Counterion Complexation by Calixarene Ligands in Cesium and

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Counterion Complexation by Calixarene Ligands in Cesium and Potassium Dodecyl Sulfate Micelles. A Small Angle Neutron Scattering Study† G. Capuzzi,‡ E. Fratini,‡ F. Pini,† P. Baglioni,*,‡ A. Casnati,§ and J. Teixeira| Department of Chemistry and CSGI, University of Florence, via G. Capponi 9, 50121 Florence, Italy, Department of Organic and Industrial Chemistry, University of Parma, Viale delle Scienze 78, 43100 Parma, Italy, and Laboratoire Le´ on Brillouin, CEA-CNRS, CEN-Saclay, 91191 Gif-sur-Yvette, France Received June 14, 1999. In Final Form: September 28, 1999 Cesium and potassium dodecyl sulfate micellar solutions have been studied in the presence of three calix[4]arene-crown ethers able to complex cesium and potassium micellar counterions in a selective way (C8Cal-6 and C3Cal-6 for Cs+ and C3Cal-5 for K+). The cesium or potassium dodecyl sulfate micelles are modeled as two-shell ellipsoids. The surfactant dodecyl chains and part of the calix-crown ligands are located in the micelle hydrophobic core. Intermicellar correlations are calculated according to a multicomponent primitive model in a mean spherical approximation with a nonadditive diameter (NARMMSA). We found that calix-crown derivatives addition to cesium dodecyl sulfate micellar solutions leads to a generalized increase of the micelle aggregation number, a consistent decrease of the micelle ionization, and a micellar grow toward elongated ellipsoids. However, the addition of calixarene molecules to potassium dodecyl sulfate produces only a slight decrease of the micellar charge.

I. Introduction The interfacial behavior of macrocyclic molecules have been largely studied.1-11 The main interest to these macromolecules is related to model carrier-mediated transport of cations across membranes, where micelles have been often used as membrane mimetic systems.12 The interfacial mechanism of interaction and macrocycle location at the micellar interface of water-soluble macrocycles (crown ethers, cryptands, etc.) are still controversial. For example, Quintela et al. investigated1 the effect of the C222 cryptand addition to sodium dodecyl (SDS) and decyl sulfate (SdeS) micellar solutions. They found a decrease of the surfactant critical micelle concentrations (cmc’s) and accounted for this effect the reorganization of * To whom correspondence should be addressed. Fax: +39.055240-865. E-mail: [email protected]. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. ‡ University of Florence. § University of Parma. | Laboratoire Le ´ on Brillouin. (1) Quintela, P. A.; Reno, R. C. S.; Kaifer, A. E. J. Phys. Chem. 1987, 91, 3582. (2) Evans, D. F.; Sen, R.; Warr, G. G. J. Phys. Chem. 1986, 90, 5500. (3) Evans, D. F.; Evans, J. B.; Sen, R.; Warr, G. G. J. Phys. Chem. 1988, 92, 784. (4) Ginley, M.; Henriksson, U.; Li, P. J. Phys. Chem. 1990, 94, 4644. (5) Carla`, M.; Gambi, C. M. C.; Baglioni, P. J. Phys. Chem. 1996, 100, 11067. (6) Baglioni, P.; Gambi, C. M. C.; Giordano, R.; Teixeira, J. Colloid Surf., A 1997, 121, 47. (7) Baglioni, P.; Bencini, A.; Dei, L.; Gambi, C. M. C.; LoNostro, P.; Chen, S.-H.; Liu, Y. C.; Teixeira, J.; Kevan, L. Colloid Surf., A 1994, 88, 59. (8) Baglioni, P.; Bencini, A.; Dei, L.; Gambi, C. M. C.; LoNostro, P.; Chen, S.-H.; Liu, Y. C.; Teixeira, J.; Kevan, L. J. Phys. Condens. Matter 1994, 6, A369. (9) Baglioni, P.; Liu, Y. C.; Chen, S.-H.; Teixeira, J. J. Phys. IV 1993, 3 (C8), 169. (10) Baglioni, P.; Kevan, L. Prog. Colloid Polym. Sci. 1988, 76, 183. (11) Baglioni, P.; Kevan, L. J. Phys. Chem. 1988, 82, 4726. (12) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982; Chapter 2.

counterions in the Stern layer, due to the formation of a hydrophobic sodium cryptate complex in the inner micellar region.1-6 Evans et al.,2,3 studied SDS micelle solutions with and without the C222 cryptand showing that the interfacial area per surfactant molecule increases from 64 Å2 for pure SDS to 136 Å2 for the SDS-C222 cryptand systems. They attributed this effect to the increase of the repulsion among the surfactant headgroups. Addition of C222 to the micellar solutions considerably increases the degree of counterions dissociation. This was explained by considering sodium ions completely caged by the macrocycle and removed from the micelle surface. Complexation resulted in a macroion with an area larger than that of the bare ion, leading to a rearrangement of the micelle polar headgroups. Therefore, encapsulation of small cations by macrocyclic compounds alters the effective micellar size and the screening of inter-headgroup electrostatic repulsion. However, the location of the macrocyclic complex within the micellar sytems was controversial. In ref 1, Quintela et al. consider the cryptand-ion complex in the inner layer near the sulfate polar headgroup, thus increasing the surfactant aggregation number and the water molecules penetration in the hydrophobic core. Conversely, Evans et al.2,3 attributed the surfactant cmc value reduction, and the micellar polar head area increase to the complexation of the counterions, which are pushed away from the micelle surface toward the aqueous phase. This would induce the decrease of the micelle aggregation number. Using small-angle neutron scattering, Baglioni et al.6 studied the complexation of counterions by C222, in sodium and lithium dodecyl sulfate. The authors found clear experimental evidence that the macrocyclic ligands are located at the micellar outer layer where they complex the micelle counterions, and in particular the selective complexation of sodium over lithium for this macrocycle was shown. The presence of the C222 macrocycle gives rise to a slight decrease of the effective charge and of the average aggregation numbers, leading to slightly smaller

10.1021/la9907580 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999

Counterion Complexation by Calixarene Ligands

and more spherical micelles. Molecular dynamics experiments have confirmed this picture.13-15 Calix[n]arenes are considered the evolution of crown and cryptand molecules. They are cavity-shaped macrocyclic molecules with good performance as receptors and cation carriers.16-19 Due to the one-pot synthesis and to the inexpensive synthetic route,16,17 these compounds are used in many industrial applications. Moreover, chemical modifications of the aromatic calix ring can be performed in simple way to functionalize these macromolecules, allowing to a wide range of applications. Industrial uses of calixarenes are related to metal cations extraction studies to the production of potentiometric sensors and ion-selective electrodes, to the recovery of cations (cesium or uranium) from nuclear waste materials.16-19 Finally, they are widely used as accelerants for cyanoacrylate instant adhesives, and as antioxidant stabilizers for organic polymers.20 It has been demonstrated that the binding properties of calix[4]arene derivatives toward a specific cation are affected by both the nature of the substituent groups in the calix ring and the stereochemical conformation. It is well-known that the calix[4]arene skeleton, formed by the aryl units, is conformationally mobile and three different structures are possible (cone, partial cone, and 1,3 alternate). However, addition of a crown ether ring to the upper rim of the cali[4]arene makes these molecules conformationally rigid (see Figure 1) and stabilizes the 1,3 alternate conformation. Ungaro et al.21,22 synthesized calix[4]arene-crown ethers forcing the molecular structure of the calix[4]arene ring in the 1,3-alternate conformation. The complexation properties of this new class of calix[4]arene compounds depend, in addition to the calix size, on the size of the crown ring.23 For example, the 1,3-alternatecalix[4]arene-crown-6 preferably complex cesium over lithium, sodium, or potassium ions, with the cesium complexation constant (log β) six times higher than those for lithium and sodium. Thermodynamic parameters and X-ray crystallographic data of the calix[4]arene-crown-6 compounds revealed that the 1,3-alternate conformation is retained in solution, as well as in the solid state. Casnati et al.22 found that the 1,3-alternate-calix[4]arene-crown5 specifically interacts with the potassium cation, with association constant values for sodium and lithium ions being half than that of the potassium ion, and the binding free energies (given as -∆G) considerably higher for the potassium-calix[4]arene-crown-5 complexes. In a previous work,24,25 we investigated the calixarenecesium complex by monitoring the surface pressure (π) vs area (A) isotherms of 1,3-dioctyloxy-calix[4]arene-crown6 (C8Cal-6) in the presence of Cs+ ions added in the aqueous (13) Wipff, G.; Lauterbach, M. Supramol. Chem. 1998, 6, 187. (14) Lauterbach, M.; Engler, E.; Muzet, N.; Troxler, L.; Wipff, G. J Phys. Chem. B 1998, 102, 245. (15) Muzet, N.; Engler, E.; Wipff, G. J Phys. Chem. B 1998, 102, 10772. (16) Gutsche, C. D., Stoddard, J. F., Ed. Calixarenes; The Royal Society of Chemistry: Cambridge, UK, 1989. (17) Vicens, J., Bo¨hmer, V., Eds. Calixarenes. A Versatile Class of Macrocyclic Compounds; Kluwer: Dordrecht, The Netherlands, 1990. (18) Shinkai, S. Tetrahedron 1993, 49, 8933. (19) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (20) Perrin, R.; Harris, S. In Calixarenes. A Versatile Class of Macrocyclic Compounds; Vicens, J., Bo¨hmer, V., Eds.; Kluwer: Dordrecht, 1990; pp 235-259. (21) Ungaro, R.; Casnati, A.; Ugozzoli, F.; Pochini, A.; Dozol, J. F.; Hill, C.; Rouquette, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1506. (22) Casnati, A.; Pochini, A.; Ungaro, R.; Bocchi, C.; Ugozzoli, F.; Schwing, M. J.; Egberink, R. J. M.; Struijk, H.; Lugtemberg, R.; de Jong, F.; Reinhoudt, D. N. Chem. Eur. J. 1996, 2, 436. (23) Casnati, A.; Pochini, A.; Ungaro, R.; Ugozzoli, F.; Arnaud, F.; Fanni, S.; Schwing, M. J.; Egberink, R. J. M.; de Jong, F.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 2767.

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Figure 1. Chemical structures of (a) 1,3-dioctyloxy-calix[4]arene-crown-6 (C8Cal-6), (b) 1,3-diisopropyloxy-calix[4]arenecrown-6 (C3Cal-6), and (c) 1,3-diisopropyloxy-calix[4]arenecrown-5 (C3Cal-5). The hydrogen atoms are not shown to simplify the figure.

subphase. We showed that C8Cal-6 selectively complexes Cs+ ions at the air/water interface and that this complexation occurs with a very efficient screening of the surface charge.25 Conversely, potassium ion complexation by the potassium selective 1,3-diisopropyloxy-calix[4]arene-crown-5 (C3Cal-5) occurs, at the water-air interface, without surface charge screening.24 The mechanism of ion capture and extraction is still unknown, even if it is stressed that the interfacial region between water and the organic solvent plays a crucial role. In this paper we extend monolayer studies to micellar solutions of cesium dodecyl sulfate (CsDS) and potassium dodecyl sulfate (KDS) that represent simple models for ions extraction across oil/water interface. We show that calix-crown derivatives are partitioned at the micelle interface, where they complex the micelle counterions (Cs+ or K+). Counterion complexation produces a consistent growth of the CsDS micelle with an almost complete screening of the micelle charge. The complexation process of potassium ions consistently differs from that of cesium. The calix-crown/K+ occurs with a negligible charge screening and micellar growth. II. Experimental Section Materials. Cesium dodecyl sulfate and potassium dodecyl sulfate surfactants were synthesized from dodecyl alcohol according to the following procedure. Dodecyl alcohol (Fluka, Milan, Italy) (14.4 g) was dissolved in 100 mL of dichloromethane under nitrogen atmosphere. Chlorosulfonic acid (Fluka) (10 g) was added to the mixture, 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) or potassium hydroxide (Fluka, Milan, Italy) was added. The solvent was evaporated and water removed by freeze-drying. The precipitate was treated (24) Capuzzi, G.; Fratini, E.; Dei, L.; LoNostro, P.; Casnati, A.; Gilles, R.; Baglioni, P. Colloids Surf. A., in press. (25) Capuzzi, G.; Pini, F.; Dei, L.; LoNostro, P.; Fratini, E.; Gilles, R.; Baglioni, P. Physica B, in press.

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twice with warm hexane to remove the unreacted dodecyl alcohol and filtered while warm. The filtrate was dissolved in ethanol to eliminate the insoluble inorganic salts and filtered, keeping the mixture at 50 °C. The ethanolic solution was cooled at 0 °C overnight, and the precipitate was filtered and washed several times with diethyl ether and hexane and recrystallized twice from ethanol. The purity of the final products was determined by atomic adsorption. The 1,3-dialkyloxy-calix[4]arene-crown ethers (see Figure 1) were synthesized by alkylating tetrahydroxycalix[4]arene, as described in the literature.22,23 The 1,3-dioctyloxy-calix[4]arene-crown-6 (C8Cal-6) and the 1,3-diisopropyloxycalix[4]arene-crown-6 (C3Cal-6) selectively complex cesium ions, while the 1,3-diisopropyloxy-calix[4]arene-crown-5 (C3Cal-5) is potassium selective. Micellar solutions were prepared in deuterium oxide (deuterium content >99.99%, Fluka). Small angle neutron scattering (SANS) measurements for the C3Cal-6 and C3Cal-5 calixarenes have been performed on the PAXE spectrometer of the Laboratoire Le´on Brillouin at Saclay (France) using an average wavelength of 5 Å with a wavelength spread, ∆λ/λ, less than 10%. The Q range investigated for all samples was 0.01 < Q < 0.35 Å-1, with a sample to detector distance of 2.3 m. The intensity, corrected for the empty cell contribution, was normalized to absolute scale by a direct measurement of the intensity of the incident neutron beam. The integration of the normalized 2-D intensity distributions with respect to the azimuthal angle yielded the 1-D scattering intensity distributions, I(Q), in the units of a differential cross section per unit volume of the sample (cm-1). The SANS experiments on the C8Cal-6/CsDS system have been performed at the Hahn-Meitner Institute (BENSC, BerlinWansee, Germany) using the V-4 spectrometer at the BER II reactor. All the experiments were performed using a wavelength λ ) 6 Å with a resolution ∆λ/λ < 10%. Scattered neutrons were detected by a two-dimensional position detector with 4096 active elements. The absolute values of the scattering vector covered the range 0.02 < Q < 0.35 Å-1. The measured intensities were corrected, cell by cell, for background scattering, transmission, and detector efficiency and calibrated for absolute intensity referring to scattering of H2O. Samples were contained in 1 mm flat quartz cells (Hellma) at the controlled temperature of 50 ( 0.1 °C.

III. Modeling of the Micelles Quantitative analysis of absolute scale SANS data was obtained by modeling the micellar solution as composed of charged two-shell ellipsoidal particles interacting with each other according to a screened Coulombic potential within the NAR-MMSA (nonadditive radius multicomponent mean sphere approximation) model.26-30 In the fitting procedure we use an overall amplitude factor which accounts for the calibration factor. The micellar solution is considered as composed of uniform-sized ellipsoidal micelles with a mean aggregation number, N, and an effective charge Z. Micelles have been modeled as formed by a hydrophobic core of spheroidal shape, with principal axes a, b, b, that contains the surfactant hydrocarbon tails, where the solvent cannot penetrate, and a hydrated hydrophilic shell of thickness d, formed by the polar heads, a fraction of counterions, some solvent molecules, and the calixarene guest. In this model the calixarene molecule is allowed to partly penetrate in the surfactant hydrocarbon tails region. (26) Liu, Y. C.; Baglioni, P.; Teixeira, J.; Chen, S.-H. J. Phys. Chem. 1994, 98, 10208. (27) Khan, S.; Morton, T. L.; Ronis, D. Phys. Rev. A 1987, 35, 4295. (28) Khan, S.; Ronis, D. Mol. Phys. 1987, 60, 637. (29) Senatore, G.; Blum, L. J. Phys. Chem. 1985, 89, 2676. (30) Senatore, G. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S.-H., Huang, J. S., Tartaglia, P., Eds.; Kluwer Publishers: Dordrecht, The Netherlands, 1992; p 175. Liu, Y. C.; Ku, C. Y.; LoNostro, P.; Chen, S.-H. Phys. Rev. E 1995, 51, 4598.

Capuzzi et al.

Within these assumptions the scattering intensity as a function of the wave vector Q can be written as26

∑i bi - VmFs)2P˜ (Q)S˜ (Q) + Ibkgd

I(Q) ) (C - cmc)N(

(1)

where C is the surfactant concentration and cmc the critical micellar concentration (both in mol/L), bi values are the scattering lengths of each atom of the surfactant and calixarene molecules, Vm is the monomer volume, and Fs is the scattering length density of solvent. P ˜ (Q) is the orientationally averaged intraparticle structure factor for ellipsoidal particles, S ˜ (Q) is the orientationally averaged center-center interparticle structure factor, and the additive term takes into account incoherent scattering and electronic background signal. P ˜ (Q) and S ˜ (Q) are given respectively by

P ˜ (Q) )

∫01 |F(Q,µ)|2 dµ

(2)

and

S ˜ (Q) ) 1 +

〈|F(Q,µ)|〉2 [SMM(Q) - 1] 〈|F(Q,µ)|2〉

(3)

The orientational-dependent form factor, F(Q,µ), is given by

F(Q,µ) ) f

3j1(v) 3j1(u) + (1 - f) u v

u ) Q[µ2a2 + (1 - µ2)b2]1/2 v ) Q[µ2(a + d)2 + (1 - µ2)(b + d)2]1/2

(4)

where µ takes into account the direction of the symmetry axis of the spheroid and the Q vector, J1(x) is a first-order Bessel function, b is the short axis of the ellipsoid, and the long principal axis is determined by equating the volume of the inner core NVm to the volume of the ellipsoid (4π/ 3)b2a. SMM(Q) has been calculated, as described by Liu et al.,26 by solving the Ornstein-Zernicke equation for the pair correlation function within the NAR-MMSA closure that yields analytical solutions.26 The dimensionless number ×a6 depends on the scattering length densities, F, of the micelle as

f ) Vt (Fcore - Fshell)/(

∑bi - VmFs)

(5)

where Fs is the scattering length density of the solvent D2O and Vt ) (Vtail + NLVL′ - 26.9nC) is the volume of the surfactant-calixarene complex “tail”. This term is defined as the sum of the volume of the surfactant tail, Vtail, and the fraction of the volume of the calixarene molecule into the micelle core, NLVL′, NL being the number of calixarene molecules per surfactant headgroup. The term 26.9nC is the classical Tanford volume for a linear surfactant tail constituted of a number n of CH2 residues in Å3. The scattering length density of the micelle-calixarene complex core is defined as

Fcore ) 1/Vtail{(NC - nC)bC + [2(NC - nC) + 1]bH} + 1/VL′NLb′L (6) where NC and nC are the total number of carbon atoms in the surfactant tail and the number of carbon atoms of the surfactant tail in the shell, respectively. The contribution of the calixarene “hydrophobic” part is also included in


Langmuir, Vol. 16, No. 1, 2000 191

Table 1. Molecular Volumes and Scattering Lengths Used for the Analysis of SANS Spectra of Cesium and Potassium Dodecyl Sulfate Micellar Solutions in the Presence of Calix[4]crown Ether Derivatives Cs+ (Å3)

V V ′hydrophilic. (Å3) V ′′hydrophobic (Å3) b (×10-4 Å) b′hydrophilic (×10-4 Å) b′′hydrophobic (×10-4 Å)

K+

-OSO3-

C12H25

25

10

57.9

360

0.542

0.367

2.607

-1.375

the term NLb′L, where b′L is the scattering length relative to the calixarene molecule “hydrophobic moiety”. The scattering density of the micellar outer layer is calculated as

Fshell ) N/Vshell [(Hbsolvent) + (1 - R)bion + b*head] (7) The presence of the calixarene molecules is also introduced in the term b*head, which is the scattering length of the surfactant polar headgroups, defined as

b*head ) bhead + (nCbC + 2nCbH) + NLb′′L

(8)

where b′′L is the scattering length of the calixarene molecule “hydrophilic part”. In eqs 1 and 5, ∑bi is the total surfactant scattering length, i.e., the sum of the surfactant hydrophobic tail and polar headgroup scattering lengths, the bound counterions, and the scattering lengths of the calixarene molecule. Finally, the volume of the surfactant monomer, Vm, is given by

Vm ) VM + NLVL - RVion

(9)

The term NLVL represents the volume fraction of the calixarene ligand per surfactant added to the total surfactant volume and RVion the volume of the free counterions fraction. The molecular volumes used in the fitting are given in Table 1. In eq 7, H is the hydration number accounting for the number of water molecules associated to the surfactant polar head defined as

H ) [(Vshell/N) - (Vm - Vt) - RVion+)]/Vsolvent

(10)

The absolute values of I(Q) have been calculated according to the above model and compared to the experimental data. Four fitting parameters have been used: the average aggregation number (N), the effective charge (Z), the short axis (b), and the shell thickness (d). The most important parameters describing the effect of calixarenes addition to the micellar solution are the average aggregation number and the ionization factor, defined as the ratio of the effective micellar charge to the average aggregation number, R ) Z/N. Small-angle neutron-scattering measurements have been performed on four sets of samples, at 1% or 4% (w/w) surfactant concentration, with the calixarene concentration added to dodecyl sulfate surfactant micellar solutions up to 15% (moles of calixarene/moles of surfactant), see Table 2. IV. Results and Discussion To analyze the SANS data of CESTO added to micellar solutions of LDS, a two-layer model for the intraparticle structure factor was used by Liuet et al.7-9,26 CESTO is a macrocyclic cage that complexes Li+ ions with high selectivity.7-9 The authors show that the two-shell model correctly describes the CESTO-LDS system giving quan-

C8CAL-6

C3CAL-6

C3CAL-5

1200 526 694 12.859 8.215 4.644

950 526 424 13.694 8.215 5.479

850 426 424 13.280 7.801 5.479

Table 2. Composition of the Four Sets of Samples Prepared for the SANS Experimentsa surfactant (% w/w)

C8CAL-6 (% mol)

C3CAL-6 (% mol)

CsDS (1%) CsDS (4%) KDS (1%)

0, 3, 5

0, 5, 8, 10, 15 0, 5, 10

C3CAL-5 (% mol)

0, 3, 5

a

Surfactant concentration in w/w. Calix-crown concentrations are in % moles of calix-crown/moles of surfactant.

titative information of 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 macrocycle is located at the micellar interface, that the average aggregation number significantly increases as the ligand is added to the surfactant solution, and that micelles grow becoming elongated ellipsoids. The formation of the CESTO/Li+ complex resulted in a consistent decrease of the effective surface charge. A similar model for the analysis of the SANS data has been used in this work. In analogy to the above-mentioned macrocycles, calix[4]arene-crown ether derivatives are able to complex counterions with high selectivity at the micellar interface. However, 1,3-alternate-calix[4]arenecrown-ethers are mostly insoluble in water and present both crown ethers and the aromatic calix ring functionalities. Therefore, they are expected to be located into the micellar core of the micelle exposing the hydrophilic crown ether ring to the water interface, as shown by molecular dynamics experiments at the water-chloroform interface.14-15 The two-shell model takes into account the possibility of different locations of 1,3-alternate-calix[4]arene-crown-ethers with respect to the micellar interface. The calix-crown derivatives studied in this work differ by the presence of two n-octyl chains and six oxyethylene groups in the crown ring (C8Cal-6) and two isopropyl chains and five (C3Cal-5) or six oxyethylene groups (C3Cal-6) in the crown ring. These differences account for the counterion specificity (Cs+ for C8Cal-6 and C3Cal-6 or K+ for C3Cal-5) and for the different location at the micelle polar headgroups region; i.e., the n-octyl chains of C8Cal-6 are inside the cesium dodecyl sulfate micellar core. The SANS experimental spectra obtained for the C8Cal6, C3Cal-6, and C3Cal-5 calix-crown added to cesium (C8Cal-6 and C3Cal-6) and potassium (C3Cal-5) dodecyl sulfate micellar solutions are shown in Figures 2, 3, 4, and 5. Symbols are the experimental data points, and the continuous lines are the spectra calculated according to the two-shell model. Figure 7 shows as an example the P(Q) and S(Q) function extracted from SANS analysis for the Cs/C3CAL-6 system at different C3CAL-6 concentrations. All the surfactant concentrations are in % w/w; the concentrations for the mixtures calixanene/surfactant are reported as moles of calixarene/moles of surfactant. The parameters obtained from the analysis of the SANS spectra are reported in Tables 3-6. A preliminary analysis of the tables shows that the calixcrown derivative addition to cesium dodecyl sulfate micellar solutions leads to a generalized increase of the

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Capuzzi et al.

Figure 2. Small angle neutron scattering spectra obtained from 1% CsDS micellar solutions (w/w surfactant/D2O) with the 1,3-dioctyloxy-calix[4]arene-crown-6 (C8Cal-6). The experimental (symbols) and theoretical (continuous line) spectra are shown. Calixarene concentrations are in % calixarene moles versus surfactant moles.

Figure 4. Small angle neutron scattering spectra obtained from 4% CsDS micellar solutions (w/w surfactant/D2O) with the 1,3-diisopropyloxy-calix[4]arene-crown-6 (C3Cal-6). The experimental (symbols) and theoretical (continuous line) spectra are shown. Calixarene concentrations are in % calixarene moles versus surfactant moles.



micelle aggregation number, a consistent decrease of the micelle ionization, and a micellar grow toward elongated ellipsoids. However, these effects are a function of the calix-crown derivative nature and of the surfactant concentration. As already mentioned above, the calix-crown derivatives are very insoluble in water. We assumed their location at the interface of the dodecyl sulfate micelle, and in particular we consider that the alkyl chains (isopropyl or n-octyl) of the calix-crown are located in the micellar core; the contrast used in the fitting procedure has been calculated accordingly. However, we allow calix-crowns to move toward the interface or the micelle core. The number of carbon atoms in the shell (see tables) is the parameter accounting for this displacement with respect to the micellar interface. A nonzero value for this parameter indicates that the calix-crown opens up the headgroup region of the micelle, allowing a change of the scattering length densities. For all the systems investigated this parameter is close to zero (a slight increase is present for the system C3Cal-6/CsDS) suggesting that the lower rim of the calix-crowns is effectively located in the

Figure 6. Schematic drawing of the location of the calix[4]arene-crown ethers (X ) 1 or 2, and R ) isopropyl or n-octyl) at the micelle interface, as deduced from SANS spectra analysis.

micelle core, while the polar crown moiety is at the micellar interface (see Figure 6). The C3Cal-6 and C8Cal-6 calix-crowns show high specificity for complexing the Cs+ ions, while C3Cal-5 is specific for K+ ions, as demonstrated by NMR21,22 and monolayer24,25 studies. In particular, monolayer studies


Langmuir, Vol. 16, No. 1, 2000 193 Table 3. Parameters Obtained from the Analysis of the SANS Experimental Spectra by Using a Two-Shell Model and the NAR-MMSA Approximationa [C8CAL-6]/[CsDS] 0% (% mol) 3% (% mol) 5% (% mol) aggregation no. (N) ionization factor (R ) Z/N) carbon atoms in the shell shell thickness, d (Å) short axis, b (Å) axial ratio, a/b av diameter, D (Å) hydration no., H association

110 13.6% 0 5 18.0 1.6 52.5 7.5

154 9.0% 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%

a Surfactant concentration 1% w/w. Calix-crown concentrations are in % mole calix-crown/mole surfactant.

Table 4. Parameters Obtained from the Analysis of the SANS Experimental Spectra by Using a Two-Shell Model and the NAR-MMSA Approximationa [C3CAL-6]/[CsDS] aggregation no. (N) ionization factor (R ) Z/N) carbon atoms in the shell shell thickness, d (Å) short axis, b (Å) axial ratio, a/b av diameter, D (Å) hydration no., H association

0%

5%

8%

10%

15%

110 13.6% 0 5 18.0 1.6 52.5 7.5

127 8% 0.3 5.5 18.1 1.9 56.4 8.2 100%

130 3% 1.3 6.4 17.0 2.3 58.2 9.4 100%

168 1.6% 2.3 7.3 16.7 2.9 63.6 8.9 100%

408 2.4 7.9 16.1 8.3 86.9 8.6 100%

a Surfactant concentration 1% w/w. Calix-crown concentrations are in % moles of calix-crown/moles of surfactant.

Table 5. Parameters Obtained from the Analysis of the SANS Experimental Spectra by Using a Two-Shell Model and the NAR-MMSA Approximationa [C3CAL-6]/[CsDS] 0%(% mol) 5%(% mol) 10%(% mol) aggregation no. (N) ionization factor (R ) Z/N) carbon atoms in the shell shell thickness, d (Å) short axis, b (Å) axial ratio, a/b av. diameter, D (Å) hydration no., H association

158 7.6% 0 5.5 18.9 2.0 59.2 8.1

320 3% 0 8.6 18.7 4.2 80.5 12.1 100%

335 3% 0.1 11.3 17.6 5.3 88.6 18.2 100%


Figure 7. P(Q) and S(Q) functions deduced from the best fitting of the SANS spectra of (a) 1% CsDS micellar solution (w/w surfactant/D2O), (b) 1% CsDS micelles with C3CAL-6 at 5%, and (c) 1% CsDS micelles with C3CAL-6 at 15%. Calixarene concentrations are in % calixarene moles versus surfactant moles.

show that calixarenes complex at the water-air interface the cesium or potassium ions with a different mechanism. Cesium is complexed by calixarene with and almost complete screening of the charge (as demonstrated by the negligible increase in the surface area of the calixarene molecule upon the complexation) while potassium complexation occurs with a weak charge screening. Tables 3, 4, and 5 clearly show that the ionization consistently decreases as the C3Cal-6 or C8Cal-6 calix-crown concentration increases. This effect is stronger for C8Cal-6, which possess two n-octyl chains in the lower rim that enhance its hydrophobicity. The complexation of the micellar

counterions is associated to three main effects: (i) an almost total micellar charge screening, micelle charge is almost zero at concentrations above 10%; (ii) a consistent growth of the micelle that becomes elongated (i.e., the a/b ratio increases from 1.6 for pure CsDS micelle to 8.3 for C3Cal-6/CsDS ) 15%); (iii) a strong increase of the aggregation number. The increase of the surfactant concentration (see Table 5) enhances the micellar growth, the micelle aggregation number being about twice that of the CsDS 1% system. The potassium dodecyl sulfate/C3Cal-5 micellar system shows a completely different behavior. The SANS experimental spectra present the typical interaction peak shown by the charged CsDS or KDS micelles. However, the addition of the calixarene molecules to KDS produces a slight decrease of the peak intensity suggesting that the micellar charge is weakly affected by the counterion complexation. This is shown in Table 6 where the ionization decreases from 26% to 19% for a 5% C3Cal-5

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Table 6. Parameters Obtained from the Analysis of the SANS Experimental Spectra by Using a Two-Shell Model and the NAR-MMSA Approximationa [C3CAL-5]/[KDS] 0% (% mol) 3% (% mol) 5% (% mol) aggregation no. (N) ionization factor (R ) Z/N) carbon atoms in the shell shell thickness, d (Å) short axis, b (Å) axial ratio, a/b av. diameter, D (Å) hydration no., H association

87 26% 0 4.7 17.0 1.5 48.6 8.4

90 21% 0 4.7 17.8 1.5 49.5 8.4 100%

95 19% 0 4.7 18 1.5 51.0 8.2 100%


added to 1% KDS micellar solutions, and the aggregation number weakly increases from 87 to 95. The other parameters reported in the table clearly show that C3Cal-5 does not modify consistently the overall micellar picture, confirming the results obtained from monolayer studies.24 Conclusions Cesium and potassium dodecyl sulfate micellar solutions have been studied in the presence of three calixarenecrown compounds, the 1,3-dioctyloxy-calix[4]arene-crown6 (C8Cal-6), the 1,3-diisopropyloxy-calix[4]arene-crown-6 (C3Cal-6), and the 1,3-diisopropyloxy-calix[4]arene-crown5 (C3Cal-5), which able to complex in a selective way Cs+ (C8Cal-6 and C3Cal-6) and K+ (C3Cal-5), respectively. These calix-crown derivatives differ by the presence of two n-octyl chains and six oxyethylene groups in the crown ring (C8Cal-6) or two isopropyl chains and five (C3Cal-5) or six oxyethylene groups (C3Cal-6) in the crown ring. These differences account for the counterion specificity (Cs+ for C8Cal-6 and C3Cal-6 or K+ for C3Cal-5) and for

the different location at the micelle polar headgroup region; i.e., the n-octyl chains of C8Cal-6 are inside the cesium dodecyl sulfate micellar core. The cesium or potassium dodecyl sulfate micelles are modeled as two-shell ellipsoids that incorporate counterions, calix-crown ligands, ligand-counterion complexes, and the associated water molecules at the polar headgroups region of the surfactant. The surfactant dodecyl chains and part of the calix-crown ligands are located in the micelle hydrophobic core. Intermicellar correlations are calculated according to a multicomponent primitive model in a mean spherical approximation with a nonadditive diameter. We found that calix-crown derivative addition to cesium dodecyl sulfate micellar solutions leads to a generalized increase of the micelle aggregation number, a consistent decrease of the micelle ionization, and a micellar grow toward elongated ellipsoids. However, the addition of the calixarene molecules to KDS produces a slight decrease of the micellar charge. The ionization decreases from 26% to 19% for a 5% C3Cal-5 added to 1% KDS micellar solutions and the aggregation number weakly increases from 87 to 95, suggesting that C3Cal-5 does not modify the overall micellar picture. This different behavior is not fully understood, since SANS does indicate that C3Cal-5 is located in the same micellar region of Cs specific calix-crown ligands. Molecular dynamic experiments could provide valuable information on this behavior. Acknowledgment. The authors acknowledge the MURST, CNR, and the Center for Large Interface Systems (CSGI) for partial financial support. Acknowledgments are also due to the European Union for support via the “HCM-Access to large scale facilities”, Contract ERB FMGE CT 950060 (BENSC-HMI, Berlin) and Contract ERB CHGE CT 950043 (LLB, Saclay, France). LA9907580

Counterion Complexation by Calixarene Ligands in Cesium and

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