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Gels of Hydrophobically Modified Hydroxyethyl Cellulose Cross-Linked by Amylose: Competition by Added Surfactants Monica Egermayer, Jens Norrman, and Lennart Piculell* Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, Box 124, S-221 00 Lund, Sweden Received April 21, 2003. In Final Form: September 2, 2003 Previous work has shown that amylose (AM) can cross-link hydrophobically modified polymers by inclusion complexation, whereby thermoreversible cold-setting gels are formed. This work investigates, mainly by rheology, the competition effect of seven different anionic, cationic, and nonionic surfactants when mixed at room temperature into preformed gels of AM and hydrophobically modified hydroxyethyl cellulose (HMHEC). The aqueous mixtures of AM, HMHEC, and surfactant are compared with reference mixtures of AM-HMHEC, AM-surfactant, and HMHEC-surfactant, respectively. All the added surfactants interact with HMHEC, giving rise to the well-known increase in shear storage modulus compared to pure HMHEC solution. In addition, all added surfactants, except Triton X-100, form inclusion complexes with AM. The mechanical spectra of the AM/HMHEC/surfactant mixtures are closely similar to those containing only HMHEC and surfactant but quite distinct from that of the AM-HMHEC gel, demonstrating that all surfactants can compete with the AM-HMHEC complexation. Heat treatment of the mixtures produced no significant changes. A detailed analysis of the competition by two surfactants, sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS), showed that much larger amounts of added SOS were required for an efficient competition. The rheological characteristics indicate that the HMHEC hydrophobes are simultaneously engaged in both mixed micelles with the added surfactant and inclusion complexes with AM over a large concentration range of added surfactant.
Introduction Hydrophobically modified polymers (HMPs) attract much interest because of their viscosifying properties. An HMP consists of a hydrophilic backbone with a small amount of chemically grafted hydrophobic constituents, here called hydrophobes.1 One particularly well-studied HMP is hydrophobically modified hydroxyethyl cellulose (HMHEC), where the hydrophobes are linear hydrocarbon chains (Figure 1). In an aqueous solution above the overlap concentration, HMP molecules create a three-dimensional network, cross-linked by the intermolecular association of neighboring hydrophobes. The hydrophobic interaction gives rise to an enhanced viscosity compared to that of a solution of the corresponding nonmodified polymer. This viscosity-enhancing effect is used in, for example, paint and paper-coating applications. The viscosity of an HMP solution can be further enhanced by the addition of other amphiphilic molecules. Best known is the effect of added surfactant.2 Numerous studies have shown that the viscosity first increases, and subsequently decreases, when increasing amounts of surfactant are added to an HMP solution.3-9 The change in viscosity may be very dramatic, especially for anionic * To whom correspondence should be addressed. (1) Landoll, L. M. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443455. (2) Piculell, L.; Thuresson, K.; Lindman, B. Polym. Adv. Technol. 2001, 12, 44-69. (3) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1-21. (4) Jime´nez-Regalado, E.; Selb, J.; Candau, F. Langmuir 2000, 16, 8611-8621. (5) Tanaka, R.; Meadow, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304-1310. (6) Gelman, R. TAPPI Proceedings of the International Dissolving Pulps Conference, Geneva, 1987; TAPPI Press: Atlanta, GA, 1987; p 159. (7) Dualeh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251-255.
Figure 1. Schematic illustration of a segment from hydrophobically modified hydroxyethyl cellulose (HMHEC). R corresponds to a grafted C16 alkyl chain.
surfactants. The origin of the effect is a mixed micellization involving the HMP hydrophobes and the surfactant molecules. The viscosity increase seems to be due to an increase in the lifetime of the micellar cross-links,4,9-12 whereas the decrease is caused by a decrease in the network connectivity. The latter effect results from an increase in the number of mixed micelles at high levels of added surfactant, which implies a decrease in the number of hydrophobes per mixed micellar cross-link. Ultimately, the hydrophobic cross-links are totally destroyed, and the viscosity at very high levels of added surfactant becomes lower than that of the surfactant-free HMP solution. Recently, a few studies have shown that amylose (AM) can also interact with HMP hydrophobes, giving rise to (8) Thuresson, K.; Lindman, B.; Nystro¨m, B. J. Phys. Chem. B 1997, 101, 6450-6459. (9) Piculell, L.; Egermayer, M.; Sjo¨stro¨m, J. Langmuir 2003, 19, 3643-3649. (10) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 7099-7105. (11) Piculell, L.; Nilsson, S.; Sjo¨stro¨m, J.; Thuresson, K. ACS Symp. Ser. 2000, No. 765, 317-335. (12) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-4918.
10.1021/la0301689 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/22/2003
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to give insights into not only the AM/HMP interactions but also the AM inclusion complexation in general. A preliminary study of surfactant competition was included in our previous study on AM-HMHEC complexation.24 There we found that added sodium dodecyl sulfate (SDS) could indeed prevent the AM-HMHEC complexation. In the present paper, we report on more extensive studies, involving several surfactants that would be expected to differ in their capacity to compete with the AM-HMHEC complexation. We also investigate the role of different sample preparation procedures, such as cold and hot mixing. The study is mainly based on rheological measurements. Figure 2. Mechanical spectrum for an AM-HMHEC gel with 0.25% amylose and 1% HMHEC: storage moduli (filled circles) and loss moduli (open circles).
a gelation of the mixed solutions.13-16 AM is the essentially linear component of starch and is composed of glucose residues linked by (1f4)-R-D-glucosidic bonds. Pure AM forms a crystal of double helices. When certain hydrophobic ligands are introduced in an AM solution, inclusion complexes may form, where the AM molecule adopts a single-helix conformation with the ligand included in the hydrophobic inner cavity. AM has long been known to form inclusion complexes with molecules such as iodine,17 alcohols,18,19 fatty acids,20,21 and surfactants.22,23 This inclusion complexation is also the basis of the AM/HMP mixed gelation where, presumably, an amylose molecule cross-links two or more HMP molecules by forming inclusion complexes with some of their hydrophobes. Recently, we demonstrated such a mixed gelation for mixtures of AM and HMHEC.24 Addition of AM to HMHEC resulted in a strongly enhanced viscosity, and sufficient amounts of added AM created a thixotropic gel, lacking a Newtonian plateau at accessible shear rates. Since AM is insoluble in cold water, it had to be dissolved by heating prior to mixing it with HMHEC, to yield a clear gel showing a strong synergism. The mixed system displayed a gellike response on oscillatory measurements, with a nearly frequency independent shear storage modulus (see Figure 2). Mixed gels melted on heating above 60 °C and reformed reversibly on subsequent cooling. The mixtures showed clear signs of saturation of the hydrophobes, when sufficient amounts of AM had been added. The effects of added surfactant on the AM/HMP interactions are of obvious interest. As detailed above, surfactants can complex with both the AM and the HMP components; moreover, they are ubiquitous in applications where HMPs are utilized. From a fundamental point of view, surfactant competition experiments may be expected (13) Okaya, T.; Kohno, H.; Terada, K.; Sato, T.; Maruyama, H.; Yamauchi, J. J. Appl. Polym. Sci. 1992, 45, 1127-1134. (14) Shogren, R. L.; Greene, R. V.; Wu, Y. V. J. Appl. Polym. Sci. 1991, 42, 1701-1709. (15) Shogren, R. L. Carbohydr. Polym. 1993, 22, 93-98. (16) Gruber, J. V.; Konish, P. N. Macromolecules 1997, 30, 53615366. (17) Rundle, R. E.; Edwards, F. C. J. Am. Chem. Soc. 1943, 65, 22002203. (18) Hinkle, M. E.; Zobel, H. F. Biopolymers 1968, 6, 1119-1128. (19) Kowblansky, M. Macromolecules 1985, 18, 1776-1779. (20) Eliasson, A.-C. Thermochim. Acta 1994, 246, 343-356. (21) Godet, M. C.; Bule´on, A.; Tran, V.; Colonna, P. Carbohydr. Polym. 1993, 21, 91-95. (22) Yamamoto, M.; Harada, S.; Nakatsuka, T.; Sano, T. Bull. Chem. Soc. Jpn. 1988, 61, 1471-1474. (23) Lundqvist, H.; Eliasson, A.-C.; Olofsson, G. Carbohydr. Polym. 2002, 49, 43-55. (24) Chronakis, I. S.; Egermayer, M.; Piculell, L. Macromolecules 2002, 35, 4113-4122.
Experimental Section Materials. Potato amylose with a molecular weight of about 800 000 g/mol was obtained from Sigma Chemical Co. Prior to use, the AM was placed in an oven at 80 °C for 1 h to remove butanol, which is used as a precipitant in the isolation of AM from starch. HMHEC with the commercial name Natrosol Plus grade 331 was obtained from Aqualon (Figure 1). According to the manufacturer, the HMHEC sample had a molar weight of 250 000 g/mol. Previous analysis of another batch of this product showed that it carried grafted C16 alkyl chains corresponding to a modification degree of 1.7 mol %, based on anhydroglucose units in the cellulose backbone, and the degree of hydroxyethyl group substitution was 3.3.10 Prior to use, HMHEC was dissolved in water to a concentration of 1%. Low molecular impurities, such as salt, were removed by dialysis against Millipore water in a Filtron Ultrasette device. The dialysis was performed until the expelled water showed a conductivity of less than 2 µS/cm. The polymer was freeze-dried in a Drywinner 6-85. In the present investigation, the HMHEC concentration was 1%, which is well above the overlap concentration (ca. 0.2%).25 The surfactants (see Table 1) were utilized as received and stored in a desiccator or a refrigerator until used. Sample Preparation. Samples containing HMHEC, AM, and surfactant were prepared by mixing stock solutions according to the following procedure. The desired amount of HMHEC was dissolved in Millipore water for a minimum of 12 h. An AM solution was prepared by heating on a Pierce Reacti-Therm to 155 °C for approximately 15 min and was then allowed to cool to just below 100 °C. The hot AM solution was added to the previously prepared HMHEC solution. The AM-HMHEC sample was agitated with a magnetic stirrer for at least 8 h until a macroscopically homogeneous sample was formed. A stock solution of the surfactant was added to the AM-HMHEC sample followed by 8 h of stirring. Finally, the sample was equilibrated for at least 8 h without any agitation. Reference samples containing only two of the solutes were prepared by similar procedures, omitting one of the stock solutions. Degassed Millipore water was used in all samples. AM and HMHEC concentrations are given as weight percent throughout this work. Rheometry. Rheological measurements were performed on a controlled stress Physica USD200 rheometer using a coneand-plate geometry (50 mm radius, 1°). All measurements were performed within the linear viscoelastic region, which was checked for each sample. Each gel was placed on the plate thermostated at 25 °C and allowed to equilibrate for at least 30 min prior to the measurements. The equilibration time was introduced since a few of the gel samples were quite viscous and thixotropic. Oscillatory tests were made in the frequency range 0.01-10 Hz. The above experimental procedure allowed the recording of the storage modulus (G′) and the loss modulus (G′′) as functions of angular frequency of oscillation.
Results and Discussion Dependence of Interactions on Surfactant Architecture. Prior to this work, only one study of surfactant competition with the AM-HMHEC complexation had (25) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443-459.
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Egermayer et al. Table 1. List of Surfactants Used
a
Values from ref 26 except where indicated. b Values from ref 27. c Values from ref 28.
been performed, for the specific case of SDS.24 Our first aim in this work was therefore to investigate a range of surfactants, to check for possible dependencies on the surfactant architecture. This part of the study had the character of a screening, involving a matrix of samples designed to reveal interaction and competition effects as sensitively as possible. Choice of Surfactants. The chosen surfactants are listed in Table 1. The list includes surfactants with different headgroups (anionic, cationic, and nonionic), all with a tail length of 12 carbons. These surfactants were SDS, C12TAC, C12E5, and C12E8. We also wished to investigate effects of the architecture of the hydrophobic part of the surfactant. Effects of tail length may be studied by comparing SDS with SOS. Moreover, some surfactants contain bulky hydrophobic groups that may be expected to affect their possibility to form inclusion complexes with amylose. Just like SOS, SOBS has an eight-carbon hydrocarbon tail, but it also contains an additional phenyl group as a ”spacer” between the tail and the anionic headgroup. The hydrophobic phenyl group facilitates surfactant micellization, and the critical micelle concentration (cmc) of SOBS is, in fact, similar to that of SDS. The nonionic surfactant Triton X-100 has an oligo(ethylene oxide) headgroup, just like C12E5 and C12E8, but its tail is very bulky, containing both a phenyl group and a branched tert-octyl hydrocarbon tail. Choice of Mixtures and Concentrations. As in our previous study, we chose HMHEC as our HMP. We chose a concentration of 1%, which represents a “standard” concentration of HMHEC, where one obtains strong effects, such as viscosity enhancements, both with added AM and with added surfactants. An AM concentration of 0.22% was chosen, since our previous investigation showed that this concentration of AM gave a maximum in G′ for mixtures with 1% HMHEC. The surfactant concentration was chosen individually for each surfactant to correspond to the viscosity maximum in a solution containing only the surfactant and 1% HMHEC. For most of the surfactants, the exact location of this maximum was not known from previous experiments. Therefore, we used for all surfactants the following
approximate relation, based on a relation that has previously been shown to roughly predict the location of the viscosity maximum in mixtures of HMHEC with various surfactants.8,9,29
cs(ηmax) ) 4ch + 0.8cmc + cin
(1)
Here cs(ηmax) is the total concentration of the added surfactant at the viscosity maximum, the numerical factor 4 represents the approximate ratio of bound surfactant molecules per hydrophobe at the viscosity maximum, ch (in this case 0.5 mM) is the concentration of hydrophobes in 1% HMHEC, and 0.8cmc is the concentration of free (monomeric) surfactant at the viscosity maximum. (See the original work29 for a detailed explanation.) The last term, cin, has been added here and represents the concentration of surfactant molecules required to saturate AM by inclusion complexation. This term was added since inclusion complexation with AM should consume some surfactant and, hence, shift the maximum slightly. Although cin should vary between the surfactants, a common value of 0.5 mM was chosen, based on an estimate on the concentration of SDS required to saturate 0.22% AM.30,31 Three series of mixtures with the various surfactants were prepared, each including a surfactant-free reference. The first series contained 0.22% AM together with the surfactants. This series gave information on AM-surfactant interactions. The second series contained 1% HMHEC mixed with surfactants and gave information on HMHEC-surfactant interactions. The final series (26) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971. (27) Tenside, Product information from Serva Feinbiochemica, Serva Feinbiochemica, Heidelberg, Germany. (28) Product Sheet, Nikko Chemicals, Tokyo, 1996. (29) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307-318. (30) Godet, M. C.; Tran, V.; Colonna, P.; Buleon, A.; Pezolet, M. Int. J. Biol. Macromol. 1995, 17, 405-408. (31) Yamamoto, M.; Sano, T.; Harada, S.; Yasunaga, T. Bull. Chem. Soc. Jpn. 1983, 56, 2643-2646.
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Table 2. Apperance of Samples Containing 0.22% (w/w) Amylose and Various Surfactants surfactant
surfactant concn (mM)
appearance of precipitate
Triton X-100 C12E5 C12E8 SOS SDS SOBS C12TAC
none 2.7 3.3 2.5 109 9.0 11.6 18.9
white, flakelike white, flakelike white, cloudy white, cloudy white, cloudy no visible precipitate no visible precipitate no visible precipitate
contained mixtures of all three components, prepared by cold mixing of a stock surfactant solution with a preformed AM-HMHEC gel. (See the Experimental Section for details on the sample preparation.) In the case of a strong surfactant competition, the rheological properties of the third mixture should be similar to that of the corresponding AM-free HMHEC-surfactant mixture. On the other hand, if there were no competition, the properties should be similar to those of a surfactant-free AM-HMHEC gel (see Figure 2). Amylose-Surfactant Mixtures. Table 2 describes the appearances of samples prepared by mixing hot amylose solutions with surfactant solutions and then letting the samples cool to room temperature. As is commonly known, AM alone falls out as a white precipitate from an aqueous AM solution below the overlap concentration, c* = 1-1.5%.32 In the surfactant-free AM sample, a white flakelike precipitate appeared, which could take several days to form, depending on the initial concentration. However, sedimentation of the precipitate took only minutes. Interestingly, one other sample was similar in appearance to the surfactant-free AM sample. This was the mixture with Triton X-100; those two samples looked identical. This suggests that Triton X-100, with its bulky, branched tail, does not form an inclusion complex with AM. No precipitation occurred in the mixtures with the three ionic surfactants with low cmc’s, i.e., SDS, SOBS, and C12TAC. This is a clear indication of a complexation with AM, whereby water-soluble complexes are formed. Presumably, the water solubility is due to the charge of the surfactants. A third type of behavior was found for the mixtures with the nonionic, nonbranched surfactants, C12E5, and C12E8, and with SOS, an ionic surfactant with a high cmc. These samples contained precipitates that looked different from the precipitate in surfactant-free AM. The precipitates were more finely dispersed, and the sedimentation took several hours. A similar precipitation was found to occur with SDS at high concentrations or with SDS and added salt.33 Our interpretation is that the finely dispersed precipitate consists of an insoluble inclusion complex. The inclusion complex is insoluble when there is no electrostatic stabilization, because either the surfactant is nonionic or, as for the other mentioned cases, the concentration of noncomplexed surfactant or added salt is sufficiently high. HMHEC-Surfactant Mixtures. As expected, these mixtures were clear without any sign of phase separation. At the concentrations chosen according to eq 1, all surfactants gave rise to an increase of the viscosity of the 1% HMHEC solution, compared to the surfactant-free reference solution. However, the magnitude of the viscosity enhancement varied greatly between the different surfactants, as is well-known from previous studies.4,5,8,9,34 AM-HMHEC Gels with Added Surfactant. When a surfactant solution was mixed cold with the preformed
AM-HMHEC gel, the gel was destroyed in all cases except for Triton X-100. Viscous liquids of varying viscosity were formed, with properties quite similar to those of the AMfree HMHEC-surfactant mixtures. The most extreme effect was found for SOS, which turned the AM-HEC gel into a quite low-viscous solution. Thus, it is quite clear that added surfactant can compete with the AM-HMHEC complexation, even if added cold to an already developed AM-HMHEC complex. Triton X-100 behaved differently from the other surfactants. When the stock surfactant solution was mixed into the AM-HMHEC gel, the result was a “smashed gel”. The smashed gel contained gel lumps surrounded by a low-viscous solution. Vigorous shaking could turn the smashed gel into a homogeneous, smooth, and viscous liquid which, on standing for a couple of hours, again transformed into a homogeneous gel. To examine this surfactant more closely, different concentrations of Triton X-100 in the AM-HMHEC gel were tested. By increase of the Triton X-100 concentration, the mixture with AM-HMHEC gel appeared less heterogeneous. The highest concentration used (10.55 mM) turned the AM-HMHEC gel to a viscous liquid which did not gel upon standing. Rheology. Oscillatory measurements were performed on all HMHEC-surfactant mixtures and all AMHMHEC-surfactant mixtures, except for the mixtures with Triton X-100, which differed quite substantially from those with the other surfactants. For each of the quaternary mixtures, the effect of heat treatment was also investigated. Each sample was divided into two equal portions. One portion was kept at room temperature, while the other portion was heated to 80 °C for approximately 5 min, whereby it lost its high viscosity, and was subsequently allowed to cool to room temperature. We have previously shown that also surfactant-free AM-HMHEC gels melt and re-form by such a heat treatment.24 For each surfactant, the mechanical spectrum looked very similar for all three samples: the AM-free sample and the heated or nonheated samples containing AM. This confirms the picture obtained from the visual inspection, that all surfactants are able to totally destroy the AMHMHEC complexes and engage the HMHEC hydrophobes in “conventional” surfactant complexes. Figure 3 compares the “mechanical spectra” for heated and nonheated samples. The SDS mixture (Figure 3a) was virtually unaffected by heat treatment. Even for the C12E8 mixture (Figure 3b), which showed the largest difference between heated and nonheated samples, the spectra look very similar. The spectra in Figure 3 also illustrate the wellknown differences in viscosifying effect for different surfactants. The spectrum for SDS shows a viscoelastic behavior, with a crossover between G′ and G′′ at high frequencies. By contrast, the spectrum for C12E8 is liquidlike, with G′ much smaller than G′′ over the accessible frequency range. Table 3 summarizes the results for all surfactants. Although the moduli differ between the heated and the nonheated samples, there is no clear trend, and we regard the differences to be within the expected experimental error. Detailed Studies for SDS and SOS. On the basis of the results for the entire set of surfactants, we decided to make detailed measurements, covering a large surfactant (32) Gidley, M. J.; Bulpin, P. V. Macromolecules 1989, 22, 341-346. (33) Karlberg, M. Lund University, 2002. (34) Panmai, S.; Prudhomme, R. K.; Peiffer, D. G. Colloids Surf., A 1999, 147, 3-15.
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Figure 3. Mechanical spectra showing storage moduli (filled symbols) and loss moduli (open symbols) for nonheated (circles) or heated (squares) mixtures of AM-HMHEC-SDS (a) or AMHMHEC-C12E8 (b). See Table 3 for sample compositions.
concentration range, for SDS and SOS. SDS was a natural choice, since its interactions with both AM and HMHEC are well-studied. SOS was chosen for the following reasons. It has the same headgroup as SDS, but a shorter tail length, and has previously been shown to interact weakly, if at all, with AM.30 It also has a high cmc. Thus, SOS is expected to compete much more weakly than SDS with the AM-HMHEC complexation. SDS. Figure 4 summarizes the results for SDS. The solid circles show the variation of the shear storage modulus at a single frequency (1 Hz) in mixtures containing 1% HMHEC and 0.25% amylose and varying concentrations of SDS, obtained by cold mixing of stock surfactant solutions with preformed AM-HMHEC gels. For reference, data on mixtures of SDS with 1% HMHEC, taken from an independent study using the same equipment and measurement protocol,9 are also included. The latter data show the normal behavior for a semidilute solution of an HMP on addition of increasing amounts of a
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surfactant: The storage modulus first increases to a maximum, in this case at around 7 mM SDS, and subsequently decreases to very low values. The molecular interpretation of this nonmonotonic behavior was summarized in the Introduction; more detailed discussions are given in previous work.3-9 The storage modulus for the series of samples containing AM starts off at the high value obtained for the surfactantfree AM-HMHEC gel (see Figure 2). With added SDS, the modulus first decreases, until a minimum is reached at ca. 6 mM SDS. After the minimum, the variation of G′ resembles that for the AM-free system, with an increase to a maximum around 8 mM SDS, followed by a decrease to very low values. We emphasize at this stage that the data in Figure 4 refer to a single frequency of oscillation. Since the frequency dependence of G′ varied among the samples, Figure 4 does not give the complete picture of the variation in rheology with surfactant content. For this, we must look at the full mechanical spectra shown in Figure 5. At and below an SDS concentration of 2 mM (Figure 5a), the rheology is characteristic of an AM complex, i.e., it shows the gel-like features of a surfactant-free AM-HMHEC gel (cf. Figure 2) with a weak dependence of G′ on frequency. By contrast, at 6 mM SDS (Figure 5c) and beyond, the systems are viscoelastic solutions with spectra characteristic of HMHEC-SDS mixtures. At 4 mM SDS (Figure 5b), the spectrum resembles neither one nor the other of the two characteristic spectra. This suggests that the HMHEC hydrophobes in this particular mixture are complexed with both AM and SDS. Yamamoto et al.22 have determined isotherms for the binding of SDS to AM. For long AM chains, as in our study, they obtained a maximum binding of 0.04 SDS molecules per anhydroglucose unit at saturation, which was reached at a free SDS concentration of 0.1 mM SDS. Using these values, we find that a concentration of 0.6 mM SDS would be sufficient to saturate 0.25% AM and that saturation should occur at a total SDS concentration of 0.7 mM in a solution free from HMHEC. Clearly, this type of binding isotherm is not obeyed in the mixtures with HMHEC. If that had been the case, the minimum in the G′ data would have appeared already at 0.7 mM SDS, since all AM-HMHEC complexes would then have been replaced by AM-SDS complexes. Evidently, the AMHMHEC complex competes rather well with the AMSDS complex (and with the HMHEC-SDS mixed micelle), since an SDS concentration of the order of 6 mM is required to completely remove the 0.5 mM of hydrophobes from the complexes with AM. In principle, the relatively inefficient competition observed for SDS could be a kinetic effect, since the samples were obtained by cold mixing stock surfactant solutions with preformed AM-HMHEC gels obtained by cooling hot mixtures. To check for this possibility, the mechanical spectra of the mixed gels containing 1 and 2 mM SDS were measured again after a heating of the mixtures to 80 °C and a subsequent cooling to 25 °C. According to the NMR measurements of our previous study,24 the AM-HMHEC complexes are broken at 80 °C. However, the storage moduli remained high after this heat treatment (see Figure 4), and the mechanical spectra were still gel-like (not shown), as before the heat treatment. A final remark on the data in Figure 4 concerns the shift in the viscosity maximum with added AM. The magnitude of this shift is ca. 1 mM, which roughly corresponds to the expected saturation binding of SDS (cf. above). That is, the shift toward higher SDS concen-
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Table 3. Comparison of Heated vs Nonheated Samples Containing 1% HMHEC, 0.22% Amylose, and Various Surfactants (taken at a frequency of 1 Hz) appearance of spectra
storage modulus G′ (Pa)
loss modulus G′′ (Pa)
surfactant
concn (mM)
nonheateda
heateda
nonheated
heated
nonheated
heated
C12E5 C12E8 SOS SDS SOBS C12TAC
3.3 2.5 109 9.0 11.6 18.9
VE LV LV VE VE LV
VE LV LV VE VE LV
5.9 0.42 1.2 34 28 2.3
4.3 0.44 0.087 39 25 3.2
6.5 1.2 3.0 24 28 4.3
5.5 2.2 1.3 22 26 6.5
a
LV ) low viscous, VE ) viscoelastic.
Figure 4. Storage moduli at 1 Hz for 1% HMHEC samples at different SDS concentrations. Symbols refer to samples containing no AM (open circles), samples produced by cold-mixing with preformed AM-HMHEC gels (filled circles), and the latter samples after heating (filled triangles, point down).
trations can be explained by postulating that ca. 1 mM of the SDS has been consumed by inclusion complexation with AM. SOS. For SOS, no previous data on the interaction with HMHEC alone were available. Hence, we made parallel rheological investigations of two series of samples, differing only by the presence or absence of 0.25% AM. The variation of G′ with surfactant concentration, given in Figure 6, shows the same trends for the two series of samples as the data for SDS in Figure 4. However, there are interesting quantitative differences. The data for the AM-free samples show a viscosity maximum of a similar magnitude as for SDS but displaced toward much higher surfactant concentrations, as expected from the much higher cmc. One can immediately note, however, that the G′ maximum for this surfactant appears at a surfactant concentration that is significantly lower than the rough prediction (109 mM; see Table 3) given by eq 1. This explains the low viscosities obtained for the SOS samples tested in the screening experiment (Table 3). The curve for the samples with AM shows that in order to reach the G′ minimum, a much higher surfactant concentration is required for SOS (ca. 25 mM), than for SDS (ca. 6 mM). Moreover, quantitative differences in the moduli of the AM-containing and AM-free mixtures persist up to surfactant concentrations far above the G′ minimum; the two curves merge only at 80 mM. This difference is significant, since the measurements were performed in parallel, in exactly the same fashion, on the two series of samples. We can think of no other explanation for this difference than that a significant fraction of AM-HEC complexes survive up to 80 mM SOS. Certainly, a possible binding of SOS to amylose cannot explain the apparent shift of the two curves along the surfactant concentration axis below 80 mM SOS. The magnitude of the shift (20 mM) is at least an order of magnitude larger than any reasonable saturation binding of surfactant to AM; see
Figure 5. Mechanical spectra showing storage moduli (filled circles) and loss moduli (open circles) for AM-HMHEC-SDS samples at SDS concentrations of 2 (a), 4 (b), or 6 mM (c).
above. Moreover, as already noted, the shift disappears above 80 mM SOS. This merging of the curves suggests
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Figure 6. Storage moduli at 1 Hz for 1% HMHEC samples at different SOS concentrations. Symbols refer to samples containing no AM (open circles) or samples produced by cold-mixing with preformed AM-HMHEC gels (filled circles).
that the amount of surfactant bound to amylose is completely negligible. In Figure 7, we compare the mechanical spectra of AMcontaining and AM-free samples for selected SOS concentrations. Figure 7a shows that at 10 mM SOS, both samples are quite similar to the corresponding surfactantfree samples: The AM-containing sample is gel-like, while the AM-free sample is a low-viscous solution. At 25 mM SOS (Figure 7b), corresponding to the G′ minimum for the series with AM, the viscosity of the AM-free sample has increased significantly, while the AM-containing
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sample shows a mechanical spectrum that resembles neither that for the AM-HMHEC gel nor those characteristic of viscoelastic surfactant-HMHEC solutions. The latter type of spectrum is, however, obtained for both samples at 50 mM SOS (Figure 7c), but the spectrum for the sample with AM is displaced toward higher oscillation frequencies. Finally, at 80 mM SOS (Figure 7d), the two mechanical spectra practically overlap. Mechanisms for Surfactant Competition. Added surfactant can compete with the AM-HMHEC complexation by two mechanisms: 1. The added surfactant can compete with the hydrophobe for inclusion complexation with AM. 2. Mixed micelles with the surfactant offer an alternative binding site for the hydrophobes. Both mechanisms may be expected to operate at least for surfactants, such as SDS, that are known to form inclusion complexes with AM. By the methods of the present investigation, it is not possible to show directly that surfactant-AM inclusion complexes are formed when surfactant solutions are mixed cold with AM-HMHEC gels, but there are some indirect indications. One such indication is the shift of the viscosity maximum with SDS, which could mean that some SDS is removed from the mixtures by binding to AM. For Triton X-100, no inclusion complexation occurs, and therefore, mechanism 1 is absent. Possibly, this is the reason for the weak effect for Triton X-100, compared to the other surfactants. This result could thus, indirectly, suggest that the stronger effect for the other surfactants is caused by a significant contribution from mechanism 1. On the other hand, the weak effect of Triton X-100 may also be due to a weak
Figure 7. Mechanical spectra showing storage moduli (filled symbols) and loss moduli (open symbols) for HMHEC solutions (squares) and AM-HMHEC-SOS samples (circles) at SOS concentrations of 10 (a), 25 (b), 50 (c), and 80 mM (d).
Surfactant Effects on Gels
competition through mechanism 2, i.e., that the micelles of this surfactant offer a less attractive site for the HMHEC hydrophobes. What seems clear, however, is that mechanism 1 is not very efficient, at least for the surfactants studied here. Thus, AM-HMHEC complexes obviously survive at an SDS concentration 1 order of magnitude higher than that required for saturation binding to HMHEC-free AM and at even higher concentrations for SOS. One reason the HMHEC hydrophobes so successfully compete with the surfactants for the inclusion sites should be their length (16 carbons, as opposed to e12 for the surfactants). The difference between SDS and SOS clearly demonstrates that the length of the surfactant tail has a strong influence on its ability to compete with the AM-HMHEC complexation. Cooperativity may be another reason; it is possible that more than one hydrophobe from a HMHEC molecule can bind to the same AM molecule. If the role of mechanism 1 is uncertain, the rheology experiments show conclusively that mechanism 2 operates, since the mixtures display all the characteristic features of HMHEC-surfactant complexation. This mechanism is also sufficient to explain the difference between SOS and SDS, since much higher concentrations of the former surfactant are needed to form mixed micellar complexes. Conclusions A number of micelle-forming surfactants, representing different types of hydrophilic headgroups and hydrophobic tails, have been tested for their ability to compete with the AM-HMHEC complexation. All tested surfactants have this ability, even when mixed cold into a preformed AM-HMHEC gel, but some surfactants compete more efficiently than others. Surfactants with straight hydro-
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carbon tails are more efficient than those with a branched tail. For homologous surfactants with straight hydrocarbon tails, the efficiency increases with the length of the surfactant tail. All tested surfactants interact with HMHEC, but not all of them interact with AM. SDS, SOBS, and C12TAC form soluble complexes with AM, while C12En and SOS probably form insoluble complexes. There are two possible mechanisms for the competition of surfactants with the AM-HMHEC complexation: 1. Surfactant molecules compete with the hydrophobes for inclusion complexation with AM. 2. Mixed micelles with the surfactant offer an alternative site for the hydrophobe. Both mechanisms probably operate, except for surfactants with bulky tails (such as Triton X-100) which do not form inclusion complexes with AM. Rheology experiments conclusively confirm the existence of mechanism 2. Both mechanisms should become more efficient for an increasing length of the hydrocarbon tail. The AM-HMHEC complexes survive even comparatively large amounts of added SDS or SOS, much larger than the amount required to saturate AM in the absence of HMHEC. Rheology suggests that AM-HMHEC and HMHEC-surfactant complexes coexist up to quite high surfactant concentrations for these surfactants, beyond the viscosity maximum of the corresponding AM-free system. Acknowledgment. This work was financed by the Centre for Amphiphilic Polymers from Renewable Resources (CAP) at Lund University (M.E.) and by the Swedish Research Council (L.P.). LA0301689
