Electroacoustic Study of Adsorption of PEO on Sunflower Oil-in-Water

The results are interpreted with the aid of O'Brien's “gel layer” model, which gives an estimate of the elasticity and the approximate thicknesses...
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Langmuir 2003, 19, 540-547

Articles Electroacoustic Study of Adsorption of PEO on Sunflower Oil-in-Water Emulsion Drops Linggen Kong, James K. Beattie,* and Robert J. Hunter School of Chemistry F11, University of Sydney, Sydney, NSW 2006, Australia Received June 4, 2002. In Final Form: November 18, 2002

The electroacoustic signals of sunflower oil-in-water emulsion droplets stabilized by sodium dodecyl sulfate (SDS) were affected by the adsorption of poly(ethylene oxide) (PEO). Large changes in the dynamic mobility of the coated oil droplets measured with an AcoustoSizer-II apparatus were observed. The results are interpreted with the aid of O’Brien’s “gel layer” model, which gives an estimate of the elasticity and the approximate thicknesses of a dense inner and an extended outer adsorbed polymer layer. The thicknesses of both the inner and the outer layer increased smoothly with increasing concentration of polymer and the rate of increase increased with molar mass of the polymer with average molecular weights of 1500, 10 000, and 35 000.

Introduction Many kinds of surfactants and polymers are used to stabilize suspensions and emulsions in the production of paint, pharmaceutical, agrochemical, cosmetic, ceramic, and ink preparations. Knowledge of the way that these stabilizers are adsorbed and their effect on the stabilization of dispersions is of importance in predicting and controlling the properties of commercial suspensions.1,2 The adsorption of polymers onto particles hinders aggregation by providing a physical (steric) barrier between the particles, which prevents them from approaching each other sufficiently closely to allow the van der Waals interaction to dominate. Therefore, a simple way that the interactions between the particles may be adjusted is by varying the thickness of the adsorbed polymer layer. This modifies the strength of the interaction between the particles and allows the attractive interparticle force to be minimized. The polymer layer thickness may be controlled by changing the molecular weight of the adsorbed polymer. Poly(ethylene oxide) (PEO) (or poly(ethylene glycol) (PEG)) has been extensively investigated due to its unique properties in a wide range of applications, including biomedicine.3 It has been shown that PEO can be adsorbed on many kinds of particle surfaces such as mica,4-6glass,7,8 * Corresponding author. Telephone: +61 2 9351 3797. Fax: +61 2 9351 3329. E-mail: [email protected]. (1) Tadros, Th. F. Adv. Colloid Interface Sci. 1993, 46, 1-47. (2) Colloid-Polymer Interactions; Farinato, R. S., Dubin, P. L., Eds.; Wiley-Interscience: New York, 1999. (3) Poly(ethylene glycol) Chemistry-Biotechnical and Biomedical Applications; Harris, J. M., Ed.; Plenum Press: New York, 1992. (4) Klein, J.; Luckham, P. F. Nature (London) 1982, 300, 429-431. (5) Klein, J.; Luckham, P. F. Macromolecules 1984, 17, 1041-1048. (6) Claesson, P. M.; Blomberg, E.; Paulson, O.; Malmsten, M. Colloids Surf. A 1996, 112, 131-139. (7) Braithwaite, G. J. C.; Luckham, P. F.; Howe, A. M. Langmuir 1996, 12, 4224-4237. (8) Braithwaite, G. J. C.; Luckham, P. F. J. Chem. Soc., Faraday Trans. 1997, 93, 1409-1415.

latex,9-16 silica,6,13,17-21 and the air-water interface.22 It can interact with neutral lipid23 and oil24 surfaces. Numerous experimental techniques have characterized the adsorbed PEO conformations at the above interfaces. These methods have rarely been applied to liquid-liquid interfaces or emulsion droplets. In our previous work,25,26 we examined a system in which a hexadecane-in-water emulsion was formed in the presence of the anionic stabilizer sodium dodecyl sulfate (SDS). Nonionic surfactants (nonyl phenol ethoxylates (9) Polverari, M.; van de Ven, T. G. M. Colloids Surf. A 1994, 86, 209-228. (10) Polverari, M.; van de Ven, T. G. M. J. Colloid Interface Sci. 1995, 173, 343-353. (11) Cosgrove, T. J. Chem. Soc., Faraday Trans. 1990, 86, 13231332. (12) Liang, W.; Bognolo, G.; Tadros, Th. F. Langmuir 1995, 11, 28992904. (13) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf. A 1999, 149, 329-338. (14) Peracchia, M. T.; Vauthier, C.; Passirani, C.; Couvreur, P.; Labarre, D. Life Sci. 1997, 61, 749-761. (15) Miraballes-Martı´nez, I.; Martı´n-Rodrı´guez, A.; Hidalgo-AÄ lvarez, R. J. Dispersion Sci. Technol. 1996, 17, 321-337. (16) O ¨ sterberg, E.; Bergstro¨m, K.; Holmberg, K.; Schuman, T. P.; Riggs, J. A.; Bruns, N. L.; Van Alstine, J. M.; Harris, J. M. J. Biomed. Mater. Res. 1995, 29, 741-747. (17) Wind, B.; Killmann, E. Colloid Polym. Sci. 1998, 276, 903-912. (18) Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993-3001. (19) Malmsten, M.; Van Alstine, J. M. J. Colloid Interface Sci. 1996, 177, 502-512. (20) Lea, A. S.; Andrade, J. D.; Hlady, V. Colloids Surf. A 1994, 93, 349-357. (21) Irvine, D. J.; Mayes, A. M.; Satija, S. K.; Barker, J. G.; SofiaAllgor, S. J.; Griffith, L. G. J. Biomed. Mater. Res. 1998, 40, 498-509. (22) Henderson, J. A.; Richards, R. W.; Penfold, J.; Thomas, R. K.; Lu, J. R. Macromolecules 1993, 26, 4591-4600. (23) Bartucci, R.; Montesano, G.; Sportelli, L. Colloids Surf. A 1996, 115, 63-71. (24) Moaddel, T.; Friberg, S. E. J. Dispersion Sci. Technol. 1995, 16, 69-97. (25) Kong, L.; Beattie, J. K.; Hunter, R. J. Phys. Chem. Chem. Phys. 2001, 3, 87-93. (26) Kong, L.; Beattie, J. K.; Hunter, R. J. Colloid Polym. Sci. 2001, 279, 678-687.

10.1021/la026034x CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002

Electroacoustic Study of Adsorption of PEO

with different molecular weights) were added both before and after the emulsification and these produced significant changes in the electroacoustic signals of oil drops. In the electroacoustic technique, a quantity known as the dynamic mobility, µd, is measured by subjecting the sample to a high-frequency electric field. The applied field causes the particles to oscillate between the electrodes, generating pressure changes in the dispersion which propagate as sound waves. This is referred to as the electrokinetic sonic amplitude (ESA) effect.27,28 The velocity of the particles in response to the field, as a function of the applied frequency, comprises the dynamic mobility spectrum, µd, of the material. The AcoustoSizer-II measures the ESA signals at 13 frequencies from 1 to 20 MHz from which the corresponding dynamic mobility values are obtained. The value of µd is related to both the ζ potential and the size of the colloidal particles so these quantities can then be obtained from the frequency dependence of the ESA signals. O’Brien’s “gel layer” model29 interprets the changes in the dynamic mobility spectra due to the adsorption of the polymeric surfactant. This model requires three parameters: the extent to which the adsorbed polymer is able to displace the shear plane out from the surface of the particle by forming a dense inner layer, of thickness ∆; the thickness of the outer adsorbed layer, d, and the elasticity of this layer. To enable comparison with dc (static) electrophoresis and other measurements, an effective static mobility thickness, ∆s, can also be calculated from the low-frequency limit of the model. In this work, the effect on the dynamic mobility spectra of sunflower oil emulsion droplets of the adsorption of PEO with molecular weights of 1500, 10 000 and 35 000 was investigated. The oil droplets were generated in a homogenizer in the presence of SDS so this anionic surfactant was adsorbed at the interface first in both systems. We showed earlier30 that the sunflower oil emulsion is not very stable at 6% concentration without the addition of SDS, but measurements on more concentrated systems suggested that there were natural stabilizers present in the oil which could complicate the situation at the interface, even at the lower oil concentration. When PEO was added to an emulsion in increasing concentration, the drops became coated with an adsorbed layer that altered the electroacoustic behavior drastically. Our objective was to determine the effects of the adsorbed layer on the dynamic mobility and hence to measure the layer thicknesses and elasticity for different polymer molecular weights. Experimental Section Chemicals and Instruments. Commercial sunflower oil (Meadow Lee Foods Ltd., Sydney, Australia) was used directly (fat total: 92 g/100 mL). Poly(ethylene oxide)s were from Fluka with average molecular weights of 1500 (1400-1600), 10 000 (8500-11 500) and 35 000 (∼35 000) and used directly. These will be denoted PEG-1500, PEG-10K, and PEG-35K, respectively. SDS (Sigma, approximately 99% GC) was used without further purification. Sodium chloride was from Ajax Chemicals (min. 99.9%). High purity Milli-Q water (Millipore) was used for all solutions; its resistivity was above 18 MΩ cm. The experimental setup is shown schematically elsewhere.31 The instrument was calibrated as recommended by the manu(27) O’Brien, R. W.; Cannon, D. W.; Rowlands, W. N. J. Colloid Interface Sci. 1995, 173, 406-418. (28) Hunter, R. J. Colloids Surf. A 1998, 141, 37-65. (29) Carasso, M. L.; Rowlands, W. N.; O’Brien, R. W. J. Colloid Interface Sci. 1997, 193, 200-214. (30) Kong, L.; Beattie, J. K.; Hunter, R. J. J. Colloid Interface Sci. 2001, 238, 70-79.

Langmuir, Vol. 19, No. 3, 2003 541 facture at 25 °C. The ESA signals were observed by oscilloscope (468 Digital Storage Oscilloscope, Tektronix, Inc.). The measuring cell was an AcoustoSizer-II slurry monitor cell with volume of 20 mL (Colloidal Dynamics Inc.). The measurement operation was controlled by the main control and function generator block (CSPU-100 central signal processing unit, Colloidal Dynamics Inc.). The emulsion was circulated by a peristaltic circulating pump (Amicon CH-2A Concentrator). The temperature of samples was controlled by a refrigerated circulator bath (Julabo F20-HC). Emulsion Preparation and Dynamic Mobility Measurement. The required quantity of water was weighed into a conical flask, and then SDS was added so that its final total concentration was 6 mM. Next NaCl was added so that its final concentration was 2 mM for all samples, to raise the conductivity to more than 0.02 S/m which was above the lower limit of calibration of the instrument of 0.01 S/m. Next, a known volume of oil was added to the water phase to give a 6% volume fraction. The solution was then stirred vigorously for about 10 minutes to produce a coarse emulsion. Finally, 200 mL of this fresh pre-emulsion was passed through the homogenizer (Milko-tester Mark III F3140, A/S N. Foss Electric) 10 times in rapid succession at 25 °C. After the particles had passed through the homogenizer, the dynamic mobility spectrum of emulsion particles was measured by the AcoustoSizer-II at 25 °C and at natural pH. Temperature, pH, and conductivity were measured and recorded automatically for each measurement (CMP-Slurry Monitor, Colloidal Dynamics Inc.) by appropriate probes immersed in the circulation cell. For all emulsions, it was assumed that the density of oil droplets was 916.0 kg m-3 for sunflower oil32 with relative dielectric constant of ∼2.33 The calculation of ζ potential and particle median diameter from the measured dynamic mobility by the AcoustoSizer-II is comparatively straightforward for systems even at oil concentrations up to about 50 vol %. The relevant relationships are given in refs 29 and 30 and are built into the instrument software. After the properties of the particles were measured, different amounts of PEO were added to the emulsions. The original concentration of PEO was 20% (w/v) for PEG-10K and PEG-35K and 40% (w/v) for PEG-1500. The PEG was added as 1, 5, 10, 20, 30, 40, 50, and 60 mL gradually into 180 mL of emulsion. Importantly, after each addition, the emulsion was circulated rapidly through the measurement circuit for more than 30 min to ensure that the PEG was completely adsorbed on the emulsion drops. Then the dynamic mobility spectrum of the emulsion droplets with adsorbed PEG was measured three times consecutively within 10 min. The final result was the average of these three measurements. If the first spectrum did not overlap with the third one, which might mean the PEO had not adsorbed completely, the emulsion was circulated for a longer time until there was little variation of the dynamic mobility between each measured spectrum. Data Processing. After the dynamic mobility values were obtained from the ESA measurement, the ratio [µd(coated)/µd(uncoated)] was calculated, where “coated” refers to the mixed SDS and PEO system and “uncoated” to the emulsion with SDS alone. The relaxation frequency ωo is estimated from the frequency of the maximum in the difference in arguments of the dynamic mobility ratio. The outer permeable elastic thickness d can then be obtained from the best fit between the experimental values and the theoretical ones for the argument difference.25,26,29 After d was determined, the average value of exp(-κ∆) and hence the thickness of the inner dense layer ∆ was obtained at selected frequencies, which were 1.32, 2.07, 3.47, 5.79, 8.80, and 15.64 MHz. The total adsorbed layer thickness is the sum of d and ∆; finally an effective static mobility thickness ∆s was calculated. This is related to the hydrodynamic thickness measured by dc techniques.29 (31) Kong, L.; Beattie, J. K.; Hunter, R. J. Chem. Eng. Process. 2001, 40, 421-429. (32) The Merck Index, 11th ed.; Merck & Co., Inc.: Rahway, NJ, 1989; p 1422, No. 8982. (33) Handbook of Chemistry and Physics, 75th ed.; CRC Press Inc.: Boca Raton, FL, 1995; pp 3-183.

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Figure 1. Schematic view of adsorption of SDS and PEO on a hydrocarbon oil droplet. Table 1. Structure Parameters for Polyethylene Glycol MW (CH2CH2O) unit L (nm) s (nm) dc (nm)

PEG-1500

PEG-10K

PEG-35K

1500 34 12.04 0.047 2.90

10 000 227 79.59 0.047 7.50

35 000 795 278.39 0.047 14.03

Evaluation of Structure Parameters for PEO. The structure parameters of poly(ethylene glycol) were calculated with the molecular-structure method used previously25 and are given in Table 1, where L is the PEO molecular chain length and s is its radius, and dc is the coil diameter. Figure 1 displays a schematic view of adsorption of SDS and PEO on an oil droplet assuming that multiple points on the PEO long thin chain can penetrate the oil phase and, hence, form the trains, loops, and tails that describe polymer adsorption. The emulsion drop radius is so large compared to molecular dimensions that the surface is considered flat. Note that only a small amount of droplet surface is physically occupied by the SDS headgroups, so there is ample room for PEO adsorption. Consequently, it is not difficult for PEO to form the loop and train configuration illustrated schematically in Figure 1. On the sunflower oil surface, the position may be more complicated since it is feasible that there are other surface active agents present, at least in small quantities, at the interface.

Results Dilution Effect on Dynamic Mobility. Since the polymer is added as an aqueous solution and dilutes the emulsion, the effect of the dilution process on the dynamic mobility of the emulsion drops was examined before investigating the effect of polymer adsorption. Figure 2 shows the variation of dynamic mobility (magnitude and argument) on diluting a sunflower oil emulsion by adding water. The ζ potential of the original sunflower oil emulsion was -99 mV, and the median diameter was 0.85 µm as measured by the AcoustoSizer-II. When the emulsion is diluted by one-third, the conductivity declines by about 20% from 0.0480 to 0.0387 S/m. The phase angle decreases negatively by about 2° in the middle of the frequency range. The change of magnitude is much more obvious; it decreases smoothly with the emulsion dilution. The calculated particle size changes by only about 2% and the absolute value of ζ potential decreases by about 10% from -99 to -89 mV. To determine the influence of PEO on the dynamic mobility of the emulsion droplets, the ESA signals from the background solution were measured. First, a solution of 6 mM SDS and 2 mM NaCl was prepared, and then the PEG-10K or PEG-35K was added gradually from 0 up to 5 wt %. The magnitude of the dynamic mobility was close to zero and the argument fluctuated between (180° due to noise, which means the PEO together with SDS has no measurable ESA signal. This is not inconsistent with the possible formation of complexes between SDS and PEO,13

Figure 2. Variation of dynamic mobility with the dilution of a sunflower oil emulsion with an initial oil volume fraction 6%, SDS bulk concentration 6 mM, and NaCl concentration 2 mM, by adding an increasing volume percent of water with decreasing conductivity (S/m) at 25 °C: ([) 0, 0.0480; (2) 5.5, 0.0445; (O) 11, 0.0437; (b) 22, 0.0418; (9) 33, 0.0387. The arrows show the direction of increasing dilution.

for the volume fraction of SDS is low and electroacoustic signals are not expected for extended polymer chains with low charge density. PEO Effects on Dynamic Mobility. Figure 3 shows the magnitude and argument of the dynamic mobility spectra for the PEG-1500 system. The sample without PEO is used to measure the size and charge of the sunflower oil emulsion drops. It can be seen that the magnitude decreases dramatically on the first addition of PEO and then decreases gradually with increasing PEO concentration. The argument shows the typical increase in phase lag with frequency due to the inertia effect. The arguments become more positive with increasing polymer concentration. These effects are made clearer in Figure 4 where the ratio of the coated to the uncoated system is plotted, and a comparison is made between the experimental data (symbols) and O’Brien’s “gel layer” model (lines) for the dynamic mobility ratio at the eight different PEG-1500 concentrations. [Note that taking the ratio of these (complex) mobilities yields the ratio of the magnitudes and the difference of the phase angles.] At the lowest concentration (0.221 wt %) the magnitude of the mobility ratio is independent of frequency with a value of about 0.49, showing a great reduction in mobility compared to the uncoated particles but no significant phase

Electroacoustic Study of Adsorption of PEO

Figure 3. Variation of dynamic mobility with frequency and PEG-1500 concentration, the initial oil volume fraction 6%, SDS bulk concentration 6 mM and NaCl 2 mM. PEG-1500 concentration (wt %) and conductivity (S/m) at 25 °C: ([) 0, 0.0513; (0) 0.221, 0.0505; (2) 1.081, 0.0485; (O) 2.105, 0.0464; (/) 4.000, 0.0426; (b) 5.714, 0.0396; (]) 7.273, 0.0369; (9) 8.696, 0.0342; (4) 10.00, 0.0321.

angle difference. As the concentration is increased, the magnitude decreases gradually and shows an increasing dependence on the applied frequency. The arguments of the mobility ratio are also affected by the increasing concentration of PEG-1500. A well-defined trend with frequency develops, consisting of a phase lead which increases progressively with concentration leading to a peak around 8 MHz. At 10 wt % PEG-1500, the phase lead is about 9° at the peak with respect to the particles covered with only SDS. When the PEO molecular weight is increased to 10,000, the magnitudes of the dynamic mobility spectra are similar to those with PEG-1500. The shift of phase angle is more obvious; it reaches 15° positive at the lowest frequency with the highest PEO concentration (Figure 5). The comparison with the theoretical model is shown in Figure 6. It is clear that the magnitude ratio in this case has a much stronger dependence on the applied frequency than with PEG-1500 while the phase lead forms a peak at around 2 MHz and reaches about 26° at 5 wt % PEG10K. The model describes the magnitude behavior well but gives a poor description of the phase angles at the higher frequencies. The most obvious influence on the dynamic mobility of the sunflower oil emulsion is with PEG-35K, which is

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Figure 4. Comparison of gel layer model (lines) with experimental data (symbols) for the dynamic mobility ratio µD(coated)/ µD(uncoated) for PEG-1500 on sunflower oil. PEG wt %: (0) 0.221; (2) 1.081; (O) 2.105; (/) 4.000; (b) 5.714; (]) 7.273; (9) 8.696; (4) 10.00. [The direction indicated by the arrows is from curve (a) to curve (h) in this and subsequent figures.] ω0 ) 2.01 × 107 s-1: (a) κ ) 0.293 nm-1, d ) 3.5 nm, ∆ ) 2.32 nm, ∆s ) 2.47 nm; (b) κ ) 0.290 nm-1, d ) 4.0 nm, ∆ ) 2.53 nm, ∆s ) 2.74 nm; (c) κ ) 0.286 nm-1, d ) 5.0 nm, ∆ ) 2.68 nm, ∆s ) 3.05 nm; (d) κ ) 0.279 nm-1, d ) 6.0 nm, ∆ ) 3.06 nm, ∆s ) 3.64 nm; (e) κ ) 0.272 nm-1, d ) 6.6 nm, ∆ ) 3.50 nm, ∆s ) 4.23 nm; (f) κ ) 0.266 nm-1, d ) 7.2 nm, ∆ ) 3.94 nm, ∆s ) 4.84 nm; (g) κ ) 0.260 nm-1, d ) 7.8 nm, ∆ ) 4.31 nm, ∆s ) 5.04 nm; (h) κ ) 0.255 nm-1, d ) 8.4 nm, ∆ ) 4.67 nm, ∆s ) 5.96 nm.

shown in Figure 7. The magnitude behavior is similar to the others but the argument displays an even more substantial effect of addition of PEG-35K. The peak of the phase angle shifts to below 2 MHz and at the highest PEG concentration the phase lag increases to near 30° at 1 MHz. Again the high-frequency magnitudes are well described but the theoretical phase angles are in poor agreement with the data (Figure 8). It should be mentioned here that the influence of viscosity of the different PEO solutions on the dynamic mobility could be ignored. The apparent viscosity of the emulsions with added PEG-1500, PEG-10K and PEG-35K seems little different from that of the untreated emulsion. From the legends to Figures 3, 5, and 7, it can be seen that the conductivity drops by the same amount from emulsion without PEO to 10 wt % PEG-1500 or 5 wt % PEG-10K or -35K. That the conductivity diminishes with the addition of PEG is more likely related to the dilution of the emulsion rather than the viscosity effect as the PEG concentration is not especially high.

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Figure 5. Variation of dynamic mobility with frequency and PEG-10K concentration of sunflower oil emulsion with initial oil volume fraction 6%, SDS bulk concentration 6 mM and NaCl 2 mM. PEG-10K concentration (wt %) and conductivity (S/m) at 25 °C: ([) 0, 0.0513; (0) 0.111, 0.0498; (2) 0.541, 0.0483; (O) 1.053, 0.0470; (/) 2.000, 0.0436; (b) 2.857, 0.0412; (]) 3.636, 0.0390; (9) 4.347, 0.0373; (4) 5.000, 0.0351.

Layer Thickness Estimation. The calculated adsorbed outer layer thicknesses d, inner layer thicknesses ∆ and static mobility thicknesses ∆s are shown in Figure 9 for the different PEO molecular weights on the sunflower oil. For PEG-1500 the permeable outer layer d increases slowly from 3.5 to 8.4 nm as its concentration increases from 0.22 to 10.0 wt %. When the PEO molecular weight is increased to 10 000, the d value increases from 10 to 25 nm with addition of PEO up to 5.0 wt %. For PEG-35K, the changes in d are about twice those of PEG10K at the same concentrations. The dense inner layer ∆ becomes thicker with increasing PEO for all systems. The ∆ value increases gradually from 2.3 to 4.7 nm as the PEG-1500 concentration increases. For PEG-10K, ∆ increases with concentration from around 2.7 to 5.5 nm. It may be significant that PEG-35K behaves similarly at the same concentrations. Figure 9 also shows that the effective static mobility thickness ∆s increases gradually with increase of PEO for all systems from 2.5 to 6.0 nm for PEG-1500, 3.0-9.3 nm for PEG-10K, and 3.9-10.9 nm for PEG-35K. There is a large difference in ∆s between PEG-1500 and PEG-10K and a smaller difference between PEG-10K and PEG-35K of about 1.5 nm at all concentrations.

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Figure 6. Comparison of gel layer model (lines) with experimental data (symbols) for the dynamic mobility ratio µD(coated)/µD(uncoated) for PEG-10K on sunflower oil. PEG10K concentration (wt %): (0) 0.111; (2) 0.541; (O) 1.053; (/) 2.000; (b) 2.857; (]) 3.636; (9) 4.347; (4) 5.000. ω0 ) 7.54 × 106 s-1: (a) κ ) 0.293 nm-1, d ) 10.0 nm, ∆ ) 2.54 nm, ∆s ) 3.04 nm; (b) κ ) 0.290 nm-1, d ) 13.5 nm, ∆ ) 2.72 nm, ∆s ) 3.68 nm; (c) κ ) 0.286 nm-1, d ) 16.5 nm, ∆ ) 3.02 nm, ∆s ) 4.46 nm; (d) κ ) 0.279 nm-1, d ) 19.3 nm, ∆ ) 3.53 nm, ∆s ) 5.50 nm; (e) κ ) 0.272 nm-1, d ) 21.7 nm, ∆ ) 4.00 nm, ∆s ) 6.48 nm; (f) κ ) 0.266 nm-1, d ) 24.0 nm, ∆ ) 4.53 nm, ∆s ) 7.53 nm; (g) κ ) 0.260 nm-1, d ) 25.5 nm, ∆ ) 5.08 nm, ∆s ) 8.47 nm; (h) κ ) 0.255 nm-1, d ) 26.6 nm, ∆ ) 5.56 nm, ∆s ) 9.26 nm.

Discussion PEO Adsorption. Despite its water solubility, PEO is surface active and has the ability to adsorb onto hydrophobic surfaces. This apparently arises from the conformational flexibility of the ethylene oxide chains, with the less polar trans conformation favored in hydrophobic environments and more polar gauche conformations in the aqueous environment.35,36 The narrow radius of the PEO polymer chain of about 0.05 nm means there is ample room on the interface between the SDS headgroups for the polymer to adsorb. While the surface area of the SDS headgroup is taken to be of the order of 0.55 nm,2,36-38 this (34) Ahlna¨s, T.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1987, 91, 4030-4036. (35) Bjo¨rling, M.; Karlstro¨m, G.; Linse, P. J. Phys. Chem. 1991, 95, 6706-6709. (36) van Voorst Vader, F. Trans. Faraday Soc. 1960, 56, 1067-1077. (37) Gerrens, H.; Hirsch, in Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; Wiley-Interscience: New York, 1975; p II-485.

Electroacoustic Study of Adsorption of PEO

Figure 7. Variation of dynamic mobility with frequency and PEG-35K concentration of sunflower oil emulsion, with initial oil volume fraction 6%, SDS bulk concentration 6 mM and NaCl 2 mM. PEG-35K concentration (wt %) and conductivity (S/m) at 25 °C: ([) 0, 0.0513; (0) 0.111, 0.0498; (2) 0.541, 0.0483; (O) 1.053, 0.0470; (/) 2.000, 0.0438; (b) 2.857, 0.0414; (]) 3.636, 0.0392; (9) 4.347, 0.0373; (4) 5.000, 0.0353.

is determined by the electrostatic repulsion between these charged moieties. The actual area occupied by the sulfate headgroup atoms is only of the order of 0.07 nm2. Consequently, adsorption of the PEO at the liquid-liquid interface is not a steric problem. Adsorption at the oil-water interface is not strong, however, even in the presence of SDS. This is apparent from the data presented in Figure 9, which shows the layer thicknesses continuing to increase at 5 and 10 wt % of PEO, where the wt % refers to the entire suspension, not just to the emulsion fraction. This weak adsorption is in contrast to the adsorption of PEO on hydrophilic particles such as silica, where strong adsorption was saturated at 0.1 wt %.39 Layer Thickness Estimation. Despite its weak adsorption, PEO is potentially an effective stabilizing agent because of the thickness of its adsorbed layer. This increases with concentration, and with polymer molecular weight (Figure 9), as expected, reaching tens of nanometers for the longer polymers. The O’Brien “gel-layer” model used to interpret the electroacoustic dynamic mobility (38) Staples, E.; Penfold, J.; Tucker, I. J. Phys. Chem. B 2000, 104, 606-614. (39) Zaman, A. A.; Bjelopavlic, M.; Moudgil, B. M. J. Colloid Interface Sci. 2000, 226, 290-298.

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Figure 8. Comparison of gel layer model (lines) with experimental data (symbols) for the dynamic mobility ratio µD(coated)/ µD(uncoated) for PEG-35K on sunflower oil emulsion. PEG35K concentration (wt %) (0) 0.111; (2) 0.541; (O) 1.053; (/) 2.000; (b) 2.857; (]) 3.636; (9) 4.347; (4) 5.000. (a) ω0 ) 1.26 × 106 s-1, κ ) 0.293 nm-1, d ) 21.5 nm, ∆ ) 2.88 nm, ∆s ) 3.89 nm; (b) ω0 ) 2.51 × 106 s-1, κ ) 0.290 nm-1, d ) 28.0 nm, ∆ ) 3.14 nm, ∆s ) 4.80 nm; (c) ω0 ) 3.77 × 106 s-1, κ ) 0.286 nm-1, d ) 32.3 nm, ∆ ) 3.30 nm, ∆s ) 5.46 nm; (d) ω0 ) 5.03 × 106 s-1, κ ) 0.279 nm-1, d ) 39.6 nm, ∆ ) 3.73 nm, ∆s ) 6.84 nm; (e) ω0 ) 6.28 × 106 s-1, κ ) 0.272 nm-1, d ) 44.0 nm, ∆ ) 4.08 nm, ∆s ) 7.84 nm; (f) ω0 ) 7.54 × 106 s-1, κ ) 0.266 nm-1, d ) 49.2 nm, ∆ ) 4.53 nm, ∆s ) 9.09 nm; (g) ω0 ) 7.54 × 106 s-1, κ ) 0.260 nm-1, d ) 52.0 nm, ∆ ) 4.98 nm, ∆s ) 10.04 nm; (h) ω0 ) 7.54 × 106 s-1, κ ) 0.255 nm-1, d ) 53.5 nm, ∆ ) 5.52 nm, ∆s ) 10.90 nm.

spectra comprises a dense inner layer, ∆, which shifts the electrical double layer outward, and a more extended outer layer, d, with its associated elasticity. With some exceptions, such as neutron scattering, other experimental approaches do not provide this level of detail, so comparisons must be made with the thickness ∆s obtained from the low-frequency limit of the dynamic mobility spectra.29 It is generally accepted that the hydrodynamic layer thickness is about twice the coil diameter dc for PEO layers adsorbed on polystyrene or silica particles in aqueous solvent,9 and about 3dc on mica.4,5 This is consistent with the results of the PEG-1500 adsorption, in which ∆s reaches 6.0 nm with its dc of 2.9 nm. On the other hand, the largest ∆s values for PEG-10K and -35K of about 10 nm are lower than their 2dc of 15 and 28 nm, respectively, but the adsorption has again not reached its saturation

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Figure 9. Outer layer thickness d, inner layer thickness ∆ (closed symbols) and static mobility thickness ∆s (open symbols) as functions of PEG concentration and PEG molecular weight on sunflower oil.

limit. However, the results are of the same order as those of other studies,9-13,17-19 indicating that the electroacoustic analysis is generally reliable. Of particular interest is the observation that the calculated inner-layer thicknesses for PEG-10K and PEG35K are almost identical. This suggests that, above a certain size, chain length is not important for the dense packing near the surface, although it has a dramatic effect on the extent of the outer extended layer, d. Lasic40 investigated various PEO conformations on liposome surfaces. The thicknesses of the polymer were 3.6, 3.6-3.8, and 7-7.5 nm, for 1.3, 4.5, and 9 mol % PEG2000, respectively. These values can be compared with those of ∆s, evaluated from the low frequency limit of the dynamic mobility, of approximately 2.8, 3.8, and 5.5 nm for PEG-1500 (Figure 9). Polymer thickness was also estimated to be about 5 nm for 5-10% PEG-2000 on liposomes from the reduction in the d.c. electrophoretic mobility.41 In contrast, Barnes and Prestidge42 calculated thicknesses of 5-20 nm for PEO-PPO-PEO block copolymers adsorbed on poly(dimethylsiloxane) emulsion drop(40) Lasic, D. D. In Poly(ethylene glycol) Chemistry-Biotechnical and Biomedical Applications; Harris, J. M., Zalipsky, S., Eds.; American Chemical Society: Washington, DC, 1997. (41) Woodle, M. C.; Collins, L. R.; Sponsler, E.; Kossovski, N.; Papahadjopoulos, D.; Martin, F. J. Biophys. J. 1992, 61, 902-910.

Kong et al.

lets. The hydrophobic PPO block creates strong adsorption. The adsorbed layer thicknesses increased with PEO chain lengths from 17 to 132 PEO units, corresponding to molecular weight from 750 to 5800. Hence the layer thicknesses from their work are much larger than found in the present electroacoustic study. This may relate to the incomplete adsorption of the PEO; the electroacoustic study should be repeated with block copolymers to make a more valid comparison with this published work. Electrophoresis gives only a single, static mobility thickness, without the detail of the two-layer model provided by the electroacoustics measurements. The volume fraction profile of adsorbed polymers can be obtained from neutron scattering or reflectivity measurements,43 but these do not appear to have been done for the PEO systems under investigation here. Again there has been a small-angle neutron scattering study of the adsorption of triblock copolymers on perfluorodecalin emulsion droplets.44 The thickness of the PEO tail layers ranged from 5 nm for an average molecular weight of 3350 to 15 nm for a molecular weight of 4300. These are comparable to the distances measured in the present work, shown in Figure 9. There was no evidence of two layers of different densities, as has been found for the adsorption of β-casein at the air-water interface.43 Again the results are not directly comparable with the electroacoustic study, as the presence of the PPO-block anchor undoubtedly affects how the PEO behaves. Phase Lead. The elementary two-layer model of O’Brien provides a good theoretical description of both the magnitude and phase of the dynamic mobility spectra for the adsorption of PEO-1500. There is a systematic discrepancy for the phase angle observed with the two longer chain PEO’s, with the experimental phase leads larger than the model predicts. It is likely that additional effects, such as chain entanglements, which are not included in the model, are responsible for these differences. There are other assumptions of the O’Brien “gel layer” model which might also be suspect in the present case: the validity of the Debye-Hu¨ckel approximation for the diffuse double layer and the random coiling of the PEO chain near the interface. The values of relaxation frequency ω0 at the higher PEG concentrations are in the range of 7.54-20.1 × 106 rad s-1, corresponding to a shear modulus γ of order 7.5420.1 × 103 N m-2 at 20 °C, which is in the upper range reported in the literature.12,45,46 [Note that in the previous data obtained from the O’Brien model25,29 the relaxation frequencies were wrongly quoted as Hz and so both the frequencies and the moduli need to be multiplied by 2π × 106 to give the modulus in N m-2.] Conclusions This work has investigated the effect of poly(ethylene oxide) adsorption on the electroacoustic behavior of sunflower oil-in-water emulsions in the presence of SDS, which provides information about the properties of the adsorbed polymer layer. The adsorption of the poly(42) Barnes, T.; Prestidge, C. A. Langmuir 2000, 16, 41164121. (43) Atkinson, P. J.; Dickinson, E.; Horne, D. S.; Leermakers, F. A. M.; Richardson, R. M. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 994998. (44) Washington, C.; King, S. M.; Heenan, R. K. J. Phys. Chem. 1996, 100, 7603-7609. (45) Faers, M. A.; Luckham, P. F. Colloids Surf. A 1994, 86, 317327. (46) Neuha¨usler, S.; Richtering, W. Colloids Surf. A 1995, 97, 3951.

Electroacoustic Study of Adsorption of PEO

mer on the SDS-stabilized oil surface is weak, but there are profound effects on the dynamic mobility spectra. These are interpreted with O’Brien’s “gel-layer” model, which works well for the PEG-100 system but shows some differences in the phase angles at higher frequencies with the longer polymer chains. Both the inner and the outer layer thicknesses became larger with increase in the polymer concentration and molecular weight, reaching up to 50 nm for PEG-35K. The effective static mobility thickness ∆s was calculated for each adsorption measurement. It is this value that is compared with the hydrodynamic layer thickness measured by other techniques. The ∆s value is about twice the radius of gyra-

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tion (dc) of the polymer chain at the highest concentration for PEG-1500, but less than 2dc for PEG-10K and PEG35K. The electroacoustic technique through the dynamic mobility spectrum is an effective method for investigating the adsorption of neutral polymers at the oil/water interface of emulsion droplets. Acknowledgment. This work was supported by the award of a Gritton Research Scholarship, at the University of Sydney, to L.K. LA026034X