Effect of Surfactants on Shear-Induced Gelation and Gel Morphology

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Effect of Surfactants on Shear-Induced Gelation and Gel Morphology of Soft Strawberry-like Particles Delong Xie,†,‡ Paolo Arosio,† Hua Wu,† and Massimo Morbidelli*,† † ‡

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland School of Chemistry and Chemical Engineering, South China University of Technology, 510640 Guangzhou, Guangdong, China

bS Supporting Information ABSTRACT: The role of surfactant type in the aggregation and gelation of strawberry-like particles induced by intense shear without any electrolyte addition is investigated. The particles are composed of a rubbery core, partially covered by a plastic shell, and well stabilized by fixed (sulfate) charges in the end group of the polymer chains originating from the initiator. In the absence of any surfactant, after the system passes through a microchannel at a Peclet number equal to 220 and a particle volume fraction equal to 0.15, not only shear-induced gelation but also partial coalescence among the particles occurs. The same shear-induced aggregation/gelation process has been carried out in the presence of an ionic (sulfonate) surfactant or a nonionic (Tween 20) steric surfactant. It is found that for both surfactants shearinduced gelation does occur at low surfactant surface density but the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. When the surfactant surface density increases above certain critical values, shear-induced gelation and eventually even aggregation do not occur any longer. For the sulfonate surfactant, this was explained in the literature by the non-DLVO, short-range repulsive hydration forces generated by the adsorbed surfactant layer. In this work, it is shown that the steric repulsion generated by the adsorbed Tween 20 layer can also protect particles from aggregation under intense shear. Moreover, the nonionic steric surfactant can also protect the strawberry-like particles from coalescence. This implies a decrease in the fractal dimension of the clusters constituting the gel from 2.76 to 2.45, which cannot be achieved using the ionic sulfonate surfactant.

1. INTRODUCTION Emulsion polymerization, as one of the most widely used processes for polymer manufacturing, produces millions of tons of polymers worldwide every year in the form of latexes. Although latexes can be applied directly, as for example in surface coating and painting,1,2 in most cases the polymer particles are extracted from the disperse medium through a proper aggregation process by forming clusters or gels, often using electrolytes. The aggregation/gelation of charge-stabilized rigid colloidal particles induced by electrolytes has been well studied in the literature under both stagnant conditions3
11 and shearing flow.12
14 In general, the added electrolytes after the aggregation cannot be completely eliminated from the polymers and lead to changes in the electronic and mechanical properties of the final polymer products as well as their color. The aggregation/gelation of rigid polymer particles without using electrolytes can be achieved by applying a sufficiently strong shear rate so as to overcome the repulsive interaction energy barrier.15
18 It is found that when a colloidal system is stabilized by the typical DLVO (Derjaguin
Landau
Verwey
Overbeek) interactions, such shear-induced aggregation/gelation does in fact take place, as has been recently explained theoretically by Zaccone et al.19 It is also found that the fractal dimension of the clusters constituting the gel is 2.4 ( 0.04, independent of the original primary particle size and concentration and the imposed shear rate.18 However, it has been observed that when ionic surfactants are r 2011 American Chemical Society

used and the density of the adsorbed surfactant on the particle surface reaches a certain value, no gelation or even aggregation is possible, even at extremely large shear rates.17,19 This somewhat unexpected behavior was attributed to the additional non-DLVO interaction, the strongly repulsive, short-range hydration force generated by the adsorbed surfactant.15
17 This is based on the fact that as the ionic surfactant progressively adsorbs on the particle surface, the hydrophilicity of the particle surface increases, eventually leading to some ordering of the water molecules, which has been confirmed by various experiments to be the source of the hydration force.20
23 With respect to rigid particles, the aggregation of soft particles has been less frequently studied in the literature.24
28 Recently, Gauer et al.29
33 systematically investigated the aggregation of elastomer particles under stagnant conditions and demonstrated that upon physical contact elastomer particles may coalesce through intermolecular diffusion or viscous flow during aggregation as they do during the film formation of latex paints and coatings.34
36 Coalescence during aggregation results in more compact clusters, and the coalescence extent depends not only on the material properties of the soft particles31 but also on the surface properties30,32 and temperature.32 Jia et al.37 investigated, Received: February 18, 2011 Revised: March 25, 2011 Published: May 04, 2011 7168

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Figure 1. Cryo-SEM (cryogenic scanning electron microscopy) picture of the primary particles showing their strawberry-like core
shell morphology.

under stagnant conditions, the aggregation of core
shell particles with a rubbery core (Tg ≈
56
C) partially covered (grafted) with a plastic shell (in pieces) (Tg ≈ 100
C), leading to a strawberry-like morphology. These are of great interest in applications for their high resistance to impact. It was found that when the temperature is below 50
C no coalescence occurs during aggregation, whereas for temperatures higher than 50
C partial coalescence has been observed and its extent increases as the temperature increases. Arosio et al.38 performed the aggregation of the same strawberry-like particles under intense turbulent flow without electrolyte addition and found that intense shear can induce partial coalescence even if the aggregation is performed at room temperature. The results of the above studies indicate that both temperature and intense shear can promote the coalescence of soft particles. In this work, we study the role played by surfactants in the shear-induced aggregation/gelation of the soft strawberry-like particles mentioned above. The main objective is twofold: first, we would like to determine if the observed partial coalescence during intense shear-induced gelation can be altered by adsorbing an ionic or a nonionic (steric) surfactant on the particle surface. Second, as mentioned above, previous studies demonstrated that when a sufficient amount of ionic surfactant is adsorbed on the particle surface, short-range hydration forces can be generated, which protect the particles from aggregating even in the presence of very intense shear. Now, we would like to explore whether similar behavior can also be obtained with nonionic (steric) surfactants.

2. EXPERIMENTS 2.1. Colloidal System. The system used for this study is a colloidal dispersion of nanosized particles supplied by BASF SE (Ludwigshafen, Germany), produced by emulsion polymerization. The particles exhibit a core
shell structure with the core of the acrylic elastomer (Tg ≈
56
C) partially covered with a shell of poly(styreneacrylonitrile). A cryo-SEM picture of the particles, as shown in Figure 1, clearly indicates their strawberry-like morphology. The hydrodynamic radius of the particles, determined by dynamic light scattering (DLS), is 51.4 nm, which is equal to the radius obtained by fitting the form factor determined by static light scattering (SLS) using the RDG (Rayleigh
Debye
Gans) expression for spherical particles. Thus, the nominal radius of the particles is set to Rp = 51.4 nm. All of the surfactants and the

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residual initiator used during the polymerization, as well as the other possible electrolytes, have been removed completely by mixing the dispersions with a mixture of cationic and anionic ion-exchange resins (Dowex MR-3, Sigma-Aldrich), according to the procedure described elsewhere.15 After the dispersions were cleaned, the surface tension of the mother liquor was very close to that of Milli-Q (Millipore) deionized water, indicating that the colloidal dispersions are free of surfactants. Note that the dispersions after cleaning are still very stable under quiescent conditions because of the presence of polymer chain-end charge groups (sulfate) on the surface, which originate from the initiator. In fact, the measured zeta potential at φ = 5.0  10
4 is
58 mV, indicating a stable colloidal system. 2.2. Surfactants. To investigate the role played by the surfactants in shear-induced aggregation/gelation, we have considered two of them: Emulgator K30/95 (Bayer) and Tween 20 (polyoxyethylene (20) sorbitan monolaurate) (AppliChem). The former is a multicomponent sulfonate (ionic) surfactant system with a mean molecular formula of C15.05H30.78(SO3Na)1.32 whose detailed information can be found elsewhere,16 and the latter is a nonionic surfactant. The adsorption (binding) of both surfactants to the particle surface mainly relies on their linear hydrophobic hydrocarbon chain, and the stability mechanism is related to charges from sulfonate groups for the former and to the steric effect of the complex polyoxyethylene group for the latter. Moreover, to investigate the effect of the hydrophobic part of the steric surfactant, we have also investigated Triton X-100 (p-(1,1,3,3-tetramethylbutyl)phenyl)poly(oxyethylene) (Merck), whose hydrophobic binding to the particle surface is related to the tetramethylbutyl-phenyl groups. 2.3. Gelation through a Microchannel. Shear-induced aggregation (gelation) has been performed in a commercially available device, a Homogenizer HC-5000 (Microfluidics, USA), equipped with a zshaped microchannel with a rectangular cross-section (with a length of 5.8  10
3 m and a cross-sectional area of 5.26  10
8 m2), referred to as the z-MC in the following text, whose fluid dynamic behavior has been characterized elsewhere.39 The high shear turbulent flow in the z-MC is generated by the pressure at the inlet through a piston pump driven by compressed air. Details of the setup were reported in our recent work.16 The relation between the shear rate G in the z-MC and the inlet pressure P, reported by the device supplier using water, is G(1/s) = 1.02  104 P (bar), where P ∈ [20,150] bar. Thus, we can obtain a shear rate in the range of G ∈ [2  105, 1.5  106] 1/s or a Peclet number, Pe (= 3πRp3μG/kBT, where μ the medium viscosity and kB is the Boltzmann constant) in the range of [55, 275] (based on water at T = 298 K). The pressure at the inlet of the capillary has been measured by means of an electronic pressure transducer and used to compute Pe in the z-MC from the above correlation. It should be pointed out that such computed shear rates have to be taken as indicative because when aggregation or gelation occurs within the z-MC the fluid properties change substantially along the microchannel and the real shear rate in the z-MC can be substantially different from that calculated by the above correlation. To explore the effect of surfactant type and surface density on shearinduced gelation, for each surfactant we have performed shear-induced aggregation (gelation) experiments at the same volume fraction, φ = 0.15, and Peclet number, Pe = 220, but with different amounts of surfactants. 2.4. Sample Characterization. As repeatedly observed in our previous studies,16
18,38 the colloidal system, after passing through the z-MC, regardless of whether it is liquidlike or solidlike, is composed of two distinct classes of clusters: class 1, primary particles plus dimers and trimers, and class 2, large clusters (or gels) with sizes at least 2 orders of magnitude larger than that of the primary particles, where clusters of intermediate size are negligible. In fact, the two classes can be easily separated by a 5-μm-opening filter. It is obvious that only the large (class 2) clusters contribute to the formation of the gel network. Therefore, for 7169

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factor of the clusters, S(q), which is defined as SðqÞ ¼

IðqÞ Ið0Þ PðqÞ

ð1Þ

where I(q) is the angle-dependent scattering intensity, I(0) is the intensity at zero angle, P(q) is the form factor of the primary particles measured using the same instrument, and q is the magnitude of the scattering wavevector, defined as   4πn θ q¼ sin ð2Þ λ 2 where λ is the wavelength of the incident light, n the refractive index of water, and θ is the scattering angle. Then, for significantly large, welldeveloped clusters, the power-law regime of S(q) leads to an estimate of the fractal dimension, Df,40 SðqÞ  q
Df for

1 1 ,q, Rg RP

ð3Þ

where Rg is the average radius of gyration of the fractal clusters. It is worth noting that because the clusters are generated from strongly stabilized particles under extremely high shear rates they are very stable under stagnant conditions. It follows that any shear rate that may be involved during sample preparation and analysis does not significantly alter their size and structure because it is at least a few orders of magnitude smaller than that in the z-MC. The reproducibility of the SALS measurements is in fact always excellent.

3. RESULTS AND DISCUSSION

Figure 2. Adsorption isotherms of surfactants on three different particles: (b) strawberry-like core
shell particles, (() particles made of the rubbery core material, and (2) particles made of the plastic shell material. (a) Tween 20 and (b) Emulgator K30/95. φ = 0.005; T = 25
C. The solid curves serve to guide the eye. each system after passing through the z-MC, we have determined the conversion of the primary particles to class 2 clusters, x. Toward this aim, we have manually homogenized each system after the z-MC using a glass bar, taken a sample, and weighted and properly diluted it with deionized water. Then, after elimination of the class 2 clusters using a 5 μm filter, the sample is characterized by a small-angle light scattering (SALS) instrument (Mastersizer 2000, Malvern, U.K.) to determine the turbidity, which from the measured correlation (calibration) curve between the turbidity value and the particle volume fraction gives the number of particles not converted to the class 2 clusters. From this, the primary particle conversion, x, can be determined. It should be mentioned that in class 1 (i.e., after the 5 μm filtration), there are not only primary particles but also certain numbers of dimers and trimers whose light extinction efficiencies are different from that of a primary particle. Because the calibration curve was measured using only the primary particles, the contributions of dimers and trimers to light extinction have to be assessed and accounted for. Details of the methodology for such corrections can be found in our previous work.17 To characterize the fractal dimensions of the class 2 clusters, we first manually homogenize the system, take a sample and properly dilute it in deionized water, and then measure with SALS the scattering structure

3.1. Surfactant Adsorption Behavior. The adsorption isotherms of Tween 20 and Emulgator K30/95 on the particles have been determined using a standard surface tension technique16 at a particle volume fraction of φ = 0.005. In the case of strawberrylike core
shell particles with the rubbery core partially covered with a plastic shell, there are two types of surfaces for surfactant adsorption. To assess if selective adsorption on different surfaces occurs, for each surfactant we have also measured the adsorption isotherm for particles of similar sizes but made of the pure rubbery core and the pure plastic shell materials, which were also supplied by BASF SE. The obtained results are shown in Figure 2a,b for Tween 20 and Emulgator K30/95, respectively, in terms of the surfactant density on the particle surface (Γ) as a function of the surfactant concentration in the aqueous phase (Cs,eq) at equilibrium. Note that in the case of strawberry-like particles, to calculate the particle surface area in order to compute Γ we have assumed that the particles are spherical and defined by the nominal radius Rp = 51.4 nm, as discussed above. It is seen that for both surfactants the adsorption isotherm for the particles made of the pure rubbery core material lies slightly above that corresponding to the pure plastic shell material, whereas that for the strawberry-like particles is in between. This may indicate a certain preference for surfactant adsorption on rubber with respect to the plastic material. All of the adsorption isotherms have the S shape, indicating changes in the adsorption mechanism as the surfactant concentration increases41
43 (i.e., at low surfactant concentrations, the adsorbed surfactant forms a homogeneous dilute gaseous-like layer of individual molecules whereas above a certain value they self-organize to form inhomogeneous hemimicelles (aggregates) on the surface, leading to a substantial increment in the surfactant surface density). The transition from a homogeneous to an inhomogeneous state in Figure 2a occurs at Cs,eq ≈ 2.4  10
5 mol/L 7170

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Figure 3. Average structure factor, S(q), plotted as a function of the normalized wavevector, q  Rg, for the clusters constituting the gel formed in the z-MC. (b) Strawberry-like core
shell particles; (2) rigid particles made of the plastic shell material.

with a surfactant surface density of Γ ≈ 2.6  10
6 mol/m2, whereas it occurs at a much lower surfactant surface density in Figure 2b. The saturation adsorption cannot be defined in both cases because the cmc occurs earlier. (See Figure S1 in the Supporting Information (SI)). However, from the experimental point at the largest surfactant concentration before the cmc, even though not at saturation, the estimated surface area Am occupied by each surfactant molecule is only 17.5 Å2 for Tween 20 and 22.2 Å2 for Emulgator K30/95, whereas from the literature it should be 60.3 Å2 for a Tween 20 molecule44 and >25 Å2 for a sulfonate molecule.45 Such a difference confirms that the adsorbed surfactants are in the inhomogeneous hemimicelle form and the obtained Am values are only their projected area on the surface. 3.2. Shear-Induced Gelation in the Absence of Surfactant. Let us first consider the case without adding any surfactant. As mentioned above, in this case the colloidal system with strawberry-like particle morphology is still very stable under quiescent conditions because of the presence of the polymer chain-end charged groups (sulfate) on the surface, originating from the initiator. Such a stable colloidal system has been forced to pass through the z-MC at a particle volume fraction of φ = 0.15 and a Peclet number of Pe = 220 (G = 8.0  105 1/s). It was found that shear-induced gelation occurred, and a solid-like gel was obtained at the outlet of the z-MC. The measured conversion of primary particles to the class 2 clusters forming the gel is x = 89.4%, and the average structure factor, S(q) measured using the SALS instrument, for the class 2 clusters in the size range from cutoff filtration between 10 and 25 μm opening filters is shown in Figure 3 (circles). It is seen that the S(q) curve exhibits a welldefined power-law regime whose slope allows us to estimate the fractal dimension of the clusters, which is Df = 2.76 ( 0.04, indicating the rather compact structure of the clusters. We have also performed shear-induced gelation for the particles made of the pure plastic shell material at the same values of φ and Pe. The conversion of primary particles to gel is x = 96%, which is significantly larger than that in the case of the strawberry-like particles. Moreover, from the average structure factor, ÆS(q)æ as shown in Figure 3 (triangles), the estimated

Figure 4. SEM pictures of the gels formed through intense shearinduced gelation after drying at room temperature. (a) Strawberry-like core
shell particles and (b) rigid particles made of the plastic shell material.

fractal dimension of the class 2 clusters is only Df = 2.4 ( 0.02, indicating a much more open structure than in the previous case. Note that the Df value of 2.4 is typical of the class 2 clusters formed from rigid (plastic) primary particles.18 Thus, the larger Df value of the class 2 clusters of the strawberry-like core
shell particles reveals that partial coalescence among the particles occurs during the shear-induced aggregation (gelation) (full coalescence would lead to Df f 3).30,31 To verify this conclusion further, we have taken SEM pictures of the gels formed from the above two types of particles after naturally drying at room temperature. The pictures shown in Figure 4 indicate that particle coalescence occurs in the case of the strawberry-like core
shell particles. It should be pointed out that this conclusion is only qualitative because the drying process can alter the extent of coalescence. We have previously investigated the coalescence behavior of a similar strawberry-like colloidal system under both stagnant37 and intense shear38 conditions and have found that when the clusters are formed through Brownian motion in the absence of shear, coalescence is insignificant at room temperature, whereas increasing temperature or forming the clusters in intense shear 7171

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surface charges by the shear stress as well as the induced strong cluster restructuring.38 The partial coalescence of the strawberrylike particles also explains why the conversion of the particles to a gel is smaller with respect to the noncoalescing particles of the pure plastic shell material. This is due to the fact that the shearinduced aggregation kinetic constant, βshear i,j , increases with the cube of the radii of the aggregating clusters, Ri and Rj:12 βshear ¼ i, j

Figure 5. Fractal dimension Df of the clusters constituting the gel and the conversion x of the primary particles to a gel as a function of the surface density of three different surfactants: (a) Emulgator K30/95 (ionic), (b) Tween 20 (nonionic), and (c) Triton X-100 (nonionic). Pe = 220 and φ = 0.15.

flow leads to partial coalescence. The insignificant coalescence at room temperature is due to the presence of the fixed surface charges that resist to their relocation,30,32 but such resistance decreases as the temperature increases, leading to coalescence at higher temperatures.32 The observed coalescence in intense shear flow is mostly related to the improved mobility of the fixed

4 GðRi þ Rj Þ3 3

ð4Þ

For given i and j primary particles in the clusters, the less the extent of coalescence, the larger their radii and therefore the faster the aggregation proceeds. Thus, at fixed values of the shear rate and residence time in the z-MC, the conversion must increase as the extent of coalescence decreases. It should be mentioned that we did not perform shear-induced aggregation on the particles made of the pure rubbery core material because when these particles are forced to pass through the z-MC they and their clusters stick to the z-MC wall, leading to clogging of the channel. 3.3. Shear-Induced Aggregation in the Presence of Ionic Surfactant (Emulgator K30/95). A set of latexes of the strawberry-like particles have been prepared at the same particle volume fraction (φ = 0.15) but with different amounts of ionic surfactant Emulgator K30/95. These latexes have been forced to pass through the z-MC to perform shear-induced aggregation at a fixed Pe = 220. Similarly to the above cases in the absence of surfactant, for each latex after passing through the z-MC, we have determined two quantities: the conversion of the particles to the class 2 clusters, x, and the fractal dimension of the class 2 clusters forming the gel, Df. The results are shown in Figure 5a as a function of the surfactant surface density. It is seen that the x value decreases monotonically as the surfactant surface density increases. Moreover, we have observed three distinct regimes as a function of the surfactant surface density, as indicated in the Figure: for Γ < 6.07  10
7 mol/m2 (Am > 274 Å2), the samples after passing through the z-MC are in a solidlike gel state, whereas for Γ > 6.07  10
7 mol/m2 (Am < 274 Å2) they are in a liquidlike state, which means that there are not enough class 2 clusters formed within the z-MC to interconnect to form a standing gel network. Thus, Γ = 6.07  10
7 mol/m2 (Am = 274 Å2) is the critical surfactant surface density for the transition from a solidlike gel to a liquidlike state. When the surfactant surface density further increases to have Γ g 1.12  10
6 mol/m2 (Am e 148 Å2), the conversion of the particles to the class 2 clusters becomes x = 0, (i.e., there is no change in the latex after passing through the z-MC). The above effect of the surfactant surface density on shearinduced aggregation has already been observed for the same surfactant but on different (plastic) colloidal particles.16 As explained in that work, this behavior has been attributed to the nonDLVO, very strong, short-range repulsive hydration force generated by the adsorbed surfactant on the particles. Such a hydration force at very low ionic strength (without adding any salt) increases as the Γ value increases, and when the Γ value reaches a certain value, the repulsive hydration force becomes extremely high such that aggregation and corresponding gelation become unachievable even at the highest collision energy that the adopted device can generate. Therefore, Γ = 1.12  10
6 mol/m2 represents the critical Γ value above which the latex becomes extremely stable against shear and no sheared-induced aggregation/gelation occurs. 7172

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Langmuir The fractal dimension of the class 2 clusters shown in Figure 5a is constant, independent of the surfactant surface density, and equal to that (Df = 2.76) in the absence of surfactant. This means that the adsorbed surfactant does not play any role in protecting the particles from coalescence. Gauer et al.,29,30 when studying the aggregation of elastomer colloids under stagnant conditions, also observed that an ionic surfactant cannot protect particles from coalescence. These results suggest that, under the shearing force or van der Waals attraction, relocation of the adsorbed surfactant molecules occurs when two particles approach each other, leading to exposure of the rubbery core surface to intermolecular diffusion (coalescence). 3.4. Shear-Induced Aggregation in the Presence of Nonionic Surfactant. For nonionic surfactant Tween 20, we have also prepared a set of latexes with the strawberry-like particles by adding different amounts of the surfactant at φ = 0.15. Each of the latexes has been forced to pass through the z-MC at Pe = 220. The conversion of the particles to the class 2 clusters, x, and the fractal dimension of the class 2 clusters forming the gel, Df, for the latexes after passing through the z-MC are shown in Figure 5b as a function of the surfactant surface density. When the conversion curve is compared to that in Figure 5a, the difference between ionic and nonionic surfactants is not substantial (i.e., in both cases, the conversion decreases as the surfactant surface density increases). In particular, we observe that the stability of the particles in the intense shear flow is similar for the ionic and nonionic surfactant, at least when the conversion is made for the same surface surfactant density, which, as seen in the contest of the adsorption isotherms in Figure 2, corresponds to rather different values of the surfactant aqueous solution. Such stabilization due to the steric effect is known46,47 to be related to two mechanisms: (1) osmotic repulsion due to the difference in the osmotic pressure of the solvent in the overlapping zone with respect to that in solution and (2) elastic repulsion corresponding to compression-induced net loss in the configurational entropy of the chains.46 Romero-Cano et al.47 experimentally investigated the steric effect of nonionic surfactant Triton X-100 on the colloidal aggregation under stagnant conditions and found that above a certain value of the adsorbed surfactant surface density no Brownian motion-induced aggregation occurs even at very high ionic strengths. Similarly, in this work, if the surface density of adsorbed Tween 20 is larger than a certain value (Γ = 1.17  10
6 mol/m2 or Am = 142 Å2), then no shear-induced aggregation occurs even at an extremely high shear rate. Therefore, similarly to ionic surfactant (with hydration), properly adsorbed nonionic surfactant can also protect colloidal particles from intense shear-induced aggregation. Somehow surprisingly, the critical value of Γ = 5.94  10
7 mol/m2 (Am = 280 Å2) at which the transition from solidlike gels to liquidlike states occurs is very similar for Tween 20 and for Emulgator K30/95. When the fractal dimension curve in Figure 5b is compared to that in Figure 5a, there are clear differences. Unlike the ionic surfactant, the adsorption of Tween 20 onto strawberry-like particles does reduce the fractal dimension of the class 2 clusters forming the gel. In particular, when the surfactant surface density is low, there is a sharp decrease in the fractal dimension as the surfactant surface density increases. This may be related to interactions of adsorbed polyoxyethylene (POE)-type surfactants that lead to bridging, thus lowing the interfacial free energy. As proposed by Zhao and Brown,41 such bridging may involve interactions between the adsorbed surfactant molecules and the neighboring polymer chains and occurs through both the

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surfactant hydrocarbon tail and its POE chain hydrogen bonding with the fixed surface charge (sulfate) groups. Cabane et al.48,49 studied the interactions between SDS (sodium dodecyl sulfate) and POE and found that POE can interact with the sulfate groups at the interface of SDS micelles to form POE-SDS aggregates. Zana et al.50,51 also suggested, on the basis of fluorescence measurements, that POE-SDS interactions mostly occur at the SDS micelle surface. Such bridging may be a general phenomenon on a “hairy” surface if the adsorbed surfactant molecule has a suitable length and reinforces the affinity between the surfactant molecule and the particle surface. On the basis of the above observations, bridging can explain the Df curve in Figure 5b. As illustrated in Figure S2 in the SI, initially, because there are very few adsorbed surfactant molecules, each of the POE chains on the Tween 20 molecule would interact with a sulfate group, leading to a very strong affinity between the Tween 20 molecule and the surface. The Tween 20 molecules adsorbed in this way are difficult to relocated when two particles approach during the intense shear-induced aggregation, thus reducing the particle coalescence. It follows that, initially, the Df value decreases as the Tween 20 surface density increases in Figure 5b. Then, as the Tween 20 surface density progressively increases, the Df value reaches a plateau. This arises because the availability of the sulfate groups is limited and now it becomes impossible for each of the POE chains to get a sulfate group for bridging. In this case, the adsorption strength of the Tween 20 molecules is substantially reduced; consequently, some of the adsorbed Tween 20 molecules can be relocated during aggregation and cannot contribute to the protection of the rubbery core from coalescence. When the Tween 20 surface density reaches the critical value (Γ = 5.94  10
7 mol/m2 or Am = 280 Å2) for the solidlike to liquidlike transition, the Df value decreases sharply as the Tween 20 surface density increases. This may be explained by two facts: (1) the particle surface is composed of both plastic and rubbery materials and the coalescence occurs only at the rubbery parts and (2) considering that each Tween 20 molecule requires 60.3 Å2,44 at criticality the area occupied by each molecule is only 4 to 5 times this value. Then, even if the surfactant molecules can be partially relocated during the particle approach, the exposed rubbery surfaces become smaller and smaller with increasing surfactant density and eventually not only coalescence but also aggregation becomes impossible. To further verify the above shear-induced aggregation behavior of the strawberry-like particles covered with a nonionic surfactant, we have also carried out the same experiments using another nonionic surfactant, Triton X-100. With respect to Tween 20, it has fewer POE groups (and thus undergoes less bridging with sulfate groups) but stronger hydrophobic interactions with the surface because of the tetramethylbutyl-phenyl groups. The results are shown in Figure 5c as a function of the surfactant surface density, which is estimated from the adsorption isotherm on a similar surface measured by Jodar-Reyes et al.42 When it is compared to Figure 5b, it is seen that both the x and Df curves are very similar. Thus, the adsorption of Triton X-100 molecules can also reduce the particle coalescence, and at substantially large surface densities, it can also protect the particles from the intense shear-induced aggregation and gelation. However, both of the critical Γ values for the solidlike to liquidlike transition and for zero conversion are larger for Triton X-100 than for Tween 20. Thus, with respect to Tween 20, Triton X-100 is less effective at protecting the particle 7173

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4. CONCLUDING REMARKS The role of the surfactant type in the shear-induced aggregation/gelation of strawberry-like particles has been investigated using a z-shaped microchannel without adding any electrolytes. The particles are composed of a rubbery core (with Tg ≈
56
C), partially covered with a plastic shell, and well stabilized by negative charges originating from the polymer-chain end groups. In the absence of any surfactant, at a Peclet number of Pe = 220 and an initial particle volume fraction of φ = 0.15, shear-induced gelation occurs as the colloidal system passes through the microchannel. Moreover, from light-scattering measurements, the fractal dimension of the clusters constituting the gel is found to be equal to Df = 2.76, which is substantially larger than that for rigid (plastic) particles (Df = 2.4) under the same gelation conditions.18 This indicates partial coalescence among the particles during shear-induced gelation. SEM pictures confirmed the observed partial coalescence. Next, an ionic (sulfonate) surfactant, Emulgator K30/95, and a nonionic (steric) surfactant, Tween 20, have been added to the system, and the same shear-induced aggregation/gelation has been carried out, again at Pe = 220 and φ = 0.15. It is found that for both surfactants the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. As the surfactant surface density increases above certain critical values, first gelation and then aggregation cannot occur any longer. In the case of ionic surfactants, this behavior has already been reported in the literature15
17 and is attributed to the occurrence of the non-DLVO, short-range repulsive hydration forces protecting the particles from aggregation. In this work, we have demonstrated that it is also possible to protect particles from aggregation under intense shear using nonionic surfactants by taking advantage of steric repulsion. Another important observation is that although the ionic sulfonate surfactant can protect the particles from aggregation it cannot protect the strawberry-like particles from coalescence once the clusters are formed, but the nonionic steric surfactant can do it. This is demonstrated by a reduction in the fractal dimension from 2.76 to 2.45. These results indicate that the adsorption of the ionic surfactant on the surface is not very strong and relocation of the adsorbed surfactant might occur as the particles approach each other during aggregation, thus leading to exposure of the rubbery core to intermolecular diffusion and then coalescence. However, the adsorption of the nonionic surfactant, mostly due to bridging between the surface sulfate groups and the groups able to hydrogen bond, such as the POE groups in Tween 20, is very strong, and its relocation during shear-induced aggregation is very difficult. In this case, the presence of the surfactant molecules between the particle surfaces can protect the rubbery cores from coalescence. ’ ASSOCIATED CONTENT

bS

Supporting Information. Semi-logarithmic plots of the surface tension of surfactant solutions as a function of surfactant concentration for Tween 20 and Emulgator K30/95. Schematic representation of the adsorption of nonionic surfactants (Tween 20) on the (sulfate) charged particle surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 0041 44 632 10 82.

’ ACKNOWLEDGMENT Financial support from BASF SE (Ludwigshafen, Germany), the Swiss National Science Foundation (grant no. 200020126487/1), and the China Scholarship Council is gratefully acknowledged. We also thank Mr. Frank Krumeich for SEM pictures. ’ REFERENCES (1) Aramendia, E.; Mallegol, J.; Jeynes, C.; Barandiaran, M. J.; Keddie, J. L.; Asua, J. M. Langmuir 2003, 19, 3212. (2) Zhang, K.; Fu, H. Q.; Huang, H.; Chen, H. Q. J. Dispersion Sci. Technol. 2007, 28, 1209. (3) Carpineti, M.; Giglio, M. Phys. Rev. Lett. 1992, 68, 3327. (4) Krall, A. H.; Weitz, D. A. Phys. Rev. Lett. 1998, 80, 778. (5) Kroy, K.; Cates, M. E.; Poon, W. C. K. Phys. Rev. Lett. 2004, 92, 148302. (6) Lattuada, M.; Wu, H.; Morbidelli, M. Langmuir 2004, 20, 5630. (7) Sciortino, F.; Tartaglia, P. Phys. Rev. Lett. 1995, 74, 282. (8) Carpineti, M.; Giglio, M. Phys. Rev. Lett. 1993, 70, 3828. (9) Mellema, M.; Heesakkers, J. W. M.; van Opheusden, J. H. J.; van Vliet, T. Langmuir 2000, 16, 6847. (10) Wu, H.; Morbidelli, M. Langmuir 2001, 17, 1030. (11) Wu, H.; Xie, J. J.; Lattuada, M.; Morbidelli, M. Langmuir 2005, 21, 3291. (12) von Smoluchowski, M. Z. Phys. Chem. 1917, 92, 129. (13) Saffman, P. G.; Turner, J. S. J. Fluid Mech. 1956, 1, 16. (14) Soos, M.; Moussa, A. S.; Ehrl, L.; Sefcik, J.; Wu, H.; Morbidelli, M. J. Colloid Interface Sci. 2008, 319, 577. (15) Zaccone, A.; Wu, H.; Lattuada, M.; Morbidelli, M. J. Phys. Chem. B 2008, 112, 1976. (16) Wu, H.; Zaccone, A.; Tsoutsoura, A.; Lattuada, M.; Morbidelli, M. Langmuir 2009, 25, 4715. (17) Wu, H.; Tsoutsoura, A.; Lattuada, M.; Zaccone, A.; Morbidelli, M. Langmuir 2010, 26, 2761. (18) Xie, D. L.; Wu, H.; Zaccone, A.; Braun, L.; Chen, H. Q.; Morbidelli, M. Soft Matter 2010, 6, 2692. (19) Zaccone, A.; Wu, H.; Gentili, D.; Morbidelli, M. Phys. Rev. E 2009, 80, 051404. (20) Bonaccurso, E.; Kappl, M.; Butt, H.-J. Curr. Opin. Colloid Interface Sci. 2008, 13, 107. (21) Goertz, M. P.; Houston, J. E.; Zhu, X.-Y. Langmuir 2007, 23, 5491. (22) Helm, C. A.; Israelachvili, J. N.; McGuiggan, P. M. Science 1989, 246, 919. (23) Major, R. C.; Houston, J. E.; McGrath, M. J.; Siepmann, J. I.; Zhu, X. Y. Phys. Rev. Lett. 2006, 96, 177803. (24) Cheng, H.; Wu, C.; Winnik, M. A. Macromolecules 2004, 37, 5127. (25) Fernandez-Nieves, A.; Fernandez-Barbero, A.; Vincent, B.; de las Nieves, F. J. Langmuir 2001, 17, 1841. (26) Roldan-Vargas, S.; Martín-Molina, A.; Quesada-Perez, M.; Barnadas-Rodríguez, R.; Estelrich, J.; Callejas-Fernandez, J. Phys. Rev. E 2007, 75, 021912. (27) Roldan-Vargas, S.; Barnadas-Rodríguez, R.; Martín-Molina, A.; Quesada-Perez, M.; Estelrich, J.; Callejas-Fernandez, J. Phys. Rev. E 2008, 78, 010902. (28) Roldan-Vargas, S.; Barnadas-Rodríguez, R.; Quesada-Perez, M.; Estelrich, J.; Callejas-Fernandez, J. Phys. Rev. E 2009, 79, 011905. (29) Gauer, C.; Jia, Z. C.; Wu, H.; Morbidelli, M. Langmuir 2009, 25, 9703. 7174

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