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Langmuir 1997, 13, 4948-4952
Articles Viscosity of Dilute Aqueous Solutions of Hydrophobically Modified Chitosan and Its Unmodified Analogue at Different Conditions of Salt and Surfactant Concentrations Anna-Lena Kjøniksen,† Bo Nystro¨m,*,† Christian Iversen,† Torgeir Nakken,‡ Odd Palmgren,‡ and Terje Tande‡ Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway, and Norsk Hydro, Research Centre Porsgrunn, Department of Polymer and Surface Chemistry, P.O. Box 2560, N-3901, Porsgrunn, Norway Received March 10, 1997. In Final Form: June 13, 1997X Viscosity measurements have been carried out on dilute acid aqueous solutions of chitosan and of hydrophobically modified chitosan (HM-chitosan) with three different degrees of C12-aldehyde substitution. The experiments were performed in the presence of different amounts of the cationic surfactant cetyltrimethylammonium bromide (CTAB) (0-30 mm) and with and without salt addition. By using Huggins equation, the intrinsic viscosity [η], and Huggins constant (k′) were evaluated at different conditions. The intrinsic viscosity was shown to decrease with increasing salt concentration. Addition of CTAB to polymer solutions without NaCl causes the values of the intrinsic viscosity to decline and the values of k′ to rise. These observations are reminiscent of those reported from solutions of chitosan and HMchitosans without surfactant but with increasing salinity. It was observed that [η] and k′ dependency on the CTAB concentration is also influenced by the degree of hydrophobic substitution. In the presence of 10 mM NaCl, both [η] and k′ seem to be independent of surfactant concentration for both the unmodified chitosan and the HM-chitosan. The features can be rationalized in terms of screening of electrostatic repulsion.
Introduction The interaction between hydrophobically modified water-soluble polymers and surfactants has attained much interest in recent years.1-10 In the present work, the effects of surfactant and salt (NaCl) additions on the hydrodynamic properties of dilute solutions of chitosan of different hydrophobicity are studied by means of viscometry. The hydrophobically modified polymer (HM-chitosan) consists of C12-aldehyde chains grafted to the polymer backbone. A schematic illustration of the structure of chitosan is displayed in Figure 1. Chitosan, partially deacetylated chitin, is a linear polysaccharide consisting of random repeating units of β-(1-4)-linked 2-amino-2-deoxy-D* Author to whom correspondence should be addressed. † University of Oslo. ‡ Norsk Hydro. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Langmuir 1986, 2, 536. (2) McCormick, C. L.; Bock, J.; Schulz, D. N. Encyc. Polym. Sci. Eng. 1989, 17, 730. (3) Polymers in Aqueous Media; Glass, J. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (4) Lundberg, D. J.; Glass, J. E.; Eley, R. R. Proc. ACS Div. Polym. Mater. Sci. Eng. 1989, 61, 533. (5) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid. Polym. Sci. 1992, 89, 118. (6) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (7) Lindman, B.; Carlsson, A.; Gerdes, S.; Karlstro¨m, G.; Piculell, L.; Thalberg, K.; Zhang, K. In Food Colloids and Polymers: Stability and Mechanical Properties; Walstra, P., Dickinson, E., Eds.; The Royal Society of Chemistry: London, 1993; pp 113-125. (8) Macromolecules Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (9) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (10) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604.
S0743-7463(97)00259-X CCC: $14.00
Figure 1. A schematic illustration of the structure of chitosan and the hydrophobically modified chitosan (HM-chitosan).
glucopyranose and 2-acetamido-2-deoxy-D-glucopyranose.11,12 The polymer chitosan is polycationic, i.e., positively charged at pH < 6. At neutral pH most chitosans (% deacetylation > 70) will loose its charge and precipitate from solution. Chitosan has become a popular biopolymer for a variety of biomedical and industrial applications.13-16 Due to the presence of protonated amino groups (the amino group in chitosan has a pKa of about 6.2-717,18 ), chitosan in dilute acid aqueous solution exhibits a polyelectrolyte character at low pH, and its hydrodynamic behavior in (11) Muzzarelli, R. A. A. In Chitin; Muzzarelli, R. A. A., Ed.; Pergamon Press: Oxford, 1977; p 1. (12) Jeuniaux, C.; Voss-Foucart, M.; Poulicek, M.; Bussers, J. In Chitin and Chitosan; Skjåk-Bræk, G., Anthonsen, T., Sandford, P., Eds.; Elsevier Applied Science: London, 1989; p 3. (13) Muzzarelli, R. A. A.; Tanfani, F.; Emanuelli, M.; Mariotti, S. Carbohydr. Res. 1982, 88, 172. (14) Chitin, Chitosan and Related Enzymes; Zikakais, J. P., Ed.; Academic Press Inc.: New York, 1984. (15) Muzzarelli, R. A. A. In Chitin and Chitosan; Skjåk-Bræk, G., Anthonsen, T., Sandford, P., Eds.; Elsevier Applied Science: London, 1989; p 491. (16) Nakatsuka, S.; Andrady, A. L. J. Appl. Polym. Sci. 1992, 44, 17. (17) Park, J. W.; Choi, K.-H. Bull. Korean Chem. Soc. 1983, 4, 68. (18) Rinaudo, M.; Domard, A. In Chitin and Chitosan; Skjåk-Bræk, G., Anthonsen, T., Sandford, P., Eds.; Elsevier Applied Science: London, 1989; pp 71-86.
© 1997 American Chemical Society
Viscosity of Aqueous Solutions of Chitosan
the solution is intricate.19-23 The physicochemical properties of solutions of chitosan are expected to be governed by factors such as temperature, pH, ionic strength, surfactant concentration, and degree of deacetylation. It is known24,25 that the charge density along the chain increases with an increase in the degree of deacetylation. In addition, the hydrophobicity of chitosan should also play an important role for the hydrodynamic behavior. In a previous paper26 we reported results from rheological experiments on aqueous semidilute systems of chitosan/ acetic acid and of a hydrophobically modified analogue at different pH and in the presence of various amounts of the cationic surfactant cetyltrimethylammonium bromide (CTAB). The results for the hydrophobically modified chitosan suggested enhanced polymer-surfactant interaction at a surfactant concentration of about 1 mM, which is close to the expected critical micelle concentration (cmc) of CTAB. The picture that emerged from the above-cited study is that strong polymer-surfactant interaction promotes the formation of “mixed micelles”, consisting of CTAB molecules and hydrophobic side chains of the polymer, and that this process increases the degree of interpolymer association and, consequently, the solution viscosity. The findings from the above-cited investigation revealed that the hydrophobicity of the polymer, pH, and the surfactant concentration had a significant influence on the rheological properties of the system. The phenomena of polymer-surfactant interaction and “mixed micelles” should also play an important role for the interaction situation in dilute solutions of HM-chitosan. Most of our attention in this work is focused on the study of the viscosity of dilute aqueous solutions of chitosan of different degrees of hydrophobic substitution in 1% acetic acid and in the presence of various CTAB and salt (NaCl) concentrations at a fixed pH of 3.0. At this pH value, chitosan in solution carries a positive charge along its backbone. The effects of hydrophobicity, surfactant, and salt addition on the intrinsic viscosity and Huggins’ constant will be reported. Experimental Section Materials. The unmodified chitosan sample was obtained from Pronova Biopolymers (Drammen, Norway). The degree of N-deacetylation was determined to be 84% by high-field 1H-NMR spectroscopy.27 By using low-angle laser light scattering (LALLS) and size-exclusion chromatography (SEC), the weight-average molecular weight was found to be Mw ) 4 × 105 and the polydispersity index was Mw/Mn ) 2.7. The SEC measurements were carried out by utilizing a RI detector. Three TSK columns (TSK 6000 PW Guard column + TSK 6000 PW + TSK 5000 PW) in series were used in these experiments. The system 0.2 M ammonium acetate at pH ) 4.5 served as the mobile phase, and the flow rate was 0.5 mL/min at room temperature. The details of these methods in connection with the characterization of chitosan have been discussed previously.28 The cationic CTAB surfactant was purchased from Fluka and was used without (19) Chen, R.-H.; Lin, W.-C. J. Fish. Soc. Taiwan 1992, 19, 299. (20) Anthonsen, M. W.; Vårum, K. M.; Smidsrød, O. Carbohydr. Polym. 1993, 22, 193. (21) Vårum, K. M.; Ottøy, M. H.; Smidsrød, O. Carbohydr. Polym. 1994, 25, 65. (22) Amiji, M. M. Carbohydr. Polym. 1995, 26, 211. (23) Jocic, D.; Julia´, M. R.; Erra, P. Colloid Polym. Sci. 1996, 274, 375. (24) Wang, W.; Qin, W.; Bo, S. Makromol. Chem., Rapid Commun. 1991, 12, 559. (25) Wang, W.; Bo, S.; Li, S.; Qin, W. Int. J. Biol. Macromol. 1991, 13, 281. (26) Kjøniksen, A.-L.; Nystro¨m, B.; Nakken, T.; Palmgren, O.; Tande, T. Polym. Bull. 1997, 38, 71. (27) Vårum, K. M.; Anthonsen, M. W.; Grasdalen, H.; Smidsrød, O. Carbohydr. Res. 1991, 211, 17. (28) Roberts, G. A. F. Chitin Chemistry; The MacMillan Press Ltd.: London, 1992.
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Figure 2. Concentration dependence of the dynamic viscosity (all the data have been collected at a fixed frequency of 2.0 s-1) of semidilute acid aqueous solutions of chitosan of different degrees of hydrophobicity. The inset plot shows a magnification of the lower concentration range. The most hydrophobic sample (10 mol %) cannot be dissolved above a polymer concentration of 0.3 wt %. further purification. The hydrophobically modified chitosans are equivalent to the unmodified chitosan sample but with C12aldehyde chains grafted to the polymer backbone. These samples were prepared by reaction with the amino groups on the polymer chains with a C12-aldehyde. The procedure used here is similar to that described elsewhere.29 In this study, the degrees of C12aldehyde substitution were 2.5, 5, and 10 mol %. Dilute aqueous solutions of chitosan and HM-chitosan in 1% acetic acid were prepared by weighing the components, and the solutions were homogenized by stirring at room temperature overnight. All the measurements were carried out at pH ) 3.0, the value that was obtained in this polymer concentration range by dissolving chitosan or HM-chitosan in 1% acetic acid. Viscosity Measurements. Viscosity measurements were performed with a standard Ubbelohde viscometer, with solvent flow times of >180 s, placed into a temperature-controlled water bath at 25 ( 0.05 °C. The solutions were filtered through a 0.8 µm Millipore filter to remove dust and other traces of impurities. The measurement was repeated several times until reproducible values were obtained. The reproducibility of an experiment was usually better than (0.3%. All the investigated samples exhibit a linear dependence of the reduced viscosity ηsp/c ) (η - η0/η0c) with concentration (see Figure 3); η is the viscosity of the solution, η0 that of the solvent, and c is the polymer concentration (g/mL). The extrapolation of ηsp/c to zero concentration provides the intrinsic viscosity [η], and the constant of Huggins equation k′ was calculated from the slope of the ηsp/c versus c using Huggins equation (see eq 1). Oscillatory shear experiments were conducted in a Bohlin VOR rheometer system using, depending on the viscosity of the sample, a double-gap concentric cylinder, an ordinary concentric cylinder geometry, or a cone-and-plate geometry, with a cone angle of 5° and a diameter of 30 mm. The measurements were carried out in the approximate frequency domain 0.03-3 Hz, and the values of the strain amplitude were checked in order to ensure that all experiments were conducted within the linear viscoelastic region.
Results and Discussion In order to confirm the effect of hydrophobic substitution, the dynamic viscosity of semidilute aqueous solutions of chitosan and of hydrophobically modified chitosans in 1% acetic acid is depicted in Figure 2. It is evident that the dynamic viscosity rises with increasing polymer concentration and that this increase is more pronounced as the hydrophobicity of the polymer increases. The hydrophobic effect on the dynamic viscosity becomes gradually more (29) Moore, G. K.; Roberts, G. A. F. Int. J. Biol. Macromol. 1981, 3, 337.
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Figure 4. The effect of surfactant addition on the intrinsic viscosity ([η]) of solutions of chitosan and hydrophobically modified chitosan in the absence (a) and in the presence (b) of NaCl.
Figure 3. Concentration dependence of the reduced viscosity (ηsp/c) of dilute solutions of a hydrophobically modified chitosan at the surfactant and salt concentrations indicated. The lines represent linear regressions of the data. The intrinsic viscosity and Huggins constant are calculated from the fitted lines with the aid of eq 1.
marked as the chitosan concentration increases. However, even at the onset of the semidilute regime, this effect can be detected (see the inset plot of Figure 2). These features show that the intermolecular associations become more dominant as the hydrophobicity of the polymer increases. This feature probably indicates formation of “mixed micelles” and enhanced cross-linking ability as the number of hydrophobic groups increases. Further rheological results on hydrophobically modified chitosan systems in the semidilute concentration regime will be presented in a forthcoming paper. Intrinsic viscosity for the samples of this study was evaluated by using Huggins equation30 which is given as follows:
ηsp/c ) [η] + k′c[η]2
(1)
The classical Huggins expansion takes care of the influence of binary hydrodynamic interactions to the viscosity. Here, k′ is the Huggins constant which depends on molecular architecture and interactions.31 The intrinsic viscosity is a measure of the hydrodynamic volume of the polymer at infinite dilution, and for flexible coils, the intrinsic viscosity is related to the radius of gyration RG and to the hydrodynamic radius RH through the relationship32
[η] = RHRG2/M
(2)
(30) Huggins, M. J. Am. Chem. Soc. 1942, 64, 2716. (31) Bohdanecky, M.; Kova´r, J. In Viscosity of Polymer Solutions; Jenkins, A. D., Ed.; Elsevier: Amsterdam, 1982. (32) Weill, G.; des Cloizeaux, J. J. Phys. (Paris) 1979, 40, 99.
where M is the molecular weight of the polymer. The effects of surfactant concentration and salt addition on [η] for chitosan and its hydrophobically modified analogues are depicted in Figure 4. The general behavior for both the unmodified and modified chitosans in the absence of NaCl is that the value of [η] falls off with increasing surfactant concentration. The trend is similar for both the unmodified and the hydrophobically modified chitosans. The decrease of [η] suggests that the size of the polymer molecules shrinks with surfactant addition. At the considered pH (3.0), the polymer chains carry positive charges and the addition of a cationic surfactant may cause electrostatic screening and neutralization of some of the charges.33 This effect is reminiscent of that observed upon addition of NaCl to chitosan or HM-chitosan solutions without surfactant (cf. the discussion below). In the presence of 10 mM NaCl, the electrostatic interactions are screened and the values of [η] for chitosan and HMchitosan are practically independent of surfactant addition (see Figure 4b). We may note that the values of [η] are higher for the hydrophobically modified chitosans, both in the presence and in the absence of salt, which suggests that the C12-aldehyde chains grafted to the polymer backbone give rise to an expansion of the molecules. In this context, we should note that CTAB interacts with the hydrophobic groups of chitosan, and steric hindrance effects may contribute to the chain expansion. We may also note that most of the expansion effect occurs between the unmodified and the 2.5 mol % hydrophobically modified chitosan. At higher degrees of hydrophobic substitution the residual effect is fairly small. It is possible that most of the structural changes of the chitosan molecules take place already at a low degree of hydrophobic substitution. Another illustration of the gradual screening of electrostatic interactions is shown in Figure 5, where the effects of salt addition on [η] for the systems chitosan/1% CH3COOH and HM-chitosan/1% CH3COOH without surfactant are depicted. In a similar way as in the case of (33) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker, Inc.: New York, 1991; pp 189-248.
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Figure 5. The effect of salt (NaCl) on the intrinsic viscosity ([η]) of chitosan and of hydrophobically modified chitosan in the absence of surfactant.
increasing surfactant concentration, the salt addition reduces the hydrodynamic volume of the molecules. A reduction of the intrinsic viscosity with increasing ionic strength is consistent with that reported19,34 previously for unmodified chitosan in aqueous acid solutions. We note that the effect of salt addition on the intrinsic viscosity is similar for the unmodified and the modified polymers, but the values of [η] are constantly lower for the unmodified chitosan. Again we can see that the effect is most pronounced when going from the unmodified chitosan to the 2.5 mol % hydrophobically modified polymer. At higher degrees of hydrophobic substitution the effect is less marked. It should also be noted that the decrease of [η] for the unmodified and hydrophobically modified chitosan solutions with increasing salinity is similar to that observed in solutions without NaCl but with increasing surfactant addition (cf. Figures 4a and 5). In principle we expect that [η] for the HM-polymers is governed by the resultant of two opposing effects: the Coulombic repulsive forces between ionic charges borne by the polymer chains and the attractive interactions between the hydrophobic segments. In a recent study35 on a charged propylene glycol ester of sodium alginate (PGA) and its hydrophobically modified analogue PGAC12 (the covalent immobilization (9 mol %) of an aliphatic long-chain amine onto PGA), the reduction of [η] with increasing salinity was much more pronounced for the HM-polymer, which was attributed to hydrophobic association. However, the present results suggest that the behavior of [η] upon addition of salt is similar for the unmodified and the hydrophobically modified analogues. This indicates that the hydrophobic effect has only a weak influence on the viscosity behavior in dilute solutions. This is in contrast to the rheological behavior observed26 in semidilute solutions of chitosan and HM-chitosan, where the hydrophobicity was found to play a dominating role (see also Figure 2). Figure 6 shows the effects of surfactant and salt addition on the Huggins constant k′ of chitosan and hydrophobically modified chitosans. Although the data are scattered, the general trend is that k′ rises with increasing surfactant concentration in the absence of salt and that the values of k′ are higher for the unmodified chitosan. The behavior of k′ with increasing CTAB concentration seems to be
consistent with that observed for the intrinsic viscosity. The lower the hydrophobicity of the polymer the more pronounced is the increase in k′ with CTAB concentration, again probably due to the interaction between surfactant molecules and the hydrophobic groups. The significance of k′ has been investigated both theoretically30,36,37 and experimentally.31,38-40 This quantity can be viewed as a measure of polymer-solvent and polymer-polymer interactions, or practically as a parameter indicating the relative effectiveness of various solvents for a given polymer and is almost independent of the molecular weight for linear polymers. High values of the Huggins constant are usually interpreted as enhanced coil-coil interactions and indicate poorer solvent
(34) Lyubina, S. Y.; Strelina, I. A.; Nud’ga, L. A.; Plisko, Y. A.; Bogatova, I. N. Vysokomol. Soedin. 1983, A25, 1467. (35) Sinquin, A.; Hubert, P.; Dellacherie, E. In Cellulose and Cellulose Derivatives: Physico-Chemical Aspects and Industrial Applications; Kennedy, J. F., Phillips, G. O., Williams, P. A., Piculell, L., Eds.; Woodhead Publishing Ltd: Cambridge, England, 1995; pp 339-344.
(36) Eirich, F.; Riseman, J. J. Polym. Sci. 1949, 4, 417. (37) Simha, R. J. Colloid Sci. 1950, 5, 386. (38) Gragg, L. H.; Sones, R. H. J. Polym. Sci. 1952, 9, 585. (39) Gragg, L. H.; Fern, R. H. J. Polym. Sci. 1953, 10, 185. (40) Antonietti, M.; Briel, A.; Fo¨rster, S. J. Chem. Phys. 1996, 105, 7795.
Figure 6. The effect of surfactant addition on the Huggins constant (k′) in solutions of chitosan and of hydrophobically modified chitosan in the absence (a) and in the presence (b) of NaCl.
Figure 7. The intrinsic viscosity ([η]) for the unmodified and the three hydrophobically modified chitosan systems. Effects of CTAB addition to solutions without any NaCl (0) and in the presence of 10 mM NaCl (b).
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quality. The detected increase of k′ with surfactant concentration in salt-free solutions of chitosan and HMchitosan (see Figure 6a) is probably associated with gradual screening of the electrostatic repulsions. In this process, the chains interact intramolecularly to shrink the coil, lowering the intrinsic viscosity (see Figure 4) and raising k′. This behavior is in agreement with the general observation31,41,42 that k′ increases when the coil size decreases. In this context it is interesting to note a recent viscosity study43 on dilute aqueous solutions of a nonionic ethyl(hydroxyethyl)cellulose (EHEC) in the presence of various amounts of the anionic surfactant sodium dodecyl sulfate (the bound ionic surfactant endows an apparent polyelectrolyte character to the initially nonionic EHEC). The results revealed a similar feature with increasing surfactant addition as in the present work, namely a strong decrease of [η] was accompanied by raising values of k′. Similar findings have also been reported44-46 for other associating polymers. In the presence of salt (see Figure 6b) the value of k′ is practically independent of surfactant concentration and the value of k′ is not too far from that (0.64) for noninteracting spheres.40,47 The conjecture is that at this ionic strength the Coulombic repulsion is effectively screened and the behavior of the coils is reminiscent of that of noninteracting spheres. Figure 7 shows a survey of the effects of surfactant addition and salinity on the intrinsic viscosity for chitosan and HM-chitosans. The overall picture that emerges from (41) Tuzar, Z.; Kratochvil, P. In Surface and Colloid Science; Matijivic, E., Ed.; Plenum Press: New York, 1993; Vol. 15. (42) Lovell, P. A. In Comprehensive Polymer Science; Booth, C., Price, C., Eds.; Pergamon Press: Oxford, England, 1989; Vol. 1, Chapter 9. (43) Holmberg, C.; Nilsson, S.; Singh, S. K.; Sundelo¨f, L.-O. J. Phys. Chem. 1992, 96, 871. (44) Witten, T. A.; Cohen, M. H. Macromolecules 1985, 18, 1915. (45) Agarwal, P. K.; Garner, R. T.; Graessley, W. W. J. Polym. Sci., Part B: Polym. Phys. Ed. 1987, 25, 2095. (46) Magny, B.; Iliopoulos, I.; Audebert, R. In Macromolecules Complexes in Chemistry and Biology, Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (47) Fixman, M. J. Chem. Phys. 1966, 45, 793.
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Figure 8. Huggins constant (k′) for the unmodified and the three hydrophobically modified chitosan systems. Effects of CTAB addition to solutions without any NaCl (0), and in the presence of 10 mM NaCl (b).
these results is that the polymer hydrodynamic volume decreases with increasing screening of the electrostatic interactions and at high level of surfactant addition [η] approaches the value observed in the presence of 10 mM NaCl. Figure 8 displays an analogous plot of the Huggins constant for the same conditions as depicted in Figure 7. The general trend is that, in the presence of NaCl, the value of k′ is practically independent of surfactant concentration, while in the absence of salt the value of k′ rises as the level of surfactant addition increases. The findings in the presence of NaCl suggest that at this salinity the intensity in hydrodynamic interaction is practically not affected by the level of surfactant addition. Acknowledgment. This work was supported by Norsk Hydro and the Norwegian Research Council through the program PROSMAT. LA9702594
