Langmuir 2001, 17, 4677-4680
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Surface Properties of Mono-, Di-, and Triglycerol Monostearate Monolayers Spread at the Air-Water Interface Masami Kawaguchi,*,† Midori Yamamoto,† Takeji Nakamura,‡ Masatsugu Yamashita,‡ Tomoharu Kato,‡ and Tadaya Kato† Department of Chemistry for Materials, Faculty of Engineering, Mie University, 1515 Kamihana, Tsu, Mie, 514-8507 Japan, and Food Ingredients Division, Taiyo Kagaku Company, Ltd., 9-5 Akahori-Shinmachi, Yokkaichi, Mie, 510-0825 Japan Received December 5, 2000. In Final Form: April 25, 2001
Introduction Polyglycerol esters of fatty acids are the commonly used emulsifiers in food and cosmetic applications.1,2 They have been used for a long time to control properties of aqueous dispersions, and they are commercially available. However, the commercial products are mixtures of many different components with variation in the degree of polymerization. Some efforts have been paid to obtain more well-defined polyglycerol ester products by esterification of purified polyols (e.g., diglycerol) with single fatty acids. The resulting diglycerol esters of various fatty acids were used to characterize their surface properties3,4 and liquidcrystalline phases5 without consideration of the effects of the polydispersity of the degree of polymerization and chemical components. To fundamentally understand the role of emulsifiers in the stabilization of emulsions and foams in the food and cosmetic industries, it is important to obtain information on their molecular packing at interfaces. Such information can be obtained from studies on monolayers spread at the air-water and oil-water interfaces using by surface and interfacial pressure measurement techniques. The surface and interfacial pressure measurements gave the interfacial packing as well as the intermolecular forces operating in the two-dimensional array of the monolayers. For polyglycerol esters of fatty acids at interfaces, it can be expected that the polar head glycerol is attached on the interface while the hydrophobic groups are oriented vertically. However, there is no systematic investigation of the surface properties of well-defined polyglycerol monoester monolayers spread at the air-water interface as a function of the degree of polymerization. In this study, we investigate the monolayer properties of three well-defined polyglycerol monostearates spread at the air-water interface in terms of the surface pressure measurement and the fluorescence microscopic observation. Experimental Section Monoglycerol monostearate (1-monostearoyl-glycerol, G1, a purity of 99%) was purchased from Funakoshi Co. Ltd. (Tokyo, Japan). Two polyglycerol monostearates such as diglycerol † ‡
Mie University. Taiyo Kagaku Co., Ltd.
(1) Friberg, S. E.; Goubran, R. F.; Kayali, I. H. In Food Emulsions, 2nd ed.; Larsson, K., Friberg, S., Eds.; Mercel Dekker: New York, 1990; p 1-40. (2) Larsson, K. In Lipids-Molecular Organization, Physical Functions and Technical Applications; Oily Press: Dundee, U.K., 1994. (3) Rodriguez Patino, J. M.; Ruiz Dominguez, M. Colloids Surf., A 1993, 75, 217-228. (4) Holstborg, J.; Pedersen, B. V.; Krog, N.; Olesen, S. K. Colloids Surf., B 1999, 12, 383-390. (5) Pitzalis, P.; Monduzzi, M.; Krog, N.; Larsson, H.; LjusbergWahren, H.; Nylander, T. Langmuir 2000, 16, 6358-6365.
Figure 1. Chemical structures of G1, G2, and G3. monostearate (3-(2,3-dihydroxypropoxy)-2-hydroxypropyl stearate, G2, a purity of 94.2%) and triglycerol monostearate (2-(2,3dihydroxy-propyl)-1-(2,3-dihydroxy-propoxymethyl)-ethyl stearate, G3, a purity of 95.5%) are kindly provided by Taiyo Kagaku Co., Ltd. (Mie, Japan). Figure 1 illustrates the chemical structures of the G1, G2, and G3 compounds. The diglycerol and diisopropyridentriglycerol (Solvay S. A., Brussels, Belgium) used to produce the G2 and G3 samples had a purity of at least 90% and 80%, respectively. The stearic acid and methyl stearate (Nippon Oil Co., Tokyo, Japan) used to produce the samples of G2 and G3 had a purity of at least 97%. The purities of the respective samples were determined by gas-liquid chromatography and IR techniques. N-(7-Nitro-2,1,3-benzoxadiazol-4-yl)-L-R-dipalymitol phosphatidyl ethanol amine (NBD-PE) (Molecular Probes Inc., Eugene, OR) was used as a fluorescence probe without further purification, and its concentration was fixed around 1.5 base mol % relative to polygylcerol monostearates in fluorescence microscopy measurements. The surface pressure was determined by a Wilhelmy system (NL-PS80-MTC, Nippon Laser & Electronics Lab., Aichi, Japan), the details of which appear elsewhere.6,7 Spreading solutions (ca. 5 mg/100 mL) were prepared by dissolving polyglycerol monostearates in chloroform, and their aliquots of 10-15 µL were spread on a deionized water surface in a Teflon-coated trough with an area of 158 × 90 mm2 with a Hamilton microsyringe. The water temperature was controlled at 25 ( 0.2 °C. After the solvent was allowed to evaporate for at least 30 min, the surface pressure was monitored by a continuous compression as a function of the area occupied by one polygylcerol monostearate molecule. The compression of the area was controlled by the barrier speed of 2.4 mm/min to guarantee making a relaxation of the molecules. The same trough mentioned above was mounted on the stage of an Olympus BHS2-UMA epifluorescence microscope attached to a DAS-512ICCD camera (Imajista Co., Tokyo, Japan).6,7 The compression isotherm measurement and the fluorescence mi-
10.1021/la001699p CCC: $20.00 © 2001 American Chemical Society Published on Web 06/08/2001
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Figure 2. Surface pressure-area (π-A) isotherms of G1 (triangles), G2 (squares), and G3 (circles) monolayers in the absence (open symbols) and presence (filled symbols) of NBDPE at 25 °C. croscope observation were carried out simultaneously. The compression speed of the area was fixed at 2.4 mm/min. Fluorescence microscopic images of the films were observed through Olympus NeoSplan 40 (40×) ultralong-working-distance objective lenses. The high-frequency blue-green 435.8 nm was used to excite the fluorescence from the film. A mercury lamp (USH-103D, Ushio, Tokyo, Japan) was used as the excitation light source. Measurements of the surface pressure and fluorescence microscopy were performed at 25 ( 0.1 °C.
Results and Discussion Surface Pressure. The surface pressure-area (π-A) isotherms of G1, G2, and G3 monolayers at 25 °C are shown in Figure 2, where A is expressed by an area per polyglycerol monostearate molecule. G1 forms a typical liquid-condensed (LC) monolayer with a steep rising zone of the π-A isotherm. When such a steep rising zone is extrapolated to zero surface pressure, the limiting area can be generally obtained. For the G1 monolayer, the limiting area is determined to be 0.26 nm2 per molecule, and it is larger than the limiting area of 0.2 nm2 per molecule for stearic acid because of the presence of the polar head of glycerol.8 In the π-A isotherm of the G1
Notes
monolayer, the collapse point is observed at a surface pressure of 55.8 mN/m and an area of 0.20 nm2 per molecule. Therefore, the collapse point of the G1 monolayer at which the stearic hydrocarbon chain is close-packed at the air-water interface is consistent with that of alkyl chains of fatty acids and phospholipids.9 In the π-A isotherm of G2, a phase transition from a liquid-expanded (LE) to a LC state is observed. The collapse of the G2 monolayer sets in at a surface pressure of approximately 49 mN/m and an area of 0.17 nm2 per molecule. The area at the collapse point is lower than 0.2 nm2 per molecule. However, it should be noticed that the monolayer exhibits a kink point at an area of 0.2 nm2 per molecule beyond the steep rising region of the surface pressure. This shows that the G2 monolayer collapses after the partial multilayer formation at the air-water interface. Furthermore, the limiting area of the G2 monolayer is determined to be 0.35 nm2 per molecule, and its magnitude is smaller than twice the limiting area for the G1 monolayer although the polar head size of G2 is double that of G1. This indicates that the polar head diglycerol of the G2 molecule is inclined into the water phase or is overlapped partially with another at the air-water interface. G3 shows a LE type π-A isotherm at 25 °C, and in the π-A isotherm of the G3 monolayer the collapse point is observed at a surface pressure of 37 mN/m and an area of 0.4 nm2 per molecule because of the presence of the larger headgroup of triglycerol. Fluorescence Images. Because the fluorescence probe is an impurity, it is necessary to examine its effect on the surface pressure. The surface pressures of polyglycerol stearate monolayers in the presence of NBD-PE were higher than those without the probe at the same area as displayed in Figure 2. Similar results have been reported for pentadecanoic acid10 and poly(n-hexyl isocyanate) monolayers.6 Figure 3 shows a series of photographs of a monolayer of G1 at 25 °C taken at various stages of compression. At larger areas than 0.5 nm2 per molecule, any morphologies in the fluorescence micrograph images were not observed and their images were similar to that of the pure airwater interface. With a decrease in the area, a lot of dark
Figure 3. Fluorescence images of G1 for compression at A ) 0.48 (A) and 0.26 (B) nm2 per molecule.
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Figure 4. Fluorescence images of G2 for compression at A ) 0.48 (A) and 0.4 (B) nm2 per molecule.
Figure 5. Transient images (A-C) of the solid-phase domains following the lapsed time after stopping the compression of the G2 monolayer at A ) 0.40 nm2 per molecule: lapsed time, t ) 0.1 (A), 10 (B), and 40 (C) min.
circular domains in the LC phase appear randomly and they partially coalesce and form larger domain structures in which some bright small domains remain (Figure 3A). At the limiting area of 0.26 nm2 per molecule, the larger dark domains fully cover the water surface and the domain structures become obscure (Figure 3B). With further compression of the area, the surface becomes dim and their images are brighter than that of the pure water surface. Fluorescence microscopic photographs of a G2 monolayer are presented at 25 °C in Figure 4. At larger areas than 0.52 nm2 per molecule, no morphology is observed and the brightness of the images increases with a decrease (6) Kawaguchi, M.; Yamamoto, M.; Kurauchi, N.; Kato, T. Langmuir 1999, 15, 1388-1391. (7) Kawaguchi, M.; Yamamoto, M.; Kato, T. Langmuir 1997, 13, 2414-2416; 1998, 14, 2582-2584. (8) Ries, H. E., Jr.; Walker, D. C. J. Colloid Interface Sci. 1961, 16, 361-374. (9) Gaines, G. L., Jr. Insoluble monolayers at liquid gas interfaces; Interscience: New York, 1966. (10) Moore, B. G.; Knobler, C. M.; Akamatsu, S.; Rondelez, F. J. Phys. Chem. 1990, 94, 4588-4595. (11) Shraiman, B.; Bensimon, D. Phys. Rev. A 1984, 30, 2840-2842. (12) Nittmann, J.; Stanley, H. E. Nature 1986, 321, 663-668.
in the area. Below the area of 0.52 nm2 per molecule, small dark nuclei appear. When the area per molecule is compressed in the LE-LC coexistence state, the nuclei grow to dark horsebean-shaped domains (Figure 4A) and some radial cusp-shaped domains appear (Figure 4B). Finally, an outline of each domain becomes unclear by further compression of the area. Cusp formation, numerically evidenced by Shraiman and Bensimon,11 is a mathematical singularity which develops in very peculiar conditions.11 No stabilizing nor smoothing influence has to be present. This means that the interfacial tension between a viscous fluid and a less viscous fluid has to be zero. According to Nittmann and Stanley,12 the system in the absence of the interfacial tension should be free of microscopic fluctuations (noise). Surely, no interfacial tension between the liquid (the bright region in Figure 4B corresponds to a less viscous fluid) and solid (the dark region in Figure 4B corresponds to a viscous fluid) phases in the LE-LC coexistence state could be present. Below an area of 0.25 nm2 per molecule, any domain structures cannot be found because of the less contrast between the solid and liquid phases. This may be due to
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partial multilayer formation because the fluorescent probe used in this study does not partition into the solid domains. Furthermore, to determine whether there are changes in the morphology of the solid-phase domain at a fixed area during lapsed time periods after stopping the compression, we obtained typical transient formation of the cusp-shaped solid-phase domains at the fixed area of 0.4 nm2 per molecule at 25 °C as displayed in Figure 5. With an increase in the lapsed time, the cusp-shaped domain wall becomes instable and some network structures are formed accompanied with partially breaking down the domain structures. Because the domain observed by the compression of the area is a nonequilibrium structure, it is necessary to cause the rearrangement of the molecules spread at the air-water interface for the formation of an equilibrium domain structure. Such morphological changes in the G2 monolayer should be attributed to a slow equilibrium. In contrast with the fluorescence microscopic images, the surface pressure at the fixed area of 0.4 nm2 per molecule almost keeps constant during the lapsed time.
Notes
On the other hand, because G3 showed a LE type isotherm at 25 °C as shown in Figure 1, any domain structures were not observed during the compression of the monolayer of G3 and the fluorescence microscopic image becomes brighter with a decrease in the area of the monolayer. In conclusion, polyglycerol monostearates form stable monolayers spread at the air-water interface, and the shape of the π-A isotherms of polyglycerol monostearate monolayers shows a strong dependence of the number of glycerol. The resulting π-A isotherm at 25 °C changes from LC to LE type through a π-A isotherm with LE to LC phase transition with an increase in the number of glycerol. Furthermore, the larger the number of glycerol, the less the collapse surface pressure. This is well correlated to the molecular packing of polyglycerol monostearates at the air-water interface. Changes in the fluorescence microscopic images of polyglycerol monostearate monolayers with an increase in the number of glycerol also correspond to the shape of their π-A isotherms. LA001699P
