for the Monolayer Langmuir Film and Transferred LB Filmsa
A
B
air/water
interface (mV)
LB film (mV),
1 layer
LB film (mV),
5 layers
air/water
interface (mV)
LB film (mV),
1 layer
LB film (mV),
5 layers
subphase
water
Cu2þ
Liþ
230
305
272
152
167
157
87
99
91
330
362
334
226
266
255
120
150
122
a The errors in these measurements were approximately 2%.
dipole associated with the fluorenylmethylamino conjugated
system compared to the simple alkylhydroxybenzene dipoles.
Figure 11 shows the corresponding derived μ^max data. It must
be emphasized that μ^ (from eq 3) depends on molecular area
values, ΔVmax (Figure 10) and ε0. The molecular dipole is made up
of contributions from chemical groups within the substituted
calixarene and the ring and also the interaction between the polar
head groups, water, and aqueous ions. The molecular dipole
moment for B is significantly larger than that of A; this is due to
the presence of the extended region of conjugation within B in the
form of the fluorenemethylamino system. Effective dipole mo-
ments measured by Weis et al.8 and Nabok et al.9 for similar
calix[4]arenemoleculesthat did not possess highly conjugatedand
polarizable electron systems were measured to be much lower
than those in materials A and B here.
3.5. ΔVmax Measurements of Monolayer on Air/Water
Interface and on Mono- and Multilayer on LB Film. In order
to understand more the behavior and orientation of A and B on
the various subphases, ΔVmax values for the floating Langmuir
layers, transferred LB films comprising a monolayer and 5 layers
deposited on a Al substrate, are compared in Table 2.
From Table 2, the values for the floating monolayer are higher
than those for the LB films deposited on the Al substrate in all
cases (water, Cu2þ, and Liþ). This is most probably due to the
difference in average orientation of the calixarenes. At the air/
water interface, the plane of the calixarene ring is parallel to the
plane of the water surface owing to the strong interaction between
the ethoxycarbonylmethyleneoxy groups and water and the
hydroxyl groups and water, within the lower rim of the calixarene
molecules.25 The presence of the water subphase provides a very
strong, dominant anchoring medium which has the effect of
inducing a well-defined molecular orientation of the calixarene
molecules within the monolayer.
architecture resulted from the 5 layer LB films, then the measured
ΔVmax from 4 sequentially deposited (Y-type) layers would cancel
out.26,27 Given that the observed ΔVmax LB monolayer values are
always higher than those for the 5 layer LB film samples, it can be
concluded that the entire multilayer film must be less aligned, on
average, than the alignment within the monolayer.
4. Conclusions
The binding interactions between Cu2þ and Liþ ions (aqueous)
and two calix[4]arenes, namely, 5,11,17,23-tetra-tert-butyl-25,27-
diethoxycarbonyl methyleneoxy-26,28-dihydroxycalix[4]arene
(material A) and 5,17-(9H-fluoren-2-yl)methyleneamino)-11,23-di-
tert-butyl-25,27-diethoxycarbonyl methyleneoxy-26,28-dihydroxy-
calix[4]arene (material B), have been investigated. The surface
pressure-area isotherms reveal that the limiting areas per molecule
lie in the range 1.9-2.5 nm2 (depending on subphase conditions)
which is consistent with an orientation in which the plane of the
calixarene ring is parallel to the plane of the water surface. Material
B shows the strongest interaction with Cu2þ and Liþ ions when both
area per molecule data and effective dipole moment data are
considered. The effective dipole moment measured for B is signifi-
cantly larger than that of A owing to its highly conjugated
fluorenemethylamino system. Furthermore, simple nonconjugated
calix[4]arenes investigated by others possessed lower effective dipole
moments than those of materials A and B.
LB deposition is successful for A and B, but some of the
molecular ordering is lost as a result of the transfer onto solid
substrates, since the measured surface potentials reduce (typically
by 30-50%) for the LB film compared to the floating Langmuir
layer. Furthermore, 5 layer LB films exhibit a lower surface
potential than the monolayer LB films, suggesting that multi-
layered films become increasingly disordered. Future work will
examine the interaction of these and other similar calix[4]arenes
with a range of mono-, di-, and trivalent cations in order to assess
the binding capability of LB films of these materials which may
find application in ion-contamination measurements within aqu-
eous environments.
However, the effect of the water subphase is absent from the
deposited LB films and some disordering of the molecular dipoles
is more likely to occur. Thus, the resulting electrical polarization
and hence ΔVmax would be expected to be lower in the case of
deposited films. Indeed, this increasing disorder is observed for
multilayer (5 layers) LB films which exhibit smaller ΔVmax
compared to monolayer LB films.
The degree of disorder thus grows as the polar molecular
system progresses from the floatingLangmuir film (little disorder)
to the transferred LB monolayer (some disorder) to the deposited
multilayer LB films (significant disorder).23 If a perfect Y-type
Acknowledgment. F.L.S. wishes to acknowledge the Universiti
Pendidikan Sultan Idris (UPSI) and Government of Malaysia for
the award of a scholarship which enabled her to undertake this work.
J.P.W. wishes to thank the Postgraduate R&D Skills Programme
and ITT-Dublin’s PhD Continuation Fund for grant support.
(27) Broniatowski, M.; Latka, P. D. Colloid Interface Sci. 2006, 299, 916.
(28) Verboom, W.; Durie, A.; Egbenink, R. J. M.; Asfari, Z.; Reinhoudt, D. N.
J. Org. Chem. 1992, 57, 1313.
(29) Creaven, B. S.; Gernon, T. L.; McGinley, J.; Moore, A. M.; Toftlund, H.
Tetrahedron 2006, 62, 9066.
(25) Korchowiec, B.; Salem, A. B.; Corvis, Y.; Vains, J. B. R. d.; Korchowiec, J.;
Rogalska, E. J. Phys. Chem. B 2007, 11, 13231.
(26) Constantino, C. J. L.; Dhanabalan, A.; O., N. O., Jr. Colloids Surf., A 2002,
198- 200, 101.
10912 DOI: 10.1021/la100808r
Langmuir 2010, 26(13), 10906–10912