Microstructure solvent diffusion

The procedure of evaluating self-diffusion data in terms of microstructure is to calculate the reduced or normalized diffusion coefficient, D/Dq, for the two solvents. Do being the neat solvent value under the appropriate conditions. Here we also have to account for reductions in D resulting from factors other than microstructure, mainly solvation effects. As discussed above, solvation will lead to a reduction of solvent diffusion that is proportional to the surfactant concentration. Normally the correction has been empirical and based on diffusion studies for cases of established structure, notably micellar solutions. We need to distinguish between corrections due to polar head-water and alkyl chain-oil interactions. The latter have often been considered insignificant, but a closer analysis (either experimental or theoretical) is lacking. However, it is probably reasonable to assume, for example, that the resistance to translation is not very different in the lipophilic part of the surfactant film and in an alkane solution. (This is supported by observations of molecular mobilities of surfactant allQ l chains on the same order of magnitude as for a neat hydrocarbon.)... 

Magnetic Resonance Imaging (MRI) becomes one of the useful microscopy and is used not only for the medical purposes but also for chemical applications. For polyurethane foams, the analysis of the distributions of many microstructural features, including strut length and window and cell shape distributions, were carried out. The diffusion behaviours of water in membrane is investigated by MRI in order to develop the polymer electrolyte fuel cells.The solvent diffusions and the swollen behaviors were investigated by MRI for hydroxy methyl cellulose,high amylose starch tablets, poly(ethylene methacrylate)/poly(2-hydroxyethyl methacrylate)-co-tetrahydro-... 

The Effect of Microstructure on Solvent and Solute Diffusion on the Micro- and Nanolength Scales... 

Anderson and Wennerstrom [33] calculated the geometrical obstruction factors of the self-diffusion of surfactant and solvent molecules in ordered bicontinuous microstructures, which serve as good approximations also for the disordered bicontinuous microemulsions and L3 (sponge) phases. The geometrical obstruction factor is defined as the relative diffusion coefficient DIDq, where D is the diffusion coefficient in the structured surfactant system and Z)q is the diffusion coefficient in the pure solvent. In a bicontinuous microemulsion the geometrical obstruction factor depends on the water/oil ratio. An expansion around the balanced (equal volumes of water and oil) state gives, to leading order. 

In a typical experimental study, D values of oil and water are obtained over a range of conditions—temperature, salinity, cosurfactant or surfactant concentration, solvent composition, etc. From the deduced D/Do values, microstructures in certain ranges are normally directly obtained. If D/Do of water and oil differ by more than one order of magnitude, discrete particles of the slowly diffusing solvent are implied, whereas if they are of the same order of magnitude a bicontinuous structure is suggested. 

We note that bicontinuity results from a particular spontaneous curvature of the surfactant films rather than from a certain solvent volume fraction, which is a secondary factor in determining microstructure. Note that for nonionic surfactants it was shown that the diffusion behavior was determined by temperature and not by solvent composition. For different systems at the same composition, we may have either water droplets, oil droplets, or a bicontinuous structure. An example is given in Fig. 17. Furthermore, one could argue that, to be consistent, all surfactant structures of infinite aggregates (including lamellar and hexagonal) should be described as percolated. 

Liquid-liquid stratified flow in microchannel is often used in biological analysis, such as during ion exchange or solvent extraction from one phase to another phase [1]. For liquid flow in microfluidics, the Reynolds number is small and the flows are always laminar. Laminar fluid diffusion interfaces (LFDIs) are generated when two or more streams flow in parallel within a single microstructure [2], as shown in Fig. la. 

---