Aqueous Nonionic Copolymer-Functionalized Laponite Clay. A

Aug 9, 2006 - The affinity of functionalized Laponite clay toward an organic material in the aqueous phase was explored. Functionalization was perform...
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Langmuir 2006, 22, 8056-8062

Aqueous Nonionic Copolymer-Functionalized Laponite Clay. A Thermodynamic and Spectrophotometric Study To Characterize Its Behavior toward an Organic Material R. De Lisi, G. Lazzara, S. Milioto,* and N. Muratore Dipartimento di Chimica Fisica “F. Accascina”, UniVersita` degli Studi di Palermo, Viale delle Scienze, Parco D’Orleans II, 90128 Palermo, Italy ReceiVed April 21, 2006. In Final Form: July 4, 2006 The affinity of functionalized Laponite clay toward an organic material in the aqueous phase was explored. Functionalization was performed by using triblock copolymers based on ethylene oxide (EO) and propylene oxide (PO) units that are EO11PO16EO11 (L35) and PO8EO23PO8 (10R5). Phenol (PhOH) was chosen as organic compound, which represents a contaminant prototype. To this purpose, densities and enthalpies of mixing as well as PhOH UV-absorption spectra were determined. The enthalpy and the spectrophotometry revealed PhOH-Laponite interactions whereas the volume did not. It emerged that the area occupied by PhOH on the Laponite surface is equal to that computed from the partial molar volume of PhOH in water, corroborating the insensitivity of the experimental volumes to the adsorption process. The situation where both PhOH and copolymer are simultaneously present in the aqueous Laponite suspension was also investigated. It turned out that the copolymer replaces PhOH from the water/Laponite clay interface, resulting in L35 being the more efficient. Moreover, the lateral copolymer-phenol interactions enhance the anchoring of PhOH to the solid surface. The reverse copolymer exercises the most important relevant effect. The UV-absorption spectra of PhOH in the water + copolymer + Laponite mixtures provided information that is consistent with those given by the calorimetric experiments. In conclusion, the aqueous copolymer-functionalized Laponite presents surface properties very different from the bare Laponite, favoring the removal of the organic compound from the solid surface.

Introduction Clay materials are investigated due to their interesting applications. One of the main advantages in their use is their low cost, with them being abundant over the world. Synthetic clays are silicates or aluminosilicates (Laponite, bentonite, montmorillonite, etc.) with a well-established structure, large surface area,1,2 and high purity. They are considered soil models3,4 within the issue of the remediation of contaminated soil. They are also investigated5,6 to establish their capability in removing from wastewater organic contaminants such as phenolic compounds,7 which damage living organisms.8-10 Clay-modified electrodes have been studied to develop electrochemical sensors and biosensors.11 Hybrid organoclay12,13 and nanocomposite films14 as well as polymer-clay nanocomposites have been prepared.15,16 The role of Laponite in the stability of emulsions has been also described.17 * Corresponding author. Phone: +39 91 6459835. Fax: +39 91 590015. E-mail: [email protected]. (1) Fraile, J. M.; Garcy´a, J. I.; Harmer, M. A.; Herrery´as, C. I.; Mayoral, J. A.; Reiser, O.; Werner, H. J. Mater. Chem. 2002, 12, 3290. (2) Barhoumi, M.; Beurroies, I.; Denoyel, R.; Saı¨d, H.; Hanna, K. Colloids and Surfaces A: Physicochem. Eng. Aspects 2003, 223, 63. (3) Sumi, K.; Takeda, Y.; Koide, Y. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 135, 59. (4) Sumi, K.; Takeda, Y.; Goino, M.; Ishiduki, K.; Koide, Y. Langmuir 1997, 13, 2585. (5) Darwish, N. A.; Halhouli, K. A.; Al-Dhoon N. M. Sep. Sci. Technol. 1996, 31, 705. (6) Viraraghavan, V.; Alfaro, F. J. Hazard. Mater. 1998, 57, 59. (7) Fang, H. H.; Chen, O. Water Res. 1997, 31, 2229. (8) Gonzalez, J. F.; Hu, W. Appl. Microbiol. Biotechnol. 1991, 35, 100. (9) Basheer, M. M.; Volpe, P. L. O.; Airoldi, C. Int. J. Pharm. 2004, 282, 163. (10) Smith, J. A.; Jaffe, P. R. Water Air Soil Pollut. 1994, 72, 205. (11) Mousty, C. Appl. Clay Sci. 2004, 27, 159. (12) Ras, R. H. A.; Ne´meth, J.; Johnston, C. T.; DiMasi, E.; De´ka´ny, I.; Schoonheydt, R. A. Phys. Chem. Chem. Phys. 2004, 6, 4174. (13) Itoh, T.; Shichi, T.; Yui, T.; Takagi, K. Langmuir 2005, 21, 3217. (14) Malwitz, M. M.; Dundigalla, A.; Ferreiro, V.; Butler, P. D.; Henk, M. C.; Schmidt, G. Phys. Chem. Chem. Phys. 2004, 6, 2977.

Functionalization of the clay surface may generate significant transformation from low value commodity products toward materials with high benefit and of considerable complexity in terms of chemical structure and molecular and supramolecular architecture. The use of appropriate capping agents may confer very specific surface properties, enhancing the performance of such materials. Copolymers based on ethylene oxide (EO) and propylene oxide (PO) units are promising molecules for this purpose. In solution they form aggregates18 with affinity for organic compounds,19,20 and they are active at the solid/solution21 and air/solution22 interfaces. Furthermore, they are low-toxicity compounds.23,24 A recent thermodynamic study21 showed that they are effective agents in building up a steric barrier around Laponite particles, forming a monolayer. Structural investigations provided similar findings.25,26 Furthermore, it was evidenced21 that EO11PO16EO11 (L35) and PO8EO23PO8 (10R5), which differ only in the architecture, form also double layers on the Laponite surface. On the basis of these findings, we thought it would be interesting to study the ability of functionalized Laponite to (15) Lendy, W.; Jannasch, P.; Maurer, F. H. J. Polymer 2005, 46, 915. (16) Alexandre, M.; Dubois, P. Mater. Sci. Eng., R Rep. 2000, 28, 1. (17) Ashby, N. P.; Binks, B. P. Phys. Chem. Chem. Phys. 2000, 2, 5640. (18) De Lisi, R.; Lazzara, G.; Lombardo, R.; Milioto, S.; Muratore, N.; Turco Liveri, M. L. J. Solution Chem. 2006, 35, 659. (19) Lazzara, G.; Gradzielski, M.; Milioto, S. Phys. Chem. Chem. Phys. 2006, 8, 2299. (20) De Lisi, R.; Lazzara, G.; Milioto, S.; Muratore, N. J. Colloid Interface Sci. 2006, 300, 368. (21) De Lisi, R.; Lazzara, G.; Lombardo, R.; Milioto, S.; Muratore, N.; Turco Liveri, M. L. Phys. Chem. Chem. Phys. 2005, 7, 3994. (22) Vieira, J. B.; Thomas, R. K.; Li, Z. X.; Penfold, J. Langmuir 2005, 21, 4441. (23) Xiong, X. Y.; Tam, K. C.; Gan, L. H. J. Controlled Release 2005, 103, 73. (24) Varshney, M.; Morey, T. E.; Shah, D. O.; Flint, J. A.; Moudgil, B. M.; Seubert, C. N.; Dennis, D. M. J. Am. Chem. Soc. 2004, 126, 5108. (25) Nelson, A.; Cosgrove, T. Langmuir 2004, 20, 2298. (26) Nelson, A.; Cosgrove, T. Langmuir 2005, 21, 9176.

10.1021/la061088i CCC: $33.50 © 2006 American Chemical Society Published on Web 08/09/2006

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interact with organic compounds. Therefore, L35 and 10R5 were chosen as capping agents and phenol was selected as organic material, which represents a contaminant prototype. To this end, thermodynamic properties (volume and enthalpy) were determined because they measure interactions (hydrophilic and hydrophobic) and their modeling provides information on the structure. On the basis of reports27-29 that UV-vis spectroscopy is very useful for studying the adsorption of additives onto the Laponite surface, UV-absorption spectra of phenol were also recorded. Experimental Section Materials. The triblock copolymers EO11PO16EO11 (L35, nominal M ) 1900 g mol-1) and PO8EO23PO8 (10R5, nominal M ) 1950 g mol-1) were obtained as gifts from BASF AG (Ludwigshafen, Germany). The water content, determined by thermogravimetry (Mettler TA 3000), is 0.57 and 0.38 w/w % for L35 and 10R5, respectively. Phenol (PhOH) is a Fluka product with purity higher than 99%. The compounds were used as received because their standard partial molar volumes, determined from density, are in a good agreement with those reported elsewhere.18,30 Laponite, RD grade (Rockwood Additives Ltd), has the molecular formula31 Si8(Mg5.45Li0.4)O20(OH)4Na0.7 constituting the unitary cell of the disklike shape clay platelet highlighted by a diameter of about 25 nm and a thickness of 1 nm.32 The procedure to prepare the aqueous Laponite dispersion is described elsewhere.21 Standard solutions of 0.1 mol dm-3 HCl (Carlo Erba) and 0.2 mol dm-3 NaOH (Fluka) were used. Buffer solutions formed by sodium tetraborate/NaOH at pH ) 10 (Fluka) and by sodium tetraborate/HCl at pH ) 8 (Riedelde-Hae¨n) were employed. The mixtures containing phenol were prepared in dark bottles. Water from reverse osmosis (Elga model Option 3) having resistivity higher than 1 MΩ cm-1 was used. The pH measurements were carried out by using a Beckman pHmeter equipped with a glass electrode. Spectrophotometry Experiments. The measurements were carried out at 298.0 ( 0.1 K by means of a Beckman spectrophotometer (model DU-640). UV-absorption spectra, reported as absorbance (A) vs wavelength (λ), were registered in the 200-400 nm range, where the phenol and the phenate ion show bands centered at 270 and 287 nm, respectively. In such a range, both Laponite and the copolymers do not absorb; notwithstanding, their spectra were subtracted from the corresponding systems containing phenol. In such a way, the light scattering effect of the clay particles was eliminated. The time effect was not observed (spectra are reported in the Supporting Information), and consequently, the experiments were performed on the mixtures after their preparation. For a given system, the same aqueous phenolic solution was used. The Laponite flocculation induced by the buffers at both pH ) 8 and 10 prevented carrying out experiments at controlled pH. This result is consistent with the finding33 that the Laponite suspensions start to flocculate at ionic strength larger than 10-2 mol dm-3. To overcome this problem, phenol solution and Laponite dispersion at the same pH values were prepared (by adding NaOH) and mixed to proceed with the UV measurements. Indeed, the pH of the final suspension was not equal to the pH of the starting mixtures, and consequently, the comparison between the UV spectra of PhOH in the absence and the presence of Laponite is unfruitful. The procedure followed to keep constant the pH is indeed disadvantageous, because either NaOH (27) Yurekli, K.; Conley, E.; Krishnamoorti, R. Langmuir 2005, 21, 5825. (28) Czimerova´, A.; Bujda´k, J.; Gaplovsky´, A. Colloids Surf. A: Physicochem. Eng. Aspects 2004, 243, 89. (29) Cione, A. P. P.; Neumann, M. G.; Gessner, F. J. Colloid Interface Sci. 1998, 198, 106. (30) Perron, G.; Desnoyers, J. E. Fluid Phase Equilib. 1979, 2, 239. (31) Thompson, D. W.; Butterworth, J. T. J. Colloid Interface Sci. 1992, 151, 236. (32) Avery, R. G.; Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 48. (33) Mourchid, A.; Lecolier, E.; Van Damme, H.; Levitz, P. Langmuir 1998, 14, 4718.

Langmuir, Vol. 22, No. 19, 2006 8057 or the buffer constituents may influence the interaction between PhOH and Laponite. On this basis, we decided to register UV spectra of some water + PhOH mixtures, CPh ) 5 × 10-4 mol dm-3, at various pH (NaOH was added) and of the water + PhOH + Laponite mixture (obtained by mixing the aqueous PhOH solution and the aqueous Laponite suspension) at CPh ) 5 × 10-4 mol dm-3, mL ) 3.27 mmol kg-1, whose pH value was 9.74. Volumetric and Calorimetric Measurements. The properties of transfer of either the additive or the solid are appropriate to investigate solid-additive interactions.21,34 Therefore, volume and enthalpy were measured. Density. The densities of the water + Laponite + phenol mixtures were determined at 298 K by using a vibrating tube flow densimeter (Model 03D, Sodev Inc.) sensitive to 3 ppm. The temperature was controlled within 0.001 K by using a closed-loop temperature controller (Model CT-L, Sodev Inc.). The densimeter was calibrated by using the procedure described elsewhere.35 The apparent molar volume (VΦ,L) of Laponite in the water + phenol mixture was calculated by means of the following equation VΦ,L )

ML 103(d - do) d ddomL

(1)

where ML is the mass of the Si8(Mg5.45Li0.4)O20(OH)4Na0.7 unitary cell, d is the density of the water + PhOH + Laponite mixture, and do is the density of the water + PhOH binary system, whereas mL represents the moles of Laponite per kilogram of water + phenol mixture. The measurements were carried out at fixed mL (6.5 mmol kg-1) by changing the phenol composition. The volume of transfer of Laponite from water to the aqueous phenol solution (∆VLPh) was calculated as the difference between VΦ,L and the value in water experimentally determined (341.8 ( 0.7 cm3 mol-1). The time effect was also analyzed (the plot is shown in the Supporting Information). The ∆VLPh independence on time reveals the stability of the dispersion over 10 h at least. Enthalpy. The experiments were carried out using a flow LKB 2107 microcalorimeter at 298.15 ( 0.01 K. The mixtures flowed into the instrument with the assistance of Gilson peristaltic pumps (Minipuls 2). The procedure used to investigate the various systems, i.e., waterPhOH-Laponite, water-copolymer in the absence and the presence of PhOH, and water-PhOH-copolymer-Laponite, is detailed below. The water-Laponite-copolymer systems were previously studied.21 (i) Water-PhOH-Laponite System. The experimental approach dealt with the mixing between the aqueous PhOH solutions (pH ≈ 6) and the aqueous Laponite suspension (pH ≈ 10) that generated the aqueous PhOH-Laponite mixtures at pH ≈ 8.5. As the baseline of the process, the dilution of the same phenolic solution with water was taken. The enthalpy of transfer of Laponite from water to the aqueous PhOH solution (∆HLPh) was calculated as the difference between the experimental enthalpy and the enthalpy of dilution of Laponite with water (∆Hd,L), whose value is reported elsewhere.21 The concentrations of the Laponite (mL) and the phenol (mPh) mixtures in the final state were calculated as mL )

mL,iΦL (ΦL + ΦPh)

mPh )

mPh,iΦPh (ΦL + ΦPh)

(2)

where mL,i and mPh,i stand for the moles of Laponite and phenol per kilogram of water in the initial state, and ΦPh and ΦL are the flows of water in the PhOH and the Laponite mixtures, respectively. Each flow was determined by weight, and the mixing ratio is about 0.5 (note that this procedure was followed for the measurements that will be described below). (34) Vignola, E.; Perron, G.; Desnoyers, J. E. Langmuir 2002, 18, 6035. (35) De Lisi, R.; Lazzara, G.; Milioto, S.; Muratore, N. J. Phys. Chem. B 2003, 107, 13150.

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Table 1. Thermodynamic Properties for the Interactions in the Aqueous Copolymer-Phenol-Laponite Clay Mixtures at 298 Ka Ki z ∆Hi b Kb b ∆Hb BPP BPPP BPh,P b g

PhOH

L35

10R5

243 ( 16 0.48 ( 0.13 -1.23 ( 0.01

3900 ( 400b (63 ( 8) × 10-3 b -14.4 ( 0.1b 1.2 118 ( 4 259.2 ( 1.6 -160 ( 19 37.8 ( 1.0 -13.8 ( 0.3 -0.42 ( 0.09

2800 ( 300b (50 ( 8) × 10-3 b -13.9 ( 0.2b 1.1 72 ( 7 262.3 ( 0.7 158 ( 12 32.0 ( 0.6 -12.9 ( 0.2 -1.20 ( 0.09

a Units: K and K , kg mol-1; ∆H , ∆H , and b, kJ mol-1; B i b i b PP and BPh,P, kJ mol-2 kg; BPPP, kJ mol-3 kg2. b From ref 21; The subscript i refers to the monolayer formation.

∆HLPh was determined at a fixed mL (6.5 mmol kg-1) as a function of mPh. The enthalpy contribution due to the acid-base equilibria was evaluated by carrying out additional experiments (details are reported in the Supporting Information); its value (40 J/mol of Laponite) is negligible, indicating that ∆HLPh reflects only the Laponite-PhOH interactions. (ii) Water-Copolymer System in the Absence and the Presence of PhOH. From the enthalpy of dilution of the copolymer solutions with water (∆Hd,P), the copolymer-copolymer pair (BPP) and triplet (BPPP) interaction parameters were calculated according to the McMillan-Mayer approach36 ∆Hd,P (mP - mP,i)

) BPP + BPPP (mP + mP,i)

(3)

where mP is the copolymer concentration after the mixing process given by mP )

mP,iΦP

(4)

(Φw + ΦP)

Figure 1. Volume and enthalpy of transfer of Laponite from water to the aqueous phenol solution as functions of the phenol concentration. The line is the best fit according to eq 7. The enthalpy of transfer of Laponite from water + PhOH mixture to the water + PhOH + copolymer system (∆HLPh+P) corresponds to the difference between the experimental enthalpy and ∆Hd,L. The final Laponite (mL) and copolymer (mP) concentrations expressed as moles per kilogram of water + phenol mixture were calculated as mP )

mP,iΦP (ΦP + ΦL)

mL )

mL,iΦL (ΦP + ΦL)

(6)

where ΦP and ΦL represent the flows of the solvent in the copolymer and the Laponite mixtures whose initial concentrations are mP,i and mL,i, respectively. The experiments were done at mL ) 6.5 mmol kg-1 and mPh ) 30 mmol kg-1 by varying mP. This mPh value was selected to measure large thermal effects.

Results and Discussion here Φw is the flow of water and ΦP is the flow of water in the copolymer solution having the initial concentration mP,i. The obtained parameters are collected in Table 1 (the experimental data are reported in the Supporting Information). The mixing between the aqueous copolymer and PhOH solutions was performed by taking as baseline the dilution process of the copolymer solution with water. The experimental enthalpy was corrected for the enthalpy of dilution of PhOH with water (∆Hd,Ph), and the enthalpy of transfer of PhOH from water to the aqueous copolymer solution (∆HPhP) was calculated. The determined ∆Hd,Ph agrees with the literature value.37 The concentrations of the copolymer (mP) and the phenol (mPh) solutions upon the mixing process are mP )

mP,iΦP (ΦPh + ΦP)

mPh )

mPh,iΦPh (ΦPh + ΦP)

(5)

where the symbols have the same meaning as above. The measurements were carried out at two phenol concentrations (mPh ) 15 and 30 mmol kg-1) by systematically changing the copolymer composition. (iii) Water-PhOH-Copolymer-Laponite System. The heat generated by mixing a water + Laponite + PhOH dispersion with a water + PhOH + copolymer system was registered by taking as baseline of the process the dilution of the water + PhOH + copolymer system with the aqueous PhOH solution. (36) McMillan, Jr. W. G.; Mayer, J. E. J. Chem. Phys. 1945, 13, 276. (37) Causi, S.; De Lisi, R.; Milioto, S. J. Solution Chem. 1990, 19, 995.

I. Water + Phenol + Laponite Mixtures. (i) Results. The properties of transfer (volume,34,38 enthalpy,21 and heat capacity34) describe stable and unstable dispersions in the absence and the presence of additives. Therefore, ∆VLPh and ∆HLPh as functions of mPh were determined and are represented in Figure 1. ∆VLPh values are nearly close to zero, whereas the enthalpy decreases with mPh, tending to a constant value at elevated concentration. The high sensitivity of the enthalpy in detecting interactions is very well recognized. For instance, the formation of hydrogen bonds causes an enthalpy39 variation of -17 kJ mol-1 and a volume40 variation of -1.2 cm3 mol-1. Therefore, the volumetric results do not rule out the PhOH adsorption onto Laponite, but they might reveal that the adsorption involves nearly null volumes. Interactions between PhOH and various clays (montmorillonite,41 kaolinite,2 and bentonite42) were evidenced. Information was also provided by the UV spectra recorded for systems where CPh was fixed at 5 × 10-4 mol dm-3 and the Laponite composition was systematically changed. As Figure 2 shows, at mL ) 0 the spectrum is typical of a phenol solution at pH ) 6. The Laponite addition causes the appearance of a band at 287 nm, enhanced by mL due to the increasing amount (38) De Lisi, R.; Lazzara, G.; Milioto, S.; Muratore, N. Thermochim. Acta 2004, 418, 95. (39) Dormindontova, E. E. Macromolecules 2002, 35, 987. (40) Gianni, P.; Lepori, L. J. Solution Chem. 2000, 29, 405. (41) Yapar, S.; Yilmaz, M. Adsorption 2004, 10, 287. (42) Banat, F. A.; Al-Bashir, B.; Al-Asheh, S. EnViron. Pollut. 2000, 107, 391.

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Figure 2. UV-absorption spectra for phenol (CPh ) 5 × 10-4 mol dm-3) in water (dotted line) and in water + Laponite mixtures (solid lines).

Figure 4. Dependence of the Laponite moles per site on the total amount of EO and PO units adsorbed onto the Laponite surface. Data are from ref 21.

the ratio between the moles of occupied sites per kilogram of water (SPh) and the total number of sites (St ) zmL). The equilibrium constant is given by

KPh )

xPh mPh,f (1 - xPh)

(8)

Moreover, for the PhOH mass balance and with SPh ) mPh,m, one may write

mPh ) mPh,m + mPh,f ) zmLxPh + mPh,f Figure 3. UV-absorption spectra for phenol (CPh ) 5 × mol dm-3) in water at several pH values (solid lines) and in water + Laponite mixture at mL ) 3.27 mmol kg-1 (dotted line). 10-4

of phenate ions. The spectra change reveals both the PhOH dissociation and the Laponite-PhOH interactions. In fact, as Figure 3 illustrates, the spectrum of PhOH in the presence of Laponite (pH ) 9.74) appears anomalous to the pH effect being comprised between those at pH ) 8.05 and 8.59. (ii) Enthalpy Data QuantitatiVe Analysis. The enthalpy function is a bulk property and only its modeling can clearly provide the mechanism of interaction. ∆HLPh may reflect the contributions for the adsorption of both phenol and phenate ions. Nevertheless, one expects that Laponite exhibits preference for phenol if one considers that a Laponite particle presents43 a net negative charge. Our statements are also supported by isotherms of PhOH adsorption onto bentonite,42 which show that the amount of PhOH adsorbed decreases with increasing pH according to a smaller affinity of PhO- toward the solid surface. Under the conditions of our experiments the fraction of phenol existing in the charged form is very small (about 4%), suggesting that it is the neutral form of the molecule that interacts with surface. On the basis of these arguments, the enthalpy of transfer of Laponite was treated by means of a recently developed approach21 that assumes that one mole of Laponite contains z moles of sites, each of which can adsorb one PhOH molecule

∆HLPh ) xPh∆HPh

(7)

where ∆HPh is the enthalpy changes for the PhOH monolayer formation and xPh is the fraction of the occupied sites given by (43) Tawari, S. L.; Koch, D. L.; Cohen, C. J. Colloid Interface Sci. 2001, 240, 54.

(9)

where mPh,m and mPh,f are the molalities of phenol in the adsorbed and the aqueous phases, respectively. The minimizing procedure was performed by means of a nonlinear least-squares fitting method. The fit (Figure 1) provided the z, KPh, and ∆HPh parameters (Table 1). The obtained St value is (3.1 ( 0.8) × 10-3 mol kg-1. (iii) Discussion. The driving forces for the interactions between PhOH and Laponite likely involve only the Laponite surface and the hydroxylic group of the additive. These arguments are supported by a few calorimetric experiments on benzene + water + Laponite mixtures.44 The attractive forces between Laponite and PhOH are controlled by the entropy change (12.4 ( 0.2 kJ mol-1), which is 1 order of magnitude higher than the exothermic ∆HPh value (Table 1). The anchoring of PhOH onto Laponite may be split essentially into three processes: (1) the phenol removal from the aqueous phase; (2) the phenol attachment onto the solid surface, and (3) the release of sodium ions from Laponite enhanced by the screening effect of the adsorbed PhOH. The latter process was already evidenced by kinetic experiments21 upon copolymers adsorption. Our interpretation disagrees with a recent view41 that the adsorption of PhOH onto the negatively charged surface of montomorillonite takes place through the polarization of π electrons. Finally, z represents the moles of Laponite necessary to adsorb one mole of PhOH and it allows calculating both the area occupied by one PhOH molecule onto the Laponite surface and the Laponite specific surface area. To this end, the following procedure was pursued. A linear correlation between 1/z, obtained for the adsorption of poly(ethylene) glycols, poly(propylene) glycols, and their copolymers onto Laponite,21 and the sum of the number of EO and PO units (Nt) in each macromolecule is observed (44) The enthalpy of transfer of 6.5 mmol kg-1 Laponite from water to the water + benzene mixture at 10 mmol kg-1 is -0.03 kJ mol-1.

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Figure 5. Enthalpy of transfer of phenol from water to the aqueous solutions of 10R5 (top) and L35 (bottom) as a function of the copolymer concentration. (O) mPh ) 15 mmol kg-1, (b) mPh ) 30 mmol kg-1. The lines are best fits according to eq 11.

Figure 6. UV-absorption spectra for phenol (CPh ) 5 × 10-4 mol dm-3) in water (dotted lines) and in the aqueous solutions of 10R5 (top) and L35 (bottom) at some copolymer concentrations (solid lines).

(Figure 4). On the basis of the z value for PhOH and the straight line illustrated in Figure 4, one may deduce that PhOH apparently behaves like a macromolecule with a nominal Nt ) 7.4. According to the literature,45 the gyration radius (Rg) of a two-dimensional polymer can be calculated as Rg ) (0.8567Ntb2/6)1/2, where b is the monomer length. In our case, we reasonably assumed that b for both the EO and the PO units are equal (b ) 3.6 Å). The calculated Rg value is 3.7 Å, which is close to that (3.2 Å) obtained from the partial molar volume of PhOH in water (86.06 cm3 mol-1)30 assuming a sphere model. This result indicates that the adsorbed PhOH occupies the same volume as that in the aqueous phase. Going further, from z one may calculate the Laponite specific surface area (Asp) by means of the following equation

Asp )

zπRg2NA ML

(10)

where NA is Avogadro’s number, whereas the other symbols have the same meaning as above. The Asp value obtained from eq 10 is 160 m2 g-1 and it is comparable to that provided by BET measurement (ca. 200 m2 g-1).46 II. Water + Phenol + Copolymer Mixtures. For both L35 and 10R5, the ∆HPhP values are positive and change linearly with mP (Figure 5), whereas they are essentially independent of PhOH concentration. The observed linearity reveals that PhOH does not induce the copolymer aggregation in the investigated domain. In the experimental conditions, phenol is nearly undissociated, as confirmed by its UV-absorption spectra in the aqueous copolymer solutions illustrated in Figure 6. Therefore, ∆HPhP was interpreted by means of the McMillan-Mayer approach36 valid for nonionic additives by considering only the pair phenol-copolymer interaction parameter (BPhP)

∆HPhP ) 2BPhPmP

(11)

The BPhP values are collected in Table 1. To understand these results, one has to analyze the interaction parameters for the copolymers (Table 1) and PhOH. The BPP for both 10R5 and L35 are positive and independent of the copolymer architecture, in agreement with the hydrophobic desolvation of the PO units. The BPPP of 10R5 and L35 differ even in the sign. The negative BPPP value for L35 can be due to the interactions between the lateral EO segments. In the case of 10R5, the lateral (45) Cardy, J. L.; Saleur, H. J. Phys. A: Math. Gen. 1989, 22, L601. (46) Kolla´r, T.; Ko´nya, Z.; Pa´linko´, I.; Kiricsi, I. J. Mol. Struct. 2001, 563564, 417.

Figure 7. Enthalpy of transfer of Laponite from water + PhOH mixture to the water + PhOH + copolymer systems as a function of the copolymer concentration. Top, 10R5; bottom, L35. The lines are best fits according to eq 17.

PO segments interact to each other, allowing a positive BPPP value. The pair interaction parameter for phenol is small and negative (-2.1 kJ kg mol-2),37 likely revealing either the interaction between the -OH group and the π electrons of the aromatic ring or the interaction between the two phenolic hydroxylic groups. The BPhP values may be a combination of the effects occurring in the PhOH + water and copolymer + water binary mixtures. Finally, it emerges that the forces engaged between the copolymer and phenol allow important calorimetric effects, whereas they do not alter the spectrophotometric behavior of PhOH itself (Figure 6). III. Water + Phenol + Copolymer + Laponite Mixtures. (i) Results. ∆HLPh+P as a function of mP is plotted in Figure 7 for both L35 and 10R5. For a given copolymer, the enthalpy decreases with mP, reaching a minimum, and thereafter it increases with an additional amount of copolymer. The study on the water + copolymer + PhOH mixture rules out that the presence of the minimum is due to the micellization process of the copolymer in the aqueous phase. Moreover, the shape of the enthalpy curve is equal to that determined for Laponite in the aqueous copolymer solutions.21 The ∆HLPh+P values in L35 and 10R5 are similar, suggesting that the architecture is not a significant factor to govern the interactions in the aqueous copolymer-Laponite-PhOH systems. (ii) Simulation of the Experimental Enthalpy. The earlier investigations showed that PhOH and the copolymers21 exhibit

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Figure 8. Excess enthalpy as a function of the copolymer concentration: top, 10R5; bottom, L35.

affinity to the Laponite surface; in particular, PhOH forms a monolayer while copolymers form single and double layers. The simultaneous presence of the copolymer and PhOH in the Laponite suspension may therefore generate different effects. Namely, the affinity toward Laponite of each component might not be influenced by the presence of the other one. The PhOH adsorption might be enhanced through its incorporation into the polymeric surfactant layers anchored to the solid surface and so on. Let us consider the simplest case for which the behavior of either PhOH or copolymer is not affected by the other. Consequently, ∆HLPh+P can be calculated as a combination of the enthalpies of the water + PhOH + copolymer, water + PhOH + Laponite, and water + copolymer + Laponite ternary mixtures. On this basis, the following equation was derived

∆HLPh+P(ideal) ) -∆HLPh + xm∆Hm + xb∆Hb + BPhPmPh(mP - mP,f) xPh∆HPh (12) mL The quantity ∆HLPh refers to the transfer of Laponite (mL ) 6.5 mmol kg-1) from water to the PhOH solution (mPh ) 30 mmol kg-1) and its value is -1.0 kJ mol-1. The xm∆Hm and xb∆Hb terms are the contributions for the formation of mono- and bilayers of the copolymer, respectively, whereas xPh∆HPh is the term for the adsorption of PhOH onto Laponite. The last quantity at the right-hand side of eq 12 is the contribution of interaction between PhOH and copolymer in the aqueous phase, where mP,f is the concentration of the copolymer in the free state. The fractions of the various adsorbed species as well as mP,f can be calculated by combining eqs 13 and 14

Km )

xm mP,f xf

mP ) mP,m + mP,b + mP,f

Kb )

xb mP,f xm

(13)

1 ) xm + xb + xPh + xf mPh ) mPh,f + mPh,m (14)

where Km and Kb are the equilibrium constants for the formation of mono- and bilayer, respectively.21 xm, xb, and xf are the fractions of sites involved in the monolayer, bilayer, and free state, respectively. The excess enthalpy (∆Hexc), i.e., the difference between the experimental ∆HLPh+P and that calculated by means of eqs 12-14 as a function of the copolymer composition, is illustrated in Figure 8 for both L35 and 10R5. For a given copolymer, ∆Hexc is negative and decreases with mP, reaching a minimum at ca. 2 mmol kg-1, and thereafter it increases, tending to zero. ∆Hexc likely reflects the lateral interactions between the

Figure 9. Dependence on the copolymer concentration of the fractions of Laponite sites occupied by phenol and copolymer in the absence (top) and the presence (bottom) of lateral interactions: solid lines, L35; dotted lines, 10R5.

adsorbed PhOH and copolymer. Short-range forces between the surface aggregates were considered47 for the adsorption of conventional surfactants onto a solid phase. Blandamer et al.48 used the Frumkin method (which assumes the lateral interactions between the adsorbed molecules) to analyze isothermal calorimetric data of aqueous polymer-surfactant systems. This approach allowed negative contributions to both free energy and enthalpy. On this basis, the equilibrium constants for the adsorption processes taking place in the present systems were rewritten as

K′m ) Km exp(gxPh) K′Ph ) KPh exp(gxm)

(15)

where g is the free energy parameter for the interactions between the adsorbed copolymer and phenol. Note that Kb remained unchanged as it was multiplied by the ratio between exp(gxPh) and itself. The partial molar enthalpies of the adsorbed copolymer (HP,m) and phenol (HPh,m) were expressed as

HP,m ) HP,mo + bxPh HPh,m ) HP,mo + bxP

(16)

where b is the enthalpy interaction parameter for the adsorbed molecules. On this basis, the following equation for the nonideal ∆HLPh+P was obtained

∆HLPh+P(nonideal) ) ∆HLPh+P(ideal) + bxPhxm (17) where ∆HLPh+P(ideal) is given by eq 12; xPh is calculated by means of eqs 8 and 15, whereas xm is determined via eqs 13 and 15. The mass balance is given by eq 14, where mP was set equal to mP,f to simplify the fractions calculations; this approximation is reliable because the adsorbed copolymer never exceeds 2%. The best fits of the experimental data according to eq 17 (shown in Figure 7) provided the parameters collected in Table 1. A careful inspection of the dependence of xPh and xm in the absence and the presence of lateral interactions on mP may reveal whether a competitive adsorption takes place and the role played by the lateral forces. As Figure 9 represents, xm increases upon the copolymer addition, whereas the opposite occurs for xPh. In other words, copolymer replaces PhOH from the water/Laponite clay interface. The macromolecule architecture plays a role to (47) Drach, M.; Narkiewicz-Michalek, J.; Rudzin˜ski, W.; Findenegg, G. H.; Kira´ly, Z. Phys. Chem. Chem. Phys. 2002, 4, 2307. (48) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Irlam, K. D.; Engberts, J. B. F. N.; Kevelam, J. J. Chem. Soc., Faraday Trans. 1998, 94, 259.

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some extent as, in the range of mP analyzed, xPh in the presence of L35 is slightly smaller than that in the presence of 10R5. The lateral copolymer-phenol interactions enhance such a difference and, consequently, PhOH is more strongly anchored to the solid surface. Moreover, such forces are more effective for the reverse copolymer. Finally, the copolymer addition to the water + Laponite + PhOH mixture does not affect the shape of the UV spectra and generates a small increase of the intensity band at 287 nm. This effect is independent of the copolymer concentration (data are represented in the Supporting Information).

Conclusions The behavior of aqueous Laponite clay functionalized with EO11PO16EO11 and PO8EO23PO8 toward phenol was studied. PhOH forms a monolayer on the aqueous Laponite surface, which generates a negative enthalpy and positive entropy. Also, the volume of one adsorbed PhOH molecule is equal to that assumed in water, corroborating the insensitivity of the experimental volumes to the adsorption process. Different is the situation when the Laponite surface is covered by the macromolecule. As a general result, the copolymer replaces PhOH from the water/

De Lisi et al.

Laponite clay interface, with EO11PO16EO11 being the more efficient. The lateral copolymer-phenol interactions enhance the anchoring of PhOH to the solid surface and they are more effective for PO8EO23PO8. In conclusion, the copolymerfunctionalized Laponite generates new surface properties that can be exploited, for example, in the processes addressed to remove contaminants adsorbed on soil. Oppositely, the bare Laponite is more efficient in removing phenol from wastewater. Acknowledgment. We are grateful to the Ministry of Instruction, University and Research for the financial support. BASF AG (Ludwigshafen, Germany) is kindly acknowledged for providing copolymers. Supporting Information Available: Time effect on the volume of transfer of Laponite from water to the water + PhOH mixtures and on the UV-absorption spectra; enthalpies of dilution of copolymer solutions with water; UV-absorption spectra of phenol in the water + Laponite + copolymer systems; description of the used procedure to calculate the acid/base equilibria. This material is available free of charge via the Internet at http://pubs.acs.org. LA061088I