Langmuir 1989,5, 1335-1340
1335
Fluorescence Studies of Polymer Adsorption. 3. Adsorption of Pyrene-End-LabeledPoly(ethylene glycol) on Colloidal Polystyrene Particles Kookheon Char, Curtis W. Frank,* and Alice P. Gast* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received March 23,1989. I n Final Form: May 30,1989 The adsorption of pyrene-end-labeled poly(ethy1ene glycol) (Py-PEG-Py)onto aqueous colloidal polystyrene (PS) particles is investigated by photostationary fluorescence, fluorescence decay, and photostationary fluorescence depolarization experiments. All of the spectroscopic observations on this system suggest the association of hydrophobic pyrene moieties with hydrophobic particles: (i) the complete disappearance of excimers accompanied by a slight wavelength shift to lower energy in the monomer emission spectrum, (ii) a significant red-shift of about 3.5 nm in the monomer excitation spectrum, (iii) an increase in the average lifetime of the fluorescence decay, and (iv) increased averaged anisotropy measured in fluorescence depolarization experiments of chromophores 1,6-diphenyl-l,3,5-hexatriene (DPH) and pyrene upon addition of PS particles. This association with the surface contrasts with previous observations of Py-PEG-Py adsorbing on hydrophilic silica particles (Char, K.; Gast, A. P.; Frank, C. W. Langmuir 1988, 4, 989). Excitation spectra of pyrene groups at various particle concentrations are much more sensitive to their state of binding than emission spectra. Qualitative displacement experiments and studies of the effect of molecular weight of Py-PEG-Py both suggest that the binding of hydrophobic pyrene to the PS surface is stronger than that of PEG segments to the surface.
Introduction The physical properties of colloidal dispersions can be controlled by the addition of polymers suitable for the specific application. Addition of nonadsorbing polymer can induce depletion flocculation due to an osmotic effect,’ while addition of polymer randomly adsorbing on the particles causes flocculation by bridging.2 Adsorption of copolymers onto particles imparts stability to colloidal dispersions by a process known as steric ~tabilization.~ The rheological properties of a suspension must often be carefully adjusted to yield desirable final performance. For paints, the viscosity a t high shear rates should be sufficient to provide brush drag and film buildup, while the low shear rate viscosity must be high enough to prevent sedimentation during storage and sagging after application but low enough to provide good flow and l e ~ e l i n g .Conventional ~ thickeners such as (hydroxyethy1)cellulose fail to provide these properties since they act to thicken paint by the entanglement of long chains. These systems suffer from very high viscosity at low shear rate and significant shear-thinning behavior: To overcome these problems, new thickeners, which are both watersoluble and associate with the colloidal particles, have recently been developed. The so-called associative thickener comprises a watersoluble chain with a small fraction of hydrophobic segments either branched uniformly along the backbone, in a comblike chain: or attached to the ends of the chaina6 The associative thickener increases the viscosity of the (1) (a) Cowell, C.; Li-In-On, R.; Vincent, B. J. Chem. SOC.,Faraday Trans. 1 1978,74,337. (b) Vincent, B.;Luckham, P. F.; Waite, F. A. J. Colloid Interface Sci. 1980,73,508. (2) La Mer, V. K.; Healy, T. W. Reu. Pure Appl. Chem. 1963,13, 112. (3) Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic: London, 1983. (4) Thibeault, J. C.;Sperry, P. R.; Schaller, E. J. Adu. Chem. Ser. 1986,213,375. (5) Landoll, L. M.J.Polym. Sci., Polym. Chem. 1982,20,443.
colloidal suspension by a mechanism wherein the polymer associates reversibly with hydrophobic surfaces such as latex particles. The desired properties are attained with a low concentration of a lower molecular weight polymer. The understanding of the effect of the associative thickener on the rheological properties or equilibrium phase behavior of a suspension remains qualitative. Recently, Santore et al.’ developed a theory for the equilibrium phase behavior of colloidal dispersions containing associative thickener using a pseudo-two-componentapproach; this provides a step toward understanding the flow behavior in such a system. Pyrene-end-tagged poly(ethy1ene glycol) (Py-PEGPy) is, in a way, very similar in structure to an associative thickener; i.e., hydrophobicchromophoresare attached to the ends of a water-soluble PEG backbone. In a fluorescence study of Py-PEG-Py dissolved in water,’ we have shown that the hydrophobic association between two ends produces a ground-state interaction causing the excimer to monomer intensity ratio (Ie/Im) to exceed that predicted by a diffusion-controlled mechanism. Excitation spectra and fluorescence decay measurements also point to the existence of a ground-state interaction between pyrenes. Finally, we have observed phase separation in low molecular weight (A&, 4800) Py-PEG-Py at high polymer concentration, indicative of extensive hydrophobic association. We have studied the adsorption of Py-PEG-Py on hydrophilic silica particles and the subsequent rearrangement and/or displacement due to the addition of untagged PEG.’ In this system, adsorption occurs mainly through hydrogen bonding between ether oxygens of the PEG backbone and silanol groups on the surface such that the hydro-
-
(6) (a) US.Patent 4,079,029,1978. (b) Valint, P. L., Jr.; Bock, J. Macromolecules 1988.21. 175. (7) Santore, M. hi.;Russel, W. B.; Prud’homme, R. K. Macromolecules 1989,22,1317-1325. (8) Char, K.;Frank, C. W.; Gast, A. P.; Tang, W. T. Macromolecules 1987,20,1833. (9) Char, K.;Gast, A. P.; Frank, C. W. Langmuir 1988,4,989.
0743-7463/89/2405-1335$01.50/0 0 1989 American Chemical Society
1336 Langmuir, Vol. 5, No. 6, 1989 phobic probes are generally excluded from the surface. From excimer fluorescence, it was possible to draw reasonably precise conclusions about the spatial distribution of the chromophores and, hence, the chain ends to which they are attached." In this paper, we investigate the adsorption of Py-PEGPy on hydrophobic polystyrene particles with a variety of fluorescence techniques. Our probe molecule in this experimental system is, in contrast to the previous work with hydrophilic silica particles, more akin to an associative thickener since it is likely that the hydrophobic groups associate with the hydrophobic surface. Thus, we will suggest, from spectroscopic measurements, a different adsorption mode in this system due to the presence of the hydrophobic probes.
Experimental Section Materials. Pyrene-end-tagged poly(ethy1ene glycols) (PyPEG-Py) were prepared by the direct esterification between hydroxy groups located at both ends of the PEG backbone and the carboxy group of 1-pyrenebutyricacid following the method developed by Cunibertiand Perico," described previou~ly.~ We denote the pyrene-tagged samples as Py-PEG-Py (4250),where the number in parenthesesrepresentsthe weight-averagemolecular weight of the original untagged PEG sample as reported by Polyscience Inc. Monodisperse, spherical polystyrene (PS) colloidal particles, of 0.13-pm diameter, prepared by the emulsifier-freepolymerization technique of Juang and Krieger" were used in deionized water su~pensions.'~The chromophore for the fluorescence depolarization experiment, 1,6-diphenyl-1,3,5hexatriene (DPH), was purchased from Molecular Probes Inc. and used as supplied. All solvents were spectroscopic grade, and water was cleaned with a Milli-Q column. Sample Preparation. In order to make a final solution of 0.05 vol % PS + 1 X lo* M Py-PEG-Py, we slowly mixed a stock solution of 0.1 vol % PS with a solution of 2 X lO* M Py-PEG-Py while vigorously stirring. We agitated the mixture with an arm shaker overnight to ensure equilibration. The pH of the final solution was around 5. We carried out supernatant analysis on 20 mL of the solution containing PS particles and polymer by ultracentrifugingfor 1 h at 60 OOOg and measuring UV absorption on about 6 mL of the supernatant. Fluorescence Measurement. Photostationaryspectrawere measured on a SPEX Fluorolog 212 spectrofluorometer as emission from the front face of the cell at an angle of 22.5O from the incident light upon excitation at 343 nm. Monomer excitation spectra were measured by scanning the excitation wavelength from 300 to 360 nm for a fixed emission wavelength of 376 nm. We measured the fluorescence lifetimes on a single photon counting Photochemical Research Associates (PRA System 3000) instrument. The excitationwavelength was 343 nm, and monomer emission was monitored at 376 nm. In order to reduce the effect of light scattering due to the turbid PS particles, we installed a 360-nm cutoff filter in front of the emission monochromator. We determined the lifetimes and preexponential factors from best fits to a multiexponentialdecay using a nonlinear least-squares deconvolution method. For photostationary fluorescence depolarization,we placed polarizers before the emission monochromatorand after the excitation monochromator. The polarizer attached to the emission monochromator,driven by a stepping motor, selected either the horizontal or the vertical polarization position. A dilute glycogen solution was used as a scattering solution in the alignment of the polarizers. Slit widths were set at 1.2 mm for both monochromators. Intensities of four combinations of the two polarizers (Zw, I,,, I,,, Zhh)were measured where Zhvdenotes horizontally polarized excitation and vertically polarized emission. (IO) Char, K.; Frank, C. W.; Gast, A. P.Langmuir 1989,5, 1096. (11) Cuniberti, C.; Perico, A. Eur. Polyrn. J. 1977,13, 369. (12) Juang, M. S.-D.; Krieger, I. M. J. Poiyrn. Sci., Polym. Chem.
E d . 1976, 14, 2089. (13) Monovoukas, Y.;Gast, A. P. J. Colloid Interface Sci. 1989, 128, 533.
Char et al. 2.17C 06 1
Wave) ength (nrnl
Figure 1. Fluorescence spectra of Py-PEG-Py (4250) for the following conditions: -, 1 x IO* M Py-PEG-Py in water; - - -, 0.05 vol % PS + 1 X lo* M Py-PEG-Py. The spectra were normalized to the highest monomer band. The ratio Zl,/ZL, unbiased by the detection system, is related to the measured intensities in four polarizer positions:
where G = Zhv/Zhhand depends on the detection system. For DPH, the excitation wavelength was 357 nm and emission was scanned from 400 to 500 nm. Pyrene was excited at 343 nm, and its emission was monitored between 360 and 500 nm. The anisotropy ( r ) is defined as
Average anisotropy values ( i )from the emission wavelength scans are reported below.
Results Emission and Excitation Spectra. Figure 1 shows photostationary emission spectra for Py-PEG-Py (4250) in water and Py-PEG-Py (4250) associated with polystyrene particles. These spectra can be divided into two parts: monomer emission bands and an excimer emission peak. Monomer emission consists of five fluorescence bands, three of which are clearly resolved, characteristic of the vibronic structure of pyrene. Excimer emission is very broad and has a peak intensity around 480 nm. While significant excimer emission is detected for Py-PEG-Py (4250) in water, i.e., I J I m = 0.46, there is no excimer detected when pyrene-end-tagged PEG chains are exposed to polystyrene colloidal particles. In addition, the structured monomeric emission of the pyrene moiety ranging from 360 nm to 420 nm is red-shifted about 1nm when PS particles are added to the aqueous Py-PEG-Py solution. Although the PS particles themselves fluoresce in the range between 360 and 590 nm, when excited at a pyrene excitation wavelength (343 nm), the fluorescence intensity is about 3 orders of magnitude smaller than that of the pyrene moieties and thus can be neglected. We confirmed that the Py-PEG-Py is adsorbed on the PS particles from analysis of the supernatant solution after centrifugation of the mixture of PyPEG-Py and PS particles. Monomer excitation spectra, monitored a t 376 nm, are given in Figure 2 for the same samples described in Figure 1. The excitation spectrum clearly shows a red-shift of about 3.5 nm when PS particles are introduced to the Py-PEG-Py solution. This result is surprising since we did not observe any wavelength shift in the monomer excitation spectrum for adsorption of Py-PEG-Py on the hydrophilic silica ~ u r f a c e .As ~ shown previously: the exci-
Langmuir, Vol. 5, No. 6, 1989 1337
Fluorescence Studies of Polymer Adsorption
Wave1 ength [nml
Figure 2. Normalized monomer excitation spectra of Py-PEGPy (4250) for the following conditions:-, 1 X 10* M Py-PEGPy in water; - - -,0.05 vol % PS + 1 X lo4 M Py-PEG-Py.
f
o'5 0.4
300
320
340
360
Wavelength (nm)
Figure 4. Effect of PS particle concentration on the monomer excitation spectra for 1 X lo4 M Py-PEG-Py(4250). The numbers shown in the figure are PS particle vol %. 10'
10'
-
0 0
0.01
0.02
0.03
0.04
0.05
Particle volume Yo
Figure 3. Effect of PS particle concentration on the excimer to monomer intensity ratio (Ze/Zm) for 1 X lo* M Py-PEG-Py
1o2
(4250).
mer excitation spectrum, monitored at 500 nm, for PyPEG-Py in water is red-shifted about 2 nm relative to the monomer spectrum, presumably due to ground-state interactions. In order to follow the transition behavior over the PS particle concentration range of 0-0.05 vol 90,we present emission spectra as well as monomer excitation spectra. Results for the emission spectra are shown in Figure 3, where the excimer to monomer intensity ratio ( I e / I m )is plotted against PS particle concentration. When PS particles are introduced, I e j I m decreases rapidly. When the PS particle concentration exceeds 0.01 vol %, no excimer is detected. The excitation spectra of monomer emission for the samples described in Figure 3 are shown in Figure 4. The peaks at 343.5 and 347 nm are marked to illustrate the transition due to the addition of PS particles. When no PS particles are in the aqueous Py-PEGPy solution, the peak intensity a t 343.5 nm is much higher than the intensity a t 347 nm. When PS particles are introduced up to 0.005 vol %, where excimers are still detected, the intensity envelopes are noticeably broadened such that they are comparable a t 343.5 and 347 nm. At a particle concentration of 0.01 vol %, where no excimer is detected, the band a t 343.5 nm has dropped, while above 0.05 vol %, the shoulder a t 343.5 nm disappears. Although the fluorescence emission spectra show no excimers for particle concentrations ranging from 0.01 to 0.05 vol 90,the excitation spectra of monomer emission shown
10'
1oo
Time (nanoseconds)
Figure 5. Fluorescence decay of monomer emission for PyPEG-Py (4250) for the following conditions: (a) 1 x lo4 M PyPEG-Py in water; (b) 0.05 vol % PS + 1 x lo4 M Py-PEG-Py. in Figure 4 continue to change, suggesting that the environment around the pyrene moieties is also changing. The disappearance of excimers after exposure to PS particles and the wavelength shift in the monomer excitation spectrum obtained with Py-PEG-Py (4250) are also observed with pyrene-end-tagged PEG (86501, implying that the adsorption of Py-PEG-Py on the PS particles involves contact between pyrene moieties and the PS surface and is not sensitive to molecular weight. Transient Fluorescence. Figure 5 shows the fluorescence decay, monitored a t 376 nm, for samples before and after the adsorption of Py-PEG-Py. As pointed out previously,' neither the monomer nor excimer Py-PEGPy fluorescence decay exhibits Birks' kinetics in water due to ground-state interactions. Thus, we have chosen
1338 Langmuir, Vol. 5, No. 6, 1989
Char et al.
Table I. Lifetime Measurements of Monomer Fluorescence* of Py-PEG-Py (4250) for Two Experimental Conditions 1 X 10"
Ab 11 A2 12
A3
3;
e
(7)
M Py-PEG-Py in water
0.05 V O ~% PS + 1 X 10- M PY-PEG-PY(4250)
0.008
0.058 169.89 0.028 65.18 1.196 0.28 1.33 149.28
153.15 0.024 54.71 0.011 8.13 1.05 99.02
'I,(t) = A, exp(-t/r,) + A, exp(-t/r,) A, exp(-t/s,). * T in ns. Standard x 2 displacement in the lifetime measurement. Time scale for fit, 1000 ns.
+
to represent the transient results in terms of an average lifetime defined by14 (7) =
x ' t I ( t ) dt/ J r n I ( t )dt
(3)
If a multiple exponential decay is assumed
(4) the average lifetime is given by
When PS particles are added, the shape of the decay is changed significantly. The contribution from the short time decay component remains very small and a slow decay component prevails over the time scale measured (ca. lo00 ns) due to the fact that excimers completely disappear when PS particles are added, asshown in Figure 1. The average lifetime for Py-PEG-Py (4250) adsorbed to PS particles increases by a factor of 1.51 over that in solution, when both are fit with triple exponential decays, as shown in Table I. This is another indication that the pyrene moieties interact with the hydrophobic PS surface. Fluorescence Depolarization. The disappearance of excimers after introduction of PS particles into aqueous Py-PEG-Py (4250) solution suggests the association of the pyrene moieties with the hydrophobic surface. Support for this may be obtained from a fluorescence depolarization experiment, which is characterized by the anisotropy ( r ) as defined in eq 2. According to the theory for polarization in a dilute vitrified solution,15 r should fall between -0.2 and 0.4depending on the angular displacement between the chromophore absorption and emission dipoles. Once a chromophore is selected, its depolarization is further affected by external factors such as rotational diffusion and electronic excitation transport15 (EET) occurring during the lifetime of the excited state. EET usually occurs by resonant interactions between excited- and ground-state chromophores in a situation where chromophores are randomly distributed in a polymer matrix at high concentration.16 In the present study, chromophores adsorb onto the PS particle surfaces with sufficient area to accommodate all of them. Since they are confined to a two-dimensional surface at very low concentration, EET is negligible. Thus, the most important factor causing fluorescence depolarization is rotation. (14) Winnik, M. A.; Egan, L. S.;Croucher, M. D. Macromolecules
1986,19, 2669. (15) Lakowicz, J. R. Principles ~. of Fluorescence Spectroscopy; - . Plenum: New York, 1983. (16) Peterson,K. A,; Zimmt, M. B.;Fayer, M. D.;Jeng, Y. H.; Frank, C. W. Macromolecules 1989,22, 874-879.
Chromophores bound to a colloidal particle remain polarized since they rotate concomitantly with the particles, generally a much slower process than rotation of the chromophore itself. The rotational diffusion coefficient is D,, = k T/(8n7),a3) (6) where a is the particle radius, qo is the solvent viscosity, and k is the Boltzmann constant. It is more sensitive to the particle size than the translational diffusion coefficient. Our experimental system, consisting of monodisperse 0.13-pm-diameter PS particles, has a characteristic rotational relaxation time17 of 1/(6Dr,,J, on the order of few milliseconds, in contrast to the lifetime of typical chromophores, 10-7-10" s. Thus, the PS particle rotation does not contribute to the fluorescence depolarization. We chose DPH rather than pyrene as our probe for the photostationary depolarization experiment for several reasons. First, the absorption and emission dipoles of DPH are almost parallel to each other so that the intrinsic anisotropy in the absence of other depolarizing processes can be as high as 0.39, in contrast to pyrene where the two dipoles are almost perpendicular to each other, yielding an intrinsic anisotropy close to zero.16 Second, the polarization spectrum of DPH, which is a plot of fluorescence anisotropy against excitation wavelength for a chromophore in a vitrified solution, remains fairly constant in the excitation region, while the pyrene anisotropy depends on ~ave1ength.l~ Third, the anisotropy of DPH in the emission region is fairly constant, while that of pyrene fluctuates over a wide range, yielding an inaccurate average. Since DPH and pyrene are both hydrophobic and practically insoluble in water, we initially dissolved them in a nonpolar solvent (hexane) and then diluted the mixture 1:lOOO with the aqueous PS suspension during vigorous stirring to allow the chromophores to associate with the PS particles. After adsorption of the chromophores, the supernatant solution from centrifugation, analyzed with UV absorption, revealed that neither DPH nor pyrene was present, implying that all of the chromophores reside on the PS surface. We corrected the anisotropy for scattering by measuring the anisotropy as a function of particle concentration and related it to the optical density at the excitation wavelength as robs = fJ( 1- K-OD) (7) where robs is the observed average anisotropy, F' is the anisotropy after the correction due to light scattering, OD is the optical density of the suspension, and K is an arbitrary proportionality constant." The correded anisotropy of DPH adsorbed on the PS particles (0.357) is 1 order of magnitude larger than that of DPH in hexane (0.044),indicating that the rotation of DPH associated with PS particles is significantly restricted. We obtained similar trends from the adsorption of pyrene on PS particles. The extrapolated anisotropy after the correction for the multiple scattering of particles (0.095) is again close to the value for l-pentylpyrene in bulk polystyrene16 (-O.l), where the depolarization of fluorescence is minimized compared to pyrene in solution. This result is consistent with the monomer excitation spectrum, where the contribution from the intensity at 343.5 (17) Berne, B. J.;Pecora, R. Dynamic Light Scattering; Wiley: New York, 1976. (18) Teale, F. W. Photochern. Photobiol. 1969, IO, 363. (19) Cohen Stuart, M. A.; Waajen, F. A. W. H.; Cosgrove, T.; Vincent, B.; Crowley, T. L. Macromolecules 1984,17, 1825.
Langmuir, Vol. 5, No. 6,1989 1339
Fluorescence Studies of Polymer Adsorption nm (which we have assigned to a loosely bound pyrene) is negligibly small. We attempted to displace the Py-PEG-Py (4250) from PS with various molecules having hydrophobic moieties. We found it impossible to displace Py-PEG-Py (4250) with 1-butanol or untagged PEG (M-22 000) at a fairly high concentrations. Only when a water-soluble nonionic surfactant (Triton X-405) was added at a high concentration was there a slight recovery of the excimer in the emission spectrum (ca. 19%). These displacement experiments indicate that the hydrophobicpyrene probes adsorb on the PS surface more tenaciously than PEG itself.
Discussion The disappearance of excimers in the emission spectrum when 1 X lo4 M Py-PEG-Py (4250) is mixed with 0.05 vol % PS particles indicates that the environment around the pyrene probes is drastically changed. We believe that the pyrene end tags are bound to the PS surface via hydrophobic attraction extensively hindering their motion and preventing excimer formation. Evidence for the association of hydrophobic pyrene groups with the PS surface also comes from the 1-nm red-shift in the monomer emission spectrum in the region 360420 nm, not observed for Py-PEG-Py adsorbed on hydrophilic silica particle^.^ Also, the independence of the emission and excitation spectra on the molecular weight of Py-PEG-Py implies an interaction between the chromophores and the PS surface. The approximate 3.5-nm red-shift in the monomer excitation spectrum of Py-PEG-Py with the PS suspension relative to that in water, as shown in Figure 2, gives further evidence of hydrophobic interaction between the chromophore and the PS surface. This is again in contrast to the monomer excitation spectrum of Py-PEG-Py adsorbed on hydrophilic silica particles.’ It is likely that alteration of the structured vibronic band due to the association of pyrene molecules with the hydrophobic PS particles is manifest in both emission and monomer excitation spectra. When the PS particle concentration exceeds 0.01 vol % ,the emission spectra show no excimerswhile the monomer excitation spectrum continues to change, as shown in Figure 4. We infer that when the planar surface of the pyrene molecule is tightly bound to the hydrophobic surface, as in the case of a PS particle concentration of 0.05 vol %, the contribution from a peak at 343.5 nm almost disappears. However, when some pyrene molecules remain loosely bound or in solution, their planar surfaces are exposed to water, and the contribution from a peak at 343.5 nm starts to grow. Thus, the excitation spectrum of monomer emission is more sensitive to the bound state of pyrene molecules on the hydrophobic surface than excimer formation. This type of information might be useful when evaluating the rheological properties of an associative thickener. We believe that the excitation at 343 nm is associated with free or loosely bound pyrenes while the excitation at 347 nm can be attributed to tightly bound pyrenes. Changing the PS particle concentration changes the relative proportion of the free and bound pyrenes. Lifetime measurements show a significant increase in average lifetime (50 ns) upon adsorption, primarily due to the disappearance of excimers. We are not able to fit the monomer emission with a single exponential in the absence of excimers because we cannot completely eliminate contributions from loosely bound pyrenes. The existence of more than one pyrene environment causes the
0.4
1k‘,
44
0.357
0.4
T
0.3
0.2
k
O.’
0.1
0.0444 DPH in hexane
0 0
0.25 0.50 0.75 1.00 1.25 1.50
Optical Density at 357 nm
Figure 6. Average anisotropy (t)of DPH as a function of optical density at the DPH excitation wavelength for PS particles at various concentrations.
0
: pyrene
- : poly(ethy1ene glycol)
Q hydrophilic surface
hydrophobic surf ace
Figure 7. Schematic picture illustrating adsorption modes for Py-PEG-Py adsorbing onto two different surfaces. time-resolved fluorescence data to deviate from a singleexponential decay. The hydrophobic interaction between pyrene and PS may be evident in the very short time response immediately after the lamp pulse. The emission spectra, excitation spectra, and lifetime data all consistently provide evidence for the binding of pyrene probes to the PS surface. If the chromophore is truly bound to the large particle, then the fluorescence depolarization is expected to be significantly reduced. As shown in Figure 6, the anisotropy of DPH adsorbed on PS particles (after correction for the multiple scattering of the exciting beam) is about 1order of magnitude larger than that of DPH in hexane. Also, the anisotropy (r) obtained for DPH adsorbed on the PS particles is close to the limiting value for DPH (0.39) in a vitrified solution, implying that the probe is tightly bound to the surface so as to minimize any motion. The trend for the pyrene molecule is the same as that of DPH. These fluorescence experiments show that when PyPEG-Py is allowed to adsorb from water, pyrene moieties are attracted to the PS surface, presumably via hydrophobic attraction. As shown in the displacement experiment, the binding energy of pyrene to the PS surface seems to be much higher than that of the PEG backbone, in contrast to adsorption onto silica. The adsorption of Py-PEG-Py on the two types of surfaces is depicted schematically in Figure 7. When Py-PEG-Py is adsorbed on a hydrophilic surface, the hydrophobic pyrene groups do not associate with the surface, so a dependence of the molecular weight of PEG on the fluorescence signal (Le.,
Langmuir 1989,5, 1340-1343
1340
Ie/Im]is ob~erved.~ However, since pyrene groups strongly associate with the hydrophobic surface, dependence of the fluorescence observables on PEG molecular weight disappears.
Summary The fluorescence properties of pyrene-end-labeled poly(ethylene glycol) (Py-PEG-Py) adsorbing on monodisperse polystyrene colloidal particles have been investigated. We conclude that hydrophobic chromophoresassociate with the PS surface more strongly than PEG segments. This contrasts with adsorption of Py-PEGPy on hydrophilic silica particles previously studied. Emission spectra, excitation spectra in the region of the pyrene monomer emission, fluorescence decay, and photostation-
ary fluorescence depolarization consistently provide evidence of pyrene adsorption on the PS surface. The experimental system studied here is similar to an associative thickener, where the hydrophobic part associates with hydrophobic colloidal particles to improve rheological properties.
Acknowledgment. This work was supported by the NSF-MRL Program through the Center for Materials Research at Stanford University. We thank Yiannis Monovoukas for donating polystyrene particles for this study. A.P.G. gratefully acknowledges the support of Xerox Foundation and a Faculty Development Award from IBM. Registry No. Py-PEGPy, 82870-83-5;PS,9003-53-6;DPH,
1720-32-7.
Retention of Molecular Oxygen in Zeolites at High Temperature S. L. Suib*>tp* and B. E. Morset Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, and Department of Chemical Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06268 Received August 18, 1988. In Final Form: June 2, 1989 X-ray photoelectron spectroscopy and residual gas analysis experiments were carried out on NaY zeolite at temperatures between -196 and 400 "C. The objective of this work was to further understand the 0 1s transition for zeolites since contributions can arise from zeolitic oxygen as well as trapped water. The residual gas analysis data show that molecular oxygen is retained by NaY zeolite to temperatures as high as 400 O C . Qualitative trends in the RGA data for 0, match those of a shoulder in the XPS 0 1s transition, which is assigned to molecular oxygen. These data indicate that zeolites can retain small amounts of 0, to very high temperature, and this may be one reason for the typical broadness of the 0 1s data for zeolites found throughout the literature.
Introduction Several surface science studies of zeolites have shown that Si/Al ratios, cation content, and oxidation states can vary considerably in these materials. Since zeolites are used in heterogeneous catalysis and in adsorption devices, the nature of their surfaces is extremely important. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry, Auger electron spectroscopy, ionscattering spectroscopy, and Rutherford backscattering are all surface techniques which have been used to study the catalytic surfaces of zeolites.'-'' XPS is the most ~~
~
* Author to whom correspondence should be addressed. Department of Chemistry.
* Department of Chemical Engineering and Institute of Materi-
als Science. (1) Vedrine, J. C. Surface Properties and Catalysis by Non-Metals; Bonnell, J. P. et al., E%.; D. Reidel: New York, 1983;pp 159-187. (2) Suib, S.L.; Winiecki, A. M.; Kostapapas, A. Langmurr 1987,3, 483. (3) Barr, T. L.;Lishka, M. A. J. Am. Chem. SOC.1986,108,3178. (4) Minachev, K. M.; Antoshin, G. V.; Shapiro, E. S. Russ. Chem. Reu. 1978,47,2097.
general of these methods; it allows the determination of oxidation states and may provide information related to the number of different types of species on a surface. XPS has been widely utilized for elemental analyses of near-surface regions (150A). In XPS, X-rays are used to eject photoelectrons. The kinetic energy (KE) of the ejected photoelectronis equal to the incident photon energy (hv)minus the binding energy of the electron (BE) minus the work function of the spectrophotometer (4). The binding energy is dependent on the nature of the bonding of the atomic species and can be calculated as follows: BE = hv - KE - 6 .'' (5) Mataumoto, Y.; Soma, M.; Onishi, T.; Tamaru, M. J. Chem. SOC.,Faraday Trans. 1 1980,76,1122. (6) Corma, A,; Fornes, V.; Pallota,0.; Cruz, J.; Ayerbe, A. J.Chem. SOC.,Chem. Comrnun. 1986,333. (7) Baumann, S.;Strathman, M. D.; Suib, S. L. J. Chem. SOC., Chem. Commun. 1986,308. ( 8 ) Turner, N. H.; Colton, R. J. Anal. Chern. l982,54,393R. (9) Suib, S. L.;Coughlin, D. F.; Otter, F. A.; Conopask, D. F. J. Catal. 1983,84,410. (10) Turner, N.H.; Dunlap, B. I.; Colton, R. J. Anal. Chern. 1984, 56,373R. (11) Ertl, G.;Kuppers, J. Low Energy Electrons and Surface Chemistry; VCH Weinheim, FDR, 1985; Chapters 2 and 3.
0 1989 American Chemical Society
