6238
J. Phys. Chem. 1987, 91, 6238-6241
Intramolecular Micelles in a Copolymer of Maleic Anhydride and Hexyl Vinyl Ether: Determination of Aggregation Number by Luminescence Quenching Jye-Ling Hsu and Ulrich P. Strauss* Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 (Received: May 1 , 1987; In Final Form: July 1 , 1987)
The quenching of the luminescence of tris(2,2'-bipyridine)ruthenium(II) by 9-methylanthracene in solutions of a hydrolyzed copolymer of maleic anhydride and n-hexyl vinyl ether (DP = 1700) was investigated in order to study the micellar behavior of this polyacid. The results indicate that the micelles inside the domain of a macromolecule encompass approximately 24 repeat units. This micelle size is unaffected by changes in the concentrations either of the polyacid or of the probe and is independent of the extent of micellization of the polyacid. The dynamics of the quenching process is compared with that reported for solutions of ordinary surfactant micelles containing the same probe and quencher.
Introduction It has been shown that the hydrolyzed alternating copolymers of maleic anhydride and n-alkyl vinyl ethers with intermediate size alkyl groups (C4-C,) undergo a conformational transition from hypercoiled to randomly coiled state as the pH is increased.' From a variety of experimental techniques including potentiometry,l calorimetry,2 sol~bilization,~ and fluorescence: it has been established that the hypercoiled state is stabilized by hydrophobic interactions between the alkyl groups. There is evidence, based on the pH dependence of the conformational transition, that in the hypercoiled state each polymer molecule contains a large number of small micelle^.^ In this paper we shall test this hypothesis with a recently developed method involving luminescence The basic idea underlying this method is that in a solution containing luminescent probe and quencher molecules, both solubilized in an excess of micelles, the quenching will decrease with an increasing number of micelles because of a decreased probability of finding both a probe and a quencher molecule in the same micelle.6 The method has been successfully applied to solutions of ordinary micelle-forming The systems investigated in the present study consisted of a hydrolyzed copolymer of maleic anhydride and hexyl vinyl ether (DP = 1700), tris(2,2'-bipyridine)ruthenium(II) ion [Ru(bpy)?+] as the probe, 9-methylanthracene (9-MeA) as the quencher, and an aqueous 0.1 M LiCl solution as the solvent. Both steady-state and time-dependent emission studies were carried out for each solution. These studies were supplemented with solubilization experiments in order to determine the distribution of 9-MeA between the polymer molecules and the solvent medium as well as the extent to which the polymer was in micellar form. Finally, a study was undertaken concerning the location of Ru(bpy),Z+. While in solutions of ordinary anionic micelles this ionic probe is considered to be situated exclusively in the micellar phase,',* in our case we have to examine the possibility that a portion of the divalent probe ions may be located on the nonmicellar parts of the polyelectrolyte. Experimental Section Materials. The copolymer of maleic anhydride and hexyl vinyl ether was our sample C described previously.' Its concentration,
. (1) Dubin, P. L.; Strauss, U. P. J. Phys. Chem. 1970, 74, 2842.
(2) Martin, P. J.; Strauss, U. P. Biophys. Chem. 1980, 1 2 , 397. (3) Dubin, P. L.; Strauss, U. P. In Polyelectrolytes and Their Applications; Rembaum, A., Selegny, E., Eds.;D. Reidel: Dordrecht, 1975; p 3. (4) Strauss, U. P.; Vesnaver, G. J . Phys. Chem. 1975, 79, 2426. (5) Barbieri, B. W.; Straws, U. P. Macromolecules 1985, 18, 411. (6) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978, 100, 5951. (7) Almgren, M.; Lofroth, J.-E. J . Colloid Interface Sci. 1981, 81, 486. (8) Turro, N. J.; Lee, P. C. C. J . Phys. Chem. 1982, 86, 3367. (9) Thomas, J. K. J . Phys. Chem. 1987, 92, 267. (10) Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91, 276.
0022-3654/87/2091-6238$01.50/0
Cp, is expressed in moles of repeat units per liter. 9-Methylanthracene (Aldrich) was recrystallized twice from ethanol. It was brought to the desired concentrations by adding 3-20 pL of M ethanolic stock solution to 3-mL portions of a 8.37 X aqueous polymer solutions. The concentration of ethanol in all final solutions was kept constant at 0.083% by additions of appropriate amounts of ethanol. Ru(bpy),Cl, (AESAR) was used as received. Methods. Steady-state fluorescence emission data were obtained with a Hitachi Perkin-Elmer MPF-3 fluorescence spectrophotometer described previ~usly.~ The excitation and emission wavelengths were 452 and 633 nm, respectively. Fluorescence lifetime measurements were carried out with a pulsed laser technique using a Lambda Physik FL 2002 dye laser containing Coumarin 47 solution, pumped by a Lambda Physik EMG-101 excimer laser operated at 308 nm with a pulse width of 14 ns. The beam with a wavelength of 450 nm strikes the sample perpendicular to the monitoring direction. The emitted light passes through an ISA Model H- 10 compact monochromator set at 633 nm. The photomultiplier tube output is captured by a Tektronix 7912AD programmable digitizer, and the data are analyzed and stored on a Tektronix 4052A desk top computer. Absorbance measurements were performed with a HewlettPackard 8450A diode array spectrophotometer. The solubility of 9-MeA in the polymer solutions was determined by following the steeply increasing turbidity resulting from increasing amounts of 9-MeA beyond its solubility limit. The pH was adjusted with 0.20 N LiOH and 0.20 N HCl solutions and measured with a Radiometer Model 26 pH meter. Sample solutions were deaerated with nitrogen. All experiments were carried out at 24 f 1 "C. Results and Discussion Location of Probe Molecules. We have used the fact that the emission intensities of Ru(bpy)?+ are larger in a hydrophobic than in a hydrophilic environment to estimate the distribution of the probe between micellized and nonmicellized portions of the hexyl M polyacid. Figure 1 shows the emission spectra of 1.22 X Ru(bpy)32+in four solutions, all at pH 5.10 and in a 0.1 M LiCl environment. One contains hexyl polyacid which, as will be shown below, is 72.6% micellized at this pH, one contains methyl polyacid which does not form micelles, and the other two are 1:l and 1:9 mixtures of these hexyl and methyl polyacid solutions. The concentrations, Cp, of both the original hexyl and methyl polyacid solutions are 8.7 X M. The emission from the hexyl polyacid solution is seen to be almost twice that from the methyl polyacid solution which, in a separate experiment, was found to have an emission spectrum identical with that of the probe in the absence of any polyacid. The spectrum from the 1:l mixture is in close proximity to that of the hexyl polyacid, indicating that very little of the probe has moved to the methyl polyacid. The difference between the emissions from the hexyl polyacid solution and the 1:9 mixture allows us to estimate the equilibrium constant for the 0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6239
Maleic Anhydride and Hexyl Vinyl Ether Copolymers a
/?
I
I
t
I
500
550
600
650
700
Figure 1. Emission spectra of Ru(bpy)t+ in polyacid solutions. (a) hexyl polyacid solution; (b) 1:l methyl/hexyl polyacid solution mixture; (c) 9:l methyl/hexyl polyacid solution mixture; (d) methyl polyacid solution. = 1.22 X M; total polyacid concentration = pH 5.10;[R~(bpy)~~’] 8.7 X mol of repeat units per liter; [LiCl] = 0.1 M. TABLE I: Solubilization of 9-MeA
sx
102n
1.210 1.210 0.879 0.707 0.647
2
3
cp
Wavelength i n m i
PH 4.20 4.50 5.10 5.56 5.83
1
om
1.000 1 .ooo 0.726 0.584 0.535
” Expressed in moles of 9-MeA per mole of repeat units of polyacid. distribution of the probe between micellized and nonmicellized polyacid to be 28.4. Since in our fluorescence experiments the micellized portion was always larger than 50%, the fraction of probe in the nonmicellized portions of the polyacid may be neglected. Solubilization of Quencher Molecules. The solubility of 9-MeA is given as a function of hexyl polyacid concentration at several values of the pH in Figure 2. The curves are linear with a common intercept, 2.79 X M, which gives the quencher’s solubility in 0.1 M LiCl. The slopes, S, represent the solubilization of the 9-MeA by the polymer and are given in Table I as a function of pH. The values of S are identical for pH 4.20 and 4.50 but decrease as the pH is raised above 4.50, reflecting a corresponding decrease in Om, the fraction of polyacid in micellized form. If Poisson statistics is assumed for the distribution of quencher among micelles,” the mass action law for the distribution of quencher between the micellized polymer and the solvent medium may be expressed in the form
where [Q,] and [Q,] are the molar concentrations of solubilized and free quencher, respectively. This equation should be valid below as well as at the solubility limit of the quencher. In the latter case, [Q,]/Cp = S and [Q,] = 2.79 X lo4, and if we take the values for these quantities at pH 4.20 or 4.50 where 0, = 1, we find K’ = (4.34 f 0.05) X lo3 M-’. The values of Om at the higher pH values which can then be calculated in an analogous manner by means of eq 1 are given in the third column of Table I. (11) Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC.1979, 101, 279.
J
4
10’
Figure 2. Solubilization of 9-MeA by hexyl polyacid: 0 , pH 4.2;0, pH 4.5;0,pH 5.1; A, pH 5.5; 0 , pH 5.8. TABLE II: Parameters of Sample Run” 1 0-6A2, 1 0 - 7 4 io-6k_, 104K, s-l 104[M] n M-I 10s[Q] s-I Fb 102A3 s-] 0 1.17 1.90 0 2.79 1.29 1.59 8.05 1.11 2.69 23 9.98 5.58 1.39 1.36 16.2 8.37 1.48 1.16 25.8 1.53
“CP= 8.35 X lo-’ M; pH 5.10;8, = 0.726;[Ru(bpy),C12]= 2.30 X M; [Q,]/[Q] = 0.963. *In arbitrary units.
Luminescence Quenching. The time dependence of the fluorescence of a probe in a micellar solution containing mobile quenchers has been shown to be given by the e q ~ a t i o n ’ ~ - ’ ~ Z ( t ) = Z(0) exp(-A,t - A3[l - exp(-A,t)]) (2) Here (3) (4)
where ko is the first-order rate constant for the fluorescence decay of the probe in the absence of quencher, kQ is the first-order rate constant for quenching inside a micelle, and k- is the exit rate constant for a quencher from a micelle. The procedure used for obtaining the desired parameters from the experimental data is illustrated with a typical sample run performed on four solutions differing only in their quencher concentration, [Q]. The results are presented in Table 11. The values of A2 were obtained from plots of In Z(t) against t , two of M, are shown which, representing [Q] = 0 and [Q] = 8.37 X in Figure 3. The first of these is linear, while the second, which is characteristic of all our quencher-containing solutions, becomes linear after an initial sharp decrease. This behavior is in accordance with eq 2 if A4 is large compared to A2, so that A , is the negative slope of the “long time” linear portion of this plot. The increase of A, with [Q], seen in Table 11, is typical of a quencher which exchanges between micelle and solvent or between (12) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (13) Infelta, P. P.; Gratzel, M.; Thomas, J. K. J . Phys. Chem. 1974, 78, 190. (14) Almgren, M.; Lofroth, J.-E. J . Chem. Phys. 1982, 76, 2734.
6240 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
Hsu and Strauss
TABLE III: Micelle Size and Related Parameters
PH
103Cp
4.50
4.60
lO5[Ru(bpy),*+] 0.90 0.90 1.10 2.30 0.90 1.10 1.10
-4.0
-4.5
Bm
1.oo 0.726 0.726 0.726 0.584 0.584 0.535
1 0 4 ~M-1 ,
10-7kQ, s-I
10"k-, s-'
1.22 1.17 1.10 1.17 1.12 1.08 1.10
1.37 1.64 1.30 1.42 1.40 1.33 1.06
1.66 1.25 1.09 1.11 1.11 1.27 1.22
9.11 11.3 9.11 9.98 10.4 11.7 9.98
1.14
1.36
1.24
10.2
10"ko,
s-I
104[M] 2.19 1.30 3.04 2.69 1.13 1.89 2.06
n 21 26 21 23 24 27 23 23.6
To estimate A4 we proceed as follows: For the large t linear region of the In t against t plot, where exp(-A4t) is negligibly small compared to unity, eq 2 leads to the relation In Z(0) = In Z ( t ) + A,t + A, ( t large) (8)
-
Using this value of Z(O), we define a functionf(t) for all values of t by the equation
.
f(t) = In Z ( t )
--
+ A2t + A, - In Z(0)
(9)
I
Then, from eq 2 we have also
H
-C -5.0
At) = A , exp(-A4t)
-5.5
-
-6.0
-
(10)
and A4 will be the negative slope of a plot of In f against t. This procedure places considerable strain on the data but improves with increasing [Q]. We were able to obtain satisfactory results only for the solution with the largest quencher concentration for which the value of A4 is given in Table 11. We shall use this value in formulas below which contain A4. To determine k-, we combine eq 3, 4, and 5 to obtain 1
0
600
300
900
1200
Time Ins 1
Figure 3. Luminescence decay curves of R~(bpy),~+ in hexyl polyacid M; [9-MeA] = 0; ( 0 *) solutions. [ R ~ ( b p y ) ~=~ +2.30 ] X (e-)
[9-MeA] = 8.37
X
M.
two micelles at a rate comparable with ko, the intrinsic fluorescence decay rate of the excited probe. Because the pulse width of our light source was too large for a satisfactory direct determination of Z(O),' we made use of steady-state fluorescence data obtained with the same solutions and under conditions essentially identical with those employed in the corresponding dynamic experiments. The emission intensities, F, expressed in arbitrary units are presented in the third column of Table 11. The relation between F and Z ( t ) is given by the equation
(6) L m Z o ( t ) dt
where the zero subscript indicates the absence of quencher. The integral in the denominator may be obtained in closed form because A3 = 0. The integral in the numerator may also be evaluated in closed form if the exp(-A4t) term of eq 2 is neglected. We have convinced ourselves that this procedure, which has also been used by others,l5 gives an excellent approximation with our data. By carrying out the integrations indicated in eq 6 and rearranging terms, we obtain A, = In
(E)
(7)
an expression previously given by Yekta et a1.I6 Values of A, are given in the fourth column of Table 11. Rodgers, M. A. J.; Baxendale, J. H. Chem. Phys. Lett. 1981,81,347. (16) Yekta, A.; Aikawa, M.; Turro,N. J. Chem. Phys. Len. 1979,63,543. (15)
Then, from the slope, D, of the linear plot of A2 against A,, we find k- by the relation D k.. = 1 D/AI
+
This value of k- is given in the sixth column of Table 11. The value of [MI, the micelle concentration, can now be determined from the slope of the plot of A3 against [ Q ] . The ratio [Q,]/[Q] which is needed for this purpose is calculated from eq 1 and is given in Table 11. The quenching rate constant, k,, is obtained from eq 5 . The micelle size n, defined as the number of hexyl groups per micelle, is then calculated by means of the relation
n = e m c P / [MI
(13)
Value of [MI and n are given in the seventh and eighth columns of Table 11. The equilibrium constant, K , for the distribution of quencher between micelles and solvent is given in the last column of Table 11. It is defined by the relation16
From eq 1, 13, and 14 one obtains the working relation K = nK'. The pertinent results of the sample run given in Table I1 together with the analogous results obtained from six other sample runs by the same procedure are presented in Table 111. The data indicate that the micelle size n is constant within the limits of error of the method and is independent of the concentration and micellization degree of the polyacid as well as of the probe concentration. The value of n is of the same magnitude as the values of 19 and 13 obtained previously for the butyl and pentyl polyacids, respectively, by a different method which depended on equating the micelle size with the cooperative unit size of the conformational transition as deduced from the dependence of the pH on the degree of deprotonati~n.~ The transition was sharp for the butyl polyacid but became increasingly fuzzy with increasing alkyl group size,
J. Phys. Chem. 1987, 91, 6241-6244 which made its application to the pentyl polyacid difficult and to the hexyl polyacid impo~sible.~ The fluorescence technique presented in this paper does not depend on the nature of the transition and is also applicable in the range where the polyacid is completely micellized. A comparison of the rate parameters with those obtained by others7,l5in similar luminescence quenching studies of sodium dodecyl sulfate micelles with the same probe and quencher shows that the values of ko and k , are of the same magnitude as ours. However, contrary to the experience with our systems, their values of A2 did not increase with quencher concentration but remained equal to ko. An explanation is therefore needed for the dependence of A2 on [Q] in our system. If we assume that transfer into the water medium is the only process by which a quencher leaves a micelle, then k+, the second-order rate constant for a quencher entering a micelle from the water phase, would be given by the equation
k+ = Kk(15) This leads to k+ being of the order of 10" M-' s- I for all our sample runs. However, this value is more than 1 order of magnitude larger than the value estimated for k+ by the Smoluchowsky equation." A more likely possibility would be a direct exchange of quencher between micelles for which several mechanisms have been prop~sed''-'~and which might be facilitated by the close (17) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1981,85, 1196.
6241
proximity of the micelles belonging to the same macromolecule. Such an intramolecular mechanism would not be possible for the sodium dodecyl sulfate systems. However, one would then expect that k-, as defined here, should increase with the intramolecular micelle concentration. While the higher value of k- at 6, = 1 in Table I11 might be considered in agreement with this expectation, the other k- data show no systematic increase with Om. Further work is needed to elucidate this mechanism.
Conclusion This investigation gives direct evidence that a large hydrophobic polyacid molecule in its hypercoiled conformation consists of a large number of small intramolecular micelles. The aggregation number of these micelles is the same in the completely hypercoiled states and in the states consisting partly of micellar and partly of random coil conformations. Acknowledgment. We are grateful to Dr. Linda Hade1 for her assistance in performing the dynamic luminescence experiments. We also wish to acknowledge a grant received in support of this work from the Charles and Johanna Busch Memorial Fund of Rutgers University. Registry No. R~(bpy),~+, 15158-62-0; 9-MeA, 779-02-2; DP-1700, 27966-67-2. (18) Lessner, E.; Teubner, M.; Kahlweit, M. J . Phys. Chem. 1981, 85, 3167. (19) Malliaris, A.; Lang, J.; Zana, R. J . Phys. Chem. 1986, 90, 655.
Kinetlcs and Mechanism of the Oxidation of Allyl Alcohol on A g ( l l 0 ) J. L. Solomon and R. J. Madix* Department of Chemical Engineerifig, Stanford University, Stanford, California 94305 (Received: May 4, 1987)
The adsorption and reaction of allyl alcohol on clean and atomic oxygen covered Ag( 110) has been investigatedwith temperature programmed reaction spectroscopy (TPRS). Allyl alcohol adsorbs and desorbs molecularly from the clean Ag( 110) surface. When the surface is covered with 0.1 monolayer (ML) of atomic oxygen, allyl alcohol forms an intermediate that reacts further to yield hydrogen, water, acrolein, and allyl alcohol at 310 K in a single rate-limiting first-order reaction. Increasing the oxygen coverage to 0.25 ML increases the amount of products formed as expected for activation of the allyl alcohol by the predosed oxygen. The results obtained are consistent with the formation of an alkoxy intermediate, which has also been observed for methanol and ethanol on this surface.
Introduction Significantly different surface reactivity may be observed in molecules containing more than one functional group than would be expected from independent studies of one or both functions alone. Thus, one group that interacts only weakly with a given surface can be held in proximity of the surface by a more strongly interacting function, possibly opening reaction channels of the weakly interacting group at elevated temperatures. The bifunctionality of allyl alcohol (H2C=CHCH20H) is revealed in the reaction of allyl alcohol with Pt, Ni, and Pd metal complexes. For example, it reacts with potassium tetrachloroplatinate(I1) (K2PtC14) to give (diallyl ether)PtC12.' The reaction of allyl alcohol with a mixture of bis( 1,5-cyclooctadienyl)nickel and triphenylphosphine (PPh,) at 30 OC causes the dismutation of allyl alcohol to yield C3Hs, Ni(CH2=CHCHO)(PPh3)2, and water in about a 1:l:l ratio.2 Allyl alcohol also reacts with Pd(PCy3), (PCy, = tricyclohexylphosphine) at 30 OC, causing condensation of the (1) Jones, R. J. Chem. SOC.A 1969, 12, 2477. (2) Yamamoto, T.; Ishizu, J.; Yamamoto, A. J. Am. Chem. SOC.1981, 103, 6863.
0022-3654/87/2091-6241S01.50/0 , I
I
-
allyl alcohol to yield diallyl ether and its coordination product, Pd(dially1 ether)(PCy3)., In each of the above reactions the 0-H bond has been activated, as evidenced by the formation of either acrolein or diallyl ether, and the double bond is involved in .rr bonding of the acrolein or diallyl ether molecule to the metal atom as shown in Figure 1. To the best of our knowledge, no studies of the bonding of allyl alcohol to the silver surface have been undertaken. Simple alcohols and unsaturated hydrocarbons, including methan01,~ethan01,~l-propanol,6 2-propanol,6 ethylene? propylene,8 and acetylene? do not react with the clean silver (1 10) surface. In the presence of adsorbed oxygen atoms, the hydroxyl proton of the alcohol reacts with the preadsorbed oxygen to yield (3) Yamamoto, T.; Akimoto, M.; Saito, 0.; Yamamoto, A. Organometallics 1986, 5, 1559. (4) Wachs, 1. E.; Madix, R. J. Surf. Sci. 1978, 76, 531. (5) Wachs, I. E.; Madix, R. J. Appl. Surf. Sci. 1978, I , 303. (6) Jorgensen, S. W.; Madix, R. J. Surf. Sci. 1983, 130, L291. (7) Barteau, M. A.; Madix, R.J. Surf. Sci. 1981, 103, L171. (8) Barteau, M. A.; Madix, R. J. J. Am. Chem. SOC.1983, 105, 344. (9) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 123, 491.
0 1987 American Chemical Society
