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Langmuir 1999, 15, 930-935
Articles Effect of Hydrotropes on the Volume Phase Transition in Poly(N-isopropylacrylamide) Hydrogel† Dibakar Dhara and Prabha R. Chatterji* Speciality Polymers Group, Organic Coatings & Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received February 18, 1998. In Final Form: October 19, 1998 With a view to understand the complex molecular forces involved in the volume phase transition (VPT) exhibited by poly(N-isopropylacrylamide) hydrogels the effect of hydrotropes on VPT was monitored. Hydroxybenzenes, hydroxybenzoates, and benzenesulfonates, which constitute the class of hydrotropes, are capable of solubilizing a variety of nonpolar organic molecules in water. Hydrotropes permit an assessment of subtle molecular and structural features on the VPT because ortho, meta, and para isomers differ in their hydrotropy. Among the compounds studied, in a selected concentration range we observed that dihydroxybenzoic acid is the most efficient in lowering the transition temperature and salicylate anion raises the transition temperature while m-hydroxybenzoate offers minimum perturbation. The results are indicative of entropic cooperativity of many interactions stabilizing a conformation as the driving force for the volume phase transition.
Introduction Poly(N-isopropylacrylamide) (PNIPA) occupies a central position in the fast-emerging field of stimuli-responsive hydrogels. Ever since the report of Tanaka et al.1 that PNIPA hydrogels can undergo temperature-induced, sharp changes in water content, there has been a surge of activities to understand this phenomenon. The volume phase transition (VPT) in PNIPA hydrogels is a direct consequence of the lower critical solution temperature (LCST) behavior of the linear polymer in water.1-4 All interpretations of LCST/VPT have focused on the structural features of PNIPA. PNIPA has a nonpolar hydrocarbon backbone with a hydrophobic isopropyl pendant group connected through a polar hydrophilic amide functionality. It has been suggested that the extensive H-bonding network of the amide group with water is primarily responsible for keeping the linear polymer in solution or the gel swollen.5-11 It is presumed that at temperatures below LCST the strong H bonding between the hydrophilic -CdO and -NH groups and water overcomes the unfavorable free energy change due to exposure of hydrophobic isopropyl groups and the hydrocarbon backbone to water and keeps the polymer in * To whom correspondence should be addressed. † IICT Communication No. 3967. (1) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (2) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (3) Hoffman, A. S. J. Controlled Release 1987, 4, 213. (4) Hirotsu, S. J. Chem. Phys. 1988, 88, 427. (5) Bae, Y. H.; Okano, T.; Kim, S. W. J. Polym. Sci., Polym. Phys. Ed. 1990, 28, 923. (6) Urry, D. W. Prog. Biophys. Mol. Biol. 1992, 57, 23. (7) Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283. (8) Tokuhiro, T.; Amiya, T.; Mamada, A.; Tanaka, T. Macromolecules 1991, 24, 2943. (9) Winnik, F. M. Macromolecules 1990, 23, 233. (10) Luan, C. H.; Urry, D. W. J. Phys. Chem. 1991, 95, 7896. (11) Luan, C. H.; Parker, T. M.; Prasad, K. U.; Urry, D. W. Biopolymers 1991, 31, 465.
solution. The possibility of hydrophobic hydration has also been hinted at.7,12-16 As the temperature rises and crosses a threshold value, the hydrogen bond network is disrupted and the polymer, if linear, precipitates or, if in the gel state, deswells. There have been several reports on the effect of additives on the VPT of PNIPA hydrogels, and a variety of parameters have been implicated.17-23 Park and Hoffman24 studied the effect of several salts and drew a parallel between the Hofmeister series for salt-induced protein precipitation in an aqueous medium and the volume phase transition in PNIPA hydrogels. Anions and cations have been ranked according to their ability to salt in/out proteins from an aqueous medium, and it has been proposed that ions which disrupt the water structure precipitate the protein and those which reinforce solubilize the protein.25,26 Anions and cations act independently, but their activities are additive; however, the activities of anions are severalfold greater than those of cations. In the same light Park and Hoffmann24 contended that anions (12) Luan, C. H.; Harris, R. D.; Prasad, K. U.; Urry, D. W. Biopolymers 1990, 29, 1699. (13) Otake, K.; Inomata, H.; Konno, M.; Saito, S. J. Chem. Phys. 1989, 91, 1345. (14) Privalov, P. L.; Gill, S. J. Adv. Protein Chem. 1988, 39, 191. (15) Privalov, P. L. Annu. Rev. Biophys. Chem. 1989, 18, 47. (16) Murphy, K. P.; Privalov, P. L.; Gill, S. J. Science 1990, 247, 559. (17) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687. (18) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H.; Saito, S. Langmuir 1993, 9, 46. (19) Inomata, H.; Otake, K.; Saito, S. Langmuir 1992, 8, 1030. (20) Schild, H. G.; Tirrel, D. A. Langmuir 1991, 7, 665. (21) Sakai, M.; Satoh, N.; Tsujii, K.; Zhang, Y.-Q.; Tanaka, T. Langmuir 1995, 11, 2493. (22) Saito, S.; Konno, M.; Inomata, H. Advances in Polymer Science; Springer-Verlag: Berlin, 1991; Vol. 109, p 207. (23) Mears, S. J.; Deng, Y.; Cosgrove, T.; Peltron, R. H. Langmuir 1997, 13, 1901. (24) Park, T. G.; Hoffman, A. S. Macromolecules 1993, 26, 5045. (25) von Hippel, P. H.; Schleich, T. Acc. Chem. Res. 1969, 2, 257. (26) von Hippel, P. H.; Wong, K. Science 1964, 145, 577.
10.1021/la980194k CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
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Figure 1. Hydroxybenzene class of hydrotropes used in this study.
which reinforce the water structure around the polymer keep it in solution and raise the transition temperature and those which break up this network in effect dehydrate the polymer chain, thus lowering the transition temperature and leading to its precipitation. There have been attempts to link the viscosity B coefficient of the anion and its structure-breaking ability.17-22 Surface adsorption and lowering of the surface tension of water have been invoked to explain the effect of alkylsulfonates. Tanaka et al.21 observed a direct relationship between alkyl chain length and the transition temperature. However, the behavior of tetraalkylammonium bromides was not well-defined.17 Even (NH4)Br does not fit into its place in the Hofmeister series. That nonionic organic additives bring down the transition temperature has been attributed to their ability to decrease the dielectric constant of water, thereby reducing interaction of water molecules with polar groups.22 In short, various factors have been implicated, but there is no consolidated comprehensive model which can explain all of the observations. We felt that to establish the role of intermolecular interactions and molecular structure in modulating the VPT in PNIPA hydrogels, hydrotropes will prove to be a powerful tool. Hydrotropes27-34 are hydroxybenzenes, hydroxybenzoates, and benzenesulfonates which, when present in high concentration in water, can enhance the solubility of hydrophobic compounds severalfold. Above a certain characteristic concentration, termed the minimum hydrotrope concentration (MHC), these compounds aggregate to offer nonpolar microdomains to solubilize hydrophobic solutes. Hydrotrope assemblies have special geometrical features which enable them to distinguish among solubilizates. For example, the ortho, meta, and para isomers differ in their hydrotropy. This makes hydrotropy distinctly different from salting in, phase mixing, or micellar solubilization.34-36 These features suggest that several balancing molecular forces are at work in hydrotropy. The hydroxybenzenes we selected (27) Lawrence, A. S. C. Nature 1959, 183, 1491. (28) McKee, R. H. Ind. Eng. Chem. Int. Ed. 1946, 38, 382. (29) Rath, H. Tenside 1965, 2, 1. (30) Friberg, S. E.; Rydhag, L. J. Am. Oil Chem. Soc. 1971, 48, 113. (31) Cox, J. M.; Friberg, S. E. J. Am. Oil Chem. Soc. 1981, 58, 743. (32) Srinivas, V.; Rodley, G. A.; Ravikumar, K.; Robinson, W. T.; Turnbull, M. M.; Balasubramanian, D. Langmuir 1997, 13, 3235. (33) Srinivas, V.; Sundaram, C. S.; Balasubramanian, D. Indian J. Chem. 1991, 30B, 147. (34) Balasubramanian, D.; Srinivas, V.; Garikar, V. G.; Sharma, M. M. J. Phys. Chem. 1989, 93, 3865. (35) von Hippel, P. H.; Schleich, T. In Structure & Stability of Biological Macromolecules; Timasheff, S. N., Fasman, G. D., Eds.; Marcel Dekker: New York, 1969. (36) Balasubramanian, D.; Mitra, P. J. Phys. Chem. 1979, 83, 2724.
Figure 2. Hydroxybenzoic acids used in this study.
Figure 3. Anionic hydrotropes used in this study.
differ from each other in minute structural features in the number and position of hydroxyl groups. We contended that if dehydration of the isopropyl group is the primary event leading to the collapse of the PNIPA chains, then hydrotropes could prevent or at least delay this process. We began this study with proven hydrotropes but felt compelled to include compounds of related structures but no manifestation of hydrotropy for a more rational and realistic interpretation of the results. The structures of various compounds included in this study are given in Figures 1-3. Experimental Section Materials. N-Isopropylacrylamide (NIPA) from Aldrich was recrystalized twice from hexane, N,N′-methylenebis(acrylamide) (BIS), ammonium persulfate (APS), and N,N,N′,N′-tetramethylethylenediamine (TEMED) special grade from SRL, Bombay, India, were used without further purification. All other chemicals and solvents were of analytical grade. A sodium salt of mhydroxybenzoic acid and a potassium salt of salicylic acid were prepared in the lab by neutralizing the acid with the corre-
932 Langmuir, Vol. 15, No. 4, 1999 sponding bicarbonate. The aqueous solution of salts were lyophilized, and the salts were reprecipitated from methanol. Double-distilled water was used for all of the experiments. Preparation of Hydrogel Disks. PNIPA gels were prepared by the usual procedure of free-radical polymerization in water at 4 °C for 8 h. The pregel solution (100 mL) containing 700 mM NIPA monomer and 8.6 mM BIS (cross-linker) was polymerized by adding 50 mg of APS and 0.05 mL of TEMED in a glass slab mould. The slab gel was dislodged carefully and punched into circular disks (diameter 1.6 cm) with a cork borer. The disks were washed extensively with distilled water, dried at room temperature to constant weight, and used for swelling studies. The degree of swelling of the gels was calculated on the basis of the dry and the equilibrium swollen weights.37,38 Synthesis of Linear PNIPA and Determination of Its LCST. NIPA was polymerized by following the method of Park and Hoffman24 in a toluene/terahydrofuran mixture (75/25, v/v) using azobis(isobutyronitrile) (AIBN) as the initiator at 60 °C for 24 h (mw ∼ 104). The cloud point (LCST) of linear PNIPA in different additive solutions was determined by reading the transmittance at 550 nm using an UV-vis spectrophotometer (Perkin-Elmer UV-vis spectrophotometer model Lambda 2). PNIPA (1% w/v) was dissolved in an additive solution of desired concentration at a suitable temperature below the LCST. The cell holder in the spectrophotometer was maintained at the required temperature by circulating water from a thermostat. The temperature was gradually raised, and the transmittance was recorded. The LCST of PNIPA was taken as the temperature which registered a 50% decrease in transmittance.
Dhara and Chatterji
Figure 4. Effect of hydroxybenzenes on the swelling behavior of PNIPA hydrogels at 24 °C.
Results and Discussion According to their ability to influence the transition temperature, additives can be classified into three categories: (1) those which raise the transition temperature, (2) those which do not affect the transition temperature, and (3) those which lower the transition temperature. The series of experiments we conducted and the data from literature show that the majority of the additives fall in the last category. Any additive which improves the solubility of PNIPA will naturally raise the transition temperature. We had expected the hydrotropes to improve the solubility of the polymer on two counts: (1) by forming aggregates which shield and protect the hydrophobic domains of the polymer and (2) by decreasing the surface tension because the lower the surface tension, the lower the energy for solubilization. Both situations would have enhanced the solubility of the polymer and consequently raised its transition temperature. However, contrary to our expectations in all cases except one, as the hydrotrope concentration increases, the transition moves to lower temperatures. Hydroxybenzenes. Figure 4 shows the effect of hydroxybenzenes on the swelling of PNIPA hydrogels. As we move from pyrogallol to 4-methylcatechol, the concentration of the additive required to effect transition at room temperature decreases. Figure 5 summarizes the relationship between the concentration of the additive and the transition temperature in each case for both linear and cross-linked PNIPA. 4-Methylcatechol is the most efficient in lowering the transition temperature, and pyrogallol, the least. In fact, they could be ranked as follows in their efficiency in lowering the transition temperature: (37) Rathna, G. V. N.; Mohan Rao, D. V.; Chatterji, P. R. Macromolecules 1994, 27, 7920. (38) Padmavathi, N. Ch.; Chatterji, P. R. Macromolecules 1993, 29, 1976.
Figure 5. Lowering of the transition temperature as a function of the hydroxybenzene concentration.
4-methylcatechol > resorcinol > hydroquinone > catechol > pyrogallol Between 4-methylcatechol and pyrogallol are the three isomers meta (resorcinol), para (hydroquinone), and ortho (catechol). It is interesting to note that structurally just one methyl group differentiates 4-methylcatechol from catechol, but this difference enhances the ability of the former to lower the transition temperature substantially. While it can be said that pyrogallol with its three contiguously placed hydroxyl groups is more hydrophilic and 4-methylcatechol more hydrophobic, the subtle differences in the behavior of ortho, meta, and para isomers emphasize the importance of structural requirements in addition to hydrophilic/hydrophobic forces. Hydroxybenzoic Acids. Hydroxybenzoic acids do not strictly fall in the category of hydrotropes; however, we included them in our study for comparison. With solubility being a limiting factor, we could operate only at very low concentrations; still they are capable of lowering the transition temperature. Here again the isomers register minute yet distinct differences and can be ranked in the following order of efficiency in lowering the transition
Effect of Hydrotopes on the VPT
Figure 6. Effect of hydroxybenzoic acids on the swelling of PNIPA hydrogels at 26 °C.
Figure 7. Lowering of the transition temperature as a function of the hydroxybenzoic acid concentration.
temperature:
o,p-dihydroxybenzoic acid > p ) m-benzoic acid > salicylic acid > benzoic acid The data presented in Figures 6 and 7 bring into sharp focus some interesting points. The behavior of para and meta isomers of hydroxybenzoic acid is superimposable, but the ortho isomer salicylic acid is less efficient and is closer to benzoic acid in behavior. In this series very surprisingly dihydroxybenzoic acid with a maximum number of hydrophilic groups is the most efficient in lowering the transition temperature. Benzoates, Hydroxybenzoates, and Benzenesulfonates. Anions and cations act independently, but their activities are additive; however, the activities of anions are severalfold greater than those of cations. Cations other than Na, K, and NH4 are difficult to study because they suffer from poor solubility. Among these three, the purity of the ammonium salt is always dubious.
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Figure 8. Effect of anionic hydrotropes on the swelling behavior of PNIPA hydrogels at 26 °C.
Figure 9. Effect of anionic hydrotropes on the transition temperature of PNIPA hydrogels.
Indeed we have carried out an extensive study of the effect of Na+ and K+ on the volume phase transition in PNIPA hydrogels.39 The results strongly indicate that the anion decides the course of events and the contribution from the cation toward influencing the LCST is negligible. In the salting out/in phenomenon too the effectiveness of cations is several orders of magnitude lower than that of the anions. Anions in general have been classified as the water structure breakers and makers, and this property has been linked to their ability to influence VPT.24 However, in the present case, the property of hydrotropy too should be considered in combination. The effect of anionic hydrotropes on the swelling behavior of PNIPA hydrogels is shown in Figure 8. Among benzoates, hydroxybenzoates, and benzenesulfonates, salicylate alone is capable of raising the transition temperature (Figure 9), while the rest of the salts lower it. All four members of this group (39) Dhara, D.; Chatterji, P. R. Swelling and deswelling pathways in nonionic PNIPA hydrogels. Langmuir, submitted for publication.
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Dhara and Chatterji
Figure 10. Effect of o-, m-, and p-hydroxybenzoates on the LCST of an aqueous PNIPA solution.
are established hydrotropes. p-Toluenesulfonate and p-xylenesulfonate anions perturb the temperature only slightly whereas benzoate lowers the temperature sharply and salicylate raises it equally high. It is necessary here to recall that the salicylic acid in fact depresses the transition temperature (Figure 7). The behavior of the salicylate anion cannot be explained in isolation. For a more relevant interpretation we compared its behavior with that of the sodium salts of meta and para isomers (Figure 10). While the salicylate anion, the ortho compound, elevates the LCST, the meta isomer has a negligible effect and the para isomer depresses it. This suggests multiple forces at work especially the possibility of electrostatic interactions stabilizing the hydrophilic conformation of PNIPA chains in an aqueous medium. From structural considerations, because of the proximity of the hydroxyl and carboxylate groups, the salicylate anion can form a stable sixmembered H-bonded complex with the amide nitrogen stabilizing the chain in solution. This will not be possible for meta and para isomers. In an attempt to consolidate the results, Figure 11 groups together selected hydroxybenzenes and hydroxybenzoic acids. It is explicit that the efficiency of these compounds in influencing the transition temperature does not conform to any pattern. Earlier, comparing the effect of nonionic organic additives such as methanol, ethanol, 1-propanol, butanol, and glycerol, Saito et al.22 reported that 1-butanol was the most efficient in decreasing the transition temperature and methanol was the least effective and the ranking is as follows:
1-butanol . 1-propanol > glycerol > ethanol > methanol ) urea Saito et al. explain this series with reference to the viscosity B coefficient which is a measure of the hydration structure around the amide group. Considering the welldocumented ability of urea to disrupt hydrogen bonds,40 its position in this series is indeed incongruous. Thus, a totally different ranking would have resulted if the operating forces were simple hydrogen-bonding, hydrophobic interactions or steric considerations alone. This necessitates us to focus on the salient features of the transition itself. (40) Stryer, L. In Biochemistry; Freeman: New York, 1981.
Figure 11. Comparison of the efficiency of hydroxybenzenes and hydroxybenzoic acids in lowering LCST.
Figure 12. Effect of hydroxybenzoic acids on the transition temperature of linear and gel PNIPA.
The most striking feature of the transition is its sharpness. The transition temperature has only a mild dependence on the molecular weight. Moreover, the LCST of the polymer and the VPT of the gel differ very little, hardly by 1 or 2 °C.24 We reconfirm this observation that while there might be a difference between various isomers, the responses of the linear polymer and that of the gel toward each additive are almost identical. The data for hydroxy acids are shown in Figure 12. These points underscore cooperative forces holding together an ordered structure. Is such a situation prevalent in PNIPA? The reversibility of the transition and the small ∆H associated with it6,41 seems to indicate so. Reversible molecular interactions in biology are mediated by three different kinds of forces: electrostatic, hydrogenbonding, and van der Waals forces. These three fundamental noncovalent bonds differ in their geometrical requirements, strength, and specificity. Furthermore, (41) Vadnere, M.; Amidon, G.; Lindenbaum, S. L. Haslam, J. L. Int. J. Pharm. 1984, 22, 5528.
Effect of Hydrotopes on the VPT
they are affected in different ways by the presence of additives. Electrostatic interactions are the weakest in water and strongest under vacuum. Hydrogen bonding demands very specific spatial organization of the interacting molecules. The strongest hydrogen bonds are those in which the acceptor and donor atoms are collinear. If the acceptor atom is at an angle to the line joining the donor and hydrogen atoms, the bond becomes weaker with increasing angle. The weakest in the group, van der Waal forces, also require sterical complementarity for effective operation. It is likely that the conformation of PNIPA chains in solution or in the swollen gel is stabilized by a combination of forces including hydrogen bonds, van der Waals interactions, electrostatic interactions, and hydrophobic effects. In each case, of course, the net contribution to the stability of the given conformational state is the difference between the strength of interaction in the collapsed state and that in the solvated state. The amide group undoubtedly enters into extensive hydrogen bonding with water.
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However, the transition is too sharp to be expected from the disruption of a single type of interaction. The entropic cooperativity of the simultaneous presence of many interactions stabilizing a single conformation seems to be the driving force for the volume phase transition. Conclusions The effect of hydrotropic compounds on the swelling behavior of PNIPA hydrogels strongly advocates that the conformation of PNIPA chains in solution or in the swollen state is stabilized by a combination of forces including hydrogen bonds, van der Waals interactions, electrostatic interactions, and hydrophobic effects. Acknowledgment. D.D. acknowledges financial help from University Grants Commission, New Delhi, India, in the form of a senior research fellowship LA980194K
