Water-in-Carbon Dioxide Emulsions with Poly(dimethylsiloxane

Ind. Eng. Chem. Res. 2000, 39, 2655-2664

2655

Water-in-Carbon Dioxide Emulsions with Poly(dimethylsiloxane)-Based Block Copolymer Ionomers Petros A. Psathas, Sandro R. P. da Rocha, C. Ted Lee, Jr., and Keith P. Johnston* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712

K. T. Lim and S. Webber Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712

The stability of water-in-CO2 (W/C) emulsions stabilized with poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) and PDMS-b-poly(acrylic acid) (PDMS-b-PAA) ionomer surfactants is reported as a function of surfactant architecture, pH, temperature, pressure, and droplet flocculation. For a given PDMS block length, the stability of the emulsion is correlated with the distance from the balanced state where the surfactant prefers the water and CO2 phases equally. When the pH starting at 3 is raised up to 5-6, the hydrophilic/CO2-philic balance of the surfactant increases, because of ionization of COOH, and the emulsion becomes more stable. At the pH of maximum stability, the emulsion becomes more stable with a decrease in the PDMS length, for a given ratio of block lengths, because of gelation of the flocculated 2-5 µm primary droplets. W/C emulsions are stable with respect to sedimentation for >24 h and are resistant to coalescence for more than 7 days. Because of gelation, the W/C emulsions are more stable than water/hexane emulsions (at ambient pressure) formed at the same conditions. The addition of 20% hexane to CO2 as a cosolvent reduced flocculation in some cases to zero. Introduction In the past decade, there has been an extensive interest in colloids utilizing supercritical fluids (SCF), as discussed in recent reviews.1-5 At The University of Texas, work in water-in-CO2 (W/C) microemulsions started in 1984 when Bob Schechter suggested that microemulsions could be used to raise the viscosity of CO2 for enhanced oil recovery. In 1989, the dissertation of his student, John Oates,6 described attempts to form these microemulsions. This pioneering work led to the first successful report of W/C microemulsions in 1994.7 We are grateful to Bob for many stimulating discussions on interfacial properties in supercritical fluids over the last 15 years. In supercritical fluids, the interfacial properties and colloid stability may be manipulated by adjusting the pressure and temperature. Microemulsions were formed first in ethane and propane,4,8 and these studies provided guidelines for the formation of W/C microemulsions.7,9 Carbon dioxide (Tc ) 31 °C, Pc ) 73.8 bar) is one of the most abundant, inexpensive, and environmentally benign solvents on Earth. It is nonflammable and essentially nontoxic. Therefore, W/C microemulsions and macroemulsions are appealing substitutes for traditional organic solvents in materials processing, solvent-free coatings, heterogeneous reactions including polymerization, and separation processes including cleaning, purification, and extraction of heavy metals. Because of their low polarizability per unit volume (R/ ν) or weak van der Waals forces, most nonvolatile hydrophilic and lipophilic compounds, such as pharmaceuticals, polymers, proteins, salts, and metals, have very low solubility. Many of these substances can be * To whom correspondence should be addressed. Tel: (512) 471-4617. Fax: (512) 475-7824. E-mail: [email protected].

solubilized in W/C microemulsions or emulsions. These emulsions are stabilized by surfactants containing tails, which are known to be solvated by CO2 fluorocarbons, fluoroethers, and siloxanes.10-12 In contrast to microemulsions, which are thermodynamically stable and optically transparent with droplet sizes from 2 to 5 nm, macroemulsions with droplets >0.1 µm require kinetic stabilization.13,14 Macroemulsions may be formed with higher interfacial tensions between water and CO2 than in the case of microemulsions, and thus lower values of surfactant adsorption at the interface. Therefore, it may be expected that macroemulsions may be formed for a wider variety of surfactants and with lower surfactant concentrations. Furthermore, almost any ratio of the two phases can be incorporated into each other, unlike the case for microemulsions. Recently, W/C emulsions containing volume fractions of water from 10 to 90% have been formed in CO2 using perfluoropolyether (PFPE) surfactants with COO- headgroups.15,16 In addition, C/W emulsions have been stabilized with poly(butylene oxide)-b-poly(ethylene oxide) and Lodyne 106A [C6F13(CH2)2SCH2CH(OH)CH2N+(CH3)3Cl-, MW ) 531.5 g/mol].17 These emulsions have also been used for phase-transfer reactions between CO2-soluble substrates and hydrophilic nucleophiles.17 Further studies of the mechanism of emulsion stabilization are needed for the rational design of surfactants for this newly emerging field. For W/C emulsions, steric stabilization against flocculation may be imparted by the physical adsorption of surfactant molecules at the interface. The CO2-philic segment of the surfactant extends outward, producing a repulsive force which opposes the attractive van der Waals (Hamaker) force between the droplets.18 As the density of CO2 is decreased, the surfactant tail collapses, reducing steric repulsion. Sharp transitions in stability

10.1021/ie990779p CCC: $19.00 © 2000 American Chemical Society Published on Web 06/16/2000

2656

Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000

function of ionic strength and pH.29-31 To further understand the effect of pH and ionic strength on the COOH groups on the hydrophilic side of the interface, control experiments were performed with hexane-brine emulsions. These emulsions are inherently simpler to understand than W/C emulsions, because hexane is a good solvent for PDMS, unlike CO2. Next, the stability of W/C emulsions was studied as a function of the surfactant architecture (HCB), pH, CO2 density, and added cosolvent (hexane). The mechanisms governing emulsion curvature, flocculation, and stability are characterized as a function of the distance from the balanced state in Figure 1, on the basis of photomicrographs, settling rates, and electrical conductivity measurements. Experimental Section

Figure 1. Schematic representation of the effect of formulation variables in the phase behavior of a ternary system, water-CO2nonionic surfactant.

have been observed for latex particles at the critical flocculation density by theory,19,20 simulation,21 and experiment.22 The curvature and stability of both microemulsions and macroemulsions are often described on a “fish”shaped plot of surfactant concentration versus a formulation variable as shown in Figure 1.23,24 In addition to the traditional variables, we include pressure, which has a large effect on the solvation of tails and the hydrophilic/CO2-philic balance (HCB) of the surfactant, which is analogous to the hydrophilic/lipophilic balance (HLB) for oil-water systems. At low HCB values, the surfactant favors the CO2 phase and low-conductivity W/C emulsions are formed. For high values of HCB, the surfactant favors water and C/W emulsions are formed with high conductivities. The head of the “fish” encompasses the three-phase region with a middle emulsion phase and excess water and CO2 phases. The vertical centerline for the fish is the balanced state where the surfactant favors the phases equally. Here the interfacial tension γ and thus gradients in γ are small, and Marangoni-Gibbs stresses, which are discussed in further detail below, provide little stabilization. In this study the stability of the emulsion will be characterized in terms of the distance from the balanced state, as has been done for W/O emulsions.25-27 The objective of this study was to design block copolymer ionomer surfactants with a hydrophobic poly(dimethylsiloxane) (PDMS) block and an ionizable poly(methacrylic acid) (PMA) or poly(acrylic acid) (PAA) block to manipulate the properties of stable W/C and C/W emulsions. The emulsions contained equal amounts of water and CO2. The CO2-philic group was chosen as PDMS to complement the above earlier studies with PFPE. Whereas only one ion was present on the PFPE COO- surfactants, the number of ions was varied in the PDMS-b-PMA or PDMS-b-PAA polyelectrolytes to achieve further control over the HCB. The Mn of the PDMS block ranged from 5 to 20K, whereas that of the polyelectrolyte varied from 300 to 2K. The pH was increased from 3 to 8.2 with buffers to adjust the HCB by varying the deprotonation of the COOH groups.28 Hydrophobically modified polyelectrolytes have been used to study the stability of O/W emulsions as a

Material Synthesis. The synthesis of the block copolymeric surfactants containing dimethylsiloxane and carboxylic acid sequences was achieved with grouptransfer polymerization (GTP). Silyl ketene acetal terminated PDMS was used as the initiator, and the free acid form was produced with hydrolysis of the ester sequences according to the literature.32 Table 1 lists the various surfactants synthesized. The preparation of PDMS5K-b-PMA1K was achieved with GTP under an argon atmosphere in a previously flamed glass reactor. A typical example is as follows: A total of 7 g (1.4 × 10 -3 mol) of dried 5K macroinitiator is introduced into the reactor. Then 20 mL of THF and 14 mg of TBABB (2 mol % based on silyl ketene acetal functionality) were transferred into the reactor by means of a cannula. After 5 min of stirring, 1.7 g of TMSMA was added slowly via syringe and the polymerization proceeded for 2 h under argon. The polymerization was quenched with degassed methanol (2 mL). After an additional 0.5 h of stirring, THF and methanol was removed by evaporation. The polymer collected was dried at 10-3 Torr for 24 h. About 2 g of the polymer was suspended in 30 mL of 5% aqueous sodium hydroxide, and the solution was stirred for 12 h at room temperature. After the pH of the solution was adjusted to 5 with hydrochloric acid, the hydrolyzed polymer was collected, washed with water, and dried under vacuum. For the synthesis of 20K PDMS monomethacrylate, anionic polymerization of hexamethylcyclotrisiloxane (D3; Aldrich) was carried out in a one-neck, roundbottomed flask equipped with a rubber septum under a prepurified nitrogen atmosphere. The cyclohexane solution of D3 was injected into the reaction flask, and 0.7 mL of sec-butyllithium was added to initiate the ringopening polymerization. The reaction was allowed to proceed for 2 h, followed by the addition of 6 mL of THF to promote propagation of the living siloxanolate species. The polymerization was terminated with [3-(methacryloxy)propyl]dimethylchlorosilane (United Chemicals) to afford the macromonomer which was then precipitated in acetonitrile containing a small amount of NaHCO3, filtered, and dried under reduced pressure at room temperature. The product was collected in a viscous form at a yield of 90%, Mn of 20 000 and Mw of 23 200. For the characterization, size-exclusion chromatography (SEC) was carried out with a Waters GPC 510 apparatus equipped with three 5-mm cross-linked polystyrene (PS) columns (linear mix and 500 and 100 Å, American Polymer Standard Co.) with THF as the eluent. 1H NMR using a Varian Unity Plus-300 determined the composition and degree of hydrolysis. Deter-

Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2657 Table 1. Preparation of Various PDMS Block Copolymersa no.b

polymerc

macroinitiator (g)d

monomer (g)d

1 2 3 4 5 6 7 8

PDMS10K-b-(PtBA0.8K-co-PAA0.5K) PDMS10K-b-PAA0.7K PDMS10K-b-PAA1.4K PDMS10K-b-PAA1.5K PDMS5K-b-PMA0.3K PDMS5K-b-PMA1K PDMS5K-b-PMA2K PDMS20K-b-PMA0.7K

10K-PDMA (7) 10K-PDMS (7) 10K-PDMS (7) 10K-PDMS (7) 5K-PDMS (7) 5K-PDMS (7) 5K-PDMS (4.8) 20K-PDMS (7)

TBA (1.4)/TMSA (0.7) TMSA (0.6) TMSA (1.1) TMSA (1.1) TMSMA (0.5) TMSMA (1.7) TMSMA (2.3) TMSA (0.3)

a 20 mL of THF was used as the solvent. b Sample nos. 1 and 5 contain approximately 35-40% of unreacted homo-PDMS. Sample no. 8 contains approximately 80% of unreacted homo-PDMS. c Composition was determined by both 1H NMR and titration. d The numbers in parentheses are the amounts which are added in the reactor.

Figure 2. High-pressure apparatus used for emulsion preparation, stability assessment, and droplet size measurements.

mination of both the degree of hydrolysis and neutralization was also carried out by titration with 0.1 N KOH/ MeOH in a THF solution using phenolphthalein as the indicator. Methods Emulsion Preparation and Stability. Emulsions involving supercritical CO2 were prepared in a 28-mL high-pressure, variable-volume view cell,33 as shown in Figure 2, and visual observations of formation and stability were recorded with time. The desired amounts of water, CO2, surfactant, and hexane (when needed) were loaded in the front of the cell. A manual highpressure syringe pump (High-Pressure Equipment Co.) was used to adjust the pressure on the back side of the piston to within 1 bar, using CO2 as the pressurizing fluid. The temperature was adjusted to within 0.1 °C by submerging the cell into a water bath equipped with a temperature controller. The emulsion was formed by recirculation through a 0.005 or 0.01 in. i.d. capillary with a high-pressure liquid chromatography pump (LDC/Milton Roy minipump) for 20 min. Cell contents were also stirred with a magnetic stir bar. The criterion used to determine the stability was the appearance of a 20% volume of excess CO2 on the top or water on the bottom of the cell after stopping recirculation and stirring. A six-port injection valve (Valco Instruments Co.) was used to inject small aliquots of a concentrated NaOH solution to adjust the pH, using an external loop of 100 µL. Microscopy. A 1.5-mL high-pressure optical cell enabled us to observe the emulsions through a microscope (Nikon OPTIPHOTZ-POL) at ×900 magnification. Two half-circled spacers were used to choose the path length between the windows, in this case 137 µm. When the six-port valve was closed and the 100-µL external

loop was isolated from the cell, the emulsion was recirculated through the optical cell which was already filled up with CO2, thus diluting the emulsion 15 times. The apparatus was equipped with a video camera (Mti, series 68) and a monitor, which allowed the droplet aggregation and coalescence to be recorded. A Polaroid Freeze-Frame apparatus was used to acquire photomicrographs of the most representative cases of flocculation. The optical cell was not heated, but the effect of temperature on emulsion stability was found to be small. Electrical Conductivity. The electrical conductivity was measured to determine the continuous phase, by taking advantage of the ultralow conductivity of supercritical CO2. The in-situ conductivity cell consists of two collinear 1/8 in. o.d. stainless steel tubes inserted in the variable-volume view cell. A two-hole alumina ceramic thermocouple insulator (Omega, 1/16 in. o.d. × 0.02 in. i.d.) was secured in the stainless steel tube with highpressure epoxy (General Fiber Optics, P/N 41-40). A 0.01 in. diameter 304 stainless steel wire was inserted into the alumina tube, forming a 2 mm loop at the end by inserting the end of the wire into the other hole secured with epoxy as well. The wire was frequently platinized with a YSI 3140 platinizing solution. Measurements were made with a conductivity meter (YSI, model 3100). The cell constant was calibrated with various NaCl solutions at 23 °C and ranged from 1.5 to 4.0 cm-1. Sonication of Brine-Hexane Emulsions. The emulsions between brine and hexane were formed in 20-mL disposable scintillation vials. Partitioning of the surfactant between the phases was recorded. Next the mixture was emulsified at room temperature using a Branson sonifier (model 250) equipped with a microtip that was submerged into the mixture. The output was controlled at 30% and the duty cycle at 25%, and the sonication was carried out for at least 3 min. The continuous phase was determined both with conductivity measurements and dilution of a droplet of the emulsion into hexane. A hexane-in-water emulsion showed no tendency to disperse into hexane, whereas a water-in-hexane emulsion dispersed immediately, forming a homogeneous turbid phase. This method was especially followed for low ionic strength solutions. The separation was recorded at certain times after, until complete phase separation. pH Measurement in the Aqueous Phase. The pH of the aqueous phase was calculated according to the biphasic model from Holmes et al.34 The pH was monitored by the UV-vis absorption profile of the hydrophilic indicator 4-nitrophenyl 2-sulfonate. In our calculations, the effect of ionic strength on the behavior of the indicator was included. The dissolution of CO2 in water renders a pH of 3 because of the carbonic acid-

2658


Figure 3. Atmospheric 50/50 (mass) hexane-water emulsions with 1 wt % overall surfactant concentration, shown 7 h after formation. The type of emulsion (W/O or O/W), pH, and ionic strength conditions are indicated, respectively. (a) PDMS5K-bPMA2K. (b and c) PDMS10K-b-PAA1.4K. The dark area corresponds to emulsified phase and the blank to excess hexane (top) or water (bottom), respectively.

bicarbonate equilibrium, and therefore large amounts of a NaOH solution (1.5-2.0 M) have to be added to achieve neutrality or alkaline conditions in the W/C microemulsions and emulsions. Results and Discussion Emulsion Formation with PDMS-b-PAA or -PMA Surfactants. Atmospheric Pressure Experiments. These emulsions were formed at room temperature with sonication of equal amounts of brine and hexane with 1 wt % surfactant. The pH was adjusted with small aliquots of 0.5 M NaOH and the ionic strength with 1.5 M NaCl. In Figure 3 we present schematic representations of the phase behavior of these emulsions, formed with two different surfactants, versus pH or ionic strength. The drawings represent the emulsions 7 h after emulsification. Immediately after sonication, hexane and brine were fully emulsified in all cases. Ranges in pH from 3 to 10.0 and in ionic strength from 0 to 0.4

M were examined, but only benchmark cases in which a significant change occurred are included in Figure 3. The dark area corresponds to the emulsified phase, whereas the transparent bottom or top regions indicate excess phases of water and hexane, respectively. The same behavior was observed for emulsions stabilized by PDMS-b-poly(tert-butyl acrylate) (PtBA)-co-PAA (10K0.8K-0.5K), PDMS-b-PMA (5K-0.3K), PDMS-b-PAA (10K-1.5K), and PDMS-b-PAA (20K-0.7K) as shown in Table 2. Each of these emulsions was hexane continuous for a pH range from 3 to 10, indicating little change in the HLB from the deprotonation of the polyelectrolyte. The length of the PAA or PMA block, ranging from 4 to 10 repeat units, appears to be too short for the COO- ions to cause the surfactant to favor the aqueous phase. Even at the highest pH where the largest fraction of the PAA or PMA block ionizes, the surfactant has a high affinity for hexane, reflecting the strong solvation of the high molecular weight PDMS block. These systems did not appear to approach the balanced state,35 because the stability changed relatively little. For example, the time for 20% settling decreased from 2 h at pH 3.0 to 1.5 h at pH 8 for PDMS5K-b-PMA0.3K. The rest of the surfactants mentioned above showed almost an identical loss of stability. The emulsion phase behavior underwent transitions because of ionization of the PMA or PAA block for the surfactants PDMS-b-PMA (5K-1K and 5K-2K) and PDMS-b-PAA (10K-1.4K) as presented in Figure 3a-c and Table 2. The molecular weights of the polyelectrolyte chains, composed of 12-23 repeat units, were significantly larger than those of the other surfactants described above. An increase in ionization with increasing pH changed the balance of the surfactant at the interface toward water (increased HLB), resulting in water-continuous emulsions over a narrow range of pH, i.e., 5-5.7. The pH for 5K-2K is approximately 5, and that for 10K-1.4K is 5.7. Given that the pKa of PMA is 4.7,36 it is likely that the inversion at pH 5.0 is due primarily to the ionization of COOH groups. Moreover, the stability decreased as the phase inversion point was approached. For example, it decreased from 2.5 h at pH 3 to 1 h at pH 5.5 for PDMS10K-b-PAA1.4K. The emulsion formed with the PDMS5K-b-PMA2K inverted from W/O to O/W at pH 5.0 with a passage through a middle-phase Winsor III emulsion37 (Figure 3a). Coalescence led to top and bottom phases or WIII behavior. A further increase in pH produced an inversion back to a W/O emulsion (Figure 3b). This type of transition has been observed for the decane/water system stabilized with 3 vol % oleic acid in the presence of 10 vol % pentanol-2.37,38 One possibility for the inversion is an increase in the ionic strength from

Table 2. Surfactant Structures and Outline of Experimental Findings

surfactant structure PDMS5K-b-PMA0.3Kb

PDMS (mol %)

atmospheric pH W/hexane

PDMS5K-b-PMA1K PDMS5K-b-PMA2K

94.3 83.3 71.4

3-10 3-5, 6-10 3-5, 6-10

PDMS10K-b-PtBA0.8K-co-PPP0.5K PDMS10K-b-PAA0.7K PDMS10K-b-PAA1.4K PDMS10K-b-PAA1.5K PDMS20K-b-PAA0.7K

88.5 93.5 87.7 86.9 96.6

3-10 3-10 3-5.5, 6-10

a

3-10

hexane/W

high pressure W/CO2 h)c

5-5.9 5-5.9

5.5-5.9

highly (>24 highly (24 h) moderately (∼24 h) d moderately (∼24 h) slightly @ pH ) 5-5.5 (>4 h)

bal. st. pHa >9.0

∼6.0

∼5.5

pH that induced transition to the balanced state. b The index denotes the molecular weight of each block. c The degree of flocculation refers to a CO2 density of 0.9564 g/mL. d Was not studied.

Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 2659

Figure 4. Photomicrographs of representative cases of flocculation encountered in 50/50 (mass) CO2-water emulsions at T ) 25-65 °C and P ) 100-350 bar.

NaOH, which could salt the surfactant out of water because of reduced headgroup solvation.37 To test this hypothesis, we formed a series of O/W emulsions with the same surfactant at a pH of 5.8 (Figure 3c) with varying amounts of added NaCl. The inversion to a W/O emulsion occurred at an ionic strength of only 0.11 M (Figure 3c), indicating that the ionic strength could also cause the inversion in Figure 3b. Moreover, the stability gradually increased when the ionic strength was raised to 0.36 M, indicating a WI to WIII to WII transition. In conclusion, a complete transition was observed from WII to WIII to WI to WIII to WII with pH. In contrast, only W/O emulsions are formed for the surfactants with a polyelectrolyte Mn of less than 1000. To further understand the properties of this surfactant, we also tested the effect of pH on the formation of micelles of the PDMS-b-PMA (5K-1K and 2K) surfactants in water (without hexane). We detected a strong effect of pH on the aggregation behavior, as observed visually on the basis of the color of the solution. At low pH, e.g., 3-5, the surfactant was almost insoluble, whereas at a pH greater than 5, the solution turned bluish, indicating aggregation of the surfactant molecules. The turbidity increased up to a value of pH 12.0, except for the most hydrophilic surfactant (2000 Mn PMA), which at the maximum pH completely dissolved in water. These experiments are consistent with the emulsion experiments in that both exhibit a transition, which indicates a large increase in solvation by water at pH 5 or greater. High-Pressure Experiments with Compressed or Supercritical CO2. Given the calibration of the surfactants at the hexane-brine interface, we now address emulsions formed with equal weights (50/50) of water and CO2. This section is organized in terms of the formulation variables, CO2 density (or pressure), pH, PDMS molecular weight, and temperature. The last section considers the addition of a cosolvent, hexane.

Four categories for the degree of flocculation have been defined for the W/C emulsions as shown in Figure 4: nonflocculated and slightly, moderately, and highly flocculated. The visual observations of the emulsions without magnification were very consistent with the photomicrographs. The nonflocculated emulsions were milky white and fine in appearance with no aggregates in the photomicrographs. The slightly flocculated emulsions appeared to be white and uniform with a few aggregates, consisting of 1-10 individual droplets. A moderate degree of flocculation indicates aggregates of approximately 10-40 droplets; these emulsions were more viscous, and slight inhomogeneities were visible. The highly flocculated emulsions were even more viscous, with a network of aggregates 20-35 µm in size (40-120 droplets per aggregate) that could not be redispersed easily by stirring. In the extreme case of very highly flocculated emulsions (not shown in Figure 4), the flocs were greater than 40 µm in size and could not be redispersed with stirring. In all cases the 2-5 µm primary droplets within the aggregates did not coalesce. (i) Effect of CO2 Density or Pressure. A CO2 density versus temperature stability plot for a W/C emulsion stabilized by 1 wt % PDMS-b-PAA (10K-1.5K) is shown in Figure 5. The isostability contours indicate the amount of time it took for the emulsions to settle to produce a clear excess phase occupying 20% of the volume. Because of the compressible nature of CO2, small variations in pressure at constant temperature have a major impact on the density and thus the solvent strength of CO2. A decrease in density of less than 0.3 g/mL reduced the stability from over 2 h to less than 20 min. The decrease in density induced rapid flocculation of the water droplets, which eventually led to coalescence and separation after long periods of time. These emulsions were resistant to coalescence even at low densities

2660


Figure 5. Stability diagram (CO2 density vs temperature) for a 50/50 (mass) CO2-water (0.1 M NaOH) emulsion with 1 wt % overall PDMS10K-b-PAA1.5K at pH ) 6.0. Solid lines represent isostability contours for 20% volume settling labeled above. Different symbols refer to degrees of flocculation.

Figure 7. Stability diagram (CO2 density vs pH) for a 50/50 (mass) CO2-water emulsion with 1 wt % overall PDMS10K-bPtBA0.8K-co-PAA0.5K. (a) Nonaqueous phase 100% CO2. (b) Nonaqueous phase 20/80 (mass) hexane-CO2 mixture.

Figure 6. Stability diagram (CO2 density vs pH) for a 50/50 (mass) CO2-water emulsion with 1 wt % overall PDMS10K-bPMA0.3K. (a) Nonaqueous phase 100% CO2. (b) Nonaqueous phase 20/80 (mass) hexane-CO2 mixture. In this and all of the remaining figures, the temperature and pressure were varied from 25 to 65 °C and 69 to 345 bar, respectively. For a given density, the effect of temperature is small (as shown in Figure 5) and, thus, these temperatures are not labeled.

of 0.6 g/mL (110 bar at 45 °C) where the solvation of PDMS by CO2 is minimal. For example, the time necessary for complete separation at a pressure of 150 bar at 45 °C was more than 3-4 h. Throughout the entire range of temperature or pH, as can be seen in Figures 5-9, a decrease in density consistently induced flocculation and a significant increase in the settling rate, primarily as a result of

Figure 8. Stability diagram (CO2 density vs pH) for a 50/50 (mass) CO2-water emulsion with 1 wt % overall PDMS5K-bPMA2.0K. Electrical conductivity of the emulsion (k) is also shown.

increased aggregate size. The collapse of PDMS tails with decreasing density has a direct impact on the ability of PDMS to shield the approaching water droplets from flocculation. A decrease in density produces larger aggregates and increases the difference between the dispersed and continuous phase densities; consequently, the settling velocity increases markedly and the emulsion separates faster. Similar features are observed for PDMS10K-b-PtBA0.8K-co-PAA0.5K in Figure 7a and PDMS20K-b-PAA0.7K in Figure 9 although they appear to be less stable at a given density. For these two surfactants, the contours in Figures 7a and 9 are


Figure 9. Stability diagram (CO2 density vs pH) for a 50/50 (mass) CO2-water emulsion with 1 wt % overall PDMS20K-bPAA0.7K.

shifted to higher densities than those in Figure 6a for a given stability. Specifically, a reduction in density from 0.95 to 0.75 g/mL accelerated separation by more than 4 h in Figure 9 and nearly 24 h in Figure 7a. (ii) Effect of pH. The effect of pH on the emulsion stability and degree of flocculation is shown in Figure 6 for PDMS5K-b-PMA0.3K. For the hexane-water emulsions, this surfactant with a very short PMA block favors hexane over water and thus W/O emulsions. Similarly in CO2 only W/C emulsions are formed. Consider a CO2 density of 0.9 g/mL. The stability is quite low at pH 3, increases with pH reaching a maximum at about pH 6, and then decreases at higher pH. Also, the maximum stability occurs at a lower pH of 4.5-5, and the right-hand branch of the U-shaped contours is shifted to lower values of pH versus the value of pH 6 in Figure 6. At the highest stabilities, the emulsions are highly flocculated for 5K Mn PDMS, only moderately flocculated for 10K, and only slightly flocculated for 20K. Despite this decrease in flocculation, the emulsion stability actually decreases in this order. This somewhat counterintuitive result will be discussed in the section on PDMS molecular weight after first focusing on the trends for changes in stability with pH. The stability of the PDMS-b-PMA (5K-0.3K) emulsion decreases markedly with the addition of hexane as a cosolvent to form a 20/80 by weight mixture of hexane and CO2 as is evident in a comparison of parts a and b of Figure 6. The pH where the emulsion is most stable shifts very little. However, the degree of flocculation decreases from moderately flocculated to slightly flocculated. Again, stability decreases even though flocculation decreases. For PDMS-b-PtBA-co-PAA (10K-0.8K0.5K) the addition of cosolvent lowers the stability but also shifts the pH of maximum stability from 4.5 to 5.5. It also shifts the right side of the U-shaped contours to higher pH. At a low pH of 3, none of the PMA or PAA groups ionize. The low stabilities of all of these emulsions suggest limited adsorption of surfactant at the interface,26 which is consistent with the weak interactions of the nonionized COOH groups with water. These surfactants are very far from the balanced state. This hypothesis is supported by the fact that the surfactant with the lowest HCB (weakest tendency toward water) PDMS-b-PAA (20K-0.7K) produces the least stable emulsion. It may be expected to have the lowest HCB for the following reason. The anchor-soluble balance

(ratio of blocks) is similar for the three surfactants, but the 20K PDMS group is the most hydrophobic (e.g., 13K Mn PDMS homopolymer is quite soluble in CO2 at a level of 4 wt % at 277 bar and 35 °C).39 As the pH is increased to 4 and then 5, the ionization of the COOH group increases the surfactant-water interaction, raising the surfactant adsorption and emulsion stability.26 As the pH is raised further, the stability goes through a maximum and then decreases very rapidly with the approach of the balanced state. These observations are in accordance with the predictions of a theoretical model at the water-oil interface, which states that a block copolymer ionomer is interfacially active when the degree of ionization is relatively small.40 In the region denoted as the balanced state, the surfactant equally favors water and CO2 and the emulsions separated in less than 15 s. Here we observed a middlephase emulsion with excess water and CO2 phases for all of the surfactants except from PDMS5K-b-PMA0.3K. An exception to this behavior was seen for PDMS-bPMA (5K-2K) in Figure 8, where the stability appeared to be low at all pH values. The HCB of this surfactant is relatively high even without any ionization. This system appears to reach the balanced state at a low pH, leading to a low emulsion stability at all pH values. Similarly for PDMS-b-PMA (5K-0.3K), only part of the transition to the unstable region (balanced state) occurs for a pH up to 8.2 with or without cosolvent (Figure 6). The high stability of this system at high pH values is caused by the low HCB, resulting from the small PMA block. The only emulsions that inverted from W/C to C/W with pH were those stabilized by PDMS-b-PMA (5K2K), the most hydrophilic surfactant (Figure 8). At pH values above 7.0, the ionization of an unknown fraction of the 23 repeat units was sufficient to give emulsions with very high conductivities, indicating a watercontinuous phase. However, separation occurred rapidly both on the top and on the bottom, to produce a WIII emulsion because this emulsion was too close to the balanced state to achieve moderate stability of C/W emulsions. Furthermore, greater amounts of base are required to deprotonate the surfactant than for the emulsions of water and hexane, because of the acidity of CO2. At a pH of 12, the ionic strength is approximately 2 M, which tends to salt the surfactant out of water (see Figure 3). Further comparisons may be made between the W/O emulsions (Figure 3) and W/C emulsions (Figures 6-9). In CO2, an inversion in curvature from W/C to C/W was observed only with a PMA Mn of 2000, whereas only a value of 1400 was required to invert the W/O emulsions. The opposite behavior would be expected by the fact that hexane solvates PDMS more effectively than CO2. The observed trend may be attributed to weaker ionization and solvation of COOH groups for the W/C emulsions at a given pH because of the higher ionic strength resulting from carbonic acid. Also, the poorer solvation of PDMS by CO2 may cause these chains to be closer to the aqueous side of the interface, where they inhibit ionization of COOH groups41-43 as well as a nonuniform distribution of counterions along the polyelectrolyte chain.44 The loss of stability at the balanced state may be explained in terms of interfacial tension gradients and the monolayer bending elasticity. Flocculation and coalescence of emulsion droplets may be impeded by

2662


Marangoni-Gibbs stabilization.45,46 The tangential interfacial flow of a continuous phase in the thin film between droplets leads to nonuniform surfactant distribution, or an unfavorable interfacial tension gradient. This gradient inhibits thin film drainage, which stabilizes emulsions. The drainage is also highly dependent upon the interfacial viscosity. As suggested recently,27,47 emulsion stability is related to the monolayer bending elasticity. Emulsion droplets coalesce when the film between the droplets ruptures, because of the formation of a hole in the film. The ability to nucleate a hole in the interfacial film is a strong function of the spontaneous curvature of the surfactant layer, and nucleation becomes faster as the curvature goes to zero in the balanced state. Another factor, which could explain the loss in stability at the balanced state, is depletion flocculation of emulsion droplets due to W/C micelles or microemulsion droplets.48-50 Both micelles and microemulsions would become more likely with ionization of COOH groups, because ions are extremely CO2-phobic. Future studies to explore the possibility of these types of organized molecular assemblies are recommended. Clearly further work is needed to understand the relative contributions of interfacial tension gradients and bending elasticity to the loss of stability of W/O and W/C emulsions at the balanced state. The important conclusion of this section is that the changes in stability with pH (and thus the HCB of the surfactant) may be rationalized in terms of the distance from the balanced state. (iii) Effect of the Molecular Weight of PDMS. The stabilization of the W/C emulsions is complex because it involves flocculation and coalescence, both of which are influenced by various thermodynamic and kinetic factors. Consider first flocculation. The longer the PDMS block is, the longer the range of the steric repulsion to counteract the van der Waals attractive forces between the water droplets. In all of the W/C emulsions in this study without hexane cosolvent, some degree of flocculation was present in the photomicrographs as shown in Figure 4. The same is true for W/C emulsions stabilized by PFPE,16 for 180-600 nm silica particles stabilized by grafted PDMS chains (5K-22K Mn), even for high graft densities,51 and for PMMA latex particles as shown recently. The Hamaker constant for two water droplets in CO2 at 345 bar and 25 °C is 10.9 × 10-21 J. For two 2 µm water droplets in CO2, the van der Waals potential becomes attractive (-3/2kT) at a distance of 90 nm. The presence of 20 wt % hexane cosolvent decreases this distance to 51 nm. According to Napper,18 the unperturbed root-mean-square (rms) end-to-end distance of 20K PDMS is approximately 7-8 nm, and it will be larger for grafted chains because of steric forces. The energy of repulsion from the grafted chains is sufficient to overcome the attractive Hamaker forces according to eq 1 by Korgel et al.52 This calculation is consistent with the observed lack of flocculation for the droplets in the photomicrographs for the emulsions with a CO2-hexane mixture. The flocculation will be prevented further by Marangoni-Gibbs stabilization. Even though the PDMS-based surfactants do not prevent flocculation, they do prevent coalescence. The emulsions were stable, in some cases for more than 24 h, and proved to be extremely resistant to coalescence, even when highly flocculated, for example in part d of

Figure 4. The large impact of the PDMS tail length can be clearly seen in Figures 6-9 and Table 2. The degree of flocculation decreased as Mn of the PDMS increased as expected, because of partial steric stabilization. PDMS-b-PMA (5K-0.3K) stabilized emulsions were highly flocculated even at the highest CO2 densities, yet they still occupied the total volume of the cell. These short PDMS tails provide limited steric stabilization such that the attractive van der Waals forces produce a gellike network of adhered droplets as shown in Figure 4c. The viscosity of the emulsion was relatively high, as reflected in the ease of redispersion after stopping recirculation and stirring. The structure of this emulsion is somewhat analogous to gelled W/O emulsions formed with Span 80.48 The gelation of these emulsions increased with a decrease in solvent quality. Similar observations were made by Bibette et al.,53 who reported emulsions indefinitely stable against coalescence because of the formation of a rigid (solidlike) monolayer of surfactant at the oil-water interface. In the case where the emulsions were not flocculated, the stability may be expected to be much lower than for the gelled emulsions, as is evident from the following discussion of the Stokes equation. Nonflocculated dilute 3 µm droplets of water would settle at a velocity of 3.17 µm/s at 310 bar and 35 °C (20% settling in 20 min) because of the large ∆F and the low continuous phase viscosity. The stability of 2 h at all pH values for the nonflocculated emulsions with hexane cosolvent is consistent with this magnitude of sedimentation velocity, after correction for hindered sedimentation due to the finite droplet volume fraction. In contrast, the felled emulsions were stable for over 24 h. As described above, the emulsions became finer, less flocculated, less viscous, and more easily redispersible with an increase in PDMS molecular weight. As the PDMS length increases, the greater steric repulsion reduces the degree of flocculation from the attractive van der Waals forces. Even though the droplets flocculated (Figure 7), coalescence of these droplets was not observed even after 7 days at a CO2 density of 0.9 g/mL. In addition, the size distribution of the individual droplets is relatively narrow, with the majority of drops between 2 and 5 µm in diameter as shown in Figure 4. (iv) Effect of Temperature. The effect of temperature is shown for PDMS-b-PAA (10K-1.5K) in Figure 5 at pH 6. Almost identical trends were also observed for a surfactant with a slightly smaller polyelectrolyte block (0.7K). Throughout the temperature range 2565 °C, the conductivity data as well as visual observation of emulsion separation indicate that CO2 was the continuous phase. At constant density the stability increases slightly with temperature. For an anionic surfactant, an increase in temperature drives the partitioning toward water. Perhaps temperature raises the adsorption of the surfactant to give this slight increase in stability. (v) Effect of Hexane Cosolvent. To enhance the solvation of PDMS in the continuous phase, two experiments were performed under exactly the same conditions, except that 20 wt % hexane was added as a cosolvent. The effect was dramatic, as shown in Figures 6b and 7b. The degree of flocculation decreased markedly. For 10K PDMS (PDMS10K-b-PtBA0.8K-co-PAA0.5K), the photomicrographs revealed no flocculation at all at the highest CO2 density and pH 4 and 5. Some flocculation was always present for the PDMS5K-b-PMA0.3K


surfactant, indicating less ability to screen the van der Waals forces. In Figure 7, a greater pH was required to reach the balanced state after adding cosolvent because of the increased affinity of the surfactant toward the nonaqueous phase. At the pH of maximum stability, the emulsions were less stable with the addition of cosolvent, because it reduced the flocculation, which in turn lowers gelation. A reduction in flocculation with the addition of cosolvent was also observed for silica particles in CO2. Conclusions The synthesis of PDMS-b-PMA and PDMS-b-PAA ionomers may be tailored to manipulate the viscosity and degree of flocculation of W/C emulsions, which are stable with respect to coalescence for days. Four categories of emulsions were identified as a function of flocculation, according to consistent observations between photomicrographs and visual observation of emulsion viscosity and homogeneity. At high pressures, the stability of most of the emulsions increases as the pH is raised above 3 because ionization raises the surfactant adsorption. Eventually, the stability decreases as the surfactant approaches the balanced state because of a loss in Marangoni-Gibbs stability and facile rupture of thin films due to the small bending energy. Inversion of W/C to C/W emulsions with pH was achieved only with the most hydrophilic surfactant, because a large amount of base was required to raise the pH above 6, and this produces a high ionic strength. The high ionic strength salts the surfactant out of water. The emulsion stability increases markedly with gelation, which results from flocculation. Without any flocculation, 2-5 µm drops would only be stable for tens of minutes in low-viscosity CO2. At the pH of maximum stability, the most stable emulsions settled 20% in more than 24 h and did not coalesce within the observation time frame of 7 days. The location of this pH shifted with the HCB of the surfactant (ratio of block lengths) in the expected manner according to solvation of the two blocks. At this pH, gelation and thus emulsion stability increase with a decrease in the PDMS length for a given ratio of block lengths. Because of gelation, the W/C emulsions are more stable than water-hexane emulsions formed at ambient pressure. The addition of 20% hexane to CO2 as a cosolvent reduces flocculation, in some cases to zero, resulting in low-viscosity emulsions which were less stable than the more flocculated and gelled emulsions without hexane. A major practical advantage of W/C emulsions is the ability to break the emulsions with pressure. The emulsions coalesced in hours as the density was reduced to 0.6 g/mL and in only seconds at ambient pressure. Acknowledgment We acknowledge support from NSF, the Welch Foundation, Unilever Research, and the Separations Research Program at The University of Texas at Austin. S.R.P.d.R. thanks CNPqsBrazil for financial support. Literature Cited (1) Bartscherer, K. A. Microemulsions in compressible fluidss A review. Fluid Phase Equilib. 1995, 107, 93-150. (2) Johnston, K. P.; Jacobson, G. B.; Lee, C. T.; Meredith, C.; da Rocha, S. R. P.; Yates, M. Z.; DeGrazia, J.; Randolph, T. W. Microemulsions, Emulsions, and Latexes in Supercritical Fluids;

Jessop, P., Leitner, W., Eds.; Wiley-VCH: Weinheim, Germany, 1999; pp 127-146. (3) McFann, G. J. Formation and Phase Behavior of Reverse Micelles and Microemulsions in Supercritical Fluid Ethane, Propane and Carbon Dioxide. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1993. (4) Fulton, J. L.; Smith, R. D. Reverse Micelle and Microemulsion Phases in Supercritical Fluids. J. Phys. Chem. 1988, 92, 2903-2907. (5) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Aggregation of Amphiphilic Molecules in Supercritical Carbon Dioxide: A Small Angle X-ray Scattering Study. Langmuir 1995, 11, 4241-4249. (6) Oates, J. The Thermodynamics of Solubilization in Aqueous Surfactant Solutions. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1989. (7) Harrison, K.; Goveas, J.; Johnston, K. P. Water-in-Carbon Dioxide Microemulsions with a Fluorocarbon-Hydrocarbon Hybrid Surfactant. Langmuir 1994, 10, 3536-3541. (8) Johnston, K. P.; McFann, G. J.; Lemert, R. M. Pressure Tuning of Reverse Micelles for Adjustable Solvation of Hydrophiles in Supercritical Fluids. ACS Symp. Ser. 1989, 406, 140-164. (9) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Water in Carbon Dioxide Microemulsions: An Environment for Hydrophiles Including Proteins. Science 1996, 271, 624-626. (10) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Synthesis of Fluoropolymers in Supercritical Carbon Dioxide. Science 1992, 257, 945-947. (11) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth: Stoneham, MA, 1994. (12) Hoefling, T. A.; Beitle, R. R.; Enick, R. M.; Beckman, E. J. Design and Synthesis of Highly CO2-Soluble Surfactants and Chelating Agents. Fluid Phase Equilib. 1993, 83, 203. (13) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Effect of Surfactants on the Interfacial Tension and Emulsion Formation between Water and Carbon Dioxide. Langmuir 1999, 15, 419428. (14) Bibette, J.; Morse, D. C.; Witten, T. A.; Weitz, D. A. Stability Criteria for Emulsions. Phys. Rev. Lett. 1992, 69, 24392442. (15) Randolph, T. W.; deGrazia, J.; Johnston, K. P.; Lee, C. T. J.; Jacobson, G. B. Emulsions of Carbon Dioxide and Water: Towards the Ultimate Green Chemistry Solvent. J. Colloid Interface Sci. 2000, to be submitted. (16) Lee, C. T.; Psathas, P. A.; Johnston, K. P. Water-in-Carbon Dioxide Emulsions: Formation and Stability. Langmuir 1999, 15, 6781-6791. (17) Jacobson, G. B.; Lee, C. T. J.; da Rocha, S. R. P.; Johnston, K. P. Organic Synthesis in Water/Carbon Dioxide Emulsions. J. Org. Chem. 1999, 64, 1207-1210. (18) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (19) Meredith, J. C.; Johnston, K. P. Theory of Polymer Adsorption and Colloid Stabilization in Supercitical Fluids. 2. Copolymer and End-Grafted Stabilizers. Macromolecules 1998, 31, 55185528. (20) Meredith, J. C. Theory of Polymer Adsorption and Colloid Stabilization in Supercritical Fluids. 1. Homopolymer Stabilizers. Macromolecules 1998, 31, 5507-5517. (21) Meredith, J. C.; Sanchez, I. C.; Johnston, K. P.; de Pablo, J. J. Simulation of Structure and Interaction Forces for Surfaces Coated with grafted Chains in a Compressible Solvent. J. Chem. Phys. 1998, 109, 6424-6434. (22) Yates, M. Z.; O’Neill, M. L.; Johnston, K. P.; Webber, S.; Canelas, D. A.; Betts, D. E.; DeSimone, J. M. Emulsion Stabilization and Flocculation in CO2. 2. Dynamic Light Scattering. Macromolecules 1997, 30, 5060-5067. (23) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. Cloud Points, Solubilization and Interfacial Tensions in Systems Containing Nonionic Surfactants. J. Chem. Technol. Biotechnol. 1990, 48, 161-171. (24) Kalhweit, M. The Phase Behavior of Systems of the Type H2O-Oil-Nonionic Surfactant-Electrolyte. J. Colloid Interface Sci. 1982, 90, 197-202. (25) Shinoda, K.; Friberg, S. Emulsions and Solubilization; John Wiley and Sons: New York, 1986.

2664


(26) Ruckenstein, E. Thermodynamic Insights on Macroemulsion Stability. Adv. Colloid Interface Sci. 1999, 79, 59-76. (27) Kabalnov, A.; Wennerstom, H. Macroemulsion Stability: The Oriented Wedge Theory Revisited. Langmuir 1996, 12, 276292. (28) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Theory of Ionizable Polymer Brushes. Macromolecules 1995, 28, 1491-1499. (29) Lockhead, R. Y. Electrosteric Stabilization of Oil-in-Water Emulsions by Hydrophobically Modified Poly(acrylic acid) Thickeners. ACS Symp. Ser. 1991, 462, 101-120. (30) Lockhead, R. Y.; Rulison, C. J. An Investigation of the Mechanism by which Hydrophobically Modified Hydrophilic Polymers Act as Primary Emulsifiers for Oil-in-Water Emulsions. 1. Poly(acrylic acids) and Hydroxyethyl Celluloses. Colloids Surf. A 1994, 88, 27-32. (31) Rulison, C. J.; Lockhead, R. Y.; Bui, H. S.; Pierce, T. D. Investigation of the Mechanism of Emulsification by Hydrophobically Modified Hydrogels. IV-Dilute Systems. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1994, 863-864. (32) Lim, K. T.; Webber, S. E.; Johnston, K. P. Synthesis and Characterization of Poly(dimethylsiloxane)-Poly[alkyl(meth)acrylic acid] Block Copolymers. Macromolecules 1999, 32, 2811. (33) McFann, G. J. Phase behavior of AOT microemulsions in compressible liquids. J. Phys. Chem. 1991, 95, 4889-4896. (34) Holmes, J. D.; Ziegler, K. J.; Audriani, M.; Lee, C. T.; Bhargava, P. A.; Steytler, D. C.; Johnston, K. P. Buffering the Aqueous Phase pH in Water-in-CO2 Microemulsions. J. Phys. Chem. 1999, in press. (35) Anton, R. E.; Jean-Louis, S. Emulsion Instability in the Three-Phase Behavior region of Surfactant-Alcohol-Oil-Brine Systems. J. Colloid Interface Sci. 1986, 111, 54-59. (36) Nemec, J. M.; Bauer, J.; Mark, H. F. Acrylic and Methacrylic Ester Polymers; Wiley & Sons: New York, 1985; Vol. 1. (37) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems: Formulation, Solvency and Physical Properties; Marcel Dekker: New York, 1988; Vol. 30. (38) Qutubuddin, S.; Miller, C. A.; Fort, T. Phase Behavior of pH-Dependent Microemulsions. J. Colloid Interface Sci. 1984, 101, 46-58. (39) O’Neill, M. L.; Cao, Q.; Fang, M.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Solubility of Homopolymers and Copolymers in Carbon Dioxide. Ind. Eng. Chem. Res. 1996, 37, 3067-3079. (40) Wittmer, J.; Joanny, J. F. Charged Diblock Copolymers at Interfaces. Macromolecules 1993, 26, 2691-2697. (41) Ma, Y.; Cao, T.; Webber, S. E. Polymer Micelles from Poly(acrylic acid)-graft-Polystyrene. Macromolecules 1998, 31, 17731778.

(42) Philippova, O. E.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. pH-Responsive Gels of Hydrophobically Modified Poly(acrylic acid). Macromolecules 1997, 30, 8278-8285. (43) Shusharina, N. P.; Nyrkova, I. A.; Khokhlov, A. R. Diblock Copolymers with a Charged Block in a Selective Solvent: Micellar Structure. Macromolecules 1996, 29, 3167-3174. (44) Eckert, A. R.; Martin, T. J.; Webber, S. E. Photoinduced Electron Transfer at the Core/Shell Interface of PolystyreneAnthracene-Poly(methacrylic acid) Diblock Copolymer Micelles in Aqueous Media. J. Phys. Chem. 1997, 101, 1646-1656. (45) Wasan, D. T. Destabilization of water-in-oil emulsions; Sjoblom, J., Ed.; NATO Advanced Study Institute Series C: Mathematical and Physical Sciences, No. 363; Plenum: New York, 1992; pp 283-295. (46) Edwards, D. A.; Brenner, H.; Wasan, D. T. Interfacial Transport Properties and Phenomena; Butterworth-Heinemann: Boston, 1991. (47) Kabalnov, A.; Weers, J. Macroemulsion Stability within the Winsor III Region: Theory versus Experiment. Langmuir 1996, 12, 1931-1935. (48) Leal-Calderon, F.; Gerhardi, B.; Espert, A.; Brossard, F.; Alard, V.; Tranchant, J. F.; Stora, T.; Bibette, J. Aggregation Phenomena in Water-in-Oil Emulsions. Langmuir 1996, 12, 872874. (49) Leal-calderon, F.; Mondain-Monval, O.; Pays, K.; Royer, N.; Bibette, J. Water-in-Oil Emulsions: Role of the Solvent Molecular Size on Droplet Interactions. Langmuir 1997, 13, 70087011. (50) Binks, B. P.; Fletcher, P. D. I.; Horsup, D. I. Effect of microemulsified surfactant in destabilizing water-in-oil emulsions containing C12E4. Colloids Surf. 1991, 61, 291-315. (51) Yates, M. Z.; Shah, P. S.; Johnston, K. P.; Lim, K. T.; Webber, S. E. Steric Stabilization of Colloids by Poly(dimethylsiloxane) in Carbon Dioxide: Effect of Cosolvents. J. Colloid Interface Sci. 2000, in press. (52) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. Assembly and Self-Organization of Silver Nanocrystal Superlattices: Ordered “Soft Spheres”. J. Phys. Chem. B 1998, 102, 83798388. (53) PDMS10K-b-PAA1.5K: Bibette, J.; Mason, T. G.; Gang, H.; Weitz, D. A.; Poulin, P. Structure of Adhesive Emulsions. Langmuir 1993, 9, 3352-3356.

Received for review October 26, 1999 Revised manuscript received February 24, 2000 Accepted February 26, 2000 IE990779P

Water-in-Carbon Dioxide Emulsions with Poly(dimethylsiloxane

Recommend Documents