Neutron scattering, microemulsion studies

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... 

Robinson BH, Toprakcioglu C, Dore JC, Chieux P (1984) Small-Angle Neutron-Scattering Study of Microemulsions Stabilized by Aerosol-Ot.l. Solvent and Concentration Variation. J Chem Soc Faraday Trans 1 80 13-27... 

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]. 

Even the traditional methods discussed in this chapter can be used for concentrated dispersions through contrast matching. For example, silica particles coated with silane coupling agents in a refractive index-matched mixture of ethanol and toluene can be used in combination with visible probe particles to study the dynamics of particles in dense systems. In the case of microemulsions (Chapter 8), selective deuteration of a component (oil, water, or surfactant) can be used in neutron scattering experiments even to measure the curvature of the oil-water interface. 

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. 

As in binary surfactant-water systems considered previously, two constraints on the geometry of the surfactant interface are active a local constraint, which is due to the surfactant molecular architecture, and a global constraint, set by the composition. These constraints alone are sufficient to determine the microstructure of the microemulsion. They imply that the expected microstructure must vary continuously as a function of the composition of tile microemulsion. Calculations show - and small-angle X-ray and neutron scattering studies confirm - that the DDAB/water/alkane microemulsions consist of a complex network of water tubes within the hydrocarbon matrix. As water is added to the mixture, the Gaussian curvature - and topology -decreases [41]. Thus the connectivity of the water networks drops (Fig. 4.20). 

Eastoe J, Cazelles BMH, Ste5Tler DC, Holmes JD, Pitt AR, Wear TJ, Heenan RK. Water-in-C02 microemulsions studied by small-angle neutron scattering. Langmuir 1997 13 6980-6984. 

Lee CT, Johnston KP, Dai HJ, Cochran HD, Melnichenko YB, Wignall GD. Droplet interactions in water-in-carbon dioxide microemulsions near the critical point a small-angle neutron scattering study. J Phys Chem B 2001 105 3540-3548. 

Chen, S.H. (1986) Small angle neutron scattering studies of the structure and interaction in micellar and microemulsions. Ann. Rev. Phys. Chem., 37, 351-399. 

Arleth, L. and Pedersen, J.S. (2001) Droplet polydispersity and shape fluctuations in AOT [bis(2-ethylhexyl)sulfosuccinate sodium salt] microemulsions studied by contrast variation small-angle neutron scattering. Phys. Rev. E, 63, 61406-61423. 

Bagger-Jorgensen, H., Olsson, U. and Mortensen, K. (1997) Microstructure in a ternary microemulsion studied by small angle neutron scattering. Langmuir, 13, 1413-1421. 

Sottmann, T., Strey, R. and Chen, S.-H. (1997) A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface Area per molecule, microemulsion domain size, rigidity. /. Chem. Phys., 106, 6483-6491. 

In Ref. [42], PEO was embedded in a w/o-droplet microemulsion and studied by small-angle neutron scattering. The authors state that this polymer does not adsorb considerably at the SDS monolayer. The important statement is that both the size polydispersity and the shape fluctuations are increased compared to the reference system without polymer. Larger shape fluctuations are also found for gelatine embedded in w/o-droplet microemulsions (see Fig. 4.10 in [43]). Here, by strong confinement, the elongated shapes... 

Zielinski, R.G., Kline, S.R., Kaler, E.W. and Rosov, N. (1997) A small-angle neutron scattering study of water in carbon dioxide microemulsions. Langmuir, 13, 3934-3937. 

Rananavare, S.B., Ward, A.J.I., Osborne, D.W., Friberg, S.E. and Kaiser, H. (1988) A small-angle neutron-scattering study of a nonaqueous 3-component microemulsion. /. Phys. Chem., 92, 5181-5183. 

Co, C.C. and Kaler, E.W. (1998) Particle size and monomer partitioning in microemulsion polymerization 2. Online small angle neutron scattering studies. Macromolecules, 31,3203-3210. 

The colloidal properties of anionically prepared poly(styrene-g-ethylene oxide) graft copolymers were studied by Candau et al. in different water/toluene/ alcohol mixtures by light and neutron scattering, NMR, and viscometry [307-309]. Aggregation numbers depend on mixture compositions with the highest values attained for water-rich systems. The micelles formed seem to have a core and shell conformation, with PS cores, in all cases studied. Dialysis experiments showed that the enhanced water-oil solubility was due to preferential solvation of the two segregated components of the copolymers by the solvent mixture and not to one specific solvent entrapment as is the case of classical microemulsions. 

The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7Hi5)-(C7Fi5)CHS04 Na [32], 667 g/mol PFPE-C00"NH4 [33], and for a partially fluorinated di-chain sodium sulfo-succinate surfactant [34]. For the PFPE-COO NH4 surfactant, the droplet radius increases from 20 A to 36 A for W o values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 A as Wo increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and Wq were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 

Eastoe, Julian Gazelles, Beatrice M. H. Steytler, David C. Holmes, Justin D. Pitt, Alan R. Wear, Trevor J. Heenan, Richard K. Water-in-C02 Microemulsions Studied by Small-Angle Neutron Scattering. Langmuir 1997, 13(26), 6980-6984. 

In most cases, it appears possible to interpret the critical behavior of microemulsion mixtures as a liquid/gas-like critical point [113-116]. Several light- and neutron-scattering studies on oil-rich ternary and quaternary microemulsions have clearly demonstrated that the structure of these media can be described as a solution of interacting water-in-oil droplets. As first shown by Calje et al. [117], the droplets may behave essentially as hard spheres. However, in many systems an attractive contribution to the interactions exists. It has been established that the strength of attractions between W/0 micelles is strongly dependent on the micellar size and on the chain lengths of both the alcohol and oil molecules. In particular, attractions have been found to increase when the micellar radius increases or the alcohol chain length decreases and the molecular volume of the oil increases [114, 115, 118-120]. 

Water-in-C02 microemulsions studied by small-angle neutron scattering, Langmuir 13,6980-6984 (1997). 

Figure 13.8 shows the first separation of small molecules by FFF. Ascorbic acid was separated from toluene through a secondary chemical equilibrium with field-retained microemulsiom droplets. Once again, the exchange between the aqueous phase and the swollen micelles is low, i. e., the efficiency is low and broad peaks are obtained (Figure 13.8). There are so many powerful techniques for small molecule separation that micellar FFF was not used for this purpose. Its interest could be in the physicochemical study of the micellar or microemulsion structure. For example, in the case of the Figure 13.8 experiment, the separation allowed the estimation of the average mass of the mobile phase microemulsion droplets (1.4x 10 %) and consequently, its radius (35 nm) [38], These values can be obtained by heavy methods such as small angle neutron scattering or high resolution NMR [38]. Micellar FFF can be an easy alternative in such studies.

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