Langmuir 1994,10, 1018-1022
1018
Effect of Surfactant Adsorption on the Polymer Bridging and Rheological Properties of Suspensions Yasufumi Otsubo Department of Image Science, Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba-shi 263, Japan Received June 28, 1993. In Final Form: December 27,199P The steady-shear viscosity and dynamic viscoelasticity were measured for suspensions flocculated by polymer bridging. Also, the sedimentation and adsorption experiments were carried out to understand the bridging conformation. For the suspensions in which the particlesare connected by irreversiblebridges, the flow is shear-thinning over a wide range of shear rates. The addition of surfactant causes a decrease in the fraction of segments of the polymer coil adsorbed in trains, and hence the adsorption-desorption of the polymer reversibly takes place. Since the bridges are constantly forming, breaking, and re-forming in a quiescent state, the flow becomes Newtonian. With increasing surfactant, the viscosity decreases at low shear rates and increases at high shear rates. The flow profiles are discussed in relation to the bridging conformation.
Introduction In polymer adsorption at a solid-liquid interface, only a portion of segments of the polymer chain are in direct contact with the surface, while the rest extend away from the surfaceinto the solution. However,polymer adsorption is considered to be irreversible, because the polymer chain may attach to the surface at many points and not be able to desorb simultaneously from all sites. When a polymer chain adsorbs onto more than two particles to bind them together, the effect is referred to as polymer bridging.'s2 Flocculation of colloidal suspensions by the bridging mechanism occurs under conditions where the polymer chain is long enough to adsorb onto two particles and has a very strong affinity for the particle surface. Since the desorption of polymer chains hardly takes place under ordinary conditions, the bridging flocculationis essentially irreversible. Therefore, the suspensions of particles with diameters comparable to the loop length of the adsorbed polymer show an irreversible increase in viscosity due to shear-induced bridging flocculation.= In this process, the particle-particle bond is formed by adsorption of the loop extending from one particle to a particle in the other floc during collision in shear flow. The bridge between particles is not broken down by thermal energy. Hence, the suspensions flocculated by polymer bridging show elastic responses at very low frequencies or plastic responses with infinite relaxation time.When the polymer chains do not have a very strong affinity for the surface, the fraction of loops which extend into the solution increases at the expense of trains which are in direct contact with the surface. Two particles can be bridged by a flexiblepolymer coil. In a previous paper? it has been reported that the suspensions in which the flexible bridges construct deformable and highly elastic flocs showunique viscosity profilesconsistingof Newtonian viscosity in the limit of zero shear rate, shear-thickening ~
@Abstractpublished in Advance A C S Abstracts, March 1,1994. (1)Iler, R. K . J. Colloid Interface Sci. 1971,37, 364. (2)Fleer, G.J.; Lyklema, J. J. Colloid Interface Sci. 1974,46,1. (3)Otaubo, Y.;Umeya, K . J. Rheol. 1984,28,95. (4)Otaubo, Y.;Watanabe, K . J.Colloid Interface Sci. 1989,133,491. (5)Otaubo, Y.;Watanabe, K . Colloids Surf. 1990,50, 341. (6)Otaubo, Y.;Watanabe, K . J. Non-Newtonian Fluid Mech. 1987, 24,265. (7)Otaubo, Y.;Watanabe, K . Colloids Surf. 1989,41,303, (8)Otaubo, Y.Langmuir 1990,6,114. (9)Otaubo, Y.Langmuir 1992,8,2336.
flow at moderate shear rates, and shear-thinning flow a t high shear rates. The Newtonian viscosity implies that the polymer bridges are constantlyforming, breaking, and re-forming in a quiescent state. The adsorption-desorption process reversibly occurs by Brownian motion. At shear rates where the time scale of coil extension is much longer than that of adsorption, the flow is Newtonian. As the shear rate is increased, the coils are subjected to rapid extension before the desorption. The shear-thickening flow may be attributed to the restoring forces of extended bridges. In suspensions flocculated by polymer bridging, the reversible bridging results in Newtonian flow and irreversible bridging in shear-thinning flow a t low shear rates. It must be stressed that the irreversibility arises from the multipoint attachment to the surface. The most important factor controlling the flow behavior of suspensions is the adsorption-desorption kinetics which in turn is closely associatedwith the fraction of segmentsadsorbed in trains. In other words the adsorption affmity determines the rheology of suspensions. An associativepolymer which consists of a nonadsorbing water-soluble backbone with grafted hydrocarbon arms adsorbs onto the particles at hydrophobic points along the chain. In the limit of very weak adsorbing groups, the associative polymer behaves as a nonadsorbing polymer. With many hydrophobes, a molecule behaves as an adsorbing polymer and strong adsorption results in bridging attraction. The associative polymers may induce different interactions between particles, depending on the number of hydrophobes and adsorption strength. A statistical mechanical theory has been developed to predict the flocculation or phase separation of a colloidal suspension by an associative polymer.lOJ1 The flocculation processes by depletion and bridgingmechanisms are explained through the interaction potentials between two flat plates in a polymer solution. In the present study, the adsorption affmityof a polymer for the particle surface is controlled by addition of surfactant. The effects of surfactant on the rheological properties of suspensionsflocculated by polymer bridging (10)Santore, M.M.; Russel, W. B.; Pmd'homme, R. K . Macromolecules 1989,22,1317. (11)Santore, M.M.;Russel, W. B.;Prud'homme,R. K.Macromoleculea 1990,23,3821.
0743-7463/94/2410-1018$04.50/00 1994 American Chemical Society
Langmuir, Vol. 10, No.4, 1994 1019
Effect of Surfactant Adsorption on Suspensions
are investigated in relation to the conformation of the adsorbed polymer.
Materials and Methods Materials. The suspensionswere compoeed of styrenemethyl acrylate copolymer particles, poly(viny1alcohol), Triton X-100, and water. The pH value was adjusted with hydrochloric acid to pH 2. The particles were formed by emulsioncopolymerization with a styrene/methyl acrylate monomer ratio of 4/60. The diameter and density of copolymer particles were 170 nm and 1.13 X 103 kg ma, respectively. Triton X-100surfactant (Union Carbide Co.) is an ethoxylated octylphenol. Poly(viny1alcohol) (PVA) with a molecular weight of M, = 1 X 106 was obtained from Kanto Chemical Co., and was used as received. The mean size of an isolated polymer chain in solution may be determined from the intrinsic viscosity [ q ] with the equation12 [VIM, = 63/2@(S2)s/2
21
.-Y,
lo-'
L
0 V
.? 10-2 >
(1)
where ( 9)1/2 is the root-mean-square radius of gyration and @ is the Flory-Fox parameter. The intrinsic viscosity of PVA was 1.2 dL g1in aqueous solution at pH 2,and the radius of gyration was 14.8 nm. The sample suspensions were prepared at concentrations up to 30% by volume. The PVA and surfactant concentrations were in the range of 0-1.5% by weight based on water. The rheological measurements were carried out for the suspensions stored at 25 "C for 1 day. Aging did not have a significant effect on the rheological behavior unlm the period exceeded 1 month. Met hods. Steady-flow and dynamic viscoelastic properties were measured using a Couette geometry on a Rheometrics RFS I1 rheometer. The diameters of the bob and cup were 30 and 32 mm,respectively. The immersion length of the bob was 30 mm. The measuring shear rates y were from 1.7 X le2to 2.7 X 102 8-1 in steady-flow measurements. The angular frequencies w were to 1.0 X 102s-l,and the strain was 0.05 in dynamic from 2.5 X le2 measurements. Adsorption of PVA on the particle surface was measured with 15 vol % suspensions which contained surfactant and sufficient polymer. To determine the concentration of nonadsorbed polymer, the particles or flocs were separated by centrifugation at 2000g, and the viscosity of the supernatant solution was measured. For Suspensions which contained surfactant at high concentrations, the supernatant solutions were slightly turbid. On the assumption that the supernatant solution has the same mixing ratio of surfactant and polymer as the initial solution, the amount of adsorbed polymer was calculated from the residual polymer concentration. The fiial sedimentation volume gives the concentration of the dispersed phase, from which the surface separation between particles can be estimated.
Results and Discussion Rheological Behavior. Figure 1shows the shear rate dependence of viscosity for 30 vol % suspensions in solutions containing PVA at different concentrations.In the absence of PVA, the suspension is electrostatically stabilizedand the flow can be approximated as Newtonian. The addition of PVA causes an increase in the viscosity over the entire range of shear rates. Since the viscosity enhancement is large at low shear rates, the flow of suspensions with PVA is shear-thinning. Especially the suspensions containing PVA at concentrations above 0.7 wt. % show nearly plastic responses at low shear rates. The appearance of plasticity is coupled with the threedimensional network of particles connected by polymer bridges. The most important feature is that, with increasingPVA concentration,the viscosity increasesand the shear-thinning behavior becomes more striking. Figure 2 shows the effect of surfactant concentrationon the viscosity behavior of 30 vol % suspensions in a 0.7 wt (12)Flory, P.J. Principle of Polymer Chemietry; Comell University Press: Ithaca, NY,1963.
IO-^
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Figure 1. Shear rate dependence of viscosity for 30 vol % suspensions in solutions of PVA at different concentrations: 0 (0);0.3 ((3); 0.5 (e); 0.7 (0);1.0 (e);1.5 wt % (0).
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Figure 2. Shear rate dependence of viscosity for 30 vol % suspensions in 0.7 w t % PVA solution containing surfactant at different concentrations: 0 (0);0.5 ((3); 0.7 (e);1.0 (0)1 ; .5 (e); 2.0 w t % (0).
% PVA solution. When the particles are covered with surfactant molecules,the polymer adsorptioncannotoccur. With increasingsurfacecoverageof particles by surfactant adsorption, the number of bridges connecting particles decreases and the concentration of the nonadsorbed polymer increases. Therefore,one can expect on the basis of the results in Figure 1that the addition of surfactant reduces the viscosity over the entire range of shear rates. However, the effect of surfactant on the viscosity behavior is very complicated. At a surfactant concentration of 0.5 wt % , the suspension shows a higher viscosity than that without surfactant. It seems likely that the addition of a small amount of surfactant enhances the flocculating power of PVA. Further increase in surfactant concentration causes a viscosity decrease at low shear rates and an increase at high shear rates. The surfactant influences the viscosity in two opposingways, dependingon the shear rate. In the range of 0.5-1.5 wt 5% ,the viscosity behavior changes from shear-thinning to Newtonian profiles with increasing surfactant concentration. Moreover, it is interesting to note that the viscosity at a shear rate of 7 X 10-1 5-1 is constant irrespective of the surfactant concentration. When the surfactant concentration is
Otsubo
1020 Langmuir, Vol. 10, No.4, 1994
IO' 4
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b
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(s-') Figure 3. Frequency dependence of the storage modulus for 30 vol % suspensions in 0.1 wt % PVA solutions containing 1.0 surfactant at different concentrations: 0 (0);0.5(0);0.7 (e); (0); 1.5 wt % (e). Angular
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Figure 4. Frequency dependence of the loss modulus for 30 vol % suspensions in 0.7 wt % PVA solutions containing surfactant at different concentrations: 0 (0);0.5 (0);0.7 (e);1.0 ( 0 ) ;1.5 wt % (0).
increased to 2.0wt % ,the viscosityis drastically decreased and the flow becomes almost Newtonian. It is considered that the suspensionis dispersedto noninteractingparticles. Under conditionswhere the partial coverageof the surface with surfactant takes place, the rheological behavior stronglydepends on the surfactant concentrationbecause the bridging conformation of PVA can be affected by surfactant adsorption. Figure 3 and 4 showthe frequency dependenceof storage and loss moduli, respectively,for 30 vol % suspensions in 0.7 w t % PVA solutions containing surfactant at different concentrations. In ordinary flocculated suspensions dispersed in polymer solutions,the viscoelastic function show a plateau at low frequencies.1s16 The plateau has been considered to be a manifestation of the network structure of particles. The network structureprovides an additional relaxationprocess. The suspensionscontainingsurfactant at low concentrations clearly show the plateau region. However, €or the suspension with 1.5 w t 5% surfactant, both the moduli rapidly decrease with decreasing frequency. The floc structure may also be constructed over the system because the viscosity is considerably higher when compared to that of the noninteracting suspension. The lack of a plateau in the frequency-dependentcurve is attributed to the rapid relaxation process. In general,bridging flocculationis essentiallyirreversible and the polymer bridge is not broken down in a quiescent (13) Onogi, 9.; Mataumoto, T.;Warashina, Y. Trans. SOC.Rheol. 1973, 17, 176.
(14) Mataumoto, T.;Hitomi, C.; Onogi, 9. Trans. SOC. Rheol. 1976,19, 641. (16) Umeya, K.; Otaubo, Y. J . Rheol. 1980,24, 239.
10-3
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1 100
1
I
I
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Shear rate ? (5-' 1 Figure 5. Shear rate dependence of viscosity for 20 vol 9% suspensions in 1.0 w t 5% PVA solutionscontaining surfactant at different concentrations: 0 (0);0.5 (@); 1.0 (0);1.5 wt % (e).
state. Since the structural breakdown is progressively induced with increasingshear rate, the flow of flocculated Suspensions is shear-thinning. The Newtonian flow observed for the Suspension with 1.5 w t % surfactant indicatesthat the polymer bridges which make up the floc structure are constantly forming,breaking,and re-forming by Brownian motion. The adsorption-desorption of the polymer on the particles reversibly takes place. The flocculationby reversiblebridging may be an equilibrium phenomenon. Figure 5 shows the effect of surfactant concentrationon the viscosity behavior of 20 vol % suspensions in a 1.0 w t 7% PVA solution. Without surfactant, the flow curve is very similar to that of a 30 vol % suspension in a 0.7 w t % PVA solution. However, the change of the viscosity profile from shear-thinningto Newtonian flow is induced at low surfactant concentrationsfor 20 vol % suspensions in spite of a higher PVA concentration. The reversible adsorption-desorptionprocess resulting in the Newtonian flow at low shear rates is achieved at a surfactant concentration of 1.0 wt %. Since the adsorption of a polymer chain extending from one particle to the other particle is less probable in dilute suspensions,the amount of surfactant required for high dispersibility can be low. The concentration of nonadsorbed polymer is relatively high in dilute suspensions at a given surfactant concentration. The suapensiof with 1.0 w t 7% surfactant shows shear-thickeningbehavior in the shear rate range of 10' to 4 X 10' s-l. In this suspension, the particles may be connectedby highly deformablebridges. Whensubjected to high shear fields, the flexible bridges are rapidly extended in the network and the entropy is decreased. The restoring forces of extended bridges provide additional resistance to flow. The shear-thickening flow appears when the time scale of extension is shorter than that of desorption. However, the very high extension causes the desorption of polymer coils from the particles. Because of the breakdown of bridges, the flow is shear-thinning at very high shear rates. The relaxation of extended bridges due to the desorption in the shear field is responsible for the shear-thickeningflow. The onset of shear-thickening can be discussed in connectionwith the relaxation process of polymer chains. From the prediction by molecular network theories, the shefv-thickeningflow occur^ when the Weissenberg number 7x1, which measures the relative strength of the shear fields, approaches unity. Here A1 denotes a characteristic relaxation time. It has been
Langmuir, Vol. 10, No. 4, 1994 1021
Effect of Surfactant Adsorption on Suspensions
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shown in previous papersgJ8 that the suspensions flocculated by reversible bridging show shear-thickening flow at high shear rates and the Weisaenberg number at the maximum viscosity is about 1. The time scale of forced desorption is estimated as 0.03 s for the suspension with 0.1 wt 5% surfactant. The mixing ratio of PVA and surfactant determines the relaxation time of a bridge. Figure 6 shows the effect of the PVA concentration on the viscosity behavior of 20 vol % suspensions with 1.0 wt % surfactant. Although the viscosity increases withPVA concentration, the flow is Newtonian at low shear rates. The adsorption-desorption process is still reversible. The decrease of the shear rate at the maximum viscosity in the shear-thickeningregion reflects an increase in the lifetime of the bridge. Figure 7 shows the effect of the particle concentration on the viscosity behavior of suspensions containing 0.7 wt % PVA and 1.0wt % surfactant. The 20 vol 5% suspension is regarded as noninteracting, while the 25 and 30 vol 7% suspensions are regarded as flocculated by reversible bridging. Except at low shear rates, a significant difference is not seen for the viscosity behavior of flocculated suspensions. It has been accepted that the viscosity of the suspension at a constant shear rate monotonously increases with an increase in particle concentration. In suspensions flocculated by bridging, the polymer concentration strongly influences the floc structure and viscosity unless the complete coverage of the particle surface takes place. At high particle concentrations, all polymer chains
adsorb onto particles and efficiently make bridges when the polymer concentration is low. It is considered that two adjacent particles are bridged in dilute suspensions which contain sufficient polymer. In such suspensions, all adsorbed coils do not always act as effective bridges. The important factor closely related to the floc formation process is the concentration ratio of particle to polymer. The similar viscosity behavior observed for 25 and 30 vol % suspensions implies that the number of bridges per unit volume is constant. In reversible bridging flocculation, the particles in flocs are cooperatively rearranged by Brownian motion. Since all the adsorbed coils effectively act as bonds connecting particles in the 25 vol 5% suspension, the increase in the particle concentration does not necessarily lead to further development of the floc structure. It looks as if the degree of flocculation does not reach the saturation in the 30 vol % suspension. Relation between Rheological Behavior and Bridging Conformation. From the rheological data, the surfactant adsorption on the particle surfaces can reduce the number of adsorption points of one polymer chain. With increasing surfactant concentration, the permanent bonds by irreversible bridging are gradually converted into the temporary bonds by reversible bridging. To understand the bridging conformation, the sedimentation and adsorption experiments were carried out for 15 vol 7% suspensions containing 1.0 wt % PVA and surfactant at different concentrations. When the interactions between the surfactant and polymer are strong, the coil conformation is varied even in solution. The radius of gyration of the isolated coil was in the range of 14.5-15.5 nm in the presence of surfactant. Since the value is almost constant at surfactant concentrations below 1.5wt % ,the addition of surfactant may not influence the coil conformation. For the suspensions without PVA and surfactant, the particle concentration in the sediment is 63.5 vol % and reasonably agrees with the maximum random sphere packing. Therefore,this value may be critical for touching particles. The sediment of suspensions which contain sufficient adsorbing polymer gives the floc structure with fully developed bridging at the maximum particle concentration. Figure 8 shows the adsorbance of PVA and surface separation plotted against the surfactant concentration. The surface separation H which corresponds to the bridging distance of adsorbed polymer in flocculated suspensionsis determinedfrom the mean distancebetween particle surfaces with the equation
H = d((CdC)1’3 - 1) (16)Otsubo, Y.J. Rheol. 1993,37,799. Eirich, F. R.J. Polym. Sci. A-1 1966, 4, 2401. (17) Rowland, F. W.; (18) Clark, A. T.; Robb, I. D.; Smith, R.J. Chem. Soc., Faraday Trans. 1, 1976, 72, 1489.
(2)
where d is the particle diameter ( ~ 1 7nm), 0 C, the critical particle concentration (=63.5vol%), and C the particle
1022 Langmuir, Vol. 10, No. 4,1994
concentration in the sedimentof the suspension containing surfactant and PVA. An increase in the surfactant concentration causes a decrease in the adsorbance of PVA, because the surfactant molecules have a stronger affinity for the particle surface. On the other hand, withincreasing surfactant, the surface separation increases at first, passes a maximum, and then decreases. In polymer adsorption on flat surfaces,the thickness of the adsorbed polymer layer is roughly of the same order as the radius of gyration of an isolated polymer chain. The loop size and hence the adsorbed layer thickness were found to decrease as the solvent changed from a good solvent to one under 6-~0nditions.~~J~ However, if the polymer chains have a strong affinity for the surface, the contraction of the coil is induced at the surface. The layer thickness varies with polymersurface interactions. The addition of surfactant causes a reduction of the fraction of segments of an adsorbed polymer in trains, leading to an increase in the adsorbed layer thickness. Therefore, the bridging distance is increased by surfactant adsorption. The maximum bridging distance is comparable to the diameter of an isolated polymer chain in solution. As expected from the rheological data, the particles can be bridged by flexible coils in the suspensions with 1.0 wt % PVA and 0.5 wt % surfactant. When the fraction of segments adsorbed in trains is decreased, the adsorptiondesorption reversibly occurs and the bridges are constantly forming and breaking by thermal energy. The polymer coils have the equilibrium conformation. Further increase in surfactant enhances the desorption of PVA. Nonadsorbed polymer chains do not contribute to the particleparticle interactions, so the particles can approach to a distance of twice the surfactant layer. On the basis of the sediment structure, schematic pictures of the bridging conformation are shown in Figure 9 for an explanation of the relation between the reversibility of the bridge and particle-particle interactions. At the intermediate concentrations of surfactant, the flocsare constructed by temporarybridges due to reversible adsorption and permanent bridges by irreversible adsorption. The permanent bridges are broken down on the application of shear, whereas the temporary bridges are constantly forming and breaking in a quiescent state. The former is responsiblefor shear-thinning flow and the latter for Newtonian flow. Therefore, the overall flow curve is characterized as a combinationof two flow profiles due to different mechanisms. For example, the 30 vol % suspension containing 0.7wt % PVA and 0.7wt % surfactant is shear-thinning at low shear rates and nearly Newtonian in the range of loo-lo2s-l (Figure 2). When the shear rate is increased above loo s-l, the permanent bridges are completely broken down and the particles are connected only by the temporary bridges. The temporary bridges are forced to desorb at high shear rates above lo2s-l. Since the shear does not affect the reversible adsorptiondesorption process at intermediate shear rates, the flow is Newtonian. The decrease in the fraction of permanent
Oteubo
)I ‘M (e) Figure 9. Conformation models of polymer bridging: (A) irreversible bridging; (B) reversible bridging.
bridges by surfactant adsorption shifts the shear-thinning region toward the low shear rate side. When the surfactant adsorption gives rise to the full conversion of permanent bridges into temporary ones, the shear-thinning region disappears. Finally, the high coverage of the surface by surfactant drastically reduces the viscosity level in the entire range of shear rate, because the particles can be completely and independently dispersed.
Conclusions The conformation of the polymer bridge can be changed by additions of a small amount of surfactant. When the fraction of segmentsof the polymer adsorbed in trains is decreased by surfactant adsorption, the adsorptiondesorption of the polymer reversibly occurs and the bridges are constantly forming, breaking, and reforming by Brownian motion. The permanent bridges by irreversible adsorption are broken down on the application of shear, so the suspensions without surfactant are shear-thinning over a wide range of shear rates. On the other hand, for the suspensions in which the flocs are constructed through temporary bridges by reversible adsorption, the flow is Newtonian at low shear rates and the viscoelasticfunction rapidly decreases with decreasingfrequency,showingrapid relaxation. With increasing surfactant, the viscosity decreases at low shear rates and increases at high shear rates. Under conditions where the particle coverage of the surface with surfactant and polymer takes place, the flow behavior can be explained by a combination of two bridging mechanisms.
