Mixed surfactant film

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Stirba and Hurt (S12), 1955 Experimental work on C02 absorption by water films in vertical tubes of length 3 and 6 ft., and dissolution of tubes of solid organic acids by water films. Effective diffusivity exceeds molecular diffusivity, even at 2VRe = 300. Dye streak experiments show that waves cause mixing surfactants damp waves to give continuous dye streak and mass transfer results in agreement with theory. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces [411]. For example, in some cases the addition of a small amount of non-ionic surfactant to a solution of anionic surfactant can enhance foam stability due to the formation of a viscous surface layer, which is possibly a liquid crystalline surface phase in equilibrium with a bulk isotropic solution phase [25,110], In general, some very stable foams can be formed from systems in which a liquid crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid crystal phase is present, stable foams are not produced. In this connection foam phase diagrams may be used to delineate compositions that will produce stable foams [25,110],... 

Film thickness for the three component mixed surfactant films in n-decane at 25 C. Total surfactant concentration = 5 g dm aqueous phaseiwater. 

Drop stabilization methods rely on the immediate stabilization of drops by encapsulation with thin polymer films or surfactants [219-221] a photomicrographic method has been employed usually after encapsulation of drops. However this method cannot always be used due to incompatibility of the encapsulating materials with some systems. The method also has the disadvantage of the influence of the chemical treatment on drop size. A special sampling apparatus has been developed to withdraw a sample of dispersed phase from the mixing vessel to stabilize drops with a surfactant and to force the dispersed sample through a capillary with a photometer assembly to measure both droplet size and concentration [222]. 

These mixed-surfactant systems are used not only for their ability to form complex condensed films at the liquid-liquid interface, enhancing the stability of the emulsion, but also because of their ability to impart body to the product, resulting in a semisolid product rather than a liquid. Mixed emulsifiers control the consistency of a cream by forming a viscoelastic network throughout the continuous phase of the emulsion. The network results from the interaction of the mixed emulsifier with water, forming a liquid crystalline phase. 

This method is particularly useful for the measurement of very low interfacial tensions (<10 mN m ) that are particularly important in applications such as spontaneous emulsification and the formation of microemulsions. Such low interfacial tensions may also be achieved with emulsions, particularly when mixed surfactant films are used. In this case, a drop of the less-dense liquid A is suspended in a tube containing the second liquid, B. On rotating the whole mass (see Figure 5.4) the drop of the liquid moves to the centre and, with an increasing speed of revolution, the drop elongates as the centrifugal force opposes the interfacial tension force that tends to maintain the spherical shape, which is that having a minimum surface area. 

Use of mixed surfactant films In many cases the used of mixed surfactants (e.g., anionic and nonionic or long chain alcohols) can reduce coalescence as a result of several effects a high Gibbs elasticity high surface viscosity and hindered diffusion of surfactant molecules from the film. 

Formation of lamellar liquid crystalline phases at the O/W interface This mechanism, as suggested by Friberg and coworkers [37], proposed that surfactant or mixed surfactant film can produce several bilayers that wrap the droplets. As a result of these multilayer structures, the potential drop is shifted to longer distances, thus reducing the van der Waals attractions. A schematic representation of the role of Hquid crystals is shown in Figure 10.32, which illustrates the difference between having a monomolecular layer and a multilayer, as is the case with hquid crystals. 

According to the thermodynamic theory of microemulsion formation, the total interfacial tension of the mixed film of surfactant and cosurfactant must approach zero. The total interfacial tension is given by the following equation. 

LANGMUIR-BLOIDGETT FILMS OF A PYRROLE AND FERROCENE MIXED SURFACTANT... 

For reduction of coalescence, one needs to dampen the fluctuations of the interface that occur during close approach of the droplets. This can be achieved by enhancement of the Gibbs elasticity. For this reason, mixed surfactant films are ideal for reducing coalescence. Such mixed films may also produce lamellar liquid crystalline phases at the interface which prevent coalescence as a result of their multilayer structure (17). 

Lipids (fatty acids, glycerides and phospholipids) are also surface-active materials and will form a mixed surfactant system in beer in competition with the surface-active proteins for space in the surface film. If the lipid... 

Microemulsions [191, 192] are transparent, optically isotropic and thermodynamically stable liquids. They contain dispersion of polar and nonpolar solvent, usually water or aqueous solutions and oils. Adding surfactants stabilizes droplets of 1-100 nm in size. Due to amphiphilic properties of the surface active substances containing lipophilic groups and one or two lyophobic C-H chains mainly collected at the interface of two liquid phases, they cannot be mixed under normal conditions. Unlike traditional macroemulsion, which is kinetically stabilized only by the external mechanical energy supply, nano-domains in the microemulsions are formed spontaneously. Their size depends on the microemulsion composition, temperature and elastic properties of the separating film of surfactant. In particular, in the case of water-oil microemulsions with spherical nanosized micelles of water dispersed in oil, water droplets can be used as nanoreactors and templates for the solid nanoparticles fabrication. Since the reaction is initiated by the spatially restricted water and micelle, heterogeneous nucleation and crystal growth can be controlled. 

When surfactant mixtures of practical interest containing multiple species were used (e.g., commercial nonionic surfactants or mixtures of anionic and nonionic surfactants), a maximum in hydrocarbon removal from polyester/cotton fabric similar to that in Figure 4.32 was again seen. For situations where the surfactant oil ratio in the system is large, the typical case for household washing, the maximum occurred at the PIT of a system for which surfactant composition in the films separating oil and water domains of the microemulsion phase was the same as the initial surfactant composition (Raney and Miller, 1987). This result is reasonable since the small amount of hydrocarbon present can dissolve only a small portion of the total surfactant, leaving the remainder, which has neariy the initial composition, to make up the films. It should be noted that here too the PIT is well above the cloud point temperature of the mixed surfactant solution. 

Lao.9oCeo.ioP04 nanocrystals were surface modified with mixed surfactants a mixture of DDA and BEHP or a mixture of OA and BEHP. The surface modified nanocrystals were dispersed at SO wt % in polystyrene, forming translucent thick films on quartz discs. Pulse height spectra from alpha particle detection (Source Am) of nanocrystal samples superimposed with the result from a highly efficient plastic scintillator, BC-400 are presented in Figure 3. 

For concentrated mixed surfactant system such as the one under investigation, would it be possible to apply the models developed primarily for "even film" drainage mode ... 

In an earlier investigation Vijayan and Woods (20) studied the stability of oil (benzene) drops at oil oil/water interface containing mixed surfactants (sodium dodecyl sulfate and dodecanol) around and below the critical micelle concentration. They measured both coalescence times and film drainage rates using interferometer-cinematography. For the range of concentration studied, they found uneven film drainage modes which finally lead the drops to coalescence. Also, anomalous coalescence time behavior was found at some concentration of the surfactant. (Note the comparison between this observation with the anomaly at 1.5% NaCl concentration in the current study.)... 

DDS and DAC at 0.1 mM bulk aqueous concentration have a methyl/methylene ratio of 1.3 and 0.8, respectively. However, the mixed surfactant film of the same cumulative bulk surfactant concentration has a ratio of 2.4. 

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