Differential Scanning Calorimetry Investigation of the Effect of Salts on

Amphiphilic Block Copolymer (Poloxamer). Paschalis ... Aqueous solution properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(et...
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Langmuir 1997, 13, 6074-6082

Differential Scanning Calorimetry Investigation of the Effect of Salts on Aqueous Solution Properties of an Amphiphilic Block Copolymer (Poloxamer) Paschalis Alexandridis*,† and Josef F. Holzwarth‡ Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, and Abteilung Physikalische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany Received April 10, 1997. In Final Form: August 20, 1997X Aqueous solution properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO13PO30EO13) were studied in the presence of various alkali halide salts (LiCl, KCl, NaCl, NaBr, and NaI), sodium thiocyanate (NaSCN), and urea ((NH2)2CO). Differential scanning calorimetry (DSC) was employed for the determination of both the unimer-to-micelle transition (critical micellization temperature, CMT) and the phase separation (cloud point, CP). DSC is particularly useful in the case of Pluronic L64 where the detection of the CMT by optical techniques is hindered by the presence of a hydrophobic impurity. The presence of LiCl, KCl, NaCl, and NaBr decreased both CMT and CP (in the order Cl- > Br- and Na+ > K+ > Li+), whereas addition of NaSCN and urea resulted in a CMT and CP increase (in the order NaSCN > urea). NaI appeared to be an intermediate case as it decreased the CMT but increased the CP. Variation of the anion type (rather than the cation) is a more effective means of modulating the CMT and CP. This is the first study where CMT and CP values were simultaneously determined, and led to the important observation CMT(no salt) - CMT(salt) ) CP(no salt) - CP(salt). Both the micellization and the phase separation of the PEO-PPO-PEO copolymer in water are endothermic; the micellization (microphase separation) enthalpy was much larger than the (macro-) phase separation enthalpy (demonstrating the dominance of the PPO-water interactions over the PEO-water interactions) and increased with increasing NaCl and NaBr concentrations and decreasing NaI and urea concentrations. The salt effects on the solution behavior of the PEO-PPO-PEO polymer were correlated to the ion radius and the solvation heat of the salts.

1. Introduction Water-soluble poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) copolymers are commercially available nonionic surface active agents.1,2 Variation of the PPO/PEO composition ratio and the copolymer molecular weight during the synthesis allows the production of molecules with hydrophobic/hydrophilic properties which meet the specific requirements of diverse applications such as detergency, foaming, dispersion stabilization, emulsification, and lubrication.3,4 In addition to chemistry (synthesis), the hydrophobic/hydrophilic character of the PEO-PPO-PEO copolymers in aqueous solutions can also be altered by physical means, e.g., by varying the solution temperature and/or modifying the aqueous solvent quality. The effects of temperature on the properties and structure of PEOPPO-PEO copolymer solutions have been studied extensively;5-7 an increase in temperature has been shown to induce micelle formation in aqueous solutions of a number of these triblock copolymers, with the micelle core consisting mainly of the hydrophobic PPO and the corona composed of hydrated PEO. The solvent quality of water can be modulated by the addition of electrolytes, such as simple salts, or nonelectrolytes, such as urea.8,9 The aqueous solution behavior of surfactants is very sensitive * To whom correspondence should be addressed. Fax: +1-7166453822. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Schmolka, I. R. Surf. Sci. Ser. 1967, 1, 300-371. (2) Whitmarsh, R. H. Surf. Sci. Ser. 1996, 60, 1-30. (3) Edens, M. W. Surf. Sci. Ser. 1996, 60, 185-210. (4) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490501. (5) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2-15. (6) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1-46. (7) Chu, B.; Zhou, Z. Surf. Sci. Ser. 1996, 60, 67-144.

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to the presence of these cosolutes.10-13 Recently, the effect of added potassium halides14-16 and urea17 on the aqueous solution behavior (cloud point and micelle structure) of PEO-PPO-PEO copolymers has been demonstrated. Most of the inorganic salts decrease the water solubility of organic solutes (salting-out phenomenon), while some of them (NaI, NaSCN) have an opposite action (saltingin).18 The anions can be classified into the so-called Hofmeister series according to their salting-out strength at a given molar concentration: SO42- ≈ HPO42- > F- > Cl- > Br- > I- > SCN-. The effect of the cation type is usually smaller than that of the anion. Two mechanisms have been proposed to explain the Hofmeister series behavior (see ref 19 and references cited therein). According to one line of thought, salts affect the “solvent quality” of water; the salts on the left-hand side of the Hofmeister series are believed to be “structure-makers” while those on the right-hand side “structure-breakers”. In an alternative interpretation, the salting-in and salting(8) Kresheck, G. C. In WatersA Comprehensive Treatise, Vol 4: Aqueous Solutions of Amphiphiles and Macromolecules; Franks, F., Ed.; Plenum Press: New York, 1975; pp 95-167. (9) Leberman, R.; Soper, A. K. Nature 1995, 378, 364-366 (10) Ray, A.; Nemethy, G. J. Am. Chem. Soc. 1971, 93, 6787-6793. (11) Carale, T. R.; Pham, Q. T.; Blankschtein, D. Langmuir 1994, 10, 109-121. (12) Schott, H. J. Colloid Interface Sci. 1995, 173, 265-277. (13) Zhang, L.; Somasundaran, P.; Maltesh, C. Langmuir 1996, 12, 2371-2373. (14) Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Langmuir 1992, 8, 1903-1907. (15) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657-667. (16) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci.sPure Appl. Chem. 1993, A30, 1-18. (17) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442-2450. (18) Colins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18, 323-422. (19) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. J. Phys. Chem. 1995, 99, 6220-6230.

© 1997 American Chemical Society

DSC Study of Salt Effects on Poloxamers

out phenomena have an interfacial origin; ions adsorb or are depleted at the water-solute “interface” and, thereby, modify the phase equilibrium.19,20 The controversy between these two mechanisms has not been resolved yet. The effects of urea, a well-known protein denaturant, on the properties of aqueous surfactant solutions have similarly been attributed to either an “indirect” mechanism, according to which urea acts as a “structure-breaker” and facilitates the hydration of nonpolar solutes, or a “direct” mechanism, whereby urea has almost no effect on the water structure but replaces some of the water molecules in the hydration shell of the solute (see ref 17 and references cited therein). In the context of a systematic study6,17,21-32 where the micellization, structural, dynamic, and surface-active properties, as well as micelle-solute interactions in aqueous solutions of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) amphiphilic copolymers have been correlated to the copolymer chemical composition and molecular weight, we investigated the effects of additives, such as alkali halide salts and urea, on the solution properties of a representative PEO-PPOPEO copolymer, Pluronic L64 (EO13PO30EO13). Besides the work of Bahadur et al.14-16 on the cloud point and micellar properties of two PEO-PPO-PEO copolymers (Pluronics L64 and P85) in aqueous potassium halide solutions, our work17 on the effects of urea on the micellar properties of another PEO-PPO-PEO copolymer (Pluronic P105), and two recent papers on the effects of alcohols and other polar molecules on PEO-PPO-PEO micellization,33,34 there is very little information published on the effects of additives on this important class of amphiphiles.35 We employed differential scanning calorimetry (DSC) in order to ascertain simultaneously both the unimer-to-micelle transition (critical micellization temperature, CMT) and the phase separation (cloud point, CP) in aqueous solutions of Pluronic L64. DSC, a very useful technique with the advantage of being nonevasive (in the sense than no additional, e.g. fluorescence probes, molecules are required), has recently been applied to the study of PEO-PPO-PEO copolymers in aqueous (20) Hall, D. G. J. Chem. Soc., Faraday Trans. 2 1974, 70, 15261541. (21) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414-2425. (22) Alexandridis, P.; Nivaggioli, T.; Holzwarth, J. F.; Hatton T. A. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 35 (1), 604605. (23) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604-2612. (24) Nivaggioli, T.; Tsao, B.; Alexandridis, P.; Hatton, T. A. Langmuir 1995, 11, 119-126. (25) Nivaggioli, T.; Alexandridis, P.; Hatton, T. A.; Yekta, A.; Winnik, M. A. Langmuir 1995, 11, 730-737. (26) Alexandridis, P.; Nivaggioli, T.; Hatton, T. A. Langmuir 1995, 11, 1468-1476. (27) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. J. Am. Oil Chem. Soc. 1995, 72, 823-826. (28) Yu, G.-E.; Altinok, H.; Nixon, S. K.; Booth, C.; Alexandridis, P.; Hatton, T. A. Eur. Polym. J. 1997, 33, 673-677. (29) Hurter, P. N.; Alexandridis, P.; Hatton, T. A. Surf. Sci. Ser. 1995, 55, 191-235. (30) Goldmints, I.; von Gottberg, F. K.; Alexandridis, P.; Holzwarth, J. F.; Smith, K. A.; Hatton, T. A. Book of Abstracts; 211th Meeting of the American Chemical Society: American Chemical Society: Washington, DC, 1996; I&CR-59. (31) Goldmints, I.; Holzwarth, J. F.; Smith, K. A.; Hatton, T. A. Langmuir, in press. (32) Alexandridis, P.; Hatton, T. A. In Dynamic Properties of Interfaces and Association Structures; Pillai, V., Shah, D. O., Eds.; AOCS Press: Champaign, IL, 1996; Chapter 12. (33) Cheng, Y.; Jolicoeur, C. Macromolecules 1995, 28, 2665-2672. (34) Armstrong, J.; Chowdhry, B.; Mitchel, J.; Beezer, A.; Leharne, S. J. Phys. Chem. 1996, 100, 1738-1745. (35) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1997, 2, 478489.

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solutions.26,33-38 DSC is particularly important in the case of Pluronic L64 where the determination of the CMT by optical (e.g., UV-visible, fluorescence, or light scattering) techniques is hindered by the presence of a hydrophobic impurity which phase-separates at the vicinity of the CMT and causes a marked increase in the sample turbidity (at temperatures a few degrees higher than the CMT, this impurity is solubilized in the micelles and the solution becomes clear again).39,40 The organization of the paper is the following: the extraction of the CMT and CP values from the DSC experiments is described first, followed by the presentation of CMT and CP data in the presence of salts, their comparison for the different salts, and their correlation with the ion radius and the salt heat of solvation. The dependence of the micellization enthalpy on the salt type and concentration is presented then. The unimer concentration at different temperatures, salt types, and salt concentrations is derived from the DSC curves. Finally, the mechanisms behind the effects of the salts on the PEO-PPO-PEO copolymer solution behavior are discussed. 2. Materials and Methods Materials. The Pluronic L64 poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer was obtained as a gift from BASF Corp., Parsippany, NJ, and used as received. The copolymer has a nominal molecular weight of 2900 and approximately 40 wt % PEO and can thus be represented by the formula EO13PO30EO13. Wu et al.41 obtained for Pluronic L64 a number-average molecular weight (Mn) of 3400 g/mol using vapor pressure osmometry, and a weight-average molecular weight (Mw) of 3700 g/mol from static light scattering measurements, thus yielding Mw/Mn ) 1.1, an indication of low polydispersity. The micellization properties and the phase behavior of L64 in water have been reported elsewhere.21,27,42-45 The various salts used were of analytical grade or better, while the water was triple-distilled. Differential Scanning Calorimetry (DSC). DSC measurements were done using a Microcal MC-2 instrument (Microcal Inc., Amherst, MA).26 The MC2 microcalorimeter contains a fixed pair of matched tantalum cells which are filled with the sample and the reference solutions, respectively. Most DSC data were collected with the reference cell filled with triple-distilled water. The (more appropriate) use in the reference cell of aqueous salt solutions of the same salt concentration as in the sample cell resulted in no difference in the CMT and CP values and less than 5% difference in the micellization enthalpy values. The baseline was drawn with the help of appropriate software supplied by the manufacturer of the DSC apparatus: points before and after the thermal transition were selected manually (and set to zero) and a baseline was fitted to pass through these points. Temperature scans were performed at a rate of 30 K/h; both up- and downtemperature scans were carried out for representative samples to ensure the reversibility of the micellization and phase separation processes. The kinetics of micelle formation for L64 are in the millisecond range30,31,46 and the kinetics of phase (36) Wanka, G.; Hoffmann, H.; Ulbricht, W. Colloid Polym. Sci. 1990, 268, 101-117. (37) Beezer, A. E.; Loh, W.; Mitchel, J. C.; Royall, P. G.; Smith, D. O.; Tute, M. S.; Armstrong, J. K.; Chowdhry, B. Z.; Leharne, S. A.; Eagland, D.; Crowther, N. J. Langmuir 1994, 10, 4001-4005. (38) Hvidt, S.; Jorgensen, E. B.; Brown, W.; Schillen, K. J. Phys. Chem. 1994, 98, 12320-12328. (39) Zhou, Z.; Chu, B. Macromolecules 1988, 21, 2548-2554. (40) Reddy, N. K.; Fordham, P. J.; Attwood, D.; Booth, C. J. Chem. Soc., Faraday Trans. 1990, 86, 1569-1572. (41) Wu, G.; Zhou, Z.; Chu, B. Macromolecules 1993, 26, 2117-2125. (42) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700-7710. (43) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 26902700. (44) Zhou, D.; Alexandridis, P.; Khan, A. J Colloid Interface Sci. 1996, 183, 339-350. (45) Alexandridis, P.; Andersson, K. J. Phys. Chem. B 1997, 101, 8101-8109.

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Figure 1. Differential scanning calorimetry curves for 1 and 10 wt % Pluronic L64 aqueous solutions. The main endothermic peak is indicative of micelle formation, while the smaller peak at higher temperatures corresponds to the phase separation. Both CMT and CP values can be obtained from such DSC experiments as indicated on the graph. separation are on the order of a second (Holzwarth, J. F., unpublished results), so the system has plenty of time to attain equilibrium in the course of the DSC measurements.

3. Results and Discussion 3A. DSC Measurements for the Determination of CMT and CP. Differential scanning calorimetry curves for Pluronic L64 aqueous solutions are shown in Figure 1 at two different copolymer concentrations (1 and 10 w/v %). A pronounced endothermic peak is indicative of micelle formation by the PEO-PPO-PEO copolymers,26,36 while the smaller peak at higher temperatures corresponds to the phase separation (cloud point), as confirmed by independent light scattering experiments (which showed an increase in the turbidity of the sample). The onset of the thermal transition due to micellization corresponds to the copolymer CMT;26 similarly, the CP can be obtained from the onset of the thermal transition due to the phase separation. Both CMT and CP values are obtained from the same DSC experiments, as indicated by the arrows on the graph. The association of PEO-PPO-PEO copolymers in aqueous solutions to form micelles is strongly endothermic; the negative (since micellization is a spontaneous process under the conditions of our study) free energy of micellization originates from a positive entropy contribution (related to an entropy gain by water when the copolymers associate and to a decrease in the polarity of EO and PO segments as temperature increases) able to overcome the positive enthalpy change.6,21 Although cooperative, the micellization phase transition is rather broad and spans a temperature range of 10-20 deg. This broadness is due to temperature dependence of the amount of unimers in equilibrium with the micelles (as temperature increases more copolymer changes from the unimer into the micellar state),21,47 as well as the size polydispersity inherent in copolymers.48 As seen in Figure 1, the onset of the micellization transition occurs at lower temperatures when the copolymer concentration increases (in agreement with data in refs 21 and 26), but its width is not affected much by the copolymer concentration. The data of Figure 1 indicate a different concentration dependence between the CMT and the CP: the former decreases with increasing polymer concentration, while the latter increases. Indeed, the CP of Pluronic L64 decreases with concentration for (46) Michels, B., Waton, G.; Zana, R. Langmuir 1997, 13, 31113118. (47) Patterson, I.; Chowdhry, B. Z.; Leharne, S. Colloids Surf. A 1996, 111, 213-222. (48) Linse, P. Macromolecules 1994, 27, 6404-6417.

Alexandridis and Holzwarth

copolymer concentrations below 2%15 but then increases at higher copolymer concentrations.15,43 The enthalpy change related to the micellization process can be calculated from integration of the Cp vs T data obtained in the DSC experiments. The great difference in enthalpies between the micellization and the cloud point transition (seen in Figures 1 and 2) is notable. The highest “caloric penalty” is paid when the PPO blocks of the PEOPPO-PEO copolymer are removed from the aqueous solution through the formation of micelles with a PPO core (microphase separation); an enthalpy change of comparable magnitude to that of micellization is observed during the macrophase separation of PPO homopolymers from aqueous solutions.49 At the macrophase separation (cloud point) of the PEO-PPO-PEO copolymer, it is the PEO blocks (at the micelle corona) which are primarily affected, and the heat required is much less. This observation points to the importance (already established in a number of studies6,21,37,50 ) of the hydrophobic PPO block as the main driving force for the PEO-PPO-PEO micellization in water. The phase transition from a micellar solution to lyotropic liquid crystals is similarly accompanied by a small enthalpy change compared to that of the micellization.50 DSC curves for aqueous Pluronic L64 solutions in the presence of sodium salts and urea are presented in Figure 2. The effects of the additives on the micellization and phase separation of the PEO-PPO-PEO copolymer are apparent: the addition of NaCl, NaBr, and NaI moves the onset of micellization to lower temperatures; the effect of NaI is relatively weak. Urea has also a weak effect on the CMT but opposite to that of the salts: urea increases the CMT (in agreement with our previous study, ref 17). NaCl and NaBr also move the onset of phase separation to lower temperatures; NaI, on the other hand increases the CP. The CP of Pluronic L64 is higher in the presence of urea. In addition to the CMT and CP values, the magnitude of the micellization enthalpy is also affected by the salt type and concentration. The information extracted from the DSC data is presented and analyzed in the subsequent sections. In addition to the CMT/CP and ∆H values, the shape of the DSC outputs originating from the block copolymer micellization and phase separation transitions can provide more information about their nature. For example, there appears to be a shoulder in the micellization DSC curve, probably related to the hydrophobic impurity mentioned in the introduction. The DSC output from a “purified” (by extraction40) L64 sample showed no shoulder and its CMT was 1-2 °C higher than that of the “as received” sample (data not shown). The area under the DSC output, proportional to the enthalpy of micellization, was the same (within the error associated with the definition of the baseline) for both samples, as was the CP. We believe that the presence of this impurity does not affect the discussion and conclusions of this paper (a possible 1-2 K shift in the CMT should be the same for all samples). Pluronic L64 is commercially available and used and there is merit in studying and reporting information on “as received” material. Furthermore, the comparison between the “purified” and the “as received” samples is not straightforward because of a possible shift in the molecular weight distribution of L64 during the extraction (purification) process. The DSC output from the macroscopic phase separation (cloud point) of Pluronic L64 resembles DSC data obtained (49) Armstrong, J.; Chowdhry, B.; O’Brien, R.; Beezer, A.; Mitchel, J.; Leharne, S. J. Phys. Chem. 1995, 99, 4590-4598. (50) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145-4159.

DSC Study of Salt Effects on Poloxamers

Figure 2. Typical DSC curves for aqueous 1 wt % Pluronic L64 solutions in the presence of (0, 0.5, 1, and 2 M) NaCl (top), NaBr (second from top), NaI (third), and (0, 1, 2, and 4 M) urea (bottom).

from the phase separation of poly(ethylene oxide) (PEO) homopolymer in aqueous solution (reported in Figure 1d of ref 49). Such a DSC output is discussed as having a very small calorimetric enthalpy, a relatively large positive increment in apparent excess Cp, and a very long sloping tail which is a manifestation of the temperature dependence of the excess heat capacity.49 The similarity between the DSC transitions corresponding to the CP of the PEOPPO-PEO copolymer and that of PEO homopolymer reinforces the suggestion advanced above that it is the

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PEO blocks at the copolymer micelle corona which are primarily affected at the cloud point. 3B. Critical Micellization Temperature and Cloud Point Data in the Presence of Salts. The effects of anion type (for the NaSCN, NaI, NaBr, and NaCl sodium salts) on the critical micellization temperature and the cloud point temperature of Pluronic L64 in aqueous solutions are summarized in Figure 3. CMT and CP are affected by both the type of electrolyte introduced and its concentration. A linear dependence of CMT and CP on the salt concentration is observed (for the concentration range investigated) for all salts studied (in agreement with refs 14-16) with the exception of NaSCN, where the CMT and CP exhibit a nonlinear but monotonic increase with concentration. The anion effectiveness follows the Hofmeister series Cl- > Br- > I- > SCN-; anions with a low lyotropic number, low radius, high ion-charge density, high hydration, or a strong water-structure-making tendency decrease the CMT and CP. While all salts studied affect both CMT and CP to the same direction, i.e., either increase or decrease (see Figures 3 and 4), the behavior of NaI strikes as being odd. NaI decreases the CMT of the PEO-PPO-PEO copolymer but increases its CP. Both CMT and CP dependencies on the NaI concentration are relatively weak (compared to the other salts) but they suggest different (opposing) contributions of the I- anion to the micellization and phase separation processes (and possibly different interactions of I- with PPO and PEO). Specific interactions between the I- anions and the PEO headgroups have been proposed in order to explain such behavior (see ref 11 and references cited therein). An adsorption of I- at the PEO blocks of the micelles renders them “effectively” charged and increases their repulsion (and their “solubility” in water), thus making it difficult for them to aggregate (and phase separate) at the cloud point. Below the CMT, NaI acts in a manner similar to that of NaCl and NaBr, by worsening the solvent conditions for the copolymer. The effects of cation type (for alkali chloride salts) on the critical micellization temperature and the cloud point temperature of Pluronic L64 copolymer in aqueous solutions are presented in Figure 4. CMT and CP values are plotted as a function of salt concentration for LiCl, KCl, and NaCl. A linear dependence of CMT and CP on the salt concentration is observed. The effectiveness of the cations in decreasing the CMT and the CP follows the series Na+ > K+ > Li+. It is interesting to note that, on a mass concentration basis, K+ is as effective in reducing CMT and CP as Li+. This fact, together with the limited (only up to 2.2%) salt concentration range examined, is probably the reason for the inaccurate report51 that “the decrease [of the critical micellization temperature] is independent of cation size (Li+, Na+, K+)” for a PEOPPO-PEO aqueous solution. The cation trend (Na+ > K+ > Li+) on CMT and CP reduction is not as straightforward as the anion trend (F- > Cl- > Br- > I-) where the stronger effect is observed for the smaller ion (the Li+ ion radius is smaller than that of Na+ and K+). Li+ is very strongly hydrated, such that water molecules are very strongly bound in its inner hydration shell. It has been proposed that, in the case of Li+, the inner hydration shell becomes part of the “ion”, such that the “effective” radius of Li+ is higher than the nominal.11 The decrease in the CP of homopolymer PEO was 14.5, 23, and 24.5 K for 1 M LiCl, NaCl, and KCl, respectively.52 We note that the values for Pluronic L64 are about half of those for the (51) Hecht, E.; Hoffmann, H. Colloids Surf. A 1995, 96, 181-197. (52) Florin, E.; Kjellander, R.; Eriksson, J. K. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2889-2910.

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Figure 3. Effect of anion type (for sodium salts) on the critical micellization temperature (top) and on the cloud point temperature (bottom) of 1 wt % Pluronic L64 copolymer in aqueous solutions. CMT and CP values are plotted as a function of salt molar (left) and weight (right) concentration for NaSCN, NaI, NaBr, and NaCl.

Figure 4. Effect of cation type (for alkali chloride salts) on the critical micellization temperature (top) and on the cloud point temperature (bottom) of 1 wt % Pluronic L64 copolymer in aqueous solutions. CMT and CP values are plotted as a function of salt molar (left) and weight (right) concentration for LiCl, KCl, and NaCl.

PEO homopolymer. Furthermore the order of the salts is different: NaCl is more efficient than KCl in suppressing the CP of L64, while KCl is more efficient than NaCl in suppressing the CP of PEO. The reasons behind these differences are not clear.

In order to contrast the effects of the different ions/ salts, we compared them on the basis of the ion radius and the salt heat of solvation in water, the ion radius being an indirect while the salt heat of solvation a direct measure of the water ion/salt interaction.13 The changes

DSC Study of Salt Effects on Poloxamers

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Figure 5. Salt effects on the micellization and the phase separation temperatures presented as a function of (left) the halide anion radius (for sodium salts) and alkali cation radius (for chloride salts) and (right) the heat of solvation of the salts. The deviations of the critical micellization temperature in the presence of salts from the CMT in the absence of salts are expressed as the slopes of the CMT vs molar (top) and mass (bottom) salt concentration linear fits to the data of Figures 3 and 4. Similarly, the deviations of the cloud point temperature when salts are present from the CP in the absence of salts are expressed as the slopes of the CP vs molar (top) and mass (bottom) salt concentration linear fits to the data of Figures 3 and 4. In the case of NaSCN, where the CMT and CP vary nonlinearly with the salt concentration, the slope reported is taken from the 0-1 M concentration range. The NaF cloud point value was taken to be the same as that for KF reported in ref 16. The ion radii values were taken from: Pauling, L.; Sherman, J. Z. Krist. 1932, 81, 1. The salt solvation heats were extracted from ref 13.

in the copolymer CMT and CP upon the addition of salts (expressed as the slopes of the CMT vs salt concentration linear fits to the data of Figures 3 and 4) are presented in Figure 5 as a function of the ion radius and the salt heat of solvation. Clearly, the halide anions with smaller radii and higher heats of solvation in water have the greater influence on the CMT and CP. Both ∆CMT ) CMT(no salt) - CMT(salt) and ∆CP ) CP(no salt) - CP(salt) decrease with increasing anion radius (in the order F- to I-) and decreasing solvation heat (in the order NaF to NaI). The dependence of ∆CMT and ∆CP on the anion radius and solvation heat even becomes linear when the salt concentration is expressed on a mass basis. For the three different alkali cations examined here, the ∆CMT and ∆CP seem to depend less (and nonmonotonically) on the ion size and solvation heat. In fact, it is the Li+ ion which is not following the expected trend, giving credence to the speculation that the effective Li+ radius is larger than the one used to plot the data in Figure 5. Even if Li+ were to follow the line connecting K+ and Na+, the slope of this line (∆CMT or ∆CP vs ion radius or solvation heat) would still have been smaller than the corresponding slope for the anion series (and the cation effect on CMT and CP weaker than the anion effect). Perhaps the most striking observation coming out of Figure 5 is that salts affect both CMT and CP to the same extend: CMT(no salt) - CMT(salt) ) CP(no salt) - CP(salt). This relationship holds for all salts examined here, with the (probably justifiable) exception of NaI. The equality

∆CMT ) ∆CP has both theoretical and practical consequences: it implies a common mechanism by which salts affect both the micellization and phase separation phenomena; it also suggests that CMT and CP measurements can be used interchangeably to examine the effects of salts on amphiphile solutions. While more experiments (at different copolymer concentrations and with different copolymer chemical compositions) are needed to establish the generality of the above observation, its importance cannot be underestimated. In a study of the micellar properties of two different PEO-PPO-PEO copolymer (Pluronics P85 and L64) in aqueous solution, the authors report that “the changes of size and shape of the micelles, revealed by the intrinsic viscosity and rheological properties of the solution, seemed to occur at the same temperature relative to the cloud point, independent of the nature of the salt”.15 This observation supports our finding; unfortunately, no CMT data were reported in ref 15 to validate the ∆CMT ) ∆CP relationship. 3C. Micellization Enthalpy as a Function of Salt Concentration and Unimer Concentration as a Function of Temperature. In addition to the valuable information on CMT and CP, the DSC curves can be analyzed to provide information on the enthalpy of micellization (directly, by integration of the Cp-T data26,37) and on the unimer concentration as a function of temperature (indirectly, based on a thermodynamic model of association21,47,49) for amphiphile solutions. The enthalpy change represented by the DSC peak area is not the

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Figure 6. Effect of sodium salts on the enthalpy uptake (∆H) due to the micellization of Pluronic L64 copolymer in water. The % change in ∆H is plotted as a function of salt concentration for NaCl, NaBr, and NaI.

standard enthalpy change (defined for the transfer of 1 mol of amphiphiles from the dilute ideal solution to the micellar state) commonly used to characterize the thermodynamics of amphiphile association, because it depends on the real states (finite concentration-intermolecular interactions) of the amphiphile before and after micellization, but it is still a very useful measure of the micellization enthalpy. Figure 6 shows the effect of sodium salts (NaCl, NaBr, and NaI) on the enthalpy uptake (∆H) due to the micellization of Pluronic L64 copolymer in water. Both NaCl and NaBr increase significantly (by 25 to 35%) the (endothermic) micellization enthalpy; the increase is more pronounced at salt concentrations up to 1 M and appears to reach a plateau at higher salt concentrations. NaI exhibits a weaker effect and while it seems to increase ∆H at 0.5 M (the error associated with deriving ∆H values from the DSC curves which may be as high as 5%), it decreases ∆H by 8% when present at 2 M concentration. For an unequivocal interpretation of the observations of Figure 6, the specific enthalpic interactions in the binary salt-water, copolymer-water, and copolymer-salt systems must be resolved (e.g., by titration calorimetry experiments). A higher positive ∆H (in the presence of NaCl and NaBr) for the same ∆G (proportional to the CMT and to the logarithm of copolymer concentration) requires a higher positive ∆S, which may be related to a gain in the entropy of the ions when the copolymers associate into micelles (similarly to the water entropy gain invoked in order to explain the positive micellization ∆H). The unimer (unassociated amphiphile) concentration in equilibrium with micelles can be calculated as a function of temperature from the DSC data using a model for the amphiphile association: (i) the association model21,26 where the 1/CMT vs ln X linear plot (CMT in K; X, mole fraction) is constructed from the CMT (determined from DSC) at 1% copolymer and the slope (inversely proportional to the ∆H determined from DSC), or (ii) the procedure described in ref 47 where the extent of unimer conversion to micelles (copolymer mass in micelles divided by the total copolymer mass) at a given temperature T is set equal to the ratio of the enthalpy, Q(T), absorbed upon heating up to a temperature T to the total enthalpy, ∆H, of the micellization transition (see ref 47 for details). Although these two methods are not equivalent, it was shown in ref 47 that there is a reasonable agreement between the unimer concentrations obtained from deconvolution of the DSC curves and independently determined critical micelle concentration data at different temperatures.21 Both methods consider the micellization enthalpy to be independent of temperature, justifiable for

Figure 7. Effect of salt type and concentration on the Pluronic L64 unimer concentration in equilibrium with micelles at different temperatures. Graphs of the unimer concentration in the presence of (0, 0.5, 1, and 2 M) NaCl, NaBr, LiCl, and NaI are shown from top to bottom.

the 20-degree-wide micellization transition. Figure 7 shows the unimer concentration in equilibrium with micelles as a function of temperature for different salts and salt concentrations. The lines plotted in Figure 7 can be viewed as a form of micellization “phase boundaries”: only unimers are present in solution at concentrations and temperatures below the lines; micelles in equilibrium with unimers are present above the boundaries. 3D. Further Considerations on the Mechanism of Salt Effects on PEO-PPO-PEO Amphiphilic Copolymers. The effects of the different ions/salts on micellization and phase separation can be correlated (Figure 5) to the ion radius and the salt heat of solvation in water (measure of the water-salt interaction); however, the fundamental mechanisms remain elusive. Below we

DSC Study of Salt Effects on Poloxamers

examine the relationship between salt effects on micellization and salt effects on the cloud point and then present possible interpretations for the effects of salt on amphiphile solutions. While the great difference in enthalpy between the micellization and cloud point transitions (see Figures 1 and 2) indicates a different underlying cause for the two phenomena (removal of PPO from water in the former case and removal of PEO from water in the latter case), the strikingly similar magnitude of the CMT and CP suppression by the salts observed in our study (see Figure 5, NaI is an exception) suggests some common mechanism of salt interaction with the PPO and PEO blocks. We must keep in mind that both PEO and PPO are polar (because of the ether oxygen) and that PEO also contributes to the micellization by a decrease in its polarity upon increasing temperature.6 Both PEO and PPO are affected by temperature in a similar manner, so they may also be affected by salts in a similar manner. The difference between the CP and CMT effects in the case of NaI can be either considered an exception (in terms of the above discussion) or an indication of a different mechanism; in the light of the specific interactions between I- and PEO reported in the literature, we would rather consider the NaI behavior an “exception”. The decrease of CP in PEO-containing surfactants caused by the presence of salts has been traditionally attributed to a decreased hydration of the PEO segments due to an increase of the water “structure”.8,52 The following explanation for the PEO-ion interaction in water has been advanced:52 The water surrounding the ion is polarized by the ionic field, resulting in a low free energy, while the water in the PEO hydration shell is in a high free energy state because of unfavorable entropy contributions. When an ion approaches PEO, the amount of intervening water decreases, leading to a repulsive (image-charge) force between the ion and PEO (since PEO is far less polarizable than water). The removal of water from the PEO hydration shell, on the other hand, leads to an attractive force (PEO-PEO interaction). The balance of these contributions to the total force will depend on the ion. The attractive component of the force is lower for smaller ions, since less structured water around PEO is expelled. The anion effects are greater since the ion size and polarizability vary more with the anions than the cations.52 Salt effects on the phase equilibrium (Winsor III system) of PEO-containing surfactants have been related to salt adsorption/depletion at the surfactant monolayer:19 the “lyotropic” salts NaF, NaCl, and NaBr were shown to desorb (depleted), whereas the “hydrotropic” salts NaI and NaSCN adsorb at the surfactant monolayer of the bicontinuous microemulsion phase. Addition of NaCl etc. made water a more polar (in a phenomenological sense) solvent and the bicontinuous microemulsion phase became water rich.19 It is interesting that ion adsorption/ depletion at the PEO interface and increased/decreased hydration of the PEO segments (due to a decrease/increase of the water “structure”) are linked, in the sense that ions which are depleted from PEO also remove water from it because they change the osmotic pressure. The effect of (alkali halide) salts on the micellization of oligo(ethylene oxide) alkyl ether surfactants has been attributed11 to a decrease in the solubility of the alkyl moieties in aqueous salt solutions, rather than an effect on the EO parts. The polarizing ions increase the interfacial free energy and thus oppose micellization, but still such salt effects on micellization are overcome by the enhanced hydrophobic driving force. The differences between PPO, the hydrophobic part of the PEO-PPO-

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PEO copolymers, and the methyl groups should be kept in mind when comparing the micellization of these copolymers to that of oligo(ethylene oxide) alkyl ether surfactants. The contribution of one PO segment to the micellization free energy is 0.2-0.3 kT, four to six times smaller than the contribution of one methylene group (1.2 kT),36,50,53 reflecting the higher solubility (and specific interactions) of PPO in water. 3E. Practical Aspects in the Use of Salts for Modulating the Solution Properties of Amphiphiles. The practical utility of using salts for modulating the solution properties of the PEO-PPO-PEO copolymer becomes evident from the data of Figures 3 and 4: a 10% NaCl solution can decrease the CMT and CP by almost 30 °C. In applications where temperature can readily be varied, the trade-off between the cost of salt and the cost of heating must be considered in order to determine the most efficient way for reaching (or avoiding) the CMT or the CP (we note that, while the “lighter” ions show a greater effect on a molar basis, they become even more effective on a weight basis). If temperature is not an operating variable, then either the copolymer concentration or the copolymer chemical composition must be varied to meet the desired amphiphile solution properties. A rough correspondence between 10 °C and a concentration decade (on a log scale) has been observed21,32 in the micellization of PEO-PPO-PEO copolymers (e.g., if a 0.1% solution has a CMT at 30 °C, then the CMT of a 1.0% solution is 20 °C). This makes the increase of copolymer concentration in order to achieve the same effect on CMT as the salt prohibitively expensive (and often impossible because the micellar solution is no longer thermodynamically stable at sufficiently high amphiphile concentrations43,44,50). The hydrophilic/lipophilic properties which are inherent to the amphiphile’s chemical nature are an important determinant of the amphiphile’s solution properties. In the case of the PEO-PPO-PEO amphiphilic copolymers, a drastic alteration of the copolymer chemical composition (by, e.g., a tripling of its molecular weight at a fixed EO/ PO ratio, or a tripling of the PPO block length at a fixed PEO block length21) would be required in order to achieve the same effects in the copolymer solution properties as the addition of 10% NaCl. Even when such amphiphiles are commercially available, there may be an advantage in employing salt rather than chemistry as a tool, since the salt allows the fine-tuning of the micellization and cloud point temperatures. Furthermore, the utilization of the same amphiphile at different solvent conditions (controlled by the addition of salt), as opposed to amphiphiles of different compositions, is expected to ease the regulatory and process validation burdens associated with, e.g., pharmaceutical applications. Lastly, we comment on the equality ∆CMT ) ∆CP observed in the PEO-PPO-PEO aqueous solutions in the presence of salts. This is useful in practice because it reduces the number of experiments needed for the CMT and CP determination, and it becomes very important when either the CP or the CMT cannot be measured. CP is usually the easier to ascertainsrequiring equipment no more sophisticated than a thermometer and a pair of eyessbut sometimes it occurs at high temperatures and cannot be measured conveniently. 4. Conclusions The aqueous solution properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) amphiphilic copolymer (Pluronic L64: EO13PO30EO13) (53) Yang Y.-W.; Deng, N.-J.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703-4711.

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were studied in the presence of various alkali halide salts (LiCl, KCl, NaCl, NaBr, and NaI), sodium thiocyanate (NaSCN), and urea ((NH2)2CO). At low concentrations and temperatures the PEO-PPO-PEO copolymer molecules are present in solution as individual coils (unimers). Thermodynamically stable micelles are formed with increasing solution temperature at a constant copolymer (and salt) concentration. At even higher temperatures the copolymer phase separates out of solution. Differential scanning calorimetry (DSC) was the technique employed for the determination of both the unimerto-micelle transition temperature (critical micellization temperature, CMT) and the phase separation temperature (cloud point, CP). DSC is particularly useful in the case of L64 aqueous solutions because the detection of the CMT by optical techniques is hindered by the presence in L64 of a hydrophobic impurity which phase separates at the vicinity of the CMT. DSC gave also valuable information of the enthalpy differences and on the unimer concentration at different temperatures above the CMT. Both the micellization and the phase separation of the PEO-PPO-PEO copolymer in water are endothermic. The micellization (microphase separation) enthalpy, ∆H, was much larger than the (macro-) phase separation enthalpy, demonstrating the dominance of the PPO-water interactions over the PEOwater interactions. ∆H increased with increasing NaCl and NaBr and decreasing NaI and urea concentrations.

Alexandridis and Holzwarth

Salts are important in modifying the copolymer aqueous solution properties. Our study examined both alkali chlorides and sodium halides. The presence of LiCl, KCl, NaCl, and NaBr decreased both CMT and CP (in the order Cl- > Br- and Na+ > K+ > Li+), whereas addition of NaSCN and urea resulted in an increase of CMT and CP (in the order NaSCN > urea). NaI appeared to be an intermediate case as it decreased the CMT but increases the CP; this behavior could be attributed to specific interactions (adsorption) between I- and PEO. The effects of the salts on the solution behavior of the PEO-PPOPEO copolymer could be correlated to the ion radius and the solvation heat of the salts. The halide ions with smaller radius and higher heat of solvation in water have the greater influence on the CMT and CP. Variation of the anion type (rather than the cation) is a more effective means of modulating the CMT and CP. This is the first study where the effects of salts on the CMT and CP values were simultaneously determined for a PEO-PPO-PEO copolymer in aqueous solution; such measurements led to the important observation: CMT(no salt) - CMT(salt) ) CP(no salt) - CP(salt). This equality has both fundamental (with respect to the mechanism of the salt effect to the different phenomena) and practical (in terms of determining experimentally CMT and CP) implications. LA9703712