424
Langmuir 1992,8, 424-428
Effect of Surfactant Charge on Polymer-Micelle Interaction: n-Dodecyldimethylamine Oxide Josephine C. Brackman and Jan B. F. N. Engberts* Department of Organic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received February 4,1991.I n Final Form: October 11, 1991 The influence of the nonionic water-soluble polymers poly(viny1methyl ether) (PVME),poly(propy1ene oxide) (PPO), and poly(ethy1ene oxide) (PEO) on the aggregation behavior of n-dodecyldimethylamine oxide (DDAO), at various stages of protonation, has been studied. Critical micelle concentration (cmc) values were determined by the pH method and revealed an increase in stabilization of the micelles by association with PVME and PPO, upon increasing the average charge of the surfactant. The micelles formed from nonionic DDAO are not stabilized by association with PVME or PPO, but association was apparent from the reduction in aggregation number. This reduction in aggregation number is even more pronounced at higher surfactant charge. The results are interpreted in terms of a reduction in electrostatic inter-head-group interaction upon formation of the smaller polymer-bound micelles in the case of the charged surfactant molecules. PEO does not exert any influence on either the cmc or the aggregation number of DDAO micelles at any degree of protonation indicating the absence of polymer-micelle interaction. The effect of neutral and protonated DDAO on the clouding behavior of PVME and PPO has also been studied.
Introduction Various micelle-forming surfactants associate with nonionic water-soluble polymers in aqueous solution forming polymer-bound micelles.'s2 Polymer-micelle complexes have found applications in many industrial products, such as paints and coatings, laundry detergents, and cosmetic products, and they also play a role in tertiary oil recovery. Several aspects of polymer-micelle interaction have been investigated, and a model for the morphology of the complex has emerged.2v3 which is almost4 generally accepted. According to this model, the polymer segments are thought to reside at, and stabilize the interface between the micellar hydrocarbon core and water. The aggregation number of polymer-bound micelles is ~ m a l l e r ~and - ~ the counterion binding is lower,1° compared to that of free micelles. Usually, the polymer asserts a stabilizing influence on the micelles, which is reflected in a lower value for the critical micelle concentration (cmch2 An unchanged cmc value of the micelles, however, does not exclude polymer-micelle association, as is illustrated by the interaction between n-octyl-&D-thioglucoside(OTG) and poly(propy1eneoxide) (PPO)" or hydroxypropyl cellulose (HPC).12 A major question that remains to be solved in order to fully understand polymer-micelle interaction is the role (1)Breuer, M.M.;Robb, I. D. Chem. Ind. (London) 1972,13,530. (2)Goddard, E. D. Colloids Surf. 1986,19,255. (3)Cabane, B. J. Phys. Chem. 1977,81,1639. (4)Gao, 2.; Wasylishen, R. E.; Kwak, J. C. T. J.Colloid Interface Sci. 1990,137,137. (5) Cabane, B.; Duplessix, R. Colloids Surf. 1985,13,19. (6) (a) Gilanyi, T.; Wolfram, E. Colloids Surf. 1981,3,181. (b) Gilanyi, T.; Wolfram, E. InMicrodomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985,p 383. (7)(a) Zana, R.; Lang, J.; Lianos, P. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985,p 357. (b) Zana, R.;Lang, J.; Lianos, P. Polym. Prep. (Am. Chem. Soc., Diu. Polym. Chem.) 1982,39 (l),39. (8) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989,36,417. (9)Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985,105, 1. (10)(a) Witte,F. M.;Engberts, J. B. F. N.J. Org. Chem. 1987,52,4767. (b) Witte, F. M. Ph.D. Thesis, University of Groningen, 1988. (11)Brackman, J. C.;van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988,4, 1266. (12)Winnik, F. M. Langmuir 1990,6, 522.
0743-746319212408-0424$03.00/0
of surfactant charge on the tendency to associate with polymers. It is known, that anionic micelles formed from sodium dodecyl sulfate (SDS)and also the decyl homologue are considerably stabilized by the quite hydrophilic polymer poly(ethy1ene oxide) (PEO).293J0 Monoanionic monodecyl phosphates show a weaker association with PEO, and dianionic monodecyl phosphate does not interact a t all.13 In contrast, micelles formed from dianionic 2dodecylmalonate salts are even more stabilized by PEO than SDS mi~e1les.l~Nonionic and cationic micelles do not associate significantly with hydrophilic polymers like PE0,15J6 although a very slight influence on the aggregation of cetyltrimethylammonium bromide (CTAB) has been observed.17J8 Cationic micelles, however, are stabilized by sufficiently hydrophobic polymers, like PPO? poly(viny1 methyl ether) (PVME),lgor HPC,20but in the case of nonionic micelles, association with polymers is not found to result in micelle stabilization."J2 Nagarajan15and Ruckenstein16have suggested that the bulky size of the usual cationic and nonionic surfactants impairs the presence of polymer segments a t the corewater interface. However, this is unlikely to be the decisive factor since (i) the large headgroup of the 2-alkylmalonate disalts does not prevent a strong interaction with PE0,14 (ii) bulky hydrophobic polymers, like HPC, do associate with surfactants having large carbohydrate12 or trimethyla"onium20 headgroups, and (iii) alkylammonium surfactants do not associate with PEO,despite the small headgr0~p.l~ (13)Brackman, J. C.;Engberts, J. B. F. N. J. Colloid Interface Sci. 1989,132,250. (14)Brackman, J. C.;Engberts, J. B. F. N. Langmuir 1991,7,46. (15)(a) Nagarajan, R.; Kalpakci, B. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985;p 369. (b) Nagarajan, R.Colloids Surf. 1985,13,1.(c) Nagarajan, R. Adu. Colloid Interface Sci. 1986,26,205.(d)Nagarajan,R.; Kalpacki, B. Polym. Prep. (Am. Chem. Soc., Diu. Polym. Chem.) 1982,23(l),41. (e) Nagarajan, R. J. Chem. Phys. 1989,90,1980. (16)Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987,3,382. (17)(a) Shirahama, K.; Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987,265, 96. (b) Shirahama, K.; Oh-ishi, M.; Takisawa, N. Colloids Surf. 1989,40, 261. (18)Hoffmann, H.; Huber, G. Colloids Surf. 1989,40, 181. (19)Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1991,7,2097. (20)Winnik, F.M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91,594.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 425
Polymer-Micelle Interaction
In order to investigate the role of surfactant charge on polymel-micelle association systematically, we have studied surfactants of which the charge can be varied, without a concomitant drastic change in headgroup structure.13J4 In the present paper, we report on the influence of PEO, PPO, and PVME on the aggregation behavior of pdodecyldimethylamine oxide (DDAO). This dipolar surfactant can be transformed into the cationic form by protonation at oxygen ( ~ K B H=+5).21-23 The amine oxide surfactant has been used for several studies of the effect of charge variation (0 to +1)on micellar proper tie^^^.^^ such as the cmc,21v25 aggregation number,21~25~26 counterion binding?' surface tension:* and phase behavior.% We have found, that themicelles formed from the cationic form are stabilized by the association with PVME and PPO, due to the loss in electrostatic repulsion upon formation of the smaller polymer-bound micelles. I n contrast, the nonionic micelles of DDAO do associate with, but are not stabilized by, PVME and PPO. Experimental Section Materials. DDAO (Fluka) and PPO (weight-averagedmo-
lecular weight lOOO, Aldrich) were used as received. The purification of PVME (50% (w/w) solution in water, inherent viscosity 0.57, Aldrich)13and PEO (weight-averagedmolecular weight 10 OOO, Sigma)l3has been described elsewhere. The molecular weight of PVME (27 OOO) was determined after purification by viscosity measurements in butanone (30 OC, K = 137 X mL-gl, a = 0 ~ 5 6 1 .The ~ quencher 9-methylanthracene (Janssen) was used as received. The fluorophore bis(2,2'-bipyridyl)(4,4'-didecyl-2,2'-bipyridyl)ruthenium(lI) perchlorate was a gift from Dr. L. A. M. Rupert of the Koninklijke/Shell Laboratorium, Amsterdam. The water used in all experiments was demineralizedand distilled twice in an all-quartz distillation unit. Preparation of Stock Solutions. To a precisely measured weight of DDAO (ca. 250 mg) was added an appropriate amount of aqueous 0.2 N HCl and water until a total volume of 25 mL, which was again precisely weighed. The pH of the stock solution containing 40 mM of DDAO (corrected for the 7% water content of the commercial surfactant) was measured. The stock solutions, producingB=0,0.24,0.75,and0.98atthecmc,hadapHof7.31, 5.21,3.19, and 2.05, respectively,and were used in all experiments. They were stored at -20 OC. Cmc Measurements. An amount of stock solution was injected stepwise in 7.5 mL of water or a 1g.dL-l aqueous polymer solution, by a homemade apparatus, connected to a PC. Since the stock solutiondid not contain any polymer,dilution of polymer upon injection of the DDAO solution was unavoidable. However, at the cmc, the added amount of stock solution never exceeded 0.8 mL, corresponding to dilution to a polymer concentration of 0.9 g.dL-l, and at the end of the experiment 2.5 mL, corresponding to a polymer concentration of 0.75 g.dL-l. Each injection of 0.85 mg was followed by a delay time of 2 s, after which the pH was measured with a Corning 130 pH meter, connected via an analog-digitalconverter with the PC. The pH was plotted against the surfactant concentration, which was corrected for volume changes. From this plot the cmc was determined as the intersection point of the tangents drawn before and after the sudden change in pH (Figure 1). The cmc values were obtained at 25 OC. (21)Herrmann, K.W. J. Phys. Chem. 1962,66,295. (22)Rathman, J. F.;Christian, S. D. Langmuir 1990,6,391. (23)Tokiwa, F.;Ohki, K. J. Phys. Chem. 1966,70,3437. (24)Chang, D.L.;Rosano, H. L.; Woodward, A. E. Langmuir 1985,1, 669. (25)Herrmann, K. W. J. Phys. Chem. 1964,68,1540. (26)Ikeda, S.;Tsunoda, M. A.; Maeda, H. J. Colloid Interface Sci. 1979,70,448. (27)Rathman, J. F.;Scamehorn, J. F. Langmuir 1987,3,372. (28)Ikeda, S.;Tsunoda, M. A.; Maeda, H. J . Colloid Interface Sci. 1978,67, 336. (29)Brandup, J.; Immergut, E. H. PolymerHandbook, 2nd ed.; Wiley: New York, 1975.
PH
58
5.6
.
50
-
Aggregation Numbers. Sample solutions were made from appropriate amounts of DDAO and polymer stock solution and water. Stock solutions of fluorophore and quencher were prepared in 96% Uvasol grade ethanol (Merck). In a typical experiment, 2 pL of the fluorophore stock solution was injected into 2 mL of the surfactant solution, yielding a probe concentration of ca. 0.5 X 10-6 M. Subsequently 2-pL aliquots of the appropriate quencher solution were injected. The concentration of the quencher solution was chosen to yield a quencher-to-micelle ratio of ca. 0.8 after injection of between 8 and 20 pL. The solution in the cuvette was stirred with a magnetic device, and thermostated at 25 f 0.1 "C. Fluorescence intensities were measured using a SLM-Aminco (SPF-500C) spectrofluorometer. Excitation and emission wavelengths were 453.5 and 626 nm, respectively. The aggregationnumbers ( h l reproducibility) were determined from plots of In Z(O)-ln I([QI) versus [QI / ([CTAB] - cmc) according to the method of Turro and Yekta.30 CloudPoint Measurements. For the determination of cloud points many slightly different methods may be found in the literature.3l These have in common that they rely on a change in light transmission upon clouding. We have taken the cloud point as the temperature at which the transmission at 400 nm passes through 50%, using a Perkin-Elmer A5 spectrophotometer. For PVME, clouding occurs within such a narrow temperature range that the outcome is hardly dependent on the method. For PPO, however, clouding takes place in a temperature range of over 10 OC, but it appeared to become more cooperativewhen micelles are bound to the polymer. The polymer concentration was 0.5 gdL-l.
Results and Discussion
Cmc Values. Cmc values for DDAO at various degrees of protonation were measured using the pH method developed for phosphate surfactants.lZ This method is especially powerful for cmc measurements in polymer solutions, when many other methods fail. The method is based on the abrupt change in pH upon increasing the surfactant concentration above the cmc (Figures 1and 2). The pH change is thought to originate from a reduced tendency of the surfactant in the micellar state to take up additional charge by protonation (in the case of DDAO) or deprotonation (in the case of the phosphates). One reason is the presence of many charged headgroups close together, and another reason is the low local polarity. More precisely, the phenomenon is a matter of surfactant activity. An attractive aspect of the method is that it (30)Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978,100,5951. (31)(a) Horne, R.-A.; Almeida, J. P.; Day, A. F.; Yu, N.-T. J.Colloid Polym. Sci. 1971,35,77. (b) Saito, S. Colloids Surf. 1986,19,351. (c) Tsuchida, E.;Osada, Y.; Ohno, H. J. Macromol. Sci., Phys. 1980,B17, 683. (d) Fujishige, S.;Kubota, K.; Ando, I. J.Phys. Chem. 1989,93,3311.
Brackman and Engberts
426 Langmuir, Vol. 8, No. 2,1992 PH 7.0
1
t
6.6 6'a
/
I
00
0 5
1.0
15
20
25
30
35
LO
[OOAO] , mM
Figure 2. Plot of pH vs DDAO concentration. Data for DDAO at fi = 0 in H20. Table I. Cmc Values. of DDAO, at Various Degrees of Protonation, in the Absence and Presence of Polymers ff
Dolvmerb
0.0
0.24
0.47
0.75
0.98
1.7
1.53
1.80
2.54
4.74
1.46 1.33 1.63 PPO PVME 1.56 1.70 2.08 PEO 10 k 1.61 1.84 2.55 a In mM. Polymerconcentration: ca. 0.9g.dL-l(see Experimental Section). Calculated from the pH at the cmc using PKa = 5.0.
makes use of an intrinsic property of the surfactant, and thus avoids the problems associated with the use of probe molecules. A drawback is, however, that for DDAO the method fails a t the extremes of unprotonated (0 = 0) and completely protonated surfactant (j3 = 1)in the presence of polymers. At j3 = 0 the pH change is very small albeit in the most sensitive pH range (pH ca. 7). At /3 = 1 (pH 2-3) the swamping amount of H+ masks the pH change. In the absence of polymers, however, it is possible to determine the cmc even a t /3 = 0 and 0 = 1. Remarkably, at j3 = 0 the pH changes to higher values a t the cmc, opposite to the above considerations (Figure 2). The surfactant picks up H+ from the bulk solution upon micellization. This probably reflects that a small degree of protonation is favorable as a result of inter-head-group hydrogen-bonding i n t e r a c t i ~ n .In ~ ~salt solution, where the electrostatic repulsions are masked, this inter-headgroup attraction causes a higher aggregation number a t /3 = 0.5 than a t either /3 = 0 or j3 = 1.26 The cmc values in the absence and presence of polymers are listed in Table I. The degree of protonation reported in this table has been calculated from the pH a t the cmc using a PKAvalue of 5.0, which seems reasonable in view of the literature data.21-23 The degree of protonation is adjusted by varying the pH of the concentrated DDAO stock solution. The pH a t the cmc, and thus /3, is not noticeably affected by the presence of polymer. The cmc values of DDAO in H20, measured by using the pH method, are relatively low in comparison with those reported in the literature (Table 11). A similar observation was made in the case of the phosphate ~urfactants.'~ Presumably the method responds to even the first stages of aggregation. Within the limit of reproducibility (55% a t /3 = 0.24,2 5% a t /3 = 0.5 and 0.75) the cmc a t /3 = 0.24 is not influenced ~
(32) Imae, T.; Ikeda, S. J . Colloid Interface Sci. 1986, 113, 449.
Table 11. Literature Data on the Cmc and Aggregation Number (n)of DDAO, at Various Degrees of Protonation, in Aqueous Solution degree of
protonation @
cmc, mM
n
0 0 0 0 0 0.5 0.5 1 1 1 1
2 2.01 2.1 1.45 1.9 2.36 2.16 8 6.55 6.40 4.7
76 76 76 f 7
a
52 89 48
ref Herrman (1962)21 Ikeda (1979)26 Faucomprb (1987)O Ikeda (1978)28 Rathman (1990)22 Ikeda (1979)% Ikeda (1978)% Herrmann (1962)21 Ikeda (1979)26 Ikeda (1978)28 Rathman (1990)2*
Faucompr6, B.; Lindman, B. J.Phys. Chem. 1987,91,383.
Table 111. AGomicpol - AGo,i, for DDAO Micelles,. at Various Degrees of Protonation, in the Presence of Polymers
B polymerb 0.24 0.47 0.75 PPO -0.1 -0.8 -1.1 PVME 0.1 -0.2 -0.5 PEO 10k 0.1 0.1 0.0 In kJ-mol-1, estimated error 0.1 kJ.mol-l. Polymer concentration, ca. 0.9 pdL-l (see Experimental Section). Calculated from the pH at the cmc using pK. = 5.0. by the presence of polymers. At higher degrees of protonation the cmc is reduced in the presence of PPO and PVME but unaffected by the presence of PEO. Although it is tempting to conclude from the cmc data that the stabilization of the micelles by PPO and PVME increases with increasing micellar charge, a more quantitative conclusion should be based on a comparison of free energies of micellization in the presence and absence of polymer. In a first approximation, the free energy of micellization is related to the cmc expressed in mole fraction units according to eq l.33Thus, the change in standard free energy of the micelles AGOmic= R T In (cmc) due to the presence of polymer is given by eq 2, in which cmcp represents the cmc in the polymer solution.33 The quantity AGomic-pol- AGomic AGomic.pol- AGOmic= RT In (cmcdcmc)
(2) denotes the change in standard free energy when 1mol of surfactant molecules is transferred from regular micelles to polymer-bound micelles, plus the change in free energy of the polymer induced by this process. The results for AGOmic-pol- AGomicare presented in Table I11 and confirm the preliminary conclusion from the cmc data, namely, that the stabilization is more pronounced at higher micellar charge. At first sight this seem to agree with current views on polymer-micelle interaction2 Deeper thought reveals that although indeed the interaction with the ionic surfactant is stronger than with the nonionic surfactant, any rationalization based on headgroup volume is misplaced, Protonation will barely influence the size of the headgroup, but particularly the hydration shell will be affected. This is expected to lead to a larger (hydrated) size of the cationic headgroup. Apparently, the size of the cationic headgroup will not be much different from that of a trimethylammonium group. Since the size of the head(33) (a) Tokiwa, F.; Tsujii, K. Bull. Chem. SOC.Jpn. 1973, 46, 2684. (b) Shirahama, K.; Ide, N. J . Colloid Interface Sci. 1976, 54, 450.
Langmuir, Vol. 8, No. 2, 1992 421
PolymepMicelle Interaction Table IV. Aggregation Numbers of Micelles of DDAO, at Various Degrees of Protonation, in the Absence and Presence of Polymers Bb
polymeP
[surfactant],mM
0.0
0.24
0.47
0.75
0.98
30 20 20 20 20
75 76
70 70 46 46
72 73 43 42 71
70
66
73
67
55' 38 PPO 57' 37 PVME PEO 10 k 73' 67 73 Polymer concentration, 0.5 g.dL-l. Calculated from the pH at the cmc using pK, = 5.0.e Calculated on the assumption that the cmc in the presence of polymer equals that in HzO.
group obviously does not play a dominant role, the effect must have a different origin. We contend that the increase in stabilization of the micelles by interaction with polymers a t increasing micellar charge stems from an increasing reduction of electrostatic repulsion. Especially a t higher micellar charge the formation of smaller, polymer-bound micelles will be favored, since electrostatic repulsion is diminished and the increase in hydrocarbon-water contact area is stabilized by the polymer. Since hitherto the influence of charge has been studied by comparing polymer-micelle interaction for SDS, CTAB, and Triton X-1002 with completely different headgroups, probably toomuch emphasis has been placed on headgroup structure and size, instead of on the role of charge proper. In view of the recent results on OTG/PPOll and OTG/ HPC,12 the occurrence of polymer-micelle interaction is not necessarily accompanied by a stabilization of the micelles. Therefore, aggregation numbers have been measured to decide whether or not the absence of a reduction of the cmc in the case of DDAO (any j3)/PEO and DDAO (j3 = 0.24)/polymer points to the complete absence of polymer-micelle interaction. Aggregation Numbers. The aggregation numbers of DDAO a t various degrees of protonation have been measured by static fluorescence quenching using the system bis(2,2'-bipyridyl)(4,4'-didecyl-2,2'-bipyridyl)ruthenium(II)/9-methylanthracene.30The same fluorophore/ quencher system has been used by Warr and G r i e ~ e for r~~ the determination of the aggregation numbers of DDAO a t various j3 values, using dynamic fluorescence. Since their measurements were performed in salt solutions, in which rodlike micelles are formed, the possibility of polydispersity necessitated the analysis of dynamic fluorescence decay curves. Unfortunately the data on aggregation numbers in salt-free DDAO solutions is limited. However, our data on DDAO in the absence of polymer (Table IV) agree well with those reported in the literature (Table 11). The DDAO concentration a t which the polymers a t 0.5 gdL-l are saturated with micelles is not known. For the system SDS/PEO this concentration is 40 mM SDS in 0.5 gdL-' of PEO. Since the cmc for this system equals 5.4 mM, about 35 mM of SDS is accommodated in polymerbound micelles. Of course, this concentration will depend on the nature of the surfactant/polymer combination. Therefore we kept the surfactant/polymer ratio the same for all experiments and well below the saturation condition for PEO/SDS. The aggregation numbers of DDAO in water (Table IV) show a decreasing trend with increasing j3 as expected in view of the enhanced electrostatic repulsion. The slightly higher aggregation number at j3 = 0.47 compared to those a t B = 0.24 and 0.75 would be in accord with inter-head(34) (a)Warr, G. G.; Grieser, F. Chem. Phys. Lett. 1985,116,505. (b) Warr, G. G.; Grieser, F. J. Chem. SOC.,Faraday Tram. 1 1986,82,1829.
group hydrogen bonding being maximal. The effect is too small, however, to exclude the possibility of an experimental artifact. We emphasize that the possibility of systematic errors that may obscure a comparison is appreciably higher within a horizontal row of Table IV than within a vertical column. The data in Table IV nicely illustrate that an unperturbed cmc may have different origins. In the case of DDAO/PEO a t various degrees of protonation, the constant aggregation numbers (within confidence limits) clearly indicate the absence of interaction. However in the case of DDAO/PPO and DDAO/PVME a t low degree of protonation, the reduction in aggregation number definitely suggests polymer-micelle association, but this interaction does not lead to stabilization of the micelle. This probably originates from counteracting contributions to the total free energy from the changes in free energy of surfactant molecules and polymer upon transferring a mole of surfactant molecules from unperturbed to polymerbound micelles. Steric hindrance between the hydrated nonionic headgroups and polymer segments will be unfavorable, whereas the transfer of polymer segments, in the case of the relatively hydrophobic PPO or PVME, to the micellar phase will be favorable. Furthermore, there will be no favorable loss of electrostatic repulsion like at higher j3. The decrease in aggregation number in the presence of PPO and PVME becomes more pronounced a t higher 8. This is not surprising since a reduction in electrostatic repulsion by increasing the surface to volume ratio of the micelles will be more important a t higher micellar charge. The influence of PPO and PVME on the aggregation number is, within the confidence limits, equal, even though AGomic-pol- AGomic is clearly more negative for PPO than for PVME despite the large difference in molecular weight. It may point to stronger hydrophobic interaction for PPO compared to PVME and to a slight difference in morphology of the polymer-micelle complex due to the lower molecular weight of PPO (1OOO) compared to that of PVME (27 OOO). This matter will be discussed in the next section in which the differences in clouding behavior of PPO and PVME are presented. Clouding of PVME and PPO. PVME13t31*and PPO both have a cloud point just above 30 "C. Clouding behavior indicates a microphase separation into a polymerrich and a water-rich phase. It is thought to result from a breakdown of the hydration layer a t higher temperatures, which facilitates interpolymer interaction.31a Especially for the hydrophobic polymers PVME and PPO, the unfavorable entropy associated with hydrophobic hydration and the cooperativity of interpolymer Londen dispersion forces may drive the system toward microphase separation. The midpoint of the clouding phenomenon of PPO (32 "C) is lower than that of PVME (34 "C). For the higher molecular weights of PPO (>2000) the polymer becomes insoluble in water. It is known that PPO coils up in aqueous solution into tight disks with most of the hydrophobic methyl groups in the center of the The cloud point of PVME is raised in the presence of DDAO micelles to an extent that is almost proportional to the charge on the micelles (Figure 3). I t has been noted before2 (35) (a)Sandell,L. S.; Goring,D. A. I. Macromolecules 1970,3,50. (b) Sandell, L,.S.; Goring, D. A. I. Macromolecules 1970,3,54. fc)Sandell, L. S.; Goring, D. A. I. Makromol. Chem. 1970, 138, 77.
428 Langmuir, Vol. 8, No. 2,1992
Brackman and Engberts
T, O C
T.
t
OC
55 3
50
0
45 r
LO
0
A
35
A
) 1
0.0
I
I
0.2
0.4
30
0.6
0.8
1.0
/3
Figure 3. Cloud point of PVME in the presence of DDAO at various concentrationsof DDAO, as a function of j3: (a) 1.25 mM (0);(b) 2.5 mM (A); (c) 5 mM (A); (d) 10 mM (m); (e) 15 mM (0); (0 20 mM (0).
that the cloud point of partly hydrolyzed P V A Cor ~ ~methyl ~ e l l ~ l o s may e ~ ~ be * elevated through binding to ionic micelles. Our results for PVME/DDAO indicate that intermicellar electrostatic repulsion is the main reason for such an elevation. Since the micelles are bound to the polymer, the polymer chain segments will be held apart if the micelles repel each other. At j3 = 0, in contrast, intermicellar interaction is small or absent as shown by light scattering data32and the effect on clouding of PVME is nil. The clouding behavior of PPO is altered by DDAO in a completely different way (Figure 4). At j3 = 1,the cloud point is raised slightly less than that of PVME. But quite unexpectedly, the cloud point is elevated also a t j3 = 0, even somewhat more than a t j3 = 1. At j3 = 0.5 and a DDAO concentration of 10 or 15 mM a shallow minimum in cloud point vs j3 is observed, which may result from the optimal inter-head-groups hydrogen bonding at that j3, leading to reduced intermicellar repulsion. The deviating behavior of PPO, compared to the anticipated characteristics of PVME, may stem from a difference in aggregate morphology. PPO appears to be more hydrophobic than PVME, thus a smaller number of chain segments will protrude as loops in the solution surrounding the micelles. Furthermore, the PPO sample has a much lower molec(36) (a) Lewis, K. E.; Robinson, C. P. J. Colloid Interface Sci. 1970, 32, 539. (b) Tadros, T. F. J. Colloid Interface Sci. 1974, 46, 528. ( c ) Saito, S.; Taniguchi, T.; Kitamura, K. J. Colloid Interface Sci. 1971,37, 154.
A
0 0
, e
,
P I
