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Active Interfacial Modifier: Stabilization Mechanism of Water in Silicone Oil Emulsions by Peptide-Silicone Hybrid Polymers Kenichi Sakai,*,† Ryosuke Ikeda,† Suraj Chandra Sharma,† Rekha Goswami Shrestha,† Naoko Ohtani,‡ Masato Yoshioka,‡ Hideki Sakai,† Masahiko Abe,*,† and Kazutami Sakamoto*,† †
Department of Pure and Applied Chemistry in Faculty of Science and Technology and Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, and ‡ Seiwa Kasei Co., Ltd., 1-2-14 Nunoichi-cho, Higashi-Osaka, Osaka 579-8004, Japan Received October 1, 2009. Revised Manuscript Received March 10, 2010
We have developed hybrid amphiphilic polymers consisting of a silicone backbone modified with hydrocarbon chains and hydrolyzed silk peptides. These polymers are molecularly soluble neither in water nor in most of organic solvent, but are attractive with these solvents. We assume that this property enables the polymers to form “an independent third phase” between immiscible two liquid phases and stabilize the emulsion system, based on a fundamentally distinguishable mechanism from the approach by conventional surfactants. We have named these amphiphilic polymers “active interfacial modifier (AIM)” and studied physicochemical properties of AIM-stabilized water-in-silicon oil emulsions. The addition of AIM to a mixture of water and decamethylcyclopentasiloxane (D5) has achieved preparation of stable W/O emulsions (droplet size = ca. 1 μm) in a wide range of the three components, even under relatively gentle vortex mixing. Interestingly, the prepared W/O emulsions are found to be nearly genuine or quasi Newtonian fluid with low viscosity when water content is in the range from 0 to 36 wt % for the fixed weight ratio of AIM/D5 = 6/4. This is a good piece of evidence that AIM forms the independent third phase, where the Newtonian shear occurs at the D5/AIM interface. The presence of AIM as third phase has also been confirmed by fluorescence probe method with confocal laser scanning microscopy. As such, AIM can activate interfaces by the least amount to cover interfaces as an independent third phase, and hence, this provides a new concept achieving a precise control of interfacial properties.
1. Introduction A living system has a multiwoven architecture of interfaces to keep its structures and functions working while materials are flowing in and out to keep the system vital. Nature established such a system as the utmost efficient way in terms of energy and material consumption to maintain life through millions years of evolution. The most remarkable example in this regard is the formation of a biomembrane by phospholipids as a primary constituent. A biomembrane has a typical bilayer lamellae structure as a self-assembly of amphiphilic molecules, i.e., surfactants. Usually, surfactants compose the bilayer lamellar structure as a liquid crystal phase at relatively high concentrations, after micelle formation through several phase transitions by increasing concentration. In contrast, phospholipids almost spontaneously compose the lamellar structure in the aqueous system, because phospholipids are amphiphiles with optimum molecular structures to form a lamellar phase but are barely soluble to water. Thus, nature utilizes the best selection of molecules with the least amount of energy and materials to construct a biomembrane, which is the most important interfacial structure to divide the living organism from the environment. The objectives of this study are try to find and establish such a system with a novel material as nature does for the control of interfacial properties in a living system. When surfactants, namely, surface active agents, are adsorbed at interfaces (e.g., gas/liquid, liquid/liquid, or solid/liquid interfaces), a remarkable change in their interfacial properties occurs. For example, emulsions consist of liquid droplets dispersed in a continuous liquid phase (where the two liquids are intrinsically *To whom correspondence should be addressed. E-mail:
[email protected]. tus.ac.jp;
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not miscible with each other), and surfactants stabilize the emulsion droplets as a result of their adsorption to the liquid/ liquid interface. The supreme goal in colloid and interface chemistry might be the precise control of such interfacial properties by the least amount of surfactants just as nature does, and from the viewpoint of material science, it is required to develop “universal” surfactants applicable to wide varieties of interfaces. In general, however, surfactants are distributed to the interface layer as a surface excess toward bulk concentration, and hence, the effective amount of surfactants to stabilize the interface is significantly decreased from an added amount in the system. In order to control interfacial properties more effectively, surfactants are required to be only present between two phases. There are several emulsion systems known to somehow adapt such requirements, where “emulsifiers” are localized at an oil/ water interface as an independent third phase or layer. Examples are “Pickering emulsion” stabilized with solid particles adsorbed at the interface,1 “three-phase emulsion” stabilized with phospholipid bilayer,2 “liquid crystal emulsion”,3 and “Cubosome” or “Isasome” stabilized with triblock PEO-PPO-PEO copolymers.4 Although these systems offer unique stability and some additional functions, selection and combination of components are specific for each system. In order to make these systems more general and useful for a wide range of emulsion preparation, we propose here a new category of amphiphilic materials to be called “Active Interfacial Modifier (AIM)”. AIM is intrinsically molecularly soluble neither in water nor in organic solvents, but has attractive (1) Pickering, S. U. J. Chem. Soc. Trans. 1907, 91, 2001. (2) Tajima, K.; Imai, Y.; Tsutsui, T. J. Oleo. Sci. 2002, 51, 285. (3) Suzuki, T.; Takei, H.; Yamazaki, S. J. Colloid Interface Sci. 1989, 129, 491. (4) Spicer, P. T.; Hayden, K. L.; Lynch, M. L.; Ofori-Boateng, A.; Burns, J. L. Langmuir 2001, 17, 5748.
Published on Web 03/16/2010
DOI: 10.1021/la100146x
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moieties to each immiscible liquid phase. Hence, AIM practically stays just at the interface to make emulsion stable. From this point of view, we have developed a new type of amphiphilic hybrid materials, consisting of silicone backbones grafting hydrocarbon chains (as hydrophobic groups) and hydrolyzed silk peptides (as hydrophilic groups).5 These polymer materials with paste-like appearance are molecularly insoluble to water and most organic solvents, but are attractive to both immiscible liquids due to their amphiphilic nature. The hybrid materials developed here, therefore, must stay as a middle phase located between two immiscible liquid phases and activate the two-phase interface, expected as AIM. This feature is very unique and fundamentally different from that of traditional surfactants. We expect that the hybrid polymer materials (AIM) are able to change interfacial properties by their own affinity to each phase. This is the same even for gas/liquid, liquid/liquid, solid/ liquid, gas/solid, or solid/solid interfaces as long as AIM stays in between such two phases. We have demonstrated in our previous papers5 that the addition of AIM to immiscible twocomponent liquid (water and oil) systems results in stable water-in-oil (W/O) emulsions with a wide concentration range of the three components, although the stabilization mechanism has not been understood fully with detailed analysis of the physicochemical data. The importance of AIM should be present in the following points (at least from the practical use of AIM in the cosmetic industry): AIM improves the emulsification process to make cold (low energy) process emulsification possible, and when W/O emulsions are applied to skin, AIM can modify the skin surface as a second function to give supple sensory feeling and water repellence. Thus, AIM can be applied to W/O sunscreens or W/O foundations with additional functionality from silk peptides (e.g., intrinsic sheen, comfortable touch, adhesive properties, and good moisture retention for skin or hair). Silicone-based amphiphilic polymers have been widely used in preparing stable water-in-silicone oil emulsions, which is very useful in the cosmetic industry. To the best of our knowledge, three stabilization mechanisms have been proposed for such silicone-type emulsifiers: (i) monolayer formation at the water/oil interface;6 (ii) nucleation, growth, and accumulation of polymer-rich particulates at the interface (in a similar fashion to Pickering emulsions);7 and (iii) gelation of the emulsifiers with a continuous bulk phase (in this particular example, n-decane was used as an oil phase).8 When compared with these studies, AIM employed in our current study exhibits fundamentally different physicochemical properties in (silicone) oils: again, AIM is not soluble in any oils (even in the case (ii) mentioned above, the silicone-type emulsifier is soluble in silicone oils7), and no direct evidence regarding the gelation of AIM in silicone oils has been suggested. This anticipates that a new stabilization mechanism lies in the AIM-stabilized W/O emulsion systems. In the following discussion, we shall present physicochemical properties of AIM-stabilized W/O emulsions in detail with a combination of optical, fluorescence, and confocal laser scanning microscopy data and viscosity measurement results. (5) (a) Koyanagi, A.; Goto, N.; Daikai, S.; Uchida, S.; Hayashi, N.; Yoshioka, M. J. Soc. Cosmet. Chem. Jpn. 2007, 41, 269. (b) Koyanagi, A. J. Cosmet. Sci. 2007, 58, 435. (c) Daikai, S. Fragrance J. 2007, 35, 49. (6) (a) Mehta, S. C.; Somasundaran, P. Langmuir 2008, 24, 4558. (b) Mehta, S. C.; Somasundaran, P.; Kulkarni, R. J. Colloid Interface Sci. 2009, 333, 635. (7) Anseth, J. W.; Bialek, A.; Hill, R. M.; Fuller, G. G. Langmuir 2003, 19, 6349. (8) Kato, T.; Naito, N. J. Soc. Cosmet. Chem. Jpn. 2002, 36, 192.
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2. Experimental Section 2.1. Materials. Seiwa Kasei PROTESIL FN (INCI name: Hydrolyzed Silk PG-Propyl Methylsilanediol Crosspolymer) was used as AIM in the current study. Hereafter, we call this particular polymer sample “AIM-FN”, and the term “AIM” is used as the general name of such amphiphiles. This polymer sample was synthesized as reported elsewhere, by coupling of silk peptide with reactive silicone monomers.5 As mentioned in the Introduction, this polymer consists of silicone backbones grafting hydrocarbon (n-C8H17) chains and hydrolyzed silk peptides (the significant amino acid components are glycine (35.5%), alanine (27.7%), and serine (13.5%) and the polymerization degree is within 5-10). We have examined the organic conceptual (OC) diagram9 of this AIM material by plotting the calculated inorganic value (IV) and organic value (OV) with use of given values for each chemical moiety. The IV/OV value of AIM-FN is calculated as ca. 5000/8700 (IV/OV = 0.57), based on its monomer composition. The IV/OV ratio corresponds to the hydrophilic-lypophilic balance (HLB) of amphiphilic materials, and the ratio calculated for AIM-FN is within the range of traditional polyoxyethylene-type emulsifiers with HLB around 3 to 6, suitable for W/O emulsion. As a control to AIM-FN, a cosmetic ingredient grade silicone emulsifier was used: Shin-Etsu Chemical KF6038 (INCI name: Lauryl PEG-9 Polydimethylsiloxyethyl Dimethicone). This material consists of a silicone backbone and hydrocarbon (n-C12H25) chains, similar to AIM-FN, but oxyethylene units are introduced to the silicone backbone instead of our hydrolyzed silk peptides. The key difference in physicochemical properties of AIM-FN and KF6038 lies in the fact that KF6038 is soluble in silicone oils whereas AIM-FN is not molecularly soluble; instead, it is dispersed in silicone oils. The IV/OV value of KF6038 is calculated as ca. 2200/8700 and the ratio is 0.25, which is again within the range of polyoxyethylene-type W/O emulsifiers. Hereafter, we call this silicone emulsifier (KF6038) “SAA (surface active agent)”. Decamathylcyclopentasiloxane (D5, Dow Corning Toray) was used as an oil phase, and the water used in the current study was filtered with a Millipore membrane filter (0.22 μm in pore size) after deionization with a Barnstead NANO pure diamond UV system. 2.2. Synthesis of Fluorescence Labeled AIM-FN. Fluorescence labeled AIM-FN was synthesized via a reaction of fluorescence probe (9-chloromethylanthracene, 9-CMA) with carbonyl groups in hybridized silk peptides. AIM-FN (5 g) was added in analytical-grade chloroform (10 cm3) and was dispersed under vortex mixing for 3 min. 9-CMA (4.8 10-5 mol, Tokyo Chemical Industry) and tetrabutylammonium bromide (1.6 10-5 mol, Wako Pure Chemical Industries) were added to the AIM-FN-chloroform mixture, and the reaction system was refluxed at 55 °C for 3 h. After evaporation, the reaction residue was thoroughly washed by analytical-grade acetonitrile (Kanto Chemical) many times with use of the vortex mixer and a centrifuge, to remove unreacted 9-CMA. Light-yellow viscous polymer materials were obtained after drying under reduced pressure. We note that no significant emission peak is present for AIM-FN itself (before fluorescence labeling), whereas characteristic emission peaks appear for the fluorescence-labeled AIM-FN when dispersed in D5 (excitation wavelength λex = 361 nm). This is a piece of evidence that the fluorescence probe is chemically bound to AIM-FN. 2.3. Emulsification. In a glass test tube, the emulsification agents (AIM-FN and SAA) were mixed with D5 and water at a fixed total weight of 1 g. The mixture was placed in a hot-water bath at 70 °C for 3 min and then was further mixed by using a vortex mixer at room temperature for 3 min. 2.4. Measurements. Optical microscope observations were made using an Olympus IMT-2. A small amount of emulsion (9) Koda, Y.; Sato, S.; Honma, Y. Organic Conceptual Diagram, Basis and Application; Sankyo: Tokyo, 1984.
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Figure 1. Phase diagrams of three-component mixtures of (a, b) AIM-FN-water-D5 and (c, d) SAA-water-D5, based on visual observations performed at fixed periods of (a, c) 1 day and (b, d) 3 weeks from preparation. samples was placed in the hollow (0.5 mm depth) of a glass slide and covered with a cover glass. The mean diameter of emulsion droplets was evaluated by the counting method over 200 droplets. Fluorescence spectra were measured using a Shimadzu RF5300PC fluorescence spectrometer at a fixed excitation wavelength of 361 nm. The sample volume was fixed at 10 cm3. A Leica TCS SP2 confocal laser scanning microscope (CLSM) was used for observations of fluorescence-labeled AIM-FN emulsions. The fluorescence detection was performed in the range 400-430 nm. Shear viscosity measurements were performed using a TA Instruments AR-G2 rheometer with the cone-plate geometry (cone angle = 2°, diameter = 40 mm). The shear rate was varied from 0.01 to 1000 s-1. All measurements reported here were performed at 25 °C.
3. Results and Discussion 3.1. Visual and Microscope Observation. For all the emulsification agents employed in the current study, W/O emulsions were easily prepared by relatively gentle vortex mixing. We have confirmed (i) the D5 continuous phase from the oil dilution method and (ii) no significant effects of the addition order of the three components on the size and stability of the prepared emulsion droplets. Figure 1 shows the “nonequilibrium” phase diagrams of three-component mixtures, based on visual observations performed at fixed periods of 1 day and 3 weeks from preparation. One can clearly see a wide range of W/O emulsions prepared with AIM-FN.10 Indeed, the dispersion stability of the W/O emulsions prepared with AIM-FN is much better than that with SAA. In order to clarify the difference in dispersion stability (10) We have also studied dispersion stability of three-component mixtures of AIM (PROTESIL LH)-water-D5, where the AIM-LH monomer composition is different from that of AIM-FN (PROTESIL FN) we have presented here. For the AIM-LH systems, we can see a wide range of stable W/O emulsion region, in a similar manner to that seen for the AIM-FN systems (see Figure S1 in Supporting Information).
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Figure 2. Dispersion stability of W/O emulsions, prepared at a fixed weight ratio of AIM-FN (or SAA)/D5 = 7/3 with various water contents: AIM-FN (open symbols, water content = 30 wt % as a typical example) and SAA (closed symbols, water content = 10, 30, and 50 wt %). The volume fraction of W/O emulsion phase is measured by visual observations of each sample. The volume fraction of W/O emulsion phase is found to be constant at 100% for the AIM-FN mixtures in the range of water contents up to 70 wt %.
of the two systems, the volume fraction of emulsion phase (measured in each test tube) is plotted in Figure 2 for a given AIM-FN (or SAA)/D5 weight ratio of 7/3. The volume fraction of the emulsion phase is significantly decreased with time in the case of SAA, whereas the value is constant at 100% for AIM-FN in the range of water contents up to 70 wt % (at least for 1 week). The W/O emulsions prepared with AIM-FN (or SAA) were observed by optical microscopy. As is seen in Figure 3, the addition of AIM-FN gives ca. 1 μm emulsion droplets in diameter with uniform distribution. No significant change in the emulsion droplet size (and their distribution) is seen in a wide concentration range of the three components, even after 3 weeks from preparation. We note that larger emulsion droplets (1-20 μm in diameter) are obtained only when the water content is increased above 70 wt %. These relatively larger emulsion droplets are not stable, DOI: 10.1021/la100146x
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Figure 3. Optical microscope images of W/O emulsions prepared with (a) AIM-FN and (b) SAA. The emulsions were prepared at a fixed composition of AIM-FN (or SAA)/water/D5 = 30/50/20 (in wt %). The images were obtained immediately after preparation.
and two-phase separation (emulsion phase þ D5 phase) occurs in two month’s time. It seems likely that this phase separation results from sedimentation of the emulsion droplets from the continuous D5 phase. On the other hand, SAA gives 3-5 μm emulsion droplets with wide distribution (see Figure 3). These results suggest primarily that the dispersion stability of W/O emulsions prepared with AIM-FN (or SAA) depends on the droplet size. In order to understand the stabilization mechanism by AIMFN, it is necessarily required to see the exact location of AIM-FN in the three-component mixtures. For this purpose, CLSM measurements have been performed by using fluorescence-labeled AIM-FN. Although the degree of fluorescence labeling per AIMFN molecule is not known in our current case, we have confirmed that the fluorescence-labeled AIM-FN gives stable W/O emulsions, being comparable with original AIM-FN-stabilized W/O emulsions in their droplet size and distribution. The W/O emulsions prepared with fluorescence-labeled AIM-FN presents characteristic emission peaks (e.g., 395, 418, and 440 nm, λex = 361 nm), similar to the fluorescence-labeled AIM-FN dispersed in D5 (see Experimental Section). This enables us to visualize AIMFN by CLSM measurements. Figure 4 shows CLSM images of W/O emulsion droplets prepared with fluorescence-labeled AIMFN, observed at various Z-heights. In this CLSM measurement, we have set the composition ratio of AIM-FN/water/D5 = 8/30/ 62 (in wt %), to observe relatively large emulsion droplets (this means that, because of the image resolution of CLSM, smaller droplets (
