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Insight into the Inversion Mechanism of an Inverse Polymer Emulsion Robert A. Gelman, John C. Harrington, and K. Abe Vaynberg* Hercules Incorporated, Research Center, 500 Hercules Road, Wilmington, Delaware 19808 ReceiVed May 1, 2008. ReVised Manuscript ReceiVed September 18, 2008 A study of the mechanism of inversion of inverse polymer emulsions showed that the likely key step in the inversion process is the swelling of polymer particles caused by added water diffusing through the oil phase. Therefore, we propose that inversion occurs as a result of polymer particle crowding caused by water diffusion and subsequent droplet rupture that results in the release of the polymer into the water phase.
Introduction An emulsion is a dispersed system wherein a liquid phase (also termed the internal or dispersed phase) is fragmented into droplets and suspended or dispersed in a second phase (termed the external or continuous phase) that is essentially immiscible in the first phase. Stability against coalescence is usually provided by a surfactant or emulsifier.1 An inverse emulsion, which is often called a water-in-oil (w/o) emulsion, is defined as an aqueous phase dispersed in a nonaqueous (oil) phase. Inverse emulsions can be used to prepare low-viscosity, high-solids dispersions of high-molecular-weight, water-soluble polymers. Inverse emulsion polymerization is the process typically used to prepare high-molecular-weight polymers from water-soluble monomers. The process involves the emulsification of an aqueous solution of the hydrophilic monomer(s) in a nonpolar organic solvent such as a paraffin oil. Surfactant stabilizes the emulsion. Nonionic surfactants typically yield a more stable emulsion than an ionic material. An oil-soluble initiator initiates polymerization. An inverse emulsion polymerization can produce a product that contains 20-40% polymer, which is an order of magnitude higher than that produced by other polymerization methods. Water-soluble polymers synthesized in the form of inverse polymer emulsions have widespread utility in personal care, oil drilling and recovery, water treatment, paper manufacturing, and other areas. Inverse polymer emulsions offer unique advantages to both polymer producers and end users. Inverse emulsion polymerization allows the direct synthesis of the product without additional purification or drying. For the end user, the inverse polymer emulsions provide a high-solids, easy-to-handle, highmolecular-weight polymer product. A key attribute of inverse polymer emulsions is the ability to invert the emulsion on demand, transferring the polymer to the continuous water phase. This is achieved by adding the inverse emulsion to a large excess of water (i.e., on the order of 1 to 100). Understanding the mechanism of inversion is important to the successful preparation and use of inverse polymeric emulsions. The scientific literature on inversion of emulsions is inconsistent, with exceptions to every proposed rule.2 It is generally accepted that the inversion of water-in-oil emulsions can occur via one of two mechanisms, transitional inversion or catastrophic inversion. Transitional inversion is a reversible phenomenon. An example is the addition of a second surfactant to change the * Corresponding author. E-mail:
[email protected]. (1) Salager, J. L.; Marquez, L.; Pena, A. A.; Rondon, M.; Silva, F.; Tyrode, E. Ind. Eng. Chem. Res. 2000, 39, 2665–2676. (2) Salager, J. L. In Encyclopedia of Emulsion Technology; Becher, P., Ed; Marcel Dekker: New York, 1988; Vol. 3, pp 79-134.
nature of the emulsion stabilization system. As the HLB of the surfactant system is changed beyond a critical value, inversion occurs spontaneously. Agitation does not impact the rate of inversion. Thus, as the name implies, inversion is a transition from one physical arrangement to another. Catastrophic inversion, in contrast, is an abrupt, irreversible change caused by a large increase in the volume of the dispersed phase.3,4 The extent of agitation, the surfactant chemistry and concentration, the phase viscosity, and the addition rate of the dispersed phase all significantly impact the inversion process.4,5 The mechanisms described above apply to emulsions where the dispersed phase is a low-viscosity fluid that readily coalesces and rearranges in response to changing thermodynamic conditions such as changes in the phase volume or the HLB of the stabilizing surfactant(s). The inverse polymer emulsions clearly differ from these simpler inverse water-in-oil emulsions. The dispersed phase of an inverse polymer emulsion contains approximately 50 wt % polymer. The high-solids-containing dispersed phase of inverse polymer emulsions renders the mechanism of inversion complex. The mechanism is not well understood. An additional surfactant, typically added after polymerization, can be used to promote inversion. Armanet et al.6 studied the impact of inverting surfactant concentration and HLB on the inversion process and found that higher HLB is beneficial to inversion efficiency. They suggested two pathways to explain the role played by surfactant HLB. The first pathway is based on the formation of a continuous hydrophilic pathway connecting polymeric particles and surrounding water in the presence of high HLB surfactant. The second pathway is based on the intrinsic instability of the inverse emulsion in the presence of higher HLB surfactant in accordance with Bancroft’s rule,7 which favors the oil-in-water emulsion state under this condition. Greenshields8 suggested two different mechanisms of inversion. The first is said to take place as a result of water/polymer particle swelling due to osmotic pressure, rupturing, and release into the surrounding water.9 The second is based on the observation that oil emulsification away from polymer gel clusters (3) Brooks, D. B.; Richmond, H. N. Chem. Eng. Sci. 1994, 49, 1065–1075. (4) Vaessen, G. E. J.; Steing, H. N. J. Colloid Interface Sci. 1995, 176, 378– 387. (5) Mira, I.; Zambrano, N.; Tyrode, E.; Marquez, L.; Pena, A. A.; Pizzino, A.; Salager, J. L. Ind. Eng. Chem. Res. 2003, 42, 57–61. (6) Armanet, L.; Hunkeler, D. J. Appl. Polym. Sci. 2006, 103, 3567–3584. (7) Rosen, M. J. Surfactants and Interfacial Phenomena, 3rd ed.; John Wiley & Sons: New York, 2004. (8) Greenshields, J. N. Annu. Surfactants ReV. 2000, 3, 66–96. (9) Larson, E. H.; Dakanayake, M.; Busler, W. R. U.S. Patent 5,925,714, June 20, 1999.
10.1021/la8013656 CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
12728 Langmuir, Vol. 24, No. 22, 2008
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leaves the latter exposed to water, which leads to polymer dissolution in the continuous water phase. A need to better understand the mechanism of inversion prompted us to undertake an experimental investigation of the inversion process. We developed a model system based on sodium chloride-containing emulsions to elucidate the mechanism of inversion.
Experimental Section Model emulsions comprising sodium chloride (Sigma-Aldrich, >99.5 wt % purity), deionized water, sorbitan monooleate (Arlacel, Uniqema), and a low-molecular-weight paraffin oil were prepared by dissolving the salt in water, adding the aqueous solution to the surfactant-containing oil, and then mixing under high-shear conditions for 10 min. Sodium chloride crystal-containing systems were prepared by dispersing 24 wt % sodium chloride solution in water (saturated concentration at room temperature) and allowing the water to evaporate overnight under mixing. Both high- and low-salt-containing dispersions were prepared as 67% of the dispersed phase. Water evaporation leading to the formation of salt crystals decreased the water-phase concentration to 62%, effectively increasing the salt content from 24 to 30 wt %. Phase contrast microscopy with a green filter was used to enhance the contrast and to improve quality. All experiments were carried out at room temperature.
Figure 1. Dispersion of 5 wt % NaCl solution in paraffin oil.
Results and Discussion Direct observation, albeit difficult, is the best way to gain insight into the inversion process. Despite references to direct observation,8 published data could not be found. Several factors complicate direct observation. First, the polymer droplets within the oil phase are submicron in size, rendering the use of light microscopy difficult. Second, inverse emulsions typically are not transparent. Third, inversion is a rapid process with the largest change, as measured by the viscosity increase, taking place within the first few minutes of inversion. Finally, inversion requires agitation to ensure proper dispersion in water and to prevent the formation of slow-to-invert agglomerates. Our model system circumvents these difficulties by replacing polymer containing droplets with droplets containing salt solution. The model system consists of two water-in-oil emulsions containing dissimilar concentrations of sodium chloride. We expect that when such emulsions are brought into contact, the higher salt-containing droplets will swell at the expense of those containing a lower sodium chloride concentration and these changes can be observed. A successful observation of this process would then serve to indicate that, driven by osmotic pressure, sufficient water flux can occur through the continuous oil phase, causing swelling that drives the inversion of inverse polymer emulsions. Figure 1 shows a water-in-oil emulsion containing 5 wt % sodium chloride. The emulsion has a broad droplet size distribution ranging from 1 to 10 µm. The droplet size distribution was observed to remain unchanged over days. Thus, the results discussed below are not due to Ostwald ripening or coalescence but rather to swelling. Figure 2 shows a water-in-oil emulsion containing 30 wt % sodium chloride. The system was prepared by dispersing a saturated salt solution (24 wt % sodium chloride) in oil and allowing the water to evaporate under stirring. The resulting emulsion contains salt crystals in equilibrium with droplets containing saturated sodium chloride solution. The two emulsions (5 and 30 wt %) are brought together by placing small droplets (∼0.01 g) of each side by side on a microscope slide. Another slide is placed on top of the droplets,
Figure 2. Dispersion containing 30 wt % NaCl. Arrows identify NaCl crystals.
and the moderate pressure generated by the weight of the second slide brings them into contact. The dynamics of water diffusion and the resulting droplet swelling are studied by observing the merged front. Figure 3 shows the merged droplets. The dashed line approximates the boundary separating them. The top left portion of the image does not contain any salt crystals and so consists of the 5 wt % sodium chloride-containing dispersion shown in Figure 1. The bottom right portion of the image, in contrast, contains readily discernible crystals, which are the same as the 30 wt % emulsions shown in Figure 2. Note that several crystals (highlighted in boxes A and B of Figure 3) are partially solubilized and appear as inclusions in small water droplets. The partial solubilization of these crystals is assumed to take place during sample preparation (3-5 min) and is consistent with the dynamics of water diffusion as discussed below. The diffusion of water was studied by observing the dissolution of the salt crystals such as those shown in boxes A and B of Figure 3. The advantage of observing the swelling of droplets that contain salt crystals is that the salt concentration remains constant during the process of swelling. This maintains the osmotic driving force and the resulting swelling as relatively constant. The images in Figure 4A,B were taken every 2 min and show
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Langmuir, Vol. 24, No. 22, 2008 12729
measures 6 µm (on the basis of microscopy data) and acts as an ideal sink (i.e., water concentration at its boundary is 0 ppm). The assumptions are that the outer boundary has a diameter of 10 µm, the size of the droplet does not change as water diffuses into it, the concentration of water at the outer boundary is 50 ppm10 (equivalent to the solubility of water in hexadecane), and the diffusion coefficient of water is on the order of 10-5 cm2/s.11 On the basis of these conditions and assumptions, we can use the following expression to describe water flux
flux ) DC
Figure 3. Image of merged 5 and 30 wt % NaCl containing emulsion droplets. The dashed line approximates the interface. Boxes A and B are the areas shown in Figure 4A,B, respectively.
Figure 4. (A) Dissolution of the NaCl crystal shown in box A of Figure 3. The images were taken 2 min apart. (B) Dissolution of the NaCl crystal shown in box B of Figure 3. The images were taken 2 min apart.
that, over time, the crystals decrease in size as the size of the water droplets increases. These observations are consistent with water effectively diffusing through the continuous oil phase and into the dispersed aqueous phase. Figure 4A,B also illustrates water transport through the continuous oil phase and can be used to estimate water flux to the droplets (calculated on an individual drop basis). The flux is estimated on the basis of droplet size changes from 5 to 7 µm (Figure 4A, frames 1-4, i.e., images containing the salt crystal) over a 6 min period and from 6 to 9 µm (Figure 4B, frames 1-8) over a 14 min period. The increase in droplet volume corresponds to average water flux values of 1.9 × 10-14 and 1.8 × 10-14 mol/s, respectively, for the droplets in Figure 4A,B. A simple diffusive model was used to verify the results of the experiment. To provide an order of magnitude estimate of water flux into a droplet, the model uses several conditions and assumptions. The boundary conditions are such that the droplet
1 1/RID - 1/ROD
(1)
where D is the diffusion coefficient, C is the water concentration at the outer boundary, and RID and ROD are the inner and outer boundary sizes, respectively. Equation 1 yields an estimated water flux of 2 × 10-13 mol/s (i.e., flux to the droplet with an assumed diameter of 6 µm). The number is an order of magnitude larger than the experimental flux values. However, considering the approximate nature of the model, the exaggerated boundary conditions and assumptions, and that the experiment limits the movement of water to the solubilizing salt crystal from only one side (i.e., the side containing the 5 wt % salt solution), we find the difference to be acceptable. We have demonstrated that water, driven by osmotic pressure, can effectively diffuse through the oil phase, causing water droplets to swell. This provides the basis for our proposed mechanism of inversion. Inversion begins with the addition of the inverse polymer emulsion to a large excess of water. With agitation, the inverse emulsion breaks into a water-in-oil-inwater emulsion where the innermost water part contains the polymer. Water located at the boundaries of the dispersed inverse emulsion, driven by osmotic pressure, begins to diffuse through the oil phase toward the polymer particles, causing them to swell. Assuming a water flux on the order of 10-14 mol/s per polymer droplet in the vicinity of the water interface, a polymer emulsion droplet with an initial size of 500 nm would double in size in 3 s. Considering the high initial volume fraction occupied by polymer containing droplets, swelling should result in droplet crowding, forcing polymer release into water continuum. It is known that the inversion of easily inverted inverse polymer emulsions results in a noticeable viscosity increase within the first few seconds after the emulsion is added to the water continuum. Our experimental observations lead us to conclude that the rapid swelling and crowding of the internal phase is the driving force behind inversion and polymer transfer into the continuous water phase.
Conclusions This work supports a catastrophic model for the inversion of inverse polymer emulsions. The addition of the water-in-oil emulsion to excess of water causes water to dissolve into the oil phase and to diffuse into the dispersed aqueous phase. This causes the particles to swell and, driven by osmotic pressure, to rupture, thereby liberating the polymer into the now continuous aqueous phase. LA8013656 (10) Ruelle, P.; Kesselring, U. W. J. Solution Chem. 1996, 25, 657–665. (11) Bird, R. B.; Steward, W. E.; Lightfoot, E. N. Transport Phenomena, 2nd ed.; John Wiley & Sons: New York, 2001.
