Langmuir 1994,10, 2954-2959
2954
Effect of Surfactants on the Volume Phase Transition of Cross-Linked Poly(acryloy1-L-prolinealkyl esters) Agneza Safranj,*'t Masaru Yoshida,? Hideki Omichi,t and Ryoichi Katakaii Department of Material Development, Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Watanuki-machi 1233, Takasaki, Gunma 370-12, Japan, and Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376, Japan Received January 31, 1994. I n Final Form: June 8, 1994@ The swelling equilibrium of cross-linked poly(acryloy1-L-prolinealkyl esters) (poly(A-ProOR),where R is methyl, ethyl, or propyl) was studied as a function of temperature in aqueous solutions of surfactants. In pure aqueous solution the poly(A-ProOMe) gel showed inverse volume phase transition at around 14 "C, poly(A-ProOEt)gel showed inverse volume phase transition at 2 "C, and the most hydrophobic gel, poly(A-ProOPr),did not show any phase transition. The effect of adding such different surfactants as sodium dodecyl sulfate (SDS, anionic), cetyltrimethylammonium chloride (CTAC, cationic), dimethyl laurylbetaine (DALB, zwitterionic), and octaethyleneglycol monodecyl ether (DOE, nonionic),on the phase transition was investigated. At the low surfactant concentration the following picture emerged: Addition ofthe anionic or cationic surfactant t o the solution raised the transition temperature as well as the swelling. Zwitterionic surfactant had no effect on the transition temperature, but the volume transition became sharper. The nonionic surfactant did not affect either the transition temperature or the volume change. With the increase of the surfactant concentrationhowever,the effects became more complex. The changes in swelling behavior of the gels in the presence of surfactants result from the association of surfactants with the polymers and formation of mixed micelles. Our results confirm that the main driving force for this association is the hydrophobic interaction between nonionic gels and surfactants, and depends on the hydrophobicity of the gel, the nature of the surfactant, and its concentration in the solution.
Introduction In recent years, polymers which exhibit good solubility in aqueous solutions at low temperatures but separate from the solvent when temperature is raised above a certain level (lower critical solution temperature, LCST) have been much investigated. When such a polymer is cross-linked, hydrogel is obtained, that shows a volume phase transition in aqueous solutions, changing from a highly swollen network a t low temperature, to a collapsed phase above the LCST. One example of such a polymer is poly(N-isopropylacrylamide). The volume phase transition of this gel has been widely studied and has been proposed for various applications ranging from devices for controlled drug release' to those for solute separation.2 The phase transition of a gel is analogous to the coilglobule transition of its linear polymer, which is an entropy-driven p h e n ~ m e n o n . ~The - ~ entropy gain originates from the breakdown of the highly ordered, icelike water around the polymer chain as the temperature increases. At temperatures above the LCST the hydrophobic interactions between polymer chains prevail, thus leading to a phase separation and the collapse ofthe Since the LCST behavior is caused by a critical balance + J a p a n Atomic Energy Research
* Gunma University.
Institute.
Abstract published inAdvanceACSAbstracts,August 15,1994. (1)Hoffman, A.S.; Afrassiabi, A.; Dong, L. C. J. Controlled Release 1986,4,213. (2)Feil, H.; Bae, Y. H.; Feijen, J.;Kim, S. W. J. Membr. Sci. 1991, 64,28. (3)Fujishige, S.;Kubota, K.; Ando, I. J.Phys. Chem. 1989,93,3311. (4)Otake, K.; Inomata, H.; Konno, M.; Saito, S.Macromolecules 1990, 23,283. (5)Binkert, T.; Oberreich, J.; Meewes, M.; NyEenegger, R.; Ricka, J. Macromolecules 1991,24,5806. (6)Schild, H. G.; Tirrell, D. A. J . Phys. Chem. 1990,94,4352. (7)Bae, Y. H.; Okano, T.; Kim, S. W. J.Polym. Sci., Polym. Phys. Ed. 1990,28,923. (8)Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26,2496. @
of hydrophobic and hydrophilic groups of the p ~ l y m e rit, ~ can be easily influenced by changing the number of hydrophobic, hydrophilic, or charged groups on the polymefl-" or by adding inorganic salts6J2J3and surfactants to the s ~ l u t i o n . ' ~ -The ' ~ effect of surfactants on the volume phase transition is especially interesting, since they are composed of a nonpolar hydrophobic tail and a polar head group that could be ionic, zwitterionic, or nonionic. The results of investigations on aqueous solutions of uncharged polymer and a surfactant carried out by variety of experimental techniquesls indicate that polymers tend to form complexes with ionic surfactants and the resulting complexes behave as polyelectrolytes. In the case of poly(NIPAAm), it was shown that anionic surfactants elevate the LCST, and the magnitude of this rise depends on the length of the surfactant tailI6 and of the surfactant concentration. Cationic surfactants have been found to induce similar effects. However, nonionic surfactants do not show any marked influence on the volume phase transition, probably due to the absence of electrostatic interactions between the surfactant head groups.15 We have been synthesizing polymers and loosely crosslinked gels with a pendant amino acid, L-proline, which (9)Taylor, L.D.; Cerankowski, L. D. J . Polym. Sci., Polym. Chem. Ed. 1975,13, 2551. (10)Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987,87, 1392. (11)Beltran, S.;Baker, J. P.; Hooper, H. H.; Blanch, H. B.;Prausnitz, J. M. Macromolecules 1991,24,54. (12)Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992,8, 687. (13)Park, T. G.; Hoffman, A. S. Macromolecules 1993,26,5045. (14)Wada.; N.;Kajima,Y.; Yagi,Y.; Inomata, H.; Saito, S.Langmuir 1993., 9. - ,46. ~(15)Kokufuta, E.;Zhang, Y.-Q.; Tanaka, T.; Mamada, A. Macromolecules 1993,26,1053. (16)Schild, H. G.;Tirrell, D. A.Langmuir 1991,7 , 665. (17)Winnik, F. M.;Ringsdorf, H.; Venzmer, J. Langmuir 1991,7, ~~
~
~
9nR
(18)For review see: Goddard, E. D. Colloids S u r t 1986,19, 255.
0743-7463/94/2410-2954$04.50/0 0 1994 American Chemical Society
shows LCST b e h a v i ~ r . ' ~ - ~The l cloud point for poly(acryloyl-L-proline methyl ester) (A-ProOMe) is at 14 "C, while the more hydrophobic ethyl ester (A-ProOEt) has its critical temperature around 2 "C, and the propyl ester (A-ProOPr) is insoluble in water. Our previous studies showed that these polymers form qomplexes with an anionic surfactant, sodium dodecyl sulfate (SDS), which results in an increase of the cloud point due to the electrostatic repulsion of the surfactants bound to the polymer.22 The elevation of t h e phase transition temperature was a linear function of the SDS concentration in the range examined. The work reported here is a systematic investigation of the effects of surfactant type a n d its concentration on the swelling behavior ofthe loosely cross-linked poly(A-ProOR) gels.
Experimental Section Materials. L-Proline methyl ester hydrochloride, L-proline ethyl ester hydrochloride, L-proline propyl ester hydrochloride, and acryloyl chloride were purchased from Kokusan Chemical Works (Tokyo, Japan). Cetyltrimethylammonium chloride (CTAC),[ H ~ C ( C H Z ) ~ ~ N ( C Has~a) ~ cationic ] C ~ , surfactant, sodium dodecyl sulfate (SDS),H ~ C ( C H Z ) ~ ~ O S as O ~an N~ anionic , surfactant, octaethylene glycol monododecyl ether (DOE), H3C(CH2)11(OCHzCH2)80H,as a nonionic surfactant, and dimethyllaurylbetaine (DALB), H~C(CHZ)~~N(CH~)ZCH~COOH, as a zwitterionic surfactant were obtained from Tokyo Kasei Kogyo Co., Ltd. (Tokyo,Japan), Merck (Darmstadt, Germany), Nippon Oil & Fats Co., Ltd. (Tokyo, Japan), and Kanto Chemical Co., Inc. (Tokyo, Japan), respectively. Synthesisof Acryloyl-L-prolineAlkyl Ester Monomers. Acryloyl-L-proline alkyl esters (A-ProOR) were synthesized by coupling reaction of L-proline alkyl ester hydrochlorides (HC1-HProOR) and acryloyl chloride. HC1.H-ProOR (0.25 mol) dissolved in chloroform (100 mL) was mixed with triethylamine (0.55 mol) and then the solution was kept at -10 "C. Acryloyl chloride (0.3 mol), which was previously dissolved in chloroform (100 mL) kept at -10 "C, was added dropwise to the HC1.HProOR solutions with vigorous stirring over a period of 60 min. This reaction was continued for further 3 h at 0 "C (ice-water system). After the solution was concentrated under reduced pressure, the crystals of triethylammonium hydrochloride were precipitated by adding ethyl acetate (400 mL), and were removed by filtration. The filtrates were washed with aqueous solution of saturated sodium chloride, dried over anhydrous sodium sulfate, concentrated under reduced pressure, and finally distilled to obtain the oily monomers. Preparationof Poly(A-ProOR)Gels. Amixture ofA-ProOR monomers (10 mM), diethylene glycol dimethacrylate (2G) as a cross-linking agent (0.02 mM), in ethanol (1 mL) was charged into a 5 mm i.d. glass ampule. Nitrogen was bubbled trough the solution to remove the dissolved oxygen. The irradiation was done at a 6OCo y source at room temperature with doses up to 30 kGy and a constant dose rate of 10 kGy/h. The gels were washed with excess ethanol to remove the unreacted monomers. The yield was about 93%in all systems. The chemical structure of the gel is shown on Figure 1. Swellingof Poly(A-ProOR)Gels. The poly(A-ProOR)gels, which were previously treated with excess ethanol, were cut into a cylindrical specimens of 5 mm and 10 mm length, washed with water at 0 "C (ice-water system) several times, lyophilized, and finally immersed in aqueous surfactant solutions of desired concentrations at 0 "C. The swelling procedure was continued under the above conditions until the equilibrium was reached. Then, the poly(A-ProOR) gels were immersed for 24 h in a surfactant solution of several preselected temperatures and weighed after wiping off the excess surface medium from the gel. (19)Yoshida, M.; Asano, M.; Kumakura, M.; Katakai, R.; Mashimo, T.; Yuasa, H.; Yamanaka, H. Drug Design Del. 1991, 7, 159. (20) Yoshida, M.; Omichi, H.; Katakai, K. Eur. Polym. J . 1992,28 (9), 1141.
(21)Miyajima, M.; Yoshida, M.; Sato, H.; Omichi, H.; Katakai, R.; Higuchi, W. I. Int. J . Pharm. 1993,95,153. (22) Safranj, A.;Yoshida, M.; Omichi, H.; Katakai, R.Langmuir 1993, 9,3338.
r+
Langmuir, Vol. 10,No. 9, 1994 2955
Swelling Equilibrium of Cross-Linked Gels
-
+ H Z C - - ~ H m~ H 2 c
c=o
c=o I
0 I CH2
HC
I
C=O
CHZ 0 I
I
0 I R
p
2
y
2
I
A-ProOR A-ProOR
0 I
*
c=o
R
I
A-ProOMe A-ProOEt
CH3 CHzCH3
--tH2C-C
A-ProOPr
CH2CH2CH3
CH3 2G
Figure 1. General formula of cross-linked poly(acryloy1-Lproline alkyl ester) gels. In the present case, the feed monomer concentration was 10 mM, and the cross-linker concentration was 0.02 mM.
0.1
L
'
0
I
1
I
I
20 40 60 Temperature ("C)
80
Figure 2. Swelling of the poly(A-ProOR) gels in water: (0) A-ProOMe, (0)A-ProOEt, (A) A-ProOPr. The swelling ratio was determined from the following equation swelling = (W - Wo)/Wo where W is the weight of the swollen gel and Wo is the weight of the dried gel. All measurements were carried out in triplicate. The reproducibility was better than 10%in all cases (the errors are around 8-10% at low temperatures and 3-5% at high temperatures). Instrumental Analyses. The surface structure of poly(AProOMe)gels was observed with a Jeol JXA-733 scanning electron microscope (SEM). The characteristic X-ray image of sodium distributed in poly(A-ProOMe) gel treated in aqueous SDS solution was observed with a Jeol JXA-733X-ray microanalyzer (XMA). For this purpose, the poly(A-ProOMe)gels swollen or deswollen in aqueous SDS solution were immersed in liquid nitrogen, and then lyophilized.
Results Swelling of the Gels in Aqueous Solution. The temperature dependence of t h e equilibrium volume phase transition for the three gels in pure water is shown in Figure 2. Poly(A-ProOMe) gel a n d poly(A-ProOEt) gel collapse from a highly swollen state to a shrunken one at around 14 a n d 2 "C, respectively, with volume change, defined as the ratio of the swelling of the gels at 0 a n d at 80 "C, of 1 7 for poly(A-ProOMe) a n d 13for poly(A-EoOEt). The phase transition temperatures of these gels agree well with the cloud point of the corresponding linear
Safranj et al.
2956 Langmuir, Vol. 10,No.9,1994 100 :
-Y
."2
30
.
20
.
10 :
:
m
3 . 2 -
'
0.3
I
I
1
20 40 60 Temperature ("C)
0
I
80
Figure 3. Swelling of poly(A-ProOMe)gels in anionic surfactant solutions.The SDS concentrationswere as follows: ( 0 ) 0; (0)0.35 d; (V)0.50 d; (V)0.70 d; (A) 3.5 mM; (0)6.9 mM; (m) 10.4 mM; (A)13.9 mM; (0)17.3 mM; (+) 173 mM.
0.2
8
0
20 40 60 Temperature ("C)
-
80
Figure 6. Swelling of poly(A-ProOPr)gels in SDS solutions of the following concentrations: (A) 0; ( 0 )173 mM; (m) 347 mM.
I
30t
0.3
0. I
1
'
0
O2
t
0.1 L 1 0
20 40 60 80 Temperature ("C)
Figure 4. Swelling of poly(A-ProOEt)gels in anionic surfactant 0; (A) solutions. The SDS concentrations were as follows: (0) 3.5 mM; (A) 17.3 mM; (0)34.7 mM; (7)173 mM; (m) 347 mM.
polymers.22 In the case of poly(A-ProOPr) the gel as well as its linear polymer does not have phase transition under the present experimental conditions. Effect of Ionic Surfactants on the Swelling. The effect of three types of ionic surfactants on the reversible volume phase transition of A-ProOMe, A-ProOEt, and A-ProOPr gels as a function of the surfactants concentration was investigated. In Figures 3-5, the effect of the anionic surfactant, SDS, on the swelling of the three gels is shown. In the case of A-ProOMe, the surfactant starts to influence the LCST a t concentrations around 0.70 mM, which is very similar to the cac (critical aggregation concentration) reported for the SDS-NIPAAm system.14J6 With the increase of the SDS concentration, the volume phase transition temperature increases. The volume ratio, on the other hand, decreases from 17 to 3 due to the fact that the gel, although the swelling is higher below the LCST, is shrinking less and less at high temperatures (Figure 3). At the surfactant concentration of 17.3 mM, the gel is swollen at all temperatures. When the surfactant concentration is above 173 mM, the gel shows a reverse change of swelling and has an upper critical solution temperature (UCST) behavior. For the other two gels, A-ProOEt and A-ProOPr, the results are similar (Figures 4 and 5). However, due to the larger surface area of the hydrophobic groups of these
I
I
20 40 Temperature ("C)
I
60
Figure 6. Swelling of poly(A-ProOMe)gels in cationic surfactant solutions. The CTAC concentrations were as follows: (0)0; (v)3.5 mM; (v)6.9 mM; ( 0 )17.3 mM; (m) 34.7 mM.
gels that results in lower LCST, as seen from swelling measurements, higher concentration of the surfactant is required to obtain the same effects. When the cationic surfactant, CTAC, is added to the aqueous solution, the LCST of the gels increases (Figure 6). This result shows that the cationic surfactant also interacts with the gel. The comparison between Figures 3 and 6 indicates that CTAC needs higher concentration than SDSto induce a similar effect on the phase transition. Contradictory to the case mentioned above, the volume ratio in CTAC solution is higher than that in pure aqueous solutions. This is due to the fact that above LCST in CTAC solutions the gels swell less than in pure water. Increasing the CTAC concentration affects the A-ProOEt and AProOPr gels similarly (data not shown). The addition of zwitterionic DALB does not affect the phase transition temperature of the gels (Figure 7). The volume phase transition on the other hand, becomes sharper and the volume ratio increases to about 70. It can be pointed out that a further increase in the surfactant concentration has no additional effect either on the LCST or on the volume ratio. Effect of a Nonionic Surfactant. on the Swelling. Although the nonionic surfactant also affects the swelling of the A-ProOR gels as shown in Figure 8, it does so only at a high surfactant concentrations (for example, at least 173 mM DOE solution for A-ProOMe). In such solutions,
Swelling Equilibrium of Cross-Linked Gels 30 20
10
1
0.3 0.2 0.1
L'
0
I
I
20 40 Temperature ("C)
60
Figure 7. Swelling behavior of the poly(A-ProOMe) gel in zwitterionic surfactant solutions of the following concentrations: (0)0; ( 0 )17.3 mM; (A) 173 mM.
0.3
I
0
I
20 40 Temperature ("C)
I
60
Figure 8. Swelling of the poly(A-PeoOMe) gel in nonionic surfactant solutions. The concentrations of the DOE were as follows: (0)0; (0)173 m M (A) 347 w,(0)694 mM.
the LCST is shifted to lower temperatures, and with further increase in the surfactant concentration the gels do not swell at all.
Discussion As was pointed out in the Introduction, the LCST behavior of an aqueous polymer solution depends on a critical balance of hydrophobic and hydrophilic groups of a p ~ l y m e r . In ~,~ our gels (structure shown on Figure l), CH2 and CH3 groups of the backbone and the proline side groups are the hydrophobic groups and the C-0 and N are the hydrophilic groups. In a pure aqueous solution below the LCST, the strong hydrogen bonding between hydrophilic groups and water triggers the formation of a highly organized water layer around the polymer chains. The formation of this structured water contributes favorably to the enthalpy of mixing, which outweighs the unfavorable free energy related to the exposure of hydrophobic groups to water thus enabling the good solubility of the p ~ l y m e r . ~ In, ~loosely cross-linked gels below LCST the polymer chains still keep the extended coil conformation, that is manifested by the swollen structure of the gels. With increasing temperature, hydrogen bonding weakens, the structured water is released, and the interactions between hydrophobic side groups increase. Above the LCST, these hydrophobic interactions become dominant (the free energy of mixing takes positive value) and lead to an entropy-driven collapse of polymer chains from an expanded coil to a compact
Langmuir, Vol. 10, No. 9, 1994 2967
globular conformation and a phase s e p a r a t i ~ n . ~ g ~ - ~ J ~ When the polymer is cross-linked, as the polymer chains refold, instead of the phase separation this will bring about the shrinking of the gel. It has been that the gel whose hydrophobic group has a larger surface area undergoes the volume phase transition at lower temperature, due to the strong hydrophobic interaction which is proportional to the number of water molecules taking part in the hydrophobic hydration. Among the gels used in our study, the A-ProOMe has the smallest, and the A-ProOPr the largest hydrophobic contact area, and accordingly, the phase transition temperature of the A-ProOMe is the highest, while the A-ProOPr does not show LCST behavior under the present conditions. If the gels are in a surfactant solution, their swelling behavior will depend as much on the kind of the surfactant and its concentration as on the temperature. Surfactant molecules are composed of a hydrophobic tail and a polar head group which can be ionic, nonionic, or zwitterionic, and this dual nature is responsible for their ability to orient themselves at various types of interfaces. In aqueous solutions, the free energy for the micelle formation consists of the following contribution^:^^-^^ the free energy of the transfer of the hydrophobic tail of the surfactant from water to a hydrocarbon core of the micelle that is accompanied by the generation of an interface between the hydrophobic core of the micelle and water; the free energy of the steric interactions between the polar head groups at the micellar surface and the electrostatic interactions between the ionic or zwitterionic head groups at the micellar surface. The driving force for micellization is provided by the free energy of the transfer of the hydrophobic tail from water to the micellar core, while the interactions between the head groups regulate the size of the micelle. When a polymer is present in the surfactant solution, mixed polymer-surfactant micelles can form. We visualize these mixed micelles as presented in Figure 9, the center of each micelle being the proline and alkyl side groups to which the hydrophobic tails of the surfactant molecules adsorb. According to the thermodynamic theory developed by Nagarajan,26in the case of a nonionic polymer-anionic surfactant system, the free energy for the polymer-bound micelles is lower than for the free micelles, irrespective of the polymer hydrophobicity. These mixed micelles are formed at a critical aggregation concentration (cac) which is lower than the cmc, and does not depend on the concentration of the p~lymer.~' In the case of the A-ProOMe gel and SDS for example, the cac is about 1order of magnitude lower than the cmc, similarly to the SDS-NIPAAm system.16J7When SDS is adsorbed to the hydrophobic groups ofthe A-ProOR gel, it converts otherwise neutral gel into a polyelectrolyte gel. These network charges will introduce an extra osmotic pressure on the network, and as a result, the volume transition temperature of the gel will increase. For example, in aqueous solution, the A-ProOEt gel is in its collapsed state at all temperatures above 2 "C, as can be seen also from the SEM photograph shown in Figure loa, which was taken at 30 "C. This is due to the hydrophobic interactions between the proline side groups. Ifhowever, the gel is in SDS solution at the same temperature, the electrostatic interactions between the surfactant head groups adsorbed around the hydrophobic side groups of the polymer prevent the interaction between those groups, (23) Inomata, H.; Goto, S.; Saito, S.Macromolecules 1990,23,4887. (24) Nagarajan, R. Adv. Colloid Interface Sci. 1986,26, 205. (25) Nagarajan, R.; Riickenstein, E. J . Colloid Znterface Sci. 1979, 71,580. (26) Nagarajan, R. J . Chem. Phys. 1989,90 (3), 1980. (27) Kresheck, G. C.; Hargraves,W. A. J . Colloid Interface Sci. 1982, 86, 283.
2958 Langmuir, Vol. 10, No. 9,1994 M
Water
Safranj et al.
Aqueous surfactant solution
vl
Figure 11. Characteristic X-ray image of Na in the poly(A-
Collapsed gel
LCSTo
LCSTi
ProOMe) gel (b), accompanied with the corresponding SEM photograph (a).The gel was immersed in 3.47mM SDS solution at 5 "C.
7
Temperature Figure9. Schematicpresentation of the formation of the mixed micelles between ionic surfactants and poly(A-ProOR)gels. In aqueous solution (lowerpart of the diagram)the gel is in swollen state at all temperatures below the LCSTo and is shrunken at the temperatures above it. In ionic surfactant solution, due to the preferential uptake of the surfactants by the gel and the hydrophobic interactions between the proline side groups and the surfactant tails, mixed micelles are formed. The charged headgroups of the surfactants convert the neutral gel t o a polyelectrolite gel, and this increased osmoticpressure elevates the phase transition temperature to LCST1.
Figure 10. SEM photographs of the poly(A-ProOEt)gel a t 30 "C: (a) in aqueous solution; (b) in 694 mM SDS solution.
and the gel remains swollen (Figure lob). The XMA photograph presented in Figure l l b shows the surfactants associated with the gel at low temperatures, and the corresponding SEM picture (Figure l l a ) shows the gel in swollen state. It is reasonableto think that the volume phase transition temperature of the gel increases with the increase of the surfactant concentrationand becomes constant at a certain concentration of the surfactant, at which all the possible sites on the gel network to which the surfactants may bind are 0~cupied.l~ With the A-ProOR gels however, this saturation value was not obtained. The phase transition temperature increases with the surfactant concentration almost linearly up to a point where it surpasses the boiling point of water, so the gel swells in the whole temperature range (Figure 3). With still further increase in the SDS concentration,the gel starts to shrink at low temperatures and swell at high temperatures. The SEM photographs
of the poly(A-ProOMe) gel in the 694 mM SDS solution (Figure 12)show this UCST behavior. Since the surfactant concentration that would equal the saturation value depends on polymer concentrationin solution,in our case, where very large gels are used, this value could be reached only at very high surfactant concentrations. Under this condition the surfactant is partially in a hydrated crystalline state at low temperature, and in this case, the osmotic pressure of the counterions might be higher outside the gel, and as a consequence, the gel is in a shrunken state. With the increasing temperatures, more surfactant will be soluble and more can diffuse into the gel. Although all sites on the gel to which surfactants can bind are occupied, at this high SDS concentrationit might be that the micelles can increase in size. The pressure balance changes, and the gel starts to swell. As it has been pointed out, A-ProOMe is the least hydrophobic, while A-PRoOPr is the most hydrophobic among our gels. For the same SDS concentration,which will saturate the A-ProOMe gel and induce the UCST behavior,A-ProOPr gel will swell below and shrink above the phase transition temperature. The effect of the cationic surfactant on the swelling of the (A-ProOR)gels is essentially the same as that of the just discussed anionic surfactant, SDS, although higher concentrationof the CTAC is needed to increase the LCST of the same gel to a similar level. The fact that the interactions of a neutral polymer with an anionic surfactant is much easier than with a cationic surfactant has been observed earlier,14J8 and was attributed to the difference in cmc of these surfactants. The surfactant with lower cmc, which is usually the anionic surfactant, is expected to induce larger effect on ~welling.~'Another plausible explanation for this behavior is based on the volume change on micellization. Namely, the volume change in a solution of cationic surfactant is less than that of anionic surfactant of comparable chain length.28 Thus the hydrophobic interaction for cationic surfactantnonionic polymer might be less. In the present study, we observed another difference between the two kind of surfactants: at the temperatures above the volume phase transition, when anionic surfactant is present, the gel swellsless than in water, while in the presence of a cationic surfactant the opposite is true. At the present stage of our work, we can only speculate about the reason for this behavior, and we offer the following explanation. Considering the structure of the CTAC, it is possible that similarly to the long alkyl chain tetraalkylammonium bromides29when adsorbed to the gel, CTAC might act as (28) Jones, M.
N.J. Colloid Interface Sei. 1967,23,36.
Swelling Equilibrium of Cross-Linked Gels
Langmuir, Vol. 10, No. 9,1994 2959
Figure 12. SEM photographs showing the UCST behavior of the poly(A-ProOMe)gel in the 694 mM SDS solution: (a) 15 "C;(b) 30 "C; (c) 80 "C. All pictures were taken at the same magnification. a cross linker and the gel will swell less. In the case of anionic surfactant, while SDSis not likely to induce crosslinks, it is also feasible that the ionic repulsion between the head groups of surfactants associated with the gel is not completely overcome by hydrophobic interactions between proline groups above the phase transition temperature, and the gel swells more in SDS solution than in pure water. To our knowledge, there are no previous studies of the interaction between LCST gels and zwitterionic surfactants. Our results show that interactions do take place and the swellingbecomes discontinuous,accompanied by a higher swelling ratio. This higher swelling ratio comes from the collapse of the gel at high temperatures, which is more pronounced than when the gel is in pure water. This collapse of the gel at high temperatures somewhat resembles the effect of cationic surfactants. Since the structure of the cationic and zwitterionic surfactants is quite similar,we presume that similar surfactant-induced cross linking might take place here too. Besides, the surfactant head groups could take such a conformation that attraction rather than repulsion between them occur. Nonionic surfactants seldom interact with such neutral gels as the NIPAAm gel. This is in agreement with the theory developed by Nagarajan,24*26 according to which a certain hydrophobic characteristics of the polymer is required for the polymer-surfactant association. Poly(A-ProOR)gels are more hydrophobic than the NIPAAm gel, and the interactionbetween them and DOE does occur. This surfactant-gel interaction does not affect the cmc of the surfactant measurably; therefore, the formation of the mixed micelles will start at much higher concentrations as compared to the ionic surfactant-gel interactions. Since the surfactanthead groups are not charged, the gel retaifis its neutral characteristics and no measurable change in the swellingbehavior can be detected. However, with the increase in the surfactant concentration, aRer all the available sites for surfactants inside the gel are occupied, the surfactant concentration outside the gel increases, and this additional osmotic pressure will result in less (29) Saito, S.;Konno,M.; Inomata,H. InAdvancesin Polymer Science; Dusek, K. Ed.; Springer Verlag: Berlin, Heidelberg, 1993; Vol. 109, p 219.
and less swelling of the gel with the increase in the surfactant concentration.
Conclusion Addition of surfactants to a solutionthat contains crosslinked poly(A-ProOR) gels will markedly influence the volume phase transition of the gels. This is interpreted in terms of preferential uptake of the surfactants by the gels and formation of mixed micelles. The driving force for the surfactants-gel interaction is the hydrophobic interaction between the surfactant tail and the hydrophobic groups on the gel. The swelling behavior of the gels depends on the surfactant concentration in the solution and the character of their head group. When the surfactant head groups are ionic, the otherwise neutral gel is converted to the polyelectrolite gel, and the introduction of this additional osmotic pressure due to ionization elevates the transition temperature. On the other hand, when the head groups are nonionic, at low surfactants concentration there are no measurable changes in the swelling behavior. In solutions containing very high surfactant concentrations, the swelling of the gel depends also on the solubility of the surfactant at the temperature in question, since this factor influencesthe micellization. Besides this factor, the high surfactant concentration outside the gel introduces additional pressure on the gel, thus facilitating its collapse. The fact that the swelling of the poly(A-ProOR)gels is dependent on the type and concentration of the surfactants in such a great extent can provide a basic idea for the device of molecular sensing, like the poly(N1PAAm)surfactant system.15 Our results indicate that the (AProOR) gels have the abilities to bind different surfactant molecules, recognize the kind of the molecule, and convert this surfactant-gel interaction to large and easily observable changes in the volume phase transition temperature and/or changes in the volume ratio. Our study is not yet complete, since many questions remain open and need further investigation. The mechanism of interaction of different surfactants with the gel and the number of surfactantsassociatedwith hydrophobic groups as well as the determination of the temperature at which this association starts are only some of them. We hope to address these questions in the near future.
