Light scattering droplet microemulsions

Table 5. Diffusion Coefficients Measured by Dynamic Light Scattering of AOT/Water Droplets in Various Alkane/AOT/Water Microemulsions as a Function of Pressure. T= 25°C, W= 5, Faot 0.015 mole fraction.

It was proved by scattering studies (1-9) that water in oil microemulsions at low water content are dispersions of identical spherical droplets in a continuous oil phase. Dilution procedure and light scattering measurements (both the intensity and the correlation function of the scattered light) allow to measure the osmotic compressibility and the diffusion coefficient of the droplets ... 

Several structural studies have shown that in the oil rich part of the phase diagram, microemulsions consist of a dispersion of mono-disperse water droplets in interaction (7-8). In recent papers (9-10), we have investigated by light scattering the effect of the micellar... 

The interpretation of the bulk properties of the microemulsions phases, close to Sx, in terms of critical phenomena, is then less satisfying. Near this boundary, the samples are further from a critical consolute point than in the case of the boundary S2. As far as bulk properties are concerned, light scattering experiments are rather sensitive to droplets elongation as it will be observed in viscosity measurements. 

Microemulsions were characterized by Dynamic Light Scattering (DLS) to determine the size distribution of the water pools. Measurements were performed on a Malvern Zetasizer. Interactions existing between aqueous droplets were neglected which is a prerequisite to use the hard sphere model. The linear mode of the apparatus was chosen this corresponds to a cumulative analysis. 

Figure 2.5 Relaxation rates F of the intensity correlation functions as a function of q2 obtained via a photon correlation spectroscopy experiment. The sample was a w/o-droplet microemulsion made of D2 0/n-octane-di8/CioE4. On the oil-continuous side of the phase diagram the scattered light intensity is usually low leading to rather large errors of the individual data points. Nevertheless, from the slope of the linear fit the translational diffusion coefficient is obtained. (Figure redrawn with data from Ref. [67].)...

Figure 2.13 Schematic elastic scattering curve of a spherical colloid (e.g. a microemulsion droplet) in solution. As a rule of thumb q 1 is an approximate measure for the spatial resolution of the used scattering experiment. At low values of q (i.e. in the Cuinier region of the scattering curve) the overall size and shape of the particles as well as correlations between different particles can be monitored (typically by static and dynamic light scattering). At high values q, the internal structure of the particles, i.e. the local structure of the interfacial film is resolved (e.g. by neutron or X-ray small angle scattering and neutron spin-echo spectroscopy (NSE)).

The analysis of the light scattering data using CONTIN also allows for a determination of the size polydispersity of the microemulsion droplets, because all the moments u = / pm G(r)rndr which describe the distribution function G(T) are computed (for details see Ref. [98]). The polydispersity index is obtained from... 

Hellweg, Th. and Langevin, D. (1998) Bending elasticity of the surfactant film in droplet microemulsions Determination by a combination of dynamic light scattering and neutron spin-echo spectroscopy. Phys. Rev. E, 57, 6825-6834. 

Elastic and inelastic light scattering are nowadays widely used techniques for the characterization of fluids. In particular, these techniques have been extensively and successfully used with microemulsion systems to obtain information about droplet sizes. Surface light scattering is a less common technique but has been used with microemulsion interfaces, in particular to measure the ultralow interfacial tensions found in these systems. In this chapter we discuss these aspects, first recalling the theoretical background and illustrating the potential of the techniques with experimental results. 

In the elastic light scattering method, one measures the scattered intensity only. At a selected scattering angle 6, the intensity scattered in the 6 direction for monodisperse spherical microemulsion droplets is given by... 

A similar difference between R and can occur if the drops are not spherical. Microemulsion droplets are usually close to spherical, and it is very difficult to separate out the contributions of solvent penetration and droplet elongation from the light scattering data alone. A useful complementary experiment is the measurement of the solution viscosity rf. For dilute enough systems,... 

In some cases, the index of refraction of the spheres can be matched by the index of refraction of the solvent. The scattered intensity should vanish if the spheres are monodisperse, because the contrast would be identically zero at this point. The droplets being actually polydisperse, the residual intensity is a direct measure of the polydispersity. This procedure has been used in light scattering experiments on AOT microemulsions [18]. It can also be used in neutron scattering experiments, where the contrast matching is more easily adjusted by using mixtures of protonated and deuterated oils or water [19,20]. 

Guizard et al. [84,85] studied the acid-catalyzed hydrolysis and condensation of titanium alkoxides [i.e., titanium isopropoxide (TIPO) and titanium tetrabutoxide (TTBO)] in polyoxyethylene- )-octylphenyl ether/decane/water microemulsions. The nonionic surfactants used were Triton X-15 (TX-15), TX-35, and TX-45. Dynamic light scattering [85] showed that the hydrodynamic radii of droplets in the TX-35/decane/water system increased from about 2,2 nm in the absence of water to about 4 nm at water/surfactant molar ratio R = 2. The radius remained approximately constant up to about R = 4, above which phase separation occurred. The constant droplet size observed above R = 2 suggests that 2 mol HiO per mole of surfactant is needed for the hydration of the polar groups. 

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