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Calorimetric Investigation of the Interaction between Lithium Perfluorononanoate and Poly(ethylene glycol) Oligomers in Water Paolo Gianni,* Arianna Barghini, Luca Bernazzani, and Vincenzo Mollica Dipartimento di Chimica e Chimica Industriale, UniVersita` di Pisa, Pisa, Italy ReceiVed April 12, 2006. In Final Form: June 30, 2006 The interaction of lithium perfluorononanoate (LiPFN) with poly(ethylene glycol) (PEG) molecules of different molecular weights (300 < MW < 20000 Da) has been investigated in water at 298.15 and 308.15 K by isothermal titration calorimetry (ITC). Density, viscosity, and conductivity measurements were also performed at 298.15 K. The aggregation process of this surfactant on the PEG polymeric chain was found to be very similar to that exhibited by cesium perfluorooctanoate (CsPFO) and appears to be consistent with the necklace model. ITC titrations indicated that a fully formed LiPFN micellar cluster can be wrapped by a PEG chain having a molecular weight (MW) of ∼3200 Da, longer than that required by the shorter perfluorooctanoate (MW ∼ 2600 Da), and also suggested a stepwise mechanism for the aggregation of successive micelles. Viscosity data indicate that the formation of polymer-surfactant complexes between PEG and LiPFN involves a conformational change of the polymer. The aggregation of preformed micelles of LiPFN or CsPFO or SDS on the PEG polymeric chain always gives rise to further stabilization.
1. Introduction Fluorinated surfactants are characterized by a much larger surface activity and chemical stability with respect to the corresponding hydrocarbon-type compounds. Their unique properties make them very interesting both for practical applications and for theoretical studies of micellar systems.1 In a previous paper, we determined some thermodynamic and transport properties for the aggregation of cesium perfluorooctanoate (CsPFO) on the polymeric chain of a series of poly(ethylene glycol) (PEG) molecules with different molecular weights (MWs).2 This study revealed the similarity of the mechanism of aggregation of this surfactant with that presented by sodium dodecyl sulfate (SDS) on the same polymer.3 Particularly, the isothermal titration calorimetry (ITC) technique proved to be very useful to put in evidence the values of the PEG MWs, which prove critical for the onset and settling of polymersurfactant (P-S) interactions. In this paper, we extend the study to a longer fluorinated carboxylate, lithium perfluorononanoate (LiPFN). The lithium counterion was chosen for solubility reasons. The system PFN-/ H2O has been investigated by several authors. Phase diagrams are known for aqueous systems involving counterions such as H+,4,5 Li+,4 Na+,4-6 Cs+,4,7 NH4+, and substituted ammonium ions.4 Structural studies of mesophases formed by concentrated NH4PFN aqueous solutions are reported by La Mesa8 and Photinos9 (see also references therein). Data concerning the * Corresponding author. Fax: 39 050 2219260. Tel: 39 050 2219263. E-mail:
[email protected]. (1) Kissa, E., Ed. Fluorinated Surfactants and Repellents, 2nd ed.; Surfactant Science Series; Marcel Dekker Inc.: New York, 2001; Vol. 97. (2) Gianni, P.; Barghini, A.; Bernazzani, L.; Mollica, V.; Pizzolla, P. J. Phys. Chem. B 2006, 110, 9112-9121. (3) Bernazzani, L.; Borsacchi, S.; Catalano, D.; Gianni, P.; Mollica, V.; Vitelli, M.; Asaro, F.; Feruglio, L. J. Phys. Chem. B 2004, 108, 8960-8969. (4) Fontell, K.; Lindman, B. J. Phys. Chem. 1983, 87, 3289-3297. (5) La Mesa, C.; Ranieri, G. A.; Terenzi, M. Thermochim. Acta 1988, 137, 143-150. (6) La Mesa, C.; Sesta, B. J. Phys. Chem. 1987, 91, 1450-1454. (7) Boden, N.; Harding, R.; Gelbart, W. M.; Ohara, P.; Jolley, K. W.; Heerdegen, A. P.; Parbhu, A. N. J. Chem. Phys. 1995, 103, 5712-5719. (8) Chidichimo, G.; Coppola, L.; La Mesa, C.; Ranieri, G. A.; Saupe, A. Chem. Phys. Lett. 1988, 145, 85-89. (9) Photinos, P. J.; Saupe, A. J. Chem. Phys. 1986, 84, 517-521.
aggregation number and the shape of micelles of LiPFN have also been reported.6,10,11 Other data relative to the micellization of PFN- salts are summarized in Table 2 of present paper. The capability of PFN- salts to form complexes in aqueous solution has been investigated for systems such as NaPFN/cyclodextrins,12-14 NaPFN/sodium tetradecyl sulfate,15 as well as for systems involving macromolecules: LiPFN/PVP,16-19 LiPFN/ lysozyme,20,21 and NaPFN/HM-PNIPAM.22 This paper aimed to investigate the aggregation process of LiPFN on oligomers of PEG and compare the behavior of this system with previously studied SDS/PEG3 and CsPFO/PEG2 systems. The LiPFN/PEG system was studied in a dilute aqueous solution mainly through calorimetry. ITC titrations of aqueous solutions of a series of PEG molecules with different MWs (300 e MW e 20000 Da), hereafter indicated as PEG MW, were systematically performed at 298.15 K. For a few polymers were also performed viscosity, conductivity, and density measurements at 298.15 K, and calorimetric measurements at 308.15 K. The data collected suggest a mechanism of surfactant aggregation on the polymer similar to that observed for CsPFO2 and SDS:3 they are consistent with the aggregation of a series of micellar clusters (10) Coppola, L.; La Mesa, C.; Ranieri, G. A.; Terenzi, M. Ann. Chim. 1990, 80, 271-281. (11) Ortona, O.; D’Errico, G.; Paduano, L.; Sartorio, R. Phys. Chem. Chem. Phys. 2002, 4, 2604-2611. (12) Wilson, L. D.; Verral, R. E. Langmuir, 1998, 14, 4710-4717. (13) Wilson, L. D.; Verral, R. E. J. Phys. Chem. B 1998, 102, 480-488. (14) Guo, W.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 446-451. (15) Nakano, T.-Y.; Sugihara, G.; Nakashima, T.; Yu, S.-C. Langmuir 2002, 18, 8777-8785. (16) Sesta, B.; D’Aprano, A.; Segre, A. L.; Proietti, N. Langmuir 1997, 13, 6612-6617. (17) Sesta, B.; Segre, A. L.; D’Aprano, A.; Proietti, N. J. Phys. Chem. B, 1997, 101, 198-204. (18) Segre, A. L.; Proietti, N.; Sesta, B.; D’Aprano, A.; D’Amato, M. E. J. Phys. Chem. B 1998, 102, 10248-10254. (19) D’Aprano, A.; Sesta, B.; Proietti, N.; Mauro V. J. Solution Chem. 1997, 26, 649-662. (20) Sesta, B.; Gente, G.; Iovino, A.; Laureti, F.; Michiotti, P.; Paiusco, O.; Palacios, A. C.; Persi, L.; Trinci, A.; Sallustio, S.; Sarnthein-Graf, C.; Caparbi, A.; La Mesa, C. J. Phys. Chem. B 2004, 108, 3036-3043. (21) Palacios, A. C.; Sarnthein-Graf, C.; La Mesa, C. Colloids Surf., A 2003, 228, 25-35. (22) Kujawa, P.; Raju, B. B.; Winnik, F. M. Langmuir 2005, 21, 1004610053.
10.1021/la0609930 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006
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Table 1. Thermodynamic Data Obtained from Calorimetric Titrations of 0.1% PEGs with LiPFN in Water PEG MW (Da)
caca,b (mmol kg-1)
Cmaxb,e C2f ∆trfHmaxc ∆aggGom d (mmol (mmol (kJ mol-1) (kJ mol-1) kg-1) kg-1) RC2g 298.15 K
no PEG 300 600 900 1500 2000 3400 4600 6000 8000 10000 11200 20000
9.4 (cmc) 9.0 6.2 5.4 4.9 4.4 3.6 3.3 3.1 3.3 3.0 3.1 2.9
no PEG 4600 8000
9.2 (cmc) 3.0 2.7
2.07 4.22 4.27 5.10 5.83 6.26 6.83 7.36 7.67 7.14 6.71 6.60
-43.27 -45.12 -45.80 -46.28 -46.82 -47.81 -48.24 -48.55 -48.24 -48.71 -48.55 -48.88
10.5 8.7 8.2 7.4 6.8 5.9 5.6 5.2 4.8 4.7 4.6 4.4
∼41 ∼39 29.5 27.6 22.4 23.8 21.3 20.7 21.9 20.3 21.5 20.0
1.09 1.02 0.81 0.91 0.81 0.79 0.84 0.78 0.83 0.77
308.15 K 1.84 2.25
-50.35 -50.70
4.5 3.8
19.3 0.73 21.0 0.82
Critical aggregation concentration. b Uncertainty: (0.2 mmol kg-1. c Calculated through eq 1 at the maximum of the endothermic peak. d Calculated as 2RT ln cac, mole fraction units. e Concentration corresponding to ∆trfHmax. f Concentration of saturation of the polymer; uncertainty (0.5 mmol kg-1 when MW g 1500 Da. g RC2 ) (C2 cac)/mEO. It represents the ratio of bound surfactant to PEG monomers.
temperature of the titrant contained in the Lund syringe. Typical titrations involved the addition of 0.8835 m aqueous LiPFN to 0.1% w/w PEG solutions and covered a surfactant concentration range of 0 < mS < 0.06 mol kg-1. The concentration of PEG (mEO), intended as the molality of the repeat unit (-CH2-CH2-O-), was 0.0227 mol kg-1, constant for all 0.1% PEG solutions. 2.3. Density. Density measurements were carried out by means of a high-precision vibrating-tube densimeter Anton Paar DMA 5000. The instrument has a built-in thermostat for maintaining the desired temperature with a precision of (0.001 K and an accuracy of (0.01 K. The densimeter was calibrated by dry air and freshly degassed water at 298.15 K. The measured densities have a precision of (3 × 10-6 g cm-3 and are accurate within (5 × 10-6 g cm-3. 2.4. Conductivity. Conductivity measurements were carried out at 298.15 ( 0.10 K with an Amel 160 apparatus. The cell constant (1.041 cm-1) was determined with a 0.01 M KCl aqueous solution. 2.5. Viscosity. Viscosity measurements were performed by means of an Ubbelohde viscosimeter equipped with an optical system for flow detection. Details of the experimental procedure are reported elsewhere.2
3. Results
a
on the PEG polymeric chain according to the necklace model.23,24 Particularly, viscosity and calorimetric measurements suggest that, at low surfactant concentrations, even for this system the polymer also wraps around a small micellar cluster assuming a compact high-energy conformation, which undergoes a conformational change to a more expanded structure with increasing concentration. 2. Materials and Methods 2.1. Materials. PEG samples with nominal MWs of 300, 600, 900, 1500, 2000, 3400, 4600, 8000, 10 000, and 20 000 Da were obtained from Aldrich. A few standard samples of PEG of certified MW, characterized by a low polydispersity index (D ) Mw/Mn) were Fluka products. For these samples, the effective MW was chosen as the average between Mw and Mn: PEG 6000 (D ) 1.03), PEG 8300 (D ) 1.02), and PEG 11 200 (D ) 1.07). Heptadecafluorononanoic acid 98% (Fluorochem) was neutralized with lithium hydroxide monohydrate 99.95% (Aldrich) in aqueous solution. After water removal, the salt was crystallized from an n-butanol/n-hexane mixture and finally dried at 80 °C under vacuum for 2 days. Doubly deionized water was used as the solvent. All solutions were prepared by weight. The concentration of LiPFN was expressed as moles per kilogram of water (m). PEG solutions were allowed to stand overnight before use, and their concentration was expressed as a weight percent. 2.2. Isothermal Titration Calorimetry. The isothermal titration calorimeter was a Thermal Activity Monitor 2277 (TAM) from Thermometric, equipped with a 612 Lund syringe pump. Titrations were performed at 298.15 (or 308.15) ( 0.02 K by adding aliquots of a few microliters (5-30) of a concentrated solution of one component into a 20 mL cell containing 15-16 g of the aqueous solution of the other component. Observed heat effects were measured mostly on the 300 µW full-scale detection range, allowing an average uncertainty of (1%. A slightly larger scatter of experimental data was observed at 35 °C because of the not perfectly constant (23) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1987, 48, 651-662. (24) Nagarajan, N.; Kalpakci, B. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1982, 23, 41.
3.1. Isothermal Titration Calorimetry. Typical enthalpy of dilution (∆dilH) curves obtained by adding concentrated aqueous LiPFN containing micelles to water in the presence or absence of a given PEG polymer are shown in Figure 1a. In this and the other figures reporting experimental data as a function of the surfactant concentration, the lines are drawn as mere guides for the eyes. The figure indicates the critical surfactant concentrations pertinent to P-S systems: the critical micelle concentration (cmc) in the absence of the polymer, the critical aggregation concentration (cac) at which the interaction with the polymer begins, the saturation concentration C2 at which no additional interaction between surfactant and polymer chains is revealed, and the concentration Cm at which free micelles are formed. The criterion for the identification of the above critical concentrations has been reported elsewhere.2 The curve relative to the dilution in water at 298 K (see Figure 1a) allows one to determine a cmc value, 0.0094 mol kg-1, which well compares with literature data, particularly with the value 0.0093 mol kg-1 also measured by microcalorimetry.25 Analytical deconvolution of the same curve also allows one to determine the enthalpy of micellization, ∆micH ) 8.0 kJ mol-1, which is in reasonable agreement with the value 6.8 kJ mol-1 measured by Johnson and Olofsson 25 and the value of ∼7.2 kJ mol-1, which can be estimated from the data reported by Ortona et al.11 Further thermodynamic data related to the micellization process of perfluorononanoates in water are given in Table 2 (see the discussion). The graphical estimation of the difference between C2 and Cm from the dilution curves (see Figure 1a) nearly equals the difference between the cac and the cmc for practically all examined PEG polymers. That is what one would expect if, after the saturation of the polymer, the free micelles begin to form only when the concentration of free surfactant has increased to the cmc value. This allowed us to interpret the calorimetric data collected below C2 as being determined only by the aggregation of the surfactant on the polymeric chain. Figure 1b reports the enthalpy of transfer at 298.15 K of one mole of LiPFN from water to 0.1% w/w PEG 8000 solutions (∆trfH) as a function of surfactant (S) concentration. The data were obtained by combining the dilution enthalpies in water and (25) Johnson, I.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1 1988, 84, 551-560.
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Table 2. Thermodynamic Data for the Micellization of Perfluorononanoates in Water S HPFN LiPFN
NaPFN KPFN NH4PFN N(CH3)4PFN
T (°C)
cmc (mol dm-3)
25 60 25
0.00275c 0.0028d 0.0108c, 0.0093e,f, 0.00487g,f, 0.0057h 0.0064i,j, 0.0080k,j, 0.0082l, 0.010m 0.0088n, 0.0090o
30 35 25 30 40 50 25 25 30
0.0106d 0.0087o 0.0042h, 0.0092q 0.0090e, 0.0091d 0.0102r 0.0085c, 0.0063d 0.00785c, 0.0091s 0.0044c,t 0.0067d
∆micH° (kJ mol-1)
6.8 ( 0.3e 8.03 ( 0.10o
∆micCp (J K-1 mol-1)
∆micV (cm3 mol-1)
Rb
-500 ( 30e
14 ( 2e
20e
0.47c
-578o
15g,p
20(2h
0.56m
32 ÷ 40i
0.53o
20 ( 2h
0.40h
23 ( 2r
0.53r 0.44c 0.48c
18 ( 2h 18.8 ( 1o 2.25 ( 0.15o 4.0 ( 0.3e
Na
10 ( 2e
a Aggregation number of the micelle near the cmc. b Dissociation degree of the counterion. c Reference 28. d Reference 33. e Reference 25. f Data are also reported at different temperatures. g Reference 34. h Reference 6. i Reference 11. j mol kg-1. k Reference 20. l Reference 17. m Reference 19. n Reference 18. o This work. p Average among different counterions. q Reference 14. r Reference 15. s Reference 35. t Data are also reported for partially methylated ions.
obtained by correcting the ∆trfH(S) data for the micellization effect:
∆trfH(S,m)WfPEG ) ∆trfH(S)WfPEG + δmicH
Figure 1. (a) Typical curves of heats of dilution of a concentrated LiPFN aqueous solution in water. (b) Enthalpy of transfer of the surfactant (S) from water to a 0.1% PEG 3400 solution: (O), ∆trfH(S), eq 1; (b), ∆trfH(S,m), eq 2.
water containing PEG, measured at the same S concentration:
∆trfH(S)WfPEG ) ∆dilH(S)PEG - ∆dilH(S)W
(1)
The difference between the partial molar enthalpies of the surfactant in water and those in PEG solutions can be obtained from eq 1 because of the negligible PEG dilution brought about by the small volumes added in the ITC titrations. Nonzero values of the ∆trfH(S) function indicate interactions of the surfactant with the polymer, apart from small cosolvent effects on the heats of dilution. Figure 1b also shows the trend of the enthalpy of transfer of the surfactant in its monomeric form, which may be
(2)
The quantity δmicH, represents the contribution of micellization to the enthalpy of transfer. This contribution is null at low surfactant concentrations (mS), whereas it approaches ∆micH at large mS values and was calculated as described in ref 2. The quantity defined by eq 2 was introduced since it avoids apparent exothermic effects caused by the shift of the micellization equilibrium (open circles of Figure 1b) and is associated to the same thermodynamic process (S monomer f S aggregated) over the whole surfactant concentration range. When not otherwise specified, throughout the paper, the transfer enthalpy of the surfactant from water to PEG solutions will be simply indicated as ∆trfH, meaning the property calculated from eq 2. Figure 2 reports values of ∆trfH at 298.15 K as a function of surfactant concentration for all polymers examined in this work. The enthalpies of transfer follow a pattern similar to that already observed for other surfactants: SDS3 and CsPFO.2 We first observe an endothermic peak at low S concentrations followed by one or two minima. The ∆trfH values finally settle at a constant common value for large surfactant concentrations where, after saturation of the polymer, free micelles are formed as in pure water. Transfer enthalpies indicate that this surfactant is able to interact with very short polymers. The cac values are always lower than the cmc, and a small endothermic shoulder is already noticed for PEG 600. This peak shifts to lower S concentrations and increases in magnitude for increasing PEG MW, pointing to a constant value for the larger MWs. The increase in the maximum of the endo peak, which may be associated with the formation of a first P-S aggregate, is generally accompanied by a parallely deeper minimum, which sometimes can even be slightly exothermic. A few measurements with polymer samples characterized by a very low polydispersity index (D < 1.07) provided results consistent with those of the other samples. Table 1 lists the values of critical concentrations relevant to the various PEGs, together with an estimate of the enthalpy and free energy of formation of the LiPFN-PEG aggregate, which is first formed at low surfactant concentrations. The cac values
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Figure 2. Enthalpy of transfer, ∆trfH, of LiPFN from water to 0.1% PEG (eq 2) at 25 °C as a function of surfactant concentration for polymers of different MW.
are regularly decreasing as the PEG MW is increased, reaching an almost constant value of 3 mmol kg-1 for MWs higher than 3400 Da. According to the charged phase separation model,26 values of the standard free energy of formation of this aggregate, starting from the free monomers, can be calculated as ∆aggGom ) (2 - R′)RT ln cac. The exact values of R′ values are not known for all PEG MWs, but fortunately they were found to be very slightly dependent on MW (see paragraph 3.2). Therefore, the ∆aggGom values of Table 1 were calculated under the assumption of R′ ) 0, in light of using them only for relative comparisons (see further the function ∆aggGom ) f(mS)). In the hypothesis that LiPFN added in the region of the maximum of the endothermic peak is 100% aggregated, the corresponding value of the transfer enthalpy (∆trfHmax, see Table 1) can be identified with the enthalpy of aggregation of LiPFN monomers (∆aggHm). The corresponding entropy of aggregation can be calculated from the Gibbs equation. The values of critical concentrations and of these thermodynamic properties of aggregation are plotted in Figure 3 as a function of the MW of the polymers. The almost constant value of these thermodynamic parameters at large MW values probably indicates a constant composition and thermodynamic stability of the P-S aggregates. The trends of the functions of Figure 3 allow one to establish that the binding of the surfactant molecules on the PEG chains reaches an almost stable condition after MW ∼ 3200 Da. This probably means that a chain of about 73 PEG monomeric units is able to bind the first fully formed micellar cluster of surfactant molecules. The fact that practically the same minimum MW is also identified by the trend of a property associated with the saturation of the polymer (see the function C2 ) f(MWPEG), Figure 3a) leads one to deduce that the number of micellar clusters per polymer chain is probably independent of the possibly different structure of the aggregates at increasing surfactant concentrations. (26) Shinoda, K.; Hutchinson, E. J. Phys. Chem. 1962, 66, 577-582.
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Figure 3. Thermodynamic parameters for LiPFN aggregation on PEG as a function of PEG MW at 25 °C: (a) surfactant concentrations at the critical aggregation (cac), at the maximum of the endothermic peak (Cmax) and at the saturation of the polymer (C2); (b) ∆aggGom, ∆aggHm ) ∆trfHmax, T∆aggSm ) ∆aggHm - ∆aggGom (see Table 1 and text).
Figure 4. Effect of polymer concentration on the heats of transfer of LiPFN from water to aqueous PEG 8000 solutions at 25 °C.
The above minimum value of MW appears to be qualitatively consistent with the analogous value of ∼2600 Da already found for CsPFO.2 The longer hydrophobic chain of the perfluorononanoate salt leads to a larger radius of the micellar clusters of the latter surfactant, which require a longer polymeric chain to be fully wrapped around. In Figure 4 is shown the effect of varying the polymer concentration. The curves of ∆trfH at increasing polymer concentration clearly reveal similar trends. As already noticed with other surfactants,2,3 an increase in polymer concentration results only in an increase in the area of the endothermic peak, indicating the involvement of a proportionally larger number of
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Figure 6. Enthalpy of dilution in water (x ) dil) and of transfer from water to aqueous 0.1% PEG solutions (x ) trf) for LiPFN at 35 °C.
Figure 5. Enthalpies of transfer of the titrant from water to the aqueous solution of the other component. Titrant: LiPFN (w f 0.1% PEG), direct titration, (b); PEG (w f 0.0227 m LiPFN), reverse titration, (O); R ) mS/mEO.
surfactant molecules. Consistently, C2, the stoichiometric surfactant concentration necessary to saturate the polymer, is also proportionally shifted to larger values. The effect of ionic strength on the reaction of the aggregation of LiPFN on the PEG chain could not be investigated: the titration with LiPFN of a solution of PEG 8000 in the presence of 0.1m NaCl resulted in the precipitation of a solid salt. Figure 5 compares the enthalpies of transfer of LiPFN from water to aqueous PEG solutions with the corresponding enthalpies of transfer of PEG from water to aqueous LiPFN solutions. The latter were obtained from reverse titrations in which a concentrated solution of the polymer is added to a surfactant solution containing free micelles. Use of the ratio R ) mS/mEO as the common abscissa leads one to read the reverse titration from right to left. We would like to point out that, at a given R value, different enthalpy values necessarily correspond because of the different criterion of normalization. Second, the two titrations involve slightly different media as the common R interval corresponds to different stoichiometric surfactant concentrations (mS). As a matter of fact, mS starts from zero, reaches the cac, and then exceeds the cac in the direct titration, while it is always larger than the cac (and even the cmc) in the reverse one. Nevertheless, the trend of the reverse function corresponds to the trend of the direct titrations, indicating a single broad endothermic maximum for PEG 4600 and two small relative maxima in the case of PEG 8000. Reverse titrations allow one to estimate the enthalpy of aggregation of micelles at the saturation. In fact, the addition of very small amounts of PEG to an aqueous solution of surfactant micelles (mS . cmc) leads to the quantitative formation of P-S aggregates. The enthalpy of aggregation observed for PEG 8000 (∆aggH(EO) ) -1.6 kJ/mol), when divided by the ratio RC2 to normalize the heat with respect to the moles of surfactant, yields the value -1.9 kJ/mol. This value, practically equivalent to that
measured for PEG 4600, represents an estimate of the overall heat of aggregation of preformed micelles on the PEG chain at the saturation. To reveal the effect of temperature on the micellization and aggregation processes, a few titrations with selected PEG polymers were also performed at 308.15 K. Figure 6 shows the dilution curve of concentrated LiPFN in pure water and the heats of transfer of this surfactant from water to aqueous PEG 4600 and PEG 8000 solutions. The dilution curve into water allows one to calculate cmc ) 0.0092 mol kg-1 and ∆micH ) 2.25 kJ mol-1. The lower cmc value, compared to that found at 25 °C, indicates a larger stability of the micelles at higher temperatures and is consistent with the endothermic micellization enthalpy. The observed change with temperature of micellization enthalpy allows one to calculate a micellization heat capacity ∆micCp ) -578 J K-1 mol-1, consistent with the value of -500 J K-1 mol-1 measured by Olofsson.25 The thermodynamic data obtained from the enthalpy curves relative to PEG 4600 and PEG 8000 at 35 °C are reported in Table 1. The transfer enthalpies of LiPFN from water to PEG aqueous solutions at 35 °C (see Figure 6) display a trend similar to that exhibited at 25 °C, but are shifted toward lower enthalpy values; for instance, in the case of PEG 8000, the endo peak, chosen as a representative of the P-S aggregate that is first formed at low surfactant concentration, decreases from the value ∆trfHmax ) 7.67 kJ mol-1 at 25 °C to the value 2.23 at 35 °C. These data allow one to calculate a negative and large heat capacity change for aggregation of the surfactant to the polymer, ∆aggCp ) -544 J K-1 mol-1, consistent with the hydrophobic nature of the aggregation process. 3.2. Conductivity. The conductivity of aqueous solutions of LiPFN at 298.15 K has been measured in water and water containing 0.1% of a few selected PEGs. The experimental results are shown in Figure 7. The ratios of the slopes of the straight lines allow one to determine the dissociation degree R of the counterions,27 which result as R ) 0.53 for the free micelles and 0.72, 0.70, and 0.69 for the aggregates formed on PEG 3400, PEG 8000, and PEG 20000, respectively. The R value of the free micelles is intermediate between the values found in the literature: 0.4728 and 0.56.19 The ionic dissociation of the aggregates appear to be practically independent of PEG MW but (27) Frohener, S. J.; Belarmino, A.; Zanette, D. Colloids Surf., A 1998, 137, 131-139. (28) Hoffmann, H.; Platz, G.; Rehage, H.; Reizlein, K. Makromol. Chem. 1981, 182, 451-481.
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Figure 8. Surfactant transfer enthalpy (∆trfH, b) and relative viscosity (ηrel, O) for 0.5% PEG 8000 aqueous solutions as a function of LiPFN concentration at 25 °C. Figure 7. Specific conductivity of aqueous LiPFN solutions at 25 °C. Vertical bars indicate critical concentrations. To increase readability, the curves have been shifted from each other by 1 mOhm-1 cm-1.
are clearly larger than that of the free micelles, as already noticed for other P-S complexes.29 3.3. Density. A few measurements were performed in aqueous LiPFN solutions, above the cmc, at 298.15 K. The volume change associated with micellization (∆micV) was evaluated as the difference between the values of the apparent molar volumes of the surfactant in the monomeric (VΦ,m) and micellar (VΦ,M) forms. VΦ,M values of LiPFN micelles were calculated as
VΦ,M )
(
)
1000 + mSMS 1000 - VΦ,mcmc /(mS - cmc) δ δ0 (3)
where mS and MS are the molality and MW of the surfactant, whereas δ and δ0 are the density of the solution and the solvent, respectively. Because of the very dilute range in which the surfactant is present as free monomer, it was not possible to obtain the VΦ,m value of LiPFN by direct experimental determination. This value was then calculated by correcting the corresponding VΦ,m value experimentally determined in our laboratory for CsPFO2 according to a well-established additivity scheme [VΦ,m(LiPFN) ) VΦ,m(CsPFO) + VΦ,m(CF2) - VΦ,m(Cs+) + VΦ,m(Li+)]30 using cation volume contributions given by Millero.31 The resulting average volume of micellization at 25 °C was found to be ∆micV ) 18.8 ( 1.5 cm3 mol-1, larger than the value of 14 cm3 mol-1 found by Johnson,25 but in fair agreement with that of 18 cm3 mol-1 measured by La Mesa.6 A comparison with analogous ∆micV values observed for the shorter perfluorooctanoate anion does not allow any useful deduction because of the spread of experimental values reported in the literature. For instance, together with a few comparable data at 30 °C (19.4 cm3 mol-1 for NaPFO and 21.5 cm3 mol-1 for HPFO),32 other values are also reported in the range 10-13 cm3 mol-1 for NaPFO and CsPFO (see Table 2 of ref 2). Density measurements in the presence of PEG were made on two aqueous LiPFN-PEG 8000 solutions above the cac. Apparent molar volumes of the surfactant aggregated on the PEG chain (VΦ,agg) were calculated accordingly to eq 3 where values of (29) Goddard, E. D. Colloids Surf. 1986, 19, 255-300. (30) Gianni, P.; Lepori, L. J. Solution Chem. 1996, 25, 1-42. (31) Millero, F. J. In Water and Aqueous Solutions; Horne, R. H., Ed.; Wiley: New York, 1972; Chapter 13. (32) Sugihara, G.; Mukerjee, P. J. Phys. Chem. 1981, 85, 1612-1616.
VΦ,M and cmc were replaced by VΦ,agg and cac, respectively, and δ0 was the density of the 0.1% PEG 8000 solution. The apparent molar volume of the S monomer was calculated with the aid of the above group contributions scheme, using VΦ,m (CsPFO) values measured in the PEG solution. The volume change associated with S aggregation on the polymer, calculated as ∆aggVm ) VΦ,agg - VΦ,m, resulted in the value 15.5 cm3 mol-1. Thus, the process of aggregation of monomeric surfactant molecules on the PEG chain also involves a positive volume change, although it is smaller than that observed for micellization. 3.4. Viscosity. Figure 8 reports the change in relative viscosity (ηrel ) η/ηo, with η being the viscosity of the sample solution, and ηo being the viscosity of the solvent) with surfactant concentration for 0.5% PEG 8000 solutions. The transfer enthalpy data of the corresponding ITC titration are also plotted for comparison. The viscosity increases up to the cac and then exhibits a clear decrease in the region where the ITC curve displays the first endothermic effect. At larger LiPFN concentrations, the relative viscosity reaches a minimum and then increases steeply and monotonically. No such viscosity minimum is observed when adding LiPFN to 0.1% PEG aqueous solutions, probably because of the low contribution to the overall viscosity of the medium by the diluted polymer.
4. Discussion Table 2 summarizes the thermodynamic data for the micellization reaction of perfluorononanoates in water. These data appear to be feebly dependent on the nature of the counterion, and often a larger difference is found among data pertaining to the same system but given by different authors. The scarce influence by the type of alkaline counterions on the micellization process has already been pointed out.34,36 The effect of the nature of the cation on the aggregation properties of anionic surfactants on PEG polymers was analogously found to be negligible,27 although others found an effect on the aggregate size.37 A few calorimetric and conductivity measurements on the LiPFO/PEG system38 confirm our previous data on CsPFO/PEG,2 thus supporting the minor relevance of the counterion effect. (33) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468-2470. (34) Muzzalupo, R.; Ranieri, G. A.; La Mesa, C. Colloids Surf., A 1995, 104, 327-336. (35) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365-369. (36) Kale, K. M.; Zana, R. J. Colloid Interface Sci. 1977, 61, 312-322. (37) Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1993, 157, 1418. (38) Unpublished results from our laboratory.
Aggregation of LiPFN on PEG Oligomers
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Figure 9. Transfer enthalpies of different surfactants from water to 0.1% PEG solutions for increasing PEG MW at 25 °C: SDS (b), CsPFO (O), LiPFN (x).
Figure 9 compares the enthalpies of transfer from water to 0.1% PEG solutions at 25 °C for the surfactants SDS, CsPFO, and LiPFN studied so far in our laboratory. The figure clearly evidences the analogous trend of the ∆trfH function for the different surfactants, which all exhibit a first endothermic maximum followed by minima and then a final plateau corresponding to the formation of free micelles. The critical concentrations indicating the onset of the cooperative interaction are always in the order LiPFN < SDS < CsPFO. However, the endothermic peak, which gives a measure of the energetics of the interaction, shows that both fluorosurfactants are able to interact with PEG polymer chains shorter than the alkyl sulfate. This peak, in fact, is already evident for the PEG 600 oligomer in the case of LiPFN and of CsPFO, while only for PEG 900 in the case of SDS. The appearance of successive minima, sometimes exothermic, suggests a kind of transition in the nature of the P-S complex, brought about by the increasing surfactant concentration. These thermal effects were attributed to an initial dehydration of the PEG chain when it hydrophobically binds a surfactant micellar cluster, followed by a rearrangement by which the oxyethylenic units can rehydrate and still bind the micelles through dipolar interactions.39,40 Viscosity measurements lead us to suggest2,3 that the observed enthalpy effects might receive an additional contribution by some conformational change of the polymer chain induced by the (39) Olofsson, G. J. Phys. Chem. 1985, 89, 1473-1477. (40) Dai, S.; Tam, K. C. J. Phys. Chem. B 2001, 105, 10759-10763.
surfactant. As above shown (see Figure 8), the titration of a 0.5% aqueous solution of PEG 8000 displays a clear decrease of viscosity at low surfactant concentrations. The viscosity decrease begins in the same concentration range where the calorimetric titration curve displays the endothermic effect, which may be associated with the early formation of some P-S aggregate. In our opinion, in this concentration range, the PEG polymeric chain wraps around a small micellar cluster of surfactant molecules, thus assuming a more compact coiled conformation characterized by a smaller hydrodynamic radius. Such a shrinking of the size of the polymer has also been predicted theoretically.41,42 These theoretical treatments predict a partial collapse of the polymer coil at a stoichiometry of one micelle per coil, thus suggesting a marked decrease in relative viscosity at small surfactant concentrations. We found no decrease with 0.1% PEG solutions, whereas a small decrease was found with 0.5% PEG 8000 (see Figure 8). A much larger decrease was found by Chary et al.43 in a careful viscosity study of PEG conformational changes following the formation of aggregates with SDS. These authors, however, adopted different experimental conditions (MW > 100 000, PEG concentration ) 1.15%, in the presence of NaCl), all of which magnify the solute contribution to viscosity changes of the solution. Probably huge effects can be observed only with very long polymers. Therefore, in our opinion, the endothermic (41) Groot, R. D. Langmuir 2000, 16, 7493-7502. (42) Diamant, H.; Andelman, D. Europhys. Lett. 1999, 48, 170-176. (43) Chary, K.; Kowalczyk, J.; Lal, J. J. Phys. Chem. B 2004, 108, 28572861.
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Gianni et al.
Table 3. Thermodynamic Parameters for the Aggregation of Surfactants on a PEG Chain in a 0.1% PEG 8000 Aqueous Solution at 298.15 Ka surfactant SDS CsPFO LiPFN
∆aggGoM b (kJ mol-1) -0.17 -0.10 -1.41
∆aggHMc (kJ mol-1) +4.66 -1.17 -0.36
∆aggSM (J K-1 mol-1)
∆aggCp,M (J K-1 mol-1)
∆aggVMd (cm3 mol-1)
+16 -4 +4
+292h
-1.4h
+109 +34
-4.0 -3.3
RC2e
Rf
R′f
0.76 1.38 0.84
0.37 0.42 0.53
0.57 0.72 0.70
a ∆aggX values (X ) G, H, S, Cp, V) refer to the aggregation on the polymer of a preformed free micelle: SDS data (ref 3), CsPFO (ref 2), LiPFN (this work). b From eq 4, cac and cmc in mole fraction units. c From eq 5. d ∆aggVM ) ∆aggVm - ∆micV. e See footnote g for Table 1. f Counterion dissociation degree of the micelle (R) and the P-S aggregate (R′). h Reference 38.
effect observed in our ITC curves might also be partially attributed to the formation of this compact, probably strained, high-energy conformation of the polymer, as already suggested in the cases of SDS3 and CsPFO.2 The increase of viscosity observed at larger S concentrations would be due to the increasing number of micellar units bound by the polymeric chain whose mutual electrostatic repulsion forces the PEG chain toward a more expanded conformation, which is consistent with the necklace model.23,24 The trend of the transfer enthalpies for some PEG MWs exhibits a couple of minima after the initial endothermic peak (see, e.g., in Figure 9 the two fluorosurfactants curves for PEG 3400 and the SDS curve for PEG 8000). The trend in the case of SDS was interpreted as being determined by the actual superposition of an exothermic and a second narrow endothermic peak. This second endo effect was actually attributed to the binding of a second micelle on the same macromolecule, each micelle binding a PEG fragment of approximately 3800 Da.3 The perfluorosurfactant micelles, characterized by a smaller size compared with SDS due to their shorter hydrophobic tail, can be fully wrapped by a shorter PEG chain. Consistently, shorter chains would be able to bind successive micelles, and multiple endo peaks can be observed at lower MWs compared with SDS. For instance, the multiple endo peaks observed for all three surfactants in the case of PEG 10000 would probably be due to the binding of a second micelle for SDS, while being due to a third one for CsPFO and LiPFN, owing to the smaller PEG fragment (MW ∼ 3000 Da) required to bind a single micellar cluster of the shorter fluorosurfactants. Unfortunately, the actual dispersion of MWs of most of the examined samples partially masks the single steps of this mechanism. We should mention, anyway, that other authors through electromotive force measurements on the PEG/SDS system,44 claim that the minimum MW for observing the aggregation of a second SDS micelle would be double that found by us,3 that is, a PEG 8000 chain would lodge only one SDS micelle. The stepwise binding of micelles on a nonionic polymeric chain was implicitly inferred by neutron scattering data45,46 and explicitly postulated in the thermodynamic model by Nikas and Blankschtein.47 To our knowledge, this would be the first time that it is experimentally revealed by a thermodynamic property. Table 3 compares the thermodynamic data of micelle aggregation on the PEG polymers for the three systems so far investigated in our laboratory. For convenience, the data refer to the aggregate formed with PEG 8000, but practically all quantities maintain the same values for all MWs larger than 5000 Da. The standard free energy of formation of the aggregate, starting from preformed surfactant micelles, was calculated according to (44) Me´sza´ros, R.; Varga, I.; Gila´nyi, T. J. Phys. Chem. B 2005, 109, 1353813544. (45) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529-1542. (46) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19-33. (47) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512-3528.
the charged phase separation model26 as
∆aggGoM(S - PEG) ) RT[(2 - R′) ln cac - (2 - R) ln cmc] (4) where R and R′ are the ionization degrees of the micelle and the aggregate, respectively. The ∆trfHmax values (see Table 1) give a measure of the enthalpy of aggregation on the polymer of the surfactant in its monomeric form (∆aggHm). Values of the enthalpy of aggregation of preformed micelles on the polymer, consistent with ∆aggGoM values calculated through eq 4, were obtained by correcting the ∆trfHmax data for the heat of micellization:
∆aggHM ) ∆trfHmax - ∆micH
(5)
Combination of the free energy and enthalpy data yielded the corresponding entropy of micelle-PEG aggregation. We should stress that we are treating our data under the approximation of the pseudo-phase model. Thus the aggregation number of the micelles is not expressly taken into account, and the effect of its decrease generally associated with the aggregation process on the polymer48,49 is therefore included in the value of the thermodynamic properties. Consequently, the data of Table 3 are properly used for mere comparison purposes. We also point out that the ∆aggXM data (X ) G, H, S, Cp) refer to the complex that is first formed at low surfactant concentrations, before its conformational change evidenced by the ITC titrations at larger concentrations. The ∆aggVM values, instead, were calculated using density data of concentrated solutions and better reflect the volumetric behavior of the complex stable at large S concentrations. Table 3 shows that the polymer-aggregated micelles are always more stable than the free micelles and the larger stability gain is exhibited by the more hydrophobic surfactant, LiPFN. While SDS and CsPFO complexes display the same stability due to an enthalpy-entropy compensation, those made by LiPFN are stabilized by both factors. All surfactants display positive heat capacity and a negative volume of aggregation. These properties have an opposite sign with respect to that of the micellization process, which appears to be consistent with the decrease in the aggregation number of the micelles when bound to the polymer chain.48,49 Concerning the dissociation of the counterions, we only observe that the general screening effect of the micellar charges provided by the bonding with the polymeric chain (R′ > R) clearly distinguishes the hydrogenated with respect to the fluorinated surfactants, but, within the latter, it does not discriminate hydrophobic chains of different length. A few observations can finally be made about the dimensions of the micellar clusters aggregated on PEG. Values of RC2 indicate (48) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41-44. (49) Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985, 105, 1-6.
Aggregation of LiPFN on PEG Oligomers
that the number of bound S molecules per PEG monomeric unit at saturation increases in the order SDS < LiPFN < CsPFO. These numbers obviously result from a combination of the number of micellar clusters bound to a specified polymeric chain and the aggregation number of the surfactant inside each cluster. Unfortunately, experimental data on such aggregation numbers are only available for SDS-PEG aggregates.45,46 We can only notice that the above order is consistent with the inverse order of the minimum PEG MW able to bind a single cluster: 3850 Da (SDS3) > 3200 Da (LiPFN) > 2600 Da (CsPFO2).
5. Conclusions A thermodynamic investigation was conducted on the LiPFN/ PEG system in dilute aqueous solution. The data were analyzed in terms of the classical charged phase separation model and qualitatively discussed in the framework of the necklace model. A more quantitative analysis of the data, for instance, through a mass action model, was determined to be unnecessary as long as one points to enlighten the slow and gradual change in behavior observed for PEG polymers with increasing MW. The adopted treatment obviously points to the possible comparison of analogously treated systems. The present investigation put in evidence an aggregation process of LiPFN on the PEG chain that is quite similar to that already revealed for CsPFO and SDS. The initial aggregation of diluted surfactant on the PEG chain involves a strong endothermic effect, probably related to both the dehydration of the PEG chain and its conformational change to a more compact structure necessary to wraparound a small micellar cluster. The addition
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of further surfactant brings about an increase in the cluster size and the binding of further micelles, with consequent relaxing of the strained polymeric structure toward a more expanded conformation, which probably also allows a partial rehydration of the PEG segments. In this mechanism, the ITC titrations were able to distinguish the heat effects associated with the stepwise binding of successive micelles. The minimum chain length of PEG required to fully wrap a single micellar cluster corresponds to an MW of ∼3200 Da. The indication of this minimum MW value is inferred both by properties related to the initial P-S aggregate and also to the final aggregate stable at the saturation, thus suggesting that the conformational change occurring with increasing surfactant concentration does not involve a change in the number of bound micelles per monomeric unit of the polymer. The final comparison of the aggregation process on the polymer by different surfactants indicates that the aggregation of preformed micelles on the PEG chains is always associated with a thermodynamic stabilization. The average number of surfactant molecules per monomer unit of the polymer is consistently found to be larger for surfactants having a shorter hydrophobic tail. The lack of structural data on the aggregation number of the perfluoroalkanoate-PEG aggregates prevents a more quantitative analysis of the present thermodynamic data. Acknowledgment. The authors are grateful to the Ministero dell’Istruzione, dell’Universita` e della Ricerca (MIUR) for financial support (COFIN 2002). LA0609930
