Surfactant Kinetics and Their Importance in Nucleation Events in (Mini

Jul 17, 2014 - Aramendia , E.; Malle , J.; Jeynes , C.; Barandiaran , J.; Keddie , J. L.; Asua , J. M. Distribution of Surfactants near Acrylic Latex ...
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Surfactant Kinetics and Their Importance in Nucleation Events in (Mini)emulsion Polymerization Revealed by Quartz Crystal Microbalance with Dissipation Monitoring Nicholas Ballard,† Jone Urrutia,† Simone Eizagirre,† Thomas Schaf̈ er,‡ Gabriela Diaconu,† José C. de la Cal,† and José M. Asua*,† †

POLYMAT and Grupo de Ingeniería Química, Dpto. de Química Aplicada, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Etorbidea 72, 20018, Donostia/San Sebastían, Spain ‡ Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain S Supporting Information *

ABSTRACT: Surfactants are vital components of almost all heterogeneous polymerizations for maintaining colloidal stability, but they also play an important role in the kinetics and mechanism of particle nucleation. Despite many decades of research, the knowledge of adsorption−desorption surfactant kinetics and their application in (mini)emulsion polymerization is largely based on qualitative arguments. In this paper we show that the use of a quartz crystal microbalance with dissipation monitoring can provide quantitative information on both the adsorption equilibrium of ionic and nonionic surfactants, and also the kinetics of adsorption/desorption, that can be applied to the understanding of nucleation processes in (mini)emulsion polymerization. We show that surfactant dynamics and nucleation phenomena in (mini)emulsion polymerization are not dominated by diffusion phenomena linked to molecular size of surfactant as previously thought but rather are driven by the large differences in the rate of surfactant adsorption and desorption at the polymer−water interface. Finally, we show the application of this knowledge to explain the differences between nucleation processes for ionic and nonionic surfactants in emulsion polymerization.

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INTRODUCTION Heterogeneous free radical polymerization techniques such as emulsion, miniemulsion, and dispersion polymerization invariably rely on the presence of surfactants to maintain colloidal stability throughout the polymerization process and during storage.1−5 The type and amount of surfactant used can significantly alter the polymerization rate, final particle size, particle size distribution, the molecular weight of the resulting polymer, and the resulting physical properties of the final polymer materials.6−13 For example, the sensitivity to electrolyte, freeze−thaw, and shear stability of polymer latexes is highly dependent on the surfactant used, and in general better properties are obtained by using nonionic surfactants.14,15 However, in polymerization of latexes solely stabilized by nonionic emulsifiers particle nucleation is inefficient, and therefore a combination of steric and ionic emulsifiers, which exhibit the opposite behavior (ionic emulsifiers provide poor © 2014 American Chemical Society

shear and freeze−thaw stability but are efficient in promoting nucleation), is often used.7 Besides this, in many instances it is important to control the process of particle nucleation in order to achieve optimum properties of the dispersion. For example, in high solids content latexes multimodal or broad particle size distributions are essential in order to limit viscosity, and hence secondary nucleation of a new crop of particles is often desirable.16−20 Similarly, if latexes with a very small diameter are desired (99.5%, Aldrich), potassium persulfate (99%, Aldrich), poly(methyl methacrylate (Mw = 350 000 g mol−1, Aldrich), potassium hydroxide (0.1 M solution in water), and Triton X-305 (70% in H2O, Aldrich) were used without purification. Doubly deionized water was used throughout the work. Methods. Latex Characterization. Z-average particle diameters were determined by dynamic light scattering performed on a Malvern Zetasizer ZS using a scattering angle of 173° at a standard temperature of 25 °C. Each measurement was conducted in triplicate, and the average of the three values was taken. Conversion was determined gravimetrically. Latex Soap Titration. The CMC and the surface area per molecule (packing area), as, of the surfactants were determined by means of surface tension measurements using a du Noüy ring (KSV Sigma 70, KSV Instruments Ltd.) equipped with automatic dosing unit. The latex used for as measurements was dialyzed until a constant conductivity was obtained. The latex or DDI water (40 g) was added to a Pyrex dish, and the surfactant solutions were titrated with measurement of the surface tension at predefined concentration intervals. Multiple measurements at each point were taken. The solids content was 9.8% 9054

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with a mean particle diameter of 461 nm and a polydispersity index (PDI) of 0.05. The packing area was calculated from the difference in CMC between the sample of pure water and that of the latex. The CMC was calculated by the intersection of the fit of the Langmuir isotherm to the sloped part of the graph and the baseline of the curve. Quartz Crystal Microbalance. Polymer-coated sensors were obtained by spin-coating a solution of poly(methyl methacrylate) (Mw = 350 000 g mol−1, 0.5 wt % in toluene) onto a gold sensor (diameter = 14 mm, Q-SENSE, Sweden) at a rate of 50 rps for 1 min using a Lot Oriel SCC 200 spin-coater. The coated thickness was typically 25 nm as measured by difference in frequency before and after coating using the Sauerbrey relationship.36 The sensors were then placed in an oven at 130 °C for 1 h. Before reuse the coated chips were submerged in THF overnight and then placed in a sonic bath, washed with ethanol, and exposed to UV/ozone cleaner for 10 min (Bioforce). QCM measurements were performed on a Q-SENSE E1 system operating at 23 °C. Before experiments DDI water was passed over the chip until a stable baseline was obtained. All solutions were passed at a rate of 150 μL min−1 using a peristaltic pump. Kinetic experiments were conducted by flowing a concentration of surfactant through the QCM for a set period of time (typically 500 s). The desorption part of the experiment was then conducted by changing the test solution to deionized water, and the frequency was monitored until the majority of the surfactant had desorbed from the surface. Unless stated otherwise, all experiments were conducted on pristine pMMA-coated chips. The QCM-D technique detects changes in the resonance frequency, Δf, and dissipation, ΔD. During the adsorption/desorption cycle the resonance frequency of the crystal changes according to changes in mass. If the mass forms an evenly distributed, rigid layer whose mass is small compared to that of the crystal, then the mass per unit area (m) can be calculated from the Sauerbrey equation.36 Δm = −

Table 1. Recipes for Seeded Emulsion Polymerizations Conducted at 70 °C reaction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 a

seed latex (g)

MMA (g)

80

10

80

10

50

2.5 5 10 2.5 5 10

50

SDS (g)

Triton X-305 (g)

0.025 0.05 0.15 0.2 0.1 0.25 0.5 0.75 0.09

0.37

KPS (mg)

conversion (%)

Dexpa (nm)

50

83 82 90 85 86 87 87 89 86 93 84 91 87 85

380 384 256 106 389 383 389 366 449 399 285 410 474 566

50

25 50 100 25 50 100

Mean particle diameter (z-average).

and Triton X-305. Triton X-305 is a nonionic octylpenol ethoxylate with an average poly(ethylene glycol) chain of 30 units and was chosen for the model nonionic surfactant because compared to most nonionics its CMC (1.916 g L−1 for according to the manufacturer) is relatively close to that of the widely employed ionic surfactant SDS (2.3 g L−1).37 For each surfactant, three concentrations were used, and the experiment was run until the system was at equilibrium, and the frequency showed a negligible change in gradient with time (approximately 500 s). Figure 1 shows the plot of the bulk

C Δf n

In this equation, C is a constant (17.7 ng cm−2 s−1 in this equipment) and n is the resonance overtone number. In this work, n = 5 was used unless otherwise stated. Kinetic fitting was performed using a Matlab routine based on solving the differential equations using a Dormand-Prince solver and using a nonlinear least-squares fitting of the parameters based on the trust region reflective Newton method. For parameter estimation the least-squares fitting routine was run from at least 50 different initial points spanning several orders of magnitude. In the majority of cases the same local minima was found which was assumed to be the global minimum of the solution. Seed Latex Preparation. Water (700 g) and MMA (80 g) were added to a 1 L double-walled glass reactor equipped with anchor type stirrer, nitrogen inlet, condenser, and thermocouple. The reaction mixture was degassed by nitrogen bubbling for 30 min, stirring constantly at 180 rpm, and then heated to 70 °C. Once at reaction temperature, potassium persulfate (325 mg dissolved in 15 g of water) was added in a single shot. After 2.5 h, the reaction mixture was cooled and filtered through a muslin lining to remove the small amount of coagulum. Seeded Emulsion Polymerization. Seeded emulsion polymerizations were performed in sealed bottles. The seed latex, monomer, initiator, and surfactant were added, and the bottle was sealed. The reaction mixture was placed in a bottle polymerization unit and tumbled end over end overnight to swell the seed latex before being heated to 70 °C. The reaction was left to polymerize for 4 h before being cooled and filtered through a muslin lining to remove any small amounts of coagulum.

Figure 1. Variation of surface concentration with increasing bulk concentration for SDS and Triton X-305 and the linear regression fit to the Langmuir equation.

concentration, [S], against the ratio of [S] to the equilibrium surface concentration, Γeq, obtained through the Sauerbrey equation.35 From the linearized form of the Langmuir equation (see eq 1) one can yield K, the Langmuir equilibrium constant, and Γ∞, the surface concentration at maximum coverage.

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[S] 1 [S] = + Γeq Γ∞K Γ∞

RESULTS Equilibrium Adsorption Isotherms. Before we consider the dynamics of surfactant adsorption, we will first determine the adsorption equilibrium for the two model surfactants: SDS

(1)

The packing area is calculated as 1/NAΓ∞, where NA is Avogadro’s number. Table 2 shows the parameters from the Langmuir equation for both surfactants. It can be seen that the 9055

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counterion can greatly influence adsorption characteristics.39,41−43,38,40−42 The significant difference between the results shown here from titration measurements and those obtained from QCM for SDS can be ascribed to the combination of several factors. First, the calculation of area per molecule assumes knowledge of the total surface area, calculated from the average particle size assuming a monodisperse system and polydispersity effects are ignored. This may be especially important where particle size is determined by light scattering that has increased sensitivity to large particles. In addition, the effects of surface curvature are ignored, although in this case this is not going to have a significant impact due to the large particle size used in the experiments. Perhaps most important are the effects of surface charges and functional groups. The QCM experiments are conducted using on a flat, homogeneous poly(methyl methacrylate) surface whereas the latex possesses surface charges originating from the persulfate initiator used. Typical surface charges for this kind of latex44,45 vary depending on the method of synthesis, but Yoon et al. have previously shown that a surface charge in the region of 5−70 μC cm−2 (corresponding to a charged functional group every 20−700 Å2) are obtained for polystyrene latexes synthesized by soap-free emulsion polymerization.46 Less adsorption is expected where the surface charge density is high because the interaction of surfactant with the surface will be severely affected by the electrostatic repulsion. Indeed, Yoon et al. showed that changing the surface charge density of polystyrene latexes altered the adsorption of bovine serum albumin by over 5 times.46,45 Kinetics of Surfactant Adsorption/Desorption. The real time measurement of QCM allows for evaluation of not only equilibrium values but also of the kinetic data. We conducted kinetic experiments of the two surfactants at different surfactant concentrations to evaluate the mechanism and dynamics of surfactant adsorption. First, the surfactant solution was passed over the surface at a constant flow rate until equilibrium was established, and then water was passed over the surface and the kinetics of desorption were monitored. The fraction of surface covered by emulsifier (θ = Γ/Γ∞) was then calculated at each time. Figures 3 and 4 present the adsorption and desorption kinetics of SDS and Triton X-305 at three different surfactant concentrations, all below the CMC. These figures show that for both surfactants the time for desorption is longer than that for adsorption. In addition, in comparison with SDS, Triton X-305

Table 2. Fitting Parameters Obtained from the Linear Regression Fit in Figure 1 Γ∞ (mol m−2) SDS Triton X-305

−6

(7.96 ± 1.70) × 10 (2.02 ± 0.06) × 10−6

K (m3 mol−1)

as (Å2)

aCMC (Å2)

0.12 ± 0.03 28 ± 5

21 ± 5 82 ± 2

42 ± 15 84 ± 21

adsorption of SDS was weaker than that of Triton X-305 (there is an order of magnitude difference in the equilibrium constant) and that the packing area, as, was smaller. The packing area at the CMC of SDS, aCMC calculated by the Langmuir equation using the CMC as the bulk surfactant concentration and the parameters given in Table 2, leads to a much higher value of the packing area. This means that the adsorption of SLS does not follow the Langmuir equation leading to a value of Γ∞ that is higher than the maximum adsorption at the CMC, ΓCMC.38,39 The ratio of these terms has previously been used as an indicator of the strength of surfactant adsorption.40 In this case, this ratio is low and, in agreement with the value of the adsorption equilibrium constant, suggests that the strength of adsorption for SDS is weak. In order to validate these results, we conducted titrations of a poly(methyl methacrylate) latex obtained by soap-free emulsion polymerization to determine the difference between the QCM technique and the widely used method of soap titration. Figure 2 shows the plots for the two surfactants titrated against a water blank and the latex at 9.8% solids content. The Triton X-305 titration curve has an inflection point that is usually observed in surfactants with a low degree of purity. The impurities initially adsorb onto the surface before being displaced by the principal component at higher concentrations. In this case, this is probably related to the distribution in the number of ethylene glycol groups in the surfactant (see Figure S3). Although SDS usually also possesses an inflection point, it was not observed here, likely due to the high purity grade of SDS used throughout this work. The surface concentration at the CMC was calculated from the difference of CMC in the absence and presence of particles and the known surface area. The calculated area per molecule, aCMC, was 60 and 83 Å2 for SDS and Triton X-305, respectively. The value for SDS is in general agreement with those found in previous works where the value of aCMC varies from 23 to 60 Å2, which have shown that particle functionality, size, and

Figure 2. Soap titrations of monodisperse poly(methyl methacrylate) latex particles (filled squares) and water (crosses) with (a) SDS and (b) Triton X-305 surfactants. 9056

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Figure 3. Time-dependent adsorption (left) and desorption (right) profiles for three concentrations (1.7, 3.4, and 5.2 mol m−3) of ionic emulsifier SDS onto poly(methyl methacrylate) substrate. Solid lines show the fitting of the mixed kinetic-diffusive model (ktr = 3.6 × 10−6 m s−1, ka = 0.0016 m3 mol−1 s−1).

Figure 4. Time-dependent adsorption (left) and desorption (right) profiles for three concentrations of nonionic emulsifier Triton X-305 (0.07, 0.16, and 0.33 mol m−3) onto poly(methyl methacrylate) substrate. Solid lines show the solution to the differential equation for mixed kinetic-diffusive transport (ktr = 2.4 × 10−6 m s−1, ka = 0.21 m3 mol−1 s−1).

Triton occupies 550 Å2. From coarse grain models it has been shown that the radius of gyration of a 30 unit ethylene oxide chain has a radius of gyration of ≈11.5 Å.48,47 This corresponds to an area of 415 Å2, which suggests that at this point there is a transition where the poly(ethylene glyol) chains are forced to interact as shown schematically in Figure 5. This has been

has a faster adsorption and a slower desorption. The observation of faster adsorption of Triton X-305 cast serious doubts on the often used argument that the performance of nonionic surfactants is almost exclusively determined by the limited diffusion of large molecules. SDS and Triton X-305 also differ in the long-time characteristics of the desorption curves; whereas SDS completely desorbs, the desorption of Triton X-305 becomes very slow after the surface coverage fraction decreases below 0.15 and appears to plateau at this value. To investigate if this was due to the presence of any impurity that may adsorb irreversibly to the surface, we performed MALDI-TOF analysis on the surfactant. The analysis showed no unexpected peaks, even in the low molecular weight range (see Figure S3), thus confirming that this is an inherent property of the surfactant. Geffroy et al. noticed similar observations in a study of the kinetics and reversibility of nonionic surfactants onto polystyrene by reflectometry which they ascribed to the nature of surfactant−surfactant interactions.46,47 Following this we also observed that after additional adsorption−desorption cycles on the same substrate the signal always returned to the same value (see Figure S4), regardless of the concentration used in the cycles. This suggested that the residual surfactant was very strongly bound and that at low concentrations the Langmuir isotherm may not be representative of the concentration dependence of adsorption. The surface concentration at which this occurs is θ ≈ 0.15. From the determined value of Γ∞ this corresponds to a concentration at which each molecule of

Figure 5. Change in conformation of Triton X-305 from mushroom to brush upon increasing surface concentration.

shown for tethered PEG44 chains on surfaces to occur at surface coverages of 5−10% and is in general agreement with results here for the shorter PEG30 chains.48 The net result of this is that at surface coverages lower than θ = 0.15 the adsorption of Triton X-305 molecules will have less steric hindrance and hence the rate of desorption will be significantly lower resulting in a higher equilibrium constant. The data were evaluated on the basis of a nonequilibrium mechanism that considered the diffusion of the surfactant through the boundary layer and reversible adsorption that was described through the Langmuir model. 9057

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d(Γ/Γ∞) ⎛ θ⎞ dθ = = ka⎜[S]surf (1 − θ ) − ⎟ ⎝ dt dt K⎠

This differs from the results reported by Geffroy et al. for nonionic surfactants with short ethylene oxide lengths.47 It is worth highlighting the differences between the kinetic effects observed with QCM-D technique and the equilibrium reported in the competitive adsorption of ionic and nonionic surfactants.8 With the help of the model, the adsorption desorption behavior for the two model surfactants can be explained as follows. In the adsorption process, the rate is largely determined by the kinetics of adsorption onto the surface. For SDS, the value of the rate constant for adsorption is low, ka = 0.0016 m3 mol s−1, and despite the fact that the concentration at which the surfactant is used is typically higher than that of nonionic surfactants (bulk concentration at CMC= 8.2 mol m−3), the initial rate of adsorption, ka[S]surf, is still relatively low. Conversely, for Triton X-305, the rate constant of adsorption is 2 orders of magnitude higher than for SDS (ka = 0.21 m3 mol s−1), and although the concentration used (bulk concentration at CMC= 1.3 mol m−3) is typically lower, the rate of adsorption, ka[S]surf, is significantly faster. The desorption rates of both surfactants is determined by the product of the value of the desorption rate coefficient and the surface concentration of adsorbed emulsifier, Γ. In this case, for SDS both the rate constant for desorption, kd (kd = ka/K = 0.013 s−1) and the surface concentration (at the CMC Γ = 2.8 × 10−6 mol m−2) are higher than that of Triton X-305 (kd = 7 × 10−3 s−1 and at the CMC Γ = 2 × 10−6 mol m−2). This results in the rate of desorption for SDS being significantly faster than for Triton X-305. These arguments are shown schematically in Figure 6.

(2)

where ka is the rate constant for adsorption, [S]surf is the surfactant concentration close to the surface (subsurface), and θ = Γ/Γ∞ is the surface coverage fraction. The transport flux of surfactant, j, through the boundary layer was described using a mass transfer coefficient, ktr. j = k tr([S] − [S]surf )

(3)

Since the flux to the surface must be equal to the rate of change at the surface (dΓ/dt = j), this equation can be used to yield the concentration of surfactant in the subsurface as a function of the surface coverage. [S]surf =

[S] +

Daθ K

1 + Da(1 − θ )

(4)

where Da is the Damkohler number, a dimensionless ratio of the adsorption rate to the rate of diffusion (Da = kaΓ∞/ktr). Equations 2 and 4 represent the dynamics of the system. A limiting case that has often been used to analyze the dynamics of adsorption−desorption of surfactants is based on the assumption that the diffusion through the boundary layer is the rate-determining step, and therefore equilibrium at the interface is reached.47 This reduces eq 4 to [S]surf =

θ K (1 − θ )

(5)

and the system is described by eqs 3 and 5. These equations contain a single unknown parameter, ktr, as the parameters K and Γ∞ were estimated from the equilibrium conditions as shown in Table 2. In addition, the dramatic change in the intermolecular interactions occurring at θ ≈ 0.15 for Triton X305 was incorporated into the model by including a simple concentration dependence of the equilibrium constant, K, such that K (θ < 0.15) = 100K (θ > 0.15). The single unknown, ktr, was estimated by fitting the experimental data in Figures 3 and 4. It was found that whereas the adsorption−desorption curves of SDS were well fitted with a single value of ktr of 1.03 × 10−8 m s−1, the desorption of Triton X-305 could not be fitted to this model (see Figures S1 and S2). Therefore, the complete model (eqs 2 and 4) was used to fit the data of Triton X-305 using ktr and ka as estimatable parameters. Using the mixed diffusion and kinetic model, it was observed that the ktr value for TritonX-305 (ktr = 2.4 × 10−6 m s−1) was 2 orders of magnitude higher than that of SDS modeled purely on the basis of diffusion limited kinetics (ktr = 1.03 × 10−8 m s−1). This difference is seemingly unexplainable, especially in light of the fact that the diffusion coefficient of SDS is higher than that of Triton X-305, and therefore a higher value of ktr would be expected for SDS compared to Triton X-305. In addition, assuming a diffusion coefficient (D) of 1 × 10‑10 m2 s−1, the depletion layer thickness (δ) would need to be on the order of 1 cm to yield a value of 1 × 10−8 m s−1 for ktr = D/δ. On this basis, we assumed the dynamics of adsorption/desorption to be described by the full model (eqs 2 and 4) for both surfactants. Figures 3 and 4 show that a good fit was achieved using this model for the two surfactants. In addition, it was found that around the reported value of ktr fitting was relatively insensitive to the actual value of ktr, which shows that the rate-determining step is the adsorption/desorption kinetics from the surface.

Figure 6. Schematic highlighting the differences in adsorption profile between SDS and Triton X-305.

It is important to note that the kinetics of particle nucleation depend on surfactant migration to nanosized, near spherical particle nuclei rather than to bulk, planar, macroscopic interfaces. It has been suggested recently that as the radius of the droplet/particle phase decreases, a shift in mechanism from diffusion limited to kinetically limited occurs.49−53 In the experiments described here we observed very limited effect of diffusion on the kinetics of surfactant adsorption, and as such the general description of the surfactant kinetics should be equally applicable to particle nuclei. The previous assumption of many authors that slow diffusion of moderate molecular weight surfactants can influence the rate of nucleation in heterogeneous polymerization is seemingly erroneous in light 9058

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Figure 7. (Left) DLS results for seeded emulsion polymerizations conducted with either SDS (runs 1−4) or Triton X-305 (runs 5−8) as surfactant. (Right) Ratio of theoretical diameter calculated assuming a constant number of particles with diameter obtained from experimental results by DLS at varying surfactant concentration in the polymerization recipe for SDS (filled squares) and Triton X-305 (stars).

Figure 8. (Left) SEM image of seeded emulsion polymerization of poly(methyl methacrylate) using SDS as emulsifier (6.5 × 10−3 M). (Right) SEM image of seeded emulsion polymerization of poly(methyl methacrylate) using Triton X-305 as emulsifier (6 × 10−3 M).

Figure 9. (Left) DLS results for seeded emulsion polymerizations conducted with either SDS or Triton X-305 as surfactant with 10 g of monomer added (reactions 11 and 14 in Table 1). (Right) Ratio of theoretical diameter calculated assuming a constant number of particles with diameter obtained from experimental results by DLS at varying monomer concentrations in the polymerization recipe for SDS (filled squares) and Triton X305 (stars).

of this evidence. The results suggest that the low monomeric concentration and slow desorption from preexisting interfaces of Triton X-305, and nonionic surfactants in general, would be responsible for the low rates of nucleation in (mini)emulsion polymerizations when solely nonionic emulsifiers are employed. Emulsion Polymerization. In order to confirm the impact of surfactant kinetics in (mini)emulsion systems, we conducted a series of seeded emulsion polymerizations. The principal behind the experiments was that if, as the QCM results suggested, desorption of the nonionic surfactant from the

polymer surface was the rate-limiting step in stabilization of primary particles, then polymerization of a surfactant covered seed latex should lead to increased secondary nucleation for SDS, which desorbs quickly compared to Triton X-305. In the first set of experiments, the seed latex was swollen with monomer in the presence of varying amounts of surfactant corresponding to various degrees of surface coverage. In all cases the concentration of surfactant was below the CMC when surface adsorption was taken into account such that micellar nucleation can be discounted. After swelling the latex overnight 9059

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Figure 10. (Left) SEM image of seeded emulsion polymerization of poly(methyl methacrylate) using SDS as emulsifier with 10 g of MMA added (reaction 11 in Table 1). (Right) SEM image of seeded emulsion polymerization of poly(methyl methacrylate) using Triton X-305 as emulsifier with 10 g of MMA added (reaction 14 in Table 1).

surface area. The observation of increased secondary nucleation at lower monomer concentrations in these experiments can be the expected result where the limiting factor in secondary nucleation is the stabilization of primary particles formed by homogeneous nucleation. At lower monomer content where the surface coverage of the swollen latex was high (around 80% coverage) increased secondary nucleation may be expected as the low surface area results in a higher concentration of surfactant in the aqueous phase. The significantly lower extent of secondary nucleation in Triton X-305 stabilized latexes can be attributed to the combination of lower aqueous phase concentration and slow dynamics of redistribution from the latex particle surface to newly formed particles. As shown from the dynamics in the QCM this is directly correlated to the slow rate of desorption from the latex surface. It is worth pointing out that the process can be designed in such a way that the need of surfactant mobility is not that important. This is the case of batch miniemulsion polymerization in which the reaction conditions are adjusted to nucleate most of the droplets. However, considering that for batch miniemulsion polymerization one-to-one copy is more an exception than a rule, and that in semicontinuous miniemulsion polymerization one-to-one copy does never occur,55 the findings reported in the present article will be of importance for most miniemulsion polymerization processes.

the reaction mixture was heated and left to polymerize for 4 h. The extent of secondary nucleation in each case could be determined from the change in diameter of the particles. Figure 7 shows that as surfactant concentration increased there was little evidence of secondary nucleation where Triton X-305 was used as surfactant. This can be expected as for Triton X-305 surfactant desorption from the latex surface is slow and the concentration in the aqueous phase is low, so primary particles occurring from homogeneous nucleation that will lead to a new crop of particles will not be stabilized in this case. Alternatively, where SDS was used as surfactant, desorption from the latex is faster and the concentration in the aqueous phase higher so the extent to which primary particles are stabilized is greater leading to increased secondary nucleation. This is particularly notable at higher concentrations as can be expected since the flux of surfactant toward these primary particles will be higher, and hence their stability increased. Scanning electron microscopy studies showed clearly that the change in size was due to secondary nucleation and the newly nucleated particles can be easily differentiated from the initial monodisperse crop of large, monodisperse particles from the seed (see Figure 8). A second set of experiments where the surfactant concentration was kept constant but the monomer concentration was varied was conducted to further support our hypothesis. In the case of SDS stabilized latexes, the extent of secondary nucleation increased dramatically with increasing monomer content (see Figure 9). This is the expected case where the increase in secondary nucleation is not limited by the stabilization of the primary particles by adsorption of surfactant molecules but by the kinetics of primary particle formation itself. As monomer content increases, the rate of growth of oligomers in the aqueous phase is increased, resulting in increased number of primary particles being formed than in turn lead to a new batch of particles.54 In this case, the SEM revealed two distinct crops of particles where SDS was used as surfactant (see Figure 10). For the Triton stabilized latex we observed much less secondary nucleation, mainly at lower monomer concentrations. It should be noted that the increased amount of secondary nucleation in observed in comparison to similar experiments using Triton X-305 in runs 5−8 is due to the lower number of particles (larger size of the seed) used in this set of experiments. The lower number of particles promotes secondary nucleation due to lower rate of capture of oligoradicals from aqueous solution caused by the lower total

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CONCLUSION A quartz crystal microbalance with dissipation monitoring has been used to determine the equilibrium and dynamic adsorption characteristics of a model ionic surfactant, SDS, and a model nonionic surfactant, Triton X-305, onto a poly(methyl methacrylate) surface. It has been shown that SDS has a significantly lower equilibrium constant than Triton X-305, leading to much higher aqueous phase concentrations. In addition, analysis of the kinetics of surfactant adsorption showed that contrary to the common belief, adsorption is not limited by the low diffusion coefficient of the bulky nonionic surfactants. We have shown that the adsorption is faster and desorption slower for nonionic surfactants due to a combination of both the large difference in concentration gradient between the bulk and the concentration near the surface caused by higher equilibrium constant and the slow desorption of surfactant from the polymer/water interface. Conversely, SDS, having a low equilibrium constant and low rate constant for adsorption, is slow to adsorb but desorbs from 9060

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the surface rapidly. Finally, we conducted a series of seeded emulsion polymerizations using these surfactants and confirmed that the high concentration in the aqueous phase and the rapid desorption for SDS leads to significantly higher secondary nucleation. Using this knowledge, it should be increasingly possible to control particle size and size distributions in emulsion and miniemulsion polymerization simply through the use of appropriate surfactants.

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ASSOCIATED CONTENT

* Supporting Information S

Diffusion-dependent fits to rate data from QCM, MALDI TOF spectra of the nonionic surfactant Triton X-305, and sequential adsorb/desorb curves. 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] (J.M.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Diputación Foral de Gipuzkoa, University of Basque Country (UFI 11/56), Basque Government (GVIT373-10 and Etortek Nanoiker IE11-304), and Ministerio de Economia y Competitividad (CTQ2011-25572) are gratefully acknowledged for their financial support.

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REFERENCES

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