Surfactant solution, light scattering

The term microemulsion was introduced by Schulman, who studied surfactant solutions as eady as 1943 (22). At that time it was widely accepted that "oil and water do not mix," and Schulman understood that an emulsion scatters light because it contains droplets whose diameters are large compared to the wavelength of light (see Emulsions). Thus, the term y /mJemulsion implies a system which (like an emulsion) contains droplets of oil or water, but in which the droplets are too small to scatter light. 

Light scattering technique was used in determining the oil solubilization rate. Debye s equation ( ) was used in the interpretation. The basic principle involves the measurement of the surfactant aggregate size during the solubilization. As the oil goes into the surfactant micelle, the increased size will be reflected by the turbidity of the solution. 

It has been well documented that surfactants self-associate in aqueous solution to minimize the are of contact between their hydrophobictails and the aqueous solution (Mukerjee, 1979 Tanford, 1980). This phenomenon occurs at a critical concentration of surfactant, the critical micelle concentration or CMC (see Figure 12.4) above where the surfactant molecules exist predominantly as monomeric units and above which micelles exist. The CMC can be measured by a variety of techniques, for example, surface tension, light scattering, osmometry, each of which shows a characteristic break point in the plot of the operative property as a function of concentration. Knowing the CMC of the particular surfactant system and understanding the conditions that may raise or lower that critical concentration is important to the design of a formulation based on micellar solubilization. 

Fig. 9.1. Synthetic microbubble histograms determined by laser-light scattering in aqueous media (at 21°C) (A) saturated solution of Filmix 3 surfactant mixture (B) distilled water alone (C ) computed difference in above two histograms. (See text for further discussion.)...

To provide a specific example, Imae and Ikeda (ref. 479) state that amine oxide is very hydrophilic and can constitute a good polar head group for nonionic surfactants at neutral pH. Dimethyl-dodecylamine oxide was first prepared by Hoh et al. (ref. 494), and its surface-active properties in aqueous solutions were investigated by measurements of surface tension (ref. 495), light scattering (ref. 496-498), and hydrodynamic properties (ref. 499). It was found that dimethyldodecylamine oxide can form only spherical micelles in water and aqueous NaCl solutions, when the micelle concentration is dilute (ref. 496,498). Similarly, the homolog dimethyltetradecylamine oxide forms only spherical micelles in water (ref. 496). 

A number of experimental techniques by measurements of physical properties (interfacial tension, surface tension, osmotic pressure, conductivity, density change) applicable in aqueous systems suffer frequently from insufficient sensitivity at low CMC values in hydrocarbon solvents. Some surfactants in hydrocarbon solvents do not give an identifiable CMC the conventional properties of the hydrocarbon solvent solutions of surfactant compounds can be interpreted as a continuous aggregation from which the apparent aggregation number can be calculated. Other, quite successful, techniques (light scattering, solubilization, fluorescence indicator) were applied to a number of CMCs, e.g., alkylammonium salts, carboxylates, sulfonates and sodium bis(2-ethylhexyl)succinate (AOT) in hydrocarbon solvents, see Table 3.1 (Eicke, 1980 Kertes, 1977 Kertes and Gutman, 1976 Luisi and Straub, 1984 Preston, 1948). 

The dispersions were obtained by emulsification via ultrasonication of a toluene solution of the unsaturated homopolymer in an aqueous surfactant solution. This was followed by exhaustive hydrogenation with Wilkinson s catalyst at 60°C and 80 bar H2 to produce a dispersion with an average particle size of 35 nm (dynamic light scattering and transmission electron microscopy analyses). The same a,co-diene was used as comonomer in the ADMET polymerization of a phosphorus-based monomer, also containing two 10-undecenoic acid moieties... 

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