Water surfactant behavior

A. Ciach, J. S. Hoye, G. Stell. Microscopic model for microemulsion. II. Behavior at low temperatures and critical point. J Chem Phys 90 1222-1228, 1989. A. Ciach. Phase diagram and structure of the bicontinuous phase in a three dimensional lattice model for oil-water-surfactant mixtures. J Chem Phys 95 1399-1408, 1992. 

Differential scanning calorimetry measurements have shown a marked cooling/heat-ing cycle hysteresis and that water entrapped in AOT-reversed micelles is only partially freezable. Moreover, the freezable fraction displays strong supercooling behavior as an effect of the very small size of the aqueous micellar core. The nonfreezable water fraction has been recognized as the water located at the water/surfactant interface engaged in solvation of the surfactant head groups [97,98]. 

The rate constants and k represent rate constants for a surface reaction and have units m mol s and s respectively. The accelerative effects are about 10 -10 fold. They indicate that both reactants are bound at the surface layer of the micelle (surfactant-water interface) and the enhanced rates are caused by enhanced reactant concentration here and there are no other significant effects. Similar behavior is observed in an inverse micelle, where the water phase is now dispersed as micro-droplets in the organic phase. With this arrangement, it is possible to study anion interchange in the tetrahedral complexes C0CI4 or CoCl2(SCN)2 by temperature-jump. A dissociative mechanism is favored, but the interpretation is complicated by uncertainty in the nature of the species present in the water-surfactant boundary, a general problem in this medium. 

R. Aveyard, B.P. Binks, S. Clark, and J. Mead Interfacial Tension Minima in Oil-Water-Surfactant Systems. Behavior of Alkane-Aqueous Sodium Chloride Systems Containing Aerosol OT. J. Chem. Soc. Faraday Trans. I 82, 125 (1986). 

Experimental Information. The review by Ekwall — offers a whole series of phase diagrams which all show similar behavior. In order to dissolve an anionic surfactant with a sodium counter ion in an alcohol a minimum water/surfactant molar ratio of about six is needed to achieve solubility. The corresponding ratio for the potassium ion is three. 

Johnston et al. l also examined the solvatochromic shift of pyridine N-oxide in an ethane/CjEj (C = 10-13 E = 5) water-in-oil microemuision, also in equilibrium with a lower liquid phase. Contrary to the behavior exhibited by the AOT system, the nonionic microemulsions display a polar environment at low pressures, which becomes progressively less polar as pressure increases. At a pressure of only 50 bar, they reported that the probe s environment resembles that observed in bulk hexane. Added water increases the polarity somewhat, yet a cosurfactant (octanol) is required to produce an environment similar to that in bulk water. The polarity of the ethane/ water/surfactant/cosurfactant system remains essentially constant as pressure increases up to 350 bar. 

As an approach to investigating the complex chemistry of natural foams, humic substances (compounds sufficiently nonpolar at pH 2.0 to be isolated by reverse phase on XAD-8 and recovered in 0.1 N sodium hydroxide) were isolated from aquatic foam and associated stream water for chemical characterization and investigations into surfactant behavior. Humic substances were chosen because they represent natural organic compounds present in natural waters that are sufficiently nonpolar at pH 2.0 to be isolated by XAD-8 adsorption. As surfactants also possess moderately nonpolar characteristics it follows that humic substances may contain a significant surfactant component. We hypothesized that foam would be enriched in humic substances compared to stream samples and would show increased hydrophobicity, aliphaticity, and decreased carboxylation in order to sustain surface-active behavior. 

Kao, C. C.-P, Pozo de Fernandez, M. E., and Paulaitis, M. E., 1993. Equation-of-state analysis of phase behavior for water-surfactant-supercritical fluid mixtures. ChiLplsiTm Supercritical Fluid Engineering Science, Fundamentals and Applications. E. Kiran and J. F. Brennecke, eds. ACS Symposium Series 514, American Chemical Society, Washington, D.C., pp. 74-91. 

This section describes how to use Hand s rule to represent binodal curves and tie lines. The surfactant-oil-water phase behavior can be represented as a function of effective salinity after the binodal curves and tie lines are described. Binodal curves and tie lines can be described by Hand s rule (Hand, 1939), which is based on the empirical observation that equilibrium phase concentration ratios are straight lines on a log-log scale. Figures 7.15a and 7.15b show the ternary diagram for a type II(-) environment with equilibrium phases numbered 2 and 3 and the corresponding Hand plot, respectively. The line segments AP and PB represent the binodal curve portions for phase 2 and phase 3, respectively, and the curve CP represents the tie line (distribntion cnrve) of the indicated components between the two phases. Cy is the concentration (volnme fraction) of component i in phase) (i or j = 1, 2, or 3), and 1, 2, and 3 represent water, oil, and microemulsion, respectively. As the salinity is increased, the type of microemulsion is changed from type II(-) to type III to type II(-i-). C, represents the total amount of composition i. 

Complete information on phase behavior including tie-lines and on diffusion coefficients is rarely available for oil-water-surfactant systems. Nevertheless, Raney and Miller used plausible phase diagrams for an anionic surfactant-NaCl brine-hydrocarbon system as a function of salinity to calculate diffusion paths that exhibited intermediate phase formation and spontaneous emulsification in agreement with experimental observations made using the vertical cell technique. For example. Figure 9.12 shows a diffusion path for a surfactant-alcohol-brine mixture of composition D in contact with oil for a case when initial salinity is high. An intermediate brine phase is predicted as well as spontaneous emulsification in the oil phase, both of which were, in fact, observed. 

Raney, K.H. and Miller, C.A., Diffusion path analysis of dynamic behavior of oil-water-surfactant systems, A/C 33, 1791, 1987. 

Endo, H., Mihailescu, M., Monkenbusch, M., AUgaier, J., Gompper, G., Richter, D., Jakobs, B., Strey, R. and Grillo, I. (2001) Effect of amphiphilic block copolymers on the structure and phase behavior of oil-water-surfactant mixtures. /. Chem. Phys., 115, 580-600. 

The part of the real world system which is most often dispensed with is the crude oil. From the practical point of view this is desirable in many cases because the dark color of the crude can make the phase behavior of the oil/water/ surfactant system difficult to observe. Crude oils are in any case unpleasant to work with. Hence, Healy and Reed (j -j>), in their study of phase behavior, used a synthetic... 

Amphiphilic molecules, when dissolved in organic solvents, are capable of self-assembly to form reversed micelles. The reversed micelles are structurally the reverse of normal micelles in that they have an external shell made up of the hydrocarbon chains of the amphiphilic molecules and the hydrophilic head-groups localized in the interior of the aggregate. Water molecules are readily solubilized in this polar core, forming a so-called water pool. This means that reversed micelles form microcompartments on a nanometer scale. The reversed micelles can host all kinds of substrate molecules whether hydrophilic, hydrophobic, or amphiphilic due to the dynamic structure of the water pool and the interface formed by the surfactant layer, in contrast with a liposome system. The properties of water molecules localized in the interior of reversed micelles are physicochemically different from those of bulk water, the difference becoming progressively smaller as the water content in the micellar system increases [1,2]. The anomalous water at low JVo =[water]/[surfactant] obviously influences the chemical behavior of host molecules in the water pools. 

Surfactant molecules commonly self-assemble in water (or in oil). Even single-surfactant systems can display a quite remarkably rich variety of structures when parameters such as water content or temperature are varied. In dilute solution they form an isotropic solution phase consisting of micellar aggregates. At more concentrated surfactant-solvent systems, several isotropic and anisotropic liquid crystalline phases will be formed [2]. The phase behavior becomes even more intricate if an oil (such as an alkane or fluorinated hydrocarbon) is added to a water-surfactant binary system and the more so if other components (such as another surfactant or an alcohol) are also included [3], In such systems, emulsions, microemulsions, and lyotropic mesophases with different geometries may be formed. Indeed, the ability to form such association colloids is the feature that singles out surfactants within the broader group of amphiphiles [4]. No wonder surfactants phase behavior and microstructures have been the subject of intense and profound investigation over the course of recent decades. 

A sudden increase in research effort on microemulsion systems was driven by the economic impact of the oil embargo in the early 1970s and the development of the so-called enhanced oil recovery processes that followed. The plentiful research funding available from both industry and governmental agencies resulted in an unprecedented improvement in the basic and advanced knowledge of very complex phenomena, in particular the surfactant-oil-water phase behavior in all its intricacies. It was found that the interfacial tension could be lowered to an ultralow 0.001 mN/m in many systems provided that a particular physicochemical condition was attained. It turned out that this so-called first optimum salinity, and then optimum formulation, coincides with the occurrence of three-phase behavior in which a bicontinuous microemulsion is in equilibrium with oil and water excess phases, i.e., the Winsor III case [20,21,109,110]. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

An example of surfactant behavior in conventional liquid solvents is shown in Fig. 3 for the nonionic C12EO5-water-heptane system [12]. The upper boundary of the narrow one-phase region, shown by open circles, is the solubilization boundary. The lower boundary, shown by filled circles, is the haze point curve. AOT systems have the same kind of phase behavior, but since AOT is anionic the relative positions of the solubilization and haze point... 

In the phenomenological model of Kahlweit et al. [46], the behavior of a ternary oil-water-surfactant system can be described in terms of the miscibility gaps of the oil-surfactant and water-surfactant binary subsystems. Their locations are indicated by the upper critical solution temperature (UCST), of the oil-surfactant binary systems and the critical solution temperature of the water-surfactant binary systems. Nonionic surfactants in water normally have a lower critical solution temperature (LCST), Tp, for the temperature ranges encountered in surfactant phase studies. Ionic surfactants, on the other hand, have a UCST, T. Kahlweit and coworkers have shown that techniques for altering surfactant phase behavior can be described in terms of their ability to change the miscibility gaps. One may note an analogy between this analysis and the Winsor analysis in that both involve a comparison of oil - surfactant and water-surfactant interactions. 

Figure 10 Microemulsion phase behavior for 5.0 wt% Igepal CO-520 in an oil-water-surfactant system. Butane and propane data are at 276 bar others are at ambient pressure (O) 2 behavior observed ( ) 2 behavior observed (A) 3 behavior observed. (From Ref. 12.)...

Such behavior appears to be quite typical regardless of the type of surfactant present. For instance, in the system sodium bis(2-ethylhexyl)phosphate-w-heptane-water, the transition from a W/O to a bicontinuous and then to an O/W microemulsion can be induced by increasing the water/surfactant molar ratio. Again in the transition region higher viscosities are observed where this increase is significantly more pronounced at the O/W to bicontinuous transition [90]. 

Another workup approach has been to use the inherent phase behavior of oil-water-surfactant systems to separate product from remaining reactants and from surfactant. A Winsor III system made with a branched-tail phosphonate surfactant was used as reaction medium for lipase-catalyzed hydrolysis of trimyristin. The enzyme resided almost exclusively in the middle-phase microemulsion together with the surfactant. The products formed, 2-myristoylglycerol and sodium myristate, partitioned into the excess hydrocarbon and water phases, respectively, and could easily be recovered [129]. A similar procedure was used for cholesterol oxidation using cholesterol oxidase as catalyst [130]. 

These are of two kinds related to each other by the difference in association structure as illustrated by the temperature variation of surfactant solubility and association. Figure 6 provides a schematic description of the interdependence. At low temperatures the solubility limit of the xmimers (s, solid line. Fig. 6) is lower than the limit for amphiphilic association (cmc, dashed line. Fig. 6), and, hence, the latter is not reached and a two-phase equilibrium, aqueous solution of monomers—hydrated surfactant, is established. At temperatures in excess of the Krafft point, Tj (Fig. 6), the association concentration (cmc, solid line, Fig. 6), is now beneath the solubility limit (s, dashed line. Fig. 6). Association takes place and the total solubility (ts. Fig. 6) is drastically increased. Hence, the water—siufactant phase diagram shows a large solubility range for the isotropic liquid solution (unimers plus micelles. Fig. 6) because the association structure, the micelle, is soluble in water. This behavior is characteristic of smfactants with Ninham R values less than 0.5. 

In this chapter, a brief theoretical background on the rheological behavior of viscoelashc worm-like micelles is given. It is followed by a discussion on the temperature-induced viscosity growth in a water-surfactant binary system of a nonionic fluorinated surfactant at various concentrations. Finally, some recent results on the formation of viscoelastic worm-like micelles in mixed nonionic fluorinated surfactants in an aqueous system are presented. 

Recently, Chatterjee et al [57] examined the problem of CMC in organic solutions in some detail in case of non-ionic surfactants of the Span and Tween series (see Section 2.2) with an eye toward the effect of the surfactant behavior on the synthesis of particles in the water pool . The choice of the relatively pH-... 

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