Langmuir 1995,11, 2493-2495
2493
Effects of Surfactants on the Phase Transition of a
Hydrophobic Polymer Gel Masahiko Sakai,* Naoki Satoh, and K a o n Tsujii Kao Institute for Fundamental Research, Kao Corporation, 2606 Akabane, Ichikai-machi, Haga-gun, Tochigi, 321-34, Japan
Yong-Qing Zhang and Toyoichi Tanaka Department of Physics and Center for Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 Received January 31, 1995. In Final Form: April 14, 1995@ The effects of surfactants having various chemical structures on the phase transition of poly(Nisopropylacrylamide) (NIPA) gel have been studied. The phase transition temperature of NIPA gel, which is 34 "C in pure water, is elevated by several tens of degrees when a small amount of some surfactant is added to water. The transition temperature increases with surfactant concentration below the critical micelle concentration and is saturated at a certain temperature. The transition temperature strongly depends on the chemical structure in the hydrophilic part as well as the length of the hydrophobic portion of the surfactants. The transition temperature ofthe gel does not depend on the counterion species. These results imply that the effects of surfactants on the phase transition of the hydrophobic polymer gel may be utilized to quantitatively classify the interfacial behavior of surfactants in addition to the widely used hydrophile-lipophile balance (HLB).
Introduction
[email protected]) NIPA gel undergoes a volume phase transition in response to temperature ~ h a n g e . l -The ~ gel is swollen at temperatures lower than 34 "C and collapsed to a compact state at higher temperatures. The competition between the network rubber elasticity and the hydrophobic interaction among isopropyl groups is responsible for the above t r a n ~ i t i o n . l - ~The hydrophobic interaction, and thus the phase behavior of hydrophobic polymers, will be significantly modified if surfactant molecules are adsorbed on the polymer chains and the polymer/polymer and polymedsolvent interactions are altered. Inomata et al.4and Kokufuta e t al.5 reported that the phase transition behavior was drastically influenced when a small amount of certain surfactant was added to the solution in which the gel was immersed. In particular, the phase transition temperature was elevated several tens of degrees centigrade in 3 mM solutions of sodium dodecyl s ~ l f a t e . ~ They . ~ concluded that the surfactant molecules were bound to polymers due to the hydrophobic interaction and practically transformed the polymer chains into polyelectrolytes. It is well-established that the ionization of polymer chain elevates the transition temperature and increases the gel volume in the swollen state at the t r a n ~ i t i o n . ~ I n this article, effects of surfactant molecules having various chemical structures on the phase transition of NIPA gel have been studied. We measured the equilibrium diameter of cylindrical gels put in the various kinds of surfactant solutions as a function of temperature in Abstract published in Advance ACSAbstracts, June 15,1995. (1)Tanaka, T. Phys. Rev. Lett. 1978,40(121,820. (2)Otake, K.;Inomata,H;Konno, M.; Saito,S. Macromolecules 1990, 23,283. (3)Hirokawa, Y.;Tanaka, T.; Matsuo, E. S. J . Chem. Phys. 1984,81 (121,6379-80. (4)Inomata, H., Goto, S.; Saito, S. Langmuir 1992,8, 1030-1031. (5)Kokufuta, E.;Zhang, Y.-Q.;Tanaka, T.; Mamada, A. Macromolecules 1993,26 ( 5 ) , 1053-9. (6) Goddard, E. D. J. Am. Chem. SOC.1994,71, 1. @
order to obtain the equation of state of NIPA gel. Firstly, phase transition behaviors in a variety of surfactant solutions have been studied to investigate the effect of hydrophilic part of surfactants. Secondly, the effect of alkyl chain length on the phase transition behaviors has been examined in alkyl sulfonate surfactant solutions.
Experimental Section The sample gel was prepared by radical polymerization. N-Isopropylacrylamide(monomer,Eastman Kodak Co.,reagent grade) and N,W-methylenebisacrylamide (cross-linking agent, Wako Pure ChemicalIndustries Ltd., electrophoresisgrade)were used as the main polymer constituents. N,N,iV',N'-Tetramethylethylenediamine and ammonium persulfate were chosen as redox initiators. All reagents were used without further purification except for NIPA, which was recrystalized three times from a toluene/petroleumether mixture (50150by volume). These constituents were dissolved into water, and the solution was saturated with nitrogen gas by bubbling for approximately 15 min. The solution was left for polymerization at 0 "C in glass capillaries of 140 pm inner diameter. After the polymerization reactionwas completed,the gel was taken out from the capillaries, cut short, and placed in a thin glass tube of 1 mm inner diameter, through which a fresh solvent was continuously flushed. Initiator and residual monomer were well washed out by flushingthe gel with pure water for 24 h. Surfactant samples used in this work and their abbreviations are listed in Table 1. Alkyl sulfonate surfactants having the alkyl chain of Cq to CE were chosen to examine the effect of chain length on the phase transition of the gel. All ofthe sulfonatesurfactants were purchased fromAldrich Chemical Co., Inc., and used without further purification. The temperature of the gel was controlled within f O . l "C by circulatingtemperature-controlledwater around the glass tube containingthe gel. Water or surfactant solution flowed through the capillary tube continuously, and the gel was allowed to completely equilibratewith the external solutions. After the gel was equilibrated at a certain temperature, the gel diameter was measured from its video image using a video measuring system (ForA. Co. Ltd.). The gel diameters were measured during both heating and cooling processes. The equilibrium diameter d was normalized by the initial diameter do (defined as the diameter on gel synthesis, i.e., the inner diameter of the capillary = 140 pm). The normalized diameters dido were plotted against temperature to obtain the phase diagram as will be shown in Figure 1.
Q743-7463/9512411-2493$09.QQlQ 0 1995 American Chemical Society
Sakai et al.
2494 Langmuir, Vol. 11, No. 7, 1995 Table 1. Surfactant Samples Used in This Work and Their Abbreviations surfactant
chemical structure
abbreviation
sodium dodecyl sulfate sodium 2-(dodecy1oxy)ethylsulfate potassium tridecanoate
obtained from
-
____
R12S04Na R120ES04Na R12COOK
sodium alkyl sulfonate triethanolammonium dodecyl phosphate dodecylamine hydrochloride dodecyltrimethylammonium chloride polyoxyethylene ( p = 9) dodecyl ether
Merck, LAB synthesized Tokyo Chemical Industry Co., Ltd. purchased as tridecanoic acid, GR Aldrich Chemical Co. Inc., GR KAO Corp. Kanto Chemical Co., Inc., GR Tokyo Chemical Industry Co., Ltd. KAO Corp.
l
f
i
RizSOiNa
E . I
c.)
3 e cc
izPOiTEA
RizEOP 3n
L W
0
1.o
0.5
1.5
7n -.,
2.0
ado Figure 1. Phase transition curves of NIPA gel in pure water (0) and aqueous R12S04Na solutions (A, 5 mM; 0 , l O mM; 0, 100 mM). Open a n d filled symbols denote t h e heating a n d the
(7) Robb, I. D. Anionic Surfactants (Surfactant Science Series Vol. 11);Marcel Dekker Inc.: New York and Basel, 1981, p 109. (8)Hall, D. G.; Tiddy, G. J.T.Anionic Surfactants (Surfactant Science Series Vol. 11);Marcel Dekker Inc.: New York and Basel, 1981; p 59.
1.0
0.5
Surfactant concentrationI M
Figure 2. Transition temperatures of NIPA gel against the concentration of various surfactants in aqueous solutions.
cooling procedure, respectively.
Results and Discussion Effects of the Hydrophilic Part. Figure 1 shows some examples of swelling curves of NIPA gel in both pure water and several aqueous solutions of sodium dodecyl sulfate (R12S04Na). NIPA gel shrinks to a collapsed state upon heating, but does so somewhat continuously in pure water. When R12S04Na is added to the solution, the transition behavior is dramatically changed from that in pure water. Firstly, a hysteresis appears in the swelling and collapsing curves in R12SO4Na solutions. This constitutes an evidence for the firstorder phase transition of the NIPNsurfactant system. Secondly, it is clearly seen from Figure 1that the phase transition temperature is elevated remarkably, and the diameter of the gel in swollen state is increased as the surfactant concentration increases. Figure 2 shows the dependence of the transition temperature of NIPA gel on the concentration of various surfactants. Roughly speaking,the effect of the surfactant on the transition temperature increases in the order of nonionics < cationics -= anionics, which agrees with the reported series of adsorbance of surfactant onto polymer^.^ No change in the transition temperature is observed in a nonionic surfactant such as polyoxyethylene ( p = 9) dodecyl ether (R12E09). When the gel is in cationic surfactant solutions, such as dodecyltrimethylammonium chloride (RIZTAC),the transition temperature is elevated up to 39.8 "C a t the concentration of 0.1 M. The transition temperature in anionic R12S04Na solution is elevated up to 95 "C. These differences in transition temperature depending upon the ionic nature of surfactant might be due to ability of each surfactant to adsorb onto the polymer
9
0.0
100
,
I
80 \
$ E
&
E ."
60
c.)
fi
40
e b 20
0
25
50
75
1
Concentrationof surfactant 1 m M
Figure 3. Transition temperatures of NIPA gel plotted against t h e concentration of alkyl sulfonate surfactants with various alkyl chain length in aqueous solutions.
chains of NIPA gel. The adsorption isotherm of each surfactant is now being measured to elucidate the above. It is reasonable for nonionic surfactant to give no effect on the transition temperature, since no electric charge can be attached to the polymer chains even if nonionic molecules adsorb onto them. Depending on the chemical structure of hydrophilic group, the transition temperature of NIPA gel varies from surfactant to surfactant. In the case of cationic surfactant, the transition temperature in the dodecylamine hydrochloride (R12NHsCl)solution is 50 "C which is significantly higher than that in R12TAC. Although the data are not shown in Figure 2, [(dodecylamido)propylltrimethylammonium chloride ( C ~ ~ H ~ & O N H C ~ H G N ( Cchanges H~)~C~) the transition temperature very little. Similar behaviors are observed also in the anionic surfactant solutions. In the solutions of sodium dodecyl sulfonate (R1$303Na),the transition temperature is elevated to 65 "C, which is lower than that in R12S04Na by 30 "C. Sodium 2-(dodecy1oxy)-
Effects of Surfactants on Phase Transition
Langmuir, Vol. 11, No. 7,1995 2495
. u
6e i Y E
*z '2 e c
8
12
16
20
Chain length n
Figure 4. Relationship between saturated phase transition temperaturesof NIPA gel in alkyl sulfonate surfactant solutions and the chain length of the alkyl group. Table 2. Effect of Counterions of Ionic Surfactant with a Cia Hydrocarbon Chain on the Transition Temperatures of NIPA Gel transition temperatureV'C counterion Li+ Na+ K+ TEA+ NH4+
sulfate phosphate
96.0
95.0 33.4
96.5 36.5
98.5 37.4
94.1
transition temperatureV'C counterion c1Brtrimethylammonium 38.0 38.9 a Data were taken in the 0.1 M surfactant solutions. ethyl sulfate (R120ES04Na),having an oxyethylene unit between hydrophilic and hydrophobic part of R12S04Na, also shows a lower transition temperature of the gel than that in the R12S04Nasolution by 20 "C.The phosphate type surfactant is of special interest. It elevates the transition temperature by only 3 "C. Furthermore, the transition temperature has a peak as a function of surfactant concentration and becomes lower than that in pure water a t higher concentrations. It was demonstrated that the transition temperature was strongly affected by the chemical structure of hydrophilic part of the surfactant added. The hydrophilic part of the surfactant can be divided into two parts, the ionic head group and the counterion. Table 2 summarizes the transition temperature of the gel in the solutions of sulfate, phosphate, and trimethylammonium type surfactant with several kinds of counterion. In all cases, the transition temperatures are almost the same value, irrespective of the counterions. We may conclude safely that the counterion shows almost no effect on the phase transition temperature of NIPA gel. (9)Griffin, W. C. J. SOC.Cosmet. Chem. 1949, 1 , 311.
Let us examine the phase transition temperature in the surfactant solutions of R I z S O ~ NR12S03Na, ~, and RlzOES04Na. Each surfactant has very similar chemical structure. The insertion or elimination ofone oxygen atom varies the transition temperature from 95 to 65 "Ca t 0.1 M surfactant concentration. Compare the two surfactants Rd304Na and R12P04TEA, where the difference is only between the S and P in the ionic group; the transition temperatures, however, are different from each other by 60 "C. Thus a small difference in the chemical structure of the ionic group of surfactant gives surprisingly large differences in the phase transition temperature of NIPA gel. It seems clear that the adsorbing power of surfactant onto polymer is strongly affected not only by the hydrophobic part but also by the hydrophilic group. These phenomena show the possibility of quantitative estimation of the hydrophilicity of surfactant through the equation of state of NIPA gel. Effects of the Hydrophobic Part. In order to examine the effects of the hydrophobic part of the surfactant, phase transition behaviors of NIPA gel in alkyl sulfonate surfactants, R,S03Na, with various alkyl chain lengths were studied. The number of carbon atoms in a chain ( n )was varied from 4 to 16. Figure 3 shows the concentration dependence of the phase transition temperature of the gel in R,S03Na solutions. Transition temperature increases with increasing concentration in every surfactant solution, and is saturated a t a certain temperature depending upon the alkyl chain length. In the solutions of surfactant with shorter alkyl chain length (n= 4,6,8), no significant change in transition temperature is observed a t 100 mM. As shown in Figure 3, the surfactant having a longer hydrocarbon chain length elevates the transition point of the gel to higher temperature a t lower surfactant concentrations. In addition, the saturated transition temperature of the gel increases linearly with the hydrocarbon chain length as shown in Figure 4. These results may indicate again that more hydrophobic (longer chain) surfactant is adsorbed onto polymer chains at lower concentrations.
Conclusions Presently, hydrophile-lipophile balance (HLB) is the only criterion used to classify the various kinds of surfactants. However, the HLB value was originally defined somewhat technologically only for nonionic surf a c t a n t ~ .It~ is particularly interesting to note that the hydrophobicity of surfactants depends heavily upon the small structural difference between the hydrophilic head group of ionic surfactants. It seems possible to classify the surfactants including the ionic ones mentioned above by examining the change of the phase behavior of the gel on addition of the surfactant. LA950070X
