594
J. Phys. Ckem. 1987, 91, 594-597
Interaction of Hydroxypropylcellulose with Aqueous Surfactants: Fluorescence Probe Studles and a Look at Pyrene-Labeled Polymer Franqoise M. Winnik,*’ Mitchell A. Winnik,* and Shigeo Tazuke Tokyo Institute of Technology, Laboratory of Resources Utilization, 4259 Nagatsuta, Midori- ku. Yokohama 227, Japan (Received: April 7 , 1986; In Final Form: July 28, 1986) The interaction of hydroxypropylcellulose (HPC) with anionic and cationic surfactants was studied by fluorescence probe and fluorescence label experiments. The changes of the 11/13ratio of the fluorescence of pyrene (probe) in aqueous solutions of HPC were measured as a function of sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (HTAC) concentrations. Mixed micelle formation with the polymer was observed, with an apparent critical micelle concentration and 3 X M, respectively. These are the first observations of mixed micelle formation between a cationic of 5.6 X surfactant and a neutral polymer. The fluorescence spectroscopy of HPC labeled with pyrene was examined in aqueous solution in the presence of HTAC and SDS. Large changes in the pyrene “excimer”fluorescence intensity and pyrene “monomern fluorescence intensity, monitored as a function of surfactant concentrations, indicate that SDS and HTAC interact with HPC to break up polymer/polymer aggregates and to change the local conformation of the polymer.
Introduction Hydroxypropylcellulose (HPC) is an interesting polymer with many commercial applications. It is soluble in a wide variety of solvents, including water from which it precipitates when heated above 42 O C 3 At high concentrations in several solvents, it forms liquid crystalline phasesS4 It is the liquid crystalline behavior which has been the focus of most of the recent research on H P C in university laboratories. There have been some attempts to understand the origin of the solubility of HPC in water. Studies directed toward determining the solubility parameter of H P C indicate a value of ca. 11, which is not consistent with solubility arising simply from a match of solvent and solute cohesive energy density. It appears that, in nonpolar solvents, HPC experiences extensive intramolecular hydrogen bonding, whereas in water, hydrogen bonding with solvent predominates.5 Nevertheless, HPC is a largely hydrophobic polymer. At temperatures above 0 OC in water, it has a tendency to form aggregates: and these become sufficiently extensive at elevated temperatures to lead to phase separation above the lower critical solution temperature. In this paper, we examine the interaction of dilute aqueous solutions of H P C with ionic surfactants. There is extensive literature on the interaction of surfactant and polymers in aqueous s o l ~ t i o n . Classical ~ studies of bulk solution properties (viscosity, surface tension, equilibrium dialysis) indicate that ionic surfactants commonly bind to nonionic polymers at surfactant concentrations below their normal critical micelle concentrations ( c ~ c ) . ~Newer ,~ techniques, particularly NMR,8 neutron scattering,8b and fluorescence quenchinggJOdemonstrate the formation of mixed micelles-micelles which bind to the polymer or, otherwise stated, micelles which incorporate segments of the polymer into their structure. In the case of sodium dodecyl sulfate (SDS) binding (1) Permanent address: Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1. (2) Permanent address: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1Al. (3). “Klucel”, a booklet distributed by the manufacturer, Hercules, Inc., Wilmington, DE 19894. (4) Gray, D. G. J . Appl. Polym. Sei.: Appl. Polym. Symp. 1983, No. 37, 179. (5) Roberts, G. A. F.; Thomas, I. M. Polymer 1978, 19, 459. (6) Neely, W. B. J . Am. Chem. SOC.1960.82, 4354. (7) (a) Nagarajan, R. Polym. Prepr. (Am. Chem. SOC.Diu. Polym. Chem.) 1981, 22, 33. (b) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (c) Gilanyi, T.; Wolfram, E. Colloids Surf. 1981, 3, 181. (d) Jones, M. N . J. Colloid Interface Sci. 1967, 23, 36. (e) Turro, N. J.; Arora, K. S . Polymer 1986, 27, 783. (8) (a) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (b) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19. (9) (a) Zana, R.; Lang, J.; Lianos, P. Polym. Prepr. (Am. Chem. Soc. Diu. Polym. Chem.) 1982, 23, 39. (b) Zana, R.; Lianos, P.; Lang, J. J. J . Phys. Chem. 1985, 89, 41. (10) (a) Turro, N. J.; Baretz, B. H.; Kuo, P.-L. Macromolecules 1984, 17, 1321. (b) Ananthapadmanabhan, K. P.; Gcddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, I , 352.
0022-3654/87/2091-0594$01.50/0
to poly(ethy1ene oxide) (PEO), one of the most extensively studied systems, the mixed micelles have smaller aggregation numbers and a more open structure than those of the surfactant micelle itself.9b Many factors affect the interaction of surfactants with nonionic polymers. Cationic detergents tend to experience weaker interactions than anionic detergent^.^ Here the size of the head group appears to be very important.” For example, no interaction has been observed between hexadecyltrimethylammonium chloride (HTAC) and PEO or poly(vinylpyrrolidone), both of which form mixed micelles with SDS.7s9J0Polymer hydrophobicity is also important. Dextran does not form mixed micelles with SDS.7a Methylcellulose interacts with SDS below the normal SDS cmc. Dye binding studies, however, suggest that hydroxyethylcellulose does not interact with SDS.Izb In these experiments we examine the interaction of HPC with SDS and with HTAC from two different points of view. First, we carry out fluorescence probe experiments with pyrene at very M) in the presence of surfactant and low concentrations (ca. polymer. We examine the influence of environment on the vibrational fine structure of the pyrene f l u ~ r e s c e n c e ~and ~ ’ ~use ~’~ this information to determine the apparent cmc (“cmc”) of the polymer-surfactant mixed micelles. In a separate series of experiments, we attempt to look at the surfactant binding process from the polymer’s perspective by examining HPC covalently labeled with a small amount (3 monomer mol %) of pyrene groups. These give excimer emission in aqueous solution. Interaction of the surfactant with the polymer simultaneously has a profound effect on the extent of excimer emission. In order to prevent loss of fluorescent label through hydrolysis, the pyrene groups were attached to the HPC via an ether linkage. The following is an idealized structure of the labeled hydroxypropylcellulose. OH
OH
I
I
OCHzCHCH3 I
OCHsCHCHi
H
&H
OCH~CHCH~
I
I
CH?
I
OH
(1 1) Kalpakci, B.; Nagarajan, R., cited in ref 7a. (12) (a) Lewis, K. E.; Robinson, C. P. J. Colloid Inferface Sci. 1970, 32, 539. (b) Goddard, E. D.; Hannan, R. B. In Micellization, Solubilizarion and Microemulsion; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2. (13) (a) Nakajima, A. J. Lumin. 1977, 15, 277. (b) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1985, 62, 2560. (c) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC.1977, 99, 2039.
0 1987 American Chemical Society
interaction of Hydroxypropylcellulose with Surfactants
Experimental Section Materials. SDS (99%) and HTAC (95% minimum) were obtained from Tokyo Kasei Kogyo Co. They were used without further purification. The distilled water used had a conductance less than 5 X mho/cm. Pyrene was chromatographed over silica gel and sublimed under vacuum. Hydroxypropylcellulose (HPC, Klucel L. Hercules Inc.) was purchased from Aldrich Chemical Co. The manufacturer’s literature claims a molecular weight of 100000. Other groups have reported measurements consistent with this value. One recent reference14areports M(sedimentation) = 82 OOO, and another,’” 73 OOO with M , = 36 000 for Klucel L. Pyrene-Labeled Hydroxypropylcellulose (HPC). The description of the synthesis of pyrene-labeled H P C is reported in detail elsewhere.lS In brief, the preparation involved reaction of 4-( 1-pyreny1)butyl tosylate with H P C (carefully dried) in N,N-dimethylformamide in the presence of sodium hydride. The polymer was purified by repeated precipitations from tetrahydrofuran (THF) solution by addition of hexanes. The polymer in THF was analyzed by gel permeation chromatography using UV-visible and refractive index detectors in tandem. These showed that the pyrene groups were attached to the polymer, that the purified polymer contained less than 0.1% low molecular weight pyrene impurities, and that the chemical transformations did not affect the (broad) molecular weight distribution of the polymer. By UV spectroscopy the polymer contained 1.23 X lo4 mol/g pyrene groups. Samples for Spectroscopic Analysis. H P C or HPC-Py solutions were prepared at room temperature (19 “C) by allowing the polymer (12 mg) to dissolve slowly (24 h) in 50 mL of water. This solution was then diluted to 100.0 mL. Aliquots (10 mL) of this solution were added to a sufficient quantity of SDS or HTAC to yield a 2 X M surfactant concentration when diluted to 25.0 mL. Solutions of lower surfactant concentration were obtained by diluting these solutions with an aqueous HPC-Py solution of identical polymer concentration. A solution of H P C (24 mg) was prepared in pyrene-saturated water (100.0 mL), previously filtered to remove pyrene microM. Solutions crystals. The pyrene concentration was ca. 3 X for study were prepared by adding sufficiently small aliquots of aqueous surfactant that the HPC pyrene solutions were diluted by no more than 10%. Fluorescence Measurements. Samples for fluorescence measurements were not degassed. Control experiments on samples deaerated by passing argon through the solutions indicated negligible differences in fluorescence intensities. Spectra were run on a Hitachi Model 4000 fluorescence spectrometer and are fully corrected. Because the excitation spectra of the monomer and excimer emissions were different at low surfactant concentrations, choice of the excitation wavelength was an important consideration. We chose 330 nm, which was 2 nm to lower wavelength of a maximum in the excimer excitation spectrum and 2 nm to longer wavelength from the corresponding peak in the monomer spectrum. Excitation slits were set at 3 nm. Spectra were run at 20 OC. ZE and ZM values were measured both as peak heights and also as integrated areas. These were proportional within a few percent. Occasional samples gave monomer and excimer fluorescence intensities quite different from the values shown in the figures. The anomalies in these spectra disappeared if the solutions were allowed to stand several hours and were then transferred carefully to a fluorescence cell. Similar unusual behavior has been described for aqueous methylcellulose at room temperature.6 Colloidal aggregates, invisible to the eye, were reported to form and to settle out on standing.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 595 HPC + Py
+ HTAC
\
i
* t
1
cmc
- 2.0
U o
”25.0
-4.0
-3.0
log [HTAC]
2.0
b.
I
I
I
HPC + Py + SDS
1’5LLL 1. -0 5.0
- 4.0
- 3.0 ‘A-
cmc
A-A-.
-2.0
log [SDS]
Figure 1. Z , / Z , intensity ratio for pyrene (ca. 3 X lo-’ M) fluorescence in aqueous solutions of HPC (0.02% by weight) as a function of surfactant concentration: upper curve, HTAC; lower curve, SDS.
Results Unlabeled HPC. When SDS or HTAC is added to a solution of HPC (0.02% by weight) plus pyrene (ca. 3 X lo-’ M) in water,
the fluorescence spectrum of the pyrene changes where the “cmc” of the system is exceeded. The spectrum shows a decrease of the (0,O)band at 373 nm ( I , ) relative to that of the (0, 2) band at 385 nm (Z3). It is well documented that changes in Zl/Z3 reflect micelle formation, both in the absence and in the presence of In Figure 1 we plot Zl/Z3 as a function of surfactant concentration. Both surfactants interact strongly with HPC. Interaction of the probe with SDS in the presence of polymer is first observed at ca. 1 X M. If we choose the inflection point in Figure l b to assign the apparent cmc of the SDS-HPC M. This is about mixed micelle, we find a value of 5.6 X 15 times smaller than the cmc of SDS itself (indicated by an arrow in Figure 1). Saturation is achieved at ca. 2 X M. For HTAC, the interaction with H P C seems more sharply defined, with an onset at ca. 3 X lo4 M. The cmc of HTAC itself occurs at 1.3 X M. This is the minimum point on the Z,/Z3 vs. log (HTAC) plot. Above this concentration, the pyrene probe partitions between the polymer-bound mixed micelles and free HTAC micelles. Even in the absence of surfactant, the Z1/Z3 ratio for pyrene + HPC (1.78) is quite different from that in water (1.87).’3b This result indicates that the pyrene in these experiments is bound to the polymer. Pyrene-Labeled HPC. Fluorescence spectra of HPC-Py are shown in Figure 2 for solutions in water and in the presence of SDS. In pure water, the emission is characterized by a large, broad excimer (of intensity Z,) centered at 489 nm and a small, locally excited (“monomer”, intensity ZM) pyrene emission with a (0,O)band at 379 nm. Excitation spectra for the monomer and the excimer emissions are similar in shape with the former blue-shifted by about 4 nm. The latter corresponds to the UV absorption spectrum of the sample. This observation indicates that the excimer originates from pairs or aggregates of pyrene groups which exist prior to excitation. This situation is different from the more common occurrence in organic solvents where excimers are formed by diffusion together of excited and unexcited pyrene groups.17 In pure water, the monomer emission originates
(14) (a) Werbowyj, R. S.; Gray, D.G. Macromolecules 1984, 17, 1512. (b) Nystrom, B.; Bergman, R. Eur. Polym. J . 1978, 14, 431. (15) Winnik, F. M.; Winnik, M. A.; Tazuke, S.; Ober, C. K., submitted for publication.
(16) Lianos, P.; Zana, R. J . Colloid Interface Sci. 1981, 84, 100; J . Phys. Chem. 1980,84, 3339. (17) Birks, J. B. In Photophysics of Aromatic Molecules; Wiley: New York, 1971.
+
596
Winnik et al.
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 HPC - Py + SDS
/
,-\,
EOS] x
-5.0
450
550
500
600
-3.0
-2.0
Figure 4. Plot of I , and ZM for HPC-Py in the presence of HTAC.
Figure 2. Fluorescence spectra of HPC-Py (0.01%in water) for a series of SDS concentrations (0.4, 1.9, 3.9, 5.8, 9.7, 19.4 X lo4 M). The spectra are arbitrarily normalized at the (0, 0) band. I
-4.0
log [HTAC]
WAVELENGTH (nm)
I
I
'\\0.4
" 400
I
I
HTAC + HPC - Py
lo4 M
I
I
I
I
I
HPC- Py + surfactants
HTAC 4.0
3.0
-s - 2.0
\
W
SDS + HPC- Py 1.0 I
-4.0
I
-3.0
I
-2.0
log [SDS] - 1
,
I
- 4.0
I
- 3.0
I
-2.0
log [surfactant]
Figure 3. ZE/ZM for aqueous HPC-Py (0.01%) in the presence of HTAC (upper curve) and SDS (lower curve) as a function of polymer concentration. The units of I , and ZM presented are proportional to the integrated areas of the fluorescence spectra after transformation to wave-
number units. from pyrene groups isolated along the chain backbone. There is only a weak dependence of Z E / I M upon polymer concentration in this concentration range. Detailed solution studies to the limits of detection ([Py] = 7 X lo-* M, [polymer] = 0.2 ppm) show that IE/ZM decreases to a value of 1.4.15 This suggests that polymer aggregation persists to very low polymer concentration. Addition of surfactant causes a large decrease in the intensity of the excimer band, as well as a shift in the excimer maximum to 485 nm. There is also a corresponding increase in ZM. The spectra in Figure 1 are normalized at 379 nm to emphasize the changes in IB/IM,the shift in the excimer maximum, and the insensitivity of the shape of the locally excited pyrene emission to the presence of surfactant. At high surfactant concentrations, the excitation spectra of the monomer and excimer emissions become very similar. In Figure 3, we plot I E / I M for HPC-Py as a function of surfactant concentration for SDS and for HTAC. Both curves are sigmoidal in shape. Z E / I M decreases sharply for surfactant concentrations above 1 X M. It is noteworthy that the changes in Figure 3 occur over a similar span of concentrations to those shown in Figure 1. Since the monomer and excimer emissions in water and at low surfactant concentrations derive from different ground-state species, it makes sense to examine separately the behavior of the two emissions. In Figure 4 we plot I , and also I , as a function
Figure 5. Plot of I , and ZM for HPC-Py in the presence of SDS.
of the logarithm of the concentration of HTAC. The excimer emission remains constant from [HTAC] = 0 to about 3 X M. The inflection point occurs at 4.0 X lo4 M, and the interaction M. The behavior of the saturates at [HTAC] = 1.0 X monomer emission is more complicated. In the high surfactant concentration domain, but well below the normal HTAC cmc, the monomer sites become saturated in binding to micelles. This HTAC concentration corresponds to that found for excimer emission. The isolated pyrene sites on the polymer are affected by HTAC at much lower concentrations than the dimer sites. The behavior of HPC-Py in the presence of SDS is qualitatively similar (Figure 5): at low concentrations of SDS the excimer emission is unaffected. Interaction between SDS and the excimer sites becomes apparent abruptly at an SDS concentration of 3 X M. The sites giving pyrene monomer emission interact with SDS at much lower concentrations. Even at the lowest concentration studied, 5 X M, ZM depends upon [SDS]. At 2X M, ZM increases suddenly. Monomer fluorescence increases at the expense of excimer emission due to binding of SDS micelles to the pyrene dimer sites. Discussion The data in Figure 1 indicate a much stronger interaction of surfactant with hydroxypropylcellulose than with poly(ethy1ene oxide). The apparent cmc for the SDS-PEO mixed micelle has M.9 Here we find a value of 5.6 been reported to be 2 X X lo-" for the SDS-HPC mixed micelle. In addition, we observe formation of a mixed micelle of HPC and HTAC, whereas previous studies indicated no detectable interaction between HTAC and PEO in aqueous s o l ~ t i o n .Both ~ results imply the existence of binding sites in H P C which are more hydrophobic than those
Interaction of Hydroxypropylcellulose with Surfactants
~
in PEO. This is not a surprising result, given the fact that half the molecular weight of HPC is due to the presence of oxypropyloxy pendant groups. The magnitude of the effect, however, is noteworthy. The pyrene probe itself provides additional information on the effective polarity of the molecular environment surrounding the polymer. The Z1/Z3 ratio (here 1.78 vs. 1.87 in water) not only indicates that the pyrene molecules bind to the polymer but also provides a measure of the polarity of the immediate probe environment. The sensitivity arises in the following way:'3b The (0,O)band ( I , ) in the pyrene fluorescence spectrum is symmetry forbidden. It is normally quite weak compared to the (0, 2) band (Z3), which becomes allowed through vibrational coupling. Polar solvents provide local electric field anisotropy which breaks the symmetry, giving ZIgreater intensity. Polarity and polarizability effects make a much larger contribution to the oscillator strength than does hydrogen For molecular pyrene bond to HPC, the 1.78 value of Zl/Z3 indicates a local polarity similar to solvents such as acetonitrile and N,N-dimethylacetamide (1.79).13b An alkyl substituent at the 1-position of pyrene has a similar symmetry-breaking effect. The pyrene groups covalently bound to the H P C polymer no longer show vibrational fine structure sensitivity to solvent polarity. The labeled polymer provides another view of the polymersurfactant interaction. The monomer emission indicates interaction with surfactant at concentrations of SDS or HTAC below the cmc of the system. One of the features of the data which we would like to try to explain is how ZM can exhibit sensitivity to surfactant interactions in a concentration range where ZE is unaffected. The excimer emission derives from preformed pyrene dimers or large intramolecular pyrene aggregates. It is not unreasonable that these different kinds of sites interact differently with surfactant a t low surfactant concentration. While we have not yet had an opportunity to carry out extensive fluorescence decay measurements on this system, it appears that, in the absence of surfactant, the locally excited isolated pyrene lifetime (170 ns) is much longer than the excimer lifetime (50 ns). Thus, ZM is intrinsically more sensitive to the presence of adventitious impurities in the system. Interaction of these pyrene groups with surfactant could provide weak protection against quenching in the system for surfactant concentrations between and 2 X lo4 M. Our experiment is unable to indicate whether micelles actually form at these pyrene loci at these surfactant concentrations. One anticipates that the preformed excimer sites would be more hydrophobic than isolated sites containing one pyrene. Nevertheless, below 2 X lo4 M surfactant, ZE is not affected. We suggest that, at these concentrations, micelles incorporate pyrene dimers and that these continue to yield excimer fluorescence. As
The Journal of Physical Chemistry, Vol. 91, No. 3, 1987 597 the surfactant concentration is raised, these sites become saturated. Further binding is inhibited by electrostatic repulsion between micelles. Additional micelle binding to polymer could occur, however, if the surfactant either disrupts the polymer aggregates or prompts changes in the local conformation of the polymer. Either phenomenon would result in a separation of pyrenes which previously had been associated. Each pyrene could bind individually to a surfactant micelle. As a consequence, according to this model, polymer-micelle interaction in this range of surfactant concentrations would be coupled to a precipitous decrease in the intensity of excimer emission and a corresponding rise in monomer fluorescence. This is what is observed. Some excimer emission persists even at high surfactant concentration. This excimer emission has the same excitation spectrum as the monomer emission. We are not yet in a position to establish whether this excimer is formed dynamically via excitation of isolated pyrenes or whether it derives from a small amount of residual dimer sites. We hope to provide further information on this point in a future publication. Summary Both SDS and HTAC bind strongly to hydroxypropylcellulose. SDS binding to H P C occurs at nearly a factor of 5 lower surfactant concentration than its binding to poly(ethy1ene oxide) or poly(vinylpyrrolidone), whereas the interaction of HTAC with poly(ethy1ene oxide) is too weak to be detected. Experiments with H P C labeled with pyrene groups provide a look at the surfactant-polymer system from the polymer point of view. The phenomena one observes occur over the same surfactant concentration range as those found by more conventional measure, indicating that the covalently bound pyrene groups do not perturb significantly the apparent cmc of the mixed micelle. Large changes are seen in the amount of pyrene excimer emission, indicating that the surfactant changes the local chain conformations responsible for pyrene dimer formation. These experiments are sensitive only to the effects of those micelles bound to the polymer; they are unaffected by surfactant or surfactant micelles in the solution phase. Consequently, the use of labeled polymers provides a new measure of polymer-surfactant interactions.
Acknowledgment. We thank Dr. C. K. Ober (Xerox Research Centre of Canada) for GPC measurements and Mr. R. Iwasaki (Tokyo Institute of Technology) for fluorescence decay measurements. Franqoise M. Winnik thanks Xerox Corporation for permission for her extended stay in Tokyo. Mitchell A. Winnik thanks JSPS for a fellowship. Registry No. HPC, 9004-64-2; SDS, 151-21-3; HTAC, 112-02-7.
