Adsorption of Sterically Stabilized Latex Particles at Liquid Surfaces

Apr 16, 2012 - Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.. •S Supporting Information. ABS...
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Adsorption of Sterically Stabilized Latex Particles at Liquid Surfaces: Effects of Steric Stabilizer Surface Coverage, Particle Size, and Chain Length on Particle Wettability K. M. Reed, J. Borovicka, T. S. Horozov,* and V. N. Paunov* Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull, Humberside HU6 7RX, U.K.

K. L. Thompson, A. Walsh, and S. P. Armes Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K. S Supporting Information *

ABSTRACT: A series of five near-monodisperse sterically stabilized polystyrene (PS) latexes were synthesized using three well-defined poly(glycerol monomethacrylate) (PGMA) macromonomers with mean degrees of polymerization (DP) of 30, 50, or 70. The surface coverage and grafting density of the PGMA chains on the particle surface were determined using XPS and 1H NMR spectroscopy, respectively. The wettability of individual latex particles adsorbed at the air−water and ndodecane−water interfaces was studied using both the gel trapping technique and the film calliper method. The particle equilibrium contact angle at both interfaces is relatively insensitive to the mean DP of the PGMA stabilizer chains. For a fixed stabilizer DP of 30, particle contact angles were only weakly dependent on the particle size. The results are consistent with a model of compact hydrated layers of PGMA stabilizer chains at the particle surface over a wide range of grafting densities. Our approach could be utilized for studying the adsorption behavior of a broader range of sterically stabilized inorganic and polymeric particles of practical importance.

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INTRODUCTION Microscopic polymer particles or latexes can be readily synthesized by various techniques such as emulsion polymerization or dispersion polymerization.1,2 Depending on the formulation, the mean particle diameter can be varied from around 25 nm to 20 μm.3,4 Latexes can be used for a diverse range of applications, including cell separation, pressuresensitive adhesives, binders for paint formulations, cement and concrete additives, photonic devices, and synthetic mimics for cosmic dust.5−10 Steric stabilization is a powerful generic mechanism for conferring colloidal stability on latex particles.11 It is conferred by an adsorbed layer of solvated polymer chains (the so-called “steric stabilizer”) and is effective in both polar and nonpolar solvents. Depending on the nature of the latex and the continuous phase, suitable steric stabilizers can be homopolymers,12 statistical copolymers,13 block copolymers,14 or macromonomers.15 Strong adsorption to the latex surface is a prerequisite for steric stabilization. This can be achieved simply by physical adsorption via hydrogen bonding,16 electrostatics,17 or hydrophobic interactions14 or by chemical grafting of the stabilizer chains.15,18 In principle, the latex surface can be easily tailored via the incorporation of functional groups to confer interfacial activity. For example, latexes can be designed that adsorb strongly at the air−water interface and hence stabilize bubbles or foams.19−23 © 2012 American Chemical Society

Similarly, pH-responsive steric stabilizers enable the preparation of latexes for the production of stimulus-responsive liquid marbles.24,25 The nature of the steric stabilizer can also be important in determining latex performance as a Pickering emulsifier.26−28 The recent development of living radical polymerization techniques now enables the rational design and convenient synthesis of a wide range of well-defined hydrophilic methacrylic macromonomers.29−35 It has been established that the interfacial activity of colloidal particles is dictated primarily by their surface wettability quantified by the particle contact angle.36 Various techniques have been developed to determine this parameter, including the colloidal probe technique,37 the gel-trapping technique (GTT),38,39 the film calliper method (FCM),40 and ellipsometry studies of adsorbed particle monolayers.41−43 However, in most cases such measurements have been confined to either inorganic oxide particles such as silica or charge-stabilized latexes. Indeed, we are only aware of a single report describing contact angle determination for a sterically stabilized latex, and this ellipsometric approach appears to be limited to relatively small particles of around 120 nm diameter.43 Received: February 20, 2012 Revised: March 30, 2012 Published: April 16, 2012 7291

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Table 1. Summary of PGMAn−PS Latexes Prepared by Alcoholic Dispersion Polymerization at 70 °C Using 1.0 wt % AIBN Initiator Based on Styrene Monomera entry no. 1 2 3 4 5 a

sample ID

stabilizer type

methanol:water mixture v/v %

stabilizer concn (wt %)

monomer conv. (%)

solids content after purification (%)

DP70/800 DP50/800 DP30/800 DP30/1100 DP30/700

PGMA70 PGMA50 PGMA30 PGMA30 PGMA30

90:10 90:10 98:2 98:2 90:10

2.5 10 10 5 10

61 100 60 82 100

4.28 9.60 4.87 4.06 8.63

DCP diam. (nm) 796 834 820 1063 679

± ± ± ± ±

87 65 90 84 79

stabilizer content by 1H NMR (%)

Γb (mg m−2)

PGMA surface coverage by XPS (%)

1.1 1.1 1.3 0.8 0.9

1.5 1.6 1.9 1.5 1.1

15 30 37 34 29

The stabilizer wt % is based on styrene monomer. bCalculated from 1H NMR data. For details see the Supporting Information.

(FCM), we also study the dynamic behavior of these latexes adsorbed at the air−water and oil−water interfaces.

Charge-stabilized latexes usually result in the formation of water-in-oil emulsions, which suggests particle contact angles greater than 90°. For example, Cayre and Paunov38,39a reported contact angles exceeding 100° for sulfate PS latex particles adsorbed at the n-decane−water interface. Similarly large contact angles (also greater than 90°) were reported by Ashby et al.44 for similar latexes that bridged oil films between two oil−water interfaces. In contrast, use of sterically stabilized latexes as Pickering emulsifiers usually produces oil-in-water emulsions, which suggests relatively hydrophilic character. Sterically stabilized latexes may exhibit complex adsorption behavior at liquid surfaces due to the response of the grafted polymer chains of the stabilizer when transferring from the water into the nonpolar phase. One would expect that strongly hydrated polymer chains on the particle surface could form a water-rich film even if this surface is in contact with air or oil phases. The presence of such strongly hydrated layers could render the latex particle hydrophilic although the bare PS surface may have a much larger three-phase contact angle. This hypothesis is borne out in the present study. Herein we examine a series of five sterically stabilized polystyrene latexes which have already been demonstrated to adsorb efficiently at the oil−water interface.45−47 These particles have mean diameters ranging from around 600 to 1100 nm and possess relatively narrow size distributions; moreover, they were prepared using near-monodisperse poly(glycerol monomethacrylate) macromonomers (PGMA) to ensure efficient chemical grafting of the steric stabilizer. We restrict our study to PGMA stabilizer chain lengths and surface coverages which provide exceptional stability to the aqueous suspensions of these latex particles even at high electrolyte concentrations.35 The latter is very important for practical applications and could be achieved at relatively high grafting densities of the steric stabilizer. The presence of strongly hydrated dense layer of polymer chains on the particle surface would imply that the contact angle of these latex particles is close to zero. This contradicts our previous findings that such particles act as a very efficient stabilizer of Pickering emulsions;45−47 hence, they should have finite contact angles at the oil−water interface. This apparent discrepancy can be resolved by assessing the effect of the stabilizer grafting density and chain length on the contact angle of these latex samples. This is the main focus of the present work. The wettability of these latexes is determined using two complementary methods38,40 after their adsorption at the air−water or oil− water interface. We investigate how the chain length of the grafted stabilizer chains and their surface coverage affects the equilibrium particle contact angle using the gel trapping technique (GTT).38,39,48 Using the film calliper method40

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EXPERIMENTAL SECTION

Materials and Methods. PGMAn Macromonomer Synthesis. Three well-defined near-monodisperse styrene-functionalized poly(glycerol monomethacrylate)-based macromonomers (denoted PGMA30, PGMA50, and PGMA70, where the subscript indicates the mean degree of polymerization in each case) were prepared as reported previously by Thompson and coworkers.35 PGMA−PS Latex Preparation by Dispersion Polymerization. PGMAn macromonomer (0.125−0.500 g) was weighed into a 100 mL three-neck round-bottomed flask fitted with a condenser, magnetic stirrer bar, and a nitrogen inlet and dissolved in the appropriate methanol−water mixture (44.5−44.9 g, see Table 1). This solution was purged with nitrogen for 30 min at 20 °C before being heated to 70 °C under a nitrogen blanket. AIBN initiator (0.050 g) was dissolved in styrene (5.00 g) and injected into the reaction vessel. The reaction mixture, which turned milky within 1 h, was stirred at 250 rpm for 24 h at 70 °C. The resulting latex was purified by three centrifugation/ redispersion cycles with each supernatant being replaced with methanol, followed by a further three cycles replacing each supernatant with deionized water. Disk Centrifuge Photosedimentometry. Weight-average diameters (Dw) of the PGMAn−PS latexes were determined using a CPS disk centrifuge photosedimentometer (model DC 24000). Samples (0.10 mL) of around 0.1% concentration were injected in an aqueous spin fluid comprising a sucrose gradient (2−8% sucrose, 15 mL). The density of the polystyrene latex particles was taken to be 1.05 g cm−3. This is a reasonable assumption for latex diameters of around 0.60−1.0 μm, since the thickness of the steric stabilizer layer is negligible compared to the mean particle diameter. Scanning Electron Microscopy. Images were obtained using a FEI Inspect instrument operating at 20 kV. All samples were dried onto aluminum stubs and sputter-coated with a thin overlayer of gold prior to inspection to prevent sample-charging effects. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were acquired using a Kratos Axis ULTRA “DLD” X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) and operating at an approximate chamber pressure of 5 × 10−8 mbar. The monochromator is set at 30° (taking the horizontal sample surface as 0°), and the analyzer is positioned normal to the sample surface. Latexes were dried from their aqueous dispersions onto indium foil prior to XPS measurements. The surface coverage of the latex particles by the steric stabilizer chains was estimated by comparing the deconvoluted CO signal at 290 eV obtained for the PGMA−PS latexes to that observed for the PGMA macromonomer reference. Peak deconvolution was conducted using Casa XPS software version 2.3.15 provided by the instrument manufacturer (Kratos). 1 H NMR Spectroscopy. All 1H NMR spectra were recorded in d5pyridine using a 400 MHz Bruker Avance-400 spectrometer. The peaks originating from the PGMA stabilizer chains at δ in the range 5.1−5.9 ppm were integrated and compared to those from the polystyrene latex core at δ in the range 7.5−8.2 ppm in order to 7292

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calculate the latex stabilizer content (see Figure S1 in the Supporting Information). Gel Trapping Technique (GTT). We used n-dodecane (from Sigma) as an oil phase after repeated chromatographic purification using an alumina column. The gelling agent used was gellan (Kelcogel, CP Kelco), which was purified as described previously.38 Ethylenediaminethetraacetic acid disodium salt (EDTA) and methanol (99.9%) were purchased from Sigma. Sylgard 184 curable silicone elastomer (PDMS) was purchased from Dow Corning. A 2.0 wt % aqueous solution of gellan was prepared as described in ref 38. Methanol−water mixture (50:50 by mass) was used as a spreading solvent to deliver the particles both at the air−water and the n-dodecane−water interface. This protocol differs slightly from the original GTT method (which uses isopropanol as a spreading solvent38,39,48) so as to minimize any possible extraction of oligomers or low molecular weight polymers from the latex particle. The spreading of particles at the air−water surface was achieved by injecting a 10 μL aliquot of 0.50 wt % latex dispersion in the spreading solvent at the surface of the hot liquid gellan solution at 55 °C, followed by cooling to 25 °C to induce gelation. A Petri dish containing this gel layer was sealed in order to prevent evaporation of water from the gel surface. Sylgard 184 curable silicone elastomer (PDMS) was used to mold the liquid interface after 30 min. For the air−water interface, this was done directly after setting the gel. For the n-dodecane−water interface, the hot gellan solution was coated with the prewarmed n-dodecane phase (55 °C), and the particle spreading and gelling of the aqueous phase was achieved as for the air−water surface, after which the n-dodecane phase was replaced with curable PDMS (1:10 Sylgard 184:cross-linker). After curing for 48 h at room temperature, the solid PDMS layer with the trapped latex particles was peeled off the aqueous gel and washed in a hot 10 mM aqueous EDTA solution and hot Milli-Q water at 80 °C to remove any gel residues from the PDMS surface. The PDMS-particle samples were prepared for imaging with SEM by coating with a carbon nanolayer (∼10 nm) using an Edwards high vacuum evaporator. Film Calliper Method (FCM). This technique has been recently developed for measuring the contact angle of individual micrometer and submicrometer particles in their natural environment in real time.40 This method exploits the behavior of bridging particles attached simultaneously to both surfaces of free-standing water films. Initially, the particles are spread at the air−water or oil−water interface in a cuvette partially filled with water using a spreading suspension of 1 wt % particles in a 50:50 methanol−water mixture. A thick water film with particles attached to its surfaces (Figure 1a) is formed by crossing the particle monolayer at the liquid interface with a glass ring initially immersed in water. Then the film is forced to thin by removing water out of the film meniscus using a syringe connected to the ring.40 As a result, some of the particles become attached to both film surfaces, thus bridging the film. Immediately after bridging, the particle is in a thicker film region and the film surface around the particle is deformed in order to match its contact angle (Figure 1b). This is an unstable high surface free energy configuration due to the excess interfacial area of the deformed film surface. Therefore, the bridging particle spontaneously moves into a thinner region of the film so as to reduce the deformation and hence minimize the surface free energy (Figure 1c). The contact angle, θ, is calculated using the formula cos θ = he/d, where he is the film thickness at the bridging particle location and d is the particle diameter (Figure 1c). The particle bridging of the film surfaces is accompanied by sliding of the three-phase contact line on the particle surface: when it moves to attain its final location in the film, the particle three-phase contact line is in its receding mode. Thus, it is expected that a receding contact angle is determined. For particles with a smooth and homogeneous surface, this should provide contact angles practically coincident with the equilibrium contact angle. However, for particles with significant surface roughness or a chemically inhomogeneous interface this may lead to the measurement of a contact angle lower than the equilibrium value due to hysteresis effects.

Figure 1. Schematic representation of the consecutive stages in the bridging of a water film in air (oil) by a spherical particle during FCM measurements: (a) The particle is only attached to one of the film surfaces before bridging. (b) Just after bridging, the water−air (oil) interface around the particle is deformed in order to match the particle contact angle. (c) The bridging particle moves in a thinner film region to reduce deformation and minimize the surface free energy. Note that the three-phase contact line shrinks (a, b) and expands (b, c) during the bridging process.

3. RESULTS AND DISCUSSION 3.1. Latex Synthesis and Characterization. The PGMAn−PS latex particles were prepared via a dispersion polymerization protocol using various methanol−water mixtures. The mean degree of polymerization (DP) of the PGMA macromonomer was either 30, 50, or 70; it behaves as a reactive steric stabilizer in such syntheses and becomes covalently grafted to the polystyrene surface via its pendent polymerizable styrene group. The stabilizer type, stabilizer concentration, and water content were varied in order to produce PGMAn−PS latexes with appropriate mean diameters (see Table 1). Very high monomer conversions were achieved for two latex syntheses, whereas somewhat lower yields were obtained for the remaining three formulations. Entries 1, 2, and 3 in Table 1 are examples of PGMAn−PS latexes of comparable diameters (∼800 nm) but with varying stabilizer chain lengths. Thus, this miniseries allows the effect of PGMA stabilizer DP on the latex contact angle to be studied. Entries 3, 4, and 5 in Table 1 comprise a second latex miniseries with a varying mean latex diameter but the same stabilizer DP of 30. Although the surface coating of these latex samples is exactly the same (PGMA30), the data in Table 1 indicate variations in surface coverage and grafting density. The interfacial energy of the latex surface with the grafted PDMA layer in contact with water depends on these parameters, so one may expect some effect of particle size on the contact angle due to these variations. All five latexes were determined to be near-monodisperse (i.e., with a coefficient of variation below 12% in each case) as judged by disk centrifuge photosedimentometry analysis. Similarly, scanning electron microscopy images of the PGMAn−PS latexes indicate that 7293

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Figure 2. Scanning electron microscopy images recorded for the five PGMAn−PS latexes used in this work. Each latex was prepared by alcoholic dispersion polymerization at 70 °C, and the final particle diameter was controlled by adjusting the methanol−water mixture used in the formulation.

polystyrene control sample. The carbonyl signal is clearly evident in both the latex and macromonomer spectra, indicating that the PGMA chains are located at the surface of the latex particles, as expected. In contrast, no carbonyl feature is observed for a macroscopic precipitate of polystyrene prepared in the absence of any macromonomer. The PGMA surface coverage determined by XPS increases from 15% to 37% as the mean DP of the macromonomer is reduced (entries 1−3 in Table 1). Its variation is smaller (29−37%) for the latexes with the same DP of 30, but different diameters (entries 3−5 in Table 1). To obtain information regarding the surface density of the PGMA chains and deduce the structure of the grafted steric stabilizer layer, we use the PGMA grafted amount per unit area, Γ (mg m−2), and the molecular weight (MW) of PGMAn macromonomer calculated from its chemical structure (see Figure 4). The MW, average number density of PGMA

each latex particle has a well-defined spherical morphology and a relatively narrow size distribution (see Figure 2). The approximate PGMA stabilizer content and hence its grafted amount per unit particle area (Γ) can be estimated by 1 H NMR analysis of these linear latexes dissolved in d5pyridine, which is a good solvent for both the PGMA chains and the polystyrene core (see Table 1 and Supporting Information). These 1H NMR results were complemented by X-ray photoelectron spectroscopy (XPS) measurements to assess the latex surface compositions, since the latter technique is highly surface-specific. The PGMA surface coverage of the latex particles was estimated by inspecting the C 1s core-line spectra: the ester carbonyl intensity observed for the PGMA− PS latexes was compared to that obtained for the PGMA macromonomer reference (see Table 1). Representative C 1s core-line spectra for both the PGMA50 macromonomer and the 834 nm PGMA50−PS latex are shown in Figure 3 along with a

Figure 4. Structure of the PGMAn macromonomer used as a steric stabilizer of PGMAn−PS latexes.35 The molecular weight (MW) of a single PGMA unit is 160 g mol−1, whereas that of the anchoring (end) group is 307 g mol−1. The cross-sectional radius of the macromonomer chain, r (the distance from the backbone center to the end of the side group including the terminal −OH group), is estimated to be ∼0.72 nm, assuming bond lengths of 0.14 nm and bond angles of 109°.

chains at the latex surface, Γn = (Γ × 10−3)NA/MW (NA is the Avogadro number), and average area per grafted chain, An = 1/ Γn, are summarized in Table 2. The coverage of PGMA chains at the PS latex surface, S = Ac/An, can be calculated using An and the cross-sectional area of one PGMA chain, Ac = πr2, if the cross-sectional radius of the macromonomer chain, r, is known. The latter can be estimated from the length of the side group (Figure 4). Assuming bond lengths of 0.14 nm and bond angles of 109° in the side group, we obtain r ≈ 0.72 nm and Ac ≈

Figure 3. XPS core-line C 1s spectra recorded for (a) PGMA50 macromonomer, (b) 834 nm PGMA50−PS latex prepared using a 9:1 methanol−water mixture at 70 °C, and (c) polystyrene precipitate prepared in the absence of any macromonomer. There is clear evidence for both C−O and CO species in the first two spectra, which confirms the presence of the PGMA chains at the surface of the latex. Similar results were obtained for the other latexes used in this study. 7294

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Table 2. Estimated Characteristics of the PGMA Steric Stabilizera no. 1 2 3 4 5

sample ID DP70/800 DP50/800 DP30/800 DP30/1100 DP30/700

stabilizer type PGMA70 PGMA50 PGMA30 PGMA30 PGMA30

MWb (g mol−1)

Γnc (m−2)

And (nm2)

S = Ac/Ane (%)

hcf (nm)

lg (nm)

Rgh (nm)

Σi

11200 8000 4800 4800 4800

× × × × ×

12.4 8.3 4.2 5.3 7.2

13 20 39 31 23

16.0 11.4 6.8 6.8 6.8

3.8 3.1 2.2 2.5 2.9

4.3 3.5 2.6 2.6 2.6

4.7 4.7 5.0 3.9 2.9

0.8 1.2 2.4 1.9 1.4

17

10 1017 1017 1017 1017

a

The contribution of the anchoring group (a few %) has been neglected. bThe molecular weight of PGMAn chain. cNumber of chains per unit area. Area per grafted chain. eThe PGMA surface coverage. fThe length of a fully extended PGMA chain. gThe average distance between centers of two neighboring chains. hRadius of gyration in water. iReduced grafting density.

d

1.63 nm2. The calculated PGMA surface coverage, S, is shown in Table 2. This parameter is smallest for latex DP70/800 (entry 1) and biggest for latex DP30/800 (entry 3) and follows the general trend observed for the XPS surface coverage data (cf. Table 1). Hence, the experimental data for the grafted amount of PGMA per unit area, Γ, determined by NMR and the PGMA surface coverage measured by XPS are selfconsistent. The length of the fully extended PGMA chains, hc, can be estimated by again assuming bond lengths of 0.14 nm and bond angles of 109° in the macromonomer backbone (Figure 4). It varies from ∼16 nm at a DP of 70 to ∼6.8 nm at a DP of 30 (Table 2). The average distance between the grafted chains, l, can be estimated from Γn assuming uniformly distributed PGMA chains at the latex surface in a hexagonal arrangement. In this case, each molecule occupies a hexagon with area An = Ahexagon = 2b2√3, b being the radius of a circle inscribed in the hexagon. Since in our notations l = 2b and Γn = 1/An, the average distance between the chain centers can be obtained by the formula l = [2/(Γn√3)]1/2. The estimated average distance between the PGMA chains, l, is also shown in Table 2. It is clear that the PGMA chain length is significantly longer (by a factor of ∼2.4−4.2) than the average lateral distance between two neighboring chains. The structure of a grafted polymer layer can be assessed by comparing the area that would be occupied by one polymer chain in a random coil conformation with radius of gyration Rg to the area per grafted chain, An, by using the so-called reduced grafting density,49 Σ = πRg2/An. Σ < 1 corresponds to a weakly interacting regime of separated random coils, while Σ > 1 corresponds to stretched polymer chains forming a polymer brush.49,50 The radius of gyration, Rg, of a polymer chain in good solvent (e.g., hydrophilic polymer in water) is Rg = aN3/5, where a is the length of the statistical segment and N is the number of segments in the chain.50 We estimate Rg of hydrophilic PGMAn chains in water assuming a = 0.6 nm,51 N = hc/a, and calculate the reduced grafting density, Σ (Table 2). The reduced grafting density of PGMAn chains, Σ, is significantly larger than unity for all PGMAn−PS latexes. Hence, the PS latex particles are covered with a dense layer of stretched hydrophilic PGMA chains acting as a very efficient steric stabilizer in aqueous media. This is consistent with our previous observation of excellent colloidal stability for similar PGMAn−PS latex dispersions in the presence of relatively high concentrations of added electrolyte (up to 0.5 M MgSO4).35 It might be expected that the PGMA chains would be only partially collapsed when exposed to nonpolar media (air or oil), thus covering the latex particle surface with a compact highly hydrated layer and making the particles hydrophilic. This hypothesis is extended in the next section where the particle contact angles at the air−water and dodecane−water interfaces are presented and discussed.

Adsorption of PGMAn-Stabilized PS Latex Particles at Liquid Surface. We studied the equilibrium three-phase contact angles of the PGMAn−PS latex particles at the air−water and the n-dodecane−water interface by using the GTT.38 Figure 5

Figure 5. SEM images of the PDMS surface with partially embedded PGMA−PS latex particles after their adsorption at the air−water (A/ W) and the n-dodecane−water (O/W) interface, respectively with the GTT. The latex properties are described in Table 1. All scale bars are 200 nm.

shows typical SEM images of PGMAn−PS latex particles protruding from the surface of the PDMS after being trapped at the air−water and n-dodecane−water surfaces. The particles appear to have significant surface roughness (see Supporting Information), and their position with respect to the PDMS surface is indicative for the particle wettability at the respective liquid surface. At least 10 SEM images of different particles were used to calculate the average particle contact angle. The results for the particle contact angles at the air−water and oil− water interface are shown in Figure 6 for PGMAn−PS latexes of similar diameters (∼800 nm) but different DP of the PGMA stabilizer, n, equal to 30, 50, and 70. The equilibrium particle contact angles measured by the GTT are rather insensitive to the variations in the DP. Their values for each interface are practically the same within the experimental error thus giving 7295

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Figure 6. Contact angles obtained for three PGMAn−PS latexes with similar diameters (∼800 nm, Table 1, entries 1−3) at (a) the air− water, θaw, and (b) the n-dodecane−water, θow, interfaces versus the degree of polymerization, n, of the grafted PGMA stabilizer chains. Triangles correspond to the equilibrium particle contact angles measured by the GTT. Circles correspond to the receding particle contact angles measured by the FCM.

Figure 7. Schematic representation of the PGMA-stabilized PS latex surface for three different degrees of polymerization of the stabilizer chains. (a) Stretched PGMA chains in water. (b) A dense layer of compacted hydrophilic stabilizer chains on the particle surface in contact with the oil or air phase. (c, d) Side view of the particles attached to the air−water and oil−water interface, respectively. The differences in the thickness of the hydrated stabilizer layers are exaggerated for clarity.

on average 73 ± 4° and 78 ± 4° at the air−water and ndodecane−water interface, respectively. The equilibrium contact angle values at the air−water interface are in good agreement with those reported for macroscopic planar silicon substrates grafted with a similar polymer, poly(glycidyl methacrylate), at high grafting densities (68 ± 3°).52 Our data confirm that the particle contact angle is lower than 90° for both the air−water and the n-dodecane−water interface. This observation is also consistent with the fact that the same PGMAn−PS latex forms oil-in-water Pickering emulsions.45 It has already been discussed in the literature that an increase of the grafting density of hydrophilic polymer chains on planar hydrophobic surfaces leads to an initial decrease of the contact angle of water droplets in air followed by a plateau at a finite contact angle above some critical grafting density.53 These authors53 use the self-consistent field theory of hydrophilic polymer brushes to demonstrate that their wetting characteristics are hardly influenced by the grafting density and chain length characterizing the polymer brush. Their main conclusion is that the finite contact angles should be expected in various systems in which bridging attraction contributes to the disjoining pressure in wetting films. In the case of PGMAstabilized latexes, this effect requires adsorption of some polymer segments at the liquid surface and favors the reduced thickness of the brush layer compared to its thickness at the particle−water interface (see Figure 7). The insensitivity of the particle contact angle to the degree of polymerization indicates that the hydrophilic PGMA chains are more likely to be compacted, thus forming a dense surface layer which limits the access of the nonpolar phase to the PS latex surface. This model of the particle surface layer is consistent with the estimated reduced grafting density of the PGMA chains, Σ > 1 (Table 2), as illustrated in Figure 7. With limited contact of the nonpolar phase to the latex surface, the wettability of the sterically

stabilized latex at the liquid interface is determined mainly by the interaction of the highly hydrated dense surface layer of PGMA chains (see Figure 7b−d) with the nonpolar phase (air or n-dodecane). The close values of the equilibrium particle contact angles of the PGMAn−PS latex particles at the air− water and the n-dodecane−water interface (θaw and θow) can be explained by considering the Young equations for the particle contact angle at the two liquid surfaces γaw cos θaw = γpa − γpw

(1)

γow cos θow = γpo − γpw

(2)

Here γaw and γow are the air−water and oil−water interfacial tensions; γpa, γpo, and γpw are the particle−air, particle−oil, and particle−water interfacial energies, respectively. Combining eqs 1 and 2 leads to the following difference in the interfacial tensions of the particle−air and particle−oil surface γpa − γpo = γaw cos θaw − γow cos θow

(3)

If one assumes that, because of the water-rich PGMA layer on the particle surface, the nonpolar phase (air or oil) does not contact the surface of the PS latex, the latter difference in eq 3 can be approximated by the difference between the air−water and the oil−water interfacial tensions γpa − γpo ≈ γaw − γow (4) Combining eqs 3 and 4 gives γ 1 − cos θow ≈ aw 1 − cos θaw γow 7296

(5)

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Given that γaw ≈ 72 mN m−1 and γaw ≈ 52 mN m−1, the simple model based on eq 5 predicts that, as θaw varies from 65° to 73°, θow varies only from 78° to 88°, which is in reasonable agreement with the GTT data within the experimental error. The latex surface roughness indicated in the SEM images shown in Figure 5 is expected to contribute to significant contact angle hysteresis due to pinning of the three-contact line at the particle surface (see also Supporting Information). In practice, this may lead to a measurable difference between the contact angle at the advancing and the receding three-phase contact line and also to a difference between the equilibrium particle contact angle and that measured under dynamic conditions. It is reasonable to assume that particle contact angles measured by the GTT38 are very close to equilibrium values because the spreading solvent creates significant agitation during the particle spreading and adsorption at the liquid interface which facilitates their equilibration. It has been shown that the FCM40 also gives particle contact angles close to equilibrium values for charge-stabilized sulfate latex particles with negligible surface roughness (see Figure S2 in the Supporting Information). For particles of significant surface roughness, one would expect that the contact angles measured by the FCM should be closer to the receding contact angle rather, than the equilibrium contact angle. This is because, although the particles are initially adsorbed at one of the film surfaces, their attachment to the other film surface (bridging) is accompanied by initial shrinking and further expansion of the three-phase contact line at the particle surface (see Figure 1). This may lead to pinning of the three-phase contact line and hence measuring lower nonequilibrium values of the particle contact angle. The data shown in Figure 6 suggest that there is indeed such an effect, especially for particles adsorbed the air−water interface. For PGMAstabilized PS latex particles adsorbed at the oil−water interface the effect is smaller, possibly due to the increased fluidity of the PGMA chains in contact with the n-dodecane phase. Figure 8 shows the particle contact angle for PGMA30−PS latexes of different mean particle diameters at a fixed degree of polymerization. One sees similar behavior for the receding contact angle compared to the equilibrium contact angle. There may be some evidence for a slight reduction of the equilibrium particle contact angle with the particle diameter at the air− water surface (Figure 8a). This might be due to variation in the particle surface roughness with the particle diameter. However, such a trend is not evident for the equilibrium contact angle at the n-dodecane−water interface (see Figure 8b). This apparently weak particle size dependence is interesting and warrants further investigation with a broader range of latex diameters. However, this is beyond the scope of the current study.

Figure 8. Contact angles of PGMA30−PS latex particles with different diameters (Table 1, entries 3−5) adsorbed at (a) the air−water, θaw, and (b) the n-dodecane−water, θow, interface. Triangles correspond to the equilibrium particle contact angles measured by the GTT. Circles correspond to the receding particle contact angles measured by the FCM.

particle contact angles at both the air−water and n-dodecane− water interfaces are less than 90° although the surface coverage of the hydrophobic PS core by the hydrophilic PGMAn chains lies in the range of 15−37%. This suggests that the PGMA stabilizer chains form compact but highly hydrated layers at the latex surface. There is some evidence for smaller particle contact angles with increasing particle diameter at a fixed stabilizer chain length, although corroboration of this effect may require further studies. The particle contact angle obtained under equilibrium and dynamic conditions show that sterically stabilized latexes exhibit significant contact angle hysteresis. The combination of GTT and FCM provides more detailed information for the particle contact angle and its hysteresis for sterically stabilized latexes adsorbed at the air−water and oil− water interfaces, thus providing useful insights regarding the structure of the steric stabilizer layer at the particle surface under both equilibrium and dynamic conditions. This approach could be utilized for studying the adsorption behavior of a broader range of sterically stabilized inorganic and polymeric particles of practical importance.

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CONCLUSIONS Sterically stabilized PGMAn-PS latex particles have been synthesized and their adsorption at both the air−water and ndodecane-water interfaces has been investigated. The effects of systematically varying the chain length of the steric stabilizer and the mean particle diameter on the latex wettability have been studied by measuring the equilibrium and the receding particle contact angles at the air−water and n-dodecane−water interfaces using the gel trapping technique and the film calliper method, respectively. It was found that the particle contact angles are insensitive to the degree of polymerization, n, of the grafted PGMAn chains in the range from n = 30 to 70. The

ASSOCIATED CONTENT

S Supporting Information *

Specimen calculation for the estimation of surface coverage from 1H NMR, representative 1H NMR spectra, and high resolution SEM images showing the surface roughness of sterically stabilized PGMAn−PS latex particles. 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] (T.S.H.); [email protected]. uk (V.N.P.). 7297

dx.doi.org/10.1021/la300735u | Langmuir 2012, 28, 7291−7298

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.P.A. thanks P & G (Newcastle, UK) for a CASE award to support an EPSRC PhD studentship for K.L.T. K.L.T. acknowledges The University of Sheffield for a University Doctoral Prize Fellowship. EPSRC is thanked for a PhD studentship for K.M.R. and for awarding an Advanced Research Fellowship to T.S.H.

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dx.doi.org/10.1021/la300735u | Langmuir 2012, 28, 7291−7298