Langmuir 1991, 7, 912-917
912
Interaction of Surfactants with Hydrophobically Modified Poly(N-isopropylacrylamides). 2. Fluorescence Label Studies Francoise M. Winnik' Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1
H. Ringsdorf and J. Venzmer Institut fiir Organische Chemie, Johannes-Gutenberg- Universitat Mainz, J.-J. Becherweg 18-20, 0-6500 Mainz, FRG Received August 31,1990. I n Final Form: October 23, 1990 The interactions between surfactants and pyrene-labeled derivatives of poly(N4sopropylacrylamide) (PNIPAM) have been examined by fluorescence measurements. The polymers are copolymers of N-iso(molarratios 200:l and propylacrylamide (NIPAM)and N-[4-(l-pyrenyl)butyl]-N-n-octadecylacrylamide 400:1, PNIPAM-Clsy 200 and PNIPAM-Clsy/400, respectively) or N-[4-(l-pyrenyl)butyl]acrylamide (molar ratio 200:1, PNI AM-Py/200). Changes in the ratio of the intensity of the pyrene excimer emission (ZE)to the intensity of the pyrene monomer emission ( I d were monitored in aqueous polymeric solutions as a function of surfactant concentration. In the case of PNIPAM-Py/ZOO, association with surfactants occurred by a cooperative mechanism at a critical surfactant concentration lower than the critical micelle concentration (cmc) of the surfactants for each of the three surfactants investigated: sodium dodecyl sulfate (SDS), hesadecyltrimethylaoniumchloride (HTAC),and n-octyl8-Pthioglucopyranoside(OTG). Association of SDS and HTAC with the hydrophobically modified copolymers PNIPAM-Clay/200 and PNIPAM-Clsy/400 took place by a noncooperativemechanism. Binding of surfactants to these polymers was detected at surfactant concentrations as low as 10-5 mol L-1 and saturation occurred well below the cmc. On the other hand, binding of OTG to PNIPAM-ClePy/200 and PNIPAM-ClaPy/400 occurred by a cooperative mechanism. The behaviors toward surfactants of PNIPAM-ClePy/200 and PNIPAM-Py 200 are compared to those of the corresponding unlabeled polymers, PNIPAM-C18/200 and P N I P A d respectively, in terms of binding mechanism and structure of the polymer/surfactant aggregates.
L
Introduction We have reported recently a fluorescence probe study of the interactions of surfactants with hydrophobically modified copolymers of N-isopropylacrylamide (NIPAM) and N-n-alkylacrylamides.' In this paper experiments with closely related fluorescently labeled amphiphilic poly(N-isopropylacrylamide) (PNIPAM) derivatives are described. Here the polymers are random copolymers of NIPAM and N-[4-(l-pyrenyl)butyl]-n-octadecylacrylamide, in which the molar ratio of pyrene-containing monomers to NIPAM units is extremely low, 1:200 in PNIPAM-Clay/200 and 1:400 in PNIPAM-Clay/400 (Figure 1). These labeled polymers differ from the amphiphilic copolymers of our previous study, such as PNIPAM-C18/200, by a small structural modification. The secondary octadecyl-substituted amides have been replaced by the tertiary N-(l-pyreny1)butyl-N-octadecylamides (see structures, Figure 1). They are identical in all other aspects, since they were synthesized under the same conditions2 (Table I). The objectives of this study were to monitor the interactions of surfactants with the amphiphilic NIPAM copolymers from the polymers's point of view, using a fluorescent dye covalently attached to the polymer strands and to compare the results of these label studies to those obtained by our first approach based on fluorescenceprobe techniques. Two facets of the polymer/surfactant inter(1) Winnik, F. M.; Ringsdorf, H.: Venzmer, J. Langmuir. Precedingpaper in this ieeue. (2) Ringdsorf, H.; Venzmer, J.; Winnik, F.M. Macromolecules 1991, 24, 1678. For a preliminary account, see Rin orf, H.; Venzmer, J.; Winnik, F. M. Polym. R e p r . (Am. Chem. S O C . Polym. ,~ Chem.) 1990, 91 (l),588.
actions will be emphasized. They concern (1) the structure of the aggregates that form upon addition of surfactants to aqueous polymeric solutions and (2) the mechanism of binding of surfactant molecules to amphiphilic polymers. Three surfactants were employed: an anionic surfactant, sodium dodecyl sulfate (SDS);a cationic surfactant, hexadecyltrimethylammonium chloride (HTAC); and a neutral surfactant, n-octyl I5-D-thioglucopyranoide(OTG). Three pyrene-labeled polymers were used: PNIPAM-CISPy/200, PNIPAM-Clay/400, and PNIPAM-Py/SOO. The third polymer was needed in control experiments, to capture the role played by the octadecyl chains in directing the interactions with surfactants. This polymer was tailored to mimic as closely as possible the labeled amphiphilic polymers both in the pyrene to NIPAM units molar ratio (1:200) and in the linkage of the label to the polymer backbone. A single spectroscopic tool was employed to study the nine surfactant/polymer pairs. I t is based on the relative intensities of the pyrene monomer and pyrene excimer emissions, as described in the first part of the paper.
Experimental Section Materials. Water was deionizedwith a Millipore Milli-Qwater purification system. n-Octylfi-Dthioglucopyranoside(OTG)was purchased from Sigma Chemicals Co. Sodium dodecyl sulfate (SDS, purum) was purchased from Fluka. Hexadecyltrimethylammonium chloride (HTAC) was obtained from Eastman Kodak Chemicals. The polymers were prepared by free radical
polymerization in tert-butyl alcohol (PNIPAM-Py/200)9or in dioxane (PNIPAM-ClgPy/200andPNIPAM-ClsPy/400).* Their (3) Winnik, F. M. Macromolecules 1990, 23, 233.
0743-7463/91/2407-0912$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 5, 1991 913
Surfactant and Modified PNZPAh4 Interactions
T
7'
T
Me2CH-NH-CO - C H
Me2CH-NH-CO - C H I CH2
Me2CH-NH-CO --CH
CHI
I
1
crm
I
W O C~~HJTNH-CO --CH
iH2
CH2
4
-1
PNIPAM-Py1200
m = 200:
PNIPAM-ClePy/200
m = 400:
PNIPAM-ClsPy1400
Py = 1-pyrenyl
Figure 1. Structure of the polymers used in this study. Table I. Physical Properties of the Polymers [SI,"
compositiona mL PYl, polymer NIPAMC,:Py g1 Mvd molgl PNIPAM-Cl@y/200 206 (18l)b:l:l 41.3 390 000 4.4 X l0-b PNIPAM-Cl@y/400 435 (365)b:1:1 40.7 380 000 2.3 X 1W6 PNIPAM-Py/POO 200*:-:1 103 1.1X 1@ 4.2 X PNIPAM-C18/200 2401:39.7 370000 G- By 1H NMR. * From UV measurements. Polymer solutions in THF.d From [ q ] = (9.59X 10-*)Mvo.m, see ref 2. physical properties are summarizedin Table I. From the numberaveraged molecular weight of the polymers, one can estimate that there are on average 16 and 8 hydrophobic groups per macromoleculein copolymers with NIPAMpyrene molar ratios of 200:l and 400:1, respectively. Fluorescence Measurements. Fluorescence spectra were recorded on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. The temperature of 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. Solutions were not degassed. All measurements were carried out at 20 "C. Excitation spectra were measured in the ratio mode. Emission spectra were not corrected. They were recorded with an excitation wavelength of 330 nm and slit widths (excitation and emission) of 1.8 nm. The excimer to monomer ratios were calculated by taking the ratio of the intensity at 480 nm to the half-sum of the intensities at 379 and 399 nm. Samples for Spectroscopic Analysis. PNIPAM-Py/200. An aqueous solution of PNIPAM-Py/2OO was prepared by allowing the polymer to dissolve in water for 24 h before it was diluted to a known volume. Aliquots of this solution (10 mL) were added to sufficient quantity of SDS, HTAC, or OTG to yield a 2 x 10-2 M surfactant concentration when diluted to 25 mL. Solutions of lower surfactant concentration were obtained by diluting these solutions with an aqueous solution of PNIPAMPy/200 of identical polymer concentration. Solutions were prepared 2 h before spectroscopic analysis. PNIPAM-Cl$y/200 and PNIPAM-Cl$y/400. Aqueous solutions of the polymers (0.05 g L-l) were prepared from stock solutions (1g L-1) kept at 5 "C in the dark. Increasing amounts of surfactants solutions were added to these solutions. The concentrations of the surfactant solutions were such that the polymer concentrations were kept constant within 10% throughout the experiments. As a control, experiments with SDS were conducted also with samples prepared by the method described for PNIPAM-Py/200. Identical results were obtained in both sets of experiments. The compositions of the solutions studied are listed in Table 11.
Results FluorescenceTechnique. In water PNIPAM-Cl&'y/ 200 and PNIPAM-Clay/400 exhibit an emission due to locally excited pyrene chromophores (intensity ZM, pyrene
Table 11. Composition of the Solutions Studied polymer PNIPAM-C&/200 PNIPAM-C&/400 PNIPAM-Py/SOO
[polymer], g L-1 0.05
[pyrenel, mol L-1
0.05
2.2 X lo4 1.1 X 104
0.04
1.7 X 1O-s
[alkyl grou~h mol L-1 2.2 X 1o-B 1.1 X 1o-B
Ymonomernemission) with the (0,O)band located at 376 nm, together with a broad featureless emission centered at 480 nm. This emission (intensity IE) originates from pyrene excimers. Identical excitation spectra were obtained for emissions monitored at 376 and 480 nm, and their maxima correspond to the UV absorption spectra. Therefore both the monomer and excimer emissions originate from excited isolated pyrene chromophoresf The dynamic nature of the excimer is confirmed by the timedependent fluorescence profile of the excimer emission. It shows both a growing-in component ( 7 = 20 ns) and a decaying component (( 7 ) = 110 ns). To allow two chromophores to interact during the lifetime of the excited pyrenes (ca. 110 ns) the polymers must adopt a conformation that keeps the pyrene groups in close proximity. This fact and other spectroscopic evidence described elsewhere2point to the existence in water of unimolecular polymeric micelles. They consist of a hydrophobic domain formed by the hydrocarbon chains and the pyrene groups, surrounded by the more polar PNIPAM chain, which lends to the polymer its solubility in water through formation of hydrogen bonds between water molecules and amide groups. Aqueous solutions of PNIPAM-PyI200 also exhibit both pyrene monomer and pyrene excimer emissions. The contribution of the excimer emission relative to the monomer emission is lower in this case, compared to the amphiphilic copolymer with identical labeling level (PNIPAM-ClePy/200). The overall fluorescence intensity from PNIPAM-Py/200 is lower, indicating a larger extent of pyrene self-quenching. These two observations are indicative of the occurrence of pyrene aggregates in aqueous solutions of PNIPAM-Py/ZOO. Such ground-state pyrene dimers or higher aggregates exist only under very special conditions. There occurrence is ascertained by the followingspectroscopic evidence.3 First, the excitation spectra for the monomer and excimer emissions are clearly different. The general features of the spectra are similar, but the former is blue-shifted by about 3 nm. Comparison with the UV spectrum reveals that it is the excitation sDectrum of the excimer that corresponds to the UV (4) Birke, J. B. Photophysics of Aromatic Molecules; Wiley-Intersciences: London, UK,1970; Chapter 7.
Winnik et al.
914 Langmuir, Vol. 7, No. 5, 1991 0.40 PNIPAM-Py / Z O O
-4.5
-4.0
-3.0
-2.0
LOG [surfactant] mol L-'
Figure 2. Plot of IE/IM for the emission of pyrene in solutions of PNIPAM-Py/POO (0.04 g L-1) as a function of surfactant concentration (logarithmic scale) for sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium chloride (HTAC), and n-octyl &D-thioglucopyranoside (OTG). Respective values of the surfactants critical micelle concentrations are indicated by the vertical arrows: temperature, 20 O C ; Lc= 330 nm.
maxima. Second,the time-dependent fluorescence profile of the excimer does not show a growing-in component, at least in the nanosecond time scale. Pyrene dimers or higher aggregateshave been detected in aqueous solutions of other pyrene-labeled polymer^.^ Their existence in aqueous polymeric solutions is attributed to a gain in free energy of mixing through hydrophobic interactions between the nonpolar pyrene groups.6 In the case of PNIPAM-Py/ 200 they form among pyrenes attached to the same chain or to different polymers. The micellar or aggregated structures do not exist in solutions of the labeled polymers in nonaqueous solvents, as evidenced for example by an extremely weak contribution of excimer emission to the fluorescence of the polymers in these solvents. Under those conditions the polymers adopt a more open conformation. The pyrene groups randomly attached to the polymer strands are kept too far apart to form excimers. The ratio of the excimer to monomer emission intensities (IE/ZM) is much smaller in methanolic solutions of the polymers compared to aqueous solutions, e.g. for PNIPAM-ClaPy/200, IE/ZM= 0.06 (MeOH),21.10 (HzO). T h d E / I M becomes asensithe tool to monitor the solution properties of the pyrenelabeled PNIPAM. I t was used in this study to follow the interactions of the polymers with surfactants. Interactions of Surfactantswith PNIPAM-Py/200. Addition of surfactants to a solution of PNIPAM-Py/ZOO (0.04 g L-l) in water caused a decrease in the intensity ZE of the excimer band, as well as a small shift of the excimer maximum from 480 to 476 nm. These changes were accompanied by a corresponding increase in the monomer intensity ZM as well as an increase in the total pyrene fluorescence intensity. They indicate that the pyrenepyrene dimers or higher aggregates are destroyed by the surfactant molecules. Curves of the ratio I E / I M for PNIPAM-Py/BOOas a function of surfactant concentration are presented in Figure 2 for SDS, HTAC, and OTG. They are sigmoidal in shape: the addition of surfactant is only sensed by the polymer above a surfactant concentration characteristic for each surfactant (Table 111, onset concentration). This concentration is well below the critical (5) Winnik, F. M.; Winnik, M. A.;Tnzuke, S.;Ober, C. Macromolecules 1987,20,38. Oyama, H. T.; Tang, W. T.; Frank, C. W. Macromolecules 1987. 20. 474. - . -(6) Yamazaki, I.; Winnik, F. M.; Winnik, M. A.; Tazuke, S. J.Phys. Chem. 1987,91, 4213.
----.--.
micelle concentration (cmc) of the surfactants. I t corresponds to the starting point of the growth of mixed clusters along the polymer chain. Critical aggregation concentrations (cac) were determined from the inflection points in the curves of I E / I M as a function of surfactant concentration for the PNIPAM-Py/200-surfactant aggregates in the three systems (Table 111). Two features of the data summarized in Table I11 are noteworthy. (1) The cac values are lower, relative to the cmc of the surfactant, for the longer chain ionic surfactants SDS and HTAC than for the neutral surfactant OTG. (2) The cac of the SDS and PNIPAM-Py/200 system is identical, within experimental error, to that reported by Schild and Tirrell for the system of SDS and PNIPAM (0.4 g L-1).7 This point emphasizes the fact that the binding of surfactants to the pyrene-labeled polymer is dominated by the interactions between the polymer main chain and the surfactants. The attached hydrophobic chromophores affect the interactions to some extent, but overall they act as rather passive reporters. This is a very useful situation encountered also in the case of the labeled amphiphilic copolymers, as described in the next section. Interactions of Surfactants with PNIPAM-C#y/ 200 and PNIPAM-C@y/400. The effect of the addition of SDS to an aqueous solution of PNIPAM-C18Py/200 on the fluorescence of the pyrene label is illustrated in Figure 3, where we show the fluorescence spectra of the polymer in water (top) and in the presence of SDS (3.2 X mol L-1, bottom). The pyrene monomer intensity is significantly higher in the presence of SDS. The contribution of the pyrene excimer is reduced to a small but still detectable emission. Phenomenologically, these SDSinduced changes in the fluorescence of PNIPAM-Clay/ 200 are identical with those observed in the case of PNIPAM-Py/ 200. However a major difference between the two polymers is uncovered when one compares the SDS concentration dependence of the changes in IE/IM in the two systems (Figures 2 and 4). In the case of the PNIPAM-CuPy/200 there is a uniformdecrease in I E / I M with increasing SDS concentration. The curve does not have a sigmoidal shape. It has the characteristics of a simple binding isotherm. There is no evidence for a cac, as was the case for PNIPAM-Py/200. The SDS-induced decrease in excimer emission was detected for SDS concentrations as low as 10-5 mol L-1, a value corresponding to ca. 10 surfactant molecules per octadecyl chain. The ratio IE/ZM decreased uniformly from its value in water (1.10) to a limiting value (0.05) for [SDS] higher than ca. 3x mol L-1, a concentration significantly smaller than the cmc of SDS (8.2 X lov3mol L-l)* but larger than the cac of SDS and PNIPAM or PNIPAM-Py/200. We carried out next a series of experiments with other surfactants and with the second amphiphilic labeled copolymer, PNIPAM-ClaPy/400, in which the number of octadecyl groups per chain is reduced by half, compared to PNIPAM-Cl$y/200. There were no unexpected effects in the interactions of HTAC with either copolymers. As in the case of SDS, addition of this cationic surfactant to aqueous solutions of either PNIPAM-C18Py/200 or PNIPAM-ClgJ?y/400 resulted in a gradual increase of the pyrene monomer emission at the expense of the pyrene excimer emission. The effect was noticed first at extremely low HTAC concentrations (ca. 10-5mol L-l, see Table 111). Saturation took place at [HTAC] slightly below the cmc of the surfactant. By comparing the results obtained with (7) Schild, H. G.; Tirrell, D. A. Polym. Prepr. (Am.Chem. SOC.,Diu. Polym. Chem.) 1989, 30 (2), 350. (8) Berr, S. S. J. Phys. Chem. 1987, 91, 4760.
Langmuir, Vol. 7, No. 5, 1991 915
Surfactant and Modified PNIPAM Interactions
Table 111. Important Surfactant Concentrations in their Interactions with Pyrene-Labeled PNIPAM Samplerr
(a) PNIPAM-Py/200 (0.04 g L-I) cmc,b mol L-1 Cowt, mol L-' 3.2 X lo-' 8.2 x 10-3 2.0 x lo-' 1.4 x 10-3 2.5 x 10-3 9.0 x 10-3
surfactanta SDS HTAC OTG ~~~
C-t,
(b) PNIPAM-Clay/m (0.05 g L-l)c cum&n, mol L-l m=400 m=200 m=400 -1 x loa 2 x 10-3 2 x 10-3
mol L-'
cac, mol L-l 0.8 x 10-3 0.5 x 10-3 7.9 x 10-3 cac, mol L-1 m=200 m=400
surfactant mc = 200 SDS -1 x loa HTAC -1 x 1o-a -1 x 1o-a 1 x 10-3 8X1o-' OTG 1.2 x 10-3 3.2 x 10-3 4.9 x 10-3 5.6 X 10-9 a SDS, sodium dodecyl sulfate;HTAC, hexadecyltrimethylammoniumchloride; OTG, n-octyl8-Dthioglucopyranoside.* From ref 8 for SDS and HTAC, from ref 9 for OTG. m = 200, PNIPAM-Clay/200; m = 400, PNIPAM-C&y/400.
1
I
1
I
1
1
PNIPAM- Cle- Pyf200
I
Water
1.0
0 . 5 10' ~
> k
v)
z W
c
z
w
0 Z
0
W
I
v, W
E v
2
I! 1
0
1.5~10'
LL
I
I
[SDS] = 3.2 x
1
I
1
LOG [SDS] mol L-'
mol L"
Figure 4. Plot of IE/IM for the emission of pyrene in a solution of PNIPAM-Cl$y/200 (0.05 g L-l) as a function of sodium dodecy1 sulfate (SDS)concentration (logarithmic scale). The arrow indicates the critical micelle concentration of SDS: temperature, 20 "C; Lc= 330 nm.
t
1.0
0.8
0 IE -
WAVELENGTH (nm)
Figure 3. Fluorescence spectra of PNIPAM-Clay 200 (0.05 g L-1) in water (top) and in the presence of sodium dO Jecyl sulfate (SDS,3.2 X mol L-I, bottom); temperature, 20 "C; )bXc= 330 nm.
two polymers of different level of octadecyl chain incorporation, one notices that there is only a weak influence of the degree of substitution of the polymer on its interactions with either SDS or HTAC. Addition of the neutral shorter chain surfactant OTG to solutions of the labeled amphiphilic copolymers was performed next. This surfactant had to be added in much larger amount than either SDS or HTAC in order to affect the fluorescence of the pyrene label. No changes in ZE/ZM occurred until a critical OTG concentration was attained. Then the ratio decreased sharply, as shown in Figure 5 in the case of PNIPAM-Clay/ 200. Critical aggregation concentration values were determined from the inflection points of the curves (Table 111). For both polymers these values are lower than the cmc of OTG (9 X mol L-1).9 They are also slightly lower than the cac measured in the OTG PNIPAM-PyI2OO system (see Table 111). (9) Teuchiya, T.; Saito, S.J. Biochem. 1984,%,1593.
IM
0.6 0.4 0.2
- 4.0
-3.0
- 2.0
LOG [OTG] mol L-'
Figure 5. Plot of IE/IM for the emission of pyrene in a solution of PNIPAM-Clsy 200 (0.05 g L-l) as a function of n-octyl f3-p thioglucopyranosi e (OTG) concentration (logarithmic scale). The arrows indicate the critical micelle concentration (cmc) of OTG and the critical aggregation concentration (cac): temperature, 20 "C; )bxe= 330 nm.
d
Discussion Our results point toward remarkable similarities in the interactions of surfactanbwith, on the one hand, PNIPAM and PNIPAM-Py/200 and, on the other hand, PNIPAMCl8/200 and PNIPAM-C1&'y/200 (Figure 6). This observation merits further exploration. PNIPAM and PNIPAM-Py/200 interact with surfactants in patterns typical of neutral water-soluble poly-
Winnik et al.
916 Langmuir, Vol. 7, No. 5, 1991 Mixed
Polymeric Micelle
-
I
Cluster /
Surfactant Addition
6.'
PNIPAM-C18/200
-
Surfactant4 Micelle
Kc
$'
KEY
0
NlPAM Unit
Octadecyl Chain 0 Pyrene Surfactant
J
Figure 6. Stylized illustration of the interactions between surfactants and PNIPAM-Py/200, PNIPAM-Clay/200, and PNIPAM-Cla/200. The structures represented schematically include the (intramolecular)polymeric micellesand the polymers aggregates existing in water in the absence of the surfactantsand the polymer/surfactant mixed clusters formed between surfactants and each polymer.
mers,lo They form mixed micelles with ionic surfactants above a critical surfactant concentration which depends strongly on structural parameters, such as the charge of the surfactant, the size of its head group, and the length of its hydrophobic tail.11J2 The binding occurs by a cooperative mechanism: further addition of surfactant results in a large increase in the amount of bound surfactant while the concentration of free surfactant remains almost constant. Surfactant clusters form along the polymer chain. In the case of PNIPAM and SDS these clusters are less polar than the corresponding SDS micelles,'which may indicate less water penetration into the mixed micelles than in the SDS micelles themselves. Neutral surfactants such as OTG interact only weakly, if at all, with PNIPAM. The sharp decrease in pyrene excimer emission from PNIPAM-Py/200 in the presence of OTG at a concentration slightly lower than its cmc is evidence of binding of the surfactant to the polymer. In this aspect, PNIPAM-Py/200 exhibits a behavior similar to that observed with pyrene-labeled hydroxypropyl c e l l ~ l o s e .This ~ ~ polymer also interacts with ionic surfactants at concentrations well below their cmc,14but with neutral surfactants, such as OTG, it undergoes binding only at concentrations close to their cmc. (10) For reviews, see for example: Goddard, E. D. colloid8 Surf.1986. 19,265 and 301. (11) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987,3,382. (12) Nagarajan, R. J. Chem. Phya. 1989,90,1980. (13) Winnik, F. M. Langmuir 1990,6,522. (14) Winnik. F. M.: Winnik. M. A.: Tazuke. S. J. Phvs. Chem. 1987. 91, 594.
Turning now to systems comprised of amphiphilic PNIPAM derivatives and surfactants, analogies can also be drawn in the behavior toward surfactants of the unlabeled and labeled polymers. The probe studies with unlabeled polymers (e.g. PNIPAM-Cu/ 200) uncovered major differences in their behavior vis-a-vis surfactants, compared to PNIPAM. The ionic surfactants HTAC and SDS with a relatively long tail bind to the amphiphilic copolymersby simple partition between the aqueous phase and the polymer and not by cooperative association. Binding occurs even at extremely low surfactant concentration (ca. 1x 10-6 mol L-I). At saturation the polymer/ surfactant aggregates consist of clusters into which one (or several) octadecyl chain is sequestered by the surfactant molecules. The neutral surfactant OTG, with a relatively shorter tail, binds to amphiphilic PNIPAM derivatives by a cooperative mechanism at a critical surfactant concentration lower than the surfactant cmc. Mixed polymer/surfactant clusters are formed. They are much more fluid than those formed with the same polymers and SDS or HTAC. The results of the experiments with the labeled amphiphilic PNIPAM reported here strongly support the conclusions drawn from the probe experiments. Evidence for a noncooperative binding of SDS and HTAC to the M polymers is given by the uniform decrease in I E / ~ with increasing surfactant concentration (see Figure 4 for the case of SDS addition). The gradual decrease in excimer emission with increasing surfactant concentration implies that the surfactant molecules destroy the unimolecular polymeric micelles into which several pyrene chromophores were kept in close proximity allowing excimer formation. The disruption caused by the surfactant can be detected at very low surfactant concentrations. At saturation the polymer/surfactant aggregates consist of surfactant clusters surrounding the hydrophobic substituents of the polymer. The pyrene groups are kept apart from each other. Excimer emission is largely prevented. It is noteworthy that a t these high surfactant concentrations it is still possible to detect a pyrene excimer emission, albeit of very low intensity. This implies that a small number of mixed polymer/surfactant clusters must include more than one pyrene group and, hence, more than one octadecyl chain. The fluorescence label experiments conducted with the neutral surfactant OTG also confirm the conclusions of the probe experiments. Here evidence for a cooperative binding of OTG to PNIPAM-Cl$y/200 stems from the sharp decrease in I E / I Mfor a critical OTG concentration (Figure 5 ) . Interestingly, the label experiments point to a similarity in the behavior toward OTG of the amphiphilic PNIPAM derivatives and PNIPAM-Py/200, in contrast to the situation encountered with ionic surfactanta. A cooperative binding operates with both PNIPAM-Cl$y/ 200 and PNIPAM-Py/200. Moreover the spectroscopic changes associated with both the onset of interaction and the critical aggregation point occur in the same surfactant concentration ranges for the two types of polymers: cowt 1X to 3 X 10-3mol L-l and cac 4.9 X lO-9to 8 X 1O-S mol L-l; see Table 111. This apparently passive role of the octadecyl chains toward OTG was beyond detection with probe experiments.
Conclusion Reported here is a study based on fluorescence label experiments of the association of surfactants with hydrophobically modified derivatives of poly-(N-isopropylacrylamide). Ionic surfactants, such as SDS and HTAC,
Surfactant and Modified PNIPAM Interactions
bind to the polymers by a noncooperative mechanism to form mixed micelles that incorporate the hydrophobic groups of the polymers. Mixed micelles are formed also between the polymers and shorter chain neutral surfactants, but by a cooperative process. A comparison of the behaviors of amphiphilic labeled copolymers and corresponding unlabeled copolymers illustrates the generality of the effect of hydrophobic alkyl chains in controlling the interactions of associating polymers. This aspect of the results provides support to a computer modeling of such phenomena recently put forward by Balazs and Hu.I6 It also highlights the relatively passive role played by the fluorescent label under the specific conditions of the (15) Balazs, A. C.; Hu,J.
Y.Langmuir 1989,5, 1230 and 1253.
Langmuir, Vol. 7, No.5, 1991 917 experiments presented in this study. The striking differences in the effects observed with the neutral surfactant and with the ionic surfactants merit further attention. In this instance it will be very useful to separate the effects related to changes in the surfactant head group to those due to changes in the length of its hydrophobic tail.
Acknowledgment. We thank Professor M. A. Winnik (University of Toronto) for many stimulating discussions as this work progressed and during the preparation of this manuscript. Financial support was provided in part by the Bundesministerfiir Forschungund 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.
