Self-diffusion droplet microemulsions

In that case the self diffusion coefficient - concentration curve shows a behaviour distinctly different from the cosurfactant microemulsions. has a quite low value throughout the extension of the isotropic solution phase up to the highest water content. This implies that a model with closed droplets surrounded by surfactant emions in a hydrocarbon medium gives an adequate description of these solutions, found to be significantly higher them D, the conclusion that a non-negligible eimount of water must exist between the emulsion droplets. 

Figure 7. Schematic diagram comparing the behavior of self-diffusion coefficients of oil (D0), water (Dw), and surfactant (Ds) expected for the droplet inversion transition and the bicontinuous transition of microemulsion depicted in Fig. 6.

Thus, by measuring oil and water self-diffusion coefficients, it was quite easy to establish whether oil or water or none of them are confined to discrete domains, i.e. to droplets . In the first work on microemulsion structure by self-diffusion [46], using both tracer techniques and NMR spin-echo measurements, it was clearly shown that microemulsions can indeed be bicontinuous over wide ranges of composition, which is manifested by both... 

Gu6ring, P. and Lindman, B. (1985) Droplet and bicontinuous structures in cosurfactant microemulsions from multi-component self-diffusion measurements. Langmuir, 1,464-A68. 

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. 

Figure 19 Double-oil diffusion experiment with nonionic surfactant, (a) Self-diffusion coefficients and (b) diffusion coefficient ratio A" as a function of temperature in a water-rich microemulsion with nonionic surfactant. A transition from oil-in-water droplets to a bicontinuous microstructure occurs with increasing temperature (decreasing spontaneous curvature of the C12E5 surfactant film). The maximum in K indicates that an attractive interaction between the micelles is operating prior to the formation of a bicontinuous structure. Kq = 1.69 is the diffusion coefficient ratio in the pure oil mixture and is indicated as a broken line in (b). Note that the initial decrease of the self-diffusion coefficients shows that the droplets grow in size before the bicontinuous transition. The phase boundary at 25.7 C is indicated as a vertical broken line. (Data from Ref 43.)...

Figure 20 Double-water experiment, the aqueous analogy of the double-oil experiment, performed on an AOT microemulsion as a function of temperature. The polar solvent is a 5% A-methyl formamide (NMF) solution in heavy water (D2O). The ratio of the water (here measured as trace impurities of HDO) and NMF diffusion coefficients is monitored as a function of temperature (c). Also shown as (a) the individual self-diffusion coefficients of water (O). NMF ( ), and AOT ( ) and (b) the relative diffusion coefficient of water. Kq = 1.73 is the diffusion coefficient ratio in the pure water-NMF mixture and is indicated as a broken line in (c). The phase boundary at 75"C is indicated as a vertical broken line. The behavior with increasing temperature is completely analogous to that of the nonionic system (Fig. 19) and illustrates a transition from reverse micelles to a bicontinuous structure via growing droplets that become attractive. (Data from Ref 49.)... 

As for solvent diffusion, surfactant diffusion follows a rather general pattern for all microemulsion systems, and we illustrate it with an example (Fig. 10). As we vary the appropriate parameter (salinity in Fig. 10), changing the spontaneous curvature from positive to negative, we see at large positive Hq values (low salinity) a slow surfactant diffusion that equals the oil diffusion value. Here, clearly, surfactant diffusion describes droplet diffusion, as also verified in measurements of droplet diffusion (by self-diffusion or... 

Decreasing spontaneous curvature further to negative values (high salinity) leads to a progressive lowering of the surfactant self-diffusion coefficient to the value of water and to that of the droplets in the W/0 microemulsions. (This decrease is not pronounced for nonionic surfactants because of the high solubility in oil and the concomitant contribution from single-molecule diffusion.)... 

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. 

NMR self-diffusion measurements indicated that all microemulsions consisted of closed water droplets and that the structure did not change much during the course of reaction. Hydrolysis was fast in microemulsions based on branched-chain anionic and nonionic surfactants but very slow when a branched cationic or a linear nonionic surfactant was employed (Fig. 11). The cationic surfactant was found to form aggregates with the enzyme. No such interactions were detected with the other surfactants. The straight-chain, but not the branched-chain, alcohol ethoxylate was a substrate for the enzyme. A slow rate of triglyceride hydrolysis for a Ci2E4-based microemulsion compared with formulations based on the anionic surfactant AOT [61,63] and the cationic surfactant cetyltrimethylammonium bromide (CTAB) [63] was observed in other cases also. Evidently, this type of lipase-catalyzed reaction should preferably be performed in a microemulsion based on an anionic or branched nonionic surfactant. Nonlipolytic enzymes such as cholesterol oxidase seem to function well in microemulsions based on straight-chain nonionic surfactants, however [64]. CTAB was reported to cause slow inactivation of different types of enzymes [62,64,65] and also, in the case of Chromobacterium viscosum lipase [66], to provide excellent stability. 

Despite the reasonable tolerance of nonionic surfactants, particularly in topical applications, microemulsions prepared from (phospho)lipids seem to be preferred over those prepared by synthetic surfactants from a toxicity point of view. As discussed by Shinoda et al. [13], lecithin in water-oil systems does not spontaneously form the zero mean curvature amphiphile layers required for the formation of balanced microemulsions but rather forms reverse structures. On decreasing the polarity of the aqueous phase by addition of a short-chain alcohol, e.g., propanol, lecithin was found to form microemulsions at low amphiphile concentrations over wide ranges of solvent composition. The structure of the microemulsions formed was investigated by NMR self-diffusion measurements, and it was found that with a decreasing propanol concentration there was a gradual transition from oil droplets in water, over a bicontinuous structure, to water droplets in oil [13]. 

The components of a microemulsion constantly self-diffuse. The process is intimately related to the internal structural state of the system. The surfactant and cosurfactanl molecules diffuse back and forth between the interfacial layer and the bulk, the water and oil molecules self-diffuse in the medium, and the droplets self-diffuse in the continuum. This dynamic process is influenced by the self-association or clustering of the droplets. Self-diffusion studies in microemulsions are therefore of considerable importance. 

By making the IL component more surfactant-like, for example, by using EmimHexSO, in the presence of SDS, a broad isotropic phase channel can be produced [16]. More comprehensive investigations show that all types of microemulsions can be observed in the phase channel. Cryo-SEM micrographs show the transition from droplets to a sponge phase, accompanied by a characteristic change of the rednced self-diffusion coefficient obtained by NMR diffusion experiments. 

With microemulsions based on long-chain alcohols (e.g. decanol), the self-diffusion coefficient for water was low, indicating the presence of definite (closed) water droplets surrounded by surfactant anions in the hydrocarbon medium. Thus,... 

---