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PEO-PPO-PEO Block Copolymers at the Emulsion Droplet-Water Interface Timothy J. Barnes and Clive A. Prestidge* Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received September 14, 1999. In Final Form: January 26, 2000 ζ potential measurements were used to determine the adsorbed layer thickness of poly(ethylene oxide)polypropylene oxide-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers at the surfaces of poly(dimethylsiloxane) emulsion droplets, silica particles, and polystyrene latex particles. The adsorbed layer thickness (δ) was shown to depend on the copolymer structure, the surface chemistry of the adsorbent, and the level of cross-linking within droplets. For a wide range of copolymers, δ values are in the range 2-20 nm and directly proportional to the PEO block length. For copolymers with PEO segment lengths > PPO segment lengths, the plateau δ values for PDMS, latex, and silica correspond to ∼35%, 20%, and 7% of the fully extended PEO segment length. Equivalent values for copolymers with PEO segment lengths < PPO segment lengths are significantly greater but follow the same dependence on the type of adsorbent. As the level of internal cross-linking within the PDMS droplets was increased, the interface became less deformable and the adsorbed copolymer layer thickness significantly decreased. Findings are discussed with respect to the adsorbed copolymer conformations, which result from different adsorption mechanisms and the influence of interfacial penetrability.
Introduction Nonionic copolymers are extensively used as stabilizers during emulsion preparation. The stabilizing performance of a copolymer is controlled by its conformation at the oil-water interface, which is in turn influenced by its functionality, molecular architecture, and weight. Numerous experimental techniques (e.g. small-angle neutron scattering,1 dynamic light scattering (DLS),2-6 nuclear magnetic resonance spectroscopy,1 infrared spectroscopy,1 and ellipsometry1,7) have characterized adsorbed copolymer conformations at the solid-liquid interface. However, these have rarely been successfully applied to liquidliquid interfaces or emulsion droplets. An obvious reason for this shortfall is that measurements are required on “bare” particles and, while this is practical for solid particles, it is impractical for emulsion droplets that coalesce in the absence of a stabilizer. In addition, for adsorbed copolymer layers of a few nanometers, hydrodynamic thickness measurements3,4 are insensitive for micrometer-sized particles, e.g. typical oil-in-water emulsions. With these thoughts in mind, the development of a relationship between copolymer characteristics, adsorbed conformation, and emulsion stability is in its infancy. Copolymers containing poly(ethylene oxide) (PEO) are of particular interest in emulsion stabilization and are the focus of our study here. Tadros8 estimated the adsorbed layer thickness of nonylphenol-PEO copolymers from shifts in the viscosity versus volume fraction profiles of concentrated paraffin oil emulsions. Adsorbed layer * Corresponding author. (1) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993; Chapter 6. (2) Killmann, E.; Maier, H.; Baker, J. A. Colloids Surf.s 1987, 31, 51. (3) Baker, J. A.; Berg, J. C. Langmuir 1988, 4, 1055. (4) Lee, J.; Martic, P. A.; Tan, J. S. J. Colloid Interface Sci. 1989, 131, 252. (5) Shar, J. A.; Obey, T. M.; Cosgrove, T. Colloids Surf., A 1998, 136, 21. (6) Baker, J. A.; Pearson, R. A.; Berg, J. C. Langmuir 1989, 5, 339. (7) Malmsten, M.; Linse, P.; Cosgrove, T. Macromolecules 1992, 25, 5, 2474. (8) Tadros, Th. F. Colloids Surf., A 1994, 91, 55.
thicknesses were reported in the range 5-11 nm and correlated strongly with PEO block lengths. The influence of the size of the anchoring block and the role of interfacial penetration were not addressed, however. Other relevant studies on PEO-based copolymers have generally been concerned with liquid-liquid interfaces rather than emulsion droplets. Neutron reflectivity9 of a PEO-PPOPEO triblock copolymer (F127) at the hexane-water interface showed that the PEO segments extended 9 nm (∼4.5 times the radius of gyration, Rg) into the water phase and the PPO segments 4 nm (∼2.5Rg) into the hexane phase. This study highlighted the importance of interfacial penetration in controlling adsorbed copolymer conformations at liquid-liquid interfaces. Interfacial rheology studies of PMMA-PEO copolymers at oil-water interfaces10 have exposed a strong correlation between the dilational elasticity, the interfacial activity, and emulsion stability. These findings were reconciled in terms of PEO chain extension into the water phase resulting in a strongly hydrated layer, while PMMA extension into the oil phase was dependent on the oil type (i.e. PMMA solvency). It is apparent that the penetrability of the liquid-liquid interface is significant in determining the structure of an adsorbed copolymer. Direct extrapolation of experimental findings obtained for solid-liquid interfaces to liquidliquid interfaces may not therefore be fully justified. The use of hydrophobic solid colloids (e.g. latex) as models for emulsion droplets, particularly when considering polymer adsorption phenomena, is questionable. It should also be noted that studies on planar oil-water interfaces9,10 may not be directly extrapolated to oil droplet-water interfaces, since droplet curvature may play an important role in controlling the adsorbed layer thickness.6 Further study is clearly warranted to fully develop the relationship between interfacial penetration, adsorbed copolymer conformation, and emulsion droplet stability. The aim of this study is to characterize PEO-PPOPEO copolymers at the interface of emulsion droplets in water. Poly(dimethylsiloxane) (PDMS) droplets were (9) Phipps, J.; Richardson, R.; Cosgrove, T.; Eaglesham, A. Langmuir 1993, 9, 3530. (10) Cardenas-Valera, A.; Bailey, A. Colloids Surf., A 1993, 79, 115.
10.1021/la991217d CCC: $19.00 © 2000 American Chemical Society Published on Web 03/31/2000
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Table 1. Composition, Molecular Weight (Mw), and Fully Extended PEO Length for PEO-PPO-PEO Copolymers19 fully extended composn approx molecular PEO segment copolymer (E ) PEO; P ) PPO) weight (g mol-1) length (nm) P84 P103 F77 F88 F108 F127
E19P39E19 E17P56E17 E52P35E52 E103P39E103 E132P56E132 E99P69E99
4 200 4 950 6 600 11 800 14 000 12 000
7.6 6.8 20.8 41.2 52.8 39.6
Table 2. Particle Size, Isoelectric Point, and Surface Area Data for PDMS Emulsion Batches, Silica Spheres, and Latex Particles
sample PDMS emulsion A (no cross-linking) PDMS emulsion B (50% TEMS; significant cross-linking) silica spheres polystyrene latex a
d(50) (µm)
surface isoelectric area point (iep) (m2 g-1)
0.80-0.90 0.80-0.90
∼3.0 ∼3.0
0.890 0.304
∼3.0 ∼2.0
7.0-9.0a 7.0-9.0a 6.44b 19.4a
Based on equivalent spherical diameter. b BET gas adsorption.
chosen since they can be prepared colloidally stable and in the absence of added stabilizer.11,12 The adsorbed layer thickness is estimated from ζ potential variation, due to movement of the plane of shear in the presence of the adsorbed nonionic polymer.13-18 The influence of PEO and PPO segment lengths on the adsorbed copolymer layer structure at the oil droplet-water interface is explored, as is its influence on emulsion stability. To further our understanding of the influence of interfacial penetration on adsorbed layer structure, we have undertaken complementary studies on silica spheres, latex particles, and PDMS emulsion droplets with various levels of internal cross-linking. Experimental Section Materials. High-purity water (Milli-Q) was used throughout and prepared by reverse osmosis, with two stages of mixed bed ion exchange, two stages of activated carbon, and a final stage involving 0.22 µm filtration. Diethoxydimethylsilane (DEDMS) and triethoxymethylsilane (TEMS) were supplied by Aldrich (Milwaukee, WI) and redistilled under nitrogen prior to use. Ammonia (Aldrich), KNO3 (Merck, Darmstadt, Germany), and other reagents used were analytical grade. Poly(ethylene oxide)polypropylene oxide-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers (Synperonics) were supplied by Orica Pty Ltd (Melbourne, Australia). Their molecular weights and structures are given in Table 1.19 PDMS in water emulsions were prepared through the basecatalyzed polymerization of DEDMS using a modified method of that reported by Obey et al.11 Aqueous solutions containing 2% DEDMS and 9% ammonia were sealed in a 250 mL reaction vessel, shaken vigorously for 30 s, and then tumbled at 30 rpm and 24.0 °C for 5 h. During the synthesis >95% of the DEDMS monomer is converted into PDMS, which forms as monodispersed droplets of 0.8-1.0 µm in diameter (see Table 2). Droplet size may be controlled by changing the ammonia-to-monomer ratio and the reaction time.11,12 Further batches of PDMS emulsion were prepared using a monomer mixture of DEDMS and TEMS, with the total monomer content remaining at 2%. Inclusion of the trimeric TEMS resulted in internal cross-linking within the PDMS droplets.12 At DEDMS:TEMS ratios greater than 1:1 the droplets formed are substantially cross-linked and solidlike, but at lower levels of MTES inclusion the droplets have viscoelastic properties; i.e., droplet deformability is controllable. Further details of the emulsion batches used in this study are given in Table 2. Directly following synthesis, PDMS emulsions were (11) Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1994, 163, 454. (12) Goller, M. I.; Obey, T. M.; Teare, D. O. H.; Vincent, B.; Wegener, M. R. Colloids Surf., A 1997, 183, 123. (13) Hunter, R. J. ζ Potential in Colloid Science; Academic Press: London, 1981. (14) Fleer, G. J. Polymer Adsorption and its Effect on Colloidal Stability. Ph.D. Thesis, Landbouwuniversiteit Wageningen, Wageningen, The Netherlands, 1971. (15) Fleer, G. J.; Koopal, L. K.; Lyklema, J. Kolloid Z. Z. Polym. 1972, 250, 689. (16) Maier, H.; Baker, J. A.; Berg, J. C. J. Colloid Interface Sci. 1987, 119, 512. (17) Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993. (18) Jenkins, P.; Ralston, J. Colloids Surf., A 1998, 139, 27. (19) Manufacturers Specifications for “Synperonic” PE, T, LF and “Ukanil” Ranges; ICI: Australia, 1996.
Figure 1. ζ potential as a function of pH for silica ()), latex (0), and PDMS emulsion A by microelectrophoresis (O) and PALS (4), with 10-3 M KNO3 as background electrolyte. dialyzed against Milli-Q water for several days to remove excess ammonia, until the pH was ∼7.5 and ionic strength < 10-4 M. Anionic polystyrene latex spheres were prepared by emulsion polymerization using a persulfate initiator;20 these were dialyzed against water prior to use. Silica spheres, supplied by Geltec Inc. (Alachua, FL), were washed and conditioned for 24 h with Milli-Q water prior to use; this ensured complete surface rehydroxylation and dispersion. Details of the particle size and ζ potential characteristics of the latex and silica particles are also given in Table 2. Methods. Particle size distributions were determined by laser diffraction (Malvern Mastersizer X) and dynamic light scattering (Brookhaven Instruments BI-200SM). Suspended samples for ζ potential measurement were conditioned in a nitrogen-purged, stirred, reaction vessel at 25.0 °C with 10-3 M KNO3 as the background electrolyte. The pH was controlled by small additions of HNO3 or KOH solutions, and copolymers were added as semidilute solutions. Electrophoretic mobilities were determined using a combination of micro-electrophoresis (Rank Bros, Mark II), laser Doppler electrophoresis (Malvern Zetasizer), and phase analysis light scattering (PALS). Electrophoretic mobilities were converted to ζ potentials using the Smoluchowski equation, since the double layer thickness is much smaller than the droplet or particle size.13
Results and Discussion Surface Electrical Properties. ζ Potential and Surface Chemistry. ζ potential versus pH plots for PDMS emulsion A, silica spheres, and latex spheres are compared in Figure 1. Estimated isoelectric point (IEP) values are given in Table 2 and are collectively in the range pH 2-3; i.e., the particles and droplets are negatively charged in the pH range from 3 to 10. With this in mind, silica and latex particles may be considered models for the PDMS emulsion droplets and comparisons of their polymer interaction behavior may reveal mechanistic details of adsorption at the liquid-liquid interface. The electrophoretic mobilities and ζ potentials of silica and latex particles reach plateaus at greater than pH 6 and pH 8, respectively, where their potential-determining surface (20) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 464.
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Figure 2. ζ potential of PDMS emulsion droplets (A) as a function of pH in the absence of copolymer (O) the presence of 100 ppm P84 ()) and 100 ppm F108 (0), with 10-3 M KNO3 as background electrolyte.
groups (i.e. Si-O- and -SO4-, respectively) are extensively deprotonated. In contrast, the PDMS emulsion droplets do not exhibit a plateau in their ζ potential over the pH range 3-10. This behavior is also independent of the experimental technique used and the equilibration time between ζ potential measurements at different pH values. To reconcile the ζ potential behavior of PDMS droplets it is useful to consider their composition and surface chemistry. The emulsion droplets are composed of linear and cyclic PDMS oligomers with an average molecular weight of 420 g mol-1 as characterized by 1H and 29Si NMR and GPC.11 The linear PDMS molecules are terminated with hydroxyl groups, which sit at the oil-water interface and undergo deprotonation as the pH is increased: pKa1
-O-Si(CH3)2-O+H2 S -O-Si(CH3)2-OH + pKa2
H+ S -O-Si(CH3)2-O- + 2H+ (1) For solid particles, with rigid interfaces, the IEP and the shape of the ζ potential versus pH curve is generally controlled by the pKa values of the surface groups and the capacitance of the interface.13 This is also likely to be the case for PDMS droplets; however, given the mobile nature of the PDMS-water interface and the complex composition of the droplets, a number of additional factors may need consideration, including the following: (a) PDMS chains of different length may have different pKa values. (b) PDMS chains are mobile within the droplet and their interfacial activity may vary with pH, as the degree of protonation changes. (c) Cyclic PDMS molecules may be hydrolyzed and cleaved at extremes of pH, thus changing the interfacial concentration of potential determining groups. Further work is required to gain a full understanding of the surface electrical properties of emulsion droplets, and although this is beyond the scope of the present study, it is a future challenge. The stabilizer-free PDMS droplets may prove a useful model system in further investigations. ζ Potential and Droplet Stability, in the Presence and Absence of Copolymer. In both the presence and absence of PEO-PPO-PEO copolymers, the influences of pH on the ζ potential and droplet size of PDMS emulsions are presented in Figures 2 and 3, respectively. In the absence of copolymer, a strong correlation between ζ potential and droplet stability is observed. That is, as the pH is decreased and magnitude of the ζ potential decreases, the droplet size increases. Rapid droplet coalescence is apparent below pH 4 and is in agreement with Friberg et al.,21 who report droplet instability for electrostatically stabilized emulsions
Barnes and Prestidge
Figure 3. Diameter of PDMS emulsion A droplets (d[0,9], i.e., particle size which 90% of the distribution is undersize) as a function of pH in the absence of copolymer (O) and the presence of 100 ppm P84 ()) and 100 ppm F108 (0).
with ζ potentials less than -30 mV. These findings also highlight the difficulty in determining droplet ζ potentials below pH 4, in the absence of stabilizer, where the halflife of a primary droplet is only a few minutes. PEO-PPO-PEO copolymers have a significant influence on both the ζ potential (Figure 2) and the stability (Figure 3) of PDMS droplets. Over the entire pH range investigated, adsorbed copolymer reduces the ζ potential, and this is more pronounced for F108 (long buoy blocks) than P84 (short buoy blocks). At pH 10, for instance, F108 reduces the ζ potential from -70 mV to less than -5 mV, whereas P84, at an equivalent concentration, only reduces the ζ potential to ∼-40 mV. In a similar manner Maier et al.16 showed that PEO homopolymers decreased the electrokinetic sonic amplitude of silica particles and that this effect was dependent on the PEO molecular weight. The droplet size data (Figure 3) further highlight the stabilizing nature of the different copolymers. In the presence of F108, negligible coalescence is observed over the entire pH range, whereas coalescence is apparent at pH < 4 in the presence of P84. It is clear that, in the presence of copolymer, droplet stability is not controlled by electrostatic considerations alone. In fact, a negative correlation between ζ potential and droplet stability is apparent; i.e., droplets with lower ζ potential have longer lifetimes. It is clear that steric stabilization is the predominant mechanism for PDMS droplet stability in the presence of PEO-PPO-PEO block copolymers. Previous studies have suggested that PEO-PPO-PEO copolymers are steric stabilizers for silica particles3 and polystyrene latex.5,22 The extent to which a particular PEO-PPO-PEO copolymer influences the ζ potential and stability of a droplet is dependent on its conformation and position at the liquid-liquid interface. The length and density of PEO tails have been shown to play a major role in the hydrodynamic adsorbed layer thickness and colloid stability of latex particles.22 With these thoughts in mind, the adsorbed conformation is controlled by the copolymer’s molecular structure, its solution concentration, and the mechanism of adsorption. The adsorption mechanism is controlled by specific polymer-surface interactions and is therefore dependent on the surface chemistry and structure of the adsorbent-water interface. The importance of the adsorbent type is highlighted in Figure 4, where the ζ potentials of silica particles, latex particles, and PDMS emulsion droplets are shown to exhibit (21) Friberg, S. E.; Quencer, L. G.; Hilton, M. L. Theory of Emulsions. In Pharmaceutical Dosage Forms; Marcel Dekker Inc.: New York, 1996; Vol. 1, pp 53-90. (22) Stenkamp, V. S.; Berg, J. C. Langmuir 1997, 13, 3827.
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Figure 4. ζ potential of silica (O), latex (0), and emulsion A (4) as a function of the concentration of F88.
strikingly different dependencies on the relative concentration of F88. These differences are related to substratedependent adsorbed layer thickness and are further explored in the following sections. Adsorbed Layer Structure. Adsorbed Layer Thickness. In this section we report determinations of the adsorbed layer thickness (δ) of PEO-PPO-PEO copolymers at the surface of PDMS droplets in water and make comparisons with equivalent data for silica spheres and latex particles in water. DLS was initially used; however, the errors associated in determining δ values of < 20 nm on droplets of diameter near 1000 nm were too large to be able to differentiate between the different copolymers on the three adsorbents. Sensitive and reproducible measurements of δ with DLS are generally restricted to particles of 100 nm or less.2-6 The variation of the ζ potential caused by the adsorbed layer was therefore used to estimate δ. There are many reported studies where electrokinetic and surface charge measurements have been used to characterize the structure of adsorbed polymers. In this work we use the highly simplified model of the electrical double layer, as was originally used by Fleer14 to determine the adsorbed layer thickness of poly(vinyl alcohol) onto silver iodide sols. In the absence of an adsorbed polymer layer the ζ potential (ζ1) may be related to the surface potential (Ψo) by13-15
tanh
zeψ0 -κ∆ zeζ1 ) tanh e 4kT 4kT
(2)
where z is the valence of the potential determining ions, e is the electron charge, ∆ is the distance from the surface to the shear plane, and 1/κ is the Debye length. In the presence of an adsorbed PEO-PPO-PEO block copolymer, it is assumed that the interfacial charge, surface potential, and charge distribution/movement of ions within the diffuse double layer are not significantly altered. This assumption has been shown reasonable for the case of adsorbed PEO layers on silica.16 Therefore, the new ζ potential (ζ2) is given by13-15
tanh
zeψ0 -κδ zeζ2 ) tanh e 4kT 4kT
(3)
Values of δ for adsorbed copolymers may therefore be estimated from ζ potential variation by solving eqs 2 and 3. Fleer and co-workers14,15 used this approach to determine the adsorbed layer thickness of poly(vinyl alcohol) onto silver iodide sols, by assuming that ∆ is equal to the (23) Koopal, L. K.; Lyklema, J. Faraday Discuss. Chem. Soc. 1975, 59, 230. (24) Koopal, L. K.; Lyklema, J. J. Electroanal. Chem. 1979, 100, 895.
Figure 5. Adsorbed layer thicknesses of PEO-PPO-PEO block copolymers at the PDMS droplet (A)-water interface as a function of concentration: F108 (0); F127 (4); F88 ()); F77 (×); P104 (O); P84 (+).
thickness of the Stern plane, which is 0.4 nm. (Koopal and co-workers23-25 have subsequently extended this method to account for double layer charge and capacitance and changes in the ion distribution within the electrical double layer.) Other workers have used a similar approach to determine the layer thickness of homopolymers16,17 and copolymers18 adsorbed at solid particle-water interfaces with reasonable success. This approach is not, however, suitable for polyelectrolytes and to our knowledge not been applied to emulsion droplets. Consideration should also be given to the relative thickness of the electrical double layer compared to the adsorbed layer. Pagac et al.17 and Koopal and co-workers23-25 have highlighted the difficulties of this approach for cases where δ , 1/κ. In line with previous studies,14-18 the current series of experiments were undertaken in the presence of an inert electrolyte, 10-3 M KNO3, where 1/κ is ∼10 nm and therefore uncertainties are anticipated for the determination of δ values < 5 nm. Given the assumptions in eqs 2 and 3, we have determined δ for PEO-PPO-PEO copolymers at the surfaces of PDMS emulsion droplets (Figure 5) and compared these with equivalent values for silica spheres and latex particles (Figure 6). It is encouraging that our δ values for silica and latex are in reasonable agreement with those previously reported using dynamic light scattering;2-6,22 see Table 3. It is also important to note that errors in the values of δ from ζ potential variation (i.e. 10 nm, increasing to (20% at lower δ values) are significantly less than those for other approaches.1,3,5 We also note that the δ values from ζ potential and DLS are invariably greater than those determined using ellipsometry on flat surfaces.7 Ellipsometry is less sensitive to polymer tails, and particle curvature may influence the layer thickness of an adsorbed copolymer.3,6 Influence of Copolymer Structure and the Nature of the Interface. Figure 5 shows that, for each of the copolymers investigated, the estimated layer thickness on the PDMS emulsion droplets exhibit a pseudoplateau when added copolymer concentrations are in the range 15-25 ppm. Similar concentration-dependent behavior was observed for silica and latex particles but in the interest of brevity is not presented here. For all three substrates, multilayer adsorption of PEO-PPO-PEO is not anticipated at these concentrations. Interestingly, PEO homopolymers do not exhibit such a plateau in their layer thicknesses;24 this has been explained in terms of adsorption fractionation of different molecular weight PEO molecules. Adsorption fractionation is apparently less prevalent for the copolymers presently under investigation. The shapes of the δ versus concentration plots are controlled by the adsorption
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Figure 7. Plateau PEO-PPO-PEO adsorbed layer thickness for silica (O), latex (0), and PDMS droplet (4) interface as a function of the number of EO units per PEO block.
Figure 6. PEO-PPO-PEO adsorbed layer thickness as a function of copolymer concentration for PDMS emulsion A (0), latex (O), and silica (4): F127 (a); F108 (b). Table 3. Plateau Adsorbed Layer Thickness Values for PEO-PPO-PEO Copolymers at the Surface of PDMS Droplets, Latex Particles, and Silica (Spheres and Flats) with Errors Provided Representing 95% Confidence Intervals copolymer emulsiona P75 P84 P85 P103 F68 F77 F88 F98 F108 F127 a
c
latexa
5.7 ( 1.1
4 ( 0.8
7.1 ( 1.4
3.8 ( 0.8
7.1 ( 1.4 14.9 ( 0.7
4 ( 0.8 8.0 ( 1.6
20.3 ( 1.1 10.2 ( 0.9 15.1 ( 0.8 8.0 ( 1.6
latexb latexc 5.0
2.6
6.0
2.6
5.7
7.0
8.0 9.5 10.6
11.7 12.9
silicaa 1.0 ( 0.2
silicad 2.5
1.3 ( 0.3 1.4 ( 0.3 3.3 ( 0.7 3.5 ( 0.8 2.5 ( 0.5
4.0 3.5
b
Current study, using electrophoresis. DLS study on latex.5 DLS study on latex.3 d Ellipsometry study on silica wafers.7
affinity of the different copolymers for the surface of the PDMS droplets, which is in turn controlled by the adsorption thermodynamics of the copolymer. This is not the focus of the present study and will be addressed in a subsequent publication. We now consider the pseudoplateau adsorbed layer thickness and describe its dependency on the copolymer structure and the nature of the adsorbent. Irrespective of the copolymer structure, significant differences in adsorbed layer thickness are apparent as the adsorbent type is varied. The layer thickness data presented in Table 3, Figure 6, and Figure 7 all confirm that adsorbed copolymer layers on PDMS droplets are significantly thicker than on latex particles, which are, in turn, considerably thicker than on silica particles. These differences can be attributed to the different equilibrium polymer conformations, resulting from different adsorption mechanisms. Hydrogen bonding is reported to be the predominant adsorption mechanism between the PEO and silica,7,26 and this is significantly influenced by specific
Figure 8. Schematic representation of the adsorbed PEOPPO-PEO conformation at the surface of different adsorbents for F-series and P-series copolymers.
Bronsted and Lewis acid-base interaction.26 However, for PEO-PPO-PEO copolymers and polystyrene latex, hydrophobic bonding between the PPO segments and latex surface group is thought to dominate the adsorption mechanism.5 The poor solvent quality of water for PPO blocks in comparison with PEO blocks and its dependency on block size is also influential here. On silica particles, therefore, both the PEO and PPO blocks may have a significant surface affinity and relatively flat conformations result. Conversely, strong PPO adsorption onto the latex particle surface causes PEO segments to extend out from the interface into solution, forming thicker adsorbed layers. The interplay between adsorption mechanism and adsorbed conformation is presented schematically in Figure 8. Given the hydrophobic nature of PDMS emulsion droplets, the adsorption mechanism would be expected to mimic that for the latex particles rather than silica particles. Adsorbed layer thickness values concur with this. On considering Figure 7, it is apparent that there is a linear correlation between the adsorbed layer thickness and the number of PEO monomeric units in the buoy blocks of the copolymer. This correlation is stronger for PDMS emulsion and latex than for silica and reflects the different adsorption mechanisms. Previous DLS studies on latex5 have confirmed the direct dependency of adsorbed layer (25) Koopal, L. K.; Hlady, V.; Lyklema, J. J. Colloid Interface Sci. 1988, 121, 49. (26) Mathur, S.; Moudgil, B. M. J. Colloid Interface Sci. 1997, 196, 92.
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Table 4. Plateau Adsorbed Layer Thickness for PEO-PPO-PEO Copolymer as a Percentage of the Fully Extended PEO Segment Length PEO extension (adsorbed layer thickness/fully extended PEO length) (%) copolymer F127 F108 F88 P103 F77 P84
emulsion A
latex
silica
38 38 36 100 34 71
20 19 19 56 19 53
6 7 8 19 7 13
thickness on the PEO block length. To further explore the factors that control PEO-PPO-PEO copolymer adsorption it is useful to characterize the relative extension of the different copolymers out from the interface. For each adsorbent, the plateau δ value is given as percentage of the full extension of the PEO block in Table 4. These data reiterate the dependency on the nature of the adsorbent but additionally show that the different copolymers behave as two distinct groups. That is, on any of the three adsorbents, copolymers with PEO block lengths < PPO block lengths (i.e. P84 and P103) behave very differently from the copolymers with PEO block lengths > PPO block lengths (i.e. F77, F88, F108, and F127). For example, for PDMS emulsion droplets, the F-series Synperonic copolymers extend ∼35% of their fully extended PEO length, whereas the P-series Synperonic copolymers extend between 70 and 100% of their fully extended PEO length. A similar trend in the plateau δ value is observed for both latex and silica particles but with decreasing relative extension, respectively. For the P-series Synperonic copolymers, the PPO blocks either contribute to the adsorbed layer thickness (as presented schematically in Figure 8) or adsorbed bilayers form. Finally, we consider the mobile nature of the PDMS emulsion droplet-water interface, since it may influence the adsorbed copolymer conformation and hence the adsorbed layer thickness. The neutron reflectivity studies of of Phipps et al.9 have determined considerable penetration of the PPO block of F127 into the cyclohexane phase at the cyclohexane-water interface. Penetration of the PPO block through PDMS droplet-water interface may also be envisaged and needs to be considered. A number of implications of PPO block penetration into the PDMS phase may be hypothesized: (a) The PPO blocks would be shielded from unfavorable interactions with water, which would reduce their intramolecular association with PEO and leave the PEO blocks with more freedom to extend into solution. (b) The surface area occupied by the PPO block at the interface would decrease, resulting in increased steric repulsion between PEO blocks in the aqueous phase. The PEO blocks would then take up a more extended conformation to minimize steric hindrance between neighboring chains. Both of these would result in a more extended PEO block conformation with a greater adsorbed layer thickness, which is in fact the case when latex and PDMS emulsion droplets are compared. To further test this hypothesis we have determined the adsorbed layer thickness values on PDMS droplets with various degrees of internal cross-linking, i.e., less deformable droplets with less penetrable interfaces. This was achieved by the inclusion of the TEMS into the emulsion droplet synthesis and has little influence on the interfacial characteristics of the emulsion droplets, as confirmed by ζ potentials. The plots of adsorbed layer thickness against concentration in Figure 9 show a strong dependency on the level of PDMS cross-linking; i.e., thicker adsorbed layers are observed on droplets with more deformable interfaces. This gives
Figure 9. PEO-PPO-PEO adsorbed layer thickness as a function of copolymer concentration for PDMS emulsion A (0), PDMS emulsion B (O), and latex (4): F127 (a); F108 (b).
credence to the hypothesis that interfacial penetration plays an important role in determining the adsorbed layer structure at the emulsion droplet-water interface, as shown schematically in Figure 8. Conclusions PDMS emulsion droplets are sterically stabilized by PEO-PPO-PEO copolymers, with the degree of stabilization dependent on the adsorbed layer structure and influenced by the copolymer structure and molecular weight. Accurate and reproducible values for the adsorbed copolymer layer thickness (δ) at the PDMS droplet-water interface may be determined from changes in ζ potential. For a wide range of copolymers, δ values are in the range 2 to 20 nm and directly proportional to the PEO block length. For copolymers with PEO segment lengths > PPO segment lengths (i.e. F-series Synperonics), the plateau δ values for PDMS, latex, and silica correspond to ∼35%, 20%, and 7% of the fully extended PEO segment length. Equivalent values for the P-series Synperonics (PEO segment lengths < PPO segment lengths) are significantly greater but follow the same dependence on the type of adsorbent. These differences reflect the adsorbed conformation resulting from different adsorption mechanisms. As the level of internal cross-linking within PDMS droplets was increased, the interface became less deformable and δ values decreased. Interfacial penetration is considered to play an important role in controlling adsorbed copolymer conformations at the liquid-liquid interface. Acknowledgment. An Australian Postgraduate Award Scholarship for T.J.B. is acknowledged. Soltec Research Pty Ltd. is acknowledged for assistance in funding this work. The Laser Light Scattering and Materials Sciences Group (University of South Australia) and Susan Kapsabelis are acknowledged for assistance in light scattering and electrophoresis. Dr. Paul Jenkins, Dr. Roland Keir, and Prof. Brian Vincent are thanked for useful discussion. LA991217D
