Negative Interfacial Tensions

Like many other topics covered here, microemulsions hold a great deal of promise in many practical applications. To date, most research in the area has been closely associated with the formation and destruction of microemulsions in the context of secondary and tertiary petroleum recovery, although more interest is being shown in cosmetic applications. While the concept of microemulsions is very attractive for potential use in many other areas, especially pharmaceuticals, their sensitivity to composition makes their application much more problematical. Since in many cases (i.e., drug delivery) one or more component (which may be somewhat surface-active) may be determined by the intended function of the system, the options for the formulator may be drastically reduced or at least greatly complicated. 

Given the following chemical structures, predict the probable location of each of the following compounds if solubilized in aqueous micellar solutions of (a) sodium dodecyl sulfate, (b) n-hexadecyl-trimethyl ammonium bromide, (c) polyoxyethylene(7)dodecylphenol  

Which of the following surfactants would be expected to be most efficient at solubilizing hexadecane sodium n-nonylbenzene sulfonate, sodium hexadecylsulfate, benzyl trimethylammonium acetate, or SDS For solubilizing cholesterol  

A system of aqueous micelles of a nonionic POE surfactant is found to solubilize an average of 2 molecules of a material per micelle at 25°C. If the temperature of the system is raised to 50°C, would you expect the solubilizing capacity per micelle to increase or decrease From the information provided, for the same total surfactant concentration, what can you say about the total solubilizing capacity of the system at the two temperatures  

If an aqueous micellar solution of sodium tetradecylsulfate is employed to solubilize a polar dye, would you expect the addition of dodecyl alcohol to increase, decrease, or not affect the capacity of the system  

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Negative interfacial tension [58,61-66] Due to adsorption of surfactants or cosurfactant molecules, the interfacial tension can become extremely low (less than 1 mN/m) and eventually transiently negative. Therefore, the interface can increase and any fluctuation can break it. 

In an attempt to explain the spontaneous movement Schulman introduced the concept of negative interfacial tension. This was used to explain why the positive... 

Areas of negative interfacial tension develop between the two phases, causing spontaneous breakup of the disperse phase... 

Groves (1978) provided an intuitive explanation based on a mechanical model in which water penetrates into the oil/surfactant system, forming liquid crystals but, more to the point, considerably expanding the interface. This is the reason why it is necessary to postulate that water is inconsiderable excess. The surface expands so that instead of a negative interfacial tension what we have is a positive surface pressure. At this point it is not unreasonable to visualize the surface expanding and stranding as postulated in the Gopal model. 

Microemulsions, like micelles, are considered to be lyophilic, stable, colloidal dispersions. In some systems the addition of a fourth component, a co-surfactant, to an oil/water/surfactant system can cause the interfacial tension to drop to near-zero values, easily on the order of 10-3 - 10-4 mN/m, allowing spontaneous or nearly spontaneous emulsification to very small drop sizes, typically about 10-100 nm, or smaller [223]. The droplets can be so small that they scatter little light, so the emulsions appear to be transparent. Unlike coarse emulsions, microemulsions are thought to be thermodynamically stable they do not break on standing or centrifuging. The thermodynamic stability is frequently attributed to a combination of ultra-low interfacial tensions, interfacial turbulence, and possibly transient negative interfacial tensions, but this remains an area of continued research [224,225],... 

A zero or negative interfacial tension also implies the compatibiliza-tion of two phases (12). An inter-diffusion at the molten stage can take place under this condition. We could expect the graft side chain to diffuse into the polymer phase and the grafted rubber main chain to diffuse into the rubber phase as shown in Figure 6. On the whole, we can conclude that grafting tends to make rubber more compatible with the polymer phase. 

The earlier concepts of microemulsion stability stressed a negative interfacial tension and the ratio of interfacial tensions towards the water and oil part of the system, but these are insuflBcient to explain stability (13). The interfacial free energy, the repulsive energy from the compression of the diffuse electric double layer, and the rise of entropy in the dispersion process give contributions comparable with the free energy, and hence, a positive interfacial free energy is permitted. 

Aqueous film stability is dependent on the adhesive force or negative interfacial tension at the two-phase (i.e., solid/liquid) boundary. The force balance at the two-phase boundary may change independently from the three-phase force balance due to surface configuration change of interfacing surface state moieties, which occurs in order to minimize interfacial tension with water as described in previous chapters. 

The concept of transient interfacial tension has been further extended by Davis and Haydon (43). They described an experiment by Ilkovic (44) in which a negative potential was applied to a mercury drop in an aqueous solution of a quaternary ammonium compound. At -8 v/cm applied potential, the spontaneous emulsification of mercury occurred. The spontaneous emulsification was observed for surfactant concentrations which exhibited negative values for interfacial tensions upon extrapolation. These results indicate that for spontaneous emulsification, the dynamic interfacial tension may approach transient negative values. Moreover, this does not mean that at equilibrium, the dispersed droplets will have a negative interfacial tension. 

Prince, L.M. (1967) A theory of aqueous emulsions. 1. Negative interfacial tension at oil/water interface. /. Colloid Interface Sci., 23, 165. 

When the in situ formed block or graft copolymer chains are accumulated in too large a volume at the interface, the copolymer chains are forced to elongate perpendicularly to the interface and destabilize the interface. In other words, the excess accumulation may lead to a negative interfacial tension coefficient, so that the interfacial area will tend to increase by the undulation. As an extreme case of undulation, the copolymer will escape from the interface to form micelles. Such an example was present in the PS-CCX)H/PMMA-epoxy system... 

It is important to note that the lamellar phase is thus stabilized by the balance of a negative interfacial tension (of the free oil/water interface covered by an amphiphilic monolayer), which tends to increase the internal area, and a repulsive interaction between interfaces. The result, Eq. (48), indicates that the scattering intensity in a lamellar phase, with wave vector q parallel to the membranes, should have a peak at nonzero q for d > d due to the negative coefficient of the q term in the spectrum of Eq. (40). just as in the microemulsion phase. This effect should be very small for strongly swollen lamellar phases (in coexistence with excess oil and excess water), as both very small [96]. Very similar behavior has been observed in smectic liquid crystals (Helfrich-Hurault effect) [122]. Experimentally, the lamellar phase under an external tension can be studied with the surface-force apparatus [123,124] simultaneous scattering experiments have to be performed to detect the undulation modes. 

The fact that they emanated from formulation activities (Prince, 1967) meant that their scientific introduction (Hoar and Shulman, 1943) and much of the subsequent treatment (Prince, 1975) focused on interfacial properties, disregarding other components of the free energy. In fact the notion of a negative interfacial tension was introduced. 

Spontaneous emulsification was first demonstrated by Gad [13], who observed that when a solution of lauric acid in oil is carefully placed onto an aqueous alkaline solution, an emulsion is spontaneously formed at the interface. As explained in Ghapter 10, such spontaneous emulsification could be due to the very low (or transient negative) interfacial tension produced by the surfactant. Using an aqueous alkaline solution causes partial neutralisation of lauric acid. A mixture of lauric acid... 

The third mechanism of spontaneous emulsification may be due to the production of an ultralow (or transiently negative) interfacial tension. This mechanism is thought to be the cause of formation of microemulsions when two surfactants, one essentially water soluble and one essentially oil soluble, are used [22, 23]. This mechanism is described in detail in Chapter 10 on microemulsions. 

The above interfadal tension results may throw some light on the mechanism of spontaneous emulsification in the present model EC. As mentioned before, there are basically two main mechanisms of spontaneous emulsification, namely creation of local supersaturation (i.e. diffusion and stranding) or by mechanical breakup of the droplets as a result of interfadal turbulence and/or the creation of an ultralow (or transiently negative) interfacial tension. Diffusion and stranding is not the likely mechanism in the present system since no water-soluble co-solvent was added. To check whether the low interfadal tension produced is sufficient to cause spontaneous emulsification, a rough estimate may be made from consideration of the balance between the entropy of dispersion and the interfacial energy, i.e. 

Schulman emphasized that micellar emulsions are systems in true equilibrium, it being proposed that the components of the surface films in these systems produce a negative interfacial tension at the hydrocarbon-water interface [172]. On mixing, a spontaneous interfacial area increase occurs until zero interfacial tension is attained. In Adamson s [173] model for micellar W/O emulsions, stability is accounted for by a balance of the Laplace pressure AP, related to the micellar radius r and interfacial tension y by... 

The van Oss-Good equation can result in either positive or negative interfacial tensions, the latter simply meaning miscible liquids. Thus, it is possible for the van Oss-Good theory to predict repulsive van der Waals forces which can be present in certain systems (van Oss et al, 1988, 1989). Because van Oss-Good can also predict negative interfacial tensions, it has been shown to predict well the solubility in aqueous polymer solutions (van Oss and Good, 1992) where Owens-Wendt fails. It has also been applied with success to biopolymers (van Oss et al,... 

Good and co-workers have frequently pointed out (e.g. van Oss et al., 1987) that the inability of Neumann theory to predict negative liquid-liquid interfacial tensions results in often poor agreement for liquid-liquid interfaces and for predicting misci-bUity (miscibUity is equivalent to zero or negative interfacial tensions). 

The original microemulsion was first detected as a distinct entity by Hoar and Schulman in 1943 [24] and consisted of water, benzene, hexanol, and potassium oleate. Most of Schulman s work dealt with four-component systems a hydrocarbon, an ionic surfactant, a cosurfactant (i.e., four- to eight-carbon-chain aliphatic alcohol), and an aqueous phase. The microemulsion was formed only when the surfactant-cosurfactant blend formed a mixed film at the oil/water interface, resulting in interfacial pressure exceeding the initial positive interfacial tension (so-called negative interfacial tension). The microemulsion was therefore produced spontaneously. During years of research many important geometrical and compositional parameters of microemulsions were studied. 

The first two terms are identical to those of de Vries. The third bending energy term accounts for the extra (positive or negative) interfacial tension of the surface of revolution with respect to the planar state ... 

As early as 1943, Professor J. H. Schulman published reports on transparent emulsions [32]. From various experimental observations and intuitive reasoning, he concluded that such transparent systems were microemulsions. Figure 13 illustrates the transparent nature of a microemulsion in comparison with a macroemulsion. He also proposed the concept of a transient negative interfacial tension to induce the spontaneous emulsification in such systems. 

Spontaneous emulsification refers to the production of an emulsified system in the absence of stirring. It is an instability mechanism in which a substance, generally a surfactant and/or a cosurfactant, is transferred from one phase to the other. There is no need to assume an unrealistic situation such as a negative interfacial tension because the decrease in chemical potential of the transferred substance is the energy source that induces the increase of surface area. How spontaneous emulsification takes place is not fully understood yet, although the diffusion and stranding mechanism seems to offer a good hypothesis [75],... 

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