Salt Tolerance of an Aqueous Solution of a Novel Amphiphilic

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Salt Tolerance of an Aqueous Solution of a Novel Amphiphilic Polysaccharide Derivative Kyoko Kawakami,*,† Takeshi Ihara,‡ Tohru Nishioka,‡ Tomohito Kitsuki,‡ and Yuji Suzuki† Tokyo Research Laboratories, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo, 131-8501, Japan, and Wakayama Research Laboratories, Kao Corporation, 1334 Minato, Wakayama, 640-8580, Japan ReceiVed October 26, 2005. In Final Form: January 9, 2006 An aqueous solution of an amphiphilic polysaccharide derivative, hydrophobically (stearyl alkyl group) and hydrophilically (sulfonic-acid salt group) modified hydroxyethylcellulose (HHM-HEC), showed increased viscosity, elasticity, and thixotropic properties in response to the addition of monovalent and divalent salts. Furthermore, the HHM-HEC solution had a transparent appearance at a NaCl concentration of 7 wt %. Since it showed superior salt tolerance to HEC, we focused attention on the two substituents of HHM-HEC and prepared HEC derivatives, namely, hydrophobically modified hydroxyethylcellulose (R-HEC), hydrophilically modified hydroxyethylcellulose (S-HEC), and nonmodified hydroxyethylcellulose (HEC). In addition, we used oscillatory, thixotropic, and fluorometric methods to compare the rheological properties of HHM-HEC with those of other derivatives in the presence of NaCl and ZnCl2, and attempted to elucidate the respective roles of the two substituents of HHM-HEC solution in the salt-tolerance mechanism. As the NaCl concentration in the HHM-HEC solution increased, the values of the elastic modulus G′ and the viscous modulus G′′ increased, and, moreover, the relative intensities of the first (I1 ) 372 nm) and the third (I3 ) 383 nm) vibronic bands of the pyrene monomer emission spectrum (the I1/I3 ratio) decreased. These results suggested that the added salt strengthened the three-dimensional network structure of the HHM-HEC polymer by the formation of cross-linkages through the association of hydrophobic substituents. This hydrophobic substituent was therefore essential in allowing HHM-HEC to exhibit a high viscosity in a salt solution. Although the R-HEC solution showed a higher viscosity than did the HHM-HEC solution in the absence of added salts, it became cloudy and lost its viscosity at high NaCl concentrations, apparently because of the shrinkage of its network structure. This signified that the hydrophilic substituent was essential for the sufficient solubility of HHM-HEC to show its rheological properties in a salt-rich solution. We propose to explain how the viscosity of HHM-HEC increases in the presence of salts as follows: Added salts weaken the electrostatic repulsion between the hydrophilic substituents, thereby enhancing the interactions of hydrophobic substituents and consequently increasing the rigidity of the HHM-HEC solution.

Introduction Many water-soluble polymers are known to have thickening and rheology-modifying properties. As a result, they have been studied intensively by many academic organizations and have been applied in various industrial products, such as toiletries, paints, and construction materials.1-6 Water-soluble polymers are useful in improving the stability of industrial products, even when they contain many different substances. However, it is common for ionic polymers to lose viscosity in the presence of ionic species, such as salts, metal ions, and ionic surfactants. Among the water-soluble polymers, polysaccharides (such as hydroxyethylcellulose and hydroxypropylcellulose) are widely used as thickeners in cosmetics because they are relatively inexpensive, comparatively safe for use in the human body, and are not greatly influenced by salts. As a result, the chemical and physical properties of polysaccharide solutions have been * Corresponding author. E-mail: [email protected]. Tel: 813-5630-9472. Fax: 81-3-5630-9338. † Tokyo Research Laboratories. ‡ Wakayama Research Laboratories. (1) Glass, J. E., Ed. Water-Soluble Polymers: Beauty with Performance; Advances in Chemistry Series 213; American Chemical Society: Washington, DC, 1986. (2) Glass, J. E., Ed. Polymers in Aqueous Media: Performance through Association; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (3) Schulz, D. N., Glass, J. E., Eds. Polymer as Rheology Modifiers; Advances in Chemistry Series 462; American Chemical Society: Washington, DC, 1991. (4) de Bruin, J. J. SOFW J. 1994, 120, 944. (5) Brown, R. Polym. Paint Colour J. 1994, 184, 267. (6) Finch, C. A., Ed. Industrial Water Soluble Polymers; The Royal Society of Chemistry: Cambridge, UK, 1996.

investigated and summarized in previous reports.1-3 We recently described an amphiphilic polysaccharide derivative, hydrophobically and hydrophilically modified hydroxyethylcellulose (HHM-HEC), which has excellent thickening and emulsification abilities coupled with salt tolerance in water.7,8 In the current study, we investigated the effects of salt addition on the rheological properties of an HHM-HEC solution. Aqueous solutions of hydrophobically modified polysaccharide derivatives, which can form three-dimensional network structures through the intermolecular associations of their hydrophobic side chains, have been investigated with respect to their viscoelastic properties, aggregation numbers, and surface tensions.9-14 Several previous reports have described the interactions between hydrophobically modified water-soluble polymers and anionic surfactants, and have discussed the unique rheological properties of these mixed polymer-surfactant solutions.15-23 In particular, (7) Ihara, T.; Nishioka, T.; Kamitani, H.; Kitsuki, T. Chem. Lett. 2004, 33 (9), 1094. (8) Akiyama, E.; Kashimoto, A.; Fukuda, K.; Hotta, H.; Suzuki, T.; Kitsuki, T. J. Colloid Interface Sci. 2005, 282, 448. (9) Williams, P. A.; Meadows, J.; Phillips, G. O.; Tanaka, R. ACS Symp. Ser. 1992, 489, 341. (10) Winnik, F. M.; Winnik, M. A.; Tazuke, S.; Ober, C. K. Macromolecules 1987, 20, 529. (11) Shaw, K. G.; Leipold, D. P. J. Coat. Technol. 1985, 57, 63. (12) Schaller, E. J. Surf. Coat. Aust. 1985, 22, 6. (13) Landoll, L. M.; J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 443. (14) Sau, A. C. Polym. Mater. Sci. Eng. 1987, 57, 497. (15) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 7099. (16) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (17) Rose´n, O.; Piculell, L.; Hourdet, D. Langmuir 1998, 14, 777. (18) Murata, M.; Arai, H. J. Colloid Interface Sci. 1974, 46, 475.

10.1021/la052877n CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006

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Figure 1. Structures of HHM-HEC and the other derivatives tested.

the effects of salt addition on the polymer-surfactant associations within these mixtures have been the focus of much interest.15-19 However, despite the growing body of literature concerning hydrophobic nonionic polymers, relatively little is known about amphiphilic polymers that contain both hydrophobic and hydrophilic substituents. We found that the viscosity of the HHM-HEC solution was much greater than that of the HEC solution at the same polymer concentration. Furthermore, the HHM-HEC solution became more elastic and thixotropic when salts were added. This profile was specific among water-soluble polymers and useful in formulating cosmetics that contain salts. We are interested in the factors that determine the superior salt tolerance of HHM-HEC and are convinced that this analysis will lead to the development of more useful cosmetic ingredients and applications not only in cosmetic formulation areas, but also in other commercial areas. In the current study, we devoted attention to both the hydrophobic and the hydrophilic substituents in the HHM-HEC molecule. We therefore compared the behaviors of several HEC derivatives with or without added salts, and explored the respective roles of the two substituents of HHM-HEC on its salt tolerance. In addition, we put forward a salt-tolerance mechanism in the HHMHEC solution. Experimental Section Materials. HHM-HEC was synthesized from HEC (having an approximate weight-average molecular weight of 1.2 × 106) with stearylglycidyl ether and 3-chloro-2-hydroxypropanesulfonic acid under alkaline conditions, as described previously.7 The average degree of replacement by the hydrophobic substituent (the stearyl alkyl group) was 0.0036 per monosaccharide unit compared with 0.11 per monosaccharide unit for the hydrophilic substituent (the sulfonic-acid salt group). By contrast, in the HHM-HEC used in fluorescence studies, the average number of hydrophobic substituents is 0.0040, and the average number of hydrophilic substituents is 0.40. Other derivatives, namely hydrophobically modified HEC (RHEC, wherein the average number of hydrophobic substituents is 0.0040) and hydrophilically modified HEC (S-HEC, wherein the average number of hydrophilic substituents is 0.21), were synthesized using the method described for HHM-HEC. The structures of the HEC derivatives are shown in Figure 1. HEC was obtained from the Union Carbide Corporation (under the commercial name of HEC QP-100MH) and had an approximate weight-average molecular (19) Sjo¨stro¨m, J.; Picullel, L. Langmuir 2001, 17, 3836. (20) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. (21) Picullel, L.; Nilsson, S.; Sjo¨stro¨m, J.; Thuresson, K. ACS Symp. Ser. 2000, 765, 317. (22) Dulaleh, A. J.; Steiner, C. D. Macromolecules 1991, 24, 112. (23) Elliott, P. T.; Tarng, M.-R.; Glass, J. E. Polym. Mater. Sci. Eng. 1999, 81, 500.

Kawakami et al. weight of 1.5 × 106. The cross-linked polymer of poly(acrylic acid) was obtained from Noveon, Inc. (under the commercial name of Carbopol 981). The polymers were used without further purification. Deionized and filtrated water was used directly. NaCl and ZnCl2 were the reagent-grade salts. There is a need for a water-soluble polymer that is tolerant to Zn2+ and can be used in cosmetics designed for ultraviolet (UV) protection, which contain zinc oxide powder. We therefore used ZnCl2 as one of the salts tested in this study. Sample Preparation. HHM-HEC was added to hot pure water (70 °C), and the polymer solution was stirred for more than 3 h to dissolve the HHM-HEC completely. Salts were added to the aqueous solution of HHM-HEC at room temperature. Throughout this work, the respective concentrations of the polymers and salts are denoted by weight. Aqueous solutions of other thickeners were also prepared using a similar protocol. The cross-linked polymer of poly(acrylic acid) was neutralized by KOH. The mixtures were cooled, stirred for 30 min to ensure complete homogeneity, and measured the next day. Rheological Measurements. Rheological measurements of the aqueous thickener solutions were performed using a rheometer (Rheometric RFS II; Rheometric Co., Ltd.) fitted with a cone-andplate geometry (50 mm) at 20 ( 0.5 °C. Care was taken not to stir the polymer solutions before measurements. The shear flow viscosities of polymer solutions over a shear rate range of 0.1 to 100 s-1 were measured by using the steady-rate sweep method. A thixotropic loop measurement was made using a standardized shearing procedure for thixotropic materials that exhibit shear-dependent and time-dependent flow behaviors.24,25 The shear stress was measured as follows: Uniform shear-rate acceleration was provided from 0.1 to 100 s-1 with a sweep time of 150 s, followed by uniform shearrate deceleration for 150 s. Oscillatory measurements were carried out over a frequency range of 0.1-100 rad/s at a fixed strain value of γ ) 20% using the dynamic frequency-sweep method. The loss tangent or tan δ, which represents the ratio of the loss modulus (G′′) to the storage modulus (G′), was calculated according to the following equation: tan δ )

G′′ G′

Fluorescence Studies. A freshly prepared pyrene solution in dimethyl sulfoxide (DMSO) at a concentration of 1 × 10-3 M was added to 0.5 wt % HHM-HEC solutions to a final pyrene concentration of 5 × 10-6 M. The mixtures were sonicated for 30 min, and measurements were taken the next day. The fluorescence-emission spectra of the polymer and pyrene probe mixtures were recorded over the range of 350-500 nm using a Hitachi F-4010 fluorescence spectrophotometer. The excitation wavelength was set at 334 nm. The relative intensities of the first (I1 ) 372 nm) and the third (I3 ) 383 nm) vibronic bands of the pyrene monomer emission spectrum were obtained as the ratio I1/I3.10,26,27 Turbidity Measurements. The turbidities of polymer solutions can be measured as optical density (OD) using a UV-visible recording spectrophotometer UV-160 (Shimadzu Corporation) at 600 nm, with water as the blank.

Results and Discussion Salt-Tolerance Properties of HHM-HEC. As we reported previously,7,8 a critical aggregation concentration (CAC) of HHMHEC in an aqueous solution was estimated to be around 0.2 wt %. The concentration of HHM-HEC used in the current report, 0.5 wt %, was sufficient to form alkyl microdomains (that is, aggregations of alkyl chains). The effects of the addition of NaCl and ZnCl2 on the shear viscosity of HHM-HEC and the other thickeners are shown in Figure 2. The concentrations of the thickeners tested in aqueous solutions were 0.5 wt %. The (24) Barry, B. W. J. Coll. Int. Sci. 1968, 28, 1. (25) Barry, B. W.; Shotton, E. J. Pharm. Pharmacol. 1968, 20, 167. (26) Winnik, F. M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91, 594. (27) Dualeh, A. J.; Steiner, C. A. Macromolecules 1990, 23, 251.

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Figure 3. Thixotropic loops of shear stress versus shear rate for aqueous solutions of HHM-HEC (a) and HEC (b) with salt (thick line) or without salt (thin line) at 20 °C. The loop of the HEC solution without salt was overlaid onto the loop with NaCl. [polymers] ) 0.5 wt %, [NaCl] ) 1 wt %. The inset shows the transition of shear rate as a function of time during the thixotropic loop measurement.

Figure 2. Shear viscosity versus shear rate for aqueous solutions of HHM-HEC (a), HEC (b), and cross-linked poly(acrylic acid) (c) with and without salt addition at 20 °C. [polymers] ) 0.5 wt %, [NaCl] ) 1 wt %, [ZnCl2] ) 2 wt %.

concentrations of NaCl and ZnCl2 were 1 wt % (0.17 M) and 2 wt % (0.15 M), respectively. The viscosity of the HEC solution is well-known to be largely unaffected by the addition of salts. Accordingly, the HEC solution in Figure 2 showed similar viscosities in the presence and the absence of salts. The viscosity of the HHM-HEC solution was significantly higher than that of HEC, even in the presence of salts. The HHM-HEC solution was non-Newtonian and maintained higher viscosity to higher shear rates before shear thinning compared to that of the HEC solution. It is shown that HHM-HEC is more useful in cosmetic applications containing salts than is HEC because the high viscosity at low shear rates is desirable in products that require a high storage stability. In addition, the viscosity of the HHM-HEC solution in the presence of NaCl was similar to that in the presence of ZnCl2. This means that HHM-HEC is tolerant to both monovalent and divalent salts. Previous investigations demonstrated that hydrophobically modified water-soluble polymers had unique rheological characteristics and exhibited high viscosities, which were attributed to the associations of adjacent hydrophobic substituents.11,12,20 We therefore predicted that the remarkable increase in the viscosity of the HHM-HEC solution at low shear rates with added salts was attributed primarily to the strength of hydrophobic association and not to the entanglement of its backbone, HEC.

Although the solution viscosity of the cross-linked poly(acrylic acid) was higher in the absence of salt than those of both HHMHEC and HEC, it decreased notably when salts were added. This means that the cross-linked poly(acrylic acid) is unsuitable for use as a thickener in a salt solution. Many water-soluble polymers, similar to the cross-linked poly(acrylic acid), show extreme reductions in viscosity in the presence of salts. One possible explanation for this phenomenon is an electric-shielding effect.28 Salt additives would screen the electrostatic interactions of the carboxylic groups of poly(acrylic acid) and make the polymer chains cohere and shrink. The 0.5 wt % HHM-HEC solution without added salts had a viscous fluid profile. Interestingly, this solution appeared to increase in thixotropic properties when salts were added. We therefore performed thixotropic loop measurements under varying shear-rate conditions, as shown in Figure 3. The thixotropic loop measurement also provided information about the behavior of the HHM-HEC solution. The loop observed for the HHM-HEC solution in the presence of 1 wt % NaCl showed a hysteresis area larger than that seen in the absence of salts, and was characterized by a single peak of the shear stress at the initial low shear rate investigated. After the shear stress peaked, the three-dimensional network structure in the polymer solution began to deform. The maximum value of the peak (namely, the apparent yield stress) might indicate the firmness of the network structure in the viscoelastic aqueous solution. In Figure 3, the peak of the HHMHEC solution loop observed with 1 wt % NaCl appeared to be higher than that observed without added salts, while the aqueous HEC solution showed no rheological changes. Much greater stress was required to break down the structure of the HHM(28) Takahashi, A.; Nagasawa, M. J. Am. Chem. Soc. 1964, 86, 543.

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Figure 5. I1/I3 ratios of the pyrene emission spectra dependence on the NaCl concentration for the HHM-HEC solution at 25 °C. [HHMHEC] ) 0.5 wt %

Figure 4. Storage (G′) and loss (G′′) moduli dependence on the angular frequency for HHM-HEC (a) and HEC (b) solutions at 20 °C. Strain ) 20%, [polymers] ) 0.5 wt %.

HEC solution in the presence of salts compared to that required in the salt-free solution; that is, the three-dimensional network structure of the HHM-HEC solution was strengthened with added salts. The presence of two substituents bound to an HEC backbone therefore resulted in notable differences in the rheological behaviors between HEC and HHM-HEC in the presence of salts. We also studied the effects of added salts on the dynamic viscoelasticity and hydrophobicity of the HHM-HEC solution. The moduli of the HHM-HEC and HEC solutions were measured at frequencies ranging from 0.1 to 100 Hz with 20% strain, and were plotted in Figure 4. The increases in the G′ and G′′ values of the HHM-HEC solution caused by the addition of NaCl indicated that its three-dimensional network structure became more elastic, whereas the corresponding moduli of the HEC solution showed no such variation. As previously reported,10,26,27,29 the I1/I3 ratio of the pyrene monomer emission spectrum was calculated by measuring the hydrophobicity or the strength of hydrophobic association in the HHM-HEC solution. A decrease in the I1/I3 ratio indicates decreased mobility of the pyrene probe, that is, enhanced hydrophobicity in a solution. The I1/I3 ratios derived from the emission spectra of pyrene in the HHM-HEC solutions as a function of the NaCl content are shown in Figure 5. The pyrene probes appeared to exist in the vicinity of the alkyl microdomains of HHM-HEC, which were the only hydrophobic areas in the aqueous solution. The I1/I3 ratio decreased rapidly as the NaCl concentration increased, and reached a plateau at NaCl concentrations above 5 wt %. This decrease in the ratio reveals a stronger association process in the alkyl microdomains of HHMHEC. The variations in the rheological properties and fluorescent spectra of the HHM-HEC solution by salt addition suggest that the enhancement of the ion intensity progressively strengthened the hydrophobic associations or increased the number of (29) Selb, J.; Biggs, S.; Renoux, D.; Candau, F. Hydrophilic Polymers: Performance with EnVironmental Acceptance; Glass, J. E., Ed.; Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996; p 251.

Figure 6. Shear viscosity versus shear rate for aqueous solutions of R-HEC (a) and S-HEC (b) with and without salt addition at 20 °C. [polymers] ) 0.5 wt %, [NaCl] ) 1 wt %, [ZnCl2] ) 2 wt %.

hydrophobic aggregations. This would allow the HHM-HEC solution to form a firmer and more thixotropic network. Respective Roles of the Hydrophobic and Hydrophilic Substituents of HHM-HEC. Comparing the solution viscosities of various HEC derivatives might reveal the respective roles of the two substituents of HHM-HEC in the salt-tolerance mechanism. The effects of the added monovalent and divalent salts on the shear viscosity of R-HEC and S-HEC are shown in Figure 6. The S-HEC solution acts as a Newtonian fluid over a wide range of shear rates and has a relatively low viscosity, regardless of the presence or absence of salts. In contrast, R-HEC showed a viscosity similar to that of HHM-HEC in the presence of salts (cf. Figure 2a). This signifies that the intermolecular associations of hydrophobic chains are important for the high viscosity of the HHM-HEC solution with added salts. The effects of the NaCl concentration on the viscosities of HHM-HEC, R-HEC, and HEC are shown in Figure 7. No variation in the shear viscosity of the HEC solution was observed over the range of salt contents investigated. It implies that the degree of chain entanglement of HEC, whose molecular weight is equivalent to that of HHMHEC, is unaffected by the NaCl concentration. In contrast, it was

Salt Tolerance of HHM-HEC Aqueous Solution

Figure 7. Effects of the NaCl concentration on the solution viscosities of HHM-HEC (a), R-HEC (b), and HEC (c) at 20 °C. [polymers] ) 0.5 wt %.

Figure 8. Turbidity as a function of the NaCl concentration for aqueous solutions of HEC derivatives at 25 °C. Each error bar represents the standard deviation of at least five measurements. [polymers] ) 0.5 wt %.

clear that the variation in the viscosity of the HHM-HEC solution differed from that of the R-HEC solution: the viscosity of the former in a Newtonian region at low shear rates increased rapidly when the NaCl concentration was increased up to 7 wt %. The HHM-HEC solutions were optically clear at all NaCl concentrations studied, similar to the HEC solutions (Figure 8). The viscosity of the R-HEC solution slightly increased when the NaCl concentration was increased up to 1 wt %, similar to the HHM-HEC solution. R-HEC containing the substituents of

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alkyl chains can form a three-dimensional network structure in an aqueous solution. Thus, the slight increase in the R-HEC viscosity suggests that the hydrophobic associations in the R-HEC solution were strengthened by the addition of salts. In contrast, we noted that the shear thinning behavior of the R-HEC solution started at lower shear rates as the NaCl concentration increased from 3 to 7 wt %. Furthermore, the R-HEC solution became turbid apparently at NaCl concentrations above 3 wt % (Figure 8). We propose that the decrease in R-HEC viscosity observed at NaCl concentrations above a certain threshold value is due to deterioration of the network structure caused by the salting-out effect and subsequent phase separation. According to the thixotropic loops of shear stress depending on the shear rate, the R-HEC solution in the absence of NaCl showed slightly greater hysteresis area than did the HHM-HEC solution (Figure 9). However, the hysteresis area of the R-HEC solution diminished as the NaCl concentration increased, and eventually disappeared at a NaCl concentration of 5 wt %, which indicated that the thixotropic nature of R-HEC had been lost. These findings reveal that the intramolecular hydrophilic substituents, the sulfonicacid groups, also play important roles in the salt-tolerance system of HHM-HEC. The values of G′ and G′′ for the HHM-HEC and R-HEC solutions were measured by varying the frequency from 0.1 to 100 Hz at 20% strain (Figure 10). The values of the two moduli of the HHM-HEC solution increased as the NaCl concentration increased (Figure 10a). The salt addition strengthens the hydrophobic associations in the HHM-HEC solution. In contrast, the values of G′ and G′′ of R-HEC decreased notably when the NaCl concentration was increased from 1 to 5 wt % (Figure 10b). The tan δ obtained from the two G values provided additional information about the individual characteristics of the two polymer solutions. The tan δ of HHM-HEC decreased as the NaCl concentration increased, while that of R-HEC increased at a moderate rate (Figure 11). This suggested that the HHMHEC solution was transformed into a more elastic gel by the addition of salts, whereas the R-HEC solution was changed from an elastic gel to a viscous fluid. For the R-HEC solution, the salt addition leads to a fragmentation of the network structure. There were obvious differences between the HHM-HEC and R-HEC solutions in terms of the effects of salt addition (Figures 7-11). These findings indicate that the behavior of HHM-HEC in salt solutions is due to the presence of both the hydrophobic and hydrophilic substituents. The decreased viscosity of the R-HEC solution at high NaCl concentrations suggests that the sulfonic-acid groups in the HHM-HEC polymer are required for the enhancement of the stability of the network structure in saltrich solutions. On the basis of the behavior of the R-HEC polymer dissolved in water, we propose the following explanation for the decrease in the solution viscosity: R-HEC solutions containing more than 3 wt % NaCl became turbid; that is, they showed a phase separation. As the ionic intensity of the R-HEC solution was increased by the addition of salts, the solubility of the hydrophobic substituents lowered, and the salting-out process began. Therefore, the three-dimensional network structure of the R-HEC solution became more fragile as the salt concentration increased. We suggest that the observed deterioration in the thickening ability of R-HEC results from the reduction in the polymer solubility and the fragmentation of the cross-linked network. It should be noted that the HHM-HEC solution became more elastic and firmer when salts were added, whereas the aqueous HEC solution showed less variation in its rheological properties (Figures 2-4 and 7). The degree of chain entanglement of HEC,

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Figure 9. Thixotropic loops of shear stress versus shear rate as a function of the NaCl concentration for aqueous solutions of HHM-HEC (a) and R-HEC (b) at 20 °C. [polymers] ) 0.5 wt %.

Figure 10. Storage (G′) and loss (G′′) moduli dependence on the angular frequency for aqueous solutions of the HEC derivatives with different concentrations of NaCl at 20 °C. Strain ) 20%, [polymers] ) 0.5 wt %.

which contributes to the viscosity behavior, is not affected by the salt addition. Hydrophobic associations appeared to induce notable differences in the rheological behaviors between HEC and HHM-HEC. The stronger thickening properties of the HHMHEC solution caused by the salt addition required the enhancement

Figure 11. Loss tangent (tan δ) dependence on the angular frequency for aqueous solutions of the HEC derivatives with different concentrations of NaCl at 20 °C. Strain ) 20%, [polymers] ) 0.5 wt %.

of hydrophobic associations. We can therefore explain the viscosity-enhancing effects of added salts on the HHM-HEC solution as follows: In the absence of salts, the electrostatic repulsion between the hydrophilic substituents of HHM-HEC tends to inhibit the hydrophobic aggregations of alkyl substituents, and thus leads to the weak formation of the three-dimensional

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Figure 12. Proposed model of the effects of added salts on the hydrophobic associations and electrostatic repulsions within the HHM-HEC solution. The respective sizes of the circles denoting the ions indicate the strengths of their electric fields.

network structure. When NaCl is added to the HHM-HEC solution, ionized groups are constricted by Na+ ions, and the electrostatic repulsion is weakened. By the charge-screening effects, the strength of hydrophobic associations is predominant in the viscoelasticity of the network structure. This idea is supported by the fact that the rheological properties (e.g., the values of the viscosity and the dynamic moduli) of the HHMHEC solution with 1 wt % NaCl were similar to those of the R-HEC solution without added salts (Figures 7, 9, and 10). Moreover, there are some reports that describe the effects of added salts on the rheological properties of a water-soluble polysaccharide, κ-carrageenan.30-32 The viscosity and the G′ value of an aqueous solution of κ-carrageenan, which also contains sulfonic-acid groups, were increased remarkably by the addition of metal ions; these effects resulted from the shielding of the electrostatic repulsion between sulfonic-acid groups and the enhancement of aggregation in the helical structure. The salt tolerance of the HHM-HEC solution can also be explained in terms of electrostatic repulsion and hydrophobic aggregation. On the basis of the observations of the effects of salt addition on the rheological properties of HEC derivatives, we propose the following mechanism for the salt tolerance of HHM-HEC (Figure 12). In an HHM-HEC solution without added salts, two forces affect the network structure: an attractive force produced by hydrophobic associations between alkyl side chains and a repulsive force produced by the electric charges of sulfonicacid groups. It is well-known that a counterion condensation takes place in a salt solution of a polyelectrolyte.33,34 Taking into consideration the fact that HHM-HEC can act as a polyelectrolyte because of the presence of the hydrophilic substituents, the repulsive force appears to be weakened by the added salts in the HHM-HEC solution. This process enables HHM-HEC to behave like R-HEC; that is, the attractive force between hydrophobic substituents becomes predominant in the salt solution. Therefore, the intensification of the hydrophobic bonds in the alkyl microdomains makes the HHM-HEC solution more elastic and rigid, while the entanglement of the backbone chains is maintained. However, from these observations of the R-HEC solution, we infer that the HHM-HEC solution cannot exhibit high viscosity at salt concentrations above a threshold value at (30) Watase, M.; Nishinari, K. Rheol. Acta 1982, 21, 318. (31) Watase, M.; Nishinari, K. Colloid Polym. Sci. 1985, 263, 744. (32) Watase, M.; Nishinari, K. Gums and Stabilisers for the Food Industry 3; Phillips, G. O., Wedlock, D. J., Williams, P. A., Eds.; Elsevier Applied Science Publishers: New York, 1986; p 185. (33) Oosawa, F. Polyelectrolytes; Marcel Dekker: New York, 1971. (34) Manning, G. S. Q. ReV. Biophys. 1978, 11, 179.

which the polymer chains are unable to remain hydrated and entangled in water. The threshold value of salt concentration, at which HHM-HEC starts to shrink, is controlled by the number of hydrophilic substituents. In future studies, we intend to investigate the effects of salt addition on the aggregation number, the radius of the domain coil, and the structural characterization of the HHM-HEC polymer to further elucidate the salt-tolerance mechanism.

Conclusions This study described the effects of the addition of monovalent and divalent salts on the solution viscosity of HHM-HEC. The solution viscosity of this amphiphilic polymer increased as the salt concentration increased. In a 1 wt % NaCl solution, HHMHEC showed higher viscosity than did HEC and S-HEC, and equal viscosity to that of R-HEC. However, while R-HEC gradually lost viscosity and became turbid at NaCl concentrations above 3 wt %, HHM-HEC did not. This indicated that the salt tolerance of the HHM-HEC solution, which was a unique property of this polymer, resulted from the presence of the two substituents within the HHM-HEC molecule. Rheological and fluorometric analyses confirmed that hydrophobic associations between the alkyl chains increased the elasticity in high-concentration salt solutions, while the hydrophilic substituents maintained the solubility of the polymer. Moreover, the values of the viscosity and the dynamic moduli of the HHM-HEC solution with 1 wt % NaCl were found to be similar to those of the R-HEC solution without added salts. On the basis of these observations, we proposed a mechanism for the increase in the HHM-HEC viscosity caused by the addition of salts as follows: Added salts decrease the electrostatic repulsion between the hydrophilic substituents and enhance the interactions of the hydrophobic substituents. This process leads to the formation of a firmer and more rigid network structure, which allows the HHM-HEC solution to maintain its unique rheological properties, even at high salt concentrations. Acknowledgment. The authors wish to acknowledge T. Kume, E. Akiyama, and N. Nagatani, co-workers at Kao Corp., for their useful discussions. Supporting Information Available: The novel polysaccharide derivative was synthesized by the method described under Japanese patent JP3329668. This material is available free of charge via the Internet at http://pubs.acs.org. LA052877N