Surfactant/oil/water mixtures

Salager JL (1977) Physico-chemical properties of surfactant-oil-water mixture phase behavior, microemulsion formation and interfacial tension. PhD Dissertation, University of Texas at Austin... 

Leaver MS, Olsson U, Wennerstrom H, Strey R, Wurz U. Phase behavior and structure in a nonionic surfactant-oil-water mixture. J Chem Soc Faraday Trans 1995 91 4269-4274. 

The integrated DLS device provides an example of a measurement tool tailored to nano-scale structure determination in fluids, e.g., polymers induced to form specific assemblies in selective solvents. There is, however, a critical need to understand the behavior of polymers and other interfacial modifiers at the interface of immiscible fluids, such as surfactants in oil-water mixtures. Typical measurement methods used to determine the interfacial tension in such mixtures tend to be time-consuming and had been described as a major barrier to systematic surveys of variable space in libraries of interfacial modifiers. Critical information relating to the behavior of such mixtures, for example, in the effective removal of soil from clothing, would be available simply by measuring interfacial tension (ILT ) for immiscible solutions with different droplet sizes, a variable not accessible by drop-volume or pendant drop techniques [107]. 

Oil-water mixture is added to a surfactant. To this emulsion, a short-chain alcohol (with four to six carbon atoms) is added continuously until a clear mixture (microemulsion) is obtained. Microemulsions will exhibit very special properties, quite different from those exhibited by ordinary emulsions the microdrops may be considered as large micelles. 

The property of interest to characterize a surfactant or a mixture of surfactants is its hydrophilic-lipophilic tendency, which has been expressed in many different ways through a variety of concepts such as the hydrophiUc-lipophilic balance (HLB), the phase inversion temperature (PIT), the cohesive energy ratio (CER), the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [1], which were found to be more or less satisfactory depending on the case. In the next section, the quantification of the effects of the different compounds involved in the formulation of surfactant-oil-water systems will be discussed in details to extract the concept of characteristic parameter of the surfactant, as a way to quantify its hydrophilic-lipophilic property independently of the nature of the physicochemical environment. 

Zhang and Rusling [66] employed a stable, conductive, bicontinuous microemulsion of surfactant/oil/water as a medium for catalytic dechlorination of PCBs at about 1 mA cm-2 on Pb cathodes. The major products were biphenyl and its reduced alkylbenzene derivatives, which are much less toxic than PCBs. Zinc phthalocyanine provided better catalysis than nickel phthalocyanine tetrasulfonate. The current efficiency was about 20% for 4,4 -DCB and about 40% for the most heavily chlorinated PCB mixture. A nearly complete dechlorination of 100 mg of Aroclor 1260 with 60% Cl was achieved in 18 hr. Electrochemical dehalogenation was thus shown to be feasible in water-based surfactant media, providing a lower-cost, safer alternative to toxic organic solvents. 

Anton, R.E. et ah. Surfactant-oil-water systems near the affinity inversion. IX Optimum formulation and phase behavior of mixed anionic-cationic systems, J. Dispersion Sci. Technol., 14, 401, 1993. Kahlweit, M. et ah. General patterns of phase behavior of mixtures of water, nonpolar solvent, amphiphiles, and electrolytes, 1,2, Langmuir, 4, 499, 1988 5, 305, 1989. 

Even today there is still some controversy about what system should be called a microemulsion. The point is that a microemulsion is not necessarily an emulsion with extremely small droplets. Actually, this may be the least interesting type of microemulsion, and many researchers think that the word microemulsion must be kept for those truly single-phase surfactant-oil-water (and often cosurfactant) mixtures in which sizable amounts of both oil and water are cosolubilized in a complex structure that has been described as percolated, bicontinuous, and transient. As will become clear later, from the point of view of applications these structures exhibit extremely interesting properties such as extremely low interfacial tension and high solubilization. 

The experimental technique used to find an optimum formulation, known a.s untdimensional scan, goes on as follows. Series of surfactant-oil-water. systems are prepared in test tubes, all with identical composition, and with the same formulation with the exception of the scanned variable, that is in general the aqueous phase salinity for ionic systems, and the average number of ethylene oxide groups per molecule (EON) if the systems contain an ethoxylated nonionic surfactant mixture. 

RE Anton, H Rivas, JL Salager. Surfactant-oil-water systems near the affinity inversion - Part X Emulsions made with anionic-nonionic surfactant mixtures. J Dispers Sci Technol 17 553—566, 1996. 

Reverse micelles are expected to form in the ternary mixtures of surfactant/oil/ water and are often in spherical shape. Despite this anticipation, this chapter... 

An emulsion (sometimes referred to as a macroemulsion in order to distinguish it from a microemulsion) is defined as a thermodynamically wn stable mixture of two immiscible liquids, one existing as a dispersed phase and the other as a continuous phase. Note that emulsions are formed from surfactant, oil, and water at concentrations found in the two-phase and three-phase regions of the surfactant/oil/water phase diagram (Figure 11.8). However, emulsions are not thermodynamically stable and thus are not labelled on the phase diagram. The... 

Therefore, it appears that this section could be concluded with the summary that the best oil recovery with fluids which bring about low interfacial tension can come either from fluids which are at their optimal salinity so that a fluid of very low interfacial tension is interposed between the oil phase and the brine phase, or, in the event that the surfactant-oil-water system is a two-phase mixture, it is essential that the displacement process occur with the Type 11+ mechanism as described by Nelson or Larson, i.e., an expanding oil phase should be part of the displacement process in the region where surfactant or alcohol concentration is increasing. 

The importance of solubility rests in large part on the fact that within true solutions, composition is a degree of freedom, and thermodynamic parameters vary smoothly with composition [16]. It is impossible to vary temperature and pressure without changing the state of a mixture, but many real situations exist in which thermodynamic states are invariant with respect to composition. Examples include b hasic mixtures in all binary systems at constant temperature and pressure, such as a salt-water mixture containing the saturated salt solution plus salt crystals (see below). Another would be any mixture in a ternary surfactant—oil—water system containing three coexisting phases (e.g., a microemulsion liquid phase, an oil-rich liquid phase, and a water-rich liquid phase). 

Another way to study a surfactant—oil-water ternary system is to study the case of OW binaries at different amounts of surfactant, so that only binary diagrams are involved. This would produce a series of two-dimensional diagrams that indicate the variation of the free energy G of a water-oil mixture at increasing surfactant content (see Fig. 3). 

Synthesis of CdS nanoparticles can be performed easily and safely by freshmen students. Based originally on research by Agostiano (P), the procedure has been adapted to use reagents commonly available to a general chemistry laboratory (JO, 11). This experiment illustrates how intermolecular forces affect the formation of micelles and how surfactants behave in oil-water mixtures. The difference in color between the bulk and nanosized CdS is visibly obvious but students can also calculate the nanoparticle size with the aid of a UV/visible spectrophotometer. The explanation for the color difference is based on quantum confinement of electrons and holes in the particle s semiconductor lattice. 

Natural Ethoxylated Fats, Oils, and Waxes. Castor oil (qv) is a triglyceride high in ticinoleic esters. Ethoxylation in the presence of an alkaline catalyst to a polyoxyethylene content of 60—70 wt % yields water-soluble surfactants (Table 20). Because alkaline catalysts also effect transestenfication, ethoxylated castor oil surfactants are complex mixtures with components resulting from transesterrfication and subsequent ethoxylation at the available hydroxyl groups. The ethoxylates are pale amber Hquids of specific gravity just above 1.0 at room temperature. They are hydrophilic emulsifiers, dispersants, lubricants, and solubilizers used as textile additives and finishing agents, as well as in paper (qv) and leather (qv) manufacture. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

In the latter the surfactant monolayer (in oil and water mixture) or bilayer (in water only) forms a periodic surface. A periodic surface is one that repeats itself under a unit translation in one, two, or three coordinate directions similarly to the periodic arrangement of atoms in regular crystals. It is still not clear, however, whether the transition between the bicontinuous microemulsion and the ordered bicontinuous cubic phases occurs in nature. When the volume fractions of oil and water are equal, one finds the cubic phases in a narrow window of surfactant concentration around 0.5 weight fraction. However, it is not known whether these phases are bicontinuous. No experimental evidence has been published that there exist bicontinuous cubic phases with the ordered surfactant monolayer, rather than bilayer, forming the periodic surface. 

In order to provide a more general description of ternary mixtures of oil, water, and surfactant, we introduce an extended model in which the degrees of freedom of the amphiphiles, contrary to the basic model, are explicitly taken into account. Because of the amphiphilic nature of the surfactant particles, in addition to the translational degrees of freedom, leading to the scalar OP, also the orientational degrees of freedom are important. These orientational degrees of freedom lead to another OP which has the form of the vector field. 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

In this section we characterize the minima of the functional (1) which are triply periodic structures. The essential features of these minima are described by the surface (r) = 0 and its properties. In 1976 Scriven [37] hypothesized that triply periodic minimal surfaces (Table 1) could be used for the description of physical interfaces appearing in ternary mixtures of water, oil, and surfactants. Twenty years later it has been discovered, on the basis of the simple model of microemulsion, that the interface formed by surfactants in the symmetric system (oil-water symmetry) is preferably the minimal surface [14,38,39]. 

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