Colloidal dynamic modeling aggregates

The highly dynamic colloidal structures described in this chapter result in considerable complexity in behaviors. This complexity has resulted in relatively slow improvement in our understanding of colloidal systems despite the fact that the structure of micelles was in essence described almost a century ago already. Results from a series of relatively recent approaches to describe colloidal aggregates are now beginning to coalesce into a model of colloidal structures incorporating the dynamic and nonhomogeneous structures of these aggregates. 

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. 

As demonstrated already in Refs. [10,20,22,71,83,84,104,123,127] by changing just the nature of the conservative interactions between the fluid particles and by introducing also larger solid particles, we can easily model the different dynamics of colloids, micelles, colloidal crystals and aggregates. We consider two types of particles solvent droplets and colloidal beads. We assume... 

Pseudophase models work for several reasons (i) Reactions in association coUoids can be carried out under conditions of dynamic equilibrium. Thus the totality of the interfacial regions of all the aggregates in micelle, microemulsion, or vesicle solutions, and the totalities of their oil and water regions can be modeled as single interfacial, oil, and water reaction volumes of uniform properties with a separate rate constant for the reaction in each volume. Scheme 4. (ii) The requirement of dynamic equilibrium is met because the rate constants for diffusion of ions and molecules in association colloid solutions are near the diffusion-controlled limit. For example, the entrance and exit rate constants in micellar solutions in Table 1 are orders of magnitude faster than the example rate constants for thermal bimolecular reactions in micellar solutions in Table 4. Many additional examples are compiled in reviews. (iii) Measured rate constants for spontaneous reactions and... 

Micellar colloids represent dynamic association-dissociation equilibria. However, the theoretical treatment of micelles depends on whether the micelle is regarded as a chemical species or as a separate phase. The mass action model which has been used ever since the discovery of micelles, takes the former point of view," " whereas the phase separation model regards micelles as a separate phase. To apply the mass action model strictly, one must know every association constant over the whole stepwise association from monomer to micelle, a requirement almost impossible to meet experimentally. Therefore, this model has the disadvantage that either monodispersity of the micelle aggregation number must be employed or numerical values of each association constant have to be assumed. " The phase separation model, on the other hand, is based on the assumption that the activity " of a surfactant molecule and/or the surface tension of a surfactant solution remain constant above the CMC. In... 

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