Langmuir 1994,10,1093-1100
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Adsorption of Cationic Polymer onto Negatively Charged Surfaces in the Presence of Anionic Surfactant Victor Shubint Physical Chemistry I, Chemical Centre, University of Lund, P.O.Box 124, S-221 00 Lund, Sweden Received September 21,1993. In Final Form: January 13,1994’ The effect of addition of anionic (sodiumdodecyl sulfate, SDS)surfactant on the structureand composition of layers of cationic hydrophobically modified (hydroxyethy1)celluloseQuatrisoft LM 200, adsorbed on negatively charged surfaces (mica and SiOd, was studied with the surface force apparatus and in situ null ellipsometry. The results were supplemented by electrokineticmeasurements. In surfactanbfree conditions the weakly charged polycation adsorbsin a rather expended conformation and overcompensatesthe substrate surface charge. It is found that associative binding of surfactant to the polymer results in a variety of interfacial behaviors dependingon the concentration of added amphiphile. Polymedsurfactant interactions affect both the adsorptivity and conformation of adsorbing macromolecules. At SDS concentrations above the critical micelle concentration the polymerlsurfactant complex desorbs from the surface. 1. Introduction
Water-soluble hydrophobically modified polymers (HMP) constitute a large and important class of industrial polymers. Due to their unique rheological behavior they are widely used as thickeners in a variety of water-based formulations, such as latex paints, building materials, drilling muds, etc. Chemically modified natural polysaccharides (starch, cellulose, etc.), being nontoxic, fiid increasingly wide application in foods and cosmetics. Adsorption of these polymers at solid-liquid interfaces is used to control the stability of colloidal dispersions as well as other interfacial properties such as wettability and adhesion. As a consequence HMPs have many applications in fine particle flotation, oil recovery, latex dispersions, skin and hair care, and emulsion polymerization. In a number of applications the polymers are used in combination with surfactants. It is well known that interaction between polymers and amphiphiles strongly affects the phase behavior, together with rheological (and other bulk) properties, of polymer solutions. The same applies to the behavior of polymers at interfaces. Various aspects of polymer/surfactant interactions-a rapidly growing field of colloid science-have been reviewed recently by Goddard et ale1Hereafter we will focus on the interfacial behavior of a particular class of polymerspolyelectrolytes-in the presence of surfactants. It has been well recognized that adsorption of polyelectrolytes on solids is determined by a variety of factors: the nature and the charge density of the surface,the charge density of the polyion, the molecular weight and concentration of the polymer, the salt concentration, nonelectrostatic interactions of macromolecules with the surface and with each other, and finally interaction with consolutes such as amphiphiles. Due to the complexityof the system and the processes involved, so far our understanding of the related phenomena remains rather limited. In the experimental field most of the efforts have been directed toward determination of the adsorptivity and the structure of the adsorbed layer. A review of experimental t On leave from the Department of Colloid Chemistry, Chemical Faculty, St. Petereburg University, St. Petereburg, Russia. .Abstract published in Advance ACS Abstracts, February 15, 1994. (1) Interactions ofeurfactante with polymers and proteins; Goddard, E. D., Ananthapadmanabhan,K. P., E&.; CRC Press: Boca Raton, FL, 1993; p 427.
techniaues providing access to this key information can be found Two complemen-ky experimental methods have been used in the present study: the surface force technique and ellipsometry. The surface force apparatus (SFA),first developed by Tabor et al.s and further elaborated by Israelachvili and Adams? has been widely used for direct measurement of forces acting between polymer-coated surfaces.s Such experiments provide fundamental information on the nature of these interactions as well as a direct measure of the adsorbed layers’ extent. A number of SFA studies involving synthetic and natural polyelectrolytes have been reported to date.”l8 It has been established that highly charged polyelectrolytes at low ionic strength adsorb in a flat conformation, while an increase of the electrolyte concentration or lowering of &hepolyion charge density results in a rather extended configuration of adsorbed molecules. Reports on the effects of surfactant on surface forces in such systems are much fewer. Argillier et al.5 studied the interaction between Quatrisoft-coated micas and found a marked increase in the force range upon addition of small amounts (2) CohenStuart, M. A.; Coegrove,T.;Vincent, B. Adv. CoUoidZnterface Sci. 1986, 24, 143. (3) Tabor, D.; Winterton, R. H.S . R o c . R. SOC.London 1969, A312, 435. Faraday Tram. (4) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC., 1 1978, 74, 975. (5) Patel, S. S.; Tirrell, M. Annu. Rev. Phye. Chem. 1989,40, 597. (6) Marra, J.; Hair,M. L. J. Phye. Chem. 1988,92,6044. (7) Luckham, P. F.; Klein, J. J. Chem. SOC.,Faraday Trom. 1 1984, 80,865.
(8)Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughton, W.; Chapman, D. Colloids Surf. 1987,26, 263. (9) Dix, L. R.; Toprakcioglu, C.; Davies, R. J. Colloids Surf. 1988,31, 147. (10) Klein, J.; Luckham, P. F. Colloids Surf. 1984,10,65. (11) Afshar-Rad,T.; Bailey,A. I.;Luckham, P. F.; MacNaughton, W.; Chapman, D. Colloids Surf. 1988,31,125. (12) Malmaten,M.;Blombeg,E.; Claeseon,P.;Carletedt,I.; Ljwegren, I. J. Colloid Interface Sci. 1992,161, 579. (13) Kawanishi, N.; Christenson,H.K.;Ninham, B. W. J. Phys. Chem. 1990,94,4611. (14) Kamiyama,Y.; Israelachvili,J. N. Macromolecules 1992,25,5081. (15) Argillier, J. F.; Ramachandran, R.; Harris,W. C.; Tinell, M. J. Colloid Interface Sci. 1991,146, 242. (16) Ananthapadmanabhan,K. P.; Mao, G. 2.;Goddard,E. D.;Tmell, M. Colloids Surf. 1991,61, 167. (17) Dhoot, S.; Goddard, E. D.; Murphy, D. S.; Tirrell, M. Colloids Surf. 1992. 6 ... -6 -. , 91. -~ (18) Dahlgren, M. A. G.; Claeeaon, P. M. h o g . Colloid Polymer Sci. 1993,93, 206. ----r
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of a nonionic surfactant. Ananthapadmanabhan et al.16 observed a somewhat increased repulsion on approach of two surfaces bearing layers of Polymer J R (cationic cellulose derivative) in the presence of sodium dodecyl sulfate (SDS)and a strong attraction on separation, which was tentatively attributed to hydrophobic interaction between SDS molecules bound to the polymer and/or bridging. T h e paucity of the reported data does not allow a physical picture of complex interactions in the interfacial region, containing both polymeric and surfactant species, to be drawn. Moreover, the technique does not permit a direct and accurate determination of the adsorbed amount, thus rendering the available information rather incom-
plete. Ellipsometry is an optical technique based on measurement of changes in the state of polarization of light upon reflection from a film-coveredsurface.lg The total surface excess (adsorbed amount), which is the quantity most commonly extracted from ellipsometric measurements, can, in principle, be resolved into the average thickness and t h e mean refractive index of an adsorbed layer. Thus, the technique allows simultaneous determination of the adsorbance, the extension of t h e polymer away from t h e surface, and t h e concentration of polymer in the layer. A good example of successful use of ellipsometry for studying the interfacial behavior of polyelectrolytes is the work of Takahashi et al.,20 who investigated adsorption of sodium poly(acry1ate) onto a platinum plate from NaJ3r aqueous
solutions. In the current paper we present an experimental study of the interfacial behavior of a cationic hydrophobically modified (hydroxyethyl)cellulose,Quatrisoft LM 200, in the presence of anionic surfactant SDS. The influence of surfactant addition on t h e structure and composition of the adsorbed polymer layers, as well as on the interaction between them, was studied via the SFA and in situ null ellipsometry. The results were supplemented by electrokinetic measurements. 2. Experimental Section 2.1. MaterialsandChemicale. The water used in this study was purified by the following consecutive steps: distillation, percolation through a Millipore water system (consisting, in sequence, of two mixed-bed ion-exchangecartridges, an Organex cartridge, an activated charcoal filter, and a 0.2-pmnucleopore filter), and final double distillation in an all-Pyrex apparatus. The conductivity of the water was less than 1 X 1 V t2-l cm-l, pH 5.6 0.2, after saturation with carbon dioxide from air. The mica used in our SFA experimenta was highest quality, optically clear muscovitegreen mica, obtained from Mica Supplies Ltd. (England). The Si/SiOz plates for ellipsometry and electrokinetic measurements were prepared as follows. Polished silicon wafers, purchased from Okmetic Ltd., were oxidized thermally in an atmosphere of pure oxygen at 920 "C with a subsequent annealing and cooling in an argon flow to produce a Si02 surface layer -30 nm thick. The wafers were then cut into slides. The substrate surfaces were cleaned by consequent immersion for 5 min in hot (80 "C) mixtures (1:1:5, by volume) of (i) 25% NH4OH (pro analysi, Merck), 30% Ha02 (pro analysi, Merck), and HzO and (ii) 32% HC1 (pro analysi, Merck), 30% H202, and H20, rinsed with pure water, and stored in ethanol until use. Prior to use the surfaces were blow-dried and then plasma-cleaned for 5 min in a glow dischargeapparatus (Harrick ScientificCorp. Model PDC3XG) in an atmosphere of residual argon (; d
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Time, min
Figure 6. Total amount of material adsorbed onto the Si02 surface from 34 ppm Quatrisoft solutions at two different SDS concentrations: 4 X 1od M (curves a and b) and 2 X 1V M (curvesc and d). The surfactant was introduced into the system at t = 0 in two different ways: after completion of polymer adsorption (b, c) and simultaneously with the polymer (a, d). The level of Quatrisoft adsorption from surfactant-freesolution is shown by a dashed line.
SDS the effect was only minor: I' reached the value of 1.95 mg/m2 shortly after surfactant addition and then decreased to the level of about 1.8mg/m2over a period of 20 h. By contrast, at CQDS= 9 X 10sM the initial increase in I' to 2.2 mg/m2 was followed by relatively quick and substantial desorption (within 1h r was down to 1 mg/ m2). In a different series of experiments the feasibility of polymer/surfactant premixture adsorption on bare substrate was addressed. The results are shown in Figure 6. The polymer/surfactant complex formed in solution at low CQDSexhibits a high affinity to the surface (see curve a), giving about the same plateau value of r as in the case of surfactant addition to the preadsorbed polymer layer (see curve b), while the polymer/2 X 10s M SDS premixture shows only little adsorption (curve d). This feature implies that interaction with surfactant promotes the polymer adsorption in the first case but inhibits it in the second case. Possible reasons for this behavior will be discussed in section 4. 3.3. Electrokinetic Measurements. Electrokinetic measurements were performed to establish charge properties of the adsorbed layers. The resulting information provides important clues to understanding the mechanism of the polycation adsorption onto the negative surface and its interaction with anionic surfactant. First, the f-potential was measured on bare plates, immersed in KC1 solutions at neutral pH. This yielded f = -22, -39,-63, and -72 mV in lO-l, 1 0 - 2 , 1 0 s , and 10-4 M KC1, respectively. These values are in remarkable agreement with those reported by Scales et al. for fused silica.33 Second, Quatrisoft was introduced into the system. It caused a rapid drop in the f-potential and even its sign reversal. At equilibrium < = +18 mV was recorded. The low potential explains the absence of any substantial long range electrostatic interaction between Quatrisoft layers (see Figure 2). The sign of the potential at the boundary of the adsorbed layer unequivocally indicates overcompensation of the substrate surface charge by adsorbed polymer. If we suppose cationic groups of Quatrisoft to be bound only to negative surface sites, the net positive charge of the adsorbed layer assumes the presence of free (33) Scalee, P. J.; Grieser, F.; Healy, T. W.; White, L. R.; Chan, D. Y. C. Langmuir 1992,8,965.
charged groups within the adlayer. Electrostatic repulsion between these free charges may be one of the factors limiting the polymer adsorption. Upon addition of 4 X 1V M SDS,the f-potential decreased to +2 mV, indicating an almost total neutralization of the polymer charge at this SDS concentration. Further addition of surfactant leads to the sign reversal of the adsorbed layer charge, and at CQDS = 2 X 10s M, f = -11 mV was measured. The charge reversal can be interpreted as a result of the surfactant aggregate formation on Quatrisoft side chains. It is noteworthy that the adlayer charge as measured by the streaming potential technique shows a clear correlation with the adsorbed amount obtained via ellipsometry: the charge m i n i " matches the maximum of adsorption and vice versa (cf. Table 1). This feature reveals that electrostatics has a determining influence on the polyelectrolyte adsorption, especially since it is almost unscreened in our case (low ionic strength). 4. Discussion
The Quatrisoft molecule bears positively charged groups which determine its ability to adsorb strongly onto negatively charged surfaces. Unlike highly charged polyelectrolytes adsorbing in a flat conformation, Quatrisoft molecules adopt a rather expanded, coillike conformation at the interface, giving an adsorbed layer thickness of about 30 nm. This finding is in agreement with both experimental and theoretical results reported in the literature. In the case of pure electrosorption, adsorption is limited by the charge compensationcondition, no matter whether polyelectrolyte charges are in physical contact with the surface or just trapped by the field of the electrical double layer. In the limit of low electrolyte concentration, when competition with small counterions is negligible, only one other factor limits adsorption-steric hindrance. This will result in undercompensation of the surface charge by the polyelectrolyte at low charge densities, a,of the latter. If, in addition to electrostatics, a nonelectrostatic, specific mechanism of adsorption is operative, the system tendency to avoid a free charge accumulation in the interfacial region can be overcome and two consequences are expected nonzero adsorption at a = 0 and possible surface charge overcompensation at low or intermediate a.34 This prediction is in agreement with observation of the l-potential sign reversal on latexes and silica in the presence of polyelectrolyte^.^^ Electrokinetic measurements, however, cannot provide a quantitative measure of the superequivalent adsorption. In this respect adsorption studies, at least in some cases, are more informative. Thus, Tanaka et al. have reported 20-30 % overcompensationin the case of low-CY cationic polyacrylamide adsorbing onto anionic latex,% and this could be easily determined as a ratio of the adsorbed polymer charge to the surface charge, the latter being due to the strong acid groups and, accordingly, constant. When the surface charging is more complex, e.g., on oxides or clays, the magnitude of the solid-phase charge, involved in electrostatic coupling with polyelectrolyte, becomes rather indefinite. Indeed, it is unclear whether one should consider the net diffuse layer charge (which,in the first approximation, can be identified (34)van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterboech, B. H. Langnwir 1992,8, 2538. (35) van der Linde, A. J.; Bijsterbosch,B. H. Croat. Chem. Acta 1990, 63, 455.
(36) Norde, W.; Rouwendal,E. J. Colloidlnterface Sci. 1990,139,169. (37) Sidorova, M.; Golub, T.; Musabekov, K. Adv. Colloid Interface Sci. 1993, 43, 1. (38) Tanaka, H.; odberg, L.; Wiigberg, L.; LmdstrBm, T. J. Colloid Interface Sci. 1990, 134, 219.
Adsorption of Cationic Polymer
with the electrokinetic one), the titratable charge, the surface functional group density, or some intermediate value. This complicates an unequivocal establishment of the overcompensation occurrence from adsorption measurements.39*40 The same applies to the adsorption of the cationic polyelectrolyte onto silica, studied in the present work. The adsorbed amount of Quatrisoft-1.6 mg/m2corresponds to the adsorbed charge density of 1.7 NC/cm2. This appears to be larger than the titratable silica surface charge at neutral pH and low ionic strength (e.g., see ref 41) and thus indicates a high probability of superequivalent adsorption. Electrokinetic measurements gave a positive potential at the boundary of the adsorbed Quatrisoft layer, confirming this supposition. One can speculate further, arguing that the first, rapid stage of the adsorption process (see Figure 5) corresponds to the electrostatics-favored situation (undercompensated surface charge),while during the second, slow stage the polymer supply to the surface is inhibited by the repulsive electrostatic field. In any case, the fact of silica surface charge overcompensation by Quatrisoft can be regarded as well established. The question of the nature of the force driving this process then arises. As was shown in ref 15, hydrophobically modified (hydroxyethy1)cellulose (HMHEC), a nonionic polymer structurally similar to Quatrisoft, does not adsorb onto a negatively charged hydrophilic surface (mica) at low salt concentrations. This finding, although rendering doubtful any substantial chemical contribution of the polymer backbone to the Quatrisoft adsorption, does not rule out the possibility of a nonelectrostatic contribution due to specific binding of the polymer cationic groups to the surface negative sites. Moreover, since the polymer adsorbs in competition with bound counterions, usually referred to as “ion pairs”,42the entropy gain upon release of the small ions into the bulk solution would enhance polyelectrolyte adsorption. All this can render the polycation attachment to the surface feasible even, to some extent, against unfavorable accumulation of the net charge brought into the interfacial region by the adsorbing polycation. This mechanism, if real, must be investigated in detail both experimentally and theoretically. In concluding this part of the discussion, we stress that electrostatics plays an important, if not decisive, role in Quatrisoft adsorption onto negatively charged surfaces. The main goal of the present study has been to improve understanding of the influence of polymer/surfactant interactions on polymer adsorption and interfacial configuration. The results obtained show that addition of anionic surfactant causes substantial changes in the structure and composition of the adsorbed layer. These changes can be interpreted in terms of surfactant binding to the polymer. Indeed, anionic surfactant association with cationic polymer commences at concentrations far below the cmc, being favored by both the electrostatic factor (the polymer and surfactant carry opposite charges) and hydrophobic interaction. At low SDS concentrations effective neutralization of the polycation both in the bulk and at the interface makes possible supplementary adsorption of the polymer/surfactant complex. Thus, at 4 X 10-5M SDS, the adsorptivity shows a maximum while the electrokinetic charge is close to zero. This favors the (39) Durand-Pima, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci. 1987,119, 414. (40)Wang, T.K.;Audebert, R. J. Colloid Interface Sei. 1988,121,32. (41)Bolt, G. H.J. Phys. Chem. 1957, 61, 1166. (42)Yatee, D. E.; Levine, S.; Healy, T. W. J . Chem. SOC.,Faraday Trans. I 1974, 70,1807.
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idea that the increase in r, compared to the surfactantfree case, can be attributed to the removal of electrostatic intralayer repulsion, limiting the polymer adsorption. Note that the polymer/surfactant complex,being overall neutral, retains its ability to interact with negative surface sites. This is demonstrated by our adsorption measurements from Quatrisoft/SDS premixtures (see Figure 6). The incorporation of additional macromolecules into the adsorbed layer can also be promoted by the formation of intermolecular hydrophobic bridges, i.e., clusters consisting of surfactant molecules and hydrophobic side groups from different polymer chains. This bridge formation can explain the occurrence of a deep attractive minimum on decompression a t CSDS = 4 X 106M. The fact that a part of the polymer, adsorbed at low SDS concentrations, can be rinsed away supports the idea that not all macromolecules are (irreversibly) attached to the surface, some of them being held within and/or on top of the original polymer layer by hydrophobic bonds. On increasing the SDS bulk concentration, progressive binding of surfactant leads to an increase in the net negative charge of the macromolecule. This results in a drop in additional polymer adsorption to zero at CSDS > 5 X 10-4 M. At 2 X 10-3 M SDS the increase in upon addition of surfactant is found to be due to the surfactant binding only. The initial adsorption increment A r = 0.35 mg/m2corresponds to an average of seven SDS molecules per polymer/hydrophobe. The number of polymer hydrophobes per unit area can be readily estimated since the amount of polymer in the mixed layer (1.6 mg/m2)and the hydrophobe content (1.12 X 10-4 mol/g) are both known. This finding does not allow estimation of the aggregation number since the fraction of unbound side chains (presumably those in close contact with the surface) is unknown. However, it clearly supports the idea of formation of multimolecular surfactant complexes on the polymer chains. An increase in the net negative charge of Quatrisoft molecules leads to a drastic reduction in the adsorptivity of the polymer even on bare substrate, as was revealed by adsorption from the polymer/surfactant premixture (Figure 6). The adlayer expansion observed at CQDS 2 X 10-3 M can be attributed to electrostatic repulsion between polymer molecules bearing bound SDS clusters. Note also the absence of an attractive minimum on the corresponding force profile (see Figure 4). Beyond the cmc, the anionic surfactant wins its competition with the surface for the polymer and highly negatively charged polymer/surfactant complexes desorb from the surface. Addressing a comparison of the adsorbed layer thicknesses determined by the two main techniques used, we note that a t both low and high SDSconcentrations addition of the surfactant leads to an increase in the ellipsometrically measured adlayer thickness, i.e., layer expansion, which is in qualitative accordance with SFA findings. At first sight, however, the thicknesses of adsorbed layers, obtained via ellipsometry, appear to be in rather poor quantitative agreement with those inferred from the force onset as measured with the SFA. For example, in the case of pure polymer the adsorbed layer optical thickness was found to be 7 nm, whereas the SFA gives the layer extent as about 30 nm. This discrepancy turns out to be rather misleading if one takes onto account the fact that ellipsometry, unlike the SFA, “sees” the most dense part of the adsorbed layer and that dfis calculated under the assumption of adsorbed layer uniformity. In reality, however,the polymer density, and consequently refractive index, changes with distance from the surface. It may be
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shown43that the values of df and nf, being the measures of an equivalent homogeneous film, can be determined by the following equations:
where n = f(d) is the refractiveindex distribution function. Unfortunately, ellipsometry does not allow determination of the type of distribution. It is possible, however, to determine two parameters of this function,f(d), if a certain reasonable refractive index profile is assumed or can be predicted theoretically. Then the refractive index distribution, calculated in this way, can be compared with information on the layer extent, obtained by other, independent techniques such as dynamic light scattering or surface force measurements. These techniques are (43) McCrackin, F. L.; Coleon, J. P. In Ellipsometry in the Measurements ofSurfacesand ThinFilms;Pasaaglia,E., Stmmberg,R. R.,m e r , J., Ede.; National Bureau of Standards Miscellaneous Publications: Washington,DC, 1964; p 61.
Shubin sensitive to the polymer tail contribution and, therefore, give a measure of the full extent of an adsorbed layer. Thus, comparative analysis of ellipsometric and SFA results can, in general, provide an indication of the character of the material distribution in the interfacial region. In our case, however, such a quantitative analysis is hardlyjustified due to the differencein the chemicalnature and charging behavior of the two substrate surfaces used. At the same time, although the details of the adsorption behavior of the polymer can differ for different substrates, the results obtained on mica SFA the and on silica using ellipsometry are consistent with respect to the surfactant effect on the adsorbed polymer layers. We have demonstrated that combination of the two main techniques used in the present work, SFA and ellipsometry,supplemented by electrokinetic results yields valuable complementary information regarding the interfacial behavior of the polymer/surfactant system. Acknowledgment. I thank B. Lindman and U.Oleson for stimulating discueaions and encouragementasthis work progressed and B. Bijsterbosch and G. Fleer for helpful comments during the preparation of the manuscript. I also thank S. Klinstr6m, who kindly provided Si/SiOz plates, and A. van der Linde for hishelp with the f-potential measurements. The work has been supported by a Karlshamn’s Research Foundation (Sweden) grant.
