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Mechanism of Stabilization of Silicone Oil-Water Emulsions Using Hybrid Siloxane Polymers Somil C. Mehta and P. Somasundaran* NSF Industry/UniVersity CooperatiVe Research Center for AdVanced Studies in NoVel Surfactants, Columbia UniVersity, New York, New York 10027 ReceiVed October 22, 2007. In Final Form: December 1, 2007 Silicone polymers, due to their high lubricity and good spreading properties, are widely used in industrial applications. Being insoluble in water and most hydrocarbons, a common mode of delivering silicones is in the form of emulsions. To stabilize silicones in the emulsion form more efficiently, it is useful to understand the mechanism of emulsion stabilization. Two different mechanisms of emulsion stabilization have been proposed in the past: film formation and precipitation (known as the Pickering mechanism). These two mechanisms are different, and there is a need to further investigate this issue. The aim of the present work was to investigate the mechanism of stabilizing silicone emulsions and to propose a generalized behavior. Several experiments including the measurement of Langmuir isotherms, rheology experiments, phase diagram studies, and microscopy experiments were conducted. All of the above techniques indicated that the functional groups interact strongly with the water phase. The emulsions were found to be stable only if the emulsifiers were soluble in silicone oil or the water phase, and the stability decreased as the emulsifier precipitated. In most cases tested here, the emulsifiers were not observed to precipitate as reported earlier for the Pickering mechanism, and the emulsion stabilization followed film formation. These results should help to predict emulsion stabilization for unknown systems.
Introduction Silicone polymers are the only class of organic as well as inorganic polymers that have been widely commercialized. Being insoluble in water and most hydrocarbons, a very common mode of utilization of silicones is in the form of emulsions.1 Silicone emulsions are used in many applications including textile softening,2 personal care,3 cosmetics,4 drug delivery,5 and printing ink formulations.6 Polymeric silicone surfactants have a silicone oil soluble backbone and water compatible hydrophilic modifying groups. Therefore, silicone surfactants are one of the favored compounds for stabilizing such emulsions.7 Because of the fundamental difference between the behavior of silicone polymers and the behavior of hydrocarbon polymers, it has been recognized in the past that the mechanisms by which conventional polymers act cannot be extended to silicone polymers. Hence, there have been some attempts to understand the mechanism of stabilization of silicone emulsions with hydrophilically modified silicone surfactants, but there are considerable discrepancies in their explanation. Sela et al. attributed the stability of emulsions by silicone surfactants to conventional steric stabilization and film formation at the surface * Corresponding author. E-mail:
[email protected]; tel.: (212) 8542926; fax: (212) 854-8362. (1) Hill, R. M. Silicone Surfactants: Surfactant Science Series; Marcel Dekker Inc.: New York, 1999. (2) Bajaj, P. Finishing of textile materials. J. Appl. Polym. Sci. 2000, 83, 631-659. (3) Starch, M. S. Personal-care emulsions comprising a siloxane-oxyalkylene copolymer; 1982. (4) Newton, J.; Stoller, C.; Starch, M. Silicone technologies as delivery systems via physical associations. Cosmet. Toiletries 2004, 119, 69-70, 72-74, 76, 78. (5) Kajihara, M.; Sugie, T.; Maeda, H.; Sano, A.; Fujioka, K.; Urabe, Y.; Tanihara, M.; Imanishi, Y. Novel drug delivery device using silicone: Controlled release of insoluble drugs or two kinds of water-soluble drugs. Chem. Pharm. Bull. 2003, 51, 15-19. (6) Rentzhog, M.; Fogden, A. Print quality and resistance for water-based flexography on polymer-coated boards: Dependence on ink formulation and substrate pretreatment. Prog. Org. Coat. 2006, 57, 183-194. (7) O’Lenick, J. A. J. Silicone emulsions and surfactants. J. Surfactants Deterg. 2000, 3, 387-393.
Figure 1. Structural representation of functionally modified silicone polymer. The grafting ratio is denoted by (a + b):n.
of a dispersed phase droplet.8,9 On the other hand, Anseth et al. attributed the mechanism of emulsion stabilization to the precipitation of silicone surfactants at the air-water interface, known as the Pickering mechanism.10 The Pickering mechanism suggests that the surfactant precipitates to form solid particles and aligns at the interface without interacting with the two phases. It is clear that a systematic effort is required to clarify the mechanism of stabilization of emulsions with modified silicone polymers. The present work utilizes four different experimental techniques to test the previous hypotheses and to understand the general behavior of hybrid silicones to stabilize emulsions. (8) Sela, Y. Y.; Magdassi, S.; Garti, N. Newly designed polysiloxane-graftpoly(oxyethylene) copolymeric surfactants: Preparation, surface activity, and emulsification properties. Colloid Polym. Sci. 1994, 272, 684-691. (9) Sela, Y.; Magdassi, S.; Garti, N. Release of markers from the inner water phase of W/O/W emulsions stabilized by silicone based polymeric surfactants. J. Controlled Release 1995, 33, 1-12. (10) Anseth, J. W.; Bialek, A.; Hill, R. M.; Fuller, G. G. Interfacial rheology of graft-type polymeric siloxane surfactants. Langmuir 2003, 19, 6349-6356.
10.1021/la7032912 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/26/2008
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Figure 2. (a) Langmuir isotherms of various functional modified silicone polymers at the air-water interface. pH ∼6.5 and I ) 0. (-----) PDMS, (- - -) acid modified silicone, (-‚-‚-‚) methylated silicone, and (-) amino silicone. (b) Langmuir isotherms of acid modified silicone polymers at the air-water interface. pH ∼ 6.5 and I ) 0. (-----) PDMS, (- - -) 1% acid modified silicone, (-‚-‚-‚) 11% acid modified silicone, and (-) 18% acid modified silicone.
Experimental Procedures Materials. Various polymeric silicone surfactants with non-ionic, cationic, anionic, and amphoteric functional modifications were synthesized at Elkay Chemical Pvt. Ltd. (Pune, India) and used for the current study. Amino modified polymeric silicone was synthesized from decamethyl cyclopentasiloxane (D5), tetramethyl ammonium hydroxide, and amino siloxane via the equilibration process. The cationic methylated amino silicone polymer was synthesized by methylation of the amino modified polymeric siloxane using p-tolyl methyl tosylate. The anionic silicone polymer was synthesized by reacting the amino modified silicone polymer with itaconic acid as described elsewhere.11 Non-ionically modified silicones were synthesized by hydrosilating the silicone polymer containing a silanic hydrogen moiety. Furthermore, in some cases, 50% of the amino groups in aminosilicone were modified to obtain 50% acid and methylated amino silicones. The ratio of reactants was selected to give an (a + b):n ratio of approximately 1:7.5 (Figure 1). The approximate molecular weights of all the compounds as measured from viscosity were 5000. The compounds had a volatile content (11) Berger, A.; Fost, D. L. Organosilicone having a carboxyl functional group thereon. U.S. Patent 5,596,061, 1997.
below 1% and were used without any further purification. Details regarding the synthesis of each polymer have been reported earlier.12 The general structure of the previously mentioned silicone surfactants is illustrated in Figure 1. Decamethyl cyclopentasiloxane was purchased from Aldrich Co. and used without any purification as silicone oil. Triple distilled water was used as the aqueous phase. Methods. Measurement of Langmuir Isotherms. A Nima MiniLangmuir trough (model 601 M) with Teflon barriers and vibrational isolation was used for surface pressure measurements. The Wilhelmy plate method using a paper plate (20 mm perimeter) provided by Nima Technology was employed for surface pressure measurements. The changes in surface pressure were measured using an electrobalance with a resolution of 0.1 mN/m. The surface of water was cleaned by using a micropipet aspirator. Chloroform, HPLC grade (with a minimum purity of 99.9%) purchased from Pharmco Products Inc., was used as the spreading solvent. A total of 25 µL of the 0.15 mg/mL solution was spread on the water surface, and chloroform was allowed to evaporate for 20 min. The surface pressure (12) Mehta, S. C.; Somasundaran, P.; Maldarelli, C.; Kulkarni, R. Effects of functional groups on surface pressure-area isotherms of hydrophilic silicone polymers. Langmuir 2006, 22, 9566-9571.
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Figure 3. (a) Phase diagram of silicone oil-water emulsions stabilized by 11.7% functional silicone polymeric emulsifiers. (b) Stable emulsion, (O) phase separation, and (×) turbid solution. pH ∼6.5 and I ) 0. (Green) water-in-oil emulsion and (blue) oil-in-water emulsion. (b) Phase diagram of silicone oil-water emulsions stabilized by various percent modifications of acid functional silicone polymeric emulsifiers. (b) stable emulsion, (O) phase separation, and (×) turbid solution. pH ∼6.5 and I ) 0. measurements were initiated only after ensuring that the pressure sensor was reading zero surface pressure. A constant temperature was maintained at 20 °C ((0.1 °C) using a Neslab water bath recirculator. The compression rate of the barriers was fixed at 5 cm2/min. All experiments were performed at pH 6 without ionic strength adjustment. Emulsion Preparation. Emulsions were prepared by first dissolving the functionalized polymeric silicone surfactant in cyclic silicone oil followed by the dropwise addition of water. The mixture was continuously stirred at high shear with an Ultraturrax T-18 homogenizer at 22 000 rpm. After the complete addition of water, the emulsification process was standardized by homogenizing for two extra minutes. The nature of emulsion (oil-in-water or waterin-oil) was determined by conductivity experiments. Rheology. The rheology of various silicone emulsions was measured using a Brookfield PVS rheometer to monitor the effect of emulsifier molecular architecture on the properties of emulsion. Coaxial cylindrical geometry with a rotating outer cylinder and stationary inner cylinder was employed. Shear transferred from the outer cylinder to the inner one was measured by the torque applied on the inner cylinder. Cylindrical bob B1, which is sensitive to viscosities in the range of 1.15 to 300 000 cp, was used for all the measurements. The temperature in all the experiments was controlled at 25 °C using a Julabo temperature controller (model F26). Microscopy. The oil-water interface was observed using a Nikon Eclipse microscope (Model ME 600) under transmitted light
geometry. A Spot Insight CCD camera (model 4.2) was utilized to capture images of the interface between the bulk silicone oil and the bulk water as well as the interface of water droplets dispersed in bulk oil media, as explained elsewhere.10 The experiments include observation of the bulk oil-bulk water interface, as well as the observation of the interface between water droplets dispersed in the bulk oil continuum. A drop of water was placed next to the drop of silicone oil containing 1% polymer, and the interface was observed under a microscope after about 5 min.10
Results Surface Pressure. Figure 2a represents the Langmuir isotherms of ionically modified hybrid silicone polymers at the air-water interface. The isotherms show different regimes under different surface concentrations. The shape of the isotherms in these regimes can be correlated to different surface conformations and orientations of the polymer chains. The modified and unmodified silicone polymers were hypothesized to conform in a linear fashion at a very low surface concentration and a coiled or helical manner at higher concentrations.12 The isotherms of different functional graft modifications look almost the same down to about 120 Å2/repeat unit (i.e., in regions I and II), with slight differences possibly because of minor variations in the equivalent weights. On further compression, between 120 and 50 Å2/repeat unit, the chains are proposed to undergo a conformational change from
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Figure 4. Rheological behavior of silicone oil-water emulsion stabilized by 11.7% functional silicone polymeric emulsifier (50% amino modified) as a function of emulsifier concentration. Shear rate ) 300 s-1, pH ∼6.5, and I ) 0.
Figure 5. Photomicrograph of silicone oil containing 1% EO modified polymer-water interface.
stretched to coil because of space constraints at the surface. This process warrants some of the functional groups to be pulled out of the water. The functional group that interacts more strongly with water will offer a higher resistance to leave the water subphase. Consequently, the chains having more affinity for water undergo a higher compression and show a higher surface pressure.12 The previous hypothesis was further corroborated by measuring the isotherms of acid modified silicone with different percent modifications, as presented in Figure 2b. Clearly, with an increase in hydrophilicity by increasing the percent modification, the interaction with the water sub-phase increased, and hence, the slope of region III increased. From the observations in Figure 2a,b, it can be proposed that the functional groups of modified silicone polymers at the interface are initially submerged in the aqueous phase and that with a reduction in area available, some of them are forced out to occupy a smaller area at the surface. A similar interaction of functional groups with water should also be present at the liquid-liquid interface in the case of emulsions. If the hybrid silicone polymers were stabilizing emulsions by the Pickering mechanism, they should be present in the silicone oil phase as a precipitate and should show minimal interaction with the aqueous phase. However, the Langmuir isotherm studies indicate that silicone emulsions are stabilized by the alternate mechanism of interfacial orientation and film formation of hydrophilic silicone polymers at the interface. Phase Diagram Study. Figure 3a,b illustrates phase diagrams of silicone emulsions stabilized by various ionic and non-ionically
modified silicone polymers.13 The emulsion stability for phase diagram purposes was noted by visual observation of phase separation when allowed to settle under gravity for 24 h. It can be seen from Figure 3a that the ability of the silicone emulsifier to stabilize emulsions depends on the nature of the hydrophilic functional group of the polymeric emulsifier employed. With an increase in the hydrophilicity from amino silicone to acid modified silicone to methylated cationic silicone, as the solubility in the oil phase decreases, their ability to stabilize the water-in-oil emulsion decreases. This leads to shrinking of the stable region in the phase diagram from amino to acid to methylated silicone. The non-ionic silicone, on the other hand, is very hydrophilic and water soluble. Therefore, it stabilizes the silicone oil-inwater emulsion instead of the water-in-oil emulsion. Such behavior could be explained based on Bancroft’s rule, which states that as the solubility of the silicone emulsifier in the oil phase decreases, its water carrying capacity decreases. This hypothesis was further corroborated by measuring the phase diagrams of acid modified silicone with different percent modifications. It can be seen from Figure 3b that with an increase in hydrophilicity due to an increase in percent modification, the compatibility of acid modified silicone with cyclic silicone decreases. This causes a decrease in the ability of the polymer to stabilize the water-in-oil emulsion as depicted by the shrinking of the stable region in the phase diagram (Figure 3b). These observations also suggest that there are interactions between the functional groups and the water phase. An emulsion appears to be stable only when the emulsifier is soluble either in silicone oil or in water. Moreover, a water-in-oil emulsion was obtained when the surfactant is oil soluble and vice versa. Rheology of Emulsions. Rheological properties of emulsions with 80% water and 20% silicone oil phase stabilized with modified silicone polymers are presented in Figure 4 as a function of the amount of silicone emulsifier. The rheology of the emulsion stabilized by 50% acid modified silicone (i.e., X in Figure 1 are amino groups and Y are 50% acid; X ) Y) is compared here with the rheology of emulsion stabilized with 100% amino silicone (both X and Y in Figure 1 are amino groups). It should be noted (13) Mehta, S. C.; Somasundaran, P. Liquid-liquid interfacial behavior of hydrophilically modified silicone surfactants: Phase diagram study, manuscript in preparation.
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Figure 6. (a) Photomicrographs of bulk silicone oil containing 1% wt functionally modified polymer (50% amino modified)-bulk water interface. (b) Photomicrographs of emulsion droplets of water in silicone oil continuum stabilized by 1% wt functionally modified polymer (50% amino modified).
that the acid modified compound considered here is only 50% modified and that the other 50% of functional groups is amino groups. This partial modification in the functional group from amine to acid leads to a drastic change in the rheological properties. The viscosity of the emulsions made with acid functional silicone is almost 300 times higher than the viscosity of silicone emulsions formulated with amine modified silicone. Such an increase in viscosity can be explained by considering the orientation of hybrid silicone polymers at the oil-water interface in such a manner that the backbone is present in the silicone oil continuous phase and the hydrophilic functional groups are submerged in the dispersed water droplets. Under such an arrangement, the acid and amine functional groups of acid modified silicone can interact with each other in the water phase, leading to the formation of a loose network among the silicone backbones in the oil continuum. Such networks lead to a significant increase in the rigidity of the oil-water interface and do not allow easy sliding of dispersed droplets under external shear. With such behavior, the viscosity of the emulsion can be expected to increase drastically.
Methylated amino silicone, on the other hand, has a limited solubility in silicone oil, as is evident from the absence of any region of stable emulsion from Figure 3a. Because of some initial solubility, a stable emulsion was obtained temporarily up to a 0.2% methylated silicone concentration. However, above a 0.2% concentration, the methylated silicone became insoluble in the silicone oil and precipitated. As a result, the methylated silicone lost its emulsifying ability and the emulsion destabilized, which is evident from the viscosity of the emulsion decreasing to a level comparable to that of water. This observation suggests that a stable water-in-oil emulsion is obtained only when the modified silicone emulsifier is soluble in silicone oil and that when the emulsifier precipitates from the silicone oil phase, the emulsion destabilizes. Mechanism of Stabilization of Liquid-Liquid Interface. Anseth et al.10 employed microscopic measurements to observe the precipitation of EO/PO modified silicone emulsifiers at the oil-water interface. To test if the Pickering mechanism also is followed by ionically modified silicone, similar experiments were performed in this study. Although the water soluble and oil
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insoluble EO modified polymers separate from the silicone oil phase as observed by Anseth et al., as can be seen in Figure 5, the oil incompatible polymer appears to exist in the form of droplets rather than precipitated polymer particles as was explained earlier.10 To further test the existence of the Pickering mechanism in the case of ionic silicone emulsifiers, microscopy experiments were repeated with various functional modified silicones at the cyclic silicone oil-water interface. Figure 6a illustrates the interface between bulk oil and bulk water, whereas Figure 6b shows the interface between water droplets dispersed in oil continuum. No separation or precipitation of aminosilicone was observed in both cases. In the case of acid modified silicone, the polymer begins to precipitate at the interface because of its proximity to the solubility limit in Figure 6a. Despite this, the interface of emulsion is still clear (Figure 6b). In contrast, the separation of silicone oil and polymer is clearly observed in the case of methylated silicone, possibly due to concentrations exceeding its solubility limit in the silicone oil phase. Thus, the presence of polymer droplets in Figure 6a,b corroborates the polymer incompatibility. The observed separation behavior of modified polymers from the silicone oil is in accordance with the measured hydrophilicity of the polymers: methylated > acid modified > amino silicone.
Discussion Silicone polymers are fundamentally different from the organic polymers in their molecular architecture due to the presence of a Si-O backbone. This raises a question as to whether the modified silicone polymeric emulsifiers would demonstrate a similar mechanism of emulsion stabilization as the organic polymers or behave differently. In the past, two different mechanisms of emulsion stabilization have been proposed: film formation at the interface and precipitation or the Pickering mechanism. In the case of the Pickering mechanism, it is proposed that the hybrid silicone particles precipitate from the oil phase and collect at the interface. If these precipitates indeed just collect at the interface, they should show minimal interaction with the aqueous phase. However, in the film formation mechanism, the hydrophilic functional groups of silicones would interact with water. In the present work, the interaction of functional groups with water was utilized as a tool to investigate the mechanism of stabilization of silicone emulsions. Langmuir isotherm studies indicated that functionalized silicones interact strongly with water and their affinity toward water increases as the degree of hydrophilic modification is increased. It was observed from phase diagram studies that with an increase in hydrophilicity of the emulsifier, its solubility in the silicone oil decreased, and consequently, its ability to stabilize the water-in-oil emulsions decreased. The network formation mechanism proposed in the rheological behavior section is consistent with the emulsion stabilization mechanism indicated by film formation.8 If the polymers were precipitating out, to avoid unfavorable contact between the functional groups and the silicone oil phase, they would form domains with functional
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groups facing toward the interior. In such a case, the network formation between the acid and the amine group mentioned earlier would not be possible, and emulsions stabilized with amine modified and acid modified silicones should show a similar rheological behavior. Microscopy experiments provided visual confirmation of droplet formation in the case of methylated and EO modified silicones, which are relatively less compatible with silicone oil. No precipitation was observed in the case of aminosilicone, and partial precipitation was observed with acid modified silicone. Moreover, the precipitated polymers showed a reduced ability as emulsifiers. When the polymer precipitates out from the oil phase, there may be temporary emulsion stabilization due to the Pickering effect, but the emulsions were stable only for a short period of time and had a significantly reduced viscosity. This suggests that precipitation is a special case of temporary emulsion stabilization but that the major mechanism is film formation.
Conclusion Most of the applications of hybrid silicone polymers utilize the behavior at interfaces, especially at liquid-liquid interfaces. In the present work, the mechanisms by which these modified silicone polymers stabilize the liquid-liquid interface was studied using a number of techniques. Langmuir isotherms and phase diagrams indicated a dependence of behavior of polymers on the functional group modifications. This dependence suggests that the functional groups are submerged into the aqueous phase, with the silicones forming a film at the liquid-liquid interface. Rheological studies further corroborated the hypothesis of film formation due to the possibility of forming loose networks among the functional groups as in the case of acid modified silicone. When such networks were not formed, the emulsion was observed to be less viscous as with the methylated silicone. The microscopy experiments provided visual evidence for film formation in oil compatible silicones (which form stable emulsions) and precipitation in the case of oil non-compatible silicones. Although there might be a temporary stability of emulsions even if the surfactant precipitates as observed in the case of methylated silicone with microscopy, in general, the emulsion was observed to be stable only if the emulsifier was soluble in the silicone oil or water phase. The major mechanism of emulsion stabilization is proposed to be film formation with hydrophilic functional groups interacting with water. It should be noted that the proposed mechanism may not represent emulsions stabilized by monomeric trisiloxane-type silicone surfactants. The insight achieved in silicone emulsification by the current analysis should make it possible to predict if a particular unknown emulsion formulation would be stable or not. Acknowledgment. The authors acknowledge financial support from the National Science Foundation and Industry/ University Center for Surfactants at Columbia University. LA7032912
