Interfacial surfactant

Recently, Figoli et al. [15] reported the use of polymerized bicontinuous microemulsion (PBM) membranes as nanostructured liquid membranes for facilitated oxygen transport. The final bicontinuous microemulsion consisting of an interconnected network of water and oil channels, stabilized by the interfacial surfactant film, in which the oil (monomer) channels were polymerized to form the polymeric matrix of the liquid membranes (Fig. 7.6) and the channel width (pore size) of the membranes could be tuned between 3 and 60 nm by adjusting the composition of the cosurfactant, while the water phase remained unchanged and it was the solvent for the novel oxygen carrier. 

The first physically sound model for adsorption kinetics, which was derived by Ward and Tordai [18], is based on the assumption that the time dependence of a surface or interfacial tension (which is directly proportional to the surface excess F, in mol m ) is caused by diffusion and transport of surfactant molecules to the interface. This is referred to as diffusion-controlled adsorption kinetics model . The interfacial surfactant concentration at any time t, T(t), is given by the following expression,... 

Ultimately, the water pool and the interfacial surfactant layer can exhibit multiple catalytic effects, which result from the concentrations of reactants localized in the nano-compartmentalized region and the physicochemical properties of the micellar environment. Accordingly, the reversed micellar systems have the possibility of controlling the multiple effects on the reactions by changing the physical factors of the reversed micellar systems such as water mobility, micropolarity, and electrostatic force. 

We describe the physicochemical aspects of a reversed micellar system, and how to control the water pools and the interfacial surfactant layer as a reaction field by changing tfj, and the hydrophilic group of the surfactant. 

In typical microemulsion systems, a prominent composition region shows bicontinuous structures. With probes and quenchers confined to either oil or water, the domains in the bicontinuous region may be so large in all dimensions that normal exponential decays are observed. Only in the region with discrete droplets, and in a transition region where droplets cluster and merge, can the micellar type of quenching be expected. However, if the amphiphilic probes and quenchers are bound to the interfacial surfactant film in the bicontinuous microemulsions, one would expect 2-D behavior. 

Figure 1. Schematic of processes and states of an interfacial surfactant layer and the adjacent solution...

In Section 3.3 we have briefly indicated that there are various factors that can affect water solubilization in reverse micelles. The importance of the oil-water interface in the context of water solubilization cannot just be overemphasized, and the entity that first comes to the forefront is the surfactant film that separates the two immiscible phases. At the very beginning, therefore, some basic and relevant points on the surfactant film are described below in brief [113, 114, 124, 125, 3]. The interfacial surfactant film can be described as a two-dimensional system in which one can consider a pressure term n [113] this term, characteristic of the film, defines the difference in the oil/water interfacial tension before and after addition of the surfactant to form an interfacial film ... 

Briefly, AFM is a technique that maps the topography of a surface by plotting (on a color scale) the measured force between the surface and a small tip attached to a sensitive cantilever spring. In most applications, the tip is in direct contact with the surface, and the AFM performs as a sensitive contact profilometer. For imaging interfacial surfactant structures, however, contact forces disrupt the liquid crystalline aggregates. Therefore the repulsive colloidal stabilization forces between the surfactant layers adsorbed to the tip and sample are used as the contrast mechanism during imaging. 

The increasing residual scattering at higher temperatures (see Fig. 9) is straightforward explained by the temperature dependent fluctuations of the interfacial surfactant layer covering the dispersed droplets (26). The fluctuations can be observed since the static contributions of the refractive index increments of the dispersed particles and the solvent (oil) are optically matched. Hence, except for the fluctuations of the surfactant molecules in the monolayer the microemulsions discussed in the present paper show a remarkable monodispersity. 

In these systems the curvature of the film is modified by the addition of short or medium chain alcohols, which partially partition between oil and water phase [25] and incorporate in the interfacial surfactant flhn. It has also been shown that the addition of salt can alter the curvature of the surfactant flhn and therefore significantly influence the phase behavior of microemulsion systems [26,28]. Figure 14.4 shows a fish-like phase diagram, obtained for the Marlowet IHF-perchloroethylene2-propanol-water system. At present these components are employed in the preparation of a macroscopic emulsion applied by the German armed forces for decontamination of HD and VX on varnished surfaces. 

In Rehbinder s concept of the stability of emulsions and other disperse systems, the focus is on the lyophilic structural-mechanical barrier as a factor responsible for the strong stabilization of disperse systems. This barrier is manifested with the interfacial surfactant adsorption layer formed predominantly with high molecular weight substances (the so-called protective colloids). This barrier on the one hand promotes the formation of a system with substantial mechanical strength that is capable of resisting coalescence and the rupture of the droplets and, on the other hand, is lyophilic with reference to the dispersion medium. The lyophilic nature of the barrier is characterized by a low value of the interfacial energy, o, on the side facing the dispersion medium. One can thus... 

Figure 9.9. Schematic representation of the different localization states of a hydrophobic solute in a microemulsion. The solute could be solubilized in the oil microdomain or at the interface. The interfacial surfactant area is noted as a and the curvature radius as R. This schematic represents the case of a nonionic surfactant where the hydration of the polar head is temperature (r)-dependant. With the pseudo-phase model, the solute concentration is considered over the volume occupied by the tails of the surfactant, whereas in the surfactant monolayer model, the binding of the solute into the surfactant monolayer is considered...

Interfacial surfactant density fluctuations (see Figure 21.14(b)) are another phenomenon not accounted for in a classical DLVO description of thin-film forces. For ionic surfactants, these fluctuations induce charge fluctuations which in turn can influence the height of the DLVO barrier, AH max- Applying a standard statistical thermodynamic approach to the interface provides a simple method for investigating which physical properties influence surfactant density fluctuations at the air-water interface. Analogous to bulk density fluctuations, surface density fluctuations can be expressed by the following ... 

Figure 35 Fluid motion in the film and droplet phases. The flow in the film phase is the superposition of the Poiseuille flow and a flow of constant velocity Ug. Due to the nonuniform interfacial surfactant distribution, surface diffusion and convective fluxes appear.

Discontinuities in interfacial tension versus temperature have been reported by Lutton et al. [99] and Krog and Larsson [100]. Lutton et al. [99] surmised that a break in y versus temperature was the result of the melting of an interfacial surfactant monolayer. The abrupt change in slope resulting from the melting could be interpreted from... 

The most important properties of a surfactant are its solubility and its interfacial activity. When a nonionic surfactant is added to a two-phase liquid-liquid system, it preferentially adsorbs at the interface forming an adsorbed layer. At low surfactant concentrations an equilibrium exists between surfactant monomers dissolved in the oil phase, surfactant monomers dissolved in the water phase and the interfacial surfactant. In the case of a separated system (constant interfacial area), as the concentration of surfactant in the system increases, the amount of surfactant at the interface reaches a maximum possible concentration and, on further increase in surfactant concentration, excess surfactant will now form micelles in either the oil or aqueous phase, or form a surfactant phase depending on its affinity for the oil and water phases. The break point is termed the critical micelle concentration (CMC). 

A surfactant micelle is generally pictured as a sphere of surfactant molecules with a liquid phase core, e.g. an aqueous micellar solution has continuous structure containing micelles with an oil phase core. This phase is thermodynamically stable and the oil within the micelle is termed solubilised. For conditions below the CMC, the effect of the adsorbed interfacial surfactant on the nSOW system s interfacial tension is governed by the Gibbs adsorption equation ... 

However, it is well recognised that the HLB concept is limited many researchers have found no correlation between emulsion type and HLB number also, changes in emulsion type have been found to depend on the water to oil ratio, surfactant concentration and temperature. " The HLB concept s main failing stems from the fact that it does not allow surfactants to have different affinities for different oils. Graciaa et al. showed that it was the HLB of the interfacial surfactant, rather than the overall surfactant HLB, that is the important affinity variable. This observation was made from the results of a model that describes the partitioning of surfactant between oil, water and interfacial surfactant phase. 

---