Interactions Between Anionically Modified Hydroxyethyl Cellulose and Cationic Surfactants

Transcription

1 Interactions Between Anionically Modified Hydroxyethyl Cellulose and Cationic Surfactants H. Lauer a, *, A. Stark a, H. Hoffmann a, and R. Dönges b a Universität Bayreuth, Physikalische Chemie I, D Bayreuth, Germany, and b Clariant GmbH, Industriepark Kalle-Albert, D Wiesbaden, Germany ABSTRACT: Macroscopic properties of aqueous solutions of several modified hydroxyethyl cellulose (HEC) samples and their interactions with cationic surfactants are studied by solubility, light scattering, electric birefringence, rheology, and surface tension measurements. Modified HEC samples carry anionic groups (an-hec D0) and anionic and hydrophobic groups in random distribution (HM-an-HEC D1 D4). The molar substitution of anionic (an) groups is about 0.07 in all samples while that of the hydrophobic (HM) groups ranges from 0 in an-hec D0 to in HM-an-HEC D4. In a 1 wt% solution this corresponds to 2.7 mm anionic and 0 to 0.46 mm hydrophobic groups. In the dilute concentration range the polymers behave like typical polyelectrolytes whereas in the semi-dilute range they resemble uncharged polymers. On addition of oppositely charged surfactants the phase behavior of all polyelectrolytes is similar. With increasing surfactant concentrations the transparent solutions become turbid and the phases separate. Finally, resolubilization takes place with excess surfactant concentrations. With the HM-an-HEC compounds viscoelastic solutions are formed with cationic surfactants. The intermolecular interaction between hydrophobic parts of the polymers and the surfactants and interactions of oppositely charged ionic groups of the two components lead to formation of a temporary network with gel-like properties. With an-hec the interaction can only take place via charges. Viscosity enhancement with increasing surfactant concentration is therefore lower with an-hec than with HM-an-HEC compounds. Paper no. S1120 in JSD 2, (April 1999). KEY WORDS: Cationic surfactants, electric birefringence measurements, modified hydroxyethyl cellulose;. Water-soluble cellulose derivatives like hydroxyethyl cellulose (HEC) are used as thickeners in food and cosmetic products and in water-based paints (1). Semidilute aqueous solutions of HEC are highly viscous. To optimize the properties for different industrial applications HEC samples are modified by ionic or hydrophobic groups. This modification causes an increase in viscosity. Properties of such systems can be improved by adding small amounts *To whom correspondence should be addressed at Lehrstuhl Physikalische Chemie I, Universität Bayreuth, Universitätsstr. 30, Gebäude NW I, D Bayreuth, Germany. Holger.Lauer@uni-bayreuth.de of surfactants. Macroscopic properties of these polymers and their ability to interact with surfactants have been studied in recent years (for a review, see Ref. 2). Investigations on the macroscopic properties of polymers carrying both hydrophobic and ionic groups show that polymer solutions exhibit higher viscosities, and that addition of oppositely charged surfactants results in stronger viscoelastic gels, than for those systems that were only hydrophobically or cationically modified. For a sample of HEC in which the substituent is both hydrophobic and charged, Goddard and Leung (3,4) showed that the phase behavior of this polymer following addition of an oppositely charged surfactant corresponds to the behavior of an unmodified cationic (cat)-hec. Association of the two oppositely charged compounds results in strong viscoelastic gels with high yield stress values. Furthermore, Goddard and Leung reported some signs of interactions between these compounds and surfactants of the same charge and even nonionic surfactants (4). For modified polymers in which the charged and the hydrophobic substituents are separately attached to the polymer backbone, Magny et al. (5) showed similar rheological behavior on the addition of an oppositely charged surfactant. An increase in viscosity is observed before associative phase separation takes place. The critical association concentration is one to three times lower than the surfactant critical micelle concentration (cmc). Furthermore, they found that the number of hydrophobic side chains forming mixed micelles at maximal viscosity are of the order of 3 6. The polymer sample also associates with surfactants of the same charge (6). Maximum viscosity increases with increasing polymer concentration. With increasing degree of hydrophobic substitution, the surfactant concentration at maximum viscosity shifts toward the surfactant cmc. Therefore, hydrophobic interactions dominate the increase in electrostatic repulsion. Kästner et al. (7) examined the macroscopic properties of samples of hydrophobically and/or cationically modified HEC and their interactions with surfactants. The results showed that cationically modified HEC (cat-hec) behaves like a typical polyelectrolyte, whereas samples that are hydrophobic as well combine the solution properties of both charged and hydrophobic polymers. On addition of an op- Copyright 1999 by AOCS Press 181

2 182 H. LAUER ET AL. positely charged surfactant, all polymer solutions show similar phase behavior of associative phase separation at certain amounts of surfactant, followed by resolubilization with excess surfactant concentrations. The hydrophobic and cationic parts of hydrophobically and cationically modified samples interact with anionic surfactant molecules, resulting in strong viscoelastic properties. Viscoelastic gels are obtained with added surfactant molecules (8 10). The surfactant concentration necessary for phase separation can be lower than the surfactant cmc (8,11). For investigations with surfactants, the degree of ionic substitution must be low; otherwise the solution separates into two phases as soon as small amounts of surfactant are added (8,12). For these highly substituted polyelectrolytes the surfactant molecules bind cooperatively to a few polymer chains which flocculate. For lower degrees of cationic substitution an increase in viscosity can be found with increasing anionic surfactant concentration (13). It is assumed that surfactant head groups bind to oppositely charged centers of cat-hec, and hydrophobic tails of surfactant molecules cooperatively connect different polymer backbones (14). The critical association concentration at which the surfactant starts to bind to the polyelectrolyte is well below the surfactant cmc. At 90% of charge neutralization, phase separation is observed (13). Thus, the precipitate consists of polyelectrolyte surfactant associatives, whereas the dilute phase consists mainly of solvent and at least one of the components (15,16). With excess surfactant concentration, resolubilization takes place. In this postprecipitation region a binding mechanism of the surfactant micelles on the polymer has been discussed (8). Systems with similar properties are obtained with perfluoralkyl hydrophobically modified HEC (F-HMHEC) and anionic or cationic surfactants (17,18). F-HMHEC interacts by association of its side chains. The maximal degree of hydrophobic substitution in the systems is about 0.3%. More highly modified F-HMHEC samples (up to 0.5%) are insoluble but become water-soluble by addition of ionic surfactants. The viscosity passes through a maximum with increasing anionic or cationic surfactant concentration. Added surfactant molecules bind to the hydrophobic parts of the polymer, resulting in viscoelastic gels (9,19). The surfactant concentration necessary for maximal viscosity depends on the degree of hydrophobic substitution (8,19). The addition of ionic surfactants to a polymer with the same charge, like dodecyltrimethyl-ammonium bromide (7) and cat-hec or the anionic compounds poly-(sodium acrylate) (PAA) and sodium dodecylsulfate (SDS) (6), increases the electrostatic repulsion between the two components and no complexes form. Only compounds with a strong hydrophobic nature seem to form complexes with surfactant molecules of the same charge, as investigated by McGlade et al. (20). In this paper we report the phase behavior, light scattering, electric birefringence, surface tension, and rheological experiments on aqueous solutions of samples of (anionically and/or hydrophobically) modified HEC and their interactions with cationic surfactants. All of the HEC samples (including the only anionically modified HEC) have comparable numbers of hydroxyethyl substituents and of sulfoethyl substituents (anionically substituent) but differ in the substitution degree of the hydrophobic alkyl-glycidyl substituent. Therefore, we are interested in explaining the differences in macroscopic properties of HEC derivatives with respect to their different degree of hydrophobic side groups. MATERIALS AND METHODS All HEC samples were used without further purification. Phase behavior was determined visually. Aqueous solutions were stirred continuously for several hours and allowed to equilibrate for at least 24 h prior to use. In the area of phase separation with surfactant a very dilute and transparent upper phase and a turbid gel-like lower phase were observed. The HEC derivatives (Clariant GmbH, Wiesbaden, Germany) have molecular weights (MW) between 1.4 and 2.2 million as determined from light-scattering experiments (see Results and Discussion section). These values are about 25 times larger than the MW of basic cellulose. For basic cellulose supplied by Clariant GmbH a MW of 80,000 was determined with viscosity measurements according to the method of Staudinger and Reinecke (21). It is assumed that the HEC samples form fringed micelles (see Results and Discussion section) which behave macroscopically like single polymer chains. The molar substitution (MS) of hydroxyethyl (HE) is per anhydroglucose unit. For different MW and degrees of MS of modified HEC samples, see Table 1. The substituent of the anionically modified sample an-hec D0 is a sodium sulfoethyl group, CH 2 CH 2 SO 3 Na. Anionically and hydrophobically modified samples HM-an-HEC D1 D4 consist of sodium sulfoethyl- and 3-alkyl-2-hydroxypropyl-groups with an alkyl chain length between 13 and 15 carbon atoms, CH 2 CHOH CH 2 O C n H 2n+1, where n = The samples in Table 1 are numbered with increasing MS of their hydrophobic groups. The surfactants are tetradecyltrimethylammonium bromide [TTAB, C 14 H 29 N(CH 3 ) 3 Br] from TABLE 1 Molecular Weight and Molar Substitution of Anionic or/and Hydrophobic Groups of All Modified Hydroxyethyl Cellulose Samples a Sample MW (10 6 g mol 1 ) MS (an) MS (HM) an-hec D HM-an-HEC D HM-an-HEC D HM-an-HEC D HM-an-HEC D a MW, molecular weight; MS, molar substitution; an, anionic; HM, hydrophobic; HEC, hydroxyethyl cellulose.

3 INTERACTIONS BETWEEN ANIONICALY MODIFIED HEC AND CATIONIC SURFACTANTS 183 Aldrich (Steinheim, Germany), and cetyl-trimethylammonium bromide [CTAB, C 16 H 33 N(CH 3 ) 3 Br] from Merck (Darmstadt, Germany). The static light-scattering experiments were done on a light-scattering photometer KMX 6 (Chromatix, Sunnyvale, CA) by using a small angle of 6 7. The differential refractive index was measured on a differential refractometer KMX 16 (Chromatix). The light source was a He-Ne laser (632.8 nm). Oscillatory rheological measurements were done using a Bohlin CS rheometer (Lund, Sweden) with a cone/plate measuring geometry. Frequency (ω) range used was s 1. Measurements were performed at 0.1 strain (linear region in the stress sweep experiments). To determine zero-shear viscosity in the very dilute concentration area we used an oscillating capillary rheometer Paar OCR-D (Graz, Austria). The temperature of all rheological measurements was 25 C. Surface tension measurements were carried out on a Lauda tensiometer (Königshofen, Germany) at a constant temperature of 25 C. For electric birefringence measurements at 25 C we placed the solution in a cell between a crossed polarizer and an analyzer. The light source was a He-Ne laser (632.8 nm). We measured the light intensity transmitted when an electric field was applied to the solution. The transient electric birefringence (TEB) apparatus with rectangular, high-voltage pulses of short rise and decay times was similar to that described in Reference 22. Experimental details appear in Reference 23. RESULTS AND DISCUSSION Aqueous solutions of polymers. (i) Phase behavior. All modified HEC samples were water-soluble and transparent at least up to 10 wt%. Because of the weak anionic modification no sol-gel transition occurs up to this concentration. (ii) Light scattering. Static light-scattering results are shown in Figure 1 where forward and backward scattering of HM-an-HEC D1 are given. Other HEC derivatives behave similarly. Scattering is linear up to concentrations of FIG. 1. Rayleigh factor (R θ ) determined from static light-scattering experiments as a function of concentration of modified hydroxyethyl cellulose samples carrying anionic and hydrophobic groups in random distribution (HM-an-HEC D1) in 10 mm NaCl solutions wt%. This indicates no interactions between the aggregates. Forward and backward scattering for HM-an-HEC are different and allow a determination of a radius of gyration (RG) of 156 nm (24): Kc / R(180 ) Kc / R(0 ) = R(0 ) R(180 ) = 1+ 16π2 (RG) 3λ 2 where K is an instrumental constant, c is the concentration, R(0 ) and R(180 ) are the forward and backward scattering, respectively, and λ is the wavelength of the laser. Radii of other polymers range between 128 and 143 nm (see Table 2). MW determined by light scattering range from 1.4 million to 2.2 million for the five polymers (see Table 1). Table 1 also includes the degrees of MS of the polymers. From the MW it is possible to calculate a contour length L K, [1] L K = n 0.63 nm 4000 nm [2] where n is the number of monomer units per chain and 0.63 nm is the approximate length of a monomer unit. TABLE 2 Experimental (exp.) and Theoretical (theor.) Data from Electric Birefringence, Light Scattering, and Rheological Measurements on Modified Hydroxyethyl Cellulose Samples a Experimental Theoretical c1 (EDB) Sample L K (nm) L(τ 2 ) (nm) RG (nm) R 0 (nm) (wt%) c 1 * (wt%) c 2 * (wt%) an-hec D HM-an-HEC D HM-an-HEC D HM-an-HEC D HM-an-HEC D a HEC, hydroxyethyl cellulose; an, anionically modified; HM, hydrophobic; L K, contour length; L(τ 2 ), molecule length detected by electric birefringence measurements; RG, radius of gyration; R 0, end-to-end distance; c1 (EDB), concentration at which the polyelectrolyte chains begin to overlap detected by electric birefringence measurements; c 1 *, concentration at which the polyelectrolyte chains begin to overlap and coil; c 2 *, concentration at which the polymer chains start to become entangled (entanglement concentration); D0 D4, sample names: as number increases, the degree of hydrophobic substitution increases.

4 184 H. LAUER ET AL. L K is about 30 times larger than the RG, which is detected by light scattering. The basic cellulose (Clariant GmbH) from which modified HEC was synthesized has a MW of 80,000. This is about 25 times lower than the MW we obtained by light-scattering measurements. With a MW of 80,000 an L K of about 190 nm is calculated and is only a little larger than the RG that is detected by light scattering. From these results we assume that the polymer coils present in solution consist of about 25 polymers each, in an extended conformation. For similar systems Burchard and Schulz (25) discuss the formation of so-called fringed micelles, which are formed of single polymer molecules in solution. These objects are built of laterally aggregated chains forming a stem and have a peripheral shell of dangling chain sections. With our systems this could be similar and these fringed micelles are detected in light scattering and explain the high MW and low values of RG. (iii) Zero-shear viscosity. In polyelectrolyte solutions there are at least two critical concentrations, c 1 * and c 2 *, at which the slope of zero-shear viscosity changes as a function of concentration. The term c 1 * denotes the concentration at which the polyelectrolyte chains begin to overlap and to coil. At this concentration the viscosity leaves the regime where the viscosity is about that of water. At c 2 * the polymer chains start to become entangled (entanglement concentration). Above c 2 * the viscosity therefore increases markedly. We also found these typical concentrations for our polyelectrolytes. Figure 2 shows a log-log plot of zero-shear viscosity [η 0 is the constant viscosity for very low shear rates (or stresses) and a characteristic parameter for polymer solutions (26)] as a function of concentration for two modified HEC samples an-hec D0 and HM-an-HEC D4. Values of the other modified HEC samples are between those of an- HEC D0 and HM-an-HEC D4 and are not shown. Below c 1 * all solutions are water-viscous, and in this concentration range the viscosity remains nearly constant. TABLE 3 Slope Above c 1 * (S1), Slope Above c 2 * (S2), c 1 *, and c 2 * from Viscosity Measurements of Modified HEC Samples S1 S2 Sample (Pa s/wt%) (Pa s/wt%) c 1 * (wt%) c 2 * (wt%) an-hec D HM-an-HEC D HM-an-HEC D HM-an-HEC D HM-an-HEC D a For abbreviations see Table 2. Above c 1 * the slopes (S1) are between 1.2 and 0.5 and decrease with increasing hydrophobic substitution of the polymer. The slope of HM-an-HEC D4 follows the scaling law for polyelectrolytes [Fuoss law (27)] [η 0 ~ (c/c 1 *) 1/2 ]. The term c 1 * shifts to lower polymer concentrations with increasing number of hydrophobic substituents on the polymer backbone. In addition, viscosity values increase with increasing hydrophobic substitution. Both effects are due to intermolecular associations of polymer hydrophobic side chains. Slopes (S2) of the plot of viscosity vs. polymer concentration of all samples above c 2 * are between 4.9 and 5.5, following the scaling law [η 0 ~ (c/c 2 *) 5.5 ] that is characteristic for uncharged polymers (28). In Table 3 data for the slopes of c 1 * and c 2 * are summarized for all modified HEC derivatives. A typical parameter for polymer network elasticity is the storage modulus (G ). For the entangled solutions of our systems G strongly depends on frequency over the entire range. A characteristic rheogram is shown in Figure 3 for a sample of HM-an-HEC D4 at 5 wt%. The characteristic crossover of G and loss modulus (G ) occur at an angular frequency of about 34 s 1. With increasing polymer concentration it shifts to lower frequencies. Thus, the structural relaxation times increase with increasing polymer concentration. They are 29 ms and 3 s, respectively, for 5 and 10 wt% HM-an-HEC D4. Also, values of the moduli FIG. 2. Zero-shear viscosity, η 0, as a function of polymer concentration of modified HEC sample carrying anionic groups [an-hec D0 ( )] and hydrophobic and anionic groups [HM-an-HEC D4 ( )]. For abbreviation see Figure 1. FIG. 3. Complex viscosity, η* ( ), storage modulus, G ( ), and loss modulus, G ( ), as a function of angular frequency, ω, of a 5 wt% solution of HM-an-HEC D4. For abbreviation see Figure 1.

5 INTERACTIONS BETWEEN ANIONICALY MODIFIED HEC AND CATIONIC SURFACTANTS 185 increase with increasing polymer concentration owing to an increase in the number density of entanglement points. At low frequencies the increase in G with increasing frequency is about 1.6 and that of G is on the order of 1 for 5 wt% HM-an-HEC D4 (Fig. 3), corresponding to the theory of viscoelastic solutions (26). With increasing polymer concentration the range for constant complex viscosity, η*, becomes smaller and the shear thinning effect starts at lower frequencies. More highly concentrated solutions show a stronger shear thinning effect which can be seen by comparing the slopes ( 0.45 and 0.66 of the complex viscosity, η* ) in the shear thinning frequency range for 5 and 10 wt% HM-an-HEC D4. G, however, does not reach a constant plateau for high frequencies. To compare storage moduli of different polymer solutions we chose an angular frequency of 12.6 s 1. Figure 4 shows G and zero-shear viscosity at this frequency as a function of polymer concentration for HM-an-HEC D4. Above c 2 * the polymer solutions become more and more viscoelastic, and G values increase with a slope m of 4.3 (log-log-plot). This increase is similar to the viscosity increase and is due to a strong enhancement in the number density of physically cross-linked polymer chains above the entanglement concentration c 2 *. (iv) Electric birefringence. The behavior of polyelectrolytes in electric fields depends on factors such as concentration and electric field strength. A signal obtained by the electric birefringence method is given in Figure 5, and represents a sample of HM-an-HEC D3 at a concentration of 0.01 wt% and 600 volts. In comparison with other studies on polyelectrolytes (29 31) it is interesting to note that no electric birefringence anomaly is observed at the crossover concentration from the dilute to the semidilute range and that the sign of electric birefringence is positive for all polymer samples over the entire concentration range. It is conceivable that the lack of this anomaly has something to do with the low charge density of the compounds. It is also interesting to FIG. 5. A signal from electric birefringence measurements of 0.01 wt% HM-an-HEC D3 at 600 V. The intensity I is in arbitrary units (a.u.). For abbreviation see Figure 1. note that the direct current signal is not symmetric. The process of orientation takes more time than the process of relaxation. This indicates that the polyelectrolytes behave as if they have a permanent dipole moment. This apparent permanent dipole moment originates from fluctuations of the counterions (32). Therefore, the dipole moment has a lifetime longer than the reorientational relaxation time. For low fields the birefringence increases with E 2 and a Kerr constant B is determined (Fig. 6): B = lim ( n/ λe 2 ) E 0 Reduced Kerr constants (B/c, Eq. 3) are plotted in Figure 7 against concentration for an-hec D0 and HM-an-HEC D3. For very low polymer concentrations the specific Kerr constants increase slightly with polymer concentration. Above a concentration c1 (EDB, Elektrodoppelbrechung ) the specific Kerr constants start to decrease. This polymer con- [3] FIG. 4. Zero-shear viscosity, η 0 ( ), and storage modulus, G ( ), at a constant angular frequency, ω, of 12.6 s 1 as a function of polymer concentration of HM-an-HEC D4. For abbreviation see Figure 1. FIG. 6. Electric birefringence, n, as a function of electric field strength E for 1 ( ), 0.5 ( ), 0.2 ( ), 0.1 ( ), and 0.05 wt% ( ) solutions of HM-an-HEC D3. For abbreviation see Figure 1.

6 186 H. LAUER ET AL. and t is the time of decay. We can also fit the decay with two amplitudes, A and B, and two relaxation times, a short time, τ 1, and a long time, τ 2 (22): n = A exp ( t/τ 1 ) + B exp ( t/τ 2 ) [5] FIG. 7. Reduced Kerr constants, B/c, vs. polymer concentration of HM-an-HEC D3 ( ) and an-hec D0 ( ). For abbreviations see Figures 1 and 2. centration c1 corresponds approximately with the concentration c 1 * detected in rheological measurements (see Table 2). Relaxation time constants were determined from electric birefringence decay. In general there are two possibilities to fit the birefringence decay. The first is a monoexponential fit, which means that the decrease of birefringence can be described with only one relaxation time τ 1 : n = A exp ( t/τ 1 ) [4] where n is the electric birefringence, A is the amplitude, We obtained the best results with a biexponential fit, as illustrated in Figure 8 for a sample of HM-an-HEC D4 at a concentration of 0.01 wt%. The short relaxation time was 51 µs and the long was 352 µs. We think that the two relaxation times belong to different kinds of aggregates in solution. The long relaxation time belongs to fringed micelles as already discussed in the light scattering section, whereas the short relaxation time belongs to single polymer molecules with MW of about 80,000 as measured for basic HEC (supplied by Clariant GmbH) with viscosity measurements according to the method of Staudinger and Reinecke (21). Figure 9 shows long relaxation times τ 2 vs. polymer concentration. All relaxation times of the hydrophobic modified samples are almost constant within the concentration range. Relaxation time is related to a rotational diffusion coefficient, D r, and a particle length, L: D r = 1/(6τ) = {3kT [ln (p) (1/ln (2p) 0.27)] 2 }/(τηl 3 ) [6] where kt is thermal energy, η is solvent viscosity, and p is axial ratio (22). Thus the relaxation times obtained from birefringence decay allow us to determine dimensions for the molecules, FIG. 8. Biexponential fit (dotted line) of the electric birefringence decay for a sample of HM-an-HEC D4 at a concentration of 0.01 wt%. The short relaxation time is 50.5 µs and the long is 352 µs. For abbreviations see Figures 1 and 5.

7 INTERACTIONS BETWEEN ANIONICALY MODIFIED HEC AND CATIONIC SURFACTANTS 187 FIG. 9. Long relaxation times, τ 2, vs. polymer concentration for all samples. For abbreviations see Figures 1 and 2. which are on the order of nm. The RG for all modified HEC samples obtained by light scattering (with added amounts of salt sufficient to screen the charges of the polyelectrolyte) are on the order of nm. The latter value is 1.3 to 1.7 times smaller than dimensions obtained by electric birefringence. This result is not contradictory because the electric birefringence method detects hydrodynamic radii of the aggregates, which are not equal to the RG detected by light scattering. The RG contains a contribution of every polymer segment in the polymer coil: <RG 2 > 0.5 = { m i [(RG) i2 /( m i )]} 0.5 [7] where m i is the mass of a polymer segment i and (RG) i is the RG of the polymer segment i. This means that the polymer segments near the coil center influence the value as well. Therefore, the RG is somewhat lower than the hydrodynamic radius detected by electric birefringence measurements. Similar results were obtained by Burchard and Schulz (25) for cellulose acetates with MW greater than 20,000. For a statistical Gaussian coil with a MW of about 2 million and complete freedom of orientation for each monomer unit, we would expect an end-to-end distance R 0 or a dimension for the coil of (<R 0 2 >) 1/2 = (n d 2 ) 1/2 [7500 (0.63 nm) 2 ] 1/2 55 nm (where n is the number of monomer units per polymer backbone and d is the length of one monomer unit). That the dimensions of the coils are about four times larger shows that polymer density inside the coil is 4 3 times smaller than that of a statistical coil. This shows that the coils are expanded and the backbone is stiffer. This can be seen by calculating a persistence length, p e, of the backbone of the polymer (33 35): If we assume a MW of about 80,000, which is the value for the basic cellulose, we calculate a p e of 18 nm. This seems more reasonable, and it represents 29 monomer molecules. The comparison of these two calculated p e is another indication that the polymers form fringed micelles of about 25 polymer molecules, with each polymer molecule existing almost entirely in its overall extended conformation (see section on Light scattering above). Table 2 shows results determined from electric birefringence, light scattering, and rheological measurements. Addition of oppositely charged surfactants. (i) Addition of TTAB. For all modified HEC samples we find very similar phase behavior: With increasing amounts of surfactant the solution phase separates, and becomes a single phase again. With increasing amounts of charge neutralization, associative phase separation takes place at a surfactant concentration of 5 mm. A sharp line between a very dilute transparent upper phase and a turbid gel-like lower phase is observed. The dense gel-like lower phase consists of the polymer surfactant associatives, whereas the supernatant upper phase is very dilute and consists of the solvent and some surfactant (11,12). We conclude the latter from viscosity measurements which show that the viscosity of the dilute phase is similar to that of water. The portion of flocculated phase is nearly constant over the entire range of phase separation up to 20 mm. The resolubilization process is characterized by a spontaneous transition into a clear solution at 50 mm TTAB. Figure 10 is a plot of the viscosity vs. TTAB concentration of an-hec D0 and all HM-an-HEC D1 D4 samples (with about the same degree of anionic substitution). For a given concentration the viscosity increases with increasing number of hydrophobic substituents. The beginning of the viscosity increase is shifted toward lower surfactant concentrations. This behavior is caused by hydrophobic interactions between polymer side chains and surfactant molecules that form mixed micelles. <R 0 2 > = n p e d p e2 {1 exp [ (n d/p e )]} [8] where R 0 is the end-to-end distance in the polymer coil. With a MW of about 2 million, this results in a p e of about 0.65 nm, which is about the length of a single monomer molecule and is therefore an unreasonably short distance. FIG. 10. Zero-shear viscosity, η 0, as a function of tetradecyltrimethylammonium bromide (TTAB) concentration of 1 wt% solutions of HMan-HEC D4 ( ), D3 ( ), D2 ( ), D1 ( ), and an-hec D0 ( ). For abbreviations see Figures 1 and 2.

8 188 H. LAUER ET AL. For all samples phase separation occurs at the same TTAB concentration, which shows that phase separation is not affected by the hydrophobic side chains. Also, range of concentration over which precipitation occurs does not depend on the degree of hydrophobic substitution. In contrast to the viscosity enhancement through hydrophobic association before phase separation, viscosity in the postprecipitation region for the HM-an-HEC D1 D4 samples is comparable with that of the an-hec D0 sample. The hydrophobic parts seemingly do not contribute to network formation in the resolubilization area. The discussion of a mixed micelle conformation from surfactant molecules and polymer side chains in the preprecipitation region implies a maximum in viscosity as also found for hydrophobic nonionic polymers (5,36,37). An increasing amount of surfactant molecules causes formation of more mixed micelles in which the number of involved polymer side chains decreases until a situation is reached where every side chain is covered by its own micelle. No further network formation is observed (15). In our case it seems possible that in the resolubilization zone of the HM-an-HEC D compounds a very low number of hydrophobic chains are already saturated with surfactants. The elastic properties in this area have their origin only in attractive interactions between polymer charges and oppositely charged surfactants. The increase in viscosity is due to an increase in elasticity. A size for the elasticity is the storage modulus G. We found a strong increase of G in the preprecipitation region. The course of G is coincident with that of the viscosity. For highly viscous solutions G reaches an almost constant value at high frequencies (3). Therefore, a constant G 0 value can be defined that corresponds to the number of effective cross-links per unit volume, υ e, which are formed in solution (26): lim G = G 0 = Aυ e kt [9] where kt is the thermal energy and A is a numerical factor that is approximately 1 for a low network density. G (respectively, G 0 ) increases in the preprecipitation area with increasing surfactant concentration. This reflects the increasing number of cross-links which are formed by polymer surfactant interactions. Polymer concentration of the polyelectrolytes in Figure 10 is well above the critical value c 1 *. Viscosity enhancement corresponds to a situation in which surfactant molecules strengthen the hydrophobic interaction of the polymer substituents, which are to some extent already present in solution. A decrease in polymer concentration decreases the interactive associations as shown in Figure 11 in which the effects on viscosity of the addition of TTAB were determined on solutions of HM-an-HEC D4 of different concentrations. Maximal viscosities in both single-phase areas decrease continuously with decreasing polymer concentration. The area of phase separation shifts to lower surfactant concentrations because of a lower charge density in solution. For a polymer concentration of 0.1 wt% no intermolecular associations of polymer substituents are formed in (the water-viscous) solution. Viscosity stays almost constant over the entire concentration range. (ii) Addition of CTAB. An increase in surfactant chain length does not influence phase behavior of modified HEC samples. Figure 12 shows viscosity as a function of CTAB concentration for all modified HEC samples. At corresponding surfactant concentrations the viscosities of all modified HEC samples are higher compared to TTAB. With 1 wt% HM-an-HEC D4, for example, the viscosity with TTAB is about 0.3 Pa s whereas with the same concentration of CTAB about 1 Pa s is reached. With increasing CTAB concentration the viscosity increases. Surfactant molecules interact with hydrophobic cross-links of the polymer network. Therefore, the number of effective cross-links increases and viscosity rises. The value of maximal viscosity thus increases with an increasing degree of hydrophobic substitution. In contrast to the addition of TTAB, the solutions become turbid at concen- FIG. 11. Zero-shear viscosity as a function of TTAB concentration for HM-an-HEC D4 solution of 1 ( ), 0.7 ( ), 0.5 ( ), and 0.1 wt% ( ). For abbreviations see Figures 1 and 10. FIG. 12. Zero-shear viscosity, η 0, as a function of cetyltrimethylammonium bromide (CTAB) concentration of 1 wt% solutions of HM-an-HEC D4 ( ), D3 ( ), D2 ( ), D1 ( ), and an-hec D0 ( ). For abbreviations see Figures 1 and 2.

9 INTERACTIONS BETWEEN ANIONICALY MODIFIED HEC AND CATIONIC SURFACTANTS 189 FIG. 13. Surface tension, σ, vs. polymer concentration of an-hec D0 ( ) and HM-an-HEC D4 ( ). For abbreviations see Figure 1 and 2. trations exceeding 2 mm CTAB which is an indication of the beginning of phase separation. A part of the polymer and the surfactant form the polymer surfactant complex which is finely dispersed and leads to solution turbidity. At higher surfactant concentrations phase separation occurs with a dense gel-like lower phase and a transparent water-viscous upper phase. (iii) Surface tension. A very sensitive method for studying polymer surfactant interactions is to measure surface tension. We measured the surface tension of both an-hec D0 and the most hydrophobically modified sample, HMan-HEC D4. Figure 13 shows surface activities as a function of polymer concentration. As expected, the hydrophobic-modified sample shows stronger surface activity than the nonhydrophobic one. Moreover, the polymers seemingly have a cmc at about 0.2 wt%. The slight increase in the surface tension values after the cmc may be caused by impurity or polymer polydispersity. FIG. 14. Surface tension, σ, vs. CTAB concentration of water ( ), a 0.3 wt% solution of HM-an-HEC D4 ( ), and a 0.3 wt% solution of an-hec D0 ( ). The dotted line represents the surface tension of pure an-hec D0 solution and the solid line that of pure HM-an-HEC D4 solution. cmc, critical micelle concentration. For other abbreviations see Figures 1, 2, and 12. In Figure 14 the variations in surface tension of an-hec D0 and HM-an-HEC D4 with the addition of CTAB are shown. The dotted line represents the surface tension of a pure an-hec D0 solution, and the solid horizontal line that of pure HM-an-HEC D4 solution without surfactant. The polymer concentration was constant at 0.3 wt% to avoid difficulties of high viscosity and phase separation. The surface tension of CTAB in water first shows a decrease and then remains constant. This surfactant concentration at which the surface tension remains at a minimum is the cmc. The surface tension of an-hec D0 shows a similar behavior with increasing surfactant concentration. First it decreases, owing to an increasing amount of free surfactant molecules, and finally the curve almost joins the CTAB water curve. In addition the surface tension of the polymer surfactant mixture is lower than that of the pure components. Finally the cmc of the mixture shifts from 1 mm CTAB in the pure surfactant solution to about 0.13 mm CTAB. Note that the two surface tension curves do not intersect. This is an indication that at this low concentration there are no interactions between the polymer and individual surfactant molecules. The surface tension of HM-an-HEC D4 with CTAB shows a somewhat different behavior. First, there is a decrease, then an intersection of the surface tension curve of CTAB, and finally an interaction with the CTAB-water curve. The decrease in surface tension for low surfactant concentrations is lower than in the absence of polymer. The cmc shifts from about 1 mm with the pure surfactant solution to about 4 mm with the polymer surfactant mixture. These effects are due to an association of surfactant molecules around the polymer. Therefore, the amount of surfactant necessary to form free micelles is somewhat higher than without polymer. At about 4 mm CTAB, where the two surface tension curves join, these free surfactant micelles form. They can again associate around the polymer backbone. The different surface tension behaviors of an-hec D0 and HM-an-HEC D4 with CTAB can be explained by the different polymer hydrophobicity. Crossing of the two surface tension curves, with and without polymer, occurs only with the more hydrophobic polymer HM-an-HEC D4. Thus it seems that there are strong interactions between the polymer at low concentration and the single surfactant molecules which are forced out by the hydrophobicity of the polymer. The an-hec D0 is obviously not hydrophobic enough to develop interactions between polymer molecules and single surfactant molecules. Moreover, this change in behavior of surface tension with increasing surfactant concentration and the well-defined crossover of the surface tension curves with and without polymer are often observed for polymers with strong hydrophobic characters. Results for poloxamer-type block-copolymers (38) and for similar hydrophobic polyelectrolytes (18) have been discussed in terms of interactions between polymer hydrophobes and surfactant molecules.

10 190 H. LAUER ET AL. Comparison with HM-cat-HEC. In this section a comparison is made between cationic and anionic systems. Kästner et al. (7) found gels with cat-hec having a cationic molar substitution of We did not find gels in our much less modified polymers with an anionic MS of about The occurrence of gels is linked with yield stresses of the solutions. This means that the systems do not flow until the yield stress is reached. With our systems we could not find yield stresses, whereas Kästner et al. (7) with the cat-hec systems did. The high cationic molar substitutions in Kästner s polymers lead to strong electrostatic repulsion and, therefore, a more stretched conformation of the polyelectrolyte, which facilitates the formation of a gel at about 4 wt% polymer concentration. Critical concentrations found by Kästner et al. (7) both in rheological and electric birefringence measurements are lower than the ones we found with our polymers. We think that this is a consequence of different polymer MW. Whereas Kästner s polymers have MW of about 950,000 (7) our single polymer molecules have MW of about 80,000. Therefore, the critical concentrations have to be much lower. Phase behavior of Kästner s polymers with oppositely charged surfactants is similar to ours. Kästner also found clear solutions at low surfactant concentrations, followed by a precipitation zone at higher surfactant concentrations, and finally found resolubilization again at high surfactant concentrations. The difference in our polymers is again the much higher viscosities. Kästner found viscosities of about 4000 Pa s just before phase separation. We found only 40 Pa s with our most viscous polymer HM-an-HEC D4. Also, viscosities for Kästner s polymers are much higher after phase separation. In this area she found viscosities up to about 4 Pa s whereas we only found about 0.04 Pa s. 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11 INTERACTIONS BETWEEN ANIONICALY MODIFIED HEC AND CATIONIC SURFACTANTS Fredericq, E., and C. Houssier, Electric Dicroism and Electric Birefringence, Claredon Press, Oxford, Schorr, W., and H. Hoffmann, Electric Birefringence Measurements on Micellar Solutions of Ionic Surfactants, J. Phys. Chem. 85:3160 (1981). 24. Kästner, U., Modifizierte Cellulose-Derivate und ihre Wechselwirkungen mit Tensid, Ph.D. Dissertation, University of Bayreuth, Bayreuth, Germany, 1995, p Burchard W., and L. Schulz, Functionality of the β(1,4) Glycosidic Linkage in Polysaccharides, Macromol. Symp. 99:57 (1995). 26. Barnes, H.A., K. Hutton, and K. Walters, An Introduction to Rheology, Elsevier, Amsterdam, Fuoss, R.M., Polyelectrolytes, Discuss. Faraday Soc. 11:125 (1951). 28. De Gennes, P.G., Reptation of a Polymer Chain in the Presence of Fixed Obstacles, J. Chem. Phys. 55:572 (1971). 29. Krämer, U., Elektrodoppelbrechungsmessungen an Tensidund Polyelektrolytlösungen, Dissertation, University of Bayreuth, Bayreuth, Germany, 1990, p Oppermann, W., Electro-optical Properties and Molecular Flexibility of Polyelectrolytes in Dilute Solution, Macromol. Chem. 189:927 (1988). 31. Yamaoka, K., and K. Matsuda, Reversing Pulse Electric Birefringence of Poly(p-styrene-sulfonate) in Aqueous Solutions: Effects of Molecular Weight and Concentration on Anomalous Signal Patterns Arising from Fast and Slow-Induced Ionic Dipole Moments, J. Phys. Chem. 89:2779 (1985). 32. Odijk, T., Polyelectrolytes Near the Rod Limit, J. Polym. Sci. 15:477 (1977). 33. Flory, P.J., Principles of Polymer Chemistry, Cornell University Press, Ithaca, de Gennes, P.G., Scaling Concepts in Polymer Physics, Cornell University Press, Ithaca, Kratky, O., and G. Porod, X-Ray Investigation of Dissolved Chain Molecules, Recl. Trav. Chim. Phys. Bas. 68:1106 (1949). 36. Dualeh, A.J., and C.A. Steiner, Bulk and Microscopic Properties of Surfactant-Bridged Hydrogels Made from an Aphiphilic Graft Copolymer, Macromolecules 24:112 (1991). 37. Steiner, C.A., Polymer Surfactant Interactions in Graft Copolymer Solutions, J. Appl. Polym. Sci. 42:1493 (1991). 38. Hecht, E., and H. Hoffmann, Interaction of ABA Block Copolymers with Ionic Surfactants in Aqueous Solution, Langmuir 10:86 (1994). [Received November 11, 1998; accepted February 2, 1999] Holger Lauer has worked for about 4 yr as a scientific employee at the Department of Physical Chemistry, University of Bayreuth (Germany). His research interests are the behavior of polyelectrolytes in aqueous solutions and their interactions with surfactants. Angela Stark has worked for about 3 yr as a scientific employee at the Department of Physical Chemistry, University of Bayreuth (Germany). Her research interests are parabolic focal cones especially in the system SDS/water/hexanol/decane. Heinz Hoffmann, born in 1935, studied chemistry at the Universities of Würzburg and Karlsruhe. After having finished his studies he worked at the Case-Western Reserve University in Cleveland, Ohio, and at the Erlangen-Nürnberg University. In 1975 he was appointed as Chair of Physical Chemistry at Bayreuth University. His research interests are colloid and interfacial science and the structure and propteries of self-assembling systems. Reihard Dönges studied chemistry at the University of Darmstadt, Germany. His dissertation dealt with Pentalen and Its Dimers. After having finished his studies he worked at the Central Research Department and at the Central Process Development Department Hoechst AG in Frankfurt. In 1985 he moved to the Research Department, and in 1999 to the Production Department, Cellulose Ethers, Clariant GmbH in Wiesbaden.