J . Phys. Chem. 1985,89, 41-44 opening of cyclopropene.21 The lowering of this value by about 8-1 0 kcal/mol to account for the effect of the carbonyl group attached to the ring (see below) would not remove the discrepancy. The same argument places a considerable barrier for the closure of biradical A to a cyclopropene ring. Biradical A necessarily has a smaller heat of formation than the transition state of reaction 4 which is given by (AHf,furan E,). In other words there is a finite barrier to ring closure, E-4 > 0, since E , E2 although E5 is probably larger than E(. This follows from the fact that no alcohol is detected in the decomposition of 2-methylfuran where both shifts might conceivably take place. In the case of furan this argument leads to AHf(A) I63 kcal/mol at 1100 K. This value should be compared with estimates based on standard methods.1° Starting with buten-3-a1 (CH2=CHCH2CHO), = -18.1 kcal/mol, assuming BDE (C2-H) = 82 kcal/mol, BDE (C,-H) = 108 kcal/mo1,26 and considering the acetonyl-type resonance (-2 kcal/mol)l' leading to the structure of biradical A depicted in Scheme I (CH=CHCH=CHO), we arrive at AHf(A) 1 63 kcal/mol at 1100 K, in excellent agreement with
+
-
(26) The value of BDE(C2-H) = 82 kcal/mol is arrived at by assuming that a normal R2C(H)-H bond (BDE = 94 kcal/mol) is stabilized by allylic resonance ( E , = 12 kcal/mol). BDE(C,-H) corresponds to the BDE(viny1H)."
41
the value derived above from present kinetic data. Thus, although it may be argued that this value actually represents a lower limit to M d A ) since allylic resonance in this species involves a carbene: :CH-CH=CH-CHO and is either forbidden27or not fully developed, we conclude that the biradical mechanism for the thermal decomposition of the furan ring presented in Scheme I is entirely consistent with available kinetic and thermochemical evidence. It should be emphasized that ring breakdown seems to represent a low-energy reaction pathway for furans in contrast with the exceptional stability of pyridines.'a2* In fact it can be expected that slow but irreversible modification of coal structure would result above 450 OC as a consequence of the destruction of furan units. On the other hand, their presence indicates that fossilization of biomass over geological times could not have occurred at temperatures far above 300 OC. Further studies are underway.
Acknowledgment. This work was supported with grants from CIC, SUBCYT, and CONICET of Argentina. Mr. E. F. Valla provided valuable technical assistance. Registry No. F, 110-00-9; MF, 534-22-5; DMF, 625-86-5. (27) Davis, J. H.; Goddard 111, W. A.; Bergman, R. G. J . Am. Chem. SOC. 1976, 98, 4015.
(28) Barton, B. D.; Stein, S. E. J . Chem. Soc., Faraday Trans. I 1981, 77, 1755.
Fluorescence Probe Studies of the Interactions between Poly(oxyethy1ene) and Surfactant Micelles and Microemulsion Droplets in Aqueous Solutions Raoul Zana,* Panagiotis Lianos,*and Jacques Langt Centre de Recherches sur les Macromol&cules,Greco Microemulsion, 67000, Strasbourg, France, and Department of Physics, University of Crete, Heraklion, Greece (Received: June 26, 1984)
The interaction between poly(ethy1ene oxide) (POE) and the aggregates present in micellar solutions of sodium dodecyl sulfate (SDS), mixed micellar solutions of SDS + 1-pentanol, and oil in water microemulsions made of SDS + 1-pentanol + oil (dodecane or toluene) has been investigated by means of fluorescence probing methods. It is shown that the addition of POE results in a decrease of the aggregation number of SDS in the aggregates present in all the systems investigated. Most likely this decrease is due to the adsorption of the POE chain in the micelle palisade layer and the ensuing increase of micelle ionization.
Introduction The study of the interaction between water-soluble polymers and surfactants in aqueous solutions is interesting from both the fundamental and applied point of views. From the fundamental point of view polymer-surfactant interactions may result in changes of conformation of polymer chains.' The unfolding of globular proteins in the presence of surfactants is partly due to such interactions. On the other hand, the addition of polymers to micellar solutions of surfactants may modify the micellar properties, if interactions occur between polymer chains and surfactant m i ~ e l l e s . ~ J From the applied point of view, a better understanding of polymer-surfactant interactions may help in the formulation of the polymersurfactant systems used in enhanced oil recovery by surfactant flooding4 Polymer-surfactant interactions play also an extremely important role in detergency, where the antiredeposition agents, used to improve the detergency by avoiding the Centre de Recherches sur les MacromolEcules. *Department of Physics, University of Crete.
0022-365418512089-0041$01.50/0
soil redeposition during the rinsing cycle, are water-soluble
polymer^.^ Many of the studies dealing with polymer-surfactant interactions have focused on the thermodynamics of these interactions and/or the stoichiometry and nature of the surfactant-polymer c o m p l e x e ~ . ~ , ~The * ~ -studies ~ ~ have shown that in the sodium (1) Isemura, T.; Imanashi, A. J . Polym. Sci. 1958, 33, 337. Takagi, T.; Tsujii, K.; Shirahama, K. J . Biochem. 1975, 77, 339. Satake, I.; Yang, J . Biopolymers 1976, 15, 2263. (2) Cabane, B. J . Phys. Chem. 1977, 81, 1639 and references therein. (3) Kresheck, G.; Hargraves, W. J. Colloid InferfaceSei. 1981,83, 1 and references therein. (4) See: "Surface Phenomena in Enhanced Oil Recovery"; Shah, D. O., Ed.; Plenum Press: New York, 1981. (5) Cahn, A,; Lynn, J. In "Encyclopedia of Technical Technology"; Wiley-Interscience: New York, 1983; p 332. (6) Sasaki, T.; Kushima, K.; Matsuda, M.; Suzuki, H. Bull. Chem. SOC. Jpn. 1980, 53, 1864. ( 7 ) Fishman, M.; Eirich, F. J . Phys. Chem. 1971, 75, 3135; 1975, 79, 2740. (8) Gilanyi, T.;Wolfram, E. Colloids Surf. 1981, 3, 181; Magy. Kem. Foly. 1982, 88, 508.
0 1985 American Chemical Society
42
Zana et al.
The Journal of Physical Chemistry, Vol. 89, No. 1, 1985
dodecyl sulfate (SDS)-poly(ethy1ene oxide) (POE) system, there is no interaction between SDS and POE for SDS concentrations C below a value which is the critical micellization concentration (cmc) of SDS in the presence of POE. For cmc < C < C,, all added SDS forms micelles bound to the POE, whereas at C > C2, bound and free SDS micelles coexist in the system. The concentration C, corresponds to the saturation of POE by SDS. Until recently there had been no investigation of the effect of the polymer on the micelle aggregation number n (number of surfactants per micelle), even though some studies suggested that the micelles formed in the range between cmc and C2have n values smaller than in the absence of polymer.8~'0,1'The reason for this situation stems from the dynamic nature of micelles which prevents a measurement of n by means of classical methods12 (light scattering, osmometry, etc.). The use of a recently developed fluorescence methodI3 has permitted us to circumvent these difficulties. In a preliminary report14 we described the results obtained in a study of the interaction between SDS and POE and poly(vinylpyrro1idone) (PVP). The micellar properties of the cationic surfactant tetradecyltrimethylammonium were found not to be affected by either PVP or POE, indicating little or no interaction in these systems. The main conclusions of this study were that (i) the polymer-surfactant interactions mostly occur at the micelle surface and very strongly depend on the nature of the surfactant head group and that (ii) the SDS micelles bound to POE are smaller than the micelles formed in the absence of POE: the larger the polymer concentration or the smaller the surfactant concentration, the smaller the micelle aggregation number. I d In the present study we have extended our previous measurements to aqueous systems containing POE and mixed SDS-1pentanol micelles and POE and oil in water (O/W) microemulsions made of SDS, 1-pentanol, and oil, using the same fluorescence probing method.
Materials and Methods The samples of POE (molecular weight 20000), SDS, l-pentanol (PeOH), dodecane, and toluene were the same as in previous investigations. l 4 The micelle aggregation number n was obtained from the analysis of the fluorescence decay curve of a micelle-solubilized fluorescence probe, pyrene, in conditions where the [pyrene]/ [micelle] concentration ratio is around 1 This analysis also provided the value of the first-order rate constant kEfor excimer formation within the micelle. It is to be noted that the n and kE values obtained as indicated above correspond to values which are averaged over all the sizes of the micelles present in the surfactant-polymer system. For the present systems, the time characterizing micelle size fluctuations is much shorter than the lifetime of pyrene in the micelles owing to the small size of the micelles." In this situation, the measured n represents the number average micelle aggregation number. Nevertheless, at C values above C,, where both bound and free micelles are present in the system, some difficulties arise concerning the precise meaning of n. The ratio 11/13of the intensities of the first and third vibronic peaks of the fluorescence spectrum of micelle-solubilized monomeric pyrene was also determined. It provided an estimate of the polarity sensed by pyrene in its micellar solubilization site,I5,I6
I
I
"
J
L
Figure 1. Effect of the POE concentration on the aggregation number of SDS micelles in SDS solutions of increasing concentration: ( 0 )0.01 M, (X) 0.025 M, (+) 0.05 M, (0) 0.1 M.
1
2 Figure 2. Effect of the POE concentration on the 1,/13intensity ratio for pyrene solubilized in SDS micelles, in solutions of increasing SDS concentration: ( 0 )0.01 M, ( X ) 0.025 M, (+) 0.05 M, (0) 0.1 M, (A) 0.5 M.
0
1
+ 0 05 M SO5 30.' M SDS
15
.13915916
(9) Moroi, Y.; Akisida, H.; Masahiko, M.; Matuura, R. J . Colloid Interface Sci. 1978, 61, 233. (10) Cabane, B.; Duplessix, R. J . Phys. (Orsay, Fr.) 1982, 43, 1529. (1 1) Shirahama, K.; Tohdo, M.; Murahashi, M. J . Colloid Interface Sci.
1982, 86, 282. (12) Parfitt, G.; Wood, J. Kolloid 2.2.Polym. 1969, 229, 5 5 . (13) Atik, S.; Nam, M.; Singer, L. Chem. Phys. Lett. 1979, 67, 75. (14) Zana, R.; Lang, J.; Lianos, P. Polym. Prepr., Am. Chem. Soc., Diu. Polym. Chem. 1982, 23, 39; 'Microdomains in Polymer Solutions"; Plenum Press: New York, in press. (15) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 72, 171; 1980, 76, 62; J . Phys. Chem. 1980, 84, 3339; J . Colloid Interface Sci. 1981, 84, 100. (16) Lianos, P.; Strazielle, C.; Lang, J.; Zana, R. J . Phys. Chem. 1982, 86, 1019. (17) Almgren, M.; Lofroth, J. E. J . Chem. Phys. 1982, 76, 2734.
Figure 3. Effect of the POE concentration on the rate constant for pyrene excimer formation in SDS micelles. Surfactant concentration: (+) 0.05 M, (0) 0.1 M.
that is the micelle palisade
Results We shall consider the systems in the order of increasing comPeOH) mixed plexity, that is POE-SDS micelles, POE-(SDS micelles, and POE-microemulsion made of SDS PeOH oil (dodecane or toluene). The results relative to the first systems were briefly presented in a preliminary report.14 They will be given again as the greatly help understanding the results relative to the more complex systems. Figures 1-3 show the changes of n, 11/13:and kE for the POE-SDS micelles with the POE concentration C, (expressed in g/100 mL of solution). The additions of POE are seen to always result in a decrease of the micelle aggregation number, an increase of 1,/13 (that is of the polarity sensed by pyrene in its micellar microenvironment), and a decrease of kE. The changes of n and 11/13are self-consistent. Indeed, as n decreases, the micelles may be thought to become more penetrable to water, and thus the micelle-solubilized pyrene, which resides mainly in the palisade layer,18 will sense an increasingly polar microenvironment. However, a decrease of n should have resulted in an increase of kE.16 This effect should have been even more pronounced as
+
+
+
(18) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133; Chem. Reu. 1980.80, 283. (19) Lianos, P.; Zana, R. J . Phjv. Chem. 1984,88, 1098.
Interactions between Poly(oxyethy1ene) and Surfactants
The Journal of Physical Chemistry, Vol. 89, No. 1. 1985 43 TABLE I1
(a) Effect of Additions of Dodecane and Toluene to the 0.2 M SDS 0.6 M PeOH + 10% POE oil vol fraction n lodkE, s-' Zl/Z3 no4
+
Y
1.2 - 400 11-300
1.20 45 31 28.5 77 38 1.16 73 2% dodecane 59 27 1.35 140 4% toluene (b) Effect of Additions of Dodecane and Toluene to the 0.5 M SDS 1 M PeOH 10% POE 0
+
- 100
30
0
2
4
6
8
1
0
Figure 4. Effect of the POE concentration on the SDS aggregation number of mixed SDS-pentanol micelles in the 0.2 M SDS + 0.6 M pentanol ( 0 )and 0.5 M SDS + 1 M pentanol (X) systems and on the ll/Z, intensity ratio of pyrene dissolved in the same systems (0)and (+), respectively. TABLE I
Effect of POE Additions on the Micelle Aggregation, Rate of Intramicellar Excimer Formation, and I1/Z3Values (a) POE + 0.2 M (b) POE + 0.5 M SDS + 0.6 M SDS + 1 M PeOH system PeOH system C,"
n
0 0.5 1 2 5 10
45 42 41 40 36 31
104kpb 28 24 27 33 28 29
l,/l, 1.02 1.05 1.09 1.13 1.13 1.20
433
104kpb 2.2
ll/lq 1.07
262 165
3.2 4.6
1.15 1.19
n
Effect of Increasing SDS and PeOH Concentrations at Consant [PeOH]/[SDS] and C, [PeOH]/[SDS] = 2; [PeOH]/[SDS] = 3;
c, = 5%
ce n 10"kEb 11/13 nod n 49 1.13 26 38 0.1 24 28 1.13 45 41 0.2 36 6 1.09 150 49 0.3 120 i 20 2.3 1.09 350 97 0.4 450 f 150 e e e 165 0.5 e
c,
= 10% 10"kEb 11/13nod 26 34 24 8.8 4.6
1.37 1.25 1.24
35 35 65 1.19 173 1.19 520
OValues in g/1OO mL. bVaIues in s-l. CVaIues in moI/L. dValues in the absence of POE from ref 16. CInsoluble. preliminary fluorescence depolarization measurements performed as part of this work, using 1,3,5-diphenylhexatrienesolubilized in the POE-SDS micelle complex, showed that the addition of POE brings about a decrease of the microviscosity of the probe environment in the micelle. We also note that the change of kE with C, depends only little on the surfactant concentration. Both the decrease of kE of Figure 3 and its near independence of the surfactant concentration can be easily understood if one assumes the existence of an interaction between pyrene and POE at the micelle surface. Indeed, such an interaction would decrease. the efficiency of excimer formation, thereby reducing kE. This interaction which pulls pyrene in a more polar environment also contributes to the increase of Zl/Z3. Recall that an interaction between pyrene and PVP a t the SDS micelle surface had been suggested in a previous paper,14 in view of the very low values found for kE in these systems. On this basis the pyrene-PVP interaction appears to be much stronger than the pyrene-POE intera~tion.'~ Before going to the results relative to more complex systems, we recall that the decrease of n upon increasing C parises from the binding of SDS micelles to the POE chains (or the "adsorption" of the POE on the micelles) and the resulting increase of the micelle ionization degree.14 As a result, the micelles formed on polymer chains are smaller than in pure water. This behavior is fairly general. It has been observed at low surfactant concentration in many instances where a compound which adsorbs on, or dissolves in, the micelles is added to the micellar solution, 15,16,20,21
dodecane vol fraction
n
0 1.75 3.5 5.25 7
165 153 147 170 200
to1u en e vol fraction
n
2.4
500
+
IO"!%,
s-'
4.6 6.7 10.8 12.1 10.9
10"kE, 2.4
S-I
1,/1?
1.19 1.07 1.01
11/13
1.21
no" 520 350 250 210 200
noa >loo0
"Values in the absence of POE from ref 16.
+
The results for the mixed SDS PeOH micelles are listed in Table I and shown in Figure 4. Two series of experiments were performed. In the first series the effect of additions of POE to two strongly differing SDS PeOH solutions was examined (see Table Ia,b). For the 0.2 M SDS 0.6 M PeOH system the n value in the absence of POE (45)suggests that the micelles are most likely spherical. On the contrary, in the absence of POE the micelles in the 0.5M SDS 1 M PeOH system are strongly anisodiametric as indicated by the very large n value.16 Nevertheless, the addition of POE results in similar trends with both systems. Thus, these additions bring about an increase of 11/Z3 and a decrease of n which is more pronounced with the more concentrated SDS PeOH system. Again the increase of Z1/Z3 is due to the interaction between POE and the micelle-solubilized pyrene. Finally, kE increases with the polymer concentration for the 0.5 M SDS 1 M PeOH system but remains nearly constant for the 0.2 M SDS 0.6 M PeOH system. These results indicate that the effects of the polymer-pyrene interaction (decrease of kE) and of the decrease of n (increase of kE) nearly compensate each other for the 0.2 M SDS 0.6 M PeOH system whereas the second effect is predominant for the more concentrated system, resulting in a net increase of kE. In the second series of experiments the effect of increasing the SDS PeOH concentrations at constant [PeOH]/ [SDS] concentration ratio and constant C, was examined. The results are listed in Table Ic,d. It can be seen that the increase of the SDS PeOH concentration results in a large increase of n, a decrease of I , / Z 3 , and a decrease of kE. Here again the results are qualitatively similar to those obtained in the absence of pentanol. As the surfactant concentration is increased, there is less and less polymer available per micelle (or the amount of adsorbed polymer per micelle decreases). This results in an increase of micelle size. The decrease of Il/Z3is due to reduced POE-pyrene interactions as well as to a lesser penetrability of water in the more compact micelles. Finally, kE decreases mostly because the micelle size increases very rapidly. It is noteworthy comparing the values of n obtained in the presence and in the absence of POE (Table Ic,d). The latter, which are taken from previous studies,16 are systematically larger than the former. Thus, both in the presence and in the absence of alcohol the addition of POE always results in a decrease of micelle size. We now turn to the effect of additions of toluene or dodecane to the mixed SDS PeOH micelles in the presence of POE. The results are listed in Table I1 where are also given the values of n for the corresponding systems in the absence of POE,I6for the
+
+
+
+
+
+
+
+
+
+
(20) Zana, R.; Yiv, S.;Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208.
(21) Zana, R., unpublished results on the effects of amines, carboxylic acids, substitutedureas, nitriles, and amides on the ionization of SDS micelles.
J. Phys. Chem. 1985,89, 44-48
44
500k\
i i
iooc \
+
Figure 5. Effect of the dodecane volume fraction on the aggregation number of SDS in the 0.5 M SDS + 1 M pentanol system with (+) and without (0)added POE (10%).
purpose of comparison (see Figure 5). As for pure SDS micelles and mixed SDS PeOH micelles, the presence of POE is seen to again result in a decrease of the surfactant aggregation number for both toluene-containing and dodecane-containing systems. The decrease of I,/Z3 upon additions of dodecane to the 0.5 M SDS 1 M PeOH system in the presence of 10% POE is qualitatively similar to that observed in the absence of POE. It appears that the pyrene is increasingly solubilized in the dodecane core which forms in the micelle. This results in a decrease of ZI/Z3 and also in an increase of kE,as the average distance between two pyrene molecules within a given micelle is reduced and the microviscosity is lowered in the dodecane core with respect to the solubilization site of pyrene in the absence of oi1.16
+
+
Discussion The results presented above clearly show that additions of POE to the aggregates present in micellar SDS, mixed SDS PeOH micellar solutions, and SDS pentanol dodecane or toluene microemulsions result in a decrease of the surfactant aggregation number. The effect responsible for this decrease is most likely the adsorption of the polymer chain in the micelle palisade layer and the ensuing increase of ionization of the aggregates.14 The results listed in Table Ic,d and in Table I1 clearly show that the changes of n with the surfactant concentration and upon
+
+
oil additions have the same trends both in the absence and in the presence of POE. There are nevertheless some minor differences between the effects of oil addition without and with added POE. Thus, upon dodecane additions to 0.5 M SDS 1 M PeOH (see Table IIa) the aggregation number remains nearly constant (or may go through a shallow minimum and then increase) in the presence of POE, whereas it decreases sharply in the absence of POE. The examination of the results of Tables Ib and IIb shows that the additions of POE and dodecane to the 0.5 M SDS + 1 M PeOH mixed micelles have qualitatively the same decreasing effect on n, even though the mechanisms responsible for these decreases are completely different. The origin of the effect of POE additions has been recalled above. As for dodecane, it is dissolved in the micelle interior where it forms a core which results in a change of micelle shape from anisotropic to more spheroidal (it has been assumed that in the absence of POE a nearly spherical shape is reached at the minimum of the n vs. dodecane volume fraction plot), with an accompanying decrease of n.I6s2'
+
Conclusions The additions of POE to SDS micelles, mixed SDS + PeOH micelles, and oil in water microemulsions containing SDS, PeOH, and toluene or dodecane bring about a decrease of the surfactant aggregation number, that is a decrease of the size of the aggregate present in the systems investigated. This effect is due to the adsorption of the polymer chain in the micelle palisade layer and the ensuing increase of micelle ionization. This effect is expected to be more pronounced with polymers interacting more strongly with the aggregates. Acknowledgment. The authors thank the PIRSEM (CNRS) for its financial support (AIP No. 2004). Registry No. POE (SRU), 25322-68-3;SDS, 151-21-3; PeOH, 7141-0; dodecane, 112-40-3; toluene, 108-88-3.
(22) Lianos,
The Electronic Structure of the Low-Lying *E+and
P.; Lang, J.; Zana, R. J . Phys. Chem. 1982, 86, 4809.
2nValence States of CO'
Nobumitsu Honjout Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21 218, and Ballistics Research Laboratory, Aberdeen, Maryland 21 005
and David R. Yarkony* Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21 218 (Received: July 2, 1984)
Potential energy curves (PEC's) for the 1, 2, and 1,2, 3'II valence states of CO determined from ab initio state averaged MCSCF/first-order CI wave functions using an extended basis of Slater type functions are presented. In the region of an avoided crossing between the 2*IIand 3*nstates a three-state averaging procedure is used as the basis for the description of 3211 state. The results are compared with the minimum basis, full valence CI results of Honjou and Sasaki and the relevance to recent from kinetic energy spectroscopy results of Curtis and Boyd discussed.
1. Introduction interest in the electronic sates Recently, there has been of CO+. Four bound electronic states have been characterized from emission spectroscopy, the X2Z+(12Z+), A211(1211), B2z+(2'Z+), and C'A (1'A) states.' Additional valence electronic states have been observed in p h o t o e l e c t r ~ n ~and - ~ ultraviolet abNAS-NRC research associate. *Supported by NSF Grant CHE 81-21810. Alfred P. Sloan
Fellow.
0022-3654/85/2089-0044$01.50/0
sorption spectroscopy5 and considered theoretically a t the INDO-CI level.6 In a theoretical investigation using ab initio (1) Huber, K. P.; Herzberg, G. 'Molecular Spectra and Molecular Structure"; Van Nostrand: Princeton, NJ, 1979. (2) Potts, A. W.; Williams, T. A. J . Electron Spectrosc. Relar. Phenom. 19,4, 3, 3. (3) Asbrink, L.; Fridh, C.; Lindholm, E.; Codling, K. Phys. Scr. 1974, IO, 183. (4) Codling, K.; Potts, A. W. J . Phys. E 1974, 7, 163.
0 1985 American Chemical Society
