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Influence of Cosolutes on Phase Behavior and Viscosity of a Nonionic Cellulose Ether. The Effect of Hydrophobic Modification Krister Thuresson,* Svante Nilsson,† and Bjo¨rn Lindman Physical Chemistry 1, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund, Sweden Received October 11, 1995. In Final Form: February 22, 1996X The phase behavior and viscosity of aqueous solutions of two nonionic polymers, ethyl(hydroxyethyl)cellulose (EHEC) and the hydrophobically modified analogue (HM-EHEC), have been studied as a function of added cosolutes, ranging from alcohols with different chain lengths to salts with different anions and to substances usually referred to as denaturants. The change in phase behavior on addition of a cosolute is dominated by the general structure of the polymer chain and is negligibly influenced by the hydrophobic tails of HM-EHEC, as they are present in such a low number. Therefore, the cosolute induced changes in phase behavior are akin for the two polymers. However, the situation is different for added cosolutes which have a particularly strong affinity for the hydrophobic moieties of HM-EHEC and at a closer inspection differences in phase behavior between the two polymers can be seen on addition of long-chained alcohols. On the other hand, changes in viscosity on addition of cosolutes are generally much more pronounced for HM-EHEC as rheology probes network formation and association resulting from small changes in the interaction energy between the polymer chains. The observations, both phase behavior and viscosity, can be qualitatively understood in terms of the distribution of cosolute between bulk solution and polymer surface. As an example, addition of NaCl or a long-chained alcohol increases the viscosity more for HMEHEC than for EHEC, while an addition of NaSCN or a short-chained alcohol rather decreases the viscosity, again more pronounced for HM-EHEC.
1. Introduction The solution behavior of hydrophobically modified polymers (HM-polymers) is to a high degree determined by their amphiphilic behavior. Surfactants are, for instance, believed to bind and self-assemble at the polymers’ hydrophobic moieties1,2 in analogy with the formation of mixed micelles in solutions containing more than one type of surfactant.3,4 The polymer we have chosen to study is an ethyl(hydroxyethyl)cellulose ether (EHEC) with a low hydrophobic modification degree (HM-EHEC) and for comparison we have also investigated the unmodified parent polymer. The interactions between the present polymers and surfactants have in a number of publications been probed by different techniques such as rheology, NMR, time-resolved fluorescence, ion specific electrode, and light scattering techniques.5-9 We have also investigated changes in phase behavior of the two polymers on addition of hexanol both experimentally and by model calculations using a Flory-Huggins approach.10 * To whom correspondence should be addressed. † Present addresss: Rogalands Research, Prof. Olav Hanssens va¨g 15, Box 2503, Ullandhaug, 4004 Stavanger, Norway. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Gelman, R. A. Hydrophobically modified hydroxyethylcellulose. International Dissolving Pulps Conference, 1987. (2) Magny, B.; Iliopoulos, I.; Audebert, R.; Piculell, L.; Lindman, B. Prog. Colloid Polym. Sci. 1992, 89, 118-121. (3) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss., in press. (4) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1-21. (5) Thuresson, K.; Nilsson, S.; Lindman, B. Effects on phase behavior and viscosity of hydrophobic modification of a nonionic cellulose ether. Influence of cosolutes. Cellucon-93; Kennedy, J. F., Phillips, G. O., Williams, P. H., Eds.; Woodhead Publishing Ltd., 1995; pp 323-329. (6) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509-4513. (7) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994-2002. (8) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730-3736. (9) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-4918.
As these polymers frequently find use in mixtures of varying compositions (as thickeners, stabilizers, emulsifiers, etc.), the interaction between the polymer and different cosolutes is of considerable concern, and it is of a general interest from both a scientific and a commercial point of view to understand and to predict the effect of adding a cosolute. Therefore, we here continue our work by investigating how addition of alcohols with different chain lengths, addition of salts with different anions, or addition of substances normally used for denaturation in biological systems influence phase behavior and viscosity of the polymer solutions. The solubility of EHEC decreases as the temperature increases, and at a specific temperature the solution demixes into one phase rich and one phase poor in polymer. The decreased polymer solubility at increased temperature has been interpreted as a less favorable interaction between the polymer and the solvent as the polymer adopts entropically favored nonpolar conformations. The model was originally developed for describing the phase behavior of poly(ethylene oxide) in a polar solvent.11 However, the model has later shown to successfully rationalize the phase behavior of clouding alkyl cellulose polymers that contain ethylene oxide side groups.12,13 Here we have studied the temperature of demixing (often referred to as the cloud point temperature, Tcp) and the viscosity of the solutions as a function of added cosolutes. It is shown that changes in phase behavior are similar for the two polymers, while the viscosity of HM-EHEC solutions is more influenced by cosolute addition than solutions of the parent polymer. For instance, depending on the chain length of the added alcohol, the viscosity for a HM-EHEC solution can either substantially increase or slightly decrease, while at the (10) Thuresson, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1995, 99, 3823-3831. (11) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962-4964. (12) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005-5015. (13) Zhang, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1994, 98, 4411-4421.
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Figure 1. Chemical structure of EHEC and HM-EHEC.
corresponding additions to the EHEC solution the viscosity is left unaffected. Another interesting finding is that in spite of the polymers being nonionic, both the viscosity of the solution and the solubility of the polymers are modulated by an electrolyte addition. The viscosity can either increase or decrease, depending on the identity of the salt, and it is observed that the salt addition influences the polymer solubility in the, compared to viscosity, opposite way. 2. Experimental Section 2.1. Materials. The two polymers, ethyl(hydroxyethyl)cellulose ether (EHEC) and the hydrophobically modified analogue (HM-EHEC), were supplied by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden, and are equivalent except for a low amount of branched nonylphenol groups grafted to the latter, Figure 1. The hydrophobic modification degree was determined to 1.7 mol % (based on repeating anhydroglucose units) by UV absorbance at 275 nm using a Shimadzu spectrophotometer with phenol as reference. Both polymer samples have the same average molecular weight (M ≈ 100 000) and degree of substitution of ethyl and hydroxyethyl groups (DSethyl ) 0.6-0.7 and MSEO ) 1.8, respectively). The molecular weights, DS, and MS were all given by the manufacturer. Prior to use, the polymers were purified as previously described.10 The purification step was important, as, for example, small amounts of free nonylphenol would have had analogous effects on phase behavior and viscosity of HM-EHEC solutions as added longchained alcohols (cf. below). The polymer concentration throughout this investigation is 1 g of polymer per 100 g of solvent (1% (w/w)) which is in the semidilute regime. The intrinsic viscosity was determined to 600 cm3/g for HM-EHEC at 25 °C, giving a value of the overlap concentration, c* ) 4/[η], of 0.7 wt %. However, the development of the specific viscosity, as well as NMR self-diffusion data,14 with polymer concentration points toward a somewhat lower value (ca. 0.2 wt %). The used cosolutes are alcohols with different chain lengths (methanol, ethanol, propanol, pentanol, and hexanol) and salts with different anions (NaCl, NaBr, and NaSCN). Apart from these also a third class of substances was made use of: urea, guanidine hydrochloride (GHCl), and guanidine hydrothiocyanate (GHSCN), see Figure 6a. Usually (14) Nyde´n, M. Unpublished results.
they find use due to their ability to denature biological macromolecules with a tertiary structure, e.g., proteins,15-17 and therefore they will be referred to as denaturants throughout this investigation. The concentration of added cosolutes is given in molal (mole substance per 1000 g of solvent). The water was of Millipore quality and all cosolutes, of high quality, were used as received. 2.2. Methods. The samples were prepared by weighing aliquots from polymer and cosolute stock solutions. Before any measurements the glass tubes were sealed with Teflon tightened screw caps and the samples were carefully mixed by turning end over end for at least 24 h. Tcp was determined by immersing the sample in a jacketed glass cell connected to a thermostated water bath. The temperature in the cell was measured with a thermocouple. On increasing the temperature of the aqueous polymer solution, it turned from a one phase (1Φ) to a two phase (2Φ) behavior (see Section 1). The sample was taken to be 2Φ when a ruled scale visually observed through the stirred solution appeared blurred. The temperature was then lowered and the solution turned from 2Φ back into the clear 1Φ region. The mean value of the two temperatures, usually within 2 °C, was taken to represent the Tcp. Viscosity experiments were carried out at 25 °C on a Bohlin VOR rheometer in the oscillatory mode, and the data are presented relative to the viscosity of the corresponding binary system (a 1% (w/w) aqueous polymer solution). For low viscous (EHEC) samples a double gap concentric cylinder was used, while for HM-EHEC samples, an ordinary concentric measuring system (cup and bob) was chosen. Before the rheology measurements, precautions were taken to prevent bubble formation in the cell that could distort the result.
3. Results and Discussion We will start this section by recapitulating some important results on related systems that have appeared (15) von Hippel, P. H.; Schleich, T. The effect of neutral salts on the structure and conformational stability of macromolecules in solution. In Structure and Stability of Biological Macromolecules; Timasheff, S. N., Fasman, G. D., Eds.; Marcel Decker: New York, 1969; pp 417-574. (16) Piculell, L.; Nilsson, S. Prog. Colloid Polym. Sci. 1990, 82, 198210. (17) Nilsson, S.; Piculell, L.; Malmsten, M. J. Phys. Chem. 1990, 94, 5149-5154.
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in the literature during the last decade. Anders Carlsson has in his thesis “Nonionic Cellulose Ethers, Interaction with Surfactants, Solubility and Other Aspects”18 investigated an EHEC molecule similar to the “parent” polymer that we have used here. He made extensive phase behavior studies of this polymer on addition of different cosolutes and combined the experimental results by theoretical calculations in a modified Flory-Huggins approach. The basic assumption in those calculations is that the polymer molecule can exist in two conformers, one polar enthalpically favored and one nonpolar entropically favored conformation. With an appropriate set of parameters describing the interactions between different molecules in the solution, this model was shown to be successful not only in reproducing the clouding phenomena of the polymer but also in qualitatively explaining some of the effects that added cosolutes had on the phase behavior. The influence of an added alcohol was rationalized by identifying the hydrophobicity of the added alcohol as the important parameter, and from a phenomenological point of view the observations were discussed on the basis of two different effects: (i) an added alcohol may make the solvent less polar and hence a better solvent for the polymer19 (especially at higher temperatures this becomes increasingly important as the number of polymer segments in the nonpolar conformation increases with increasing temperature11,12), or (ii) an alcohol may associate with the polymer, giving a net hydrophilic or hydrophobic contribution to the polymeralcohol complex. The first explanation was used for short-chained alcohols and the latter for alcohols with a longer hydrocarbon chain. These simplified explanations are, as we will see, not enough to explain all our experimental data. However, later work by Zhang et al.20 showed that it is still possible that an addition of a cosolute that is less polar than the polymer can favor a one-phase behavior if the attraction between polymer and cosolute is rather weak. If, on the other hand, the additive strongly prefers either the polymer or the solvent, then a phase separation will be promoted, provided the system is on the limit for phase separation before the addition (temperatures close to Tcp). The first example (strong attraction between polymer and cosolute) results in an associative phase separation, while the latter (where the cosolute strongly prefers the solvent) induces a segregative phase separation. Regarding addition of alcohols with longer chain lengths, we have previously shown10 that there is a fundamental difference between the phase separation phenomenon of the EHEC solution at lower hexanol concentrations together with the phase separation of HM-EHEC over the entire investigated concentration interval on the one hand and the phase separation of EHEC at higher alcohol concentrations on the other. This conclusion emerged from ternary phase diagrams.10 The former is a separation into one polymer-rich and one water-rich phase, while the latter rather is a separation of the alcohol from the water (and polymer)-rich phase. The difference arises since hexanol does not interact with EHEC markedly at low temperatures, and it is only at higher temperatures, when the EHEC molecule adopts more of its nonpolar conformations according to Karlstro¨m,11 that the interaction becomes strong enough to promote a phase separation (18) Carlsson, A. Nonionic Cellulose Ethers Interactions with Surfactants. Solubility and Other Aspects. Thesis, University of Lund, 1989. (19) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Langmuir 1986, 2, 536-537. (20) Zhang, K.; Karlstro¨m, G.; Lindman, B. Coll. Surfaces 1992, 67, 147-155.
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of the polymer (seen as a small decrease in the Tcp at low hexanol concentrations, see below). The modulation of the phase behavior on addition of different salts and denaturants that we find in this investigation mostly follows the lines that previously have been reported, not only for related systems but also for a wide range of both synthetic polymers21,22 and biomolecules.15,23 The “salting in” and “salting out” phenomena, with the former promoted by salts comprising anions with a large size and the latter seen on addition of salts with small anions, are reproduced. The different salts are often arranged in a so-called lyotropic series according to their relative effectiveness in “salting in” and “salting out”. The effect can, in analogy with the addition of alcohols, be explained in two ways: (i) Addition of NaCl to the aqueous polymer solution results in a solvent with higher effective polarity. Due to the increased difference in polarity between polymer and aqueous environment the polymer molecules experience a less good solvent. (ii) The second, and generally more appropriate explanation, as discussed by Piculell and Nilsson,16 is that the polymer surface either has a positive or negative excess of the salt which will influence the interfacial energy between the polymer surface and solvent. Small anions show a depletion from a hydrophilic/hydrophobic interface, while large anions are enriched. As an argument, Piculell and Nilsson used results from investigations by Aveyard et al. where it was shown that the interfacial tension (which is a macroscopic measurable quantity related to surface energy) between an aqueous salt phase and an oil phase could be increased by addition of salt with small anions, while salt with large anions induced a decreased interfacial tension.24,25 Aveyard et al.26 also discussed the interfacial tension changes on addition of different electrolytes in similar terms as Piculell and Nilsson.16 To summarize, addition of NaCl, which is depleted from the polymer surface, results in a higher surface energy and, thus, a force to minimize the contact between polymer and solvent appears, resulting in a reduced Tcp. SCN-, on the other hand, enriches at the polymer surface; the polymer surface energy decreases, which is followed by an increased polymer solubility seen as an increased Tcp. 3.1. Addition of Alcohols. The general trend seen in Figure 2 is that long-chained alcohols decrease the Tcp; the longer the hydrocarbon chain, the larger the drop. The short-chained alcohols methanol and ethanol, on the other hand, slightly increase the Tcp. It could be mentioned that methanol, ethanol, and propanol are fully miscible with water27 and that the polymers are insoluble in ethanol at a concentration of 2% (w/w) (from room temperature to 80 °C). It seems reasonable that an addition of methanol should favor the 1Φ region by making the solvent less polar, while the increased 1Φ behavior on addition of ethanol (which seems to be slightly more hydrophobic than the polymers) must be explained according to Zhang et al.20 Thus, ethanol shows no strong interaction either with the polymer or with the solvent. Addition of propanol, (21) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci.: Pure Appl. Chem. 1993, A30, 1-18. (22) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657-667. (23) Norton, I. T.; Morris, E. R.; Rees, D. A. Carbohydr. Res. 1984, 89-101. (24) Aveyard, R.; Saleem, S. M. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1609-1617. (25) Aveyard, R.; Saleem, S. M.; Heselden, R. J. Chem. Soc., Faraday Trans. 1 1977, 73, 84-94. (26) Aveyard, R. Can. J. Chem. 1982, 60, 1317-1326. (27) CRC Handbook of Chemistry and Physics, 66th ed.; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press, Inc.: Boca Raton, FL, 1986.
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Figure 3. The relative viscosity (see text) of a 1% (w/w) EHEC solution (open symbols) or a 1% (w/w) HM-EHEC solution (filled symbols) as a function of added ethanol (0, 9), pentanol (4, 2), or hexanol (3, 1). At higher concentrations of pentanol or hexanol the phase separation concentration in the HM-EHEC solution at 25 °C is indicated with hatch lines. The insert shows the relative viscosity of an EHEC (O) and of a HM-EHEC (b) solution as a function of added SDS.
Figure 2. The Tcp for a 1% (w/w) EHEC solution as a function of added alcohol, methanol (O), ethanol (0), propanol ()), pentanol (4), or hexanol (3). Below, or to the left of, the lines the mixture displays a 1Φ behavior, while above, or to the right of, the lines the solution demixes into two coexisting phases. The full thick lines represent the solubility of pentanol in water at different temperatures (the line to the right) and of hexanol (the line to the left). (b) Same as part a but for HM-EHEC. The different alcohols are represented by the corresponding, but filled, symbols.
which has an intermediate chain length, neither increases nor decreases the solubility of EHEC, while for HM-EHEC a slight decrease in Tcp is seen. Pentanol can be expected to interact stronger with the hydrophobically modified polymer, and thus, the phase separation is favored.20 The literature gives us additional information for alcohols with longer chain lengths, and included in Figure 2 are the solubilities of pentanol and hexanol in water.28 As was stressed in an earlier publication10 the Tcp decrease on addition of hexanol is smoother with HM-EHEC compared to the drop seen in the EHEC solution. This behavior was reproduced on addition of pentanol; Tcp shows a marked decrease over the entire investigated concentration interval with HM-EHEC, while initially the Tcp of the EHEC solution shows a more moderate change. Therefore, hexanol and pentanol seem to interact with HM-EHEC over a broad concentration regime, while a major impact on the behavior of the EHEC solution appears at concentrations close to the solubility limit of the alcohol in water (as indicated by Figure 2). The explanation can be found in our previous investigation and is due to a crossover between two different 2Φ regions in the case of the unmodified polymer.10 The viscosity data, Figure 3, given at 25 °C, support the proposed explanation for the phase behavior. The viscosity of the EHEC solution is not affected by the addition of an alcohol, independent of the hydrocarbon chain length of the alcohol, which suggests that the alcohols do not (28) von Erichsen, L. V. Brennstoff Ch. 1952, 33, 166-172.
associate with EHEC at this temperature. The viscosity of the HM-EHEC solution, on the other hand, is slightly decreased by an addition of a short-chained alcohol (ethanol). This can be interpreted as a weakening of the hydrophobic bonds between different HM-EHEC chains. (These bonds are responsible for the higher viscosity of the HM-EHEC solution as compared to the EHEC solution in the binary systems.7) The decrease in viscosity may be explained by some hydrophobic polymer tails associating with the short chain alcohol molecules instead of with other hydrophobic polymer tails. Since short chain alcohols are completely miscible with water, the net result is a reduced association between the hydrophobic parts of the polymer chains. Effectively this can be regarded as making the solvent less polar. In this context it could be mentioned that an investigation by Sivadasan and Somasundaran29 performed in the dilute regime (i.e., at a polymer concentration below c*) showed that the intrinsic viscosity of HM-HEC (hydrophobically modified hydroxyethyl cellulose) increased by an addition of ethanol. The observation was explained in terms of a decreased intramolecular association followed by an expansion of the polymer chains as the solvent was made less polar, in analogy with the discussion above where the intermolecular associations in the semidilute polymer regime are believed to decrease on addition of ethanol. On addition of alcohols with longer hydrocarbon chains the viscosity is increased. The difference with long chain alcohols is that they have a limited solubility in water. Added long chain alcohols therefore prefer to associate with the hydrophobic cross-linking zones (consisting of polymer hydrophobic tails). The association can be expected to be stronger as the hydrocarbon chain length of the alcohol becomes longer. A larger aggregation number of the alcohol molecules at the hydrophobic tails of the polymers gives a smaller surface to volume ratio, or in other words a more effective shielding of the hydrophobic parts from the water. Since it is the shielding from water that is the driving force for creating the (29) Sivadasan, K.; Somasundaran, P. Colloids Surf. 1990, 49, 229239.
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Figure 5. Relative viscosity of a 1% (w/w) EHEC solution (open symbols) or a 1% (w/w) HM-EHEC solution (filled symbols) as a function of added NaCl (O, b), NaBr (0, 9), or NaSCN (), ().
Figure 4. (a) Tcp curves for a 1% (w/w) EHEC solution as a function of added salts with different anions: NaCl (O), NaBr (0), or NaSCN ()). (b) Same as part a but for HM-EHEC. The different salts are represented by the corresponding, but filled, symbols.
hydrophobic cross-linking zones that are responsible for the generally higher viscosity of HM-EHEC solutions compared to EHEC, it is natural that the viscosity increases on addition of long chain alcohols. The situation is similar to the viscosity increase seen for HMEHEC solutions on addition of surfactants at low concentrations; see insert in Figure 3. However, above a certain surfactant concentration the surfactant binding becomes cooperative, which imposes that at this point the mixed micelles are dominated by surfactants.3 When the average number of hydrophobic tails in each mixed micelle drops below a certain value, the viscosity again decreases, because the physical cross-links between different polymer chains vanish.17 An important difference between surfactants and long chain alcohols is that the latter do not form spherical aggregates (micelles) at higher concentrations. Rather than a “microscopic phase separation”, as with surfactants, a macroscopic phase separation is observed. 3.2. Addition of Salt. When salt is added to an aqueous polymer solution, the Tcp may either increase or decrease, depending on the identity of the salt, Figure 4. Salts with small anions, as Cl- or Br-, tend to decrease the Tcp, while salts with large anions tend to increase the Tcp. This can be explained by a depletion/enrichment of added salt at the polymer surface as discussed above. While the changes in Tcp are similar for the two polymers, the changes in viscosity on addition of salt differ, with the viscosity changes for the HM-EHEC solution being more pronounced than for the EHEC solution, Figure 5. On addition of NaCl the viscosity is increased, while a decrease is observed on addition of NaSCN. NaBr has an intermediate effect and the viscosity is slightly increased. The reason for the relatively larger effect seen in the viscosity is that this is a rather sensitive measure for small changes in the degree of, or strength of, association
Figure 6. Tcp curves for a 1% (w/w) EHEC solution as a function of added denaturants, urea (O), GHCl, (0), or GHSCN ()). The Tcp as a function of added ethanol (4) is included in the figure. The insert shows the structure of the denaturants. (b) Same as part a but for HM-EHEC. The different denaturants are represented by the corresponding, but filled, symbols.
between polymer hydrophobic tails. When salt with a small anion (cf. NaCl) is added to the HM-EHEC solution, the energy cost to break the physical bond between two hydrophobic tails is increased due to the increased surface energy. This is expected to increase the viscosity. The opposite is seen on addition of NaSCN; the anion enriches at the surface of the hydrophobic junction zones which is accompanied by a decreased energy cost in breaking the hydrophobic bonds. This is followed by a decreased viscosity. The analogy between surface energies and molecular association has been discussed by Nilsson et al.16,17 The fact that the viscosity changes for the EHEC solution (though less than for the HM-EHEC solution) on addition of salt probably reflects the EHEC by itself has
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influence on Tcp of different denaturants correlates to the relative effectiveness known from denaturation of biological systems (GHCl and GHSCN are stronger denaturants than urea).15 It is interesting to observe that GHSCN is more effective than GHCl, which can be understood from what was obtained from the investigation of addition of salts. We recall that SCN- increases while Cl- decreases the solubility of the polymers. Thus, SCN- promotes while Cl- opposes the denaturation effect of GH+. Related to this observation von Hippel and Schleich found that the melting temperature, Tm, of a collagen gel obeyed the same pattern and that the addition of different destabilizing cosolutes, including salts and denaturants, almost produced an additive effect on the shift of Tm.15 As expected from the increased polymer solubility on addition of a denaturant, the viscosity decreases, Figure 7. Again the drop is more pronounced with the hydrophobically modified polymer and probably the explanation given for salts applies. 4. Conclusions
Figure 7. (a) Relative viscosity of a 1% (w/w) EHEC solution as a function of added urea (O), GHCl (0), or GHSCN ()). The relative viscosity as a function of added ethanol (4) is included in the figure. (b) Same as part a but for HM-EHEC. The different denaturants are represented by the corresponding, but filled, symbols.
parts that are more hydrophobic than other parts of the backbone. The heterogeneity can be expected to generate some association between the more hydrophobic parts of the polymers. 3.3. Addition of Denaturants. The addition of denaturants is quite analogous to the addition of salts and will be discussed in similar terms. Chemicals that are referred to as denaturants have a disrupting or dissociative effect on aggregates of biological macromolecules in an aqueous solution.15-17 The dissociating effect can be regarded as a result of a decreased surface energy between hydrophobic parts of the molecule and the surrounding aqueous media (cf. the discussion of salts comprising large anions). Then it seems reasonable that a solution of a clouding polymer should display an increased Tcp on addition of a denaturant, Figure 6. The
We have demonstrated that the strength of association of a hydrophobically modified polymer is strongly affected by a wide range of cosolutes, including alcohols, simple electrolytes, and chemicals that are used as denaturants for biomolecules. The relative strength of the association has been probed by measuring the relative viscosity which can either increase or decrease, depending on the cosolute. In addition to viscosity we have also followed the solubility of the polymers and we showed that a qualitative discussion of the effect of an added cosolute relates the observations to the distribution of cosolute molecules between bulk solution and polymer surface. The observations have been rationalized in the frame of a changed surface energy of the polymer molecules on addition of a cosolute. An alternative way to express the explanation given above may be to regard a decreased polymer solubility (decreased Tcp) as an effectively decreased polymer-polymer repulsion. As the repulsion decreases, the hydrophobic tails can be expected to show an enhanced association followed by a strengthening of the physical inter polymer-polymer bonds giving an increased viscosity. Acknowledgment. The present work has been financially supported by a grants from the National Board for Industrial and Technical Development (NUTEK) and from Akzo Nobel Surface Chemistry AB. The department for Food Technology is gratefully acknowledged for placing the Bohlin rheometer at our disposal. LA9508607
