Self-diffusion technique, surfactant systems

In principle, we can distinguish (for surfactant self-assemblies in general) between a microstructure in which either oil or water forms discrete domains (droplets, micelles) and one in which both form domains that extend over macroscopic distances (Fig. 7a). It appears that there are few techniques that can distinguish between the two principal cases uni- and bicontinuous. The first technique to prove bicontinuity was self-diffusion studies in which oil and water diffusion were monitored over macroscopic distances [35]. It appears that for most surfactant systems, microemulsions can be found where both oil and water diffusion are uninhibited and are only moderately reduced compared to the neat liquids. Quantitative agreement between experimental self-diffusion behavior and Scriven s suggestion of zero mean curvature surfactant monolayers has been demonstrated [36]. Independent experimental proof of bicontinuity has been obtained by cryo-electron microscopy, and neutron diffraction by contrast variation has demonstrated a low mean curvature surfactant film under balanced conditions. The bicontinuous microemulsion structure (Fig. 7b) has attracted considerable interest and has stimulated theoretical work strongly. 

The alternative NMR approach that has provided information on microemuisions is relaxation. However, on the whole, relaxation has been less informative than anticipated from earlier studies of micellar solutions and has provided little unique information on microemulsion structure, although in the case of droplet structures it is probably the most reliable way of deducing any changes in droplet size and shape, particularly for concentrated systems. The reason for this is that NMR relaxation probes the rotational diffusion of droplets, which is relatively insensitive to interdroplet interactions. This is in contrast to, for example, translational collective and self-diffusion and viscosity which depend strongly on interactions. Furthermore, NMR relaxation is a useful technique for characterizing the local properties of the surfactant film. 

Measurement of self-diffusion coefficients by means of PGSE techniques has evolved to become one of the most important tools in the characterization of surfactant systems. In particular, this is true of those surfactant systems that are isotropic liquid solutions such as micellar systems and microemulsions. The technique has been described in a number of review articles [9,11-13]. An account of the most recent developments of the method can be found in Ref 9. We do not dwell on the technical aspects here but merely note that the technique requires no isotopic labeling (avoiding possible disturbances due to addition of probes) furthermore, it gives component-resolved diffusion coefficients with great precision in a minimum of measuring time. 

Complementary information can be obtained from surfactant self-dilfusion. In a typical NMR self-dilfusion experiment, the value of the surfactant diffusion coefficient is obtained along with the values of the solvents. However, for two reasons the accuracy in self-diffusion coefficient of a surfactant is lower than that of a solvent. First, surfactant is typically present at a considerably lower concentration, and second, the transverse relaxation rate is higher and thus more unfavorable. Depending on the experimental conditions and the system, it may turn out to be impossible, to measure surfactant diffusion. However, by using stimulated echo techniques this problem can be diminished. 

To make the significance of the NMR technique as an experimental tool in surfactant science more apparent, it is important to compare the strengths and the weaknesses of the NMR relaxation technique in relation to other experimental techniques. In comparison with other experimental techniques to study, for example, microemulsion droplet size, the NMR relaxation technique has two major advantages, both of which are associated with the fact that it is reorientational motions that are measured. One is that the relaxation rate, i.e., R2, is sensitive to small variations in micellar size. For example, in the case of a sphere, the rotational correlation time is proportional to the cube of the radius. This can be compared with the translational self-diffusion coefficient, which varies linearly with the radius. The second, and perhaps the most important, advantage is the fact that the rotational diffusion of particles in solution is essentially independent of interparticle interactions (electrostatic and hydrodynamic). This is in contrast to most other techniques available to study surfactant systems or colloidal systems in general, such as viscosity, collective and self-diffusion, and scattered light intensity. A weakness of the NMR relaxation approach to aggregate size determinations, compared with form factor determinations, would be the difficulties in absolute calibration, since the transformation from information on dynamics to information on structure must be performed by means of a motional model. 

To confirm that the systems change from discrete aggregates to becoming bicontinuous, NMR self-diffusion experiments were performed. The results are shown in Figure 3.12. This technique [28, 29] measures the diffusion of the surfactants and the oils, and can thus be used to determine whether the studied structures consist of discrete aggregates or are bicontinuous. 

During surfactant dissolution the two diffusion processes can be identified. On the molecular scale a molecule undergoes self diffusion where the diffusion coefficient is determined from its mean squared displacement. Various NMR techniques have been used to study quantitatively self diffusion processes (21-24), It is important to note that each component in the system will have a self diffusion coefficient. Diffusion coefficients for a number of mesophase systems have been collected where values of order 10 - 10 m s" were reported (25). The self diffusion coefficients of the solvent are typically reduced no more than an order of magnitude in the presence of mesophases which essentially act as obstacles to the solvent (25, 26). 

The microstructure of complex fluids such as ILs, surfactant systems, and liquid crystals can be profitably investigated by means of pulsed gradient spin-echo nuclear magnetic resonance (PGSE-NMR) experiments, a technique that allows the determination of the self-diffusion coefficients. 

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