Effects of Surfactant and Temperature on Rheological and Structural

Oct 11, 2005 - The frequency dependencies of the storage (G') and loss (G' ') moduli for some polymer systems can be portrayed by the simple Maxwell ...
0 downloads 0 Views 267KB Size
Download PDF
Langmuir 2005, 21, 10923-10930

10923

Articles Effects of Surfactant and Temperature on Rheological and Structural Properties of Semidilute Aqueous Solutions of Unmodified and Hydrophobically Modified Alginate Huaitian Bu,† Anna-Lena Kjøniksen,† Kenneth D. Knudsen,‡ and Bo Nystro¨m*,† Department of Chemistry, University of Oslo, P. O. Box 1033, N-0315 Oslo, Norway, and Department of Physics, Institute for Energy Technology, P. O. Box 40, N-2027 Kjeller, Norway Received May 3, 2005. In Final Form: August 3, 2005 The dynamic and structural perturbations that result from the interactions between the anionic surfactant sodium dodecyl sulfate (SDS) and the hydrophobically modified biopolymer alginate (HM-alginate) have been studied with the aid of rheological methods, turbidimetry, and small-angle neutron scattering (SANS). The rheological results for a semidilute HM-alginate solution in the presence of SDS disclose strong interactions between HM-alginate and SDS at a low level of surfactant addition, and this feature is accompanied by enhanced turbidity. At higher surfactant concentrations the association complexes are disrupted. A strong temperature effect of the viscosity is observed in HM-alginate solutions at moderate SDS concentrations, where an elevated temperature leads to enhanced chain mobility, which promotes a breakup of the association complexes. The SANS results reveal a pronounced peak in the plot of scattered intensity versus wavevector q at intermediate q values for SDS concentrations above the critical micelle concentration (cmc). With contrast-matching conditions, using deuterated SDS instead of SDS, no interaction peak appears but an “upturn” in the scattered intensity is observed at small q value. The magnitude of this effect decreases with increasing surfactant concentration, showing clearly that SDS is capable of breaking up the large aggregates created.

Introduction Interaction between hydrophobically modified watersoluble polymers (HMWSP) and ionic surfactants has attracted significant interest in recent years.1-4 This type of amphiphilic polymer consists of a hydrophilic chain to which small amounts of hydrophobic substituents are incorporated as pendant chains, blocks, or terminal groups. Aqueous solutions of HMWSP exhibit unusual dynamic and rheological features because of intra- and intermolecular associations via the hydrophobic groups. The solution properties of these polymers can be controlled and manipulated by design, hydrophobicity, and interaction with cosolutes. In the presence of an ionic surfactant, the structural and rheological behaviors of these systems are governed by an intricate interplay between hydrophobic, hydrophilic, and ionic interactions.4 The addition of a surfactant to an aqueous solution of a hydrophobically modified polymer usually leads to a viscosification of the solution at a moderate level of surfactant addition. Several studies1-4 on mixtures of a hydrophobically modified polymer and an ionic surfactant have been reported in † ‡

University of Oslo. Institute for Energy Technology.

(1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Macromolecular Complexes in Chemistry and Biology; Dubin, P., Bock, J., Davies, R. M., Schulz, D. N., Thies, C., Eds.; Springer-Verlag: Berlin, 1994. (3) Polymer-Surfactant Systems; Kwak, J. C., Ed.; Marcel Dekker: New York, 1998; Vol. 77. (4) Malmsten, M. Surfactants and Polymers in Drug Delivery, Drugs and the Pharmaceutical Sciences; Marcel Dekker: New York, 2002; Vol. 122.

recent years. However, there is a lack of investigations on hydrophobically modified polymers that are biocompatible and, therefore, attractive for pharmaceutical applications. This property makes them potential materials for drugrelease systems and tissue engineering.5 Polysaccharides such as pectin, alginate, and chitosan are biocompatible and belong to the category of polyelectrolytes with either negative or positive charges on the backbone. In this study, we have chosen alginate and synthesized a hydrophobically modified analogue (HM-alginate). Alginate can be classified as an anionic copolymer, and the modified analogue can be considered as a hydrophobically modified anionic polyelectrolyte. Alginate is regarded as a biocompatible, nontoxic, nonimmunogenic, and biodegradable polymer, making it an attractive candidate for biomedical applications. It is a naturally derived linear polysaccharide comprised of β-D-mannuronic acid (M block) and R-L-guluronic acid (G block) units arranged in blocks rich in G units or M units, separated by blocks of alternating G and M units.6 A schematic of the alginate building blocks is shown in Figure 1. The physical properties of alginates depend not only on the uronic acid composition but also on the relative proportion of the three types of blocks.7 In this work, the interaction between the anionic surfactant sodium dodecyl sulfate (SDS) and alginate or HM-alginate has been studied with the aid of rheology, (5) Polymer Based Systems on Tissue Engineering Replacement and Regeneration, Reis, R. L., Cohn, D., Eds.; Kluwer Academic: Dordrecht, 2002. (6) Haug, A.; Larsen, B.; Smidsrød, O. Acta Chem. Scand. 1966, 21, 691. (7) Haug, A.; Myklestad, S.; Larsen, B.; Smidsrød, O. Acta Chem. Scand. 1967, 21, 768.

10.1021/la051187g CCC: $30.25 © 2005 American Chemical Society Published on Web 10/11/2005

10924

Langmuir, Vol. 21, No. 24, 2005

Figure 1. Structure of the chemical units of alginate (M ) mannuronic acid and G ) guluronic acid). The hydrophobically modified alginate (HM-alginate) has n-octylamine (C8) side chains attached to the carbonyl atom (in the COO- group) with a concentration of 5.4 mol %.

turbidity, and small-angle neutron scattering (SANS). Effects of surfactant addition and temperature on rheological and structural properties will be reported. This work provides new insights into the interactions between hydrophobically modified alginate and a surfactant. Rheological data yield information about dynamics of the polymer network and on the strength of the associations. It will be demonstrated that some of the characteristic rheological features related to polymer-surfactant associations will appear in a similar way in the turbidity results. This reflects the formation of large-scale complexes, and the findings indicate major changes of the thermodynamic conditions. Some unexpected strong temperature effects on the viscosity of HM-alginate solutions at moderate and high levels of SDS addition are reported. The SANS experiments have been conducted both with SDS and at contrast-matching conditions (the surfactant is “invisible” in the measurements) by using deuterated SDS (d-SDS), and the results reveal novel structural differences. Experimental Section Materials. An alginate sample, designated LF 10/60 LS, was supplied by FMC Biopolymers, Drammen, Norway. According to the specifications from the manufacturer, this sample has a weight-average molecular weight of 152 000 and the guluronic acid to mannuronic acid (G/M) ratio is 0.75. Dilute alginate solutions were dialyzed against pure water for several days to remove the salt and other low-molecular-weight impurities. Regenerated cellulose with a molecular weight cutoff of 8000 was used as dialyzing membrane. After dialysis, the solutions were freeze-dried and the polymer was kept in the refrigerator. Hydrochloric acid (HCl) was obtained from Merck and was of analytical grade. A concentration of 0.1 M HCl was prepared by dissolving a suitable amount of concentrated HCl in Millipore water (used for all preparations). Formaldehyde, n-octylamine, and cyclohexyl isocyanide were supplied by Merck or Fluka and were all of analytical grade. Sodium dodecyl sulfate (SDS) was purchased from Fluka with 99% purity, and the deuterated SDS utilized in the SANS experiments was obtained from Cambridge Isotope Laboratories, Andover, MA. These chemicals were used without any further purification. Synthesis of HM-Alginates. Unmodified alginate was dissolved in water at room temperature by weighing the components (20 g; 2.5 wt %), and it was gently stirred overnight to obtain a

Bu et al. homogeneous solution. The solution was then slightly acidified with 0.1 M HCl to obtain the acidity (pH ≈3.6) necessary for the Ugi reaction to proceed,8 and the polymer concentration was diluted to 2.0 wt %. The n-octylamine groups (C8) were grafted onto the backbone of the alginate chain via the Ugi multicomponent condensation reaction.9 The molar amount of the noctylamine groups (C8) was calculated with respect to the molar amount of carbohydrate monomers. The other components were added to the reaction mixture in an excess of about 40%. In the present case, formaldehyde (0.018 mol), n-octylamine (0.013 mol), and cyclohexyl isocyanide (0.018 mol) were added to the solution successively. After the addition of each component, the solution was stirred vigorously for 15 min to disperse the components homogeneously in the solution. The solution was then stirred at room temperature for 24 h before the reaction mixture was diluted to 0.7 wt % and purified by dialyzing against distilled water for 7 days, and subsequently freeze-dried. The reaction yield was 84%. The chemical structure and purity of the HM-alginate was ascertained by 1H NMR (D2O, 85 °C) with a 500 MHz Bruker DRX 500 spectrometer (Bruker Biospin, Fa¨llanden, Switzerland): δ (ppm) ) 5.08 (C1H, G unit), 4.68 (C1H, M unit), 0.88 (CH3, hydrophobic part), 1.30-1.34 (-(CH2)6-, hydrophobic part); other protons of the alginate unit and octyl chains overlapped and were unresolved. As a reference for 1H NMR at 0 ppm, 3-trimethylsilylpropionic acid sodium salt-d4 was used. The hydrophobic modification degree was determined from the peak ratios between the anomeric protons and methyl protons of the octyl chain. The degree of modification determined from NMR analysis was 5.4 mol %. The total polymer concentration was kept constant at 1 wt % (semidilute regime) for both alginate and HM-alginate in all experiments. The samples were prepared by weighing the components, and the solutions were homogenized by stirring at room temperature for several days. Rheological Experiments. Shear viscosity and oscillatory sweep measurements were conducted in a Paar-Physica MCR 300 rheometer using a cone-and-plate geometry, with a cone angle of 1° and a diameter of 75 mm. The samples were introduced onto the plate, and to prevent evaporation of the solvent, the free surface of the sample was always covered with a thin layer of low-viscosity silicone oil. (The viscoelastic response of the samples is not observed to be affected by this layer.) The measuring device is equipped with a temperature unit (Peltier element) that provides rapid alteration of the temperature and gives good temperature control over an extended time for all the temperatures investigated in this work. The values of the strain amplitude were checked to ensure that all measurements were performed in the linear-viscoelastic regime, where the dynamic storage modulus (G′) and loss modulus (G′′) are independent of strain amplitude. The oscillating sweep experiments were carried out over an extended angular frequency (ω) domain. The shear viscosity measurements were conducted over an extended shear rate range. Turbidity Measurements. The transmittances of alginate and HM-alginate solutions in the presence of various amounts of SDS were measured with a temperature-controlled Helios Gamma (Thermo Spectronic, Cambridge, UK) spectrophotometer at different temperatures at a wavelength of 500 nm. The apparatus is equipped with a temperature unit (Peltier plate) that gives an accurate temperature control over an extended time. The turbidities, τ, of the samples were calculated from

τ ) (-1/L) ln(It/I0)

(1)

where L is the light path length in the cell (1 cm), It is the transmitted light intensity, and I0 is the incident light intensity. Small-Angle Neutron Scattering (SANS). Small-angle neutron scattering experiments were carried out at 25 °C (temperature controlled to within (0.1 °C) at the SANS instal(8) de Nooy, A. E. J.; Capitani, D.; Masci, G.; Crescenzi, V. Biomacromolecules 2000, 1, 259. (9) Ugi, I.; Lohberger, S.; Karl, R. The Passerini and Ugi Reactions. In Comprehensive Organic Synthesis; Trost, B. M., Heathcock, C. H., Eds.; Pergamon: Oxford, 1991; Vol. 2, pp 1083-1107.

Properties of Semidilute Solutions of HM-Alginate

Langmuir, Vol. 21, No. 24, 2005 10925

Figure 2. Shear rate dependencies of viscosity for 1 wt % solutions of alginate and HM-alginate at 25 °C and SDS concentrations indicated. lation at the IFE reactor at Kjeller, Norway. The wavelength was set with the aid of a velocity selector (Dornier), using a high full width at half-maximum for the transmitted beam with a wavelength resolution (∆λ/λ) of 20%, and maximized flux on the sample. The beam divergence was set by an input collimator (18.4 or 12.2 mm diameter) located 2.2 m from the sample, together with a sample collimator that was fixed to 4.9 mm. The detector was a 128 × 128 pixel, 59 cm active diameter, 3He-filled RISØ type detector, which is mounted on rails inside an evacuated detector chamber. The 1 wt % solutions of alginate and HM-alginate with heavy water as solvent and in the presence of various levels of SDS or d-SDS addition were filled in 2 mm Hellma quartz cuvettes (with stoppers), which were placed onto a copper base for good thermal contact and mounted in the sample chamber. The space between the sample and the detector was evacuated to reduce air scattering. In the SANS measurements, D2O was used as a solvent instead of light water to obtain good contrast and low background for the neutron-scattering experiments. However, with d-SDS as the additive, a mixture of 91% D2O and 9% H2O, with scattering length densities equal to, respectively, -0.56 × 1010 and 6.3 × 1010 cm-2, was used to contrast match the surfactant. Each complete scattering curve is composed of three independent series of measurement, using three different wavelength-distance combinations (5.1 Å/1.0 m, 5.1 Å/3.4 m, and 10.2 Å/3.4 m). These combinations were utilized to yield scattering vectors q ) (4π/λ) sin (θ/2) (where λ is the wavelength of the incident beam, and θ is the scattering angle) in the range of 0.008-0.25 Å-1. Standard reductions of the scattering data, including transmission corrections, were conducted by incorporating data collected from empty cell, beam without cell, and blocked-beam background. When relevant, the data were transformed to an absolute scale (coherent differential cross section (dΣ/dΩ)) by calculating the normalized scattered intensity from direct beam measurements.

Results and Discussion Rheological Characteristics. Figure 2 shows the shear rate dependence of the viscosity for 1 wt % solutions of alginate and HM-alginate in the presence of different amounts of SDS. A virtually Newtonian behavior, over the considered shear rate domain, is observed for the alginate solution at all levels of SDS addition. The small change of the viscosity with the SDS concentration indicates weak interactions between SDS and the unmodified polymer. In contrast, strong interactions are induced in the HM-alginate solutions at moderate concentrations of SDS, whereas the associations are disrupted at high levels of surfactant addition. In HM-alginate solutions with low amounts of SDS, shear thickening is visible at low shear rates, followed by shear thinning at higher shear rates. The former effect suggests a shear-

Figure 3. Effect of addition of SDS on zero-shear viscosity (a) and on turbidity at a wavelength of 500 nm (b) for 1 wt % solutions of alginate and HM-alginate at 25 °C.

induced reorganization of the association network, probably due to stretching and alignment of the chains and thereby facilitating the formation of more polymersurfactant junctions. The most pronounced shear-thinning behavior at higher shear rates is observed for the system that exhibits the strongest association network and viscosity enhancement. The progressive decrease in viscosity as the shear rate rises is ascribed to the breakdown of the network junctions; that is, the rate of network disruption exceeds the rate at which cross-links are reformed. In Figure 3a, effects of the addition of surfactant on the zero-shear viscosity η0 are illustrated for 1 wt % solutions of alginate and HM-alginate. A conspicuous feature is that for the HM-alginate-SDS system η0 passes through a pronounced maximum (located at about 2 mm SDS), whereas for the alginate-SDS system the value of η0 is much lower and the value depends only weakly on the level of surfactant addition. This behavior supports the conjecture of weak interactions between the unmodified polymer and the surfactant. In the case of the HM-alginate, the enhancement of the viscosity signals the formation of micellar-type cross-links between the polymer chains and SDS (the surfactant works as a cross-linking agent). It is usually assumed that the binding of surfactant to the polymer occurs at the critical aggregation concentration (cac), above which the surfactant will associate cooperatively and bring polymer chains together. However, in the case when the hydrophobic tails are attached to the polymer backbone, it has been reported10,11 that the value of cac is very low or nonexistent; that is, the surfactant is adsorbed in a noncooperative way as soon some surfactant molecules are present. This may explain why the viscosity maximum is located at such a low SDS concentration. Piculell and co-workers have performed extensive studies on similar systemsshydrophobically modified hydroxyethylcellulose (HM-HEC) and ethyl(10) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994. (11) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909.

10926

Langmuir, Vol. 21, No. 24, 2005

(hydroxyethyl)cellulose (HM-EHEC).12,13 It was shown that a large range of surfactants (both anionic and cationic) give rise to a viscosity maximum, but that the surfactant concentation corresponding to this maximum varies with the amount of free (monomeric) surfactant (thus being a function of the critical micelle concentration (cmc) value) and with the surfactant chain length and nature of the headgroup. The latter influences the lifetime of the junctions made up of surfactant and polymer side chains. It is interesting to note that while for the HM-HEC/SDS system the surfactant concentation corresponding to the viscosity maximum was found to be just below the cmc (8.1 mM), with HM-alginate we find a maximum well below the cmc value. The difference may be due to the fact that, for the HM-HEC/SDS system, a considerable amount of surfactant may bind also to the backbone of the polymer,12 thus not contributing to the junctions, whereas for the alginate backbone the interaction with SDS surfactant molecules is probably much lower, as also indicated by the flat curves seen in Figure 3 for the unmodified alginate. At the viscosity maximum, the number of mixed-micelle intermolecular links, and the lifetime of the hydrophobes inside them, has reached its optimum. At higher levels of SDS addition, a progressive “solubilization” of the hydrophobic moieties occurs. This gives rise to a reduction of the number of effective cross-links and the network is gradually disrupted (the viscosity decreases): there is a changeover from polymer-dominated to surfactantdominated micelles. It is interesting to note that the turbidity enhancement (Figure 3b) at low surfactant concentrations for the HMalginate solution is reminiscent of the viscosity feature. This announces the formation of large association complexes that affect the turbidity of the system, and this is a result of the deteriorated thermodynamic conditions. The drop of the turbidity at higher SDS concentrations is consistent with the conjecture that the associations break up and the thermodynamic conditions are improved. Polymer-surfactant interaction in solutions of amphiphilic polymers has recently been addressed in theoretical14-16 and simulation17 studies. The picture that emerges is that the hydrophobic tails of the polymer and hydrophobes on the surfactant aggregate into mixed micelles that act as cross-link junctions in the evolution of the associative network. At low or moderate surfactant concentrations, the network chains are tightly connected to each other by the hydrophobic junction zones, whereas in the excess of surfactant many junctions are solubilized, yielding a loosely bound network. To gain more insight into the viscoelastic response in the polymer-surfactant systems at different intensities of the associations, it may be worthwhile to consider the frequency dependence of the complex viscosity η*. It is generally found that this behavior can be described in terms of a power law η* ∼ ω-m, where m assumes values of 0 and 1 for a liquid and a solid, respectively.18 The frequency dependences of η*, as measured in smallamplitude oscillatory shear experiments, for 1 wt % solutions of alginate and HM-alginate at various levels of SDS addition are displayed in Figure 4. For the unmodified (12) Piculell, L.; Egermayer, M.; Sjo¨stro¨m, J. Langmuir 2003, 19, 3643. (13) Olsson, M.; Bostro¨m, G.; Karlson, L.; Piculell, L. Langmuir 2005, 21, 2743. (14) De Gennes, P.-G. J. Phys. Chem. 1990, 94, 8407. (15) Tanaka, F. Macromolecules 1998, 31, 384. (16) Diamant, H.; Andelman, D. Macromolecules 2000, 33, 8050. (17) Groot, R. D. Langmuir 2000, 16, 7493. (18) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: New York, 1999.

Bu et al.

Figure 4. Frequency dependence of complex viscosity (loglog plot) for 1 wt % solutions of alginate and HM-alginate at 25 °C and SDS concentrations indicated.

Figure 5. Shear rate dependencies of relative viscosity (every second point is shown) for 1 wt % solutions of HM-alginate (solid symbols) and alginate (open symbols) at SDS concentrations and temperatures indicated. The inset plot shows the temperature effect of the turbidity for 1 wt % solution of HMalginate in the presence of 8 mm SDS.

analogue, a liquidlike behavior is observed at all surfactant concentrations. This is another demonstration of weak polymer-surfactant interactions. For the HM-alginate solutions, the elastic (solidlike) response is more pronounced and the highest values of m are observed for the systems (2 and 4 mm levels of SDS addition) that exhibit the most marked viscosity enhancements. This strong elastic response is typical for systems with well-developed association networks. Temperature is a variable that in many cases affects the viscoelasticity of associating polymer systems. To take into account trivial changes of the solvent viscosity with temperature, the viscosity results are presented in terms of the relative viscosity ηrel (ηrel ≡ η/ηsolvent, where ηsolvent is the viscosity of water or the viscosities of the waterSDS mixtures). At the SDS concentrations considered in this work, the temperature dependence of the viscosity is virtually the same as in water without surfactant.19 The effect of temperature on the relative shear viscosity for 1 wt % solutions of alginate and HM-alginate in the presence of different amounts of SDS is depicted in Figure 5. No effect of temperature on the relative viscosity is observed for the unmodified alginate at the considered surfactant (19) Cavallaro, G.; Giammona, G.; La Manna, G.; Palazzo, S.; Pitarresi, G.; Liveri, V. T. Int. J. Pharm. 1993, 90, 195.

Properties of Semidilute Solutions of HM-Alginate

Langmuir, Vol. 21, No. 24, 2005 10927

concentrations (0 and 8 mm SDS), whereas for the hydrophobically modified analogue ηrel drops with increasing temperature in the low-shear regime, and this effect is pronounced at moderate and high levels of surfactant addition. This type of temperature dependence of the viscosity has been reported20 for semidilute solutions of HM-chitosan and the viscosity decrease was attributed to enhanced mobility of the polymer chains at elevated temperature, leading to loosening of the associative network. We can probably resort to a similar interpretation of the viscosity results for the semidilute solution of HMalginate with different surfactant concentrations. It is interesting to note that the most marked temperature effect of the viscosity is found for HM-alginate with 8 mm SDS (approximately two decades in viscosity), whereas no impact of temperature is visible at the condition (2 mm SDS) where the strongest network is formed. This shows that the association network in the presence of 2 mm SDS is sufficiently strong to resist the temperature-induced perturbations. The association complexes between the hydrophobic moieties of the polymer and the surfactant thus have a varying sensitivity to temperature changes, depending on the amount of surfactant present. We also note that the turbidity decreases slightly with increasing temperature (see the inset plot in Figure 5c), and this trend announces amended thermodynamic conditions and reduced associations at elevated temperature. We should consider that the binding of the anionic surfactant to the HM-polymer endows an apparent polyelectrolyte character to the polymer and thereby a reduced tendency to form association complexes. As the surfactant concentration increases, there is a gradual transition from polymerrich to surfactant-dominated mixed micelles. It is reasonable to assume that, at sufficiently high surfactant concentration, the cross-link junctions that stabilize the network are surfactant-rich and weaker than the polymerdominated cross-links at low amounts of SDS, and therefore more prone to breakup when exposed to enhanced thermal motion. It is interesting to note that, even in the presence of 20 mm SDS (Figure 5d), a significant temperature-induced change of the relative viscosity occurs. This signals that even at this high level of surfactant addition there exist several effective polymersurfactant junction zones, but in this case the intensity of the associations is weaker. Effects of surfactant addition and temperature on the relative zero-shear viscosity for 1 wt % solution of HMalginate are illustrated in Figure 6. A close inspection of the data in Figure 6a reveals that the temperature effect is small at low SDS concentrations, and the maximum of the viscosity is shifted toward lower SDS concentration values as the temperature increases. At higher levels of SDS addition, the value of η0/ηwater is strongly temperature dependent. Increasing temperature generates lower values of η0, suggesting a temperature-induced disruption of the association complexes. It is clearly demonstrated in Figure 6b that the temperature dependence of the relative viscosity is strongest for the SDS (8 mm)/HM-alginate (1 wt %) system. The pronounced drop of the relative viscosity for this system shows that the junction zones that strengthened the network at low temperature are disrupted by thermal motions at elevated temperature. This finding favors the conjecture that fairly weak cross-links are established by surfactant-rich mixed micelles. The frequency dependencies of the storage (G′) and loss (G′′) moduli for some polymer systems can be portrayed

by the simple Maxwell model, suggesting that the viscoelastic response is governed by a single relaxation time. However, many associating polymer systems exhibit a more complex viscoelastic behavior, with a distribution of relaxation times. The deviation from the Maxwellian response in the analysis of dynamic moduli data has recently been described10,21 with a procedure where it was noted that, if the loss modulus is plotted as a function of the storage modulus, a linear relation in a log-log plot is obtained (see the inset plot of Figure 7). It was reported10 that the value of the power-law exponent e (G′′ ∼ (G′)e) varies with the strength of the association network. The deviation of the value of e from 0.5, the value of a single Maxwell element, was traced to a change in the number of relaxation modes. It was argued that the departure from the value of 0.5 reflects a change in the relative

(20) Nystro¨m, B.; Kjøniksen, A.-L.; Iversen, C. Adv. Colloid Interface Sci. 1999, 79, 81.

(21) Thuresson, K.; Lindman, B.; Nystro¨m, B. J. Phys. Chem. B 1997, 101, 6450.

Figure 6. Surfactant concentration and temperature dependencies of relative viscosity for 1 wt % solutions of HM-alginate at the conditions indicated.

Figure 7. Plot of power-law exponent, e (see text), as a function of surfactant concentration for 1 wt % solutions of HM-alginate at the temperatures indicated. The inset shows a plot of G′′ versus G′ for 1 wt % HM-alginate solutions at the conditions indicated.

10928

Langmuir, Vol. 21, No. 24, 2005

Bu et al.

Figure 8. SANS scattered intensity plotted versus scattering vector q (every second point is shown) at 25 °C for 20 mm SDS and 1 wt % solution of alginate and solutions of HM-alginate at the surfactant (SDS or d-SDS) concentrations indicated. The line going through the 20 mm SDS data is a fit with a coreshell spherical model including electrostatic interaction (see text for details).

strength of the associations, giving rise to the different relaxation modes. Effects of temperature and surfactant concentration on the value of e for 1 wt % solutions of HM-alginate are depicted in Figure 7. The value of e is found to vary with the temperature and the level of surfactant addition. The maxima of the curves are located approximately at a SDS concentration where an “optimum” of the network strength is observed for the investigated temperatures. The behavior at moderate SDS concentrations indicates that there is not a single relaxation time that controls the time scale. A more complex picture, with a broad distribution of relaxation times, emerges. At higher SDS concentrations, a situation evolves that approaches a simple Maxwellian response. It seems that the deviation of e from the value of 0.5 in the considered frequency regime is related to the structural enhancements of the network. Small-Angle Neutron Scattering. SANS is an ideal tool with which to investigate structures in the size range of 5 Å to several hundred angstroms, and alginate, HMalginate, and SDS together with the complexes they form fall in this range. An advantage of using neutrons in these systems is the ability to suppress selectively the scattering of either component by adjusting their scattering length densities relative to the solvent. For example, the scattering contrast of deuterated SDS (d-SDS) in a mixture of 91% D2O and 9% H2O is practically zero, whereas those of the protonated samples of alginate and HM-alginate are fairly strong. These contrast-matching conditions facilitate a more thorough understanding of the structure of the polymer-surfactant complexes. Figure 8 shows the variation of the SANS scattering intensity as a function of q for a surfactant solution without polymer, for a 1 wt % solutions of alginate, and for 1 wt % solutions of HM-alginate without surfactant and with SDS or d-SDS. For the SDS/D2O solution without polymer, the concentration is above the normal critical micelle concentration (cmc ) 8.1 mM); hence a structure peak is observed that accounts for interparticle correlations in charged systems. The maximum of the structure peak is located at approximately 0.04 Å-1, and this corresponds to a center-to-center separation of the micelles of ca. 157 Å (separation ≈ 2π/q). The observed value is consistent with values reported22,23 from other SANS studies on the SDS/D2O system. At the same level of surfactant addition

Figure 9. SANS scattering curves at 25 °C from 1 wt % HMalginate solutions in the presence of various amounts of SDS (a) or deuterated SDS (b). The data in (a) have been fitted (solid line) with a core-shell model including electrostatic interaction. The inset plot in (a) shows how the volume fraction (Vf) of the micellar structures varies with SDS concentration. The inset plot in (b) shows the effect of d-SDS addition on the power-law exponent N (I(q) ∼ q-N) in the small q range.

in the presence of 1 wt % HM-alginate, a similar profile of the scattered intensity evolves and the peak is less pronounced but located at the same q value. It is interesting to note that when SDS is replaced by d-SDS, a significantly different profile of the scattered intensity is displayed with no structure peak. In this case the surfactant is invisible, and this finding suggests that the peak originates mainly from surfactant micelles and is not a signature of the polymer structure. Since the peak usually is ascribed to electrostatic interactions and it is often detected in “salt-free” polyelectrolyte solutions,24,25 its absence in the HM-alginate/d-SDS (20 mm) system may be a harbinger of a weak polyelectrolyte character of the polymer. This seems to signal that, although the binding of surfactant to the polymer would endow it with charges, the impact of this effect is not strong. In this context it is worth noting that SANS studies23,26,27 on other amphiphilic polymer systems in the presence of SDS or d-SDS have not revealed a disappearance of the interaction peak in the presence of d-SDS. The divergence in behavior between the different polymer systems can probably be traced to structural differences of the polymers and to the degree of hydrophobicity. For a polymer with high hydrophobicity, as in the present work, it is possible that the hydrophobic interactions can counteract the development of electrostatically induced structures that may be important for the appearance of the interaction peak in the SANS spectra. In this context it should also be (22) Mears, S. J.; Deng, Y.; Cosgrove, T.; Pelton, R. Langmuir 1997, 13, 1901. (23) Wesley, R. D.; Cosgrove, T.; Thompson, L.; Armes, S. P.; Baines, F. L. Langmuir 2002, 18, 5704. (24) Ermi, B. D.; Amis, E. J. Macromolecules 1997, 30, 6937. (25) Nishida, K.; Kaji, K.; Kanaya, T.; Shibano, T. Macromolecules 2002, 35, 4084. (26) Cosgrove, T.; White, S. J.; Zarbakhsh, A.; Heenan, R. K.; Howe, A. M. Langmuir 1995, 11, 744. (27) Kjøniksen, A.-L.; Knudsen, K. D.; Nystro¨m, B. Eur. Polym. J. 2005, 41, 1954.

Properties of Semidilute Solutions of HM-Alginate

Langmuir, Vol. 21, No. 24, 2005 10929

Figure 10. Schematic drawing of the effect of shear and surfactant addition on the sturctural organization of hydrophobically modified alginate.

mentioned, as demonstrated by Piculell and co-workers,28 that hydrophobic modification of polyelectrolytes may actually reduce the tendency of phase separation; thus the addition of hydrophobic moieties can in some cases help in providing amended themodynamic conditions. We notice in Figure 8 that the upturn in the small q range is weak for solutions of the unmodified analogue and for the HM-alginate with a high level of added d-SDS, whereas a strong upturn is visible for the HM-alginate solution without d-SDS. This trend indicates that large association structures are formed in the HM-alginate/ D2O system, while for solutions of unmodified alginate and in HM-alginate solutions with a large amount of surfactant the tendency to create association complexes is reduced. This picture is compatible with the rheological results presented above. In Figure 8 is also shown a fit that has been done to the 20 mm SDS solution (i.e., without polymer), using a spherical core-shell model for the form factor P(q)29 and a Hayter-Penfold formalism30 for the structure factor S(q) to account for the interparticle interference effects due to Coulomb interaction between the charged micelles. In the combined model, P(q)‚S(q), several parameters may be kept fixed, since they can be calculated independently. For the scattering length densities (SLD) the following values were used: SLD(core) ) -0.41 × 1010 cm-2; SLD(shell) ) 3.96 × 1010 cm-2; SLD(solvent) ) 6.3 × 1010 cm-2. The sample had been purified by dialyzing against distilled water, so the salt concentration (which determines the screening length) was here set to zero. However, there will still be Na+ counterions present from the surfactant headgroup. The micellar volume fraction, polydispersity, core size, and surface charge were then fitted, returning a volume fraction of micelles equal to 0.0042 and core radius of 16 Å. The volume fraction is lower than the maximum possible (0.0058 for 20 mm SDS), demonstrating that the micelles are in equilibrium with free (monomeric) surfactant, and the radius of the core fits reasonably well with the length of the SDS alkyl chain, 16.9 Å.31 The modeling of the SDS system without polymer described above was done mainly to obtain good starting values for the subsequent analysis of the combined HMalginate/SDS system, as discussed below. (28) Sjo¨stro¨m, J.; Piculell, L. Colloids Surf., A 2003, 183-185, 429. (29) Bartlett, P.; Ottewill, R. H. J. Chem. Phys. 1992, 4, 96. (30) Hansen, J.-P.; Hayter, J. B. Mol. Phys. 1982, 46, 651. (31) Kurakake, M.; Hagiwara, H.; Komaki, T. Cereal Chem. 2004, 81, 108.

A comparison of the effect of SDS or d-SDS addition to a 1 wt % solution of HM-alginate on the SANS scattering intensity is displayed in Figure 9. In the absence of SDS no structure peak is visible in the scattered intensity, but as the amount of added SDS increases a gradually stronger shoulder is developed in the intermediate q range, and at surfactant concentrations above cmc a peak appears. However, when d-SDS is added to the HM-alginate solution instead of SDS, a different pattern of behavior emerges (Figure 9b). In this case, no peak is detected, but an upturn in the small q range is observed. This upturn can be described by a power law I(q) ∼ q-N, and the values of N are shown in the inset plot. The value of N falls off as the surfactant concentration increases. This feature can again be attributed to the intensity of the hydrophobic interactions. As the surfactant concentration rises, the progressive solubilization of the hydrophobic microdomains will give rise to smaller association structures, and this effect is reflected in the magnitude of the upturn of the scattered intensity at low q values. In Figure 9 is also shown the results of the fits to the HM-alginate/SDS mixtures of the core-shell model described previously. The quality of the fit is reasonable, apart from in the low-q regime, where there is contribution from large-scale structures that are not straightforward to include in the model. The inset plot of Figure 9a shows how the fitted volume fraction of the charged micellar structures decreases continuously as the amount of SDS is reduced from 20 to 2 mm. This shows that the shoulder in the intermediate q range is mainly due to the surfactant micellar structures, either from the micelles in solution or from complexes attached to the polymer. The data obtained with d-SDS indicate that the main contribution is from bulk micelles. However, for the HM-alginate/ surfactant mixture the average micellar core radius was found to increase slightly compared to that of SDS alone, varying between 20 and 22 Å compared to the original 16 Å. This suggests that a small part of the interparticle term seen in the SANS data is due to micellar-like hydrophobe/surfactant complexes. The picture that emerges when the rheological data and the SANS data are viewed together is shown schematically in Figure 10. The application of shear to the hydrophobically modified polymer promotes a stretching and alignment of the chains, facilitating the formation of hydrophobe-hydrophobe and probably also hydrophobe-surfactant interactions. This results in shear thickening at low shear rates (cf. Figure 2) as long as the

10930

Langmuir, Vol. 21, No. 24, 2005

surfactant concentration is not so high that the intermolecular junctions are dissolved by the surfactant (up to approximately 4 mm SDS). At higher shear rates the junctions start to disrupt faster than they are created, and the viscosity decreases markedly. The peak observed in viscosity with varying surfactant addition (as seen in Figure 3) is most likely due the combined effects of (i) an increasing residence time for the hydrophobes inside the mixed micelles as the surfactant concentration increases and (ii) a decrease in the overall degree of cross-linking, as in the mechanism proposed by Piculell et al.12 based on their detailed rheological study on the HM-HEC system. The SANS data show the existence of micellar structures, giving rise to a correlation peak due to electrostatic interaction. A slight effect can be detected also at surfactant concentrations below the cmc, showing that micellar-like complexes exist before there are free surfactant micelles in the solution; i.e., the mixed-micellar junctions are initially in equilibrium with monomeric surfactant (as illustrated schematically in Figure 10). However, the high hydrophobicity of this polymer tends to counteract structural rearrangements from electrostatic interaction of the adsorbed surfactant, so that interparticle effects from the mixed-micellar junctions are weak. In addition to the small-scale inhomogeneities in the sample presented by the polymer-surfactant junctions, the SANS, as well as turbidity data, indicates the presence of also structures of larger sizes, giving rise to enhanced scattering in the very low q regime. Conclusions In this paper we have investigated the rheology, turbidity, and structure of semidilute alginate and HMalginate solutions in the presence of various levels of surfactant concentrations. Only a weak interaction is detected between alginate and SDS, whereas a strong viscosification and shear thickening are observed upon addition of SDS to the hydrophobically modified analogue at low shear rates. The network structures break up at higher shear rates, and shear thinning is the dominating feature. At higher surfactant concentrations, the associa-

Bu et al.

tion complexes formed at low shear rate are disrupted and this phenomenon is accompanied by a lower turbidity, announcing amended thermodynamic conditions. At the stage of optimal viscosity enhancement, a simple Maxwell model cannot describe the frequency dependences of the dynamic moduli, but the rheological behavior is complex with a broad distribution of relaxation times. The SANS results reveal a pronounced interaction peak in the intermediate q range for HM-alginate with SDS concentrations above the cmc, whereas no interaction peak is observed at the same conditions in the presence of the “invisible” d-SDS. These results suggest that the peak has its origin mainly from electrostatic interactions generated by the surfactant micelles in the bulk, but there is also a slight contribution from mixed-micellar junctions between surfactant and hydrophobic side chains. In the HM-alginate solution a marked upturn in the scattered intensity is observed at low q values (a much weaker effect is detected for the unmodified analogue), and this upturn becomes less pronounced as the d-SDS concentration is increased. This trend indicates that the large aggregates break up when the level of surfactant addition increases. No effect of temperature on the viscosity is found for the alginate solution with different amounts of SDS, whereas for the HM-alginate solution the influence of temperature on the viscosity is slight at low surfactant concentrations and drastic at moderate levels of SDS addition (around 8 mm). The general hypothesis is that the viscosity of the system will be reduced at elevated temperature, because of enhanced chain mobility. For the strongest polymer associations (low surfactant concentration), the temperature-induced motions are too weak to break up the network, whereas at moderate surfactant concentrations the surfactant-rich junction zones are weaker and more easily disrupted by thermal perturbations. Acknowledgment. B.N., H.B., and K.D.K gratefully acknowledge support from the Norwegian Research Council through a NANOMAT Project (158550/431). LA051187G