Counterion complexation of calixarene ligands in monolayers and micellar solutions

Colloids and Surfaces A: Physicochemical and Engineering Aspects 167 (2000) 105 – 113 www.elsevier.nl/locate/colsurfa

Counterion complexation of calixarene ligands in monolayers and micellar solutions Giulia Capuzzi a, Emiliano Fratini a, Luigi Dei a, Pierandrea LoNostro a, Alessandro Casnati b, Ralph Gilles c, Piero Baglioni a,* b

a Department of Chemistry and CSGI, Uni6ersity of Florence, Via Gino Capponi 9, 50121 Florence, Italy Department of Organic and Industrial Chemistry, Uni6ersity of Parma, 6iale delle Scienze 78, 43100 Parma, Italy c Hahn-Meitner Institut, BENSC, 14109 Berlin, Germany

Abstract Calix[n]arenes and their derivatives are currently the object of several studies, due to their peculiar cavity, that is suitable for very specific and efficient complexation of ions and small organic molecules, with a high degree of efficiency and selectivity, and form host–guest systems in the solid and in the liquid state. Besides some very important applications as ion carriers and cages, they can be used for studying the counterion distribution in micellar aggregates formed by anionic amphiphiles such as alkyl-sulfates. In this paper we report the studies on the complexing properties of a new calixarene derivative, namely the 1,3-dioctyloxy-calix[4]arene-crown-6-ether (CAL), that shows a high selectivity for cesium ions, at the air/water interface and in aqueous micellar solutions of cesium dodecyl sulfate. Langmuir surface pressure (p/A) isotherms were performed in order to study the stability of CAL films and the effect of cesium and potassium ions on the monolayer properties. In addition, small-angle neutron-scattering (SANS) experiments were carried out in order to determine the structure of the micellar system, and the interactions between the ligand and the micellar interface. Our study shows that the charge screening at the micellar interface is the predominant phenomenon that rules over the ion transport across liquid membranes, that is usually performed by macrocyclic ligands. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Counterion complexation; Calixarene ligands; Host – guest systems

1. Introduction The term ‘calix[n]arenes’ commonly refers to the cavity-shaped cyclic molecules made up of n phenol units linked via alkylidene groups. Calixarenes, along with cyclodextrins and crown-ethers * Corresponding author. Tel.: +39-55-275-7567; fax: +3955-240-865; http://csgi.unifi.it. E-mail address: [email protected] (P. Baglioni)

are considered as the third generation of supramolecules [1–3]. The interest in studying these compounds arises from their host–guest properties, and their selective affinity for several metal ions and small organic molecules. These macrocyclic host molecules with pre-organized cavities can be successfully used in liquid membranes and in the preparation of ion-selective electrodes to detect and separate alkali ions [4–6].

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 6 7 - 7

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Furthermore, the study of the host – guest interactions is crucial in understanding the assembly of biological structures such as ribosomes, enzymes and nucleic acids’ complexes. The tetramer calix[4]arenes show a particular binding efficiency towards alkali and alkaline earth metal ions, depending on their stereochemical conformation [7 – 12]. The rate of complexation depends on the molecules pre-organization and on the desolvation of the ligand’s sites prior to complexation. The Corey – Pauling–Koltun (CPK) molecular models, also called space filling molecular models, are very useful in predicting conformational and geometrical properties (volume, cross section and cavity dimension) of macromolecular systems. CPK models of several calix[4]arenes indicate that the aryl units are conformationally mobile, because of the interconversion between different structures, i.e. the cone structure (all the aryl units are at the same side of the calixarenical ring), the partial cone (three aryl units are at the same side of the calixarenical ring), and the 1,3-alternate (where the opposite aryl units are at the same side of the calixarenical ring) [13–16]. Even a little change in the nature or position of the substituting groups attached to calix[4]arenes, or in their conformation, can result in significant changes in their selectivity for metal complexation. When the tert-butyl groups are present in the para position, the thermodynamic stability of calix[4]arene-alkali ion complexes shows a sequence of binding efficiency for alkali ions in solution that decreases in the order partial cone\ 1,3-alternate\ cone [13 – 16]. On the other hand, the 1,3-alternate conformation is favored for unsubstituted derivatives, their efficiency in forming complexes, and their selectivity toward alkali ions increases in the order 1,3-alternate \ partial cone\cone [16 – 18]. Recently 1,3-dialkoxy-calix[4]arene-crown-5 and -crown-6-ethers have been synthesized in a 1,3-alternate conformation through a crown ether bridge, as shown by the 1H-NMR and 13C-NMR spectra [16– 18]. The rigid 1,3-alternate conformation results in their high capability to form inclusion complexes with potassium and cesium ions. Moreover, the lower polarity of the 1,3-dialkoxycalix[4]arene-crowns, with respect to the other

conformers, reduces the calixarene–sodium complex stability but increases that of other ions (cesium or potassium), through cation/p interactions [16–18]. This non-covalent force is involved when cation interacts with non-spherical charge distribution on the p face of an aryl unit [16–18]. 1 H-NMR experiments show that the 1,3-dioctyloxy-calix[4]arene-crown-6 is selective for Cs+ with a complexation constant of about 8.6 [16– 18]. We investigated the interactions of 1,3-dioctyloxy-calix[4]arene-crown-6 in the presence of Cs+ solution, by carrying out surface pressure (p) versus area (A) isotherms. Small-angle neutronscattering (SANS) experiments were performed in order to study the calixarene effect at the cesium dodecyl sulfate (CsDS) micellar interface. The study of systems containing micelles and macrocyclic ligands is of great help in understanding the cation–carrier interactions, and provides new insights on the mechanism and applications of ionic transport across fluid membranes. As a matter of fact, SANS measurements are a powerful tool to determine the micellar structure, the intermicellar interactions, as well as the micelle– macrocyclic cage interactions at the micellar interface [19,20]. Micelles can be depicted as particles formed by a hydrophobic core made up of the surfactant tails, and surrounded by a hydrophilic outer shell that contains the surfactant’s polar head groups and the associated counterions, as well as some water molecules. The [macrocyclic-ion]+ complexes, and the hydration water molecules are tightly bound to the hydrophilic layer, and an increment in the ligand-to-surfactant mole ratio modifies the surfactant’s self-assembling behavior, and induces significant variations in the geometrical micellar parameters [21–23].

2. Experimental section The 1,3-dioctyloxy-calix[4]arene-crown-6 (CAL, see Fig. 1) was synthesized by alkylating tetrahydroxy-calix[4]arene, as described in the literature [17]. For the monolayer experiments, calixarene solutions in chloroform were spread onto the clean

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surface of the subphase, by using a 100 ml pressure lock microsyringe (Hamilton). Surface pressure (p) measurements as a function of the molecular area (A) were carried out with a computer-controlled Lauda Filmwaage balance at different temperatures. The temperature was controlled by a Haake PG 40 thermostat within9 0.1°C. The accuracy of p and A measurements was9 0.1 mN m − 1 and 90.20 A, 2/ molecule, respectively. Films were compressed in a discontinuous way, by moving the teflon barrier about 0.6 cm per step, and then allowing the film to reach the equilibrium condition before another compression. Before spreading the film, the purity of the subphase was checked in the whole area interval (p B0.1 mN m − 1). Small Angle Neutron Scattering experiments were performed at the Hahn – Meitner Institute (BENSC, Berlin, Germany), using the V-4 spectrometer at the BER II reactor. All the experiments were performed using a wavelength l= 6 A, with a resolution Dl/l B10%. Scattered neutrons were detected by a two-dimensional position sensitive detector with 4096 active elements. The absolute values of the scattering vector covered the range 0.02 B Q B0.35 A, . The measured intensities were corrected, cell by cell, for background scattering, transmission and detector efficiency and calibrated for absolute intensity referring to scattering of water. Samples were contained in 1 mm-flat-quartz cells at the controlled temperature of (40 9 0.1°C), and were prepared by fixing the surfactant con-

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centration and scaling up the calixarene-to-surfactant mole ratio. Micellar solutions were prepared using cesium dodecyl sulfate, 1,3-dioctyloxycalix[4]arene-crown-6 calixarene, and deuterium oxide (deuterium content\ 99.99%, Fluka, Milan, Italy) as continuous phase. The cesium dodecyl sulfate was synthesized from dodecyl alcohol according to the following procedure. 14.4 g of dodecyl alcohol (Fluka, Milan, Italy) was dissolved in 100 ml of dichloromethane under nitrogen atmosphere. The mixture was cooled in an ice bath, and 10 g of chlorosulfonic acid (Fluka) was added, keeping the temperature under 10°C. The mixture was stirred for 30 min at 5°C, and then 100 ml of 8 M cesium hydroxide (ACROS, Belgium) was added. The solvent was evaporated, and water was eliminated by freeze-drying. The precipitate was treated twice with hot hexane to remove the unreacted dodecyl alcohol, and filtered while warm. The filtrate, after solvent evaporation, was dissolved in warm ethanol to eliminate the insoluble inorganic salts, and then filtered keeping the mixture at 50°C; this procedure was repeated twice, and finally the ethanolic solution was cooled at 0°C overnight. The precipitate was filtered and washed several times with diethyl ether and hexane, and then recrystallized twice with ethanol. The purity of the final products was assessed by atomic adsorption and found to be \99%.

3. Data analysis and results

3.1. Langmuir films

Fig. 1. 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 (CAL) molecular structure.

A preliminary result of the selective complexation of cesium ions was obtained by studying the isotherms of CAL films spread on a 1M CsCl or KCl subphase at the air/water interface. In Fig. 2 we report the p/A isotherms recorded on a pure water subphase at different temperatures. The 1 pcoll, Alim, and C − data, listed in Table 1, indis cate that CAL produces stable Langmuir films at the air/water interface between 20 and 35°C. The collapse pressure (pcoll) is the highest pressure to which a monolayer can be compressed without detectable expulsion of molecules to form a new

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Fig. 2. Spreading isotherms of CAL at the air/water interface at () 20°C; ( ) 25°C; (") 30°C and () 35°C.

phase. Practically, it was obtained as the highest value reached by the surface pressure in the p/A isotherm. The collapse pressure values show a decreasing trend when temperature increases, indicating that the collapse of the monolayer is favored by thermal motion. The limit area (Alim) value was determined as the intercept on the x axis of the tangent straight line to the isotherm in the most condensed region of the monolayer [24]. The experimental Alim values do not depend on temperature, and are about 939 1 A, 2/molecule, when pure water is the subphase. These values are in agreement with the value of 100 A, 2/molecule that we have calculated using the CPK molecular model, assuming a perpendicular orientation of calixarenes molecules at the air/water interface, with the crown ether bridge in the aqueous subphase, and the octyl chains in the gaseous phase. Table 1 The collapse pressure, molecular area, and compressibility modulus values of the 1,3-dioctyloxycalix[4]arene-crown-6 (CAL) spreading isotherms at the air/water interface T (°C)

pcoll (mN m−1)

Alimit (A, 2/molecule)

C−1 S (mN m−1)

20 25 30 35

43 91 42 91 41 9 1 40 9 1

939 2 949 3 929 2 929 2

230 293 291 330

Fig. 3. Spreading isotherms of CAL at the air/ water interface at () 20°C; ( ) 25°C; (") 30°C and () 35°C, using a CsCl 1 M solution as a subphase.

Because of CAL’s rigid structure, and since the 1,3-alternate conformation is fixed through the crown ether bridge, we can exclude any conformational change. Thus, the unique alternative scenario would imply that the calixarene’s molecules assume a parallel orientation at the interface, but in this case the molecular area calculated from CPK models would be 216 A, 2/molecule. The com1 pressibility modulus, C − (mN m − 1), is defined S as [24]:

 

1 C− S = −A ·

(p (A

(1)

According to the literature [24], the compressibility modulus values range from 12.5 to 50 mN m − 1 for liquid expanded films, while for a liquid condensed phase it varies from 100 to 250 mN 1 m − 1. Table 1 reports the maximum values of C − S for each case, that correspond to the closest packing of the monolayer. The calixarene investigated is in liquid condensed-solid phase at the water–air interface as evidenced by the data reported in Table 1. In order to verify the selective complexation operated by CAL towards cesium ions, the spreading isotherms were studied using a 1 M CsCl solution as subphase. Between 20 and 35°C, the presence of Cs+ ions in the aqueous subphase produces a consistent change in the spreading isotherms, and the molecular limit area of CAL

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increases to an average value of 101 A, 2/molecule (see Fig. 3). This increment can be explained in terms of increased electrostatic repulsions, due to the formation of the stable Cs+ – CAL complex. The measurements were repeated using a 1 M KCl solution as subphase. In this case, the isotherms do not show any effect related to the presence of potassium ions, in fact the average molecular limit area results to be about 96 A, 2/ molecule, the same value found in the case of pure water, within the experimental error. Fig. 4 reports, as an example, the spreading isotherms of CAL obtained at 20°C using CsCl, KCl, and pure water as subphases. In order to determine the effect of CAL complexation on the monolayer stability, the Gibbs free energy of the spreading films, DG (kJ mol − 1), has been calculated according to the following formula [24]: DG =

&

p0

0

Asalt dp −

&

p0

Awater dp

(2)

0

where p0 = 30 mN m − 1 is the upper limit pressure used to calculate the integral, Asalt is the experimental area as function of the surface pressure in presence of the saline subphase (KCl, CsCl) and Awater is the experimental area as function of the surface pressure in presence of the pure water subphase. The DG values are reported in Table 2,

Fig. 4. Spreading isotherms at constant temperature (20°C) using as a subphase: (, dashed line) pure water; (", continuous line) KCl 1M and (, continuous line) CsCl 1 M solutions are shown.

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Table 2 The Gibbs free energy values calculated in the presence of CsCl 1M and KCl 1M solutions as subphase, with respect to pure water, for the limit pressure value 30 mN/m T (°C)

DGKCl (kJ mol−1)

DGCsCl (kJ mol−1)

20 25 30 35

0.54 0.67 0.94 0.67

1.5 1.3 1.6 0.96

as a function of temperature and for the different subphases. When Cs+ and K+ solutions are used as subphases, the DG values are always positive, indicating that monolayers are destabilized by the presence of the cations, due to their interaction with the CAL molecules at the air/water interface. However, this effect is enhanced by the presence of cesium ions, because of steric and electrostatic repulsive interactions. This evidence confirms the selective complexation of cesium ions operated by the ligand at the air/water interface.

3.2. Small-angle neutron scattering Small-angle neutron-scattering measurements were performed on a set of samples, where the surfactant concentration is kept constant at 1% (w/w), whereas 0, 3 and 5% of CAL was added to cesium dodecyl sulfate (CsDS) micellar solutions. We focused our attention on the changes of the alkyl sulfate micelles’ properties upon addition of CAL, and on the ability of the ligand to form stable complexes with the alkali counterions. Calixarene complexes are poorly soluble in water, and must associate with the surfactant film. This effect decreases the film rigidity, and increases its deformability, because of the increased disorder in the interfacial region. When the ligand molecules associate with the micelles, they increase the effective area of the heads, and decrease the surfactant’s critical packing parameter, Pc = 6 al − 1 (where 6, a, and l are the volume of the hydrocarbon chain, the area per polar head group, and the length of the aliphatic tail in the fully stretched conformation, respectively). These effects would produce a lowering in the aggregation number [25]. On the other hand, the counterion complexa-

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tion operated by the ligands screens the charges at the micellar interface, resulting in a lowering of the surface charge and therefore in an increase in the aggregation number. These two phenomena i.e. steric hindrance and charge screening-play opposite effects at the micellar surface, and can be modulated in such a way to control both the charge and the size of the micellar particles. Dodecyl sulfate micellar (DS) solutions with different alkali counterions have been extensively studied by SANS, as a function of temperature, surfactant concentration, and added salt solutions [26–29]. Missel et al. [27 – 30] found that alkyl sulfate micelles exhibit a sphere-to-rod transition that depends on the surfactant concentration and on the surfactant counterion. These results are also in agreement with those found by Chen and coworkers [31–33], who studied CsDS micellar solutions at a surfactant concentration of 3%. A two-shell model for the intraparticle structure factor was used by Liu et al. [20,24 – 26] in order to study micellar solutions of LDS added to CESTO, a macrocyclic ligand that complexes Li+ ions with high selectivity. The authors show that the model correctly describes the CESTO–LDS system, giving quantitative information on the complex formation and a description of the aggregational behavior of the micelle in the presence of the ligand. In particular, they found that the average aggregation number significantly increases as the ligand is added to the surfactant solution, and micelles become more and more elongated ellipsoids. The formation of the CESTO/Li+ complex results in a decrease in the effective surface charge. In this study we used a similar approach for the analysis of the SANS spectra. A FORTRAN code has been developed to calculate I(Q) and to compare it with the experimental intensity distribution in absolute scale, by using four fitting parameters: the aggregation number (N), the effective charge (Z), the short axis (b), and the thickness (d). The I(Q) values have been calculated assuming a twoshell model for the micellar form factor F(Q), and the Non-Additive Radius-Mean Spherical Approximation (NAR-MSA) for the structure factor S(Q), as described by Liu et al. [20].

For neutrons scattered by a micellar solution, I(Q) depends on both the intermicellar interactions-described by the structure factor, S(Q)-and on the intramicellar interactions-described by a form factor P(Q). The single-particle form factor, P(Q), was calculated by using a two-shell model, that describes the micelles as formed by a hydrophobic core of aliphatic chains, surrounded by a hydrophilic outer layer that includes the surfactant polar head groups, the counterions, and the hydration water molecules as well. The micellar shape is assumed to be that of a prolate ellipsoidal particle, with semi-axis a, b and c= b (where a is the major axis), and shell thickness d. Based on this model, P(Q) and S(Q) are defined as: P(Q)=

&

1

F(Q, m) 2 dm

(3)

0

S(Q)= 1+

 F(Q, m) 2 [S (Q)− 1]  F(Q, m) 2 MM

where SMM(Q) is the macroion–macroion structure factor. The orientation dependent form factor, F(Q, m), is given by the following formula, where m takes into account the direction of the symmetry axis of the spheroid and the Q vector: 3j1(u) 3j (6) + (1− f) 1 u 6

F(Q, m)=f

(4)

u= Q[m 2a 2 + (1− m 2)b 2]1/2 6= Q[m 2(a+d)2 + (1− m 2)(b + d)2]1/2 where j1(x) is a first order Bessel function. The dimensionless number f depends on the scattering length densities, r, of the micelle as: f= Vt(rcore − rshell)/(Sbi − Vmrs)

(5)

where Vt and Vm are the dodecyl tail and the surfactant monomer volumes respectively, and are assumed to be Vt = 360 A, 3 [19,26] and Vm = (VM − aVc), where VM = 443 A, 3 (CsDS) and Vc = 25 A, 3(for Cs+) [34–36]. The sum of the scattering length of all the atoms in a surfactant monomer is Sbi = btail + bhead + (1− a)bCs + , whereas rs is the scattering length of the solvent D2O. The ionization factor, a=Z/N, is related to the effective micellar charge (Z), and to the aggregation number (N). The hydration number H is then determined by:

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Table 3 Parameters obtained from SANS data fitting using the two shell model and the NAR-MSA approximation. Surfactant concentration 1% w/w [CAL]/[CsDS]

Aggregation number (N) Ionization factor (a=Z/N) Carbon atoms in the shell Shell thickness, d (A, ) Short axis, b (A, ) Axial ratio, a/b Average diameter, D (A, ) Hydration number, H Association

0%

3%

5%

110 13.6% – 5 18.0 1.6 52.5 7.5 –

154 9.0% – 5.4 19.2 2.0 59.5 7.7 100%

278 4.5% 1.4 7 20.0 3.0 73.0 7.5 100%

H=[(Vshell/N) − (Vm −Vt)(1 −a) − (Vm −Vt −VCs +)a]/Vwater

(6)

where Vshell is the total volume of the shell and Vwater is the volume of water molecule. SANS measurements were performed on a set of three samples, by fixing the CsDS surfactant concentration at 1%, and increasing the CAL-tosurfactant mole ratio from 0 to 3% and then to 5%. The parameters derived from the spectra analysis are shown in Table 3, while in Figs. 5 and 6a, and Fig. 6b we reported the spectra, P(Q), and S(Q) factors from the fitting of the data.

Fig. 5. Small-angle neutron-scattering measurements of cesium dodecyl sulfate (CsDS) solutions at 1% (w/w) constant surfactant concentration, and () 0%, (") 3% and () 5% of CAL. Symbols indicate the experimental data, while continuous lines are the calculated intensities.

Fig. 6. (a) The form factors and (b) the structure factors of the cesium dodecyl sulfate micellar solutions extracted from SANS data analysis. 1% (w/w) constant surfactant concentration and () 0%, (") 3% and () 5% of CAL. Symbols indicate the experimental data, while continuous lines are the calculated intensities.

Table 3 shows that the average aggregation number considerably increases as CAL concentration is increased, and the micelles grow in size, as described by the increase of either the shell thickness d or the short axis b, as well as the ellipsoidal ratio a/b. The latter parameter shows that micellar growth results in an elongation of the ellipsoid shape more pronounced as more ligand is added to the surfactant solution. Another important parameter to describe the effect of CAL addition to the micellar solution, is the ionization factor a, i.e. the effective micellar charge to the average aggregation number ratio (Z/N). As shown in Table 3, the ionization strongly decreases as CAL is added. This fact can

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be ascribed to the presence of the ligand in the micellar interface. In fact, CAL, by entrapping the cesium counterions, approaches more closely the surfactant’s head groups, and thus screens more efficiently the negative electrostatic charge density at the micellar surface.

4. Conclusions In this paper we report the monolayer properties of a new calixarene, namely 1,3-dioctyloxycalix[4]arene-crown-6-ether (shortly CAL), studied at the air/water interface, as a function of temperature and in the presence of potassium or cesium ions in the subphase. Furthermore, we have investigated the effect of CAL on the micellar charge of cesium dodecyl sulfate (CsDS) micelles by small-angle neutron-scattering experiments. CAL forms stable monolayers at the air/water interface at all temperatures, and selectively complexes Cs+ ions. SANS data analysis indicates that the addition of CAL to CsDS micellar solutions results in an increment of the micelles size and in a more elongated shape of the particles. The micellar ionization factor, a =Z/N, strongly decreases as CAL is added. Our results confirm that CAL is adsorbed at the micellar interface and entraps the cesium counterions, providing an efficient screening of the surfactant’s negative charge. These results will be relevant in understanding the efficient ion transport across liquid membranes, that is usually performed by macrocyclic ligands.

Acknowledgements The authors acknowledge the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST), CNR and the Consorzio per lo Sviluppo dei Sistemi a Grande Interfase (CSGI) for partial financial support. Acknowledgment is also due to the European Union and BENSC, contract ERBFMGECT950060.

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