Langmuir 1991, 7,905-911
905
Interactions of Surfactants with Hydrophobically Modified Poly(N-isopropylacrylamides). 1. Fluorescence Probe Studies Francoise M. Winnik' Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll
H. Ringsdorf and J. Venzmer Institut fiir Organische Chemie, Johannes- Gutenberg- Universitiit Mainz, J.-J.Becherweg 18-20,D-6500 Mainz, FRG Received August 30, 1990. In Final Form: October 23, 1990 The interactions between surfactants and copolymers of N-isopropylacrylamide (NIPAM) and N-nalkylacrylamides (n-alkyl = decyl, tetradecyl, and octadecyl groups; NIPAM to n-alkylacrylamide molar ratios, 1OO:l and 2OO:l) have been examined by fluorescence measurements with two hydrophobic probes: pyrene and bis(1-pyrenylmethyl) ether (Dipyme). Strong association between the copolymers and the charged surfactants sodium dodecyl sulfate (SDS) and hexadecyltrimethylammoniumchloride (HTAC) occurs by partition of the surfactants in a noncooperative mechanism. The interactions between the neutral surfactants n-octyl 8-D-glucopyranoside (OG) and n-octylj3-D-thioglucopyranoside(OTG) take place by a cooperative mechanism at a critical aggregation concentration (cac) well below the critical micelle concentration (cmc) of the surfactants. The association results in the formation of mixed clusters consisting of the n-alkyl substituents of the polymers surrounded by surfactant molecules (ca. 30 per n-alkyl substituent for the CSand surfactants and ca. 15 for the Cl6 surfactant). The values of the to pyrene excimer emission (intensityIE) of Dipyme ratio IE/ZM of pyrene monomer emission (intensityIM) in aqueous solutions of copolymers, surfactants, and surfactant/polymer mixtures indicate that the mixed clusters are more rigid than the corresponding surfactant micelles.
Introduction Over the last few years the industrial importance of water-basedfluids has grown significantly,due to concerns about the usage of organic solvents in industrial processes and the proper disposal of waste solvents. Aqueous systems of sophisticated physical and rheological properties are sought to fulfill new needs in a breadth of technological applications. Examples include fluids used in mineral processing, in oil recovery, and in coating operations, as well as paints and inks.' It has become apparent to the practitioners that the physical properties of aqueous media can be controlled to an amazing degree through the use of additives, mostly polymers and detergents. This situation has given rise to a growing need to unravel the detailed mechanism of the interactions between macromolecules and surfactants.23 Aqueous solutions comprised of an uncharged polymer and surfactants have been studied by a variety of experimental techniques. In most cases binding of the surfactant to the polymer is observed to start at a rather well-defined surfactant concentration, somewhat below the critical micelle concentration (cmc). The binding initially is cooperative: further addition of surfactant results in a large increase in the amount of bound surfactant while the concentration of free surfactant remains almost constant. Upon further increase of the surfactant concentration, the binding levels off and a point is reached where surfactant micelles coexist in solution. One system investigated in great detail is that of poly~~
(1) For recent revie-, nee Polymers in Aqueous Media; Advancee in Chembtry Serb 229; G h ,J. E., Ed.; American Chemical Society: Wmhington, DC,1989. (2) Robb., T. D. In Anionic Surfoctante, Physical Chemistry of Surfactant Actron;Lu"em-Reiiden,E. H., Ed.;MarcelDekker: New York, 1981; p 109. (3) For review, nee for example: Goddud, E. D. Colloids Surf. 1986, 19,256 and 301, and referencee therein.
(ethylene oxide) (PEO) and sodium dodecylsulfate (SDS). Elucidated largely by Cabane et al.' using NMR and neutron scattering techniques, the picture that emerges is one in which the surfactant forms complexes in the form of small spheres adsorbed onto the PEO strands. While the clusters are similar in size to pure SDS micelles, they have a lower critical aggregation concentration (cac) and a mean aggregation number (N= 38 f 30%) significantly smaller than that of pure SDS micelles (iV= 60 to 65). The spacing of the clusters is determined by an interplay between their mutual coulombic repulsion and their need to adhere to the polymer chain. A related system is that of (hydroxypropy1)cellulose (HPC) and SDS. Here also, SDS binds to the polymer above a critical concentrati~n.~ Small clusters form along the polymer chain. These grow in size as more SDS is added, but no new SDS clusters add to the polymere6 Above the cmc of SDS the clusters continue to grow in size, but their growth is now in competition with the formation of free SDS micelles. This situation is very different from that of the PEO-SDS interaction where, above the cac, spherical SDS aggregates of unique stoichiometry bind to the polymer. The clusters increase in number, but not in size as more SDS is added. One interpretation of this difference in the binding of SDS to the two polymers is that HPC provides hydrophobic sites to which SDS binds preferentially. Such sites may be, for example, small poly(propy1ene oxide) side chains protruding from the main carbohydrate backbone. It is well recognized that commercial samplesof HPC are rather heterogeneousmaterials. To test the role of hydrophobic binding sites on the polymer, we set about to study the (4) Cabane, B.; Dupleasix, R. J. Phys. 1982, 43, 1629. Cabme, B.; Dupleasix, R. J . Phy8. 1987, 48,6511. (5) Winnik,F.M.;Winnik, M.A,;Tazuke, S. J. Phys. Chem. 1987,91, 594. (6) Winnik, F. M.;Winnik, M.A. Polym. J. 1990,22 (61, 482.
0 1991 American Chemical Society
906 Langmuir, Vol. 7,No. 5, 1991
Winnik et al. Table I. Physical ProDerties of the Polymers
compositiona NIPAMC, 1:-
polymer
PNIPAM PNIPAM-C10/200 PNIPAM-Clo/ 100 PNIPAM-C14/200 PNIPAM-Clr/ 100 PNIPAM-C18/200 PNIPAM-Cle/ 100 a By 1H NMR. b From
[ q ] = (9.59 X
[?I* mL g-1
MVb
39.0 39.9 39.5 40.3 40.5 39.7 39.7
360 OOO 370 OOO 360 OOO 370 OOO 380 OOO 370 OOO 370 OOO
2401 114:l 2201 108:l 240 1 126:l 10-3)
[email protected] GPC (relative to polystyrene).
interactions of surfactants with a series of well-characterized water-soluble polymers bearing a small number of hydrophobic groups. The polymers belong to the class of "hydrophobically modified" polymers that have important industrial applications in the control of the rheology of aqueous solutions.' They are often used in conjunction with other additives, such as Surfactants. While the macroscopic features of the effects of added surfactants on the properties of such fluids have been examined? there have been relatively few studies devoted to the detailed molecular mechanism of the interactions. Dualeh and Steiner have reported recently astudy of the properties of a waterinsoluble amphiphilic polymer in aqueous surfactants solutions.8 This polymer, a (212-grafted (hydroxyethy1)cellulose, undergoes strong association with sodium dodecy1 sulfate (SDS), in contrast with its water-soluble "parent' polymer, (hydroxyethyl)cellullose,which has been shown to interact only weakly with SDS.g Another study relevant to this study originates in the group of Winnik.lo It focuses on the interaction between SDS and a poly(ethylene oxide) with pyrene groups attached at its ends. Formation of mixed micelles incorporating the hydrophobic substituents was observed at SDS concentrations well below the cmc. The objectives of the experiments described here were to study the interactions of surfactants with a series of amphiphilic copolymers of N-isopropylacrylamide (NIPAM) and N-n-alkylacrylamides of different chain lengths ((210, C14, and C18) present in small amounts (e.g., ratios of n-alkyl group to NIPAM units, 1:lOO and 1:200,see structures). We used fluorescence probe techniques to monitor the interactions of these copolymers with an anionic surfactant, sodium dodecyl sulfate (SDS),a cationic surfactant, hexadecyltrimethylammonium chloride (HTAC),and two neutral surfactants, n-octyl fl-D-glucopyranoside (OG)and n-octyl fl-Dthioglucopyranoside(OTG). f"3 HC-NH
I
CH3
H3C '(CH2)n-
n
I I
-CO-CH
'HZ
NH -CO-CH
I
m=1m
n=9:
PNIPAM-~10
m=200
n=13: n =17:
PNiPAM-Cl4 PN I PAM C18
-
I
:"2 -1
Formation of surfactant clusters along the polymer chain was detected in all cases, but for each pair, the type of clusters and the mechanism of cluster formation proved to depend on the molecular structures of the surfactant (7) For a recent study, see Carlsson, A.; Karlstrcm, G.; Lindman, B. Colloids Surf. 1990, 47, 147. (8)Dualeh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251. (9) Goddud, E. D.; Hannan, R. B. In Micellization, Solubilization, and Microemuleions;Mittal, K. L., Ed.; Plenum Press: New York, 1977; VOl. 2, p 835. (10)Hu, Y.-Z.; Zhao, C.-L.;Winnik, M. A.; Sundararajan, P. R. Langmuir, 1990,6, 880.
M.' 19 OOO 19 OOO 24 OOO 26 OOO 22 OOO 17 OOO
23 OOO
MlV'
Mw/Mn'
31 OOO 31 OOO 41 OOO 43 OOO 37 OOO 27 OOO 38 OOO
1.63 1.67 1.72 1.64 1.68 1.64 1.66
Table 11. Composition of the Solutions Studied probe, mol L-1 PNIPAM-C,/100 Dipymeb (2 X 10-7) pyrene (5 x io-') PNIPAM-C,/200 Dipymeb (2 X 10-7) pyrene (5 x 10-7) PNIPAM-C10/100 pyrene (5 x lo-') PNIPAM-C10/200 Dipyme' (2 X lo-') pyrene (5 x 10-7) polymep
[polymer], [alkyl groupl, g L-1 mol L-1 1.87 1.62 X lo-' 1.62 X lo-' 1.87
1.87 8.2 X 106 1.87 8.2 X loa 1.87 1.62 X lo-' 1.87 8.2 X 1Oa 1.87 8.2 X 104 I, n = 14,18. b Solutions prepared by method A (see Experimental Section). Solutions prepared by method B (see Experimental Section).
and the polymer. We have examined the effects on the polymer-urfactant interactions of parameters such as the length of the alkyl substituent of the polymers and the charge and chain length of the surfactant. A comparison of these results with experiments carried out with PNIPAM itself emphasizes the extent of the disruptions brought about by the presence on the polymer of extremely small amounts of hydrophobic substituents.
Experimental Section Materials. Water was deionized with a Millipore Milli-Qwater purification system. Dipyme was a gift from Professor M. A. Winnik. It is the same material as that used in a study of the use of this probe in solutions of hydrophobically modified polymers.ll Pyrene (99%, Aldrich Chemicals Co.) was purified by repeated recrystallizations from absolute ethanol and subsequent sublimation. n-Octyl B-D-thioglucopyranoside (OTG) and n-octyl8-D-glucopyranoside(OG) were purchased from Sigma Chemical Co. Sodium dodecylsulfate (SDS,.purum) was purchased from Fluka. Hexadecyltrimethylammonium chloride (HTAC) was obtained from Eastman Kodak Chemicals. The polymers were prepared by free radical polymerization in dioxane.12 Their physical properties are summarized in Table I. From the number-averaged molecular weight of the polymers, one can estimate that there are on average 33 and 16hydrophobic groups per chain in copolymers with NIPAMn-alkylacrylamide molar ratios of 100:1 and 200.1, respectively. Fluorescence Measurements. Fluorescence spectra were recorded on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. The temperatureof the water-jacketed cell holder was controlled with a Neslab circulating bath. The temperature of the sample fluid was measured with a thermocouple immersed in the sample. Emission spectra were not corrected. For measurementswith Dipyme, band paths were set at 3.6 nm (excitation) and 1.6 nm (emission). The excitation wavelength was 348 nm. The excimer to monomer ratio (ZE/ZM) was calculated as the ratio of the emission intensity at 495 nm to that of the emission at 399 nm. The [Z1/ZslDp ratio was calculated as the ratio of the emission intensity at 378 nm to that of the emission at 388 nm. For experiments with pyrene, the excitation band paths were 3.6 nm (excitation) and 0.9 nm (emission). The excitation wavelength was 336 nm. The [ZI/ Z3lPy ratio was calculated as the ratio of the intensity at 376 nm (11)Winnik, F. M.; Winnik, M. A,;Ringsdorf, H.; Venzmer, J. J. Phys. Chem. 1991,95, 2583. (12) Ringsdorf, H.; Venzmer, J.; Winnik, F. M. Macromolecules 1991, 24,
1678.
Surfactant and Modified NIPAM Interactions
Langmuir, Vol. 7, No. 5, 1991 907
Table 111. Spectroscopic Data for Dipyme and Pyrene in Aqueous Solutions of Amphiphiic NIPAM Copolymers and in Surfactant Micelles Dipyme Dipyme pyrene pyrene polymers IElh [I1/ W D P [IllI3l@ surfactantso IE/IM [Il/I31DP [Ii/ZaI@ PNIPAM-Cls/ 100 0.11 1.20 1.10 SDS 0.60 1.62 1.11 PNIPAM-C18/200 0.13 1.29 1.22 HTAC 0.88 1.30 1.34 PNIPAM-Clr/ 100 0.12 1.23 1.21 OTG 0.88 1.11 PNIPAM-C14/200 0.20 1.19 1.35 OG 0.96 1.14 1.44 PNIPAM-Clo/ 100 1.39 PNIPAM-C10/200 1.69 SDS, sodium dodecyl sulfate; HTAC, hexadecyltrimethylammoniumchloride; OTG, n-octyl 8-D-thioglucopyranoside;OG, n-octyl 8-b glucopyranoside; concentration > cmc.
to that of the intensity at 386 nm. Solutions were not degassed. All measurements were carried out at 20 “C. Samples for Spectroscopic Analysis. Dipyme i n Amphiphilic Copolymers. Polymer stock solutions (5g L-l) were prepared. Aliquots of the stock solutions were diluted to the desired concentration. Samples containing Dipyme (2.6 X 10-’ mol L-l) were prepared by adding a concentrated solution of Dipyme (5 pL, 1.3 X 1Vmol L-l in acetone) to aqueous solutions of the polymers (2.5 mL, 1.87 g L-l). The solutions were sonicated for 10min in an ultrasonic bath (Cole-Parmer). They were kept at room temperature in the dark until equilibrated (2 to 4 days). The compositions of the solutions studied are listed in Table 11. Dipyme i n Surfactant Micelles. The solutions of Dipyme in surfactants were prepared by adding a concentrated solution ofDipyme (5pL, 1.3 X lVmolL-linacetone) toaqueoussolutions of the surfactants (2.5 mL, [SDS] = 8.82 X 10-a mol L-l; [HTAC] = 1.64 X le3 mol L-l; [OTG] = 1.67 X 10-2 mol L-l; [OG] = 2.9 x lo-* mol L-l). The solutions were sonicated for 10 min in an ultrasonic bath. They were kept at room temperature in the dark until equilibrated (2 days). Pyrene in Solutions of Polymers and Surfactants: Aqueous solutions of the polymers containing pyrene were prepared by dissolvingthe polymers (1.87 g L-9 in pyrene-saturated water, previously filtered to remove pyrene microcrystals. Small aliquots of surfactant solutions were added to the polymer solutions in amounts such that the polymer-pyrene solutions were diluted by no more than 10%. Solutions were prepared 2 h before spectroscopic analysis. Dipyme in Solutions of Polymers a n d Surfactants. Method A (for PNIPAM-C14/ 100,PNIPAM-C14/200, PNIPAMCls/100, PNIPAM-C18/200). Increasing amounts of a concentrated surfactant solution were added to the Dipyme-containing polymer solutions (2.5 mL, 1.87 g L-l). Spectra were measured after each addition. Method B (for PNIPAM-Clo/100, PNIPAM-C10/200, and PNIPAM-C14/200). An aliquot of polymer stock solution (0.95 mL, 5 g L-l) was added to Dipyme-containingsurfactant solutions (1.55 mL, [SDS] = 1.48 X 1 0 - 2 mol L-1, [OTG] = 1.52 X 10-2 mol L-1, [HTAC] = 1.0 X 10-2 mol L-l, [OG] = 4.0 x 10-2 mol L-l). The samples were kept at room temperature in the dark for 4 h prior to spectroscopic measurements.
Results The Fluorescence Probes. To study the interactions of surfactants with the entire series of PNIPAM copolymers, it was necessary to employ two fluorescent probes, pyrene (Py)and bis( 1-pyrenylmethyl)ether (Dipyme). No single probe gave meaningful results in all cases, as will become apparent from a brief description of the fluorescence properties of the two probes. Pyrene is a hydrophobic probe with modestly low solubility in water (ca. 6 X mol L-9. The fluorescence spectrum of Py at low concentration possesses a vibrational band structure which exhibits a strong sensitivity to the polarity of the Py environment.l3 The relative intensity of the peaks undergoes a significant perturbation upon going from non(13) Nakajima, A. J. Lumin. 1977, 15, 277.
polar to polar solvents. In particular, the intensity (ZI) of the (0, 0) band increases in polar solvents whereas that (13) of the (0, 2) band is essentially unaffected. As a consequence the ratio [11/13]b is sensitive to solvent polarity and has been shown to correlate well with other scales of solvent p01arity.l~ It increases with increasing polarity. For example in hydrocarbon solvents it has a value of ca. 0.70, while in water its value is ca. 1.80. Pyrene has also been used as a probe to determine critical micelle concentration^.'^ In these experiments the value of [11/ I31bis monitored as a function of surfactant concentration. Below the cmc pyrene resides in water. Its fluorescence is characteristic of a highly polar environment. As the surfactant concentration is increased through the cmc, a large decrease in [Il/I3]b is observed, reflecting the incorporation of the hydrophobic probe into the micellar environment. The technique has been extended to the study of the formation of polymer/surfactant clusters.16 The ratio [Il/I3]b was determined for aqueous solutions of each NIPAMIN-n-alkylacrylamidecopolymer.17 In solutions of the copolymers (1.87 g L-l) its value ranged from 1.69 (PNIPAM-C10/200) to 1.10 (PNIPAM-ClS/ 100) (Table 111). Such a wide range of [Il/I~]fivalues implies that Py experiences different environments in the various polymer solutions. The low value of [I1/13]fi indicates that by himself the CIScopolymer provides pyrene with hydrophobic domains into which it is entirely solubilized. By analogy with the structure adopted in water by related CIS-substituted NIPAM copolymers,12it can be inferred that these domains are mostly unimolecular polymeric micelles in the case of the CIS-substituted copolymers. The length of the alkyl substituents dictates the extent of hydrophobic microdomain formation in the aqueous solutions of the other hydrophobically modified PNIPAM. Pyrene partitions between the polymeric microphasesand water, as reflected by the range of [Il/I3]@ values. The ratios [!1/13]b were also measured in micellar solutions of the surfactants used in this study (Table 111). These are too close to those measured for pyrene in the (214-and CIS-NIPAM copolymers to serve as a useful tool to detect interactions between these polymers and the Surfactants. Therefore in these systems Py is not a useful probe. Another hydrophobicprobe, Dipyme, proved to be more effective in the study of these systems. Unlike pyrene it is a probe not only of local polarity but also of local viscosity.1s Like the closely related 1,341’-pyreny1)pro(14) Dong, D. C.; Winnik, M.A. Con. J. Chem. 1986,62,2660. (15) Kalyanasundaram,K.; Thomas, J. K. J. Am. Chem. SOC. 1977,99, 2039. (16)Turro, N. J.; Baretz, B. H.; Kuo, P.-L. Macromolecules 1984,17, 1321. (17) Ringadorf, H.; Venzmer, J.; Winnik, F. M.Polym. Prepr., Am. Chem. SOC. Diu. Polym. Chem. 1990,31 (l), 568. (18) Georgescauld, D.; Desmasbz, J. P.; Lapouyade, R.; Babeau, A.; Richard, H.; Winnik, M.A. Photochem. Photobiol. 1980,31, 539.
Winnik et al.
908 Langmuir, Vol. 7, No. 5, 1991 I
I
I
a.
I-".' 1 /I
1
-hu
-hu'
monomer emission
excimer emission
l
d
I
I
I
I1 13'
b.
@cH2-o-cH2@
Py-CH2-O-CH2-Py
(Dipyme)
Figure 1. Representation of intramolecular excimer formation in Dipyme (bis((1-pyrenylmethyl) ether).
pane,19 Dipyme forms an intramolecular excimer (Figure 1). The extent of excimer emission in both speciesdepends upon the rate of conformational change. The motion is resisted by the local friction imposed by the environment. As a consequence, the excimer to monomer intensity ratio, I E / I M provides , a measure of the fluidity of the Dipyme environment. The vibrational fine structure in the Dipyme monomer emission is also sensitive to the polarity of the probe microenvironment. The ratio [11/131DPreports on local polarity in much the same way as the ratio [11/13]w. For both probes the ratio increases with increasing micropolarity, but the absolute values are not the same for the two probes in identical environments. In Figure 2 are shown representative emission spectra of Dipyme in OTG micelles and in an aqueous solution of PNIPAM-C18/100. It is immediately apparent that the extent of excimer emission differs greatly for Dipyme in these two environments. In OTG, the ratio of excimer to monomer emission intensity, I E / I M , is 0.88, while in PNIPAM-C18/100 it takes a value of 0.11. Differences in I E / I Mwere noted also for Dipyme solubilized by the other surfactants and copolymers. These data, collected in Table 111, reveal the following trends. (1) The I E / I Mvalues in the polymeric solutions are markedly lower than those measured in ionic and neutral surfactant micelles. (2) The [I1/I3IDpand I E / I Mratios do not show a significant dependence on either the length of the alkyl group on the polymer or the level of alkyl group incorporation. The IE/IMresults point to a more rigid structure for the alkyl chain clusters formed from these polymers than in simple surfactant micelles. The [I1/I3IDPratios indicate that in each of the types of polymeric micelles the Dipyme probe is located in a very similar and strongly hydrophobic environment. Unlike pyrene, Dipyme, which has a much lower water solubility, is not solubilized in aqueous solutions of PNIPAM and PNIPAM-Clo. Therefore its use is limited to the C14- and Cls-NIPAM copolymers. Interactions of Surfactants with PNIPAM-Clo (Pyrene Probe Experiments). The variations of [I1/ 13]bwere measured for pyrene (ca. 5 X 10-7 mol L-1) in aqueous solutions containing fixed amounts of PNIPAMCl0/200 (1.87 g L-l) and increasing amounts of SDS or HTAC. (Figure 3). Solubilization of pyrene into a (19) Zachariasse,K. A.; Vaz,W. L.C.; Sotomayor,C.; KGhnle,W. Biochim. Biophys. Acta 1982,688,323.
I/
Excimer
400
450
500
600
550
WAVELENGTH (nm)
Figure 2. Fluorescence spectra of Dipyme (a) in micelles of n-octyl @-wglucopyranoside(1.67 X mol L-1) and (b) in an aqueous solution of PNIPAM-Cla/ 100 (1.87 g L-Y: temperamol L-l. ture, 20 OC; bxc= 348 nm; [Dipyme] 2 X
-
1.60
-
A
[:IPY 1.40
1.20
-
1.00L -5.0
"
"
'
- 4.0
'
"
'
' -3.0
c m c , j
"
- 2.0
LOG [surfactant] mol L-'
Figure 3. Plot of [Il/&]~forthe emissionof pyrene in an aqueous solution of PNIPAM-C10/200 (1.87 g L-l) as a function of surfactant concentration (logarithmic scale) for SDS and HTAC. Values of [11/13]b for pyrene in SDS and HTAC micelles are indicated by an open square and a full square, respectively: temperature, 20 "C; )bxc = 336 nm.
hydrophobic environment, as detected by a decrease in [11/13]-, was observed for SDS concentrations higher than ca. 1 X lo4 mol L-l. With increasing SDS concentration, the ratio decreased until a saturation value was reached for [SDS] 1.6 X 10-3 mol L-l, a concentration signifimol L-1).20 cantly lower than the cmc of SDS (8.2 X One aspect of these data is particularly noteworthy: the decrease in [I1/l,]l'Y with increasing SDS concentration occurs over a large surfactant concentration range rather than as a sharp transition centered at a critical surfactant concentration, the behavior more commonly observed with other neutral polymers, such as PE0,16PVP,16HPC,6p6or PNIPAM.21
-
(20)Berr, S. S. J. Phys. Chem. 1987,91,4760.
Langmuir, Vol. 7, No. 5, 1991 909
Surfactant and Modified NIPAM Interactions
slightly less rigid than the original polymeric micelles. But the clusters are very different in micropolarity and fluidity from free SDS micelles. Addition of HTAC to solutions of the CU- and CISNIPAM copolymers resulted qualitatively in the same changes in Dipyme emission as those observed during SDS addition: (1)The ratio IE/IMincreased in all cases. The onset of the increase was observed at [HTAC] 5 X 10-5 mol L-l. Saturation was achieved at the cmc of HTAC. The limiting values of I E / I M were lower than I E / I Mfor the emission of Dipyme in HTAC micelles (Table I11and Table IV). (2) The [Z1/I3lDPvalues also increased in this HTAC concentration range. No sharp transition was apparent in the plots of either IE/ZM or [I1/Z3IDPas a function of HTAC concentration. We turned our attention next to the interactions of a neutral surfactant, n-octyl&D-thioglucopyranoside (OTG), with the amphiphilic NIPAM copolymers. To our surprise, the fluorescence of the Dipyme probe in these systems revealed clearly that this surfactant interacts with the copolymers by a mechanism different from that observed for SDS and HTAC. This is illustrated here for the case of PNIPAM-C18/200 (Figure 6). Addition of OTG to Dipyme-containing solutions of PNIPAM-C18/200 solutions had no effect on I E / I Mbelow a critical concentration mol L-l. Above this concentration the ratio of 1X increased sharply to reach a maximum value which remained unaffected by further surfactant addition. The saturation value of IE/IMwas attained for OTG concentrations well below the cmc of the surfactant in the absence of polymer (9.0 X mol L-1).24 Concomitant with this increase in IE/IM,a sharp drop in [I1/I3IDPoccurred as well. The limiting value of [I1/I3IDPis almost identical with that of Dipyme in OTG micelles.
1 . 8 0 1 -
cmc
The concentration dependence of the interaction between PNIPAM-C10/200 and HTAC seems even less sharply defined than in the case of SDS (Figure 3): a smooth decrease of [11/13]& takes place with increasing [HTAC], with a leveling-off for [HTAC] > 7 X lo4 mol L-l, a value lower than the cmc of HTAC (1.3 X mol L-1).22 One notices that the value of [I1/I3IpY at this saturation concentration is lower than that of Py in HTAC micelles, indicating that in the polymer/HTAC clusters Py senses a less polar environment than in the micelles. The changes in p p e n e emission in solutions of PNIPAM-Clo/ 200 were monitored also during addition of a neutral surfactant, OG. In this case a plot of [I1/I3]fi versus [OG] exhibits a sharp transition centered 1.4 X mol L-l, a concentration lower than the cmc of OG (2.32 X mol L-1)23(Figure 4). Whether these data support the formation of polymer/surfactant clusters will be discussed further in light of results obtained with Dipyme. Interaction of surfactants with PNIPAM-Cl4 and PNIPAM-ClB copolymers (Dipyme probe experiments). When increasing amounts of SDS were added to solutions of the C14- and CIS-NIPAM copolymers in water containing Dipyme, two features of the Dipyme emission were affected: (1)the pyrene excimer emission increased with increasing SDS concentration at the expense of pyrene monomer emission and (2) the ratio [I1/I3IDpof the pyrene monomer emission increased with increasing [SDS] for all four copolymers (Tables I11 and IV). The changes in IE/ZM and [I1/ZslDPare represented in Figure 5 in the case of the interactions of SDS with PNIPAM-C18/200. The overall increase in ZE/ZM is small. It occurs over a wide range of surfactant concentrations and the value levels off for [SDS] < cmc. The value at saturation is much lower than the value recorded for Dipyme in SDS micelles. On the other hand, the increase in [Z1/13]DPtake place over a narrower concentration range, the midpoint of the transition (6.3 X lO-9mol L-l) corresponding to the SDS concentration at which IE/IMreaches saturation in this same system. Taken together these two trends are indicative of the formation of surfactant/ polymer clusters (21)Schild,H.G.; Tirrell, D. A. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) lS89,30 (2), 350. (22) Malliarie, A.; Lang,J.; Zana, R. J. Colloid Polym. Sci. 1986,110, an"
-
Discussion The interaction of poly(N-isopropylacrylamide) with surfactants is typical of that of neutral polymers. Schild and Tirrel121have shown that PNIPAM forms mixed micelles with anionic surfactants at a critical aggregation concentration which becomes increasingly lower relative to the cmc as the length of the surfactant tail increases. The polymer/surfactant clusters are less polar than the correspondingfree micelles which may indicate less water penetration into the mixed micelles than in the surfactant micelles themselves. We have found that mixed micelles are formed also between PNIPAM and cationic surfactants but not neutral surfactants.26 When interactions take place, they occur by a cooperative mechanism, as observed with other neutral polymers. The situation with amphiphilic PNPAM copolymers is different in two major aspects: (1)the binding of charged surfactants occurs by a noncooperative mechanism and (2) the copolymers interact also with neutral Surfactants. These bind to the polymer chains by a cooperative mechanism. The differences in binding mechanism among surfactants are emphasized when the changes in probe spectral properties are reported as a function of the number of surfactant chains per n-alkyl substituent on the polymer chain,rather than as a function of surfactant concentration. As an illustration the values of I E / I Mof Dipyme fluorescence are plotted as a function of the ratio of surfactant concentration, [SDS] or [HTAC], to the octadecyl chain concentration in solutions of the CIS-NIPAM copolymers (Figure 7). The ratio increases sharply and reaches a plateau when the relative surfactant to polymer concen-
Zdl.
(23) De Grip, W. J.; Bovee-Geurta, P. H. M. Chem. Phys. Lipids 1979,
29, 321.
(24) Tsuchiya, T.; Saito, 5.J . Eiochem. 1984,96,1693. (26) Winnik, F. M.; Ringedorf, H.; Venzmer, J. Unpublished multa.
910 Langmuir, Vol. 7, No. 5, 1991
Winnik et a.1.
Table IV. Spectroscopic Data for Dipyme in Surfactant/Amphiphilic NIPAM Copolymer Clusters SDS/polymer cluster@ HTAC/polymer clustersb OTG/polymer clustersc polymers PNIPAM-Cls/ 100 PNIPAM-C18/200 PNIPAM-Clr/ 100 PNIPAM-C14/200
IEIIM
[IlIZ3lDP
IElIM
[IIII31DP
IEIIM
0.36 0.31 0.30 0.31
1.28
0.32 0.37 0.33 0.35
1.32 1.39 1.46 1.34
0.40 0.60 0.80 0.85
1.40 1.35 1.33
[IiIIaIDP 1.15 1.05 1.22 1.10 1.1od
0.W
*
[SDS] = 7.0 X 10-3mol L-1. [HTAC] = 1 X 10-3 mol L-1. c [OTG] = 3.2 X 10-3 mol L-1. d [OTG] = 9.12X by method B. 0
1.6
1
cmc I
1
1.3
10-8
mol L-1, solution prepared
-
[%IDP1.2
1.1
t
la2 1.1 1 0.6
IE
I
-
..-
1.o
m -
8
0.4,
IM 0.2
-.--.,.
'
cmc 01 -4.0
- 3.0
-2.0
1I
LOG [SDS] mol L-'
0
-4.0
n
a
n
/*
'
.
*
*
n
-3.0
- 2.0 '
.
LOG [OTG] mo1L-l
Figure 5. Plots of [Z!/Za]DPand ZE/ZM for the emission of Dipyme in an aqueous solution of PNIPAM-C18/200 (1.87 g L-l) as a function of SDS concentration (logarithmic scale). Values of [Zl/Z3]DPand ZE/ZM for Dipyme in SDS micelles are indicated by a full square in each plot: temperature, 20 "C; bxc = 348 nm.
Figure6. Plotsof [Z1/ZalDPandZE/Z~for theemissionofDipyme in an aqueous solution of PNIPAM-C18/200 (1.87 g L-I) as a function of OTG concentration (logarithmic scale). Values of [ Z l / l ~and ] ~ ZE/ZM ~ for Dipyme in OTG micelles are indicated by a full square in each plot: temperature, 20 OC; bxc= 348 nm.
trations correspond to a situation were there are approximately, in the case of HTAC, 15 surfactant molecules per octadecyl chains. This behavior resembles a binding isotherm and suggests simple partition of HTAC between the aqueous phase and the polymeric environment. In contrast, a behavior typical of cooperative binding is observed in the case of neutral surfactants. A sigmoidal curve is obtained when the changes in I E / I Mare plotted as a function of the ratio of the concentration of neutral surfactant [OTG] to the concentration of polymer n-alkyl substituents (Figure 8). The onset of the increase in IE/ IMoccurs when there are approximately 12 OTG molecules per alkyl chain, in the case of PNIPAM-C18/200. The ratio saturates at a surfactant concentration corresponding to about 30 OTG molecules per alkyl groups on the polymer. The spectroscopic data point to the following description of the interactions of surfactants with amphiphilic NIPAM copolymers (Figure 9). Extensive micellar structures exist in aqueous solutions of the PNIPAM-C, copolymers with tetradecyl and octadecyl chains. These form rigid hydrophobic microdomains surrounded by the more polar poly(N-isopropylacrylamide)chains. Addition of minute amounts of surfactants causes a disruption of the polymeric micellar structures. The hydrophobic tails of the surfac-
tant molecules adsorb on the polymer alkyl substituents. The formation of mixed micelles begins to take place. At very low surfactant concentrations, the structure of these mixed micelles fluctuates, but a regime is achieved where they are, it seeems, unaffected by further surfactant addition. This is evidenced by the insensitivity of the pyrene and Dipyme fluorescence to surfactant concentrations higher than a minimal value, in all cases well below the cmc of the surfactant (see, for example, Figures 7 and 8). One can picture the polymer/surfactants aggregates as consisting of clusters into which one (or several) alkyl substituents of the polymer is sequestered by surfactant molecules. The size, fluidity, and polarity of these clusters depend more on the structure of the surfactants than on the length of the polymeric substituents. While our experiments do not allow us to determine the size of individual clusters, it is possible to extract from the data the minimum number of surfactant molecules per alkyl substituent (Table V). In all cases this number is lower than the aggregation number of the corresponding surfactant micelles. It is higher for the short chain surfactants (ca. 30 for the C8 and C12 surfactants) than for the CISsurfactants (ca. 15). The fluidity of the mixed clusters for each polymer/surfactant pair can be compared to that
Langmuir, Vol. 7, No. 5, 1991 911
Surfactant a n d Modified NIPAM Interactions
Mixed
Cluster
Polymeric Micelle HTAC
- e ,
Surfactant Addition
0.10r
f
a?
Surfactant Micelle
a
Key:
:) PNIPAM-Clr1200
0
NIPAM Unit c18"37
Surfactant
Figure 9. Stylized illustration of the interactions between surfactants and PNIPAM-Cia ([surfactant] > cmc). Polymeric micelles (intramolecular) form in water. They are disrupted by the addition of surfactant. Mixed clusters form, consisting of surfactant molecules and octadecyl chains. These coexist with free surfactant micelles, when the surfactant concentration exceeds the cmc.
1.2
t
Table V. Number of Surfactant Molecules per Alkyl Substituents on the Polymers in the Surfactant/Copolymer Clusters.
PNI PAM - C j4/200
OTG SDS HTAC polymers onset aaturation saturation saturation PNIPAM-C18/100 10 30 25 15 PNIPAM-C18/200 12 30 30 15 PNIPAM-Cu/lOO 8 30 30 a 7 30 50 5 PNIPAM-C14/200 PNIPAM-C1o/2OOb 6 30 15 10 a From plots of zE/ZM vs [surfactant]/[alkyl group]. From plots of 11/13 vs [surfactant]/[alkyl group] (experiments with pyrene).
*
I 0
20
40
60
80
100
[OTG] : [cnl
Figure 8. Plot of ZE ZM of Dipyme emission for Dipyme in aqueous solutions o PNIPAM-C18/200 (full circles) and PNIPAM-C14/200 (full triangles) as a function of the molar ratio of OTG to Cl8 or C14 substituents of the copolymers: temperature, 20 OC, Iexc= 348 nm.
I
of either the pure surfactant micelles or the polymeric micelles through the values of IE/IM of Dipyme solubilized in each environment (Tables I11and IV). In all cases the mixed micelles are more fluid (e.g. larger value of IE/ IM)than the polymeric micelles, irrespective of the length of the surfactant tailor the polymeric substituent. Among the three surfactants, OTG forms the most fluid clusters with all polymers.
Conclusion Reported here is a study by fluorescence probe experiments of the association of surfactants with amphiphilic water-soluble polymers. Ionic surfactants, such as SDS and HTAC, bind to the polymers by a noncooperative mechanism to form mixed micelles that sequester the hydrophobic groups of the polymer. Mixed micelles are
formed also between the polymers and neutral surfactants, but by a cooperative process. The results highlight the importance of both the charge of the surfactant and the length of its tail on polymer/surfactant interactions. In this last aspect they provide support to a computer model of the interactions between self-assembling polymers and surfactants, recently put forward by Balazs and H u . ~Other ~ parameters such as the charge and the size of the surfactant head groups are deemed to be important also in directing the interaction^.^^^^^ These effects cannot be seen from the data presented here. The issue will be addressed in a systematic investigation with series of anionic and cationic surfactantsof increasing chain lengths but identical head groups.
Acknowledgment. We thank Professor M. A. Winnik (University of Toronto) for a gift of a Dipyme sample and for many stimulating discussions during the preparation of this manuscript. Financial support was provided in part by the Bundesminister fiir Forschung und Technologie (FRG) (H.R. and J.V.). Part of J.V.'s expenses for his stay in Mississauga was provided by an NSERC (Canada) grant to Professor M. A. Winnik. (26) Balm, A. C.; Hu, J. Y. Langmuir, 1989,6, 1230 and 1263.
(27) Ruckenetein,E.;Huber, G.; Hoffmann, H. Langmuir 1987,3,382. (28) Nagarajan, R. J. Chem. Phye. 1989,90,1980.
