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Characterizations and properties of hairy latex particles
Journal of Colloid and Interface Science 285 (2005) 136–145 www.elsevier.com/locate/jcis
Characterizations and properties of hairy latex particles Pascal Borget a , Françoise Lafuma a,∗ , Cécile Bonnet-Gonnet b a Université Pierre et Marie Curie, Laboratoire de Physico-Chimie Macromoléculaire UMR CNRS 7615, ESPCI, 10 rue Vauquelin,
F-75231 Paris 05, France b Centre de Recherches d’Aubervilliers Rhodia 52, rue de la Haie Coq, 93308 Aubervilliers, France
Received 26 May 2004; accepted 8 November 2004 Available online 15 January 2005
Abstract Industrial latex composed of a hydrophobic core surrounded by a charged hydrophilic layer exhibits excellent stability toward monovalent salt. That feature is classically attributed to a steric effect due to a loss of entropy during overlapping of coating materials. The so-called electrosteric stabilization is, however, not a straightforward function of the nature of the hydrophilic corona. This suspension was characterized in dilute solution by scattering and electrophoresis techniques. In contrast to spherical brushes the interface between the core and the corona is not well defined. The layer is more similar to a highly hydrated nonuniform gel with few longer strands that control the hydrodynamic properties than to a polyelectrolyte brush whose dependence on ionic strength reflects the concentration of counterions inside a well-defined structure. Thus the steric contribution to stabilization of these hairy particles appears to be insignificant in the range studied. The highly hydrated nature and the global charge of the layer are two predominant factors for the stability of the particles. 2004 Elsevier Inc. All rights reserved. Keywords: Electrosteric stabilization; Hairy latex; Core–corona particles; Spherical brushes; Electrophoretic mobility; Dynamic light scattering; Small-angle neutron scattering
1. Introduction The control of the stability of suspensions is important for many industrial applications such as adhesives, coatings, and texture modifications. To prevent the coalescence of the colloidal particles due to van der Waals attractive forces, specific groups must be present on their surfaces in order to generate repulsive forces. During classical emulsion polymerization, the introduction of a large amount of ionic initiator leads to the appearance of several groups such as sulfate on the surface of the particles; the origin of the stabilization is electrostatic. Some suspensions are obtained with charged surfactant species that adsorb on the surface by hydrophobic interactions, enhancing electrostatic repulsions. However, the stability of such suspensions is strongly dependent on the ionic strength of the medium and the range * Corresponding author. Fax: +33-140-79-46-40.
E-mail address: [email protected] (F. Lafuma). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.019
of repulsive interactions must be greater than the size of the particles to overcome the van der Waals attraction. Other forms of stabilization less dependent on the Debye screening have been proposed. This effect can be achieved by building a hydrophilic layer onto the surface of the particles to add a steric protection. When the corona is composed of charged units or when ionic groups are already present on the surface, the stabilization is called electrosteric. At low ionic strength, electrostatic interactions generate long-range repulsions and the suspension is kinetically stable. At high ionic strength, colloidal particles are prevented from aggregating by the steric protection, which ensures thermodynamical stability. But these short-range repulsions are strongly dependent on the thickness of the layer, i.e., solvation, and the length of the hydrophilic chains. A steric barrier is already obtained by adsorbing surfactants with a sufficient chain length [1–3]. Although these molecules are easy to handle, controlling the thickness of the layer can only be achieved with macromole-
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cules, homopolymers [4] such as poly(ethylene oxide) [5] or block [6,7] and graft copolymers [8,9]. The adsorption process is mainly governed by the affinity for the surface and the solvency. The introduction of an additional electrostatic component by adsorbing polyelectrolytes [10,11] increases the stability of the suspension when the architecture is adapted. But the stabilization by physisorption is extremely dependent on variations of the medium. Grafting a polymeric brush on colloidal particles is an alternative strategy to enhance stability and build a permanent protective layer. The amount of polymer (contour length and grafting density) is determined according to the synthesis conditions and two ways are studied, the “grafting onto” [12–14] and the “grafting from” technique [15–17]. In fact, if a lot of theoretical work has been realized to describe the properties of polymeric brushes [18–21], there is a lack of experimental studies due to the synthesis difficulties [22]. Another interesting way to prevent aggregation is to introduce hydrophilic monomers during the synthesis. Depending on the polymerization parameters and conditions, long hydrophilic sequences can be formed around the hydrophobic core of the particle and induce a steric stabilization. Moreover, if hydrophilic units are charged, the particles are said to be electrosterically stabilized. They can be obtained with new techniques, e.g., “frozen micelles” [23] or layer-by-layer assembly [24], or with methods more convenient with industrial targets such as emulsion polymerization [25–27]. Contrary to spherical brush synthesis, numerous different morphologies of core–shell particles have been studied and extensively commented [28]. However, most of the characterization experiments concern the radial repartition of comonomers or surface analyses [24,27,29–37]. Surprisingly, the properties in solution of core–corona particles and the nature of the hydrophilic layer have been related in few studies [26,38–40], since polyelectrolyte brushes have been more precisely described in terms of interfacial distribution of monomers and thickness of the brush according to molecular characteristics (grafting density, contour length of the chains, and curvature of the surface) and the nature of the surrounding medium (ionic strength and pH) [13,17,22, 41–45]. In what follows, we present a detailed characterization of the properties of industrial core–corona particles in solution. These suspensions can be used as binders for the papermaking industry. Films obtained from the drying of such latex are characterized by a great cohesion between particles [46] and adhesion properties with the paper fiber or pigments lead to the rationalization of coating formulations [47]. Moreover, the destabilization of core–corona particles by classical flocculation with cationic agent if mentioned was briefly examined from academic point of view. Adsorption mechanisms of macromolecules on a soft surface and structures of aggregates in relation with structural characteristics of flocculating agents will be presented in another paper. Here the mode of electrosteric protection induced by the properties of the surrounded hydrophilic layer is discussed and compared
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to previous results on model systems such as polyelectrolyte brushes. 2. Materials and methods 2.1. Materials The latex dispersion was supplied by the Centre de Recherches d’Aubervilliers (Rhodia, France) and synthesized through emulsion copolymerization of styrene (59 wt%), butadiene (37 wt%) and acrylic (2 wt%), and fumaric (2 wt%) acids. The thermodynamic arrangement between the components leads to a hydrophobic soft core (Tg ≈ 11 ◦ C) surrounded by a hydrophilic negatively charged polymeric layer. In addition to the dispersed particles, the aqueous phase also contains soluble species such as surfactant, amphiphilic polymers, salt, residual monomer, preservatives. These are removed by tangential ultrafiltration through a 100-kDa cutoff polyethersulfone membrane (Millipore). The purification method was checked by conductivity and total organic carbon measurements (Dohrmann DC 80 analyzer). The pH of the dispersion (typically 2 wt%) was adjusted to 8 with NaOH and the washing process was stopped when the conductivity of the filtrate was around 10 µS/cm and the carbon concentration less than 5 ppm. The density of acid groups was measured by conductometric titration. The latex has a charge density of 274 µeq g−1 with a higher proportion of weak acid groups (247 µeq g−1 ) than of strong ones (27 µeq g−1 ). All water used was freshly purified and deionized with a Milli-Q Plus water treatment system (Millipore). KNO3 salt was Normapur grade. 2.2. Methods 2.2.1. Small-angle neutron scattering (SANS) The scattering patterns were obtained with small-angle scattering instrument D11 at the Institut of Lauë–Langevin and analyzed according to standard procedures [48]. The scattered intensity was recorded according to the magnitude of the scattering vector, 4π sin(θ/2), (1) λ where λ is the wavelength of the incident neutron beam and θ the scattering angle. At wavelength 1.25 nm three experimental configurations were used to cover a large range of q values (8.45×10−3 < q [nm−1 ] < 1.58), which corresponds to distances between 750 and 4 nm. Sample concentrations were 0.1 wt% in a 10/90 H2 O/D2 O mixture. All measurements were done in 2-mm-thick quartz cells (Hellma). Under these conditions it was verified that there is no correlation between latex particles and no multiple scattering. In the case of a core–corona latex the scattering intensity depends on the structure, the homogeneity, and the proporq=
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Table 1 Scattering length densities
H2 O D2 O Latex core (59% styrene–37% butadiene) Latex corona
b × 1015 (m)
V¯ (cm3 /mol)
ρ/1014 (m2 )
−1.675 19.145 15.256 36.7675
18 18 99 × 0.59 + 60.7 × 0.37 = 80.9 66.5
−0.560 6.405 1.136 3.229
tion of each part of the particle [49,50]. To get a rough idea of the contribution of the core and the corona, Table 1 gives a list of scattering length densities according to the values of scattering length b [51] and partial volume V¯ [52]. In the H2 O/D2 O mixture the scattering length density of the solvent ρs is equal to 5.7085 × 1014 m−2 . The corona is assumed to be highly hydrated [37] and the exchange between H2 O and D2 O molecules inside this structure sufficiently fast. Assuming that the core and the corona have the same continuous-phase scattering length density, (ρcore )2 = 20.9 × 1028 m−4 and (ρcorona )2 = 6.1 × 1028 m−4 . According to the chemical nature of each component of the particle, the scattering intensity of the core is 3.5 times higher than that of the corona. Considering their relative proportion (core 96%, corona 4%), neglecting the contribution of the corona to the total scattering intensity is a reasonable assumption. The scattering intensity is given by
2.2.2. Dynamic light scattering (DLS) Light scattering measurements were carried out at a wavelength of 514.5 nm (Spectra Physics M2000 Ar ion laser) with a Malvern goniometer, a correlator, and a detector from ALV. The solutions were cleared from dust by filtering several times through a Millex HV 0.45-µm-pore-sized filter before being transferred into scattering cells. DLS measurements were performed at 25 ◦ C for a particles concentration of 1 ppm. Preliminary experiments showed that concentration dependence was negligible below 5 ppm and probably up to 100 ppm [38]. Multiple diffusion or particular interactions can be ruled out. The distribution function of the decay rates G(Γ ) is described by a distribution of relaxation times τr with the Repes method from the Gendist program. For each q value an apparent diffusion coefficient Dapp is calculated from the mean decay rate,
I (q) = cΦ(ρ)2 vP (q),
Γ = Dapp q 2 =
(2)
with c the concentration of the dispersion, Φ the volume fraction of particles, v the volume of one particle, and P (q) the form factor which for a homogeneous sphere of radius R is given by sin(qR) − qR cos(qR) 2 . P (q) = 3 (3) (qR)3 At high q values the form factor decays as q −4 under the condition that the scattering density change at the particle surface is abrupt. This is called the Porod law, S 1 , (4) V q4 where S/V is the specific surface of the scatterers. This ratio can be used to calculate the radius of Porod, defined as V RP = Φ . (5) S This information is useful to characterize the diffuse or sharp nature of the interface between the hydrophobic core and the surrounding layer of particles. But the determination of the specific surface is strongly dependent on the contrast of the scattering length density. A value of (ρ)2 more precise than those presented in Table 1 can be obtained by calculation of the invariant H of the system, Iq→∞ = c2π(ρ)2
∞ q 2 I (q) dq = c2π 2 (ρ)2 Φ.
H= 0
(6)
1 . τr
(7)
The concentration and angular dependence of Dapp is usually written as Dapp (q, c) = Dz 1 + C Rg2 z q 2 + · · · (1 + kD c + · · ·), (8) where C is a dimensionless factor depending on the structure of the scatterers, Rg2 z the z-average mean square radius of gyration, and kD the dynamic second virial coefficient, which describes hydrodynamic and thermodynamic interactions between particles and solvent. The hydrodynamic radius is calculated according to the Stokes–Einstein equation,
1 Rh
−1 z
=
kB T , 6πηDz
(9)
where kB is the Boltzmann constant, T the absolute temperature, and η the viscosity of pure solvent. 2.2.3. Electrophoresis Electrophoretic mobilities of particles at various ionic strengths were determined with a Laser Zee Meter Model 501 zetameter (Pem Kem Inc.). Measurements were performed each 100 µm from the upper wall of the rectangular cell to get the whole profile of mobility. The two stationary layers were calculated according to the Komogata equation [53]. This apparatus displays the zeta potential for an aqueous suspension at 20 ◦ C according to the Smoluchowski
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model, ε µ = ζ. η
139
(10)
Given the Smoluchowski zeta-potential the original mobility is easily recovered. The value of the zeta potential can be related to the electrophoretic mobility µ of particles and depends on the dimensionless quantity κa, where a is the radius of the particle and κ −1 the Debye length: µ=
2ε f (κa)ζ, 3η
(11)
where ε is the electric permittivity of the dispersing medium and η the viscosity of the dispersing medium. In this study electrophoretic measurements of colloidal particles with radius around 80 nm are performed in media whose ionic strength ranges between 10−4 and 1 mol L−1 . The value of κa lies between 3 and 270. Thus, at ionic strength greater than 10−2 mol L−1 , the values of f (κa) are calculated according to f (κa) =
330 3 9 75 − . − + 2 2 2(κa) 2(κa) (κa)3
(12)
At lower ionic strength the correction factor is obtained by interpolation with Henry’s values [53]. 2.2.4. Turbidity measurements The introduction of salt in the medium diminishes the interparticular repulsive forces. The salt concentration which leads to the destabilization of the suspension can be obtained by turbidity measurements at different wavelengths [54]. Transmittance measurements were performed with a Hewlett–Packard Type 8453 UV–visible spectrometer. The turbidity is defined as OD , (13) l where l is the pathlength and OD the optical density. Generally τ ∼ = λ−γ , where γ is the slope of the plot of the turbidity versus the wavelength in logarithmic coordinates [55]. With Rayleigh scatterers γ = 4 and the value of this gradient decreases as the size of the objects increases. τ = ln(10)
3. Results and discussion 3.1. Structure of the core 3.1.1. Radius of the core and distribution of size SANS measurements were performed at different ionic strengths ([KNO3 ] = 10−3 –1 mol L−1 ), but no difference between the patterns could be detected. No aggregation phenomenon takes place even at the highest ionic conditions. Taking into account a slight polydispersity, a hard sphere model without interactions is sufficient to fit the data (Fig. 1). Generally the Schultz–Flory distribution is used to describe
Fig. 1. SANS spectrum of the suspension and hard-sphere model.
the polymolecularity of the system [56,57]. But this function can be described with a Gaussian curve when the polydispersity is not too high: ¯ 1 (r − R) , w(r) = √ exp − (14) 2σ 2 σ 2π where R¯ is the mean radius of particles and σ the standard deviation. For a more classical characterization of the system, the polydispersity index is given as
2
2 σ σ Ip = 1 + 7 (15) 1+6 . ¯ R R¯ Since the diffusion of a sphere of radius R is proportional to the square of its volume, the scattering intensity is written as ∞ sin(qr) − qr cos(qr) 2 3 w(r) dr, I (q) = I0 ¯ 3 (q R)
(16)
0
with I0 = I (q)q→0 = CΦ(ρ)2 v.
(17)
This model leads to R¯ = 77 nm and σ = 7 nm. The polydispersity index is around 1.11. Calculations of scattering length densities in the experimental part (Table 1) show that the scattered intensity only results from the core of the particle. The plot of I (q)q 2 = f (q) (Fig. 2) leads to a more consistent value of (ρ)2 as reported in Eq. (6). The integration of the experimental plot or the fitting curve gives the same value H = 3620 ± 20 µm−4 . For a volume fraction of 0.108% corresponding to 0.1 wt%, the contrast is equal to (ρ)2 = (17.0 ± 0.1) × 1028 m−4 . Considering that the scattering length density of the solvent, ρcore is around (1.59 ± 0.02) × 1014 m−2 , which is in relative agreement with the value 1.43 × 1014 m−2 proposed by De Bruyn for a polystyrene core surrounded by a hairy layer of perdeuterated acrylic acid [37]. According to this model the value at q = 0 can be extrapolated to I0 = 3900 cm−1 . The corresponding radius is
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Fig. 2. Calculation of the invariant.
Fig. 3. Calculation of the specific surface of the particle.
found equal to 79.8 nm, which is close to the precedent value. 3.1.2. The interface between the core and the corona of the particle After correction of the background the plot of I q 4 vs q leads to asymptotic behavior at high q corresponding to the discontinuity of the scattering length densities between the core and the corona (Fig. 3). The plateau value (5.0 ± 0.2) × 1034 m−5 is proportional to the total specific surface of particles. According to Eq. (5), the radius of Porod is RP = 21.6 ± 1.3 nm. This corresponds to a radius of a spherical particle around 65 ± 4 nm, which is significantly lower than the mean radius (R¯ = 77 nm). At high q values, the model does not describe perfectly the system. With the assumption that the surface of the core is not sharp but rough, the specific surface of the scatters increases and the radius of Porod diminishes. Thus this feature can be explained by a diffuse transition between the core and the corona as a consequence of the incorporation of hydrophobic units during the copolymerization of carboxylic monomers. This is consistent with most of the emulsion polymerization studies that conclude to “buried” hydrophilic units that compose 1–5% of the total core volume [37,58,59]. 3.2. Hydrodynamic properties of the corona 3.2.1. Dynamic light scattering To characterize the hydrophilic corona of the particle, the suspension is analyzed by dynamic light scattering. When the particle moves in the aqueous phase, it is supposed that water molecules are enclosed in the corona and that the shear plane is located near the outer surface of the particle. The hydrodynamic radius is related to the extension of the corona. The pH of the suspension is systematically adjusted to 8 in order to be sure that most of the charges are potentially dissociated. As already mentioned in SANS experiments, the introduction of monovalent salt KNO3 does not induce the coagulation of the suspension. DLS measurements were
Fig. 4. Diffusion coefficient versus q 2 at different ionic strengths.
performed at different ionic strengths ranging from ∼10−4 (pure water) and 1 mol L−1 . The plot of the diffusion coefficient Dapp vs q 2 (Fig. 4) also indicates that there is no angular dependence. This information is in agreement with the slight polydispersity of the sample. In order to get the best precision, the diffusion coefficient is averaged out over the whole angular domain. Fig. 4 clearly shows an increase of the diffusion coefficient with the ionic strength. This variation can be converted into size effects by the Stokes–Einstein relation (Eq. (9)). The viscosity of the medium diminishes when the salinity increases; its influence becomes significant only for ionic strengths higher than 10−1 mol L−1 [60]. In the case of polydisperse systems, the hydrodynamic ¯ radius Rh obtained by DLS is not equal to the mean radius R. According to SANS measurements the size distribution can be described by a normal law and Rh must be corrected as follows: σ 4 σ 6 σ 2 + 45 R + 15 R 1 + 15 R R 6 ¯ ¯ ¯ ¯ . Rh = 5 = R (18) σ 2 σ 4 R 1 + 10 R¯ + 15 R¯ On the assumption that the corona has no influence on the size distribution, this correction leads to a variation of the hy-
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Fig. 5. Variation of the hydrodynamic radius of the particle with the ionic strength.
Fig. 6. Variation of the corona thickness of the particle with the ionic strength.
drodynamic radius between 84.4 ± 0.5 and 78.1 ± 0.5 nm (Fig. 5). Thus by comparison with the radius of the core (R¯ = 77 nm), the maximal thickness of the corona is around 7 nm in pure water. At high ionic strength the hydrodynamic influence of the corona is close to the experimental error, that is 1 nm. In the range 10−4 –10−1 mol L−1 the layer thickness δ seems to obey a power law δ ∼ I −a (Fig. 6) with an exponent a equal to 0.3. This variation does not correspond to the gel or polyelectrolyte shrinkage (a = 0.5). Starting from the blob model proposed by Daoud and Cotton [61], scaling laws with 0.1 < a < 0.17 depending on the curvature radius of the sterically stabilized particles were obtained [17]. Our results could be related to the theory for planar brushes proposed by Borisov et al. [20], which forecasts an exponent equal to −1/3. Finally the diminution of the thickness is only observed over one decade and it is hard to conclude because of the poor precision. But Fig. 6 brings out a monotonous decrease of the thickness δ until an ionic strength around 10−1 mol L−1 at which the corona is totally collapsed. This kind of variation is not conform to planar brushes theories that predict no diminution of the brush thickness in the osmotic regime, that is as
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Fig. 7. Evolution of the zeta potential ζ and the reduced electrophoretic mobility E versus the concentration of monovalent salt.
long as the ionic strength does not exceed the counterion concentration inside the charged corona. This property has been checked by Tran et al. [22] for dense planar brushes obtained by chemical grafting on silica wafers. The concentration of counterions in the brush is evaluated to 1.4 mol L−1 . The ionic effect appears for salt concentrations higher than 1 mol L−1 and the brush thickness follows a power law with −0.27. In the case of the studied particle, the properties in solution are different. As reported previously, the charge density of the core–corona particle is rather high and the confinement in a structure of small thickness leads to an inner concentration of counterions around 1 mol L−1 . Thus no variation of the thickness of the corona should be observed below a salt concentration of 1 mol L−1 . Although the thickness is small in comparison to the radius of the particle, the structure of the corona cannot be considered as a charged brush on a planar surface but rather is like a gel. 3.2.2. Electrophoresis Zeta potential measurements on a freshly ultrafiltered sample are performed at different ionic strengths controlled by the concentration of potassium nitrate (Fig. 7). These values are obtained according to the procedure described above, which is first conversion into the corresponding mobilities (Eq. (10)) and then calculation of zeta potential as reported in Eq. (11). In pure water the zeta potential is around −70 mV. The introduction of a small amount of monovalent salt (10−3 mol L−1 ) leads to a diminution of ζ (−77 mV), followed by a monotonic increase to 1 mol L−1 . This last evolution corresponds to the classical effect of charge screening with increasing salt concentration. Notice that the value of zeta potential at the highest ionic strength (−30 mV) is still sufficient to ensure the stability of the suspension already observed by scattering techniques. The conversion into reduced electrophoretic mobility1 confirms the presence of an extremum around 10−3 mol L−1 . 1 E = 3 e η µ. 2 kB T ε
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Table 2 Electrophoretic study of core–corona latices at different ionic strengths in confrontation with the theory of the relaxation term [KNO3 ] (mol L−1 )
κa
Emeasured
Emax (O’Brien and White)
ζ (mV) (Henry)
ζ (mV) (O’Brien and White)
0 10−3 10−2 10−1 1
2.8 8.4 26 81 257
3.00 ± 0.12 3.65 ± 0.05 3.02 ± 0.04 2.56 ± 0.05 1.69 ± 0.02
3.12 3.3 4.3 5.9 >7
−71 −77 −57 −45 −29
−107 – −61 −48 −29
The values obtained do not reflect the high charge density of the particles. Furthermore, the classical theory of Gouy, Chapman, Grahame, and Stern does not predict such a variation with the ionic strength. This premium model supposes in particular the invariance of the dielectric constant in the near of the interface solid/liquid and a uniform repartition of charges on a sharp surface, which does not correspond to the core–corona particle. The improvement of this theory by O’Brien and White leads to a maximum on the plot of the reduced electrophoretic mobility E versus the reduced zeta potential ζ˜ [62].2 A relaxation term resulting from an electrical field induced by the polarization of the double layer in opposed direction to the external field is taken into account. For highly charged systems the distortion of the double layer leads to a retardation effect on the particle in movement. This term is proportional to the square of the reduced zeta potential. A simple model based on this theory allows the calculations of E vs ζ˜ curves [53,63]. Thus for κa > 3 the presence of a maximum suggests two possible values of the zeta potential for the same electrophoretic mobility. If appropriately applied, this theory makes it possible to obtain monotonic plots ζ = f (I ) [64,65]. Even with highly charged latex (30–80 µC cm−2 ), good correlations between theory and experimental measurements were observed in the range 10 < κa < 50, where the contribution of the relaxation term is maximal [66]. In the case of the core–corona particle the relaxation term theory predicts a zeta potential in pure water greater in absolute values than that obtained via the Henry relation. But for κa = 8.4 the measured electrophoretic mobility (µ = −4.8 (µm/s)/(V/cm) and E = 3.65) is higher than the maximal theoretical value predicted by O’Brien and White (E = 3.3). This fact has already been mentioned and discussed elsewhere [67,68]. All results are summarized in Table 2. The relaxation term model cannot be applied to the whole experimental data because this analysis is based on the Gouy–Chapman theory, whose foundations are not in agreement with the structure of the core–corona particle. Two other models have been proposed. To interpret more precisely the electrokinetic and conductometric properties of colloidal systems, Zukoski and Saville 2 ζ˜ = e ζ . kT
[69] have suggested specific adsorption of ions on the surface of the particle. The Stern layer is considered to be the place of lateral movement of ions by diffusion and electromigration [70]. The confrontation of the model with experimental data leads to the evaluation of different factors such as the adsorption capacity and their relative mobility inside the layer [71]. At weak ionic strength the diminution of the potential is explained by preferential interactions between co-ions and hydrophobic parts or by a great adsorption– desorption constant. This model does not take into account the position of the shear plane which depends on the ionic strength in the case of a core–corona particle. The concept of a layer of loose polymer strands on the surface of the latex particles has been proposed to explain the observed electrophoretic properties. This “hairy” model, introduced by van der Put and Bijsterbosch [72], was applied to polystyrene latex [73,74] or to particles surrounded by an hydrophilic corona [67,75,76] in order to explain the variation of the zeta potential with the ionic strength. Flexible chains on the surface latex particles are assumed to be charged, either carrying a terminal ionic group or being composed of charged monomers. At low ionic strength these hairs adopt an extended conformation due to electrostatic repulsions between surface and chains and/or between neighboring segments. This leads to a shift of the shear plane away from the surface and the possibility that some charges are located in the Stern layer and/or in the diffuse layer. The zeta potential measured at the shear plane is therefore lower than that of a bare particle carrying the same charge. The continuous collapse of the layer with ionic strength has the effect of moving the shear plane closer to the surface, thus increasing the zeta potential of the particle. At high ionic strength, chains are completely shrunk and the potential decreases classically owing to electrical double-layer compression. Each model (“hairy layer” and “ion adsorption”) has been criticized and seems not to be able to explain all experimental data [77]. In the case of the studied particle, specific ion adsorption is very unlikely, according to dynamic light scattering results that show a continuous decrease until an ionic strength around 10−1 mol L−1 . Finally, the hairy model is more consistent with the present experimental observations since the corona of the particles is essentially composed of hydrophilic charged strands. No supplementary information was obtained by applying the model of soft spheres proposed by Oshima [78] because of the nonuniform and complex structure of the corona, whose effective charge depends on
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the ionic environment (screening, neighbor carboxylic sites) and thickness of the layer. 3.2.3. Electrosteric stabilization The stability of the suspension at 0.1 wt% was studied by visible spectroscopy in monovalent (KNO3 ) and divalent (Ca(NO3 )2 ) ionic medium (Fig. 8). In pure water the turbidity gradient γ is around 3.5, since the size of the particle is greater than that of a Rayleigh scatterer. In monovalent electrolyte solution, no significant variation of γ is observed over a wide range of ionic strength. The particles do not show any aggregation until at least 3 mol L−1 . Generally the stability of a suspension is correlated with the surface charge density. The core–corona particle is characterized by a higher structural charge (274 µeq g−1 ) compared to commonly studied systems. If all ionic groups were situated on the surface of the particle, the charge density would be equal to 70 µC cm−2 . The high proportion of potentially charged sites carried by the particle undoubtedly contributes to the peculiar stability toward monovalent ions. Tsaur and Tamai have studied the stability in ionic medium of core–shell particles (styrene/styrene sodium sulfonate) obtained by seed polymerization, respectively BA51
Fig. 8. Stability of the suspension in monovalent (KNO3 ) and divalent (Ca(NO3 )2 ) ionic medium.
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and P(StNaSS) (Table 3). Within the same series increasing the proportion of hydrophilic comonomer leads to a greater stability. Comparison between the different systems shows that the critical coagulation concentration does not depend only on the charge density and on the thickness of the stabilizing layer. The internal structure of the corona (inhomogeneities, density, solvation) also influences the efficiency of the protection. The studied latex is characterized by a quite thin corona, taking into account the relatively high proportion of incorporated monomer and consequently a high charge density. In comparison to the systems of Tsaur and Tamai, the corona of the particle is denser and more hydrophilic. The studied particle is rather analogous to electrosterically stabilized latices with a poly(acid acrylic) hydrophilic layer, prepared in a manner similar to that commonly employed industrially [37]. At low ionic strength and pH 8.5, the thickness is found to range between 5 and 7 nm, the shell hydration is around 95 vol%, and no destabilization of the suspension is observed, even at high ionic strength (KCl 2.5 mol L−1 ). The dispersion of poly(styrene–co–butyl acrylate) particles surrounded by chains of poly(methacrylic acid) displays the same stability towards monovalent salt (NaCl 3.5 mol L−1 ) [39]. However in the case of the studied particle, the steric contribution of its hair is not straightforward. Table 4 summarizes all results concerning the behavior of the particle in ionic medium and yields a comparison between hydrodynamic and electrophoretic effects. The Debye length is roughly related to the extent of interparticle electrostatic repulsions. With smooth particles this parameter is defined from the surface plane where the charges are situated. In the case of a hydrophobic core surrounded by a hydrophilic charged layer, ionic groups are located in a volume and there is no well-defined plane. At low ionic strengths some hydrophilic chains are in extension but still located inside the electrical double layer: stabilization is governed by electrostatic repulsions. When the ionic strength reaches 10−1 mol L−1 , the corona thickness is more or less equal to the Debye length and the charge density (see the value of the measured electrophoretic mobility) is sufficient to ensure a good stability of the suspension. If there is a purely
Table 3 Correlation between the critical coagulation concentration (ccc) in monovalent ionic medium and the characteristics (hydrophilic comonomer proportion, surface charge density σ , thickness of the corona measured at ionic strength 10−2 mol L−1 ) of Tsaur and Fitch [81] and Tamai et al. [75] particles and the studied latex at pH 8 Latex
Hydrophilic comonomer proportion ( wt%)
σ (µC cm−2 )
(nm)
ccc (mol L−1 )
BA51I BA51D BA51C BA51H
– – – –
10.2 9.1 5.0 1.0
– – – –
1.35 1.22 0.80 0.32
P(St/NaSS2 ) P(St/NaSS3 )II P(St/NaSS4 )II P(St/NaSS5 )II
0.4 0.6 0.8 1
7.5 15.1 17.0 20.7
1.1 2.0 2.7 3.6
0.36 0.45 0.86 1.23
Rhodopas
4
70
2
>3
144
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Table 4 Hydrodynamic and electrophoretic properties of a core–corona latex [KNO3 ] (mol L−1 ) 0 10−3 10−2 10−1 1
κ −1 (nm) 30.4 9.6 3.0 1.0 0.3
Electrophoretic mobility (µm s−1 cm V−1 ) −4.0 ± 0.2 −4.9 ± 0.07 −4.0 ± 0.05 −3.4 ± 0.07 −2.3 ± 0.02
Corona thickness (nm) 7.4 ± 0.5 4.0 ± 1 2.0 ± 0.5 1.0 ± 0.5 1.0 ± 0.5
electrosteric effect, it must happen at very high salt concentrations. In electrolyte solutions controlled by a divalent salt (CaCl2 ), an aggregation phenomenon is observed beyond 3×10−2 mol L−1 , although at the same ionic strength monovalent ions do not lead to destabilization of the suspension. This discrepancy is attributed to the chemical affinity between carboxylate groups of the corona and calcium ions. Specific interactions between poly(sodium acrylate) chains and multivalent ions have indeed been extensively described in the literature [79]. The aggregation mechanism is the result not only of the compression of the electrical double layer but also of equilibrium between free and specifically adsorbed cations. Moreover, no restabilization is observed even at very high salt concentration. This feature must be related to the fact that the complex between polyacrylates and Ca2+ does not redisperse in excess calcium [80].
4. Summary Although industrial latices are obtained by emulsion polymerization, the hydrodynamic properties of core–shell particles in solution have mainly been studied on monodisperse samples with well-defined hydrophilic layers. The studied suspension is composed of core–corona colloidal species according to the hydrophilic/hydrophobic character of the comonomers. Small angle neutron scattering measurements display no modification of the core protected by the charged layer with increasing ionic strength. Moreover, the interface between the core and the corona is not sharp but rather rough. A progressive variation from a hydrophobic center to a highly hydrophilic edge is in agreement with the synthesis method. The behavior of this layer with added salt observed by dynamic light scattering and electrophoresis differs from that of ideal objects such as polyelectrolyte brushes. If referred to as “hairy,” the surrounding structure is closer to a nonuniform charged gel of small thickness, whose hydrodynamic properties are controlled by a few strands. The expected purely steric effect from those chains is not straightforward, according to the high charge density of the colloidal particles. In contrast to smooth particles or to spherical brushes, the main part of the charge is confined in a layer of small thickness and its radial repartition depends on the proportion of
charged units and the density of the structure at a specific distance from the center. Furthermore, along a given chain, the distance between acid groups is lower than the Bjerrum length in pure water (∼0.7 nm). Thus the structural charge obtained by titration is greater than the effective charge and only a part is actually dissociated. Analogously the counterions concentration inside the corona is not uniform and the accessibility of sites depends on the position of the charged group. The collapsing process of the longest strands leads to a denser layer and an increasing of the concentration of dissociable sites. The special repartition of charges in a layer whose thickness and structure depend on the ionic strength will induce original interactions with cationic macromolecules.
Acknowledgments We thank Rhodia for financial support and synthesis of the core–corona particles. Peter Lindner from Institut Lauë Langevin is gratefully acknowledged for his assistance during SANS measurements, data analysis, and many valuable discussions.
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Characterizations and properties of hairy latex particles Pascal Borget a , Françoise Lafuma a,∗ , Cécile Bonnet-Gonnet b a Université Pierre et Marie Curie, Laboratoire de Physico-Chimie Macromoléculaire UMR CNRS 7615, ESPCI, 10 rue Vauquelin,
F-75231 Paris 05, France b Centre de Recherches d’Aubervilliers Rhodia 52, rue de la Haie Coq, 93308 Aubervilliers, France
Received 26 May 2004; accepted 8 November 2004 Available online 15 January 2005
Abstract Industrial latex composed of a hydrophobic core surrounded by a charged hydrophilic layer exhibits excellent stability toward monovalent salt. That feature is classically attributed to a steric effect due to a loss of entropy during overlapping of coating materials. The so-called electrosteric stabilization is, however, not a straightforward function of the nature of the hydrophilic corona. This suspension was characterized in dilute solution by scattering and electrophoresis techniques. In contrast to spherical brushes the interface between the core and the corona is not well defined. The layer is more similar to a highly hydrated nonuniform gel with few longer strands that control the hydrodynamic properties than to a polyelectrolyte brush whose dependence on ionic strength reflects the concentration of counterions inside a well-defined structure. Thus the steric contribution to stabilization of these hairy particles appears to be insignificant in the range studied. The highly hydrated nature and the global charge of the layer are two predominant factors for the stability of the particles. 2004 Elsevier Inc. All rights reserved. Keywords: Electrosteric stabilization; Hairy latex; Core–corona particles; Spherical brushes; Electrophoretic mobility; Dynamic light scattering; Small-angle neutron scattering
1. Introduction The control of the stability of suspensions is important for many industrial applications such as adhesives, coatings, and texture modifications. To prevent the coalescence of the colloidal particles due to van der Waals attractive forces, specific groups must be present on their surfaces in order to generate repulsive forces. During classical emulsion polymerization, the introduction of a large amount of ionic initiator leads to the appearance of several groups such as sulfate on the surface of the particles; the origin of the stabilization is electrostatic. Some suspensions are obtained with charged surfactant species that adsorb on the surface by hydrophobic interactions, enhancing electrostatic repulsions. However, the stability of such suspensions is strongly dependent on the ionic strength of the medium and the range * Corresponding author. Fax: +33-140-79-46-40.
E-mail address: [email protected] (F. Lafuma). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.11.019
of repulsive interactions must be greater than the size of the particles to overcome the van der Waals attraction. Other forms of stabilization less dependent on the Debye screening have been proposed. This effect can be achieved by building a hydrophilic layer onto the surface of the particles to add a steric protection. When the corona is composed of charged units or when ionic groups are already present on the surface, the stabilization is called electrosteric. At low ionic strength, electrostatic interactions generate long-range repulsions and the suspension is kinetically stable. At high ionic strength, colloidal particles are prevented from aggregating by the steric protection, which ensures thermodynamical stability. But these short-range repulsions are strongly dependent on the thickness of the layer, i.e., solvation, and the length of the hydrophilic chains. A steric barrier is already obtained by adsorbing surfactants with a sufficient chain length [1–3]. Although these molecules are easy to handle, controlling the thickness of the layer can only be achieved with macromole-
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cules, homopolymers [4] such as poly(ethylene oxide) [5] or block [6,7] and graft copolymers [8,9]. The adsorption process is mainly governed by the affinity for the surface and the solvency. The introduction of an additional electrostatic component by adsorbing polyelectrolytes [10,11] increases the stability of the suspension when the architecture is adapted. But the stabilization by physisorption is extremely dependent on variations of the medium. Grafting a polymeric brush on colloidal particles is an alternative strategy to enhance stability and build a permanent protective layer. The amount of polymer (contour length and grafting density) is determined according to the synthesis conditions and two ways are studied, the “grafting onto” [12–14] and the “grafting from” technique [15–17]. In fact, if a lot of theoretical work has been realized to describe the properties of polymeric brushes [18–21], there is a lack of experimental studies due to the synthesis difficulties [22]. Another interesting way to prevent aggregation is to introduce hydrophilic monomers during the synthesis. Depending on the polymerization parameters and conditions, long hydrophilic sequences can be formed around the hydrophobic core of the particle and induce a steric stabilization. Moreover, if hydrophilic units are charged, the particles are said to be electrosterically stabilized. They can be obtained with new techniques, e.g., “frozen micelles” [23] or layer-by-layer assembly [24], or with methods more convenient with industrial targets such as emulsion polymerization [25–27]. Contrary to spherical brush synthesis, numerous different morphologies of core–shell particles have been studied and extensively commented [28]. However, most of the characterization experiments concern the radial repartition of comonomers or surface analyses [24,27,29–37]. Surprisingly, the properties in solution of core–corona particles and the nature of the hydrophilic layer have been related in few studies [26,38–40], since polyelectrolyte brushes have been more precisely described in terms of interfacial distribution of monomers and thickness of the brush according to molecular characteristics (grafting density, contour length of the chains, and curvature of the surface) and the nature of the surrounding medium (ionic strength and pH) [13,17,22, 41–45]. In what follows, we present a detailed characterization of the properties of industrial core–corona particles in solution. These suspensions can be used as binders for the papermaking industry. Films obtained from the drying of such latex are characterized by a great cohesion between particles [46] and adhesion properties with the paper fiber or pigments lead to the rationalization of coating formulations [47]. Moreover, the destabilization of core–corona particles by classical flocculation with cationic agent if mentioned was briefly examined from academic point of view. Adsorption mechanisms of macromolecules on a soft surface and structures of aggregates in relation with structural characteristics of flocculating agents will be presented in another paper. Here the mode of electrosteric protection induced by the properties of the surrounded hydrophilic layer is discussed and compared
137
to previous results on model systems such as polyelectrolyte brushes. 2. Materials and methods 2.1. Materials The latex dispersion was supplied by the Centre de Recherches d’Aubervilliers (Rhodia, France) and synthesized through emulsion copolymerization of styrene (59 wt%), butadiene (37 wt%) and acrylic (2 wt%), and fumaric (2 wt%) acids. The thermodynamic arrangement between the components leads to a hydrophobic soft core (Tg ≈ 11 ◦ C) surrounded by a hydrophilic negatively charged polymeric layer. In addition to the dispersed particles, the aqueous phase also contains soluble species such as surfactant, amphiphilic polymers, salt, residual monomer, preservatives. These are removed by tangential ultrafiltration through a 100-kDa cutoff polyethersulfone membrane (Millipore). The purification method was checked by conductivity and total organic carbon measurements (Dohrmann DC 80 analyzer). The pH of the dispersion (typically 2 wt%) was adjusted to 8 with NaOH and the washing process was stopped when the conductivity of the filtrate was around 10 µS/cm and the carbon concentration less than 5 ppm. The density of acid groups was measured by conductometric titration. The latex has a charge density of 274 µeq g−1 with a higher proportion of weak acid groups (247 µeq g−1 ) than of strong ones (27 µeq g−1 ). All water used was freshly purified and deionized with a Milli-Q Plus water treatment system (Millipore). KNO3 salt was Normapur grade. 2.2. Methods 2.2.1. Small-angle neutron scattering (SANS) The scattering patterns were obtained with small-angle scattering instrument D11 at the Institut of Lauë–Langevin and analyzed according to standard procedures [48]. The scattered intensity was recorded according to the magnitude of the scattering vector, 4π sin(θ/2), (1) λ where λ is the wavelength of the incident neutron beam and θ the scattering angle. At wavelength 1.25 nm three experimental configurations were used to cover a large range of q values (8.45×10−3 < q [nm−1 ] < 1.58), which corresponds to distances between 750 and 4 nm. Sample concentrations were 0.1 wt% in a 10/90 H2 O/D2 O mixture. All measurements were done in 2-mm-thick quartz cells (Hellma). Under these conditions it was verified that there is no correlation between latex particles and no multiple scattering. In the case of a core–corona latex the scattering intensity depends on the structure, the homogeneity, and the proporq=
138
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Table 1 Scattering length densities
H2 O D2 O Latex core (59% styrene–37% butadiene) Latex corona
b × 1015 (m)
V¯ (cm3 /mol)
ρ/1014 (m2 )
−1.675 19.145 15.256 36.7675
18 18 99 × 0.59 + 60.7 × 0.37 = 80.9 66.5
−0.560 6.405 1.136 3.229
tion of each part of the particle [49,50]. To get a rough idea of the contribution of the core and the corona, Table 1 gives a list of scattering length densities according to the values of scattering length b [51] and partial volume V¯ [52]. In the H2 O/D2 O mixture the scattering length density of the solvent ρs is equal to 5.7085 × 1014 m−2 . The corona is assumed to be highly hydrated [37] and the exchange between H2 O and D2 O molecules inside this structure sufficiently fast. Assuming that the core and the corona have the same continuous-phase scattering length density, (ρcore )2 = 20.9 × 1028 m−4 and (ρcorona )2 = 6.1 × 1028 m−4 . According to the chemical nature of each component of the particle, the scattering intensity of the core is 3.5 times higher than that of the corona. Considering their relative proportion (core 96%, corona 4%), neglecting the contribution of the corona to the total scattering intensity is a reasonable assumption. The scattering intensity is given by
2.2.2. Dynamic light scattering (DLS) Light scattering measurements were carried out at a wavelength of 514.5 nm (Spectra Physics M2000 Ar ion laser) with a Malvern goniometer, a correlator, and a detector from ALV. The solutions were cleared from dust by filtering several times through a Millex HV 0.45-µm-pore-sized filter before being transferred into scattering cells. DLS measurements were performed at 25 ◦ C for a particles concentration of 1 ppm. Preliminary experiments showed that concentration dependence was negligible below 5 ppm and probably up to 100 ppm [38]. Multiple diffusion or particular interactions can be ruled out. The distribution function of the decay rates G(Γ ) is described by a distribution of relaxation times τr with the Repes method from the Gendist program. For each q value an apparent diffusion coefficient Dapp is calculated from the mean decay rate,
I (q) = cΦ(ρ)2 vP (q),
Γ = Dapp q 2 =
(2)
with c the concentration of the dispersion, Φ the volume fraction of particles, v the volume of one particle, and P (q) the form factor which for a homogeneous sphere of radius R is given by sin(qR) − qR cos(qR) 2 . P (q) = 3 (3) (qR)3 At high q values the form factor decays as q −4 under the condition that the scattering density change at the particle surface is abrupt. This is called the Porod law, S 1 , (4) V q4 where S/V is the specific surface of the scatterers. This ratio can be used to calculate the radius of Porod, defined as V RP = Φ . (5) S This information is useful to characterize the diffuse or sharp nature of the interface between the hydrophobic core and the surrounding layer of particles. But the determination of the specific surface is strongly dependent on the contrast of the scattering length density. A value of (ρ)2 more precise than those presented in Table 1 can be obtained by calculation of the invariant H of the system, Iq→∞ = c2π(ρ)2
∞ q 2 I (q) dq = c2π 2 (ρ)2 Φ.
H= 0
(6)
1 . τr
(7)
The concentration and angular dependence of Dapp is usually written as Dapp (q, c) = Dz 1 + C Rg2 z q 2 + · · · (1 + kD c + · · ·), (8) where C is a dimensionless factor depending on the structure of the scatterers, Rg2 z the z-average mean square radius of gyration, and kD the dynamic second virial coefficient, which describes hydrodynamic and thermodynamic interactions between particles and solvent. The hydrodynamic radius is calculated according to the Stokes–Einstein equation,
1 Rh
−1 z
=
kB T , 6πηDz
(9)
where kB is the Boltzmann constant, T the absolute temperature, and η the viscosity of pure solvent. 2.2.3. Electrophoresis Electrophoretic mobilities of particles at various ionic strengths were determined with a Laser Zee Meter Model 501 zetameter (Pem Kem Inc.). Measurements were performed each 100 µm from the upper wall of the rectangular cell to get the whole profile of mobility. The two stationary layers were calculated according to the Komogata equation [53]. This apparatus displays the zeta potential for an aqueous suspension at 20 ◦ C according to the Smoluchowski
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model, ε µ = ζ. η
139
(10)
Given the Smoluchowski zeta-potential the original mobility is easily recovered. The value of the zeta potential can be related to the electrophoretic mobility µ of particles and depends on the dimensionless quantity κa, where a is the radius of the particle and κ −1 the Debye length: µ=
2ε f (κa)ζ, 3η
(11)
where ε is the electric permittivity of the dispersing medium and η the viscosity of the dispersing medium. In this study electrophoretic measurements of colloidal particles with radius around 80 nm are performed in media whose ionic strength ranges between 10−4 and 1 mol L−1 . The value of κa lies between 3 and 270. Thus, at ionic strength greater than 10−2 mol L−1 , the values of f (κa) are calculated according to f (κa) =
330 3 9 75 − . − + 2 2 2(κa) 2(κa) (κa)3
(12)
At lower ionic strength the correction factor is obtained by interpolation with Henry’s values [53]. 2.2.4. Turbidity measurements The introduction of salt in the medium diminishes the interparticular repulsive forces. The salt concentration which leads to the destabilization of the suspension can be obtained by turbidity measurements at different wavelengths [54]. Transmittance measurements were performed with a Hewlett–Packard Type 8453 UV–visible spectrometer. The turbidity is defined as OD , (13) l where l is the pathlength and OD the optical density. Generally τ ∼ = λ−γ , where γ is the slope of the plot of the turbidity versus the wavelength in logarithmic coordinates [55]. With Rayleigh scatterers γ = 4 and the value of this gradient decreases as the size of the objects increases. τ = ln(10)
3. Results and discussion 3.1. Structure of the core 3.1.1. Radius of the core and distribution of size SANS measurements were performed at different ionic strengths ([KNO3 ] = 10−3 –1 mol L−1 ), but no difference between the patterns could be detected. No aggregation phenomenon takes place even at the highest ionic conditions. Taking into account a slight polydispersity, a hard sphere model without interactions is sufficient to fit the data (Fig. 1). Generally the Schultz–Flory distribution is used to describe
Fig. 1. SANS spectrum of the suspension and hard-sphere model.
the polymolecularity of the system [56,57]. But this function can be described with a Gaussian curve when the polydispersity is not too high: ¯ 1 (r − R) , w(r) = √ exp − (14) 2σ 2 σ 2π where R¯ is the mean radius of particles and σ the standard deviation. For a more classical characterization of the system, the polydispersity index is given as
2
2 σ σ Ip = 1 + 7 (15) 1+6 . ¯ R R¯ Since the diffusion of a sphere of radius R is proportional to the square of its volume, the scattering intensity is written as ∞ sin(qr) − qr cos(qr) 2 3 w(r) dr, I (q) = I0 ¯ 3 (q R)
(16)
0
with I0 = I (q)q→0 = CΦ(ρ)2 v.
(17)
This model leads to R¯ = 77 nm and σ = 7 nm. The polydispersity index is around 1.11. Calculations of scattering length densities in the experimental part (Table 1) show that the scattered intensity only results from the core of the particle. The plot of I (q)q 2 = f (q) (Fig. 2) leads to a more consistent value of (ρ)2 as reported in Eq. (6). The integration of the experimental plot or the fitting curve gives the same value H = 3620 ± 20 µm−4 . For a volume fraction of 0.108% corresponding to 0.1 wt%, the contrast is equal to (ρ)2 = (17.0 ± 0.1) × 1028 m−4 . Considering that the scattering length density of the solvent, ρcore is around (1.59 ± 0.02) × 1014 m−2 , which is in relative agreement with the value 1.43 × 1014 m−2 proposed by De Bruyn for a polystyrene core surrounded by a hairy layer of perdeuterated acrylic acid [37]. According to this model the value at q = 0 can be extrapolated to I0 = 3900 cm−1 . The corresponding radius is
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Fig. 2. Calculation of the invariant.
Fig. 3. Calculation of the specific surface of the particle.
found equal to 79.8 nm, which is close to the precedent value. 3.1.2. The interface between the core and the corona of the particle After correction of the background the plot of I q 4 vs q leads to asymptotic behavior at high q corresponding to the discontinuity of the scattering length densities between the core and the corona (Fig. 3). The plateau value (5.0 ± 0.2) × 1034 m−5 is proportional to the total specific surface of particles. According to Eq. (5), the radius of Porod is RP = 21.6 ± 1.3 nm. This corresponds to a radius of a spherical particle around 65 ± 4 nm, which is significantly lower than the mean radius (R¯ = 77 nm). At high q values, the model does not describe perfectly the system. With the assumption that the surface of the core is not sharp but rough, the specific surface of the scatters increases and the radius of Porod diminishes. Thus this feature can be explained by a diffuse transition between the core and the corona as a consequence of the incorporation of hydrophobic units during the copolymerization of carboxylic monomers. This is consistent with most of the emulsion polymerization studies that conclude to “buried” hydrophilic units that compose 1–5% of the total core volume [37,58,59]. 3.2. Hydrodynamic properties of the corona 3.2.1. Dynamic light scattering To characterize the hydrophilic corona of the particle, the suspension is analyzed by dynamic light scattering. When the particle moves in the aqueous phase, it is supposed that water molecules are enclosed in the corona and that the shear plane is located near the outer surface of the particle. The hydrodynamic radius is related to the extension of the corona. The pH of the suspension is systematically adjusted to 8 in order to be sure that most of the charges are potentially dissociated. As already mentioned in SANS experiments, the introduction of monovalent salt KNO3 does not induce the coagulation of the suspension. DLS measurements were
Fig. 4. Diffusion coefficient versus q 2 at different ionic strengths.
performed at different ionic strengths ranging from ∼10−4 (pure water) and 1 mol L−1 . The plot of the diffusion coefficient Dapp vs q 2 (Fig. 4) also indicates that there is no angular dependence. This information is in agreement with the slight polydispersity of the sample. In order to get the best precision, the diffusion coefficient is averaged out over the whole angular domain. Fig. 4 clearly shows an increase of the diffusion coefficient with the ionic strength. This variation can be converted into size effects by the Stokes–Einstein relation (Eq. (9)). The viscosity of the medium diminishes when the salinity increases; its influence becomes significant only for ionic strengths higher than 10−1 mol L−1 [60]. In the case of polydisperse systems, the hydrodynamic ¯ radius Rh obtained by DLS is not equal to the mean radius R. According to SANS measurements the size distribution can be described by a normal law and Rh must be corrected as follows: σ 4 σ 6 σ 2 + 45 R + 15 R 1 + 15 R R 6 ¯ ¯ ¯ ¯ . Rh = 5 = R (18) σ 2 σ 4 R 1 + 10 R¯ + 15 R¯ On the assumption that the corona has no influence on the size distribution, this correction leads to a variation of the hy-
P. Borget et al. / Journal of Colloid and Interface Science 285 (2005) 136–145
Fig. 5. Variation of the hydrodynamic radius of the particle with the ionic strength.
Fig. 6. Variation of the corona thickness of the particle with the ionic strength.
drodynamic radius between 84.4 ± 0.5 and 78.1 ± 0.5 nm (Fig. 5). Thus by comparison with the radius of the core (R¯ = 77 nm), the maximal thickness of the corona is around 7 nm in pure water. At high ionic strength the hydrodynamic influence of the corona is close to the experimental error, that is 1 nm. In the range 10−4 –10−1 mol L−1 the layer thickness δ seems to obey a power law δ ∼ I −a (Fig. 6) with an exponent a equal to 0.3. This variation does not correspond to the gel or polyelectrolyte shrinkage (a = 0.5). Starting from the blob model proposed by Daoud and Cotton [61], scaling laws with 0.1 < a < 0.17 depending on the curvature radius of the sterically stabilized particles were obtained [17]. Our results could be related to the theory for planar brushes proposed by Borisov et al. [20], which forecasts an exponent equal to −1/3. Finally the diminution of the thickness is only observed over one decade and it is hard to conclude because of the poor precision. But Fig. 6 brings out a monotonous decrease of the thickness δ until an ionic strength around 10−1 mol L−1 at which the corona is totally collapsed. This kind of variation is not conform to planar brushes theories that predict no diminution of the brush thickness in the osmotic regime, that is as
141
Fig. 7. Evolution of the zeta potential ζ and the reduced electrophoretic mobility E versus the concentration of monovalent salt.
long as the ionic strength does not exceed the counterion concentration inside the charged corona. This property has been checked by Tran et al. [22] for dense planar brushes obtained by chemical grafting on silica wafers. The concentration of counterions in the brush is evaluated to 1.4 mol L−1 . The ionic effect appears for salt concentrations higher than 1 mol L−1 and the brush thickness follows a power law with −0.27. In the case of the studied particle, the properties in solution are different. As reported previously, the charge density of the core–corona particle is rather high and the confinement in a structure of small thickness leads to an inner concentration of counterions around 1 mol L−1 . Thus no variation of the thickness of the corona should be observed below a salt concentration of 1 mol L−1 . Although the thickness is small in comparison to the radius of the particle, the structure of the corona cannot be considered as a charged brush on a planar surface but rather is like a gel. 3.2.2. Electrophoresis Zeta potential measurements on a freshly ultrafiltered sample are performed at different ionic strengths controlled by the concentration of potassium nitrate (Fig. 7). These values are obtained according to the procedure described above, which is first conversion into the corresponding mobilities (Eq. (10)) and then calculation of zeta potential as reported in Eq. (11). In pure water the zeta potential is around −70 mV. The introduction of a small amount of monovalent salt (10−3 mol L−1 ) leads to a diminution of ζ (−77 mV), followed by a monotonic increase to 1 mol L−1 . This last evolution corresponds to the classical effect of charge screening with increasing salt concentration. Notice that the value of zeta potential at the highest ionic strength (−30 mV) is still sufficient to ensure the stability of the suspension already observed by scattering techniques. The conversion into reduced electrophoretic mobility1 confirms the presence of an extremum around 10−3 mol L−1 . 1 E = 3 e η µ. 2 kB T ε
142
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Table 2 Electrophoretic study of core–corona latices at different ionic strengths in confrontation with the theory of the relaxation term [KNO3 ] (mol L−1 )
κa
Emeasured
Emax (O’Brien and White)
ζ (mV) (Henry)
ζ (mV) (O’Brien and White)
0 10−3 10−2 10−1 1
2.8 8.4 26 81 257
3.00 ± 0.12 3.65 ± 0.05 3.02 ± 0.04 2.56 ± 0.05 1.69 ± 0.02
3.12 3.3 4.3 5.9 >7
−71 −77 −57 −45 −29
−107 – −61 −48 −29
The values obtained do not reflect the high charge density of the particles. Furthermore, the classical theory of Gouy, Chapman, Grahame, and Stern does not predict such a variation with the ionic strength. This premium model supposes in particular the invariance of the dielectric constant in the near of the interface solid/liquid and a uniform repartition of charges on a sharp surface, which does not correspond to the core–corona particle. The improvement of this theory by O’Brien and White leads to a maximum on the plot of the reduced electrophoretic mobility E versus the reduced zeta potential ζ˜ [62].2 A relaxation term resulting from an electrical field induced by the polarization of the double layer in opposed direction to the external field is taken into account. For highly charged systems the distortion of the double layer leads to a retardation effect on the particle in movement. This term is proportional to the square of the reduced zeta potential. A simple model based on this theory allows the calculations of E vs ζ˜ curves [53,63]. Thus for κa > 3 the presence of a maximum suggests two possible values of the zeta potential for the same electrophoretic mobility. If appropriately applied, this theory makes it possible to obtain monotonic plots ζ = f (I ) [64,65]. Even with highly charged latex (30–80 µC cm−2 ), good correlations between theory and experimental measurements were observed in the range 10 < κa < 50, where the contribution of the relaxation term is maximal [66]. In the case of the core–corona particle the relaxation term theory predicts a zeta potential in pure water greater in absolute values than that obtained via the Henry relation. But for κa = 8.4 the measured electrophoretic mobility (µ = −4.8 (µm/s)/(V/cm) and E = 3.65) is higher than the maximal theoretical value predicted by O’Brien and White (E = 3.3). This fact has already been mentioned and discussed elsewhere [67,68]. All results are summarized in Table 2. The relaxation term model cannot be applied to the whole experimental data because this analysis is based on the Gouy–Chapman theory, whose foundations are not in agreement with the structure of the core–corona particle. Two other models have been proposed. To interpret more precisely the electrokinetic and conductometric properties of colloidal systems, Zukoski and Saville 2 ζ˜ = e ζ . kT
[69] have suggested specific adsorption of ions on the surface of the particle. The Stern layer is considered to be the place of lateral movement of ions by diffusion and electromigration [70]. The confrontation of the model with experimental data leads to the evaluation of different factors such as the adsorption capacity and their relative mobility inside the layer [71]. At weak ionic strength the diminution of the potential is explained by preferential interactions between co-ions and hydrophobic parts or by a great adsorption– desorption constant. This model does not take into account the position of the shear plane which depends on the ionic strength in the case of a core–corona particle. The concept of a layer of loose polymer strands on the surface of the latex particles has been proposed to explain the observed electrophoretic properties. This “hairy” model, introduced by van der Put and Bijsterbosch [72], was applied to polystyrene latex [73,74] or to particles surrounded by an hydrophilic corona [67,75,76] in order to explain the variation of the zeta potential with the ionic strength. Flexible chains on the surface latex particles are assumed to be charged, either carrying a terminal ionic group or being composed of charged monomers. At low ionic strength these hairs adopt an extended conformation due to electrostatic repulsions between surface and chains and/or between neighboring segments. This leads to a shift of the shear plane away from the surface and the possibility that some charges are located in the Stern layer and/or in the diffuse layer. The zeta potential measured at the shear plane is therefore lower than that of a bare particle carrying the same charge. The continuous collapse of the layer with ionic strength has the effect of moving the shear plane closer to the surface, thus increasing the zeta potential of the particle. At high ionic strength, chains are completely shrunk and the potential decreases classically owing to electrical double-layer compression. Each model (“hairy layer” and “ion adsorption”) has been criticized and seems not to be able to explain all experimental data [77]. In the case of the studied particle, specific ion adsorption is very unlikely, according to dynamic light scattering results that show a continuous decrease until an ionic strength around 10−1 mol L−1 . Finally, the hairy model is more consistent with the present experimental observations since the corona of the particles is essentially composed of hydrophilic charged strands. No supplementary information was obtained by applying the model of soft spheres proposed by Oshima [78] because of the nonuniform and complex structure of the corona, whose effective charge depends on
P. Borget et al. / Journal of Colloid and Interface Science 285 (2005) 136–145
the ionic environment (screening, neighbor carboxylic sites) and thickness of the layer. 3.2.3. Electrosteric stabilization The stability of the suspension at 0.1 wt% was studied by visible spectroscopy in monovalent (KNO3 ) and divalent (Ca(NO3 )2 ) ionic medium (Fig. 8). In pure water the turbidity gradient γ is around 3.5, since the size of the particle is greater than that of a Rayleigh scatterer. In monovalent electrolyte solution, no significant variation of γ is observed over a wide range of ionic strength. The particles do not show any aggregation until at least 3 mol L−1 . Generally the stability of a suspension is correlated with the surface charge density. The core–corona particle is characterized by a higher structural charge (274 µeq g−1 ) compared to commonly studied systems. If all ionic groups were situated on the surface of the particle, the charge density would be equal to 70 µC cm−2 . The high proportion of potentially charged sites carried by the particle undoubtedly contributes to the peculiar stability toward monovalent ions. Tsaur and Tamai have studied the stability in ionic medium of core–shell particles (styrene/styrene sodium sulfonate) obtained by seed polymerization, respectively BA51
Fig. 8. Stability of the suspension in monovalent (KNO3 ) and divalent (Ca(NO3 )2 ) ionic medium.
143
and P(StNaSS) (Table 3). Within the same series increasing the proportion of hydrophilic comonomer leads to a greater stability. Comparison between the different systems shows that the critical coagulation concentration does not depend only on the charge density and on the thickness of the stabilizing layer. The internal structure of the corona (inhomogeneities, density, solvation) also influences the efficiency of the protection. The studied latex is characterized by a quite thin corona, taking into account the relatively high proportion of incorporated monomer and consequently a high charge density. In comparison to the systems of Tsaur and Tamai, the corona of the particle is denser and more hydrophilic. The studied particle is rather analogous to electrosterically stabilized latices with a poly(acid acrylic) hydrophilic layer, prepared in a manner similar to that commonly employed industrially [37]. At low ionic strength and pH 8.5, the thickness is found to range between 5 and 7 nm, the shell hydration is around 95 vol%, and no destabilization of the suspension is observed, even at high ionic strength (KCl 2.5 mol L−1 ). The dispersion of poly(styrene–co–butyl acrylate) particles surrounded by chains of poly(methacrylic acid) displays the same stability towards monovalent salt (NaCl 3.5 mol L−1 ) [39]. However in the case of the studied particle, the steric contribution of its hair is not straightforward. Table 4 summarizes all results concerning the behavior of the particle in ionic medium and yields a comparison between hydrodynamic and electrophoretic effects. The Debye length is roughly related to the extent of interparticle electrostatic repulsions. With smooth particles this parameter is defined from the surface plane where the charges are situated. In the case of a hydrophobic core surrounded by a hydrophilic charged layer, ionic groups are located in a volume and there is no well-defined plane. At low ionic strengths some hydrophilic chains are in extension but still located inside the electrical double layer: stabilization is governed by electrostatic repulsions. When the ionic strength reaches 10−1 mol L−1 , the corona thickness is more or less equal to the Debye length and the charge density (see the value of the measured electrophoretic mobility) is sufficient to ensure a good stability of the suspension. If there is a purely
Table 3 Correlation between the critical coagulation concentration (ccc) in monovalent ionic medium and the characteristics (hydrophilic comonomer proportion, surface charge density σ , thickness of the corona measured at ionic strength 10−2 mol L−1 ) of Tsaur and Fitch [81] and Tamai et al. [75] particles and the studied latex at pH 8 Latex
Hydrophilic comonomer proportion ( wt%)
σ (µC cm−2 )
(nm)
ccc (mol L−1 )
BA51I BA51D BA51C BA51H
– – – –
10.2 9.1 5.0 1.0
– – – –
1.35 1.22 0.80 0.32
P(St/NaSS2 ) P(St/NaSS3 )II P(St/NaSS4 )II P(St/NaSS5 )II
0.4 0.6 0.8 1
7.5 15.1 17.0 20.7
1.1 2.0 2.7 3.6
0.36 0.45 0.86 1.23
Rhodopas
4
70
2
>3
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Table 4 Hydrodynamic and electrophoretic properties of a core–corona latex [KNO3 ] (mol L−1 ) 0 10−3 10−2 10−1 1
κ −1 (nm) 30.4 9.6 3.0 1.0 0.3
Electrophoretic mobility (µm s−1 cm V−1 ) −4.0 ± 0.2 −4.9 ± 0.07 −4.0 ± 0.05 −3.4 ± 0.07 −2.3 ± 0.02
Corona thickness (nm) 7.4 ± 0.5 4.0 ± 1 2.0 ± 0.5 1.0 ± 0.5 1.0 ± 0.5
electrosteric effect, it must happen at very high salt concentrations. In electrolyte solutions controlled by a divalent salt (CaCl2 ), an aggregation phenomenon is observed beyond 3×10−2 mol L−1 , although at the same ionic strength monovalent ions do not lead to destabilization of the suspension. This discrepancy is attributed to the chemical affinity between carboxylate groups of the corona and calcium ions. Specific interactions between poly(sodium acrylate) chains and multivalent ions have indeed been extensively described in the literature [79]. The aggregation mechanism is the result not only of the compression of the electrical double layer but also of equilibrium between free and specifically adsorbed cations. Moreover, no restabilization is observed even at very high salt concentration. This feature must be related to the fact that the complex between polyacrylates and Ca2+ does not redisperse in excess calcium [80].
4. Summary Although industrial latices are obtained by emulsion polymerization, the hydrodynamic properties of core–shell particles in solution have mainly been studied on monodisperse samples with well-defined hydrophilic layers. The studied suspension is composed of core–corona colloidal species according to the hydrophilic/hydrophobic character of the comonomers. Small angle neutron scattering measurements display no modification of the core protected by the charged layer with increasing ionic strength. Moreover, the interface between the core and the corona is not sharp but rather rough. A progressive variation from a hydrophobic center to a highly hydrophilic edge is in agreement with the synthesis method. The behavior of this layer with added salt observed by dynamic light scattering and electrophoresis differs from that of ideal objects such as polyelectrolyte brushes. If referred to as “hairy,” the surrounding structure is closer to a nonuniform charged gel of small thickness, whose hydrodynamic properties are controlled by a few strands. The expected purely steric effect from those chains is not straightforward, according to the high charge density of the colloidal particles. In contrast to smooth particles or to spherical brushes, the main part of the charge is confined in a layer of small thickness and its radial repartition depends on the proportion of
charged units and the density of the structure at a specific distance from the center. Furthermore, along a given chain, the distance between acid groups is lower than the Bjerrum length in pure water (∼0.7 nm). Thus the structural charge obtained by titration is greater than the effective charge and only a part is actually dissociated. Analogously the counterions concentration inside the corona is not uniform and the accessibility of sites depends on the position of the charged group. The collapsing process of the longest strands leads to a denser layer and an increasing of the concentration of dissociable sites. The special repartition of charges in a layer whose thickness and structure depend on the ionic strength will induce original interactions with cationic macromolecules.
Acknowledgments We thank Rhodia for financial support and synthesis of the core–corona particles. Peter Lindner from Institut Lauë Langevin is gratefully acknowledged for his assistance during SANS measurements, data analysis, and many valuable discussions.
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