Mixed Solutions of an Associating Polymer with a Cleavable Surfactant

University of Technology, SE-412 96 Go¨teborg, Sweden, and Camurus AB, Ideon Science. Park, Gamma 2, So¨lvegatan 41, SE-223 70 Lund, Sweden...
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Mixed Solutions of an Associating Polymer with a Cleavable Surfactant Maria Karlberg,† Maria Stjerndahl,‡ Dan Lundberg,‡,§ and Lennart Piculell*,† Center for Chemistry and Chemical Engineering, Lund University, Box 124, SE-221 00 Lund, Sweden, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden, and Camurus AB, Ideon Science Park, Gamma 2, So¨ lvegatan 41, SE-223 70 Lund, Sweden Received May 5, 2005. In Final Form: August 17, 2005 Mixtures of hydrophobically modified hydroxyethyl cellulose (HMHEC) and alkali-sensitive cleavable betaine ester surfactants have been studied by viscometry, 1H NMR, absorbance measurements, and birefringence determinations. Before the hydrolysis, the surfactants behaved as conventional nondegradable surfactants in terms of the effect on the viscosity of increasing surfactant concentration. As the surfactants were hydrolyzed, systems with time-dependent viscosity were obtained. The viscosity either decreased monotonically or went through a maximum as a function of time, depending on the initial surfactant concentration. Different surfactant chain lengths gave rise to different viscosity profiles. The rate of hydrolysis, and thus the time-dependency of the surfactant concentration, could be controlled by changing the pH of the solution.

Introduction A hydrophobically modified polymer (HMP) consists of a hydrophilic polymer backbone to which a small amount of strongly hydrophobic groups (so-called hydrophobes) have been grafted. Compared to solutions of nonmodified polymers, semidilute solutions of HMPs exhibit enhanced viscosity because of reversible intermolecular associations between the hydrophobes. Interactions between HMPs and surfactants have been a subject of intensive research during the past decades.1 The mixed aggregates between surfactants and polymer hydrophobes have been investigated by various techniques, such as rheology, NMR, cloud point measurements, dynamic light scattering, and time-resolved fluorescence quenching.2-7 The solution is strongly affected by the presence of the mixed aggregates. For instance, the viscosity of a mixed solution of HMP and spherical micelles goes through a pronounced viscosity maximum at a specific surfactant-to-polymer hydrophobe ratio (Figure 1).5,8-12 * Corresponding author. E-mail: [email protected]. † Lund University. ‡ Chalmers University of Technology. § Ideon Science Park. (1) Piculell, L.; Thuresson, K.; Lindman, B. Polym. Adv. Technol. 2001, 12, 44. (2) Thuresson, K.; Joabsson, F. Colloids Surf., A 1999, 151, 513. (3) Nilsson, S.; Thuresson, K.; Lindman, B.; Nystro¨m, B. Macromolecules 2000, 33, 9641. (4) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (5) Piculell, L.; Egermayer, M.; Sjo¨stro¨m, J. Langmuir 2003, 19, 3643. (6) Winnik, F. M.; Regismond, S. T. A. Colloids Surf., A 1996, 118, 1. (7) Panmai, S.; Prud’homme, R. K.; Peiffer, D. G.; Jockusch, S.; Turro, N. J. Langmuir 2002, 18, 3860. (8) Jimenez-Regalado, E.; Selb, J.; Candau, F. Langmuir 2000, 16, 8611. (9) Gelman, R. In International Dissolving Pulps Conference 1987, Proceedings of the Technical Assocaition of the Pulp and Paper Industry (TAPPI), Geneva, Switzerland, Mar 24-27, 1987; Tappi Press: Atlanta, GA, 1987; p 159. (10) Dualeh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251. (11) Piculell, L.; Nilsson, S.; Sjo¨stro¨m, J.; Thuresson, K. ACS Symp. Ser. 2000, 765, 317. (12) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304.

Figure 1. Schematic illustration of the viscosity effects of the addition of a micelle-forming surfactant to a solution of an HMP.

The viscosity maximum is the result of two competing effects of an increasing fraction of surfactant in the mixed aggregates: a longer residence time of the polymer hydrophobes in the mixed micellar junctions (change in dynamics) and a decreasing number of polymer hydrophobes per mixed micelle (change in structure).5,8,9,13 A pronounced maximum, such as that seen in Figure 1, is found for surfactants that form spherical micelles. The addition of salt or long-chain alcohols to a surfactant solution changes the packing parameter so that the micellar shape is changed.14,15 Added salt and/or alcohol induces a growth of the micelles from spherical to ellipsoidal and finally to rodlike or wormlike micelles. When sufficiently long rodlike micelles are formed, the viscosity of the mixed solutions of HMPs and surfactants (13) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (14) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1. (15) Lundberg, D.; Holmberg, K. J. Surfactants Deterg. 2004, 7, 239.

10.1021/la051205u CCC: $30.25 © 2005 American Chemical Society Published on Web 09/15/2005

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Figure 2. The base-catalyzed hydrolysis of a betaine ester surfactant, exemplified by dodecyl betainate.

then continues to increase with increasing surfactant concentration, and a maximum as in Figure 1 is absent because of an increased possibility to form bridging interactions.16,17 In this paper, we introduce a new aspect to HMPsurfactant mixtures by investigating cleavable surfactants, that is, surfactants with a built-in weak bond. The novel feature of using degradable surfactants instead of traditional surfactants is that the properties of the mixture change as a function of time and, in this case, pH. This investigation concerns mixtures of hydrophobically modified ethyl hydroxyethyl cellulose (HMHEC)sa “classical” HMPswith betaine esters of long-chain alcohols. Today, there is a substantial interest in cleavable surfactants.18-20 Environmental concerns are one of the major driving forces behind the development of new surfactants that break down into biocompatible components. Most cleavable surfactants contain a hydrolyzable bond that breaks down in either acid or basic solutions. In this investigation, we used betaine esters of long-chain alcohols. These surfactants show exceptional pH-dependent hydrolysis kinetics, and they are degraded by the mechanism of base-catalyzed hydrolysis, even at neutral pH (Figure 2).15,21,22 The hydrolysis of these surfactants results in the formation of trimethylglycine (betaine) and a long-chain alcohol. The alcohol created during the degradation may act as a cosurfactant. NMR self-diffusion experiments of a betaine ester of dodecanol have shown that the nondegraded surfactants form spherical micelles, which grow with increasing alcohol concentration.15 Experimental Section Materials. HMHEC, with the commercial name Natrosol Plus grade 331, was obtained from Aqualon. The average molecular weight and the degree of hydroxyethyl substitution per glucose unit given by the manufacturer were MW ≈ 250 000 g/mol and MSEO ) 3.3. The polymer material was analyzed according to the method described by Landoll23 and the dry sample was found to contain 0.765 wt % C16 alkyl chains. This corresponds to 0.34 mM alkyl chain concentration in a 1 wt % aqueous polymer solution. Prior to use, the HMHEC was purified as described elsewhere.24 The synthesis of the betaine esters followed a two-step route based on the procedure described by Thompson and Allenmark.21 In the first step, the alcohol was converted into the corresponding chloroacetate. The chloroacetate was further reacted with trimethylamine to give the final product, which was a fluffy precipitate that was purified by a wash with diethyl ether to remove unreacted alcohol and chloroacetate. The betaine esters (16) Panmai, S.; Prud′homme, R. K.; Peiffer, D. G. Colloids Surf., A 1999, 147, 3. (17) Sarrazin-Cartalas, A.; Iliopoulos, I.; Audebert, R.; Olsson, U. Langmuir 1994, 10, 1421. (18) Stjerndahl, M.; Lundberg, D.; Holmberg, K. In Novel Surfactants: Preparation, Applications and Biodegradability; Marcel Dekker: New York, 2003; Chapter 10. (19) Hellberg, P.-E. Lipid Technol. 2003, 15, 101. (20) Hellberg, P.-E.; Bergstro¨m, K.; Holmberg, K. J. Surfactants Deterg. 2000, 3, 81. (21) Thompson, R. A.; Allenmark, S. Acta Chem. Scand. 1989, 43, 690. (22) Thompson, R. A.; Allenmark, S. J. Colloid Interface Sci. 1992, 148, 241. (23) Landoll, L. M. J. Polym. Sci. 1982, 20, 443. (24) Thuresson, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1995, 99, 9, 3823.

Langmuir, Vol. 21, No. 21, 2005 9757 had a purity of >97%, as determined by NMR. No resonance peaks originating from the starting material were visible in the NMR spectrum, and no significant amounts of any other impurities were detectable. Tetradecyltrimethylammonium chloride was purchased from TCI. Sodium hydroxide, sodium dihydrogen phosphate dihydrate (extra pure), and disodium hydrogen phosphate were obtained from Merck. Sodium chloride (>99%) was purchased from Prolabo. Formic acid was obtained from Fluka. Tris(hydroxymethyl)aminomethane was purchased from Sigma. Dodecanol was purchased from BDH Chemicals. Water of Millipore quality (resistivity: ∼18 MΩ cm-1) was used to prepare all solutions, except the samples with heavy water, in which deuterium oxide (99.8 atom-% D), which was obtained from Dr. Glaser AG (Basel, Switzerland), was used instead. All chemicals were used without any further purification. Sample Preparation. The measurements in this investigation were performed at pH 7.5, with the exception of a few cases in which comparative measurements were prepared at a different pH and samples with heavy water in which a pD value of 7.5 (pD ) pH(meter reading) + 0.4)25 was used to compare with earlier studies.15 The samples for the viscosity measurements were prepared by mixing appropriate amounts of stock solutions of surfactants dissolved in formate buffer (pH 4) with stock solutions of HMHEC dissolved in phosphate buffer (pH 7.5) so that the final concentration of HMHEC was 1.0 wt % and the surfactant concentration was in the range of 0.5-18 mM. The amount of formate buffer was small in comparison to the amount of phosphate buffer so that the final solution was buffered at pH 7.5. A few comparative viscosity measurements were performed in TRIS buffer (pH 9.0) and phosphate buffer (pH 6.6). The viscosity measurements performed on the mixtures with D2O were prepared as described for the NMR samples. The samples used in the NMR experiments were prepared by mixing appropriate amounts of a 24 mM stock solution of dodecyl betainate in D2O with either a 1.8 wt % solution of HMHEC in a D2O-based 200 mM phosphate buffer with pD 7.5 or pure phosphate buffer and pure D2O when necessary. The final HMHEC concentration in the mixtures was 0.9 wt %. The phosphate buffer was prepared by mixing 200 mM solutions of sodium dihydrogen phosphate and disodium hydrogen phosphate in a proportion that gave rise to a meter value of 7.1 on a glass electrode pH meter calibrated in H2O-based buffers. This buffer has an effective pD value of 7.5.25 All samples had a final buffer concentration of 100 mM, and the ionic strength was adjusted with NaCl to approximately 270 mM. Viscosity Measurements. Flow curves were measured using a controlled stress CarriMed CSL 100 rheometer with a plateplate geometry (40-mm radius) in which the top plate was serrated to prevent slip when the fatty alcohol was created. The experiment started as soon as the samples were properly mixed and it was inevitable that some surfactants had already been subjected to hydrolysis. The measurement started with an increase in the shear rate for 15 min, then the shear rate was decreased for 15 min, and finally the sample was allowed to rest for 30 min. This procedure was repeated 10 times to monitor the effect of surfactant hydrolysis with time. The samples were shear-thinning (Figure 3), and the results reported from the viscosity measurements are the apparent viscosities at 10 s-1. NMR Measurements. The 1H NMR experiments were performed on a Varian Unity Inova 600 MHz spectrometer at 25 °C. Spectra were recorded every 30 min for a period of 11 h. The degree of hydrolysis at different times was calculated from the integrals of the N-methyl signals from the intact dodecyl betainate and the betaine formed during the hydrolysis. To remove air bubbles, the samples were gently centrifuged prior to the NMR experiments. Spectroscopy. The absorbance was measured using a GBC 920 UV/VIS double-beam spectrometer. The measurements were performed in a 10-mm quartz cuvette. To remove air bubbles, the samples were gently centrifuged prior to the experiments. (25) Covington, A. K.; Paabo, M.; Robinson, R. A.; Bates, R. G. Anal. Chem. 1968, 40, 700.

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Figure 3. Viscosity vs share rate of 1.0 wt % HMHEC alone (4), with 4 mM dodecyl betainate (O), 10 mM decyl betainate (0), and 0.5 mM tetradecyl betainate (×).

Figure 4. The percentage of remaining intact dodecyl betainate as a function of time at pD 7.5 in solutions with 0.9 wt % HMHEC (empty symbols) and without (filled symbols) HMHEC in which the surfactant concentration was 5 (circles), 8 (squares), or 12 mM (triangles). Birefringence. The birefringence of the samples was studied through crossed polarizers.

Results and Discussion Surfactant Hydrolysis. The rate of hydrolysis at pD 7.5 in D2O solutions of dodecyl betainate alone or in mixtures with HMHEC was investigated by NMR to establish whether the presence of the polymer affected the result. Figure 4 shows the percentage of remaining surfactant as a function of time for 5, 8, and 12 mM dodecyl betaine with or without 0.9 wt % HMHEC. Clearly, in the studied range of surfactant concentration, the hydrolysis rate constant was affected only to a very limited degree by either the initial surfactant concentration or the presence of the polymer. Absorbance. As the betaine ester surfactant is hydrolyzed, the corresponding alcohol is formed. Initially, the alcohol functions as a cosurfactant in the micelles, but when the fraction of alcohol reaches the solubilization limit, the alcohol separates out.15 As a result, the solution becomes turbid.

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The turbidity was uniform throughout the sample, and no macroscopic phase separation was seen during the experiment. The increase in turbidity was followed for HMHEC-dodecyl betainate mixtures both in normal water at pH 7.5 and in heavy water at pD 7.5 (Figure 5). The D2O samples were included to allow comparison with the 1H NMR hydrolysis experiments. One sample with decyl betainate was also included. Initially, the absorbance was constant. In the samples prepared with heavy water (Figure 5a), the absorbance started to increase after approximately 2 h. The pronounced increase in the turbidity is due to phase separation because solid alcohol separates out. The measurement of the rate of degradation (Figure 4) showed that the percentage of intact dodecyl betainate at 2 h is ∼80. This agrees with the findings of Lundberg et al.,15 who showed that, in the relevant concentration range, solid alcohol separated out when 10-20% of the betaine ester had degraded. The increase in absorbance for the samples in normal water (Figure 5b) occurred after approximately half the time it took to occur in heavy water. This suggests a faster surfactant hydrolysis in these solutions. It has previously been shown that changing the solvent from heavy water to normal water decreases the rate of degradation.26 The major reason for the faster rate of hydrolysis in normal water here is therefore not an expected isotope effect but is most likely due to the higher concentration (∼9 times) of hydroxide ions at pH 7.5 compared to the concentration of deuterium oxide ions at pD 7.5. The higher concentration of hydroxide ions is due to a lower pKw for normal water (pKw ) 14.0) compared with that for heavy water (pKw ) 15.0). The dramatic increase in absorbance for the sample containing decyl betainate appeared much later than it did in the samples with dodecyl betainate. This is probably an effect of the lower rate of degradation15 and the shorter chain length of the corresponding alcohol, as will be further discussed below. Birefringence. The samples containing HMHEC and degradable surfactant showed birefringence under certain conditions, which depended on the temperature and the specific betaine ester that was used. Mixtures containing dodecyl betainate became birefringent ∼2 h after sample preparation. The birefringence did not appear throughout the sample, but rather appeared as patches. In samples that contained dodecyl betainate, the birefringence was visible both when the sample was at rest and while it was being stirred. The samples with tetradecyl betainate showed birefringence after being stirred, but the birefringence disappeared when the samples were allowed to rest for a long time. The decyl betainate system was not birefringent at rest or during shearing, but birefringence appeared if the samples were kept at a temperature below the melting point of decanol (∼7 °C). The samples that showed birefringence also did so at long times when all the surfactant had degraded and the only components in the sample that could provide birefringence were the alcohol and HMHEC. A sample that contained HMHEC and 8 mM dodecanol was mixed to establish whether the birefringence was a unique feature for samples that contained degradable surfactants. Birefringence was not obtained at rest in the mixtures in which the polymer and the alcohol were mixed directly but appeared when the samples were stirred before checking for birefringence. It is therefore likely that the birefringent structures were (26) Bruice, T. C.; Fife, T. H.; Bruno, J. J.; Benkovic, P. J. Am. Chem. Soc. 1962, 84, 3012.

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Figure 5. Absorbance as a function of time after sample preparation in samples containing polymer and alkyl betainate. (a) Samples containing 0.9 wt % HMHEC with 5 (O) or 8 (0) mM dodecyl betainate in heavy water at pD 7.5. (b) Samples containing 1.0 wt % HMHEC with 5 (O), 8 (0), or 10 (4) mM dodecyl betainate in normal water at pH 7.5. (c) Sample containing 1.0 wt % HMHEC with 10 mM decyl betainate in normal water at pH 7.5.

point of the alcohol (approximately room temperature for dodecanol and ∼ 37 °C for tetradecanol). The varying behavior of the different betaine esters might have been due to differences in the lifetime of the birefringent structures, depending on whether the temperature used in the investigation was above or below the melting point of the alcohol. Viscosity Before Hydrolysis. The viscosity as a function of the concentration of intact surfactant molecules was studied by investigating the viscosity directly after mixing, when nearly all the surfactant molecules were still intact (Figure 6). The overall trend in Figure 6 is that the viscosity went through a viscosity maximum as a function of increasing surfactant concentration, a result well-known (see Introduction) for conventional surfactants. It has been shown that the critical micelle concentration (cmc) for a betaine ester is approximately the same as for an n-alkyltrimethylammonium chloride with two additional methylene groups in the alkyl chain.27 The two surfactants may then be expected to affect the viscosity of a HMHEC solution in approximately the same way. The viscosity for HMHEC with an increasing concentration of tetradecyltrimethylammonium chloride (C14TAC) was compared with the results for dodecyl betainate in Figure 6b. Figure 6 shows that intact betaine esters behaved as conventional surfactants in terms of viscosity as a function of increasing surfactant concentration, and the magnitude of the viscosity is comparable with that of the viscosity obtained with the noncleavable C14TAC. The viscosity before hydrolysis did not vary in a completely smooth way for dodecyl and tetradecyl betainate. The measurements were performed with a serrated plate to prevent slip as the alcohol was obtained during the degradation (see Experimental Section), and repeated measurements showed that the absolute viscosity could vary. A drawback with the serrated plate is that the sensitivity in the viscosity measurements is somewhat reduced, but a conventional plate could not be used because slip occurred as the surfactant degraded. To check whether the scatter in the data was caused by an incipient phase separation in the samples due to the salt present, samples prepared in water were titrated with salt, but no phase separation was obtained, even at high salt concentrations. For a homologous series of conventional surfactants, the maximum viscosity increases and the surfactant concentration required for the maximum viscosity decreases with increasing hydrophobic chain length. The trends in this investigation were somewhat obscured by scatter in the data but were obeyed at least for the decyl and the dodecyl betaine esters. Time-Dependent Viscosity. Because the viscosity of a mixed polymer-surfactant solution is dependent on the surfactant concentration, the viscosity of HMHEC and a cleavable surfactant was expected to change as a function of time because of the decreased surfactant concentration. Figure 7 shows the viscosity for HMHEC-surfactant mixtures as a function of time at pH 7.5 for various initial concentrations of the three different betaine esters. The viscosity was followed for 9 h in most of the experiments. Included in Figure 7, for selected initial surfactant concentrations, is the viscosity after a time (120 h) long enough that virtually no intact betaine ester was left in the solution. In those cases (compare with Figure 6) in which the initial viscosity was higher than that of the polymer alone,

due to the alcohol and HMHEC and that the structures appeared when the temperature was below the melting

(27) Rozycka-Roszak, B.; Przestalski, S.; Witek, S. J. Colloid Interface Sci. 1988, 125, 80.

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Figure 6. Viscosity before hydrolysis for (a) decyl betainate, (b) dodecyl betainate (b) and C14TAC (O), and (c) tetradecyl betainate as a function of surfactant concentration in mixtures containing 1.0 wt % HMHEC.

the viscosity varied with time before it eventually reached a constant value. However, the observed time dependence varied with surfactant type and concentration. In some cases, the viscosity decreased monotonically, whereas in other cases, the viscosity went through a pronounced maximum as a function of time. This will be discussed in

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Figure 7. Viscosity as a function of time at pH 7.5 for 1.0 wt % HMHEC in mixtures with different initial concentrations of (a) decyl betainate: 1 (b), 3 (O), 5 (9), 8 (0), 10 (2), 14 (4), and 18 (1) mM; (b) dodecyl betainate: 1 (b), 2 (O), 3 (2), 4 (4), 5 (1), 6 (3), 7 (9), 8 (0), and 12 (×) mM; and (c) tetradecyl betainate: 0.5 (b), 1 (O), 2 (9), 3 (0), 4 (2), 5 (4) and 10 (×) mM. The viscosity after 120 h is included for 5, 8, and 18 mM decyl betainate; 1, 4, 8, and 12 mM dodecyl betainate; and 3 and 10 mM tetradecyl betainate.

more detail below. The viscosity maximum for decyl betainate was not as pronounced as for the other betaine ester used in the study. The magnitude of the viscosity

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Figure 8. Viscosity as a function of time for 1.0 wt % HMHEC and (a) 6 mM dodecyl betainate at pH 7.5 (b) or 9 (9) or (b) 8 mM dodecyl betainate at pH 7.5 (b) or 6.6 (9).

leveled off slowly after the maximum viscosity had been reached. After 9 h, the viscosity had not reached a low level for all of the initial surfactant concentrations. The viscosity after a long time (120 h) was therefore measured in some of these cases. The long-time viscosity for the samples that initially contained decyl betainate was somewhat higher than the viscosity for the polymer alone. This will be discussed in more detail below. Viscosity at Different pH. The rate of degradation of the surfactants increases with increasing pH. Changing pH thus gives a tool to modify the time dependence of the viscosity. Figure 8 compares the viscosities of HMHEC with initially 6 mM dodecyl betainate at pH 7.5 and 9 (Figure 8a) and with initially 8 mM dodecyl betainate at pH 7.5 and 6.6 (Figure 8b). At pH 7.5, the viscosity went through a maximum as a function of time. The rate of degradation at pH 9 was higher, and the viscosity decreased almost immediately before it attained a constant value. Because of the rapid kinetics, the shape of the viscosity curve could not be determined at pH 9. However, on the basis of the results at pH 7.5, a nonmonotonic curve would be expected. After 9 h, both experiments had reached the same viscosity. The difference in the initial viscosity between the two experiments may be due to the somewhat reduced sensitivity in the data and the difference in the

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rate of hydrolysis, meaning that the surfactant concentration at the start of the measurement was not the same in the two cases. The rate of degradation at pH 6.6 is lower than that found at pH 7.5. The viscosity at pH 6.6 increased continuously during the 9 h of measurement. Molecular Interpretation. The concentration-dependent effect adding surfactant has on the viscosity is due to the formation of mixed micelles between the surfactants and the polymer hydrophobes (see Figures 1 and 6). The viscosity then changes as the surfactant degrades. The simplest behavior would occur for the hypothetical case in which the surfactant vanishes upon degradation without forming any degradation product. Then the time dependence would be totally determined by the varying surfactant concentration, as illustrated in Figure 6. This would imply that when the initial surfactant concentration is on the left side (lower concentrations) of the viscosity maximum at time zero (Figure 6), the viscosity should decrease continuously as the surfactant degrades. In contrast, when the initial surfactant concentration is higher than the concentration corresponding to the viscosity maximum, the viscosity would go through a maximum as a function of surfactant degradation. The real situation in this investigation is not that simple, however, because the viscosity is also influenced by the alcohol incorporated in the mixed micelles. Both the total amount of alcohol and surfactant and the surfactant/ alcohol ratio are of importance. The surfactant/alcohol ratio influences the shape and aggregation number of the micelles and possibly the lifetime of the hydrophobes in the mixed micelles as well. Lundberg et al.28 showed that the micellar phase for dodecyl betainate is in equilibrium with crystalline, pure dodecanol. For the dodecyl and tetradecyl betainates, the alcohols formed at the solubilization limit are thus solid at room temperature. As the surfactant progressively degrades to alcohol, the resulting scenario may be divided into before the crystalline alcohol precipitates out and after. Before precipitation, the total concentration of surfactant and alcohol is preserved in the aqueous phase. Essentially all of the formed alcohol molecules are in the mixed micelles. When the solubilization limit of alcohol in the mixed micelles is reached, alcohol separates out, and the chemical potential of the alcohol then remains constant; that is, no more alcohol enters the micelles. For mixed micelles containing only surfactant and alcohol, the composition of the mixed micelles will be fixed after this point. Further degradation only results in the formation of more pure alcohol in a separate phase while the number of micelles, preserving a fixed composition, decreases. The solubilization limit of the alcohol is reached at a lower mole fraction of alcohol in the mixed micelles for surfactants with longer hydrophobic chain lengths. Hence, the formed alcohols with the longer hydrophobes may be expected to affect the number and the shape of the mixed micelles to a lesser extent. This is confirmed by the behavior of tetradecyl betainate. When the initial concentration was on the left side (up to 4 mM) of the viscosity maximum (Figure 6), the viscosity decreased with time (Figure 7). With initially 10 mM tetradecyl betainate, which was on the right side (higher concentrations) of the viscosity maximum in Figure 6, the viscosity went through a maximum as the surfactant degraded (Figure 7). Because of the rather high salt concentration and the long surfactant chain length, it is possible that the micelles (28) Lundberg, D.; Ljusberg-Wahren, H.; Norlin, A.; Holmberg, K. J. Colloid Interface Sci. 2004, 278, 478.

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that are formed by tetradecyl betainate are nonspherical already when no degradation product has been incorporated in the micelles. The somewhat wide viscosity maximum before surfactant hydrolysis (Figure 6c) indicates that the micelles might be elongated.16 The shorter hydrophobic chain length of dodecyl betainate implies that more alcohol is solubilized in the mixed micelles and that the micellar shape might change as the surfactant degrades. The uncharged alcohol has a greater preference for being in the micelles than the charged surfactant. Hence, initially upon degradation, the total concentration of micelle-bound surfactant and alcohol increases, and the effective cmc decreases. As long as the proportion of alcohol is small, the micellar shape is not affected, and the number of mixed micelles should thus increase. When the degradation proceeds, the micelles contain larger proportions of alcohol, and longer micelles may form, which agrees with the findings of Lundberg et al.15 As a consequence, the number of micelles may actually decrease. The viscosity is thus influenced both by the changing fraction of bound surfactant plus alcohol per se and by the formation of longer mixed micelles. The viscosity data does not provide any information on which of these effects dominates, but we propose that one or both of these effects are responsible for the occurrence of a viscosity maximum with time for initially 3 mM dodecyl betainate, which would not be expected if no alcohol were incorporated in the micelles because the initial surfactant concentration was on the left side (lower concentration) of the viscosity maximum in Figure 6. The melting point of decanol is lower (∼7 °C) than that of the other alcohols in the study. This means that the neat alcohol that eventually separates out upon the degradation of the surfactant is in the liquid state. This also implies a higher solubilization limit of the alcohol in the mixed aggregates. This is confirmed by the results in Figure 5c, in which the strong increase in absorbance, indicating the formation of a separate alcohol phase, did not occur until after 9 h, which is approximately the time when our viscosity measurements were finished. The fact that a higher mole fraction of decanol can be incorporated in the mixed aggregates implies that the structure should eventually switch to a lamellar phase.29-36 To check for this possibility, the degradation of a solution of decyl betainate alone was followed with time through crossed polarizers. No signs of birefringence, indicating a lamellar phase, was found. However, we cannot exclude the possibility that closed vesicles, rather than infinite lamellae, were formed during the surfactant degradation. The above considerations mean that the formed alcohol does not separate out during the time span of our viscosity measurements and that the mixed aggregate shape might change considerably during the hydrolysis of decyl betainate. The shape of the viscosity curves in Figure 7 was different with decyl betainate, compared to the two other betaine esters. The viscosity actually increased initially for all surfactant concentrations, and it is likely that large (29) Fontell, K.; Khan, A.; Lindstro¨m, B.; Maciejewska, D.; PuangNgern, S. Colloid Polym. Sci. 1991, 269, 727. (30) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1977, 61, 519. (31) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1977, 61, 519. (32) Ekwall, P.; Mandell, L. J. Colloid Interface Sci. 1979, 69, 384. (33) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1969, 31, 508. (34) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1969, 31, 530. (35) Ekwall, P.; Mandell, L. Acta Chem. Scand. 1967, 21, 1612. (36) Fontell, K.; Danielsson, I.; Mandell, L.; Ekwall, P. Acta Chem. Scand. 1962, 16, 2294.

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Figure 9. The time position of the viscosity maximum as a function of the initial concentration of (a) decyl betainate and (b) dodecyl betainate in mixtures with 1.0 wt % HMHEC. The line is a guide to the eye in panel a and a linear fit in panel b (see text).

aggregates were formed. As discussed above, the effective cmc also decreases as alcohol is incorporated in the micelles. This effect should also be more significant if the cmc of the ionic surfactant alone is high (short alkyl chains). The only components in the samples at long times after preparation are the polymer, salt, and the alcohol, which was obtained during the surfactant degradation. For decyl betainate (Figure 7a), in which the viscosity after a long time is higher than that for HMHEC alone, it is possible that some alcohol is solubilized in the hydrophobic domains formed by the polymer hydrophobes, thus influencing the viscosity. The Viscosity Maximum. The time at which the viscosity maximum appeared, tmax, for different initial concentrations of decyl and dodecyl betainate is shown in Figure 9. The maximum was evaluated by drawing a smooth curve through the experimental points. Dodecyl betainate (Figure 9b) follows a linear trend, and it can be seen by extrapolation that the straight line crosses the x-axis at approximately 1.2 mM. This value is somewhat lower than the dodecyl betainate concentration needed to obtain the maximum viscosity (Figure 6b), and it is likely that the difference is due to the decrease in cmc and the formation of elongated micelles. The trend of tmax with

Associating Polymer/Cleavable Surfactant Mixtures

decyl betainate (Figure 9a) is nonmonotonic. More detailed information would be required to rationalize this behavior because the viscosity maximum in this case most likely results from a combination of many changes: an increase in the concentration of the surfactant + alcohol in the micelles, a change in micellar shape, and a possible change in the lifetime of the bound hydrophobes in the mixed micelles. In contrast to the time of the viscosity maximum, the measured viscosity at the maximum did not vary in a regular fashion with the initial surfactant concentration (see Figure 7). Repeated measurements showed that the shape of the curves and the time at which the maximum viscosity occurred was the same in all tests, but the absolute values of the viscosity varied. Conclusions This is the first investigation in which a mixture of degradable surfactants and an HMP is used to obtain a system with time-dependent viscosity. The use of surfactants with pH-dependent hydrolysis kinetics makes it possible to control the surfactant degradation as a function of time and thereby control and adjust the viscosity of the mixture with time. The initial viscosity, before any surfactant degradation has occurred, is comparable with the viscosity for conventional surfactants; that is, the viscosity goes through a pronounced viscosity maximum with increasing surfactant concentration. As the surfactant molecules degrade, the viscosity of the mixture varies with time before it reaches a constant value. The viscosity either decreases continuously or goes through a viscosity maximum, depending on the initial surfactant concentration. Changing the initial surfactant concentration and/

Langmuir, Vol. 21, No. 21, 2005 9763

or the surfactant hydrophobic length can be used as tools to control the magnitude of the viscosity and the viscosity profiles. The hydrophobic chain length affects the number and shape of the mixed micelles directly, as well as indirectly, via the alcohol produced as a result of the surfactant degradation. The rate of degradation increases with increasing pH, which gives another tool to control the viscosity. At a sufficiently low pH, the rate of degradation is so slow that the betaine esters can be considered stable. This provides the mixture with start and stop possibilities, meaning that the addition of a sufficient amount of acid will stop the degradation of the surfactant molecules, and the viscosity will be constant. Adding alkali to the mixture will again start the surfactant degradation, and the viscosity will decrease. To obtain further possibilities to control the viscosity, an interesting possibility would be to use surfactants or conditions in which the surfactant aggregates will switch from micelles to other aggregate geometries as the alcohol concentration in the mixture increases. The viscosity of the mixture will then also be dependent on the type of surfactant aggregate. Acknowledgment. The authors thank Krister Holmberg for suggesting these experiments. Krister Thuresson and Håkan Wennerstro¨m are acknowledged for valuable discussions. This work was financed by the Centre for Amphiphilic Polymers from Renewable Resources (CAP) (M.K.), the Centre for Surfactants Based on Natural Products (SNAP) (M.S) and by the Swedish Research Council (L.P.). LA051205U