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Langmuir 1997, 13, 5558-5562
Polymer/Surfactant Complexes at the Air/Water Interface Detected by a Simple Measure of Surface Viscoelasticity S. T. A. Regismond,† K. D. Gracie,† F. M. Winnik,*,† and E. D. Goddard‡ Department of Chemistry, McMaster University, 1280 Main Street W., Hamilton, Ontario, Canada L8S 4M1 and 349 Pleasant Lane, Haworth, New Jersey 07941 Received February 28, 1997. In Final Form: July 31, 1997X The rheology of the air/water interface of mixed polymer/surfactant systems has been examined by the talc particle test. Polymer/surfactant pairs studied range from strongly interacting polyelectrolytes/ oppositely charged surfactant systems, such as sodium hyaluronate/alkyltrimethylammonium halides, carboxymethylcellulose/hexadecyltrimethylammonium chloride (HTAC), and an alternating copolymer of acrylamide and diallyldimethylammonium chloride (Merquat-550)/sodium dodecyl sulfate (SDS) to less strongly interacting systems of neutral polymers and charged surfactants, such as ethylhydroxyethyl cellulose/SDS or /HTAC, hydroxypropyl cellulose/SDS, poly(N-isopropylacrylamide)/SDS and poly-(Nvinylpyrrolidone)/SDS. Clear indications of surface viscoelasticity at the air/water interface were obtained over a wide range of compositions for several, but not all, polymer/surfactant pairs examined. These results extend our previous findings (Regismond, S. T. A.; Winnik, F. M.; Goddard, E. D. Colloids Surf., A 1996, 119, 221-228.) on mixed solutions of SDS and a cationic derivative of hydroxyethyl cellulose (polymer JR400). Generally, viscoelasticity provides evidence of a synergistic adsorption of the two components. Factors influencing the development of surface viscoelasticity are assessed.
Introduction In work on combinations of polycations and anionic surfactants reported some twenty years ago the discovery was made that not only was interaction in these systems general but that interaction at the air/water interface seemed to mirror that in the bulk phase, and complex formation occurred in the surface as well.1 The evidence was based on synergistic lowering of surface tension observed in the mixed systems.2 The observations were particularly striking as the polycations examined were themselves only feebly surface active while the complexes had pronounced surface activity. A similar effect was reported subsequently by Buckingham et al.3 for combinations of poly(L-lysine) and sodium dodecyl sulfate (SDS) and for a system charged in the opposite sense, carboxymethylcellulose/alkyltrimethylammonium bromide (HTAB) by Barck and Stenius4 and by Bergeron et al.5 for the system acrylamide-acrylamidomethyl propanesulfonate copolymer/dodecyltrimethylammonium bromide (DTAB). In related work, Goddard and Hannan2 reported both synergistic surface tension lowering and development of surface viscosity and surface viscoelasticity in monolayers of docosyl sulfate on introducing a minute amount (10 ppm) of a cationic cellulosic polymer into the subsolution. This development, monitored by the simple “talc particle” test, was interpreted as indicating that the polymer was drawn into the surface phase as reflected by a pronounced change in two-dimensional rheology. Recently this cationic polymer in combination with SDS was studied over a wide range of composition by the talc particle test, and a “topological phase map” was generated.6 The data †
McMaster University. Haworth, NJ. X Abstract published in Advance ACS Abstracts, September 15, 1997. ‡
(1) Goddard, E. D.; Hannan, R. B. J. Am. Oil Chem. Soc. 1977, 54, 561. (2) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73. (3) Buckingham, J. H.; Lucassen, J.; Hollway, F. J. Colloid Interface Sci. 1978, 67, 423. (4) Barck, M.; Stenius, P. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 89, 59. (5) Bergeron, V.; Langevin, D.; Asnacios, A. Langmuir 1996, 12, 1550.
S0743-7463(97)00228-X CCC: $14.00
agreed with the concepts outlined above inasmuch as they provided clear evidence of polymer in the surface over much of the concentration regime. They also indicated that the surface was denuded of polymer when the relative concentration of surfactant was high and that, beyond the cmc, the extent of the micellar interface became dominant. Mixed-film formation7 at the air/water interface has been invoked to account for a number of properties dependent on the detailed nature of such interactions, for example, wetting8 and, especially, foaming5,9 in which the surface viscosity is recognized to be potentially important. The purpose of the current work was to use the talc method to examine a series of polyanion/cationic surfactant mixtures and to determine its applicability to other systems by investigating a number of uncharged polymer/ charged surfactant systems known from previous work to interact in solution (see Table 1). Experimental Section Materials. Water was deionized using a Nanopure water purification system. SDS was obtained from Sigma Chemical Co. (purity >99%). Hexadecyltrimethylammonium chloride (HTAC) was purchased from Kodak Chemicals; hexadecyltrimethylammonium bromide (HTAB), tetradecyltrimethylammonium bromide (TTAB) and DTAB were obtained from Sigma, and dodecyltrimethylammonium chloride (DTAC) was obtained from TCI America. Alginic acid, sodium salt (medium viscosity), carboxymethyl cellulose, sodium salt (NaCMC, high viscosity), and poly(vinylpyrrolidone) (PVP) were obtained from Aldrich Chemical Corp. Merquat-550, an alternating copolymer of acrylamide and diallyldimethylammonium chloride was a gift from Calgon Corp. Hyaluronic acid, sodium salt (HA), was a gift of Hyal Pharmaceutical Corp. (Mississauga, ON, Canada). Ethylhydroxyethyl cellulose (EHEC) and a hydrophobically modified ethylhydroxyethyl cellulose (HM-EHEC) were obtained from Dr. B. Lindman (University of Lund, Sweden). Poly(Nisopropylacrylamide) (PNIPAM) and HM-poly(N-isopropylacryl(6) Regismond, S. T. A.; Winnik, F. M.; Goddard, E. D. Colloids Surf. A: Physicochem. Eng. Aspects 1996, 119, 221. (7) See, for example: Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf. 1995, 94, 125. Chari, K.; Hussain, T. Z. J. Phys. Chem. 1991, 95, 3302. Cohen-Addad, S.; DiMeglio, J. M. Langmuir 1994, 10, 773. Duffy, D. C.; Davies, P. B.; Creeth, A. M. Langmuir 1995, 11, 2931. (8) Kilau, H. W.; Voltz, J. I. Colloid Interface Sci. 1991, 57, 17. (9) De Gennes, P. G. J. Phys. Chem. 1990, 94, 8407.
© 1997 American Chemical Society
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Table 1. Polymer/Surfactant Systems Investigated polymers cationic polymers polymer JR-400 (MW 400 000) Quatrisoft LM-200 (MW 100 000) Merquat-550 (MW 5 × 106) anionic polymers carboxymethyl cellulose (MW 300 000) sodium alginate (medium viscosity) sodium hyaluronate (MW 500 000-800 00) acrylamide-acrylamide sulfate copolymer (AN125, MW 2 × 106) neutral polymers hydroxypropyl cellulose (MW 100 000) ethylhydroxyethyl cellulose (MW 100 000) HM-ethylhydroxyethyl cellulose (MW 100 000) poly(N-vinylpyrrolidone) (MW 10 000, 1.3 × 106) poly(N-isopropylacrylamide) (Mv 300 000) HM-poly(N-isopropylacrylamide) (Mv 200 000)
surfactants SDS SDS SDS HTAC HTAC HTAC, TTAB, DTAB DTAB SDS SDS, HTAC SDS, HTAC SDS SDS SDS
amide) (HM-PNIPAM) were prepared as previously described.10 A random copolymer of acrylamide (AM) and acrylamidomethylpropanesulfonate (AMPS) (AAS, AN125) was a gift from SNF Floerger (St.-Etienne, France). It is a statistical copolymer composed of 75 mol % AM and 25 mol % AMPS, with molecular weight of 2 × 106 Da. It was purified by ultrafiltration with a 30 000 cutoff membrane. Methods. Solution Preparation. Stock solutions of each polymer (2 g L-1) were prepared by dissolving the polymer in water. They were allowed to equilibrate for 24 h. Surfactant solutions of varying concentrations, ranging from 0.01 to 10 g L-1, were obtained by dilution of a stock solution in water. Mixed surfactant/polymer solutions subsequently prepared spanned surfactant concentrations from 0.005 to 5.0 g L-1. For measurements, the solutions were mixed and allowed to stand in Petri dishes for at least 30 min. Solubility diagrams of the polymer/ surfactant systems, when known, were used to guide the choice of absolute and relative concentrations of the two components. Emphasis was placed on fairly dilute solutions of polymer (e1 g L-1) in order to lessen possible contributions of bulk phase effects on the observed surface behavior. Talc Method. A small quantity of calcined talc powder was sprinkled on the aqueous surface of a solution contained in a Petri dish 10 cm in diameter. A gentle current of air was directed tangentially at the talc particles for 1 or 2 s and then removed. The observed movement of the particles along the surface was recorded. The following qualitative characterizations of the interface were made: F, fluid; V, viscous; VE, viscoelastic; G, gel; S, solid. While in category G no net flow results, in category VE, a net movement of the particles is observed, together with some recovery upon removal of the air current. This procedure has long been used as a sensitive, qualitative measure of the fluid/mechanical conditions of spread or adsorbed monolayers.11
Results and Discussion Polyelectrolytes/Surfactant Systems. The interactions between polyelectrolytes and oppositely charged surfactants have been characterized in great detail over the past decades.2 The bulk physical properties of the mixed solutions are interpreted in terms of the formation of polymer/surfactant complexes and the distribution of the surfactant between the free state and the aggregated or micellar states. Macroscopic phase separation is observed when charge neutralization by surfactant molecules yields hydrophobic surfactant/polymer aggregates. This effect was noticed in early studies of polymer/ surfactant systems, especially with anionic polysaccharides. Scott,12 for example, found for anionic polysaccharides with weak acidic groups, such as hyaluronic acid (10) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991, 7, 905. (11) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; pp 257-258. (12) Scott, J. E. Chem. Ind. 1955, 168.
Figure 1. Surface-phase maps of the systems consisting of sodium hyaluronate (HA) and the three surfactants, HTAC, TTAB, and DTAB (pH 6.27). The circles represent the observation points. The bulk phase appearance of each composition is indicated by the shading of the circles (open, clear; full, precipitation or turbidity).
or alginic acid, that precipitation with HTAC could be inhibited as a result of deionization of their carboxy groups. Sodium hyaluronan (HA) is a linear polysaccharide made up of alternating glucuronic acid, sodium salt, and N-acetylglucosamine. It is known to interact strongly with cationic surfactants.13 Clear indications of surface viscoelasticity were obtained over substantial concentration regions of the alkyltrimethylammonium halide/HA mixtures, for TTAB and HTAC but not for DTAB. In the latter system, however, the surface was viscous at low surfactant concentration. It became fluid for [DTAB] > 1.0 g L-1. Phase maps representing dilute aqueous mixtures of these solutions are shown in Figure 1, together with the corresponding bulk phase diagrams recorded for the same solutions. These are comparable to those reported by Thalberg and Lindman for a HA sample of higher molecular weight.13 For HTAC/HA and TTAB/HA solutions, the surface viscoelasticity occurs for systems on the left-hand side of the phase diagram, which corresponds to the regions of relatively low surfactant concentrations. The surface VE corresponds, in general, to systems shown by conductivity measurements to undergo cooperative binding of surfactant to the polymer backbone.13 The fluidity of the surface is recovered only at surfactant concentrations above the respective critical aggregation concentration (cac). Control experiments were carried out with solutions of HA alone at various pH (from 3.0 to 12.0) and of increasing ionic strength. None of these solutions exhibited surface viscoelasticity, confirming the polymer/surfactant synergism in the development of surface viscoelasticity. Another observation is that many of the systems which show bulk phase incompatibility display particularly pronounced viscoelasticity. Thus, while interactions at the surface mirror those in the bulk phase, the changes in surface rheology can be more sensitive and, for example, are still clearly detectable at polymer concentrations (0.3 g L-1) too low to produce macroscopically detectable changes in bulk properties. The (13) Thalberg, K.; Lindman, B. J. Phys. Chem. 1989, 93, 1478.
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Figure 2. Surface-phase map of the HTAC/sodium alginate system (pH 7.0). See Figure 1 for symbols used. The arrow indicates the concentration corresponding to the cmc.
polymer alone is weakly surface active but does not exhibit surface viscoelasticity. The system consisting of HTAC and sodium alginate, a random copolymer of glucuronic acid and mannuronic acid, was analyzed by the same method. A surface phase map for this system (Figure 2) shows that the three types of surface rheology are detected. Although the area of surface viscoelasticity is very limited, a large zone of enhanced surface viscosity supports the notion of mixed film formation of polymer and surfactant. As a control, we also recorded the surface rheology of solutions of sodium alginate (1.0 g L-1) and the anionic surfactant SDS, two species that are not expected to exhibit any attraction owing to electrostatic repulsion between the surfactant head groups and the polyanion. Indeed, the air/water interface remained fluid over all surfactant concentrations tested. A third polysaccharide, the anionic cellulose derivative NaCMC, was tested for any development of surface viscoelasticity in the presence of alkyltrimethylammonium chlorides. Surface VE was seen with solutions of NaCMC of 1-0.1 g L-1 containing DTAC in the range 0.5-2.5 g L-1 (or from 1.9 × 10-3 to 9.5 × 10-3 mol L-1) where the mixtures were cloudy or, in some cases, phase separated. For mixtures of NaCMC and HTAC, surface VE was less pronounced and found to center around HTAC concentrations of 0.25-1.5 g L-1 (or 7.8 × 10-4 to 4.7 × 10-3 mol L-1). We note that this situation is quite different from that offered by the HA/alkyltrimethylammonium bromide systems, where the strongest effect was observed with HTAC. In the case of the lowest polymer concentration (NaCMC, 0.1 g L-1), all surfaces were fluid, although some solutions were slightly cloudy. The three polymers described so far are all polysaccharides, as is polymer JR400, the polymer examined in our original report. Surface viscoelasticity of mixed polyelectrolyte/surfactant solutions is not limited to polysaccharides. As examples, we describe results obtained with two commercial polyelectrolytes: the polycation Merquat-550 and the polyanion AAS. Merquat550 solutions of concentration ranging from 0.5 to 2.5 g L -1 were observed in the presence of SDS. The surfaces of the most concentrated Merquat-550 solutions exhibited modest viscoelasticity for 0.5 < cSDS < 1.0 g L-1, but there
Regismond et al.
Figure 3. Surface phase map of the DTAB/acrylamideacrylamidosulfonate (AAS) system (pH 7.0). See Figure 1 for symbols used.
were again extensive areas of enhanced surface viscosity at the lowest surfactant levels. More dilute polymer solutions underwent an abrupt transition from surface fluidity to surface viscosity, with no intermediate surface viscoelasticity. The transition occurred for a surfactant concentration of 0.5 g L-1. The surface rheology of the solutions correlates well with the bulk phase diagrams reported by Goddard and Hannan.1 They observed no macroscopic phase separation in mixed SDS/Merquat-550 solutions for polymer concentrations lower than 0.5 g L-1. At higher polymer content, maximum precipitation occurred for 0.5 < cSDS < 1.5 g L-1 (Merquat-550, 1.0-2.5 g L-1). The anionic copolymer AAS was mixed with DTAB at polymer concentrations ranging from 0.02 to 0.37 g L-1. Surface VE was observed in all mixed AAS/HTAC of sufficiently low surfactant concentration (see Figure 3). Mixed solutions were clear in the high and low surfactant concentration domains. In all cases there exists an intermediate surfactant concentration domain where phase separation takes place. Evidence for macroscopic phase separation is clearest for the mixed solutions of highest polymer content. For mixed solutions containing less than 0.05 g L-1, only a slight bulk phase turbidity was noticeable, although surface VE was preserved. In fact, solutions of polymer alone exhibited surface viscoelasticity albeit less pronounced than the mixed solutions, despite the fact that at the concentrations studied here the polymer is at most only feebly surface active:14 a tentative explanation is that the apparent surface viscoelasticity of the solutions is a surface and bulk phase phenomenon caused by self-association of this high molecular weight copolymer. The surface “phase diagram” of this polymer/surfactant system is in fact significantly different from the other polyelectrolyte/oppositely charged surfactant pairs examined so far. Studies of the influence of such parameters as molecular weight of the polymer and comonomer ratio on its behavior with added surfactant would be desirable. We should add that purification of the polymer by ultrafiltration/freeze-drying had little effect on the properties we observed. (14) Asnacios, A; Langevin, D.; Argillier, J.-F. Macromolecules 1996, 29, 7412.
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Table 2. Surface Rheology Data for Mixed EHEC/ Surfactant Systems surfactant concn marking the SVE-SV transition polymera
surfactant
g L-1
mol L-1
bulk phase study (ref)
EHEC EHEC EHEC EHEC EHECb HM-EHEC
DTAB TTAB HTAC SDS SDS SDS
1.00 0.25 0.10 0.20 0.15 1.50
3.2 × 10-3 7.4 × 10-4 3.1 × 10-4 6.9 × 10-4 5.2 × 10-4 5.2 × 10-3
13 13 14 16 16 17
a Polymer concentration 1.0 g L-1. b Polymer concentration 0.1 g L-1.
Other Polymer/Surfactant Pairs. The systems chosen so far represent those in which electrostatic attraction between the polymer and the surfactant is the main driving force for interaction, resulting in extremely low values of the cac of the surfactant in such systems. There are, however, a large number of interacting pairs that have been widely investigated and do not involve these forces. We decided to examine a representative group of these by the surface rheological “talc test”, especially since there are several recent pointers to the existence of mixed surfactant/polymer compositions in the surface of such systems.7 Two examples are hydroxypropyl cellulose (HPC)/SDS15 and EHEC/SDS.16 Over a wide range of composition, HPC/ SDS mixed solutions showed no trace of surface viscoelasticity and no development of enhanced surface viscosity, whereas EHEC in combination with ionic surfactants displayed surface viscoelasticity. A phase map was prepared for mixed surfactant/EHEC solutions, with either anionic or cationic surfactants. Dilute polymer solutions were monitored (concentration below 1 g L-1). Mixed solutions remained clear for all surfactant concentrations. Surface viscoelasticity was observed in the low surfactant concentration domain. For each surfactant, the breadth of the surface viscoelasticity domains decreased with decreasing polymer concentration. The cationic surfactant series tested, DTAB, TTAB, and HTAC, allowed us to monitor the importance of the length of the hydrocarbon tail. The breadth of surface viscoelasticity domains increased with decreasing surfactant chain length (Table 2), a trend in agreement with the well-known increase in surfactant cmc and polymer/surfactant cac values with decreasing chain length. We noted that solutions of EHEC alone exhibited surface viscoelasticity for polymer concentrations as low as 0.01 g L-1. This was also the case with HM-EHEC.17 In fact the surface rheology of mixed HM-EHEC/SDS solutions followed the trends described for EHEC/SDS, but the surface viscoelasticity was enhanced and covered a wider surfactant concentration range. For example, in polymer solutions of 1.0 g L-1, mixed HM-EHEC/SDS had viscoelastic surfaces for cSDS < 1.5 g L -1, whereas for EHEC/SDS solutions of identical polymer concentration, surface viscoelasticity vanished for cSDS > 0.2 g L-1. In a recent study of the interaction phenomena at the air/liquid interface of mixed EHEC/SDS solutions Nahringbauer reported an extraordinary acceleration of polymer adsorption at the interface following the addition of (15) Winnik, F. M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91, 594. (16) Kamenka, N; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (17) Thuresson, K.; Nilsson, S.; Lindman, B. In Cellulose and Cellulose Derivatives; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Woodhead Publ. Ltd: Cambridge, UK, 1995; p 323.
SDS.18 Dynamic surface tension measurements performed over a wide SDS concentration range with fixed concentrations of EHEC (2 g L-1, 12 ppm, and 2 ppm) were interpreted in terms of the adsorption at the air/ water interface of charged clusters of EHEC/SDS complexes, which, Nahringbauer argued, become firmly adsorbed at the air/water interface due to the polyelectrolytic properties of the EHEC/SDS complexes. The development of surface viscoelasticity observed in our studies of this mixed system confirms the synergism in surface activity. The absence of surface viscoelasticity in the case of other cellulose ethers deserves further comments. Nonionic alkyl celluloses are generally surface active, and time-dependent variations in surface tension and surface pressure were observed in all cases. These polymers alone in aqueous solution adsorb at the air/water interface, forming monolayers with properties such as compressibility similar to those formed by spreading from a solvent, as shown by studies of HPC15,19,20 and EHEC.21 Polymer surface monolayer formation is thought to reflect a competition between the surface activity of the polymer segments, which tends to spread the polymer segments on the interface, and the entropy of the polymer chain, which tends to preserve the coil. The lag time observed in the development of surface activity was attributed to this effect.18 The fact that we did not observe surface viscoelasticity in solutions of HPC might be indicative of an extremely slow surface layer formation, especially in the case of dilute polymer solutions. For the sake of consistency, all our measurements were carried out 30 min after sample preparation. Periods as long as 160 h were needed for some EHEC solution to reach equilibrium in terms of surface activity.18 A current study of the surface rheology time dependence in these solutions will clarify this point. Finally the PVP/SDS and PNIPAM/SDS mixed systems were analyzed, as they are neutral polymer/surfactant pairs known to interact in water. Neither system showed any trace of surface viscoelasticity or development of any enhanced surface viscosity over a wide range of composition. On the other hand, the hydrophobically modified PNIPAM, HM-PNIPAM, was different inasmuch as it displayed domains of surface viscoelasticity in combination with SDS. Mixed solutions of SDS/HM-PNIPAM (0.19 g L-1) had fluid surfaces at low (cSDS < 0.02 g L-1) and high SDS contents (cSDS > 0.11 g L-1), and in the intermediate surfactant concentration range, the solution surface was viscous, viscoelastic, then viscous again, with increasing surfactant concentration. The formation of polymer/ surfactant aggregates in bulk solution was detected previously by fluorescence measurements (cac 6.3 × 10-3 M L-1 for a 1.8 g L-1 HM-PNIPAM solution).10 It should be pointed out that solutions of this surface-active polymer by itself give no trace of surface viscoelasticity. Conclusions We have confirmed the findings of our previous publication that mixed film formation in solutions containing a combination of a polyelectrolyte and an oppositely charged surfactant can lead to the development of pronounced surface viscoelasticity. Furthermore, we have shown that the simple talc particle test used for this purpose demonstrates that solutions of uncharged polymer that contain hydrophobic groups can also develop, or show (18) Nahringbauer, I. Langmuir 1997, 13, 2242. (19) Chang, S.; Gray, D. G. J. Colloid Interface Sci. 1978, 67, 255. (20) Johnson, B. A.; Kreuter, J.; Zografi, G. Colloids Surf. 1986, 17, 325. Gau, C.-S.; Yu, H.; Zografi, G. Macromolecules 1993, 26, 2524. (21) Nystrom, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994 and references therein.
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increased, surface viscoelasticity when combined with surfactants. The following factors can be listed as being important for the development of viscoelasticity or viscosity in mixed films from mixtures of polymer and surfactant in aqueous solution. (1) Electrostatic attraction, as between a polycation and an anionic surfactant or a polyanion and a cationic surfactant. The driving force toward surface association can be assessed by synergistic lowering of surface tension in such systems and by the fact that the polyion in most cases shows no surface activity on its own. (2) Hydrophobic interaction, as between a polymer with hydrophobic groups and a surfactant. In this case, a driving force exists for formation of a mixed surface film, since both compounds are intrinsically surface active. Hydrophobicity in the polymer can be furnished from a multitude of short alkyl groups, as in the case of EHEC, a small number of long alkyl chains, as in the octadecyl groups in HM-PNIPAM, or a combination of these as in HM-EHEC. (3) The length of the surfactant chain will increase the interaction and the size of the polar head group would be expected to play a role.
Regismond et al.
The chief advantage of the talc test lies in its utility as a simple screening test to study polymer/surfactant interactions. It was shown to be applicable to a number of systems in which the polymer is a polyelectrolyte or is hydrophobically modified. It has limitations when the polymer itself leads to solutions displaying surface viscosity or viscoelasticity. The test could be used as a diagnostic tool in the formulation of fluids for practical applications when dilute polymer solutions are required in order to achieve the fast wetting and quick adsorption. Its relevance to the subject of foaming is the subject of a forthcoming publication.
Acknowledgment. This work was supported by a grant of the Natural Sciences and Engineering Council of Canada to F.M.W. We thank Dr. K. Thuresson (Lund University, Sweden) for purifying ethylhydroxyethyl cellulose. LA9702289
