Microemulsion SANS study

SANS studies and fluorescence correlation spectroscopy have been successfully combined to study the size of water-in-oil (heptane) microemulsions stabilized by sodium Z A-2-ethylhexyl sulpho succinate (AOT) with and without a fluorescently labeled peptide (phalloidin, a fungal toxin of mass 789 Da) and protein (ot-chymotrypsin, a serine protease of mass 25 kDa). In incorporation of the small peptide, phalloidin did not increase the size of the microemulsion droplets whereas the presence of ot-chymotrypsin significantly increased the size of the microemulsion droplets. Furthermore, the studies suggested that while all the phalloidin was in the disperse water phase, the a-chymotrypsin appears to be dispersed in the oil phase in monomeric form and protected from contact with the oil by a shell of surfactant. 

Sottmann, T., Strey, R. and Chen, S.H. (1997) A SANS study of non-ionic surfactant molecules at the water-oil interface Area per molecule, microemulsion domain size and rigidity. /. Chem. Phys., 106, 6483. 

Schtibel, D., Bedford, O.D., Ilgenfritz, G., Eastoe, J. and Heenan R.K. (1999) Oligo- and polyethylene glycols in water-in-oil microemulsions. A SANS study. Phys. Chem. Chem. Phys., 1,2521-2525. 

The behaviour of a real microemulsion is much more complex than the idealised system described above and the SANS studies are capable of revealing much more detailed information on the structure. A few illustrative examples are provided below. Since the microemulsion consists of three different regions the use of H/D isotope substitution (see Table 2 in previous chapter) can be used to vary the p-value of each region independently. In practice, it is much easier to use D2O in the central core and hydrogenated surfactant and oil components as shown in Figure 7a but some experiments have also been done with the inverted system shown In Figure 7b or for various H/D combinations (Figure 7c). 

The size, shape, formation mechanism, and interior polarity of the different IL microemulsions have been studied extensively by various techniques, such as freeze-fracture electron microscopy (FFEM), dynamic light scattering (DLS), conductivity study, UV-Vis spectroscopy with various probe molecules, and SANS study [30-33]. 

The relationship between the interfacial curvatures and phase behavior in bicontinuous microemulsions - a SANS study... 

In order to clarify the relation between the phase behavior, interactions between droplets, and the Ginzburg number, we have undertaken further SANS studies of critical phenomenon in a different three-component microemulsion system called WBB, consisting of water, benzene, and BHDC (benzyldimethyl-n-hexadecyl ammonium chloride). This system also has a water-in-oil-type droplet structure at room temperature and decomposes with decreasing temperature. Above the (UCST) phase separation point, critical phenomena have been investigated by Beysens and coworkers [9,10], who obtained the critical indexes, 7 = 1.18 and v = 0.60, and concluded that their data could be interpreted within the 3D-Ising universality. However, Fisher s renormalized critical exponents were not obtained. 

The structure of microemulsions have been studied by a variety of experimental means. Scattering experiments yield the droplet size or persistence length (3-6 nm) for nonspherical phases. Small-angle neutron scattering (SANS) [123] and x-ray scattering [124] experiments are appropriate however, the isotopic substitution of D2O for H2O... 

Water-in-fluorocarbon emulsions, stabilised with fluorinated nonionic surfactants, were investigated by small angle neutron scattering (SANS) spectroscopy [8,99]. The results indicated that the continuous oil phase comprised an inverse micellar solution, or water-in-oil microemulsion, with a water content of 5 to 10%. However, there was no evidence of a liquid crystalline layer at the w/o interface. A subsequent study using small angle x-ray scattering (SAXS) spectroscopy gave similar results [100]. 

Small-angle neutron scattering (SANS) can be applied to food systems to obtain information on intra- and inter-particle structure, on a length scale of typically 10-1000 A. The systems studied are usually disordered, and so only a limited number of parameters can be determined. Some model systems (e.g., certain microemulsions) are characterized by only a limited number of parameters, and so SANS can describe them fully without complementary techniques. Food systems, however, are often disordered, polydisperse and complex. For these systems, SANS is rarely used alone. Instead, it is used to study systems that have already been well characterized by other methods, viz., light scattering, electron microscopy, NMR, fluorescence, etc. SANS data can then be used to test alternative models, or to derive quantitative parameters for an existing qualitative model. 

For the study of colloidal interactions, SANS gives higher signal to noise ratio and can be used to lower Q than SAXS. It has been widely used for the characterization of synthetic microemulsions. Application to food systems has been more limited, but one example is described below. A study of voids in food solids is noted. 

Another polar solvent that has been used in SDS-stabilized microemulsions is glycerol. Hexanol or decanol have been used as cosurfactants and systems both with and without oil have been studied. The ternary system with hexanol as cosurfactant was examined with SANS and NMR self-diffusion measurements by two different groups and both found the microemulsions to be structureless solutions [130,131], Similar behavior was found from a self-diffusion study of the quaternary systems with p-xylcnc or decane as the oil component [131,132],... 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

Based on the success of these fluoro-sulfosuccinates described above di-fluorocarbon phosphates have also been investigated. In terms of synthesis and raw materials costs these surfactants have significant advantages over the sulfosuccinates. Surfactants of this kind have also been studied by DeSimone et al (27 c), and the synthesis and purification are described elsewhere (27b, c). Detailed SANS experiments are described in these papers (27b, c), and it is clear that surfactants of diis kind stabilize aqueous nano-droplets. Hence, anionics other than sulfosuccinates may be employed in water-in-C(>2 microemulsions. Significantly, one of these conq>ounds (di-HCF6-P, see ref 27 b) stabilizes microemulsions in liquid CO2 at vapor pressure a potentially useftil result that may be of importance in facilitating applications. 

---