Anion Effects on Calixarene Monolayers: A Hofmeister Series Study

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Anion Effects on Calixarene Monolayers: A Hofmeister Series Study Barbara Lonetti,† Pierandrea Lo Nostro,*,† Barry W. Ninham,†,‡ and Piero Baglioni*,† Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy, and Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Canberra, Australia 0200 Received July 16, 2004. In Final Form: November 30, 2004

Due to their amphiphilic structure, calixarenes adsorb at the air/water interface and form stable Langmuir films. We have explored the effect of salts on calix[6]- and calix[8]arene spreading isotherms at the air/ water interface. A wide range of different potassium salts was used in the subphase: KCl, KI, KBr, KSCN, KNO3, CH3COOK, K2SO4, and K3PO4. The differences in Langmuir isotherms are due to the presence of different anions in the subphase, to the different conformations of the ligands at the interface, and to the different complexing affinities of calix[6]- and calix[8]arene for potassium ions. The two systems show a significant specific ion effect that can be discussed in terms of Hofmeister series. Characteristic monolayer parameters, e.g., limiting area (Alim), collapse pressure (πcoll), modulus of compressibility (Cs-1), and surface potential (∆V), are discussed in terms of some physicochemical parameters that reflect dispersion forces: in particular, anion polarizabilities, lyotropic number (N), molar surface tension increment (σ), and partial molar volume (νs).

Introduction Calixarenes are macrocyclic compounds obtained through condensation of a para-substituted phenol with formaldehyde. These basket-shaped macromolecules can act as hosts for ions and small neutral molecules.1-3 The capacity of calixarenes to complex metallic ions has been extensively studied. The selectivity and the efficiency of the binding process depend on ring size, on the nature of the attached groups, and on the stereochemical conformations of these versatile macrocyclic molecules.4-9 Because of their considerable conformational flexibility, calixarenes can rotate freely around σ bonds of Ar-CH2Ar groups and generate a large number of conformations. This is especially so for the larger rings. This conformational freedom is a key factor in the optimization of the particular host-guest interactions between calixarene ligands and their guests. In this work we focus on p-tert-butylcalix[6]arene (CAL6) and p-tert-butylcalix[8]arene (CAL8). Their structures are shown in Figure 1. These molecules exhibit large cavities * To whom correspondence should be addressed. E-mail: pln@ csgi.unifi.it (P.L.N.); [email protected] (P.B.). Fax: +39 055 457-3032. URL: http://www.csgi.unifi.it/. † University of Florence. ‡ Australian National University. (1) Gutsche, C. D. In Calixarenes; Stoddard, J. F., Ed.; The Royal Society of Chemistry: Cambridge, England, 1989. (2) Shinkai, S. Tetrahedron 1993, 49, 8933. (3) Bo¨hmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (4) Dei, L.; Lo Nostro, P.; Capuzzi, G.; Baglioni, P. Langmuir 1998, 14, 4143. (5) Capuzzi, G.; Fratini, E.; Dei, L.; Lo Nostro, P.; Casnati, A.; Gilles, R.; Baglioni, P. Colloids Surf., A 2000, 167, 105. (6) Dijkstra, P. J.; Brunink, J. A. J.; Bugge, K.-E.; Reinhoudt, D. N.; Harkema, S.; Ungaro, R.; Ugozzoli, F.; Ghidini, E. J. Am. Chem. Soc. 1989, 111, 7567. (7) Ikeda, A.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 3102. (8) Beer, P. D.; Drew, M. G. B.; Gale, P. A.; Leeson, P. B.; Ogden, M. I. J. Chem. Soc., Dalton Trans. 1994, 3479. (9) Arduini, A.; Fanni, S.; Manfredi, G.; Pochini, A.; Ungaro, R.; Ugozzoli, F. J. Org. Chem. 1995, 60, 1448.

Figure 1. Chemical formulas of CAL6 and CAL8.

and higher flexibility than do the p-tert-butylcalix[4]arenes. While calix[4]arene can exist in four conformations which differ in the relative positions of phenolic units (cone, partial cone, 1,2-alternate, 1,3-alternate),1 calix[6]arenes and calix[8]arenes possess eight and sixteen different updown conformations, respectively. In particular, CAL8 presents both pleated-loop and cone-shaped conformations. The former is preferred in the solid state as the -OH groups lie either above or below the ring plane, while the

10.1021/la048211v CCC: $30.25 © 2005 American Chemical Society Published on Web 02/11/2005

Anion Effects on Calixarene Monolayers

Figure 2. Schematic picture of CAL8 conformations reported in Table 2: (a) 1,3,5,7-alternate, (b) anti cone/cone, (c) syn cone/ cone.

latter maximizes the hydrogen-bonding interactions with the subphase in spreading monolayers (see Figure 2c). The complexation of alkali-metal cations by calixarenes has been extensively studied by Izatt and co-workers. By measuring the cation transport through a chloroform phase that separates two aqueous compartments, they found that the transport ability through the organic hydrophobic layer is efficient only in the case of strongly basic solutions, and inhibited at lower pH.1,10,11 In particular, the potassium flux through the membrane is 0.7 × 108, 13 × 108, and 10 × 108 s-1 m-2 for CAL4, CAL6, and CAL8, respectively. These findings brought Izatt to conclude that potassium-calixarene complexes are neutral and obtained through the deprotonation of the ligand molecule. On the other hand, crown ethers are more effective than calixarenes in transporting alkali-metal cations at neutral pH.10 In these cases, the anions must accompany the positively charged complexes through the membrane. In another paper, Izatt reported the binding constants for K-CAL6 and K-CAL8 complexes, obtained from experiments on extration in THF from picrates. The values are log K ) 4.13 for CAL6 and 3.11 for CAL8, indicating that K+ interacts more strongly with the cavity of the hexamer ligand than with that of the octamer.12 McKervey and co-workers accumulated several data on the extraction capabilities of carboalkoxymethyl esters of calixarenes (where the phenolic OH group is replaced by a polar but aprotic OCH2COOR residue).13 They found similar trends that show the optimal affinity of calix[6]arene derivatives for K+ with respect to calix[4]- and calix[8]arene, even when the deprotonation of the ligand is not possible. On the other hand, Davis has demonstrated that a CAL8 derivative with a longer hydrophobic chain does not bind potassium ions.14 (10) Izatt, R. M.; Lamb, J. D.; Hawkins, R. T.; Brown, P. R.; Izatt, S. R.; Christensen, J. J. J. Am. Chem. Soc. 1983, 105, 1782. (11) Izatt, S. R.; Hawkins, R. T.; Christensen, J. J.; Izatt, R. M. J. Am. Chem. Soc. 1985, 107, 63. (12) Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. Rev. 1991, 91, 1721. (13) McKervey, M. A.; Seward, E. M.; Ferguson, G.; Ruhl, B.; Harris, S. J. J. Chem. Soc., Chem. Commun. 1985, 388. (14) Davis, F.; O′Toole, L.; Short, R.; Stirling, C. J. M. Langmuir 1996, 12, 1892.

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Hanna15 and Murayama16 concluded that, especially in the case of K+, cation-π interactions are established between the metal ion and the coordinating aromatic moieties. It is interesting to note that, in the case of CsCAL8 complexes, the complexation leads to relevant perturbations of the calix shape.1 When the cation binding occurs at the air/water interface, and there is no lipophilic membrane to be crossed, the behavior can be substantially different. As pointed out in our previous studies on the monolayers of CAL6 in the presence of different subphases (pure water and chlorides of different alkali-metal cations),17 the higher the monolayer expansion with respect to pure water, the stronger the complexation of the cation by the ligand. In the same work we observed that spreading isotherms are not affected by pH between 0 and 7, and we concluded that K-CAL6 complexes are positively charged at the air/water interface. Moreover, surface potential measurements and limiting area values indicated that the conformation of the complexed species (parallel versus perpendicular) does depend on the nature of the metal cation. The calculation of the Gibbs free energy of spreading change (∆Gspread) for the different cations confirmed the evidence of the destabilization of the monolayer compactness upon complexation, due to electrostatic repulsions. The effect of complexation on the conformational state of calixarenes and that of electrostatic repulsions have been discussed by Shinkai for monolayers obtained from ester derivatives of calixarenes, in the presence of Li+, Na+, K+, and Rb+ ions.18 Despite the fact that the complexation properties of calixarenes at the air/water interface have been extensively studied, as a function of ring size and of the coordinated cation,17,19,20 the effects induced by counterions in the aqueous subphase still need further investigation. An indication of some salt effects on monolayers was given by Coleman et al.21 They observed that the properties of monolayers formed by p-dodecanoylcalix[4]arene at the air/water interface do depend on the kind of salt in the aqueous solution. Ion specificity is involved in many interfacial phenomena, such as salting-out of proteins,22-24 bubble-bubble interactions,25 at the air/water and oil/water interfaces, direct measurement of molecular forces between interfaces,26 conformations of polyelectrolytes,27 floc dimensions of hydrophobic particles in salt solutions,28 formation of (15) Hanna, T. A.; Liu, L.; Angelez-Boza, A. M.; Kou, X.; Gutsche, C. D.; Ejsmont, K.; Watson, W. H.; Zakharov, L. N.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 6228. (16) Murayama, K.; Aoki, K. Inorg. Chim. Acta 1998, 281, 36. (17) Dei, L.; Casnati, A.; Lo Nostro, P.; Baglioni, P. Langmuir 1995, 11, 1268. (18) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736. (19) Davis, F.; O’Toole, L.; Short, R.; Stirling, C. J. M. Langmuir 1996, 12, 1892. (20) Ludwig, R.; Matsumoto, H.; Takeshita, M.; Ueda, K.; Shinkai, S. Supramol. Chem. 1995, 4, 319. (21) Shahgaldian, P.; Coleman, A. W. Langmuir 2001, 17, 6851. (22) Baldwin, R. L. Biophys. J. 1996, 71, 2056. (23) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18, 323 (24) Hincha, D. K. Arch. Biochem. Biophys. 1998, 358 (2), 385. (25) Craig, V. S. Y.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192. (26) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F.; Brady, J. J. Phys. Chem. 1986, 90, 1637. (27) Tomas, S.; Sarkar, M.; Ratilainen, T.; Wittung, P.; Nielsen, P.; Norde´n, B.; Gra¨slund, A. J. Am. Chem. Soc. 1996, 118, 5544. (28) Yaminsky, V. V.; Pchelin, V. A. Dokl. Akad. Nauk SSSR 1973, 310, 154.

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polypseudorotaxanes,29 hydrophobic chromatography,30 water absorbency by wool fibers,31 and transitions in surfactant condensed phases.32 However, the same trends are found also in bulk aqueous solution properties such as osmotic coefficients, partial molar volumes, activity coefficients, freezing point lowering, viscosity, and so forth.33 Consistently, all phenomena show regular and repeated trends for a series of anions or cations. This indicates that a dominant factor behind the phenomena is related to some specific inherent physicochemical properties of the ionic species. Since the first studies conducted in the late 19th century, such empirical ionic sequences have always been referred to as “Hofmeister series”. Despite the wide range of phenomena where Hofmeister effects take place, a self-consistent theoretical frame for its explanation is still debated.34,35 But it appears evident that ionic polarizability and ionization potentials which control the dispersion forces experienced by ions in solution and their hydrationsare the key factors in the microscopic description of these phenomena.36 We use the term “dispersion forces” for brevity, and mean the totality of electrodynamic fluctuation forces besides nonspecific electrostatic forces acting on ions. These determine specific ion adsorption, and conspire with electrostatics to determine hydration. All frequency ranges (infrared, microwave optical, UV, ...) contribute. That they can be expected to show strong specificity can be seen by considering, e.g., that, in the visible frequency range, dispersion forces depend on ionic polarizabilities and ionization potentials. Anions are richer in electrons and have a greater variation in excess polarizabilities and hence dispersion forces than do cations. In this study the effect of different anions on the monolayer properties of CAL6 and CAL8 at the air/water interface was investigated by recording the π/A and ∆V/A isotherms in the presence of different aqueous subphases. These are pure water, KCl, KBr, KI, KSCN, KNO3, CH3COOK, K2SO4, and K3PO4, all at 0.5 M concentration and at 20 °C. For each case, the limiting area, the compressibility modulus, the collapse pressure, and the surface potential at maximum packing values were determined. Materials and Methods CAL6 and CAL8 were purchased from Acros Organics (Milan, Italy). CAL6 was purified as described in the literature.37 The salts KCl, KBr, KI, KSCN, KNO3, CH3COOK, K2SO4, and K3PO4 were supplied by Aldrich (Milan, Italy); CHCl3 (99.8% grade) and CCl4 (99.9% grade) were purchased from Fluka (Milan, Italy). Solutions of CAL6 and CAL8 were prepared by dissolving about 4 mg of compound in 10 mL of CHCl3 or a CHCl3/CCl4 (1:4) mixture, respectively, and were stored at 4 °C. A 100 µL sample of calixarene solution was spread on the aqueous subphase by using a 100 µL pressure lock microsyringe (Hamilton). Nine series of measurements were performed for each calixarene using different subphases: bidistilled water, KCl, KBr, KI, KSCN, KNO3, CH3COOK, K2SO4, K3PO4 (0.5 M). Bidistilled (29) Lo Nostro, P. J.; Lopes, R.; Ninham, B. W.; Baglioni, P. J. Phys. Chem. B 2002, 106 (9), 2166. (30) Hirayama, N.; Umehara, W.; Makizawa, H.; Honjo, T. Anal. Chim. Acta 2000, 409, 17. (31) Lo Nostro, P.; Fratoni, L.; Ninham, B. W.; Baglioni, P. Biomacromolecules 2002, 3, 1217. (32) Lo Nostro, P.; Ninham, B. W.; Ambrosi, M.; Fratoni, L.; Palma, S.; Allemandi, D.; Baglioni, P. Langmuir 2003, 19, 9583. (33) Zavitsas, A. A. J. Phys. Chem. B 2001, 105, 7805. (34) Ninham, B. W.; Yaminsky, V. V. Langmuir 1997, 13, 2097. (35) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 1. (36) Tavares, F. W.; Bratko, D.; Blanch, H. W.; Prausnitz, J. M. J. Phys. Chem. B 2004, 108, 9228. (37) Gutsche, C. D.; Iqbal, M. Org. Synth. J. D. Rd. 1989, 68, 238.

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Figure 3. Spreading isotherms of CAL6 at the air/water interface for different subphases. All measurements were performed at 20 °C.

Figure 4. Spreading isotherms of CAL8 at the air/water interface for different subphases. All measurements were performed at 20 °C. water was obtained with a Millipore Milli-Q system (Organex) and had a resistance greater than 18 MΩ‚cm. We used a computer-controlled Lauda Filmwaage balance to perform surface pressure measurements (π) as a function of molecular area (A) at 20 °C. The temperature was controlled within (0.5 °C by a Haake PG 40 thermostat. The accuracy of π and A measurements was (0.1 mN/m and (1 Å2/molecule, respectively. The compression was carried out with discontinuous steps of a Teflon barrier, each step being about 0.6 cm. After each compression step, the monolayer was allowed to reach equilibrium. Before spreading of the film, the surface of the subphase was cleaned and its purity was checked in the whole area interval (π < 0.1 mN/m).38 Surface potential measurements were carried out by using two 241Am electrodes (1.04 cm2, 200 µCi) with an accuracy of 25 mV, following the procedure described elsewhere.39

Results and Discussion CAL6 and CAL8 Complex Potassium Ions.17,40 Figures 3 and 4 show the spreading isotherms of CAL6 and CAL8 at the air/aqueous solution interface at 20 °C, respectively. Table 1 lists the corresponding characteristic parameters. There is a significant ion effect on spreading isotherms, both on the position and on the shape of the curves. The limiting area of the films increases upon addition of salt in the subphase. This behavior can be explained in terms of electrostatic repulsion between the positive, K+-complexed headgroups. However, the extent of such na increment does depend on the type of anion. (38) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; Chapter 5. (39) Jaycock, M. J.; Parfitt, G. D. Chemistry of Interfaces; Ellis Horwood Ltd.: Chichester, U.K., 1981; pp 60-84. (40) Shi, X. F. Y.; Wang, P.; Liu, Y. S. Chem. J. Chin. Univ. 1999, 20, 193.

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Table 1. πcoll (mN/m), Alim (Å2/molecule), Surface Molecular Orientation, Maximum Value of Compressibility Modulus (Cs-1) (mN/m), and Average Surface Potential at Maximum Packing (∆V) (mV) Values for CAL6 and CAL8 Monolayers at 20 °C for Different Subphases CAL6

CAL8

subphase

πcoll

Alim

orientation

Cs-1

∆V

πcoll

Alim

orientation

Cs-1

∆V

water chloride bromide iodide thiocyanate nitrate acetate sulfate phosphate

56.6 51.5 52.6 54.0 56.0 51.8 54.7 59.5 59.0

170 172 184 175 180 178 184 198 192

⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥

138 134 96 233 184 135 129 95 170

170 325 313 240 320 280 300 300 185

47.6 59.0 58.3 55.8 56.0 58.0 55.3 57.3 58.0

95 127 156 262 270 171 264 162 302

⊥ ⊥ ⊥ | | ⊥ | ⊥ |

115 105 82 137 77 90 107 87 123

105 170 370 298 190 180 385 320 150

Table 2. Typical Conformations of p-tert-Butylcalix[6]arene and p-tert-Butylcalix[8]arene Referred to Two Different Orientations at the Interface: A| (Å2/molecule) with the Macrocyclic Ring Parallel to the Interface and A⊥ (Å2/molecule) with the Macrocyclic Ring Perpendicular to the Interface CAL6 CAL8

conformation

A|

A⊥

pleated loop anti cone/cone syn cone/cone 1,3,5,7 -alternate

250-260 322 244 273 183

170-180 140 201

In the case of CAL6, the limiting area (Alim) values span from 170 Å2/molecule for pure water to 198 Å2/molecule in the presence of sulfate. Instead, in the case of CAL8, Alim values range between 95 Å2/molecule (pure water) and 302 Å2/molecule for phosphate. In both cases, SO42- and PO43- anions produce Alim values of 198 and 192 Å2/molecule for CAL6 and 162 and 302 Å2/molecule for CAL8, respectively. This is to be expected as, at 0.5 M, sulfate and phosphate carry more potassium ions in solution than the other salts do. Mass action then dictates that calyxarene-K+ binding is enhanced: surface charge and area are therefore increased. But this, as it will be seen, is not the whole story. Further, acetate and phosphate take part in a hydrolysis reaction in solution, with a significant pH change that can also affect the coordination process of CAL hosts with potassium. The pH measured with a glass electrode in bulk was 8.8 for acetate and 12 for phosphate. This can alter the charge of the phenolic groups in the ligands. The Corey-Pauling-Koltun (CPK) space-filling molecular models are useful in envisaging conformational and geometrical properties (volume, cross section, and cavity dimension) of supramolecular systems. Table 2 shows the CPK values relative to different conformations and orientations at an interface of CAL6 or CAL8. By comparing the experimental values shown in Table 1 for Alim to the theoretical calculations reported in Table 2 for parallel and perpendicular arrangements (A| and A⊥), we can deduce that CAL6 always possesses a perpendicular orientation with respect to the air/water interface, regardless of the subphase composition. This is in agreement with previous reports in the literature, indicating that when the available area is very large compared to the calixarene dimensions, the macrocyclic molecule lies parallel to the subphase. However, during monolayer compression the available surface is decreased, the parallel orientation is progressively lost, and a perpendicular alignment, stabilized by intermolecular hydrogen bonds between OH groups of facing adjacent molecules, is favored (see Figure 5).17 CAL8 is a more complicated system, due to the high flexibility of its larger ring. As already reported in the

literature,4,41 CAL8 adopts a pleated loop conformation with a perpendicular alignment when the ligand is spread on pure water. The presence of salts induces an increase in the limiting area values due to the formation of ioncalixarene complexes.17,41 The Alim values for Cl-, Br-, and NO3- can be ascribed to a pleated-loop conformation with a perpendicular alignment (see Tables 1 and 2). The larger values obtained for SCN-, CH3COO-, and I- may indicate a change of both conformation and orientation: in fact, CAL8 can assume a parallel orientation and a cone/cone conformation at the air/water interface. The collapse pressure (πcoll) for CAL6 monolayers decreases in the presence of salts, indicating a destabilization of the monolayer. On the other hand, CAL8 monolayers show an increase of the collapse pressure in the presence of salts, which reflects a stabilization of the monolayer. This effective reversal of the Hofmeister sequence is surprising and provides a clue to the process. In Table 1 we list the maximum value of the compressibility modulus Cs-1, referred to the highest packing of each monolayer. The compressibility modulus (mN/m) is defined as Cs-1 ) -A(∂π/∂A)T. Its value can be used to characterize the phase state of the monolayer and to track the phase transitions during isothermal compression. Cs-1 ranges approximately between 1000 and 2000 mN/m for solid-condensed films, between 100 and 250 for liquidcondensed monolayers, and between 10 and 50 for liquidexpanded phases and is about π for ideal surface films.38 In the case of CAL6, Cs-1 values are typical of a liquidcondensed phase regardless of the ions dissolved in the subphase (see Table 1). When the subphase contains KBr, KSCN, and KNO3, the CAL6 isotherms show an inflection point that cannot be attributed to a collapse phenomenon (see Figure 3), as the time dependence of the surface pressure is typical of a stable monolayer. In the case of KSCN and KNO3 this inflection can be ascribed either to a conformational change of the molecules in the monolayer or to a phase transition from a liquid-condensed to a liquid-expanded phase as indicated by the decrease of Cs-1 from 135 to 90 mN/m in the case of KNO3 and from 184 to 77 mN/m for KSCN. In the case of KBr, the inflection is not due to such a phase transition, as the difference in Cs-1 (from 96 to 82 mN/m) indicates that the system remains in a liquidexpanded phase. Rather, it can be related to an orientational change accompanied by the formation of a bilayer. In fact, the area value after the transition is 131 Å2/molecule, which is about half the area predicted by the CPK model in the case of a parallel orientation for the p-tert-butylcalix[6]arene (see Table 2). The existence of such a bilayer implies very strong Br- adsorption-binding. (41) Lo Nostro, P.; Casnati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf., A 1996, 116, 203.

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Figure 5. Perpendicular (⊥) and parallel (|) orientation of CAL6 at the water-air interface.

Evidently, electrostatic interactions in the subphase are irrelevant in determining that adsorption. Monolayers formed by CAL8 have Cs-1 values typical of a liquid-condensed phase (see Table 1). In the presence of Cl-, SCN-, NO3-, and SO42- an inflection point again appears. This is associated with a decrease in Cs-1. It can be explained by invoking a phase transition from a liquidcondensed to a liquid-expanded phase. Note that the inflections and the decreased values of Cs-1 are less marked than for CAL6. Hofmeister Correlations. In an attempt to bring some kind of rationalization to these anion effects, we now explore correlations between variations in the monolayer parameters and some physicochemical parameters that are standard fingerprints of Hofmeister effects generally. These are the lyotropic number (N), the molar surface tension increment (σ), the partial molar volume (νs), and the anion polarizability in solution (R). Their values are obtained from the literature and reported in Table 3.29,31 The lyotropic number N is an empirical parameter introduced by Voet,42 related to the swelling effect of salts with starch. Like other quantities (such as the Setschenow constant),43 it is a typical parameter considered as the fingerprint of Hofmeister-related phenomena. Sometimes ions are defined as “kosmotropic” or “chaotropic” depending on their ability to stabilize or break the structure of water, reflecting the effect of a single species alone on the water structure, rather than pinning down the properties and/ or the mechanism through which the anion plays its role in the phenomena. Kosmotropes possess low N values, and chaotropes large lyotropic numbers. However, this distinction between structure makers and structure breakers is still debated. The partial molar volume (νs) is obtained from the literature.44 The molar surface tension increment is defined as σ ) ∂(∆γ)/∂c. Surface tension increments or polarizabilities in solutions are much affected by often

large experimental uncertainties. The literature provides several values for surface tension increments, with large discrepancies for the same anion. Ionic polarizabilities of gaseous species are very different from the values for crystalline lattices and electrolyte-diluted solutions.45 However, all these parameters reflect ion specificity due to many-body electrodynamic dispersion forces, missing from theories based on electrostatics alone. Dispersion self free energies and dispersion interaction (adsorption energies) are coupled. Both ultimately depend on ionic polarizabilities and ionization potentials. We have already studied different phenomena (formation of pseudopolyrotaxanes from β-cyclodextrin29 and coagels from ascorbic acid esters32 and water absorbency of wool fibers31). A number of other studies show how these physicochemical parameters have a crucial role in Hofmeister effects.35,46,47 It is worth stressing that Hofmeister series can change dramatically from one (interfacial) system to another. The trend of effects induced by a series of ions can be completely or partially reversed, depending on the specific interface that is studied. E.g., the magnitude of interfacial tensionss reflecting usually negative adsorptionsby ion pairs at the water/air interface gives one Hofmeister series. The series can be just the opposite of that found for an oil/water surface that shows positive adsorption for, e.g., KBr. A key factor involved that determines the sign of ion pair adsorption is the difference between the refractive indices (i.e., molecular polarizabilities) of the two phases (see ref 34 and references therein). In the present study the condensed Langmuir monolayer is essentially a triple film, formed from a thin penetrable oil-like film (calixarene) on a water/air interface. The film has strong binding affinity for potassium, equal to essentially the change in the sum of Born electrostatic and dispersion free energies of transfer from water to the hydrocarbon-like calixarene interior. At the high concentration of electrolyte, 0.5 M for our work, electrostatic interactions in the subphase are almost completely screened. Cations and anions in the bulk solution “see” a self-consistent adsorption potential dominated by dispersion interactions. In general, these include temperature-dependent many-body dipolar effects.34 At large distance the ions see an air/water interface. At close distance they interact with effectively an oil-like surface. Moreover, during the monolayer compression the air/water interface is progressively covered with the calixarene monolayer, and so replaced effectively by an oil/water interface. This also changes effective image charges that ions experience in the gaseous or in the hydrophobic phase change. The dispersion potential felt by an ion at a distance x from an interface,

(42) Voet, A. Chem. Rev. 1937, 20 (2), 169. (43) Cacace, M. G.; Landau, E. M.; Ramsden, J. J. Q. Rev. Biophys. 1997, 30, 241. (44) Millero, F. J. In Water and aqueous solutions; Horne, R. A., Ed.; Wiley-Interscience: New York, 1972; pp 565-595.

(45) Jungwirth, P.; Tobias, D. J. J. Phys. Chem. B 2002, 106, 6361. (46) Bostrom, M.; Williams D. R.; Ninham, B. W Eur. Phys. J. E 2004, 13, 239. (47) Bostrom M.; Williams D. R.; Stewart P. R.; Ninham B. W. Phys. Rev. E 2003, 68, 041902.

Table 3. Lyotropic Number (N), Molar Surface Tension Increment (σ, mN‚L/(m‚mol)), Anion Polarizability (r, Å3), and Partial Molar Volume (νs, cm3/mol) for the Different Anions subphase

N

σ

R

νs

chloride bromide iodide thiocyanate nitrate acetate sulfate phosphate

10.0 11.3 12.5 13.2 11.6

1.63 1.31 1.02 0.54 1.18 0.93 2.74

3.76 5.07 7.41 6.74 4.47 5.50 6.32

26.85 33.73 45.1 47 38.02 48 32

2.0 3.2

Anion Effects on Calixarene Monolayers

Figure 6. πcoll for CAL6 (b) and CAL8 (0) as a function of the lyotropic number N.

Figure 7. Alim for CAL6 (b) and CAL8 (0) as a function of the partial molar volume νs.

schematically Udispersion(x) ≈ -[(nwater2 - nsubstrate2)RI*(0)hωi]/ [16πx3], changes sign according to whether the refractive index of the substrate is larger or smaller than that of water. That expectation seems to be borne out for the case of CAL6. It is very interesting to note here that the least hydrophilic anions produce the highest effect, both on the collapse pressure and on the limiting area (see Table 1). The presence of cations in the subphase causes a decrease in πcoll. This implies monolayer destabilization because of complexation. The intriguing effect is that the less hydrophilic, i.e., chaotropic, the anion, the less destabilized the monolayer. This, because of the oil-like nature of the monolayer interface formed by calixarene molecules, appears to be a consequence of positive anionic adsorption at the interface, just as for the oil/water interface discussed above. The trend of the limiting area values is also in accord with the anion adsorption. It is smallest for Cl-. In the case of the less hydrophilic anions they appear indeed to adsorb so strongly that they penetrate into the monolayer interface and cause a steric rearrangement. This results in higher limiting area values in the monolayer. Figures 6-10 show the plots of Alim and πcoll as a function of Hofmeister parameters. With the exception of acetate and sulfate, all plots indicate a consistent trend. The acetate ion is more effective than SCN- probably because of its basic properties. In general, πcoll for CAL6 increases for chaotropic anions, while Alim remains almost constant. For CAL8 the results

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Figure 8. Alim for CAL6 (b) and CAL8 (0) as a function of the lyotropic number N.

Figure 9. πcoll for CAL6 (b) and CAL8 (0) as a function of the partial molar volume νs.

Figure 10. Alim for CAL6 (b) and CAL8 (0) as a function of the molar surface tension increment σ.

are influenced by the anions according to the Hofmeister trend: the higher the lyotropic number (N), and the partial molar volume (νs), the lower the πcoll and the higher the limiting area increase. The πcoll trend may be due to the fact that different anions in the subphase induce different orientations of CAL8 molecules at the interface during the collapse. We recall again that the Hofmeister effects are more or less reversed for CAL8 as compared with CAL6. This makes sense only if dispersion potentials

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Figure 11. Molecular area A for CAL6 as a function of the anion polarizability R at different surface pressures: 0.5 (b), 5 (0), 20 ([), and 40 (2) mN/m.

Figure 12. Molecular area A for CAL8 as a function of the anion polarizability R at different surface pressures: 0.5 (b), 5 (0), 20 ([), and 40 (2) mN/m.

Table 4. Molecular Area Values (Å2/molecule) and Orientation for CAL6 at 20 °C in the Presence of Different Anions in the Subphase and at Different Surface Pressures

Table 5. Molecular Area Values (Å2/molecule) and Orientation for CAL8 at 20 °C in the Presence of Different Anions in the Subphase and at Different Surface Pressures

π) orienπ) orienπ) orien0.5 mN/m tation 5 mN/m tation 20 mN/m tation water chloride bromide iodide thiocyanate nitrate acetate sulfate phosphate

192 193 233 184 188 189 195 205 222

⊥, | ⊥, | ⊥, | ⊥, | ⊥, | ⊥, | ⊥, | ⊥, | ⊥, |

167 170 182 169 175 173 178 192 193

⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥

144 148 145 157 157 151 155 171 170

⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥ ⊥

acting on anions are involved. But more than that is implicated. The two monolayer interfaces, CAL6 and CAL8, display different “oil-like” properties (e.g., thickness, dielectric properties) that affect the adsorption potentials acting on anions. These depend self-consistently on the film thickness, i.e., steric rearrangements, themselves caused by the anion adsorption. It seems and must be stressed that Cl-, Br-, and NO3-, which show a lower affinity for the interface, do not cause a change of orientation of CAL8 molecules with respect to pure water. In contrast, SCN- and I-, both of which adsorb strongly at the air/water interface,45 do produce differently shaped spreading isotherms and, in the more condensed phase, different orientations at the interface. In the case of CAL6, it is interesting to plot the molecular area at different surface pressures π (from 0.5 to 50 mN/ m) extrapolated from the spreading isotherms as a function of the anion polarizability (Figure 11 and Table 4). We notice an inversion of the trend. In fact, below 2 mN/m the sequence in areas is Cl- > NO3- > I- > SCN-. Above this π value it is Cl- < NO3- < I- < SCN -. An analogous change can be found in the trend relative to the other Hofmeister parameters (figure not shown). This effect can be ascribed to the fact that at low pressure the anions see an air/water interface and at higher pressure a triple air/ oil/water film. In the case of CAL8 (see Figure 12 and Table 5) the trend of A as a function of R at constant π reflects the destabilization of the monolayer due to the anions, and particularly to those with higher polarizability (I- and SCN-). Table 1 reports the average values of the surface potential (∆V) for the monolayer at maximum packing.

π) orienπ) orienπ) orien0.5 mN/m tation 5 mN/m tation 20 mN/m tation water chloride bromide iodide thiocyanate nitrate acetate sulfate phosphate

126 155 271 274 295 193 279 184 344

⊥, | ⊥ | | | | | | |

97 127 177 255 255 163 255 159 299

⊥ ⊥ ⊥, | | | ⊥ | ⊥ |

79 105 123 225 205 136 218 129 256

⊥ ⊥ ⊥ ⊥, | ⊥, | ⊥ ⊥, | ⊥ |

∆V values are similar for CAL6 and CAL8. For CAL6, ∆V decreases slightly with the polarizability of the anion; this effect is presumably due to the selective adsorption of the anions at the monolayer interface that partially reduces ∆V. In the case of CAL8 the values are more scattered. The different qualitative trends for CAL6 and CAL8 must be related to the different complexation affinities of the ligand for potassium, and to the different conformations of the calix molecules at the interface. These two effects can operate in opposite ways and produce different behaviors. Conclusions Spreading monolayers of CAL6 and CAL8 at the air/ water interface in the presence of different potassium salts in the subphase indicate that (1) for CAL6, the limiting area changes slightly with the anion, while πcoll increases from the most hydrophilic (hardest) ions such as chloride to the most hydrophobic (softest) such as iodide and thiocyanate, and (2) for CAL8, Alim changes a great deal with the type of anion, and increases with the most hydrophobic (softest) ions, because they preferably adsorb at the interface. π decreases largely in going from the most hydrophobic ions that destabilize the monolayer packing. CAL6 produces more rigid and compact films than CAL8, because CAL6 possesses a narrower and less flexible ring. CAL8 produces more flexible and open conformations (loop) at the surface. This fact determines the closer proximity of the aromatic organic rings to the water subphase. The ions “feel” the presence of a hydrocarbon/water interface at close separations, and therefore

Anion Effects on Calixarene Monolayers

react in the opposite manner with respect to the air/water interface. This difference appears to explain the opposite trends recorded in all πcollapse plots against the different (Hofmeister) physicochemical parameters. The specific Hofmeister anion effects cannot be understood in terms of the usual electrostatic description of interactions, or of water structure alone. These results convincingly support a relevant role of dispersion forces in the observed phenomena. Rearrangements of the monolayers as a function of anion and calixarene and sometimes even

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reversal of the Hofmeister series as a function of pressure for the same calixarene all seem to make sense. Taken together, the results we believe demonstrate convincingly that the role of anions in calixarene chemistry is a matter of some importance. Acknowledgment. We are grateful to CSGI and to MIUR (PRIN 2003) for partial financial support. LA048211V