Langmuir 1994,10, 4048-4052
Effect of Temperature on the Interactions between Neutral Polymers and a Cationic and a Nonionic Surfactant in Aqueous Solutions Olivier Anthony and Raoul Zana* Institut C. Sadron (C.R.M.), 6 rue Boussingault, 67083 Strasbourg Cedex, France Received May 5, 1994. I n Final Form: August 22, 1994@ Many studies performed around 25 "C concluded that the interactions between nonionic polymers such as poly(oxyethy1ene) (POE) or poly(vinylpyrro1idone)(PVP) and cationic surfactants such as alkyltrimethylammonium halides or nonionic surfactants such as poly(oxyethy1ene) monoalkyl ethers are nonexistent or extremely weak. In the present work we have investigated the effect of temperature on these interactions by means of conductivity,spectrofluorometry,and time-resolvedfluorescence quenching. The results show that an interaction sets in between POE and the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) at a temperature T above 35 "C, resulting in the formation of POEbound 'M'AB micelles which are smaller and more ionized than free 'M'AJ3 micelles, the more so the larger the POE content ofthe system. This behavior is attributed to the decreased polarity of POE upon increasing temperature. Nevertheless, the decreased polarity of POE at high Tis not sufficient to induce interactions with the nonionic surfactant C12& even at 60 "C, as indicated by the fact that the aggregation number of C&8 micelles remained unchanged by the addition of 2% POE in the whole temperature range. No interaction occurs between PVP and TTAB even at T = 60 "C because the polarity of this polymer is not reduced at high T. Some measurements of micelle aggregation numbers have also been performed in systems containing 1%POP (poly(oxypropy1ene glycol)), which is much less polar than POE at all temperatures. The results show a very strong interaction with TTAB but only a very weak interaction with C12E8, in the range 5-20 "C.
Introduction Mixed solutions of neutral polymers and surfactants have been widely studied, and the topic has recently been reviewed. 1-4 The general conclusion is that anionic surfactants like sodium dodecyl sulfate (SDS)interact strongly with neutral polymers such as poly(oxyethy1ene)(POE) or poly(vinylpyrrolidone) (PW). This interaction takes place when the surfactant concentration, C , reaches a value often referred to as the critical aggregation concentration (cac). At C > cac, the surfactant starts binding to the polymer under the form of aggregates which are always smaller and more ionized than the micelles formed by the surfactant in the absence of polymer. Besides, the cac is smaller than the critical micelle concentration (cmc) of the surfactant in the absence of polymer. The polymer becomes saturated with surfactant when C reaches a sufficiently large value and free micelles appear in the ~ remarks must be made concerning system at C > C Z . Two the cac and C2. First, the term "cac" is not so useful. It refers to the onset of mixed micelle formation between polymer and surfactant, and there is no more significance attached to it than in the case of mixed micellization between a surfactant and any additive, an alcohol, for instance. Nevertheless, this term is used below to keep
* To whom correspondence should be addressed. Abstract published inAdvanceACSAbstracts, October 1,1994. (1)Hayakawa, K.;Kwak, J. C. T. In Cationic Surfactants; Surfactant Science Series; Rubingh, D. N., Holland, P., Eds.; M. Dekker Inc.: New York, 1991;Vol. 37,Chapter 5, p 189 and references therein. (2)Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993;Chapter 4,p 219 and references therein. (3)Saito, S.In Nonionic Surfactants. Physical-Chemistry; Schick, M. J., Ed.; M. Dekker Inc.: New York, 1989;Chapter 15,p 881 and references therein. (4)Lindman, B.; Thalberg, K. In Interaction of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993;Chapter 5,p203 and references @
therein. _.._.
(5)Tondre, C. J.Phys. Chem. 1985,89,5101. (6)Gilanyi, T.; Wolfram, E. Colloids Surf. 1981,3, 181,
in line with the literature. Second, contrary to what is sometimes stated? the concentration C2 does not correspond to the concentration of saturation, CS, of the polymer. The concentration CZ is reached when the concentration of free surfactant in the system is close to the cmc in the absence of p ~ l y m e r . ~The , ~ following relationships holds: C2 = CS cmc - cac. Contrary to anionic surfactants, cationic surfactants, such as alkyltrimethylammonium bromides, and nonionic surfactants, such as poly(oxyethy1ene) monoalkyl ethers, interact very weakly, if at all, with POE or This has been explained by the fact that these surfactants have a very large head group which prevents polymer penetration in their micelles.lS2 It has also been suggested' that the overlap between the hydration shells of the polymer and surfactant head groups in the micelles largely determines the effect of the surfactant charge type. Nevertheless, interactions between neutral polymers more hydrophobic than POE, as for instance poly(oxypropy1ene glycol), poly(methy1vinyl ether), poly(viny1alcohol), and poly(viny1 alcohol-co-vinyl acetate) and the cationic surfactant cetyltrimethylammonium bromide (CTAB),have been evidenced?-l' There has been no report of interaction between cationic surfactants such as alkyltrimethylammonium chloride or bromide and the neutral polymers POE and Interactions have been reported in systems where the halide counterions are substituted by more lyotropic counterions such as iodide or thi~cyanate,~ or salicylate.12J3 However, all of the above studies were performed at 25
+
(7) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989,36,417. (8) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1991,7,2097. (9)Shirahama, K.; Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987,265,96. (10)Reekmans, S.;Gehlen, M.; De Schryver, F. C.; Boens, N.; Van der Auweraer, M.Macromolecules 1993,26,687. (11)Sierra, M.; Rodenas, E. J.Phys. Chem. 1993,97,12387. (12)Brackman, J.C.; Engberts, J. B. F. N. J.Am. Chem. SOC.1990, 112,872. (13)Wong, T.; Liu, C. S.;Poon, C. D.; Kwoh, D. Langmuir 1992,8, 460.
0743-746319412410-4048$04.50/00 1994 American Chemical Society
Interactions between Neutral Polymers and Surfactants "C, where both POE and PVP are hydrophilic. Since POE is known to become less polar as the temperature is increased,14 as indicated by the existence of the cloud temperature above which aqueous solutions ofPOE phaseseparate,15 this polymer may interact with cationic or nonionic surfactants at high temperature. Such would not be the case for PVP, which shows no clouding phenomenon even at high temperature and, thus, probably remains sufficiently polar. The present work was undertaken to check this possibility by studying the effect of temperature on the micellar properties of two typical surfactants: the cationic tetradecyltrimethylammonium bromide (TTAB) and the nonionic octaoxyethyleneglycolmonododecyl ether (C12E8), in the presence and in the absence of POE by conductivity, spectrofluorometry,and time-resolved fluorescence quenching. For the sake of comparison we also investigated the effect of the more hydrophobic polymer poly(oxypropy1ene glycol) (POP) on the aggregation number of TTAB and C I Z Emicelles. ~ Some measurements were also performed on the system PVP/"AB. It is shown below that an interaction sets in between POE and TTAB at a temperature above about 35 "C, resulting in POE-bound TTAB micelles smaller and more ionized than the free micelles. POE does not interact with C12E8 even at a temperature as high as 60 "C. POP interacts very stronglywith'M'AB but weakly with C12E8. As expected, no interaction occurs between PVP and TTAB.
Experimental Section Materials. The investigated samples of POE (Hoechst), POP
(Aldrich), and PVP (Polymer Consultants Ltd.) had molecular weights of 20 000, 2000, and 57 000, respectively, provided by the manufacturers. They sample of POP used was of low molecular weight in order to permit studies in a sufficiently large range of temperatures. Indeed, higher molecular weight POP samples are insoluble in water. The sample of lTAl3 (Aldrich, 99%)was recrystallized thrice from a mixture ofethyl acetate and ethanol. The sample ofC1ZE8 was purchased from Nikko Chemicals (Japan) and used as received. The sample of purified pyrene (fluorescence probe) was the same as in a previous investigation.l6 Hexadecylpyridinium chloride (Fluka), recrystallized once from ethyl acetate and twice from a mixture of acetone and water, was used as a quencher of the pyrene fluorescence. Methods. The conductivity measurements were performed using an autobalanced conductivity bridge Wayne Kerr B905. The critical micelle concentration was obtained from the break in the plot of the conductivity against the surfactant concentration. The fluorescence emission spectra of pyrene solubilized in the investigated solutions were recorded using a Hitachi F4010 spectrofluorometer in the range 350-500 nm at an excitation wavelength of 335 nm. The ratio 11/13 of the fluorescence intensities of the first and third vibronic peaks was then calculated. This ratio gives a measure of the polarity of the microenvironment of pyrene in the micelles.17J8 The value of the micelle aggregation number, N , Le., the number of surfactants constituting a micelle, and the pyrene fluorescence lifetime, T, were determined in the absence and in the presence of polymer using the well-described time-resolved fluorescence quenching method.'6,18-23 The overall errors in N and t are 10 and 2%,respectively. (14)Karlstrom, G. J. Phys. Chem. 1986,87,4762. (15)$ellander, R.;Florin, E. J . Chem. Soc., Faraday Trans. 1 1981, 77,2053. (16)Binana-Limb616,W.; Zana, R. Macromolecules 1990,23,1731. (17)Kalyanasundaram, IC;Thomas, J. K J.Am. Chem. SOC. 1977, 99,2039. (18)Zana, R.Insurfactant Solutions. New Methodsoflnvestigation; Surfactant Science Series; Zana, R., Ed.; M. Dekker Inc.: New York 1987;Vol 22,Chapter 5, p 241. (19)Infelta, P. Chem. Phys. Lett. 1979,61, 88. (20) Tachiya, M.Chem. Phys. Lett. 1975,33,289. (21)Almgren, M.Adv. Colloid Interface Sci. 1992,41, 9. (22) Gehlen, M.;De Schryver, F. C. Chem. Revs. 1993,93,199.
Langmuir, Vol. 10, No. 11, 1994 4049 450,
,
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1200 1000
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*
2oo
POEZ%+TTAB TTAB
1/51
, c (mM)
0 0
I
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6
8
10
12
1
14
Figure 1. Variation of the conductivity with the TTAB concentration (A) at 25 "C and (B)at 60 "C,in water (A)and water-POE (0)mixtures. The transition range is represented by a dotted line. The plots in water have been shifted upward by 50 pS (A) and 200 pS (B)for the sake of clarity. The fluorescencedecay curves were recordedusing a previously described single-photon counting apparatus.23 In the present study, pyrene was used as a fluorescenceprobe, at a concentration [PI such that [PHM] < 0.02, [MI being the molar micelle concentration, to avoid the formation of pyrene excimers. The fluorescence quencher, hexadecylpyridinium chloride, was used at a concentration [Ql such that [Ql/[Ml 1.
Results and Discussion POE/l"AB and POPPITAB Systems. Comparison of the Effect of the Surfactant Concentration at 25 and 60 "C. Figure 1 shows the variation of the conductivity, K , of TTAB solutions with the surfactant concentration, C . At 25 "C (Figure lA), in the absence as well as in the presence of 1.7% POE, the plots are made up of two nearly linear parts with different slopes. The change of slope takes place in a narrow range of concentration which defines the cmc or the cac, here about 4 mM. The effect of 1.7% POE on the value of the cmc is seen to be small (+3%) and may reflect some minor effect of the polymer on water. At 60 "C (Figure lB), the K vs C plot in POEfree systems is still made up of two linear parts with a sharp break at the cmc which is now close to 5 mM. However, in the presence of polymer the two linear parts are separated by a curved part which stretches from about 4 to 9 mM. Such a behavior is usually found in polymersurfactant systems where a n interaction takes place between polymer and surfactanL2 The curvature prevents the determination of the cac. Below the transition range, the slope of the K vs C plot in the presence of POE is some 7.5%lower than in water whereas above this range the (23)Binana-Limb&, W.Doctorate Thesis, University Louis Pasteur, Strasbourg, 1991.
Anthony and Zana
4050 Langmuir, Vol. 10,No. 11, 1994
1.8
POE 2 % + T A B 25'C l 7 AE B.. O. T 0
t
POE2%+TTAB6O0C POE 5% + T A B 60'C
I?+\
-/ 1
c (mM)
r
1.2
0
5
10
15
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1
20
25
Figure 2. Variation of 11/13 with the TTAB concentration at 25 "C in water (A) and in water-2% POE (0)and at 60 "C in water (A),water-2% POE (01, and water-5% POE (W.
slopes of the plots in the absence and in the presence of POE are equal within the experimental error. The slopes of the K vs C plots at 60 "C show the same characteristics as a t 25 "C. The variation of the pyrene polarity index 11/13with the TTAB concentration represented in Figure 2 also reveals that an interaction takes place between POE and 'M'AB at high temperature. At 25 "C, the two 11/13 plots in the absence and in the presence of POE are quite close, with a rapid and large decrease over the 1mM concentration range just before the cmc, yielding a cmc of about 4 mM, slightly larger in the POE-containing system than in water, in agreement with the conductivity measurements. At 60 "C, the rapid decrease of 11/13is still present in the plot for the polymer-free system, with a cmc value of about 5 mM. However, at this temperature, the decrease 0fZl/Z3 for the POE-containing systems is much less steep and stretches between 3 and 11mM for the 2% POE system and between 3 and 20 m M for the 5%POE system. This behavior reveals the occurrence of interactions between POE and 'M'AB.At higher C theZ1l13values in the absence and in the presence of POE become identical. As is shown below, the size of the polymer-bound micelles increases significantly but progressively between cac and CS with C, resulting in a decrease of micelle ionization and a better shielding of micelle-solubilized pyrene. This explains the progressive changes of K and Z1/13 with C in the presence of POE. It constitutes the essential difference with polymer-free systems where the changes are more rapid because the micelle aggregation number generally increases slowly with C above the cmc. The variations of the pyrene fluorescence lifetime, t , with C represented in Figure 3 provide even clearer evidence for the occurrence of an interaction between POE and TTAB at high temperature. At 25 "C, measurements performed at C = 10 and 25 mM yielded equal t values in the absence and in the presence of 1 or 2% POE, indicating no interaction between POE and TTAB at this temperature. A decrease o f t takes place when the 'M'AB micelles appear in the solution, from 197 ns a t C < cmc to 176 ns at C = 10 or 25 mM. The data at 60 "C in the absence of polymer are more extensive than at 25 "C and are represented in Figure 3. They show that t remains nearly constant at low C and below cmc and then decreases rapidly at C > cmc owing to an increased partitioning of pyrene in favor of the TTAB micelles, where it is strongly quenched by the micelle-bound bromide i o n ~ . ~ *Such J~ a variation of t is opposite to that observed for surfactants
120' 0
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Figure3. Variation ofthe fluorescence lifetime with the 'ITAB concentration at 60 "C in water (A),water-2% POE (01, and water-& POE (m). The inset represents the same data on a more extended scale. Measurements were performed at C = 150 mM and yielded t values of 136 and 140 ns in water and water-2% POE, respectively. For the sake of comparison we have also represented the results at 25 "C in water (+) and in water-2% POE (0). with nonquenching counterions, sodium dodecyl sulfate for instance,l' where the micelles formed in the system shield pyrene from water, resulting in large increases in quantum yield and fluorescence lifetime. At 60 "C, the value of z in the presence of POE is about 20% larger than in water, revealing the existence of an interaction between pyrene and POE at this temperature (it is shown below that this interaction still persists a t 20 "C).As C is increased, t first remains constant and then, contrary to its behavior in pure water, increases when C becomes larger than about 3-3.5 mM. This value probably corresponds to the cac of the system at 60 "C. The POEbound TTAB micelles formed at C =- cac must be strongly ionized and thus rather small since the corresponding conductivity plot (Figure 1B)remains linear up to 4 mM and shows a small change of slope up to C = 6 mM. These micelles further shield pyrene from water, resulting in the observed initial increase o f t with C since there are only few micelle-bound bromide ions quenching the fluorescence of micelle-solubilized pyrene. As C is increased, the bound micelles grow in size, providing an even better shielding to pyrene and increasing further t . However, as these micelles grow, they bind a n increasing amount of bromide counterions which quench pyrene and, thus, decrease t . Owing to these two opposite effects, and since the second effect is predominant at high C, t is expected to go through a maximum, as observed. At still higher C values, free TTAB micelles form in the system, and pyrene is increasingly partitioned in these micelles. As t is smaller in free than in bound micelles, the occurrence of free micelles results in a further decrease of t at high C, as is indeed observed. The value of the cac in the system containing 2% POE is about 3 mM at 60 "C, i.e., below the value of the cmc in the absence of POE a t the same temperature (Figure 3), as expected in systems where interactions take place between polymer and surfactant.z6 "he plot for the system containing 5%POE is less well defined. Nevertheless, it indicates a cac value close to that for the 2% POE system. Recall that the cac value for the POEISDS system was found to be nearly independent of the POE content whereas the concentration Cz, above which free micelles form, (24) Malliaris, A.; Le Moigne, J.; Sturm, J.; Zana, R. J . Phys. Chem. 1985,89,2708. (25) Zana, R.; Binana-Limb&, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992,96,5461. (26) Nagarajan, R. J. Chem. Phys. 1989,90, 1980.
Interactions between Neutral Polymers and Surfactants
0
10
20
30
40
50
60
Figure 4. Variation of the fluorescence lifetime with temwater-0.5% POE (+I, water-2% POE perature in water (O),
(e),water-lOmMTTAB(r), water-0.5%POE-l0mMTTAB (O), water-1% POE-10 mM TTAB (v),and water-2% POE10 mM "TAB (0).
increased nearlylinearlywith the POE content.z A similar behavior is expected for the POE/"AB system. All the above results concur to indicate the occurrence of an interaction between POE and TTAB at 60 "C but not at 25 "C. We therefore investigated the effect of temperature at constant POE concentration in order to find out above which temperature the interaction can be detected by the fluorescence probing techniques used. Effectof Temperature in Systems ofFixed Composition. Figure 4 shows the variation of the pyrene fluorescence lifetime as a function of temperature, T , in micellar solutions of TTAB in water and in water-POE mixtures. Within the experimental error, the different plots are coincident up to T = 30 "C, showing no effect of the added POE below this temperature. At higher T , in water, t keeps decreasing upon increasing T whereas it goes through a minimum and increases with Tin water-POE systems: above 35 "C, the larger the POE content, the steeper the increase of t with T . This reveals that the polymer has become sufficiently less polar above 35 "C to be able to bind some of the initially free TTAB micelles. In this process the micelle size decreases (see below) and the micelle ionizationdegree increases,resulting in a lesser quenching and in the observed increase oft. At a given T ,the system containinga larger amount ofpolymer shows a larger increase of t because the amount of bound TTAB probably increases with the POE content, as for the POE/ SDS system.z Note that the shape of the t vs T plots for the POE-containing systems is very unusual. It arises from the presence of the bromide counterions which quench pyrene and whose bindingto the POE-bound TTAB micelles is increased with T . Peculiar t vs T plots have also been reported for the ethyl hydroxyethyl cellulose/ cetyltrimethylammonium bromide system where the hydrophobicity of the polymer also increases with T.26 It could be argued that the increase of t at T =- 35 "C is due to the fact that POE has become sufficiently less polar to bind pyrene, thereby partially shielding it from water and increasing the lifetime of its excited state, without interacting with TTAB. Some kind of partition of pyrene between POE coils and TTAB micelles would then be responsible for the minimum in the variation of t with T . This explanation does not hold in view of the values of the pyrene lifetime in water and in water-POE mixtures in the absence of TTAB obtained as part of this work and represented in Figure 4. The larger values of t found in the two water-POE mixtures with respect to water indicate that some interaction exists between pyrene and POE even below 20 " C ,in agreement with previous
Langmuir, Vol. 10, No. 11, 1994 4051
report^.^^^^^ These results also show that, at a given T below 30 "C, t is reduced in the presence of 10 mM TTAB to a value close to that in the absence of POE, indicating that pyrene is completely partitioned in TTAB micelles, whether bound to POE or free. At last, the results in water and in water-POE mixtures show a monotonous decrease of t upon increasing Tup to 60 "C. The absence of a minimum in these plots is a clear indication that the effect on t of direct POE-pyrene interactions in the presence of POE-boundTTAB micelles must be very small. Note that the value o f t in water-2% POE-10 mM TTAB is larger than in water-2% POE (the t values are nearly equal in the corresponding systems with only 0.5% POE) in spite of the strong quenching of pyrene by the bromide counterions, which reduces its lifetime from about 300 ns (valuein tetradecyltrimethylammoniumchloride micelles) to about 215 ns at 60 "C. The relative decrease o f t in water-POE mixtures is smaller than in water, probably as a result of the increased interaction between pyrene and POE, with T . Nevertheless, our results show that the presence of 10 mM TTAB is sufficient t o nearly eliminate direct POE-pyrene interactions. A more quantitative view of the effect of the POE/TTAB interactions on the micellar properties is provided by the values of the micelle aggregation number determined by time-resolved fluorescence quenching. Two difficulties arose in these determinations. First, the calculations require the knowledge of the cac or cmc.19-z3,z8The cmc and cac were determined with good accuracy (f4%)from the conductivityvs C plots at low T as long as no interaction occurred between POE and "TAB. At 60 "C, where interactions took place, the t vs C plot was used to obtain the cac values because the large ionization of the POEbound TTAB micelles at C close to the cac resulted in conductivity plots with a very gradual change of slope in the cac range (Figure 1B). Note that the error in the cac value obtained from the t vs C plots can nevertheless be large (Figure 3). In the calculations we assumed a linear change of the cac from 4 mM at 25 "C to 3 mM at 60 "C (Figures 1B and 3). The second difficulty concerns the knowledge of the concentration CZ above which free micelles are present in the system (see above). The measurements were performed at C = 10 and 25 mM, on the assumption that CZ> 25 mM. The amount of bound surfactant expressed as the ratio number ofmoles ofbound surfactant per mole ofpolymer repeat unit (stoichiometry of binding) does not appear to depend much on the polymer hydrophobicity.2 For instance, values of about 0.3 have been reported for the PVP/SDS and POE/SDS systems? even though PVP depresses the cmc of anionic surfactants more than POE at 25 "C. If the corresponding ratio is even a quarter of this value for the POE/TTAB systems at high T ,then CZwould still be higher than the maximum TTAB concentration used for a given POE concentration. The error, if any, due to an incomplete binding of the surfactant would probably affect only the results at C = 25 mM. Figure 5 represents the variations of N as a function of temperature. Both in the absence and in the presence of POE, the aggregation number is a decreasing function of the temperature. However, at a given T above 40 "C, N decreases upon addition of POE, and the magnitude of this decrease increases with the POE content as for the POE/SDS s y ~ t e m , where ~ ~ - ~interactions ~ have been fully characterized.z A decrease of N occurs as the polymer concentration is increased at constant C because the same amount of surfactant is available for an (27) Zana, R.;Lianos, P., Lang, J. J . Phys. Chem. 1985,89,41. (28) Zana, R.; Lang, J.; Lianos, P. In Miemdomains in Aqueous Polymer Solutions; Dubin, P., Ed.;Plenum Press: New York, 1985; p 357.
Anthony and Zana
4052 Langmuir, Vol. 10, No. 11, 1994 90. 80
-
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TTABIOmM PO€ 05% +TTAB 10 mM POE 1% t TTAB 10 mM POE 2% + TTAB 10 mM POP 1 % + TTAB 10 mM
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Figure 5. Variation of the aggregation number of TTAB micelles with temperaturein water (A),water-O.5% POE (o), water-1% POE (v),water-2% POE (O), and water-1% POP (0). Surfactant concentration: 10 mM.
increasing number of sites, This results in more polymerbound micelles with a lower N . Figure 5 also shows the variation of N with T in the 1% POP-10 mM TTAB system. The measurements were restricted to temperatures below 10 "C because the values of N became too small for the time-resolved fluorescence quenching method to yield accurate results. The values of N in the POP/T"AB system are much smaller than for the corresponding POE/TTAB system, indicating a much stronger interaction, expected on the basis of the more hydrophobic character of POP with respect to POE. Our results are in line with those reported for the interaction between POP and cationic surfactants.8J0J1 They are also qualitatively similar t o those found for the interaction between SDS and POE27-30and POP.31 POE/C1&e and POP/C&e Systems. The measurements of fluorescencelifetimes yielded no information on the occurrence of interaction because of the absence of quenching counterions in these systems. So, only the measurements of micelle aggregation numbers are presented. Figure 6 shows that the micelle aggregation number of the nonionic ClzE8 is not modified in the presence of 2%POE, up to a temperature of 60 "C,contrary to what was observed with TTAB. With POP an interaction sets in, as indicated by the decrease of N at T above 5 "C. Nevertheless, the decrease ofN is relatively modest compared to that for the POE/"AB system under similar experimental conditions (Figure 5). Overall, the results indicate that the interaction between POE (or POP) and the typical nonionic surfactant ClzE8 is much weaker than that with the typical cationic surfactant TTAB. This conclusion may look surprising. Indeed, ClzEs contains a large poly(oxyethy1ene)moiety which also becomes less polar as the temperature is increased, and one may have expected the POE interaction with the nonionic surfactant to become even stronger than with TTAB upon increasing T. It should be recalled that an interaction between a polymer and a surfactant takes place only if it lowers the overall free energy of the system. At higher T both POE and ClzE8 becomes less polar. Nevertheless, the interactions between the surfactant poly(oxyethy1ene)moieties in the outer shell of the micelles may be more favored with respect to interactions between these moieties and some polymer segments. Indeed, the latter involve (29) van Stam, J.; Almgren, M.; Lindblad, C. Prog. Colloid Polym. Sci. 1991,84, 13. (30) van Stam, J.; Brown, W.; findin, J.;Almgren, M.; Lindblad, C. In Colloid-Polymer Interaction; Dubin, P., Ed.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993, p 194. (31)Binana-Limb&, W.; Zana, R. Colloids Suf. 1986,21,483.
15 25 35 45 60
189 172 167 156 148
84 70 63 53 42
194 180 172 165 159
80 70 60 52 37
188 176 168 165 174
87 76 69 54 28
entropic and also conformational contributionswhich may forbid the interactions between polymer and ethoxylated nonionic surfactants. PWATAB Systems. Table 1lists the values oft and N for the 2% PVPI10 mM TTAB system at various temperatures. The corresponding values for the 10 mM TTAB solution in water are also listed. Within the experimental error, the values of N are equal to those in the absence of PVP, indicating the absence of interaction. The values of t are slightly larger in the presence than in the absence of PVP, probably because pyrene binds slightly to PVP, as in the case of POE (see above).
Conclusions The above results show that POE can interact with the cationic surfactant TTAB at temperatures higher than 35 "C, where POE becomes sufficiently less polar. This interaction results in the formation of POE-bound TTAB micelles which are smaller and more ionized than free TTAB micelles. These results are very similar t o those for the much investigated POE-SDS system around 25 "C. However, POE does not interact with ClzE8 even at a temperature of 60 "C. Contrary to POE, an increase in temperature does not induce interactions between TTAB and PW because this polymer remains hydrophilic at high T. The more hydrophobic polymer POP interacts very strongly with TTAB even at 5 "C but rather weakly with C12& in the range between 5 and 20 "C. Overall, our results confirm the conclusionof previous studies, namely, that it is the hydrophilic-lipophilic balance of the polymer which determines whether it will interact with a given surfactant. However, the relationship between polymer nature and binding stoichiometry is not clear and requires further studies.
