Langmuir 1998, 14, 3517-3523
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Characterization of Water-in-Oil Microemulsions Formed in Silicone Oils D. C. Steytler,* P. J. Dowding, and B. H. Robinson School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
J. D. Hague, J. H. S. Rennie, and C. A. Leng Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral L63 3JW, U.K.
J. Eastoe School of Chemistry, Cantock’s Close, University of Bristol, Bristol BS8 1TS, U.K.
R. K. Heenan ISIS, Rutherford Appleton Laboratory, Chilton, Oxon OX11 OQX, U.K. Received November 17, 1997. In Final Form: March 25, 1998 The phase stability and structure of water-in-oil microemulsions stabilized by the surfactant Aerosol OT have been examined in the low molecular weight silicone oils hexamethyldisiloxane (HMDS) and diphenyltetramethyldisiloxane (DPTMDS). The solubilization capacity ωmax (where ω ) [H2O]/[AOT]) determined as a function of temperature defines a limited single phase microemulsion region with relatively low water solubilization (ωmax < 40). Addition of NaCl shifts this single phase region to a higher temperature. SANS and dynamic light scattering measurements show the presence of strong attractive interdroplet interactions in HMDS, which are relatively absent in DPTMDS. Addition of n-octanol as a cosurfactant dramatically increases the solubilization of water in HMDS, giving an optimal solubilization capacity at a specific cosurfactant:surfactant molar ratio, x. Small-angle neutron scattering (SANS) measurements made close to the optimized condition show elimination of attractive interactions. A Porod analysis of the SANS data demonstrates an increase in the area of the surfactant/cosurfactant layer and commensurate reduction in droplet size with increasing x.
1. Introduction Traditionally, water-in-oil (w/o) and oil-in-water (o/w) microemulsions have been prepared using hydrocarbon oils and hydrocarbon-based surfactants. With increased use of silicone-based chemicals in commercial formulations (e.g., cosmetics), there is now growing interest in emulsions and microemulsions incorporating silicone-based oils1 and surfactants. Of all silicone-based oils, poly(dimethylsiloxane)s (PDMS) and their derivatives are probably the most widely used and are available covering a wide range of molecular weight. The structure and dynamic features of these oils differ from hydrocarbon oils in a number of fundamental respects, giving rise to important physical properties. First, the large Si-O-Si bond angle and absence of substituents on the bridging oxygen atom permits enhanced chain mobility. Second, the high density of methyl groups imparts a highly hydrophobic character. As a result, silicone oils have melting points, glass transition temperatures, and solubility parameters lower than hydrocarbon oils of comparable structure. High molecular weight PDMS oils will spread on water and lower the surface tension. Due to a difference in polarity between the methyl groups and silicone-oxygen backbone these insoluble monolayers show a degree of orientation with the oxygen atoms protruding into, and the methyl groups away from, the water surface.2 Low molecular weight
Figure 1. Chemical structure of the silicone oils (a) HMDS and (b) DPTMDS.
oils are available in both linear and ring configurations and also as phenyl derivatives. Although much attention has focused on silicone-based surfactants, particularly “superwetters”,3-7 there have been surprisingly few published reports of surfactant(1) Messier, A.; Schorsch, G.; Rouviere, J.; Tenebre, L. Prog. Colloid Polym. Sci. 1989, 79, 249. (2) Jarvis, N. L. J. Colloid Interface Sci. 1969, 29, 647. (3) Lin, Z. X.; Stoebe, T.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1996, 12, 345-347. (4) Lin, Z. X.; Hill, R. M.; Davis, H. T.; Scriven, L. E.; Talmon, Y. Langmuir 1994, 10, 1008-1011.
S0743-7463(97)01251-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/03/1998
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stabilized self-assembled structures such as microemulsions involving silicone oils. Messier et al.1 reported the phase behavior including Winsor I type microemulsion systems and showed electron microscope images of emulsions stabilized by nonylphenol nonionic surfactants in hexamethyldisiloxane (HMDS) and a silanol. For nonionic surfactants (CnEm) based on n-alcohols, liquid crystal structures were also examined for binary (surfactant/ water) and ternary (surfactant/water/oil) mixtures. In both cases, lamellar phases were formed, giving characteristic SAXS diffraction peaks. Microemulsion formation in high molecular weight amine-derivatized PDMS oils has also been studied by Katayama8 using the dimethylethanolamine salt of myristic acid as surfactant. Structural and dynamic features of w/o microemulsions stabilized by the surfactant Aerosol OT or AOT (sodium bis(2-ethylhexyl) sulfosuccinate) have been extensively studied9 in a range of hydrocarbon oil media. As a result, a detailed picture has emerged with all evidence in support of the formation of microemulsions composed of spherical water droplets in oil with a low degree of polydispersity.10 The water core radius Rc is found to be essentially independent of the oil medium. In accord with simple theoretical arguments based on all surfactant being located in the interfacial layer, Rc is found to increase linearly with the molar ratio of water-to-surfactant (ω ) [H2O]/[AOT]).
Rc ∝
3Vw ω NavAs
(1)
where Nav is the Avogadro number, As is the area occupied by the surfactant at the water interface, and Vw is the molar volume of water. In this paper we report the formation of w/o microemulsions stabilized by AOT in low molecular weight silicone oils. Microemulsion structure was examined in HMDS (Figure 1a) and its phenyl derivative diphenyltetramethyldisiloxane, DPTMDS (Figure 1b).
flight LOQ spectrometer11 using the ISIS pulsed neutron source of the EPSRC Rutherford Appleton Laboratory. The magnitude of the momentum transfer vector Q is given by
Q)
4π θ sin λ 2
()
(2)
where λ is the incident wavelength (2.2-10.0 Å), determined by time-of-flight, and θ is the scattering angle. The intensity of scattered neutrons was recorded on a position-sensitive 64 × 64 pixel 2-D detector at a fixed sample-to-detector position (4.43 m). The raw data were corrected for transmission, incoherent background scattering, and normalized to absolute scattering probabilities using standard procedures. Further details of technical and experimental aspects together with data reduction procedures are given elsewhere.11,12 Samples were contained in stoppered, matched 1.0 mm Hellma cells and thermostated to (0.1 °C. Owing to the unavailability of deuterated silicone oils or surfactant AOT, all measurements were made using a “core” contrast, i.e., D-water/H-surfactant/H-oil. The mean scatteringlength density difference (∆F) at the water/surfactant interface of the droplets was approximately 6.9 × 1010 cm-2. 2.4. Dynamic Light Scattering (DLS) Measurements. DLS measurements were made using a Spectra Physics argon ion laser (maximum output power 2 W) operating at
