5342
Langmuir 1998, 14, 5342-5346
Articles Interaction between Poly(acrylic acid) and Nonionic Surfactants with the Same Poly(ethylene oxide) but Different Hydrophobic Moieties Dan F. Anghel,* Shuji Saito,† Adriana Ba˜ran, and Alina Iovescu Department of Colloids, Institute of Physical Chemistry “I. G. Murgulescu”, Spl. Independent¸ ei 202, 77208 Bucharest, Romania Received July 15, 1997. In Final Form: May 18, 1998 The effect of homogeneous nonionic surfactants (CnE8 with n ) 10, 12, and 14) on the solution behavior of poly(acrylic acid) (PAA) was investigated by surface tension and viscometry. The first method allowed determination of the critical micelle concentration (cmc), the critical aggregation concentration (cac or T1), the saturation of the polymer and the onset of free micelles into solution (T2) and an intermediate concentration (T2′) that was defined as the stoichiometric concentration for binding. The T1 points were lower than cmc’s, and the change of T1 vs the number of carbon atoms in the alkyl chain of the surfactants obeys the same rule as the cmc does. The energy change in transferring one methylene unit from micellar to water environment was nearly the same for micelles and complex and proves that both phenomena have similar driving forces. Viscometric data evidenced in turn the T1 and Tv points. Tv is the minimum point that appears in the viscosity curve of PAA-nonionic surfactant systems and T2 by surface tension was a little higher than Tv by viscometry. The longer the alkyl chain of the surfactant, the lower was Tv. The composition of complexes at Tv was nearly constant and suggested that a considerable number of ethylene oxide groups do not participate in complex formation. The effect of surfactants on PAA at Tv was compared to that of inorganic electrolytes (i.e., HCl and NaCl), and the following order was established: surfactant < NaCl e HCl. The results revealed the important role played by hydrogen bonding and hydrophobic forces in PAA-nonionic surfactant interaction, and the data were discussed in light of the latest experimental and theoretical achievements about the mechanism of complex formation.
Introduction Aqueous mixtures of polymers and surfactants have both fundamental and technical importance. The unusual behavior of these systems can be traced by complex formation between the micellized surfactant and polymer. Complexes have properties that are quite different of the individual components, and this is beneficial in the practice. Many industrial processes and products can be cited as examples, including enhanced oil recovery, emulsion polymerization, foodstuffs, pharmaceuticals, cosmetics, detergents, coating fluids, inks, and paints.1 The specific functions of biological membranes may also depend in some subtle way on the interaction between proteins and lipids.2 In synthetic polymer-surfactant systems, polyelectrolytes and oppositely charged surfactants produce the strongest association.3,4 Although less effective, the interaction between ionic surfactants and nonionic polymers is evident for anionics, and the classical example † Present address: Nigawa-Takamaru 1-12-15, Takarazuka 665, Japan.
(1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 395. (2) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 113. (3) Hayakawa, K.; Kwak, J. T. In Cationic Surfactants. Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 189. (4) Wei, Y. C.; Hudson, S. M. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1995, C35, 15.
keeping the stage for more than 40 years is the sodium dodecyl sulfate (SDS)-poly(ethylene oxide) (PEO) system.5-14 As for ethoxylated nonionic surfactants, they bind easily to polymers that develop hydrogen bondings. Such polymers are poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA), and they were particularly studied in relation to ethoxylated nonionic surfactants with normal distribution of the PEO chain length.15-22 For instance, the main findings of these investigations with PAA were as follows: (a) The complexes form when surfactant exceeds a distinct concentration (i.e., the critical aggregation concentration, cac or T1), that is usually lower than the critical micelle concentration (cmc). (b) In a (5) Saito, S. Kolloid Z. 1957, 154, 19. (6) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (7) Schwuger, M. J. J. Colloid Interface Sci. 1972, 43, 491. (8) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (9) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41. (10) Winnik, F. M.; Winnik, M. A. Polym. J. 1990, 22, 482. (11) van Stam, J.; Almgren, M.; Lindblad, C. Prog. Colloid. Polym. Sci. 1991, 84, 13. (12) Veggeland, K.; Austad, T. Colloids Surf., A 1993, 76, 73. (13) Nassar, P. M.; Nogueira, L. C.; Bonilha, J. B. S.; Leiva, O. W.; Quina, H. F. J. Brazil. Chem. Soc. 1995, 6, 173. (14) Minatti, E.; Zanette, D. Colloids Surf., A 1996, 113, 237. (15) Saito, S.; Taniguchi, T. J. Colloid Interface Sci. 1973, 44, 114. (16) Saito, S. Colloid Polym. Sci. 1979, 257, 266. (17) Saito, S. In Nonionic Surfactants. Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 881. (18) Saito, S. J. Am. Oil Chem. Soc. 1989, 66, 987. (19) Saito, S. Rev. Roum. Chim. 1990, 35, 821. (20) Saito, S. J. Colloid Interface Sci. 1993, 158, 77. (21) Baranovsky, V. Yu.; Shenkov, S.; Borisov, G. Eur. Polym. J. 1993, 29, 1137. (22) Saito, S. J. Colloid Interface Sci. 1994, 165, 505.
S0743-7463(97)00788-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/15/1998
Polymer-Surfactant Mixtures
Langmuir, Vol. 14, No. 19, 1998 5343
Table 1. Cmc for CnE8 Surfactants and Their T1 and T2 by Surface Tension and Tv by Viscosity with 10.2 mM PAA, and the Composition of Complexes at Tv and 25 °C surfactant C10E8 C12E8 C14E8 a
cmc (mol/L) 10-3
1× 8 × 10-5 9 × 10-6
T1 (mol/L) 10-4
7× 6.3 × 10-5 7 × 10-6
T2 (mol/L) 10-3
4× 2.8 × 10-3 4 × 10-3
Tv (mol/L) 10-3
3.0 × 2.4 × 10-3 2.1 × 10-3
EO(Tv-cmc)/PAAa 1.6 1.8 1.7
EO ) 8.
homologous surfactant series, T1 depends on the hydrophobic moiety and is independent of PEO chain length and temperature. Recently, the response of polymeric acids to surfactants homogeneous in the PEO hydrophilic group was examined.23-25 The data collected on systems containing PAA and octaethylene glycol mono(n-dodecyl) ether (C12E8) showed no interaction and an adsorbed film made of surfactant at pH greater than or equal to 8. At pH ) 3 and above T1, the surface tension isotherm had a higher plateau and the adsorbed layer was much thicker than that of the pure surfactant.24 Investigations on PAA-C12E6 and -C12E8 systems revealed several critical surfactant concentrations for interaction and showed the importance of ethoxylated and hydrophobic moieties of the surfactant in complex formation.24 Subsequent examination of these systems by dynamic fluorescence probe technique pointed out aggregation numbers for the free C12E6 and C12E8 micelles of about 105 and 83, respectively.25 It was also found that PAA wrapped around the surfactant clusters and decreased the aggregation numbers by factors of about 15%. More recent studies unveiled that the PAA coil is a spheroid,26 and the addition of C12E8 alters both the volume and the shape of the polymer coil.27 This paper is a continuation of our previous work and aims to investigate the effect of hydrocarbon chain length of the surfactant on the interaction between PAA and nonionic surfactants with the same homogeneous PEO moiety (i.e., octaethylene oxide units) but having different alkyl chain lengths (i.e., decyl, dodecyl, and tetradecyl). The first objective was to check up whether the onset of complex formation obeys the same relationship as that existing between the cmc and the alkyl chain length of the surfactants. Second, we determined the composition of complexes and found that the amount of surfactant bound on the polymer at the viscosity minimum (Tv) point was roughly identical despite the different alkyl chain length of the surfactants. Finally, the mechanism of complex formation was discussed. Experimental Section Materials. Poly(acrylic acid) was obtained from Wako Pure Chemical Industries Ltd., Osaka, Japan. The sample was an aqueous solution with a concentration of 25.5%. It had a molecular weight determined viscometrically of about 150 000 and was used as received. Octaethylene glycol mono(n-decyl) (C10E8), and -(n-tetradecyl) (C14E8) ethers were supplied by Fluka Chemie AG (Buchs, Switzerland), whereas the octaethylene glycol mono(n-dodecyl) ether (C12E8) was a Nikko Chemicals Co., (Tokyo, Japan) product. They were used without purification. All the other chemicals were of reagent grade. The water used to prepare the solutions had an electrical conductivity lower than 1.5 µS/ cm. (23) Maloney, C.; Huber, K. J. Colloid Interface Sci. 1994, 164, 463. (24) Anghel, D. F.; Saito, S.; Iovescu, A.; Ba˜ran, A. Colloids Surf., A 1994, 90, 89. (25) Vasilescu, M.; Anghel, D. F.; Almgren, M.; Hansson, P.; Saito, S. Langmuir 1997, 13, 6951. (26) Raicu, V.; Ba˜ran, A.; Iovescu, A.; Anghel, D. F.; Saito, S. Colloid Polym. Sci. 1997, 275, 372. (27) Raicu, V.; Ba˜ran, A.; Anghel, D. F.; Saito, S.; Iovescu, A.; Ra˜doi, C. Prog. Colloid. Polym. Sci. 1998, 109, 136.
Figure 1. Surface tension isotherms of C10E8 and C14E8 without and with 10.2 mM PAA at 25 °C. Methods. Surface tension measurements were made by the platinum ring method using the same procedure and corrections as previously described.28 The viscosities were determined with an Ostwald viscometer having a flow time for water of about 100 s. The reduced viscosity was calculated by dividing the specific viscosity (ηsp) to the polymer concentration (cPAA) in weight percent. In the ηsp calculations either the surfactant or the HCl or the NaCl solution was considered as the solvent. Throughout the measurements, the PAA concentration was kept constant at 10.2 mM. It was selected according to the findings in previous studies on PAA-nonionic surfactant interaction.15-17,24 The pH of PAA solution was of 3.4. The pH of the PAA-CnE8 systems lies within 3.4 and 3.6 and is optimal for complex formation in the soluble state. Neither the pH nor the ionic strength was adjusted during the determinations. The measurements were carried out at 25 °C.
Results Figure 1 shows the surface tension results for C10E8 and C14E8 in the absence and presence of PAA. The isotherms are similar to those reported for the same polymer and C12Em24 and homogeneous and polydisperse PEO nonylphenols.29 The cmc and T1 values are summarized in Table 1. For comparison reasons, the data for C12E8 and C12E8-PAA are also included. Cmc’s agreed with the literature values.30 For surfactants with the same PEO moiety, the longer the alkyl chain, the lower T1. The surface tension curve for C10E8 or C14E8 in the presence of 10.2 mM PAA abruptly broke at T1 below the cmc. Beyond the T1 it ran first flat, then dropped at T2′, and finally joined at T2 with the surface tension curve of the respective surfactant solutions without PAA. The T1 and T2 values are given in Table 1. It may be that above T1 the surfactants are preferably bound to PAA in a (28) Radu, M.; Popescu, G.; Anghel, D. F. Kolloid Z. Z. Polym. 1973, 251, 1039. (29) Anghel, D. F.; Winnik, F. M.; Galatanu, N. Colloids Surf., A, in press. (30) Meguro, K.; Ueno, M.; Esumi, K. In Nonionic Surfactants. Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 109.
5344 Langmuir, Vol. 14, No. 19, 1998
Anghel et al.
Figure 2. Dependence of cmc and T1 on the alkyl chain length of CnE8.
solution was of 6.31 dL/g. The Tv and T2 data are separately given in Table 1. Even though each Tv is a little lower than the respective T2, the unbound surfactant concentration at Tv was approximated as being equal to the cmc. This allows an estimate of the amount of surfactant bound to the PAA, or the mole ratio of EO per carboxyl of the complex at the viscosity minimum point. The ratios are of about 1.7 in the three surfactants (see Table 1). The figure is comparable to those found in PAApolydisperse PEO nonionic surfactants15 and in PAAC12Em systems24 and reveals that at Tv a considerable number of ethylene oxide units are free or they form loops or cross each other in the complex. A recent study showed that at 10 mM PAA T2 and Tv are coincidentally close to each other, but at higher PAA concentrations T2 is increasingly higher than Tv.32 Table 1 and Figure 3 show that the longer the alkyl chain, the lower Tv, but their (Tv-cmc) nearly agree considering the experimental error. At the same time, the height of the viscosity curve at the bottom point is roughly the same. These trends were also observed in previous studies on PAA-C8E20, -C12E20, and -C16E20 systems and on PAA-C8E8 and -C12E8 systems, though C8 surfactants showed somewhat higher height, and also on PAA-C12E6,8,20&50 systems, all with polydisperse PEO moieties15,17 and on PAA-C12E6 and -C12E8 systems with homogeneous PEO moieties.24 Though the contraction of complex at Tv was unaffected, it was suggested that the dominant factor in complex formation is the hydrogen bonding between EO and carboxyls. The change induced by additives on the reduced viscosity of PAA is due either to contraction or to network formation. The former reduces the dimension of the polymer coil and decreases the viscosity. The latter has an opposite effect and leads to gelation, which obviously is not our case because the concentration of PAA is low. Discussion
Figure 3. Changes in the reduced viscosity of 10.2 mM PAA solution induced by CnE8 surfactants and electrolytes at 25 °C.
micellar state (complex formation), and at T2 the complexes and free micelles begin to coexist; that is, the free surfactant concentration reaches the cmc. The relationships between cmc or T1 and the carbon number (nC) in the alkyl moiety of the surfactants are illustrated in Figure 2. The plots are linear, almost parallel to each other, and obey the eq 1 formerly proposed by Shinoda et al.31
ln cmc (or T1) ) -nCω/kT + constant
(1)
where ω, k, and T are, respectively, the energy change in transferring one methylene unit from micellar to water environment, the Boltzmann constant, and the absolute temperature. In parallel measurements, the reduced viscosity of the PAA-nonionic surfactant systems showed a transition point at T1. When the surfactant concentration was increased, it took a valley form with a minimum point at Tv. The results for the three surfactants with PAA are presented in Figure 3. On the same figure are plotted the data for PAA-NaCl and PAA-HCl. Although not shown in Figure 3, but for comparison reasons, we have to mention that the reduced viscosity of the 10.2 mM PAA (31) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223.
To interpret the data, one has to admit that poly(acrylic acid) associates with the nonionic surfactants and forms complexes. The data in Figure 1 show that polymer induces serious changes in the surfactant surface tension isotherm. Unlike the surfactant, where only the premicellar and the micellar regions are present, the polymersurfactant system has three break points (T1, T2′, and T2) that delineate four separate domains of surfactant concentration (Cs). In the first domain, below T1, the polymer and surfactant molecules are supposed to be free in solution, and T1 represents the beginning of surfactant fixation on the polymer chain. Let us consider latter the T2′ point, which in surface tension isotherms is rather a domain, and to refer first to the literature data concerning the overall polymer-surfactant interaction.33 They stress that in the domain, T1 < Cs < T2, most of the surfactant is bound to the polymer in equilibrium with the free molecules. T2 corresponds to the polymer saturation and to the formation of surfactant micelles in the bulk. It is the limit of complex formation that can be due at least to three reasons:34 polymer saturation with surfactant molecules, too large interaction between the bound molecules, and polymer-free micelle formation in the bulk when the surfactant activity becomes sufficient. By applying eq 1 to the cmc and the T1 data in Figure 1, one obtains for the free micelle and the complex ω values (32) Anghel, D. F.; Saito, S.; Ba˜ran, A.; Iovescu, A. Unpublished data. (33) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 123 and 171, and references therein. (34) Gilanyi, T.; Wolfram, E. Colloids Surf. 1981, 3, 181.
Polymer-Surfactant Mixtures
of -1.15 kT (-2.85 kJ/mol) and -1.18 kT (-2.92 kJ/mol), respectively. It means that the alkyl moieties in the free and PAA-bound micelles are in the same state and supports the model that micelles in the complex are wrapped around by the PAA chain through hydrogen bondings with the PEO moieties.20 Such a dependence was initially observed in systems of poly(vinylpyrrolidone) (PVP) and sodium alkyl sulfates with ω ) -1.1 kT (-2.73 kJ/mol) for both free micelle and complex, and the authors concluded that the surfactant molecules are bound on polymer not singly but in a micellar form.31 The present ω values compare favorably with the increment per methylene group to transfer alkyl hexaethylene glycol monoethers from water to micelle (ω ) -1.16 kT, or -2.87 kJ/mol).35 For nonionic polymer-anionic surfactant systems, the T2′ concentration deserved less attention, and it may be due to the fact that this point is not defined so sharp as T1 and T2 are. However, an attentive examination of the early experiments reported by Jones,6 Schwuger,7 and Shinoda et al.,31 clearly indicate a change in the variation of the surface tension for a concentration near the T2′ value. Francoise et al.,36 observed in turn three changes in the slope of the PEO-SDS conductivity curves and assigned them to the T1, T2′, and T2. In our previous data on PAA-C12Em,24 and PAA-homogeneous and polydisperse PEO nonylphenols,29 these points are likewise detected. It was assumed that just above T1, all the added surfactant molecules bind to the polymer, the concentration of free surfactant remaining constant and equal to T1 up to a surfactant concentration T2′, which is defined as the stoichiometric concentration for binding.36 In an intermediate region, between T2′ and T2, the concentration of free surfactant is assumed to increase from T1 to cmc.37 In this model, the amount of bound surfactant is (T2cmc). One could suggest that T2′ corresponds to the saturation of the available sites with isolated molecules or micelles of low aggregation number and that, in the region T2′-T2, the added surfactant molecules partially contribute to increasing the aggregation number of the micelles and partially are dissociated in the bulk. In the following discussion we will try to make more clear the case of PAA-nonionic surfactant complexes. To understand what is happening in an aqueous solution of PAA and PEO nonionic surfactant, let us consider first the simpler case of proton-acceptor polymers. There is enough evidence at present to prove that polymers such as PEO and PVP bind small protic molecules such as carboxylic acids or phenols in aqueous solution.38,39 These polymers also form complexes with poly(carboxylic) acids by cooperation of many hydrogen bondings. The importance of hydrogen bondings in complex formation is stressed by the fact that complexation appears in a narrow pH range. For example, there is no PAA-PEO complex above pH 5, and below pH 3 it is insoluble and has stoichiometric composition with respect to proton donor and acceptor,40 except each of them are too bulky.41 The equilibrium of complex formation between proton-donor (35) Tanford, C., The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1973; p 45. (36) Francois, J.; Dayantis, J.; Sabbadin, J. Eur. Polym. J. 1985, 21, 165. (37) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (38) Inoue, M.; Otsu, T. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 1933 and 1939. (39) Molyneux, P. Water-Soluble Synthetic Polymers: Properties and Behavior; CRC Press: Boca Raton, FL, 1984; Vol. 2. (40) Ikawa, T.; Abe, K.; Honda, K.; Tsuchida, E. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 1505. (41) Jaycox, G. D.; Sinta, R.; Smid, J. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 162.
Langmuir, Vol. 14, No. 19, 1998 5345
and proton-acceptor polymers depends on the molecular weight of both components.40,42 To illustrate, high molecular weight polymeric acids generate stable complexes with PEO of polymerization degree above 200 for PAA and 40 or less for poly(methacrylic acid) (PMA), which is more hydrophobic than PAA.40,43 In contrast to PAAPPO systems, the clouding of PMA-poly(propylene oxide) (PPO) solutions in appropriate conditions does not appear even at 90 °C, though without the polymeric acid, the PPO solution is cloudy at room temperature.44 These results emphasize the importance of hydrophobic attraction in interpolymer complexation. The above findings hold also true in polymeric acid interactions with ethoxylated nonionic surfactants, and this is supported by both the present results and the literature data.15,17,24,45 Our data in Figure 3 refer to the change of viscosity in the PAA-CnE8 mixtures. The reduced viscosity is a function of hydrodynamic volume, polymer-polymer interaction, and polymer-solvent interaction.46 Since the solvent power of water is not greatly affected by the presence of the surfactant, the changes in reduced viscosity can be considered to be due to the changes in the hydrodynamic volume of the polymer and the polymer-polymer interactions. The latter may also be neglected in our case because the polymer concentration employed was quite low. Therefore, the only component contributing to the change of our reduced viscosity data is the hydrodynamic factor. Within the investigated surfactant concentration range, the pH was between 3.4 and 3.6, and no precipitate appeared. The curve has a deep minimum and compares favorably with that of Sivadasan and Somasundaran47 reported for the interaction of C12E8 with hydrophobically modified hydroxyethylcellulose (HMHEC). It indicates that at a particular PAA/surfactant ratio the complex is mostly contracted and more stretched in both of its sides. By PEO, however, the reduced viscosity of the PAA solution was affected to a much lesser degree.15,17 Because of carboxyl fixation with EOs, the pH of solution rose, but the increase was also much higher by nonionic surfactants than by free PEO. Thus, the PAA-EO interaction is remarkably reinforced in nonionic surfactants by the presence of a big hydrophobic moiety. In other words, two-point binding of the surfactant is the basis of a strong interaction as described in ref 48. Additional information is obtained by substituting the surfactants with inorganic electrolytes. The data displayed in Figure 3 show that both sodium chloride and hydrochloric acid lower the viscosity. The effect by NaCl is not so strong as that by HCl, but they are leveling at high concentrations. These results suggest that in aqueous 10.2 mM PAA solution, which has a pH of about 3.4, not all the carboxylic groups of the polymer are in the H form. A very small part of them are dissociated, and this allows for both the initial high viscosity of polymer solution (i.e., 6.31 dL/g) and also for its behavior in the presence of inorganic additives. The differences between the inorganic electrolytes arise because the salt only shields the polyelectrolyte charges whereas the acid suppresses (42) Kokufuta, E.; Yokota, A.; Nakamura, I. Polymer 1983, 24, 1031. (43) Eagland, D.; Crowther, N. J.; Butler, C. J. Eur. Polym. J. 1994, 30, 767. (44) Saito, S. Colloids Surf. 1986, 19, 351. (45) Musabekov, K. B.; Abdiev, K. Zh.; Aidarova, S. B. Kolloid Zh. 1984, 46, 376. (46) Moore, W. R. Prog. Polym. Sci. 1967, 1, 3. (47) Sivadasan, K. B.; Somasundaran, P. Colloids Surf. 1990, 49, 229. (48) Saito, S.; Anghel, D. F. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; p 357.
5346 Langmuir, Vol. 14, No. 19, 1998
them all. It seems that the big molar volume of the surfactant does not affect significantly upon the collapsed PAA coil, and the hydrophobic moiety in the range of C10C14 has no influence on the size of complex at Tv. One implies that complex contraction is due to the consecutive multiple H-bonds whose number is independent of the hydrophobic moiety, as the EO(Tv-cmc)/PAA is the same. The surface tension data in Figure 1 show that PAACnE8 systems have several break points and behave similarly to those of nonionic polymer-anionic surfactants already discussed.6,7,31,36 Except for these and for our former results,24,29 there is according to our knowledge no information to reveal such a comportment in PAAnonionic surfactant mixtures. Early works by one author of this group inferred to a minimum point of viscosity,15,17 which proved to be very closed to the present T2. More recently, the data collected by surface tension and ellipsometry on systems containing PAA and C12E8 at pH 3 confirmed the existence of T1 and revealed an adsorbed layer above T1 much thicker than that of surfactant alone.23 It would have been, however, profitable if those authors had not limited their investigations to a concentration range around T1. The results in Figures 1 and 3 illustrate that interaction begins at T1. To explain why it does come about in the PAA-CnEm systems, one has to admit that, as for the PAA-PEO systems, the binding implies the cooperativity of many hydrogen bondings,40,41,44 to which the hydrophobic effect is added. This account may explain the decrease of viscosity observed in Figure 3, but it does not make clear why the minimum point Tv appears. The answer may arise by taking into consideration the information by fluorescence24,25 and dielectric spectroscopy.26,27 Static fluorescence measurements unveiled that in the presence of PAA the aggregation starts at T1, which is slightly lower than cmc, and the polarity sensed by the pyrene probe is about the same in polymer-bound and normal micelles.24,25 These findings confirm the early statement made by Saito that nonionic surfactants and PAA interact and form aggregates below the cmc.17,20 He explained this phenomenon in terms of interactions of the surfactant molecules with the PAA chains through their PEO parts, which facilitate the subsequent aggregation by hydrophobic attraction. Dynamic fluorescence allowed calculation of an aggregation number of about 70 for the C12E8 micelles bound to PAA in the T1-T2 range.25 The method also revealed that the polymer wraps the surfactant aggregates, and on an average, in the same T1-T2 range, the number of aggregates per PAA chain was not higher than three. Dielectric spectroscopy measurements carried out on a PAA sample similar to that in the present study, put into evidence that the polymer random coil is a spheroid. It has a radius of the equivalent sphere of about 22 nm,26 which roughly agreed with the gyration radius of some 15 nm determined by quasi-elastic light scattering.49 Dielectric spectroscopy also demonstrated that the volume (49) Anghel, D. F.; Alderson, V.; Winnik, F. M.; Mizusaki, M.; Morishima, Y. Polymer 1998, 39, 3035.
Anghel et al.
of polymer coil continuously decreases after T1 whereas the shape-dependent depolarization factor (A) suddenly changes at T2′.27 The A parameter increases from a value of 0.33 corresponding to a sphere to another of 0.8 for oblate ellipsoids. The change is not visible from viscosity data because the decrease of complex volume is stronger and dominates over the effect induced by modification of shape. This lasts until the viscosity minimum point is reached. Beyond Tv the rising is steeper no matter the alkyl chain length of the surfactant. Therefore, the minimum point displayed on the viscosity curves is the result of the competition between these two effects: the volume contraction before Tv and the change of the shape afterward. In the interaction of polymeric acid and nonionic surfactant, besides the mode of surfactant organization on the polymer chain, two factors should be taken into consideration: the binding of surfactant as a whole molecule, and the degree of hydrogen bonding in the hydrophilic moiety. The change of the reduced viscosity is a hydrogen-bond-sensitive phenomenon and not directly related to the binding as a whole molecule. Conclusions We examined the behavior of PAA and homogeneous PEO nonionic surfactants as a function of the hydrophobic character of the surfactants. As revealed by the surface tension, the interaction begins at T1 (below cmc) and ends at T2. The change of T1 with the alkyl carbon number in the surfactant molecule obeys the same rule as the cmc does. As a consequence, the computed energy changes in transferring one methylene unit from an ethoxylated nonionic surfactant micelle to water or from a PAAsurfactant complex to water were practically equal to each other and prove the important role played by hydrophobic forces in polymer-surfactant interaction. T2 is coincidentally close to the viscosity minimum point Tv at low PAA concentration. The composition of complexes at Tv suggests that not all the ethylene oxide units participate in complex formation. Many of them are free or form loops or cross over each other in the complex. Another point T2′ , which lays between T1 and T2, comes to the attention. Although not visible in the viscosity and rather ill-defined in the surface tension curves, it was clearly evidenced by previous dielectric spectroscopy measurements as a point where the polymer coil suddenly changes the shape under the effect of the surfactant.27 To our opinion, this is the first paper that confirms the early finding of T2′ in nonionic polymer-anionic surfactant systems36 and makes it clear that T2′ has a more general meaning than initially stated. The surfactant affects both the dimension and the shape of the polymer coil, and the competition between them is the key factor that generates the minimum of viscosity curve. Acknowledgment. The authors thank Dr. V. Raicu from Department of Physiology, Kochi Medical School, Kochi, Japan, for helpful discussion and the reviewers for helpful criticism. LA9707888
