J. Phys. Chem. B 2007, 111, 9821-9827
9821
Structures in a Microemulsion System of an Ethoxylated Polymethylsiloxane Surfactant, Water, and Oil Studied by NMR Self-Diffusion Measurements Harald Walderhaug* Department of Chemistry, UniVersity of Oslo, P.O.B. 1033 Blindern, N-0315 Oslo, Norway ReceiVed: April 19, 2007; In Final Form: June 20, 2007
Microemulsion samples of an ethoxylated polymethylsiloxane surfactant, water, and 1-dodecanol or 1-decanol as the oil component are investigated using pulsed field gradient NMR to determine the components’ selfdiffusion coefficients. It is demonstrated that the structure of the liquids depends heavily on their composition, in that, for low water content, the structure is water-in-oil (w/o), gradually changing to a bicontinous structure in a concentration range ca. 40-60 wt % water, and, finally, to an oil-in-water (o/w) structure for more water rich samples. In the water poor samples, the surfactant molecules apparently do not form extended aggregates (inverted micelles). In the water rich samples, the surfactant and oil (if present) form ordinary micelles, and it is demonstrated for the binary water/surfactant system that the micelles are spherical at very low surfactant concentrations and grow into oblate (disk) shaped aggregates at surfactant concentrations above ca. 5 wt %. From density and viscosity measurements of binary mixtures of oil (1-decanol) and surfactant, it is demonstrated that these components form solutions that are not far from ideal.
Introduction Silicone surfactants form a group of small or polymeric surface active molecules made of silicone and polyethers.1,2 One class of these materials consists of a hydrophobic permethylated siloxane group coupled to poly(oxyethylene). These surfactants find applications in cosmetics, in manufacture of plastic foams, and as spreading and wetting agents. A number of studies of phase behavior and structures formed when surfactants of this kind are mixed with water and various cosurfactants or oil have recently been published.3-8 Garti and co-workers have published a study of phase behavior and structure of microemulsions formed by a class of nonionic ethoxylated polymethylsiloxane (so-called “Silwets”) surfactants when mixed with water and various oils or long chain alcohols.9 In general, these systems form isotropic and thermodynamically stable solutions in a large composition range. On the basis of conductivity and microcalorimetry measurements together with small-angle X-ray scattering (SAXS) measurements on samples containing water, these authors draw some conclusions regarding the structures formed in the microemulsions as a function of composition. A lamellar local structure for the amphiphiles is suggested, with water closely bound to the hydrophilic poly(oxyethylene) chains of the surfactants, and with the oil component confined in layers between the surfactant/water bilayers. The pulsed field gradient Fourier transform NMR (PGSEFT NMR)10-12 method has proven to be a very powerful method to investigate structures formed in organized liquids like microemulsions and related systems.13-18 With this method, it is possible to determine self-diffusion coefficients for all components in a single experiment without perturbing the system by any means. On the basis of observation of self-diffusion coefficients for the various components of the samples, compared to the self-diffusion coefficients for the pure components at the same temperature, it is possible to determine * Phone: + 47 22 85 55 88. Fax: + 47 22 85 54 41. E-mail:
[email protected].
some structural details in these liquids. In an oil continuous and water discontinous system (w/o), a comparatively slow water diffusion is expected, while, in a water continuous and oil discontinous (o/w) system, a rather slow oil diffusion is expected. In a system that is continuous in both components (i.e., a bicontinous microemulsion), the self-diffusion of these two components is expected to be reduced by approximately the same factor.13,14,17 In the present work, one of the surfactants in the work of Garti and co-workers, i.e., the so-called Silwet L 7607 surfactant, is used to make aqueous microemulsion samples together with 1-dodecanol or 1-decanol as “oil”. Measurements of selfdiffusion coefficients of the various components are used to investigate possible structures formed in this system as a function of its composition. Materials and Methods The surfactant was supplied by GE Bayer Silicones and was used as received. 1-Decanol (99%) was from Aldrich, and 1-dodecanol (puriss grade) was from Fluka. Light water was of “Type 1” (Millipore) quality with a conductivity of
