Surfactant micellar aggregate size distributions

Figure 16. Natural crude oil surfactant micellar aggregate size distributions for Long Beach crude/caustic system. The aqueous phase containing 0.05M NaOFI without hexanol (0) and with 0.50% hexanol ( ).

Micelle size and structure are stabilized by surfactant interactions and bonding. Therefore additives that destabilize micellar structure also disrupt the interactions and bonding of the surfactants adsorbed at the oil-aqueous interface. The disruption of the surfactant interactions and bonding at the interface leads to a weakening of the interfacial film and thereby promotes coalescence. The micellar aggregate size distributions for surfactant systems under consideration for chemical flooding,... 

The objectives of this study was to determine the changes in micellar aggregate size distributions caused by oil/water ratio, co-surfactant and three phase development in petroleum sulfonate systems and by a co-surfactant in a caustic system. 

It has been shown in our recent publications (6 9 7, 27) that the addition of a co-surfactant greatly enhances the coalescence rates of oil droplets. The co-surfactant must have disrupted the surfactant interactions at the oil-aqueous interface. We have also previously reported the significant changes in micellar aggregate size distribution caused by the equilibration of the aqueous surfactant solution against the crude oil. 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

The structure of the AOT micellar system, as well as the state of water entrapped inside swollen micelles, have been characterized using different techniques, such as photon correlation spectroscopy (25), positron annihilation (26), NMR (27, 28), fluorescence (29-32) and more recently small angle neutron scattering (33). The existence of reversed micelles has been demonstrated in the domain of concentrations explored by protein extraction experiments. Their size (proportional to the molar ratio of water to surfactant known as wo), shape and aggregation number have been determined. Furthermore, the micelle size distribution is believed to be relatively monodisperse. 

In addition to these technological developments, the range of applications of aqueous SEC has increased dramatically. Size and size distributions can be determined for colloidal systems, pre-eminently micellar aggregates of either synthetic or natural surfactants. In principle, one may also obtain information about the association equilibrium in such systems. The use of aqueous SEC to evaluate equilibrium constants for the binding of smaller molecules to larger ones by the Hummel-Dryer method and related techniques is well documented. Ligand/macromolecular systems studied in this way include small ion/protein, substrate/enzyme, and protein/polyelectrolyte mixtures. The perturbation of ionic concentrations by polyelectrolytes, i.e. the Donnan effect, is also susceptible to a similar approach. 

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