Hydrophilic surfactant mixtures

Figure 18 [99] shows the optimum formulation for three-phase behavior (as the optimum salinity) as a function of the composition of a mixture of anionic (sodium dodec)4 sulfate) and cationic (tetradecyltrimethylammonium bromide) species loaded with a considerable amount of alcohol to avoid the formation of liquid crystals. The surfactant pair was selected so that both individual surfactants produced a three-phase microemulsion-oil-water behavior at about the same salinity, i.e., 5-10% NaCl. As some cationic surfactant is added to the anionic one, the shaded region that indicates the three-phase behavior goes down (from left to right). This downward displacement and the fact that three-phase behavior is still exhibited means that the addition of a small amount of the cationic surfactant to the anionic one results in a less hydrophilic surfactant mixture. 

When mixing two surfactants species in a SOW system, an equilibrium takes place between the oil and water phases and the interface for each species. Since the two species do not necessarily exhibit the same affinity for the interface and the oil and water bulk phases, the compositions of the surfactant mixtures at interface and in the phases might be different. For instance if a very hydrophilic species is mixed with a very lipophihc one, as often recommended in the old formulation literature, then the hydrophihc surfactant has a strong tendency to partition in water, whereas the lipophihc one would partition in the oil. In this case the surfactant mixture in water will contain a large majority of hydrophilic species, i.e., it will be very hydrophilic, whereas the oil phase will predominantly contain the hpophihc species, with the remaining adsorbing at interface. This situation in which each species actuates on its own, more or less independently of the other, has been called non-collective behavior. Since the surfactant mixture composition at interface is often the one that commands the actual property of the system, such as the interfacial tension or the stabihty of the emulsion, it is most important to know how to calculate or measure the characteristics of the mixture present at interface. Such methods will be discussed in the next section. 

Figure 7 indicates the phase behavior of SOW systems containing ternary nonionic surfactant mixtures that in turn contain a very hydrophilic surfactant (Tween 60 Sorbitan -i- 20 EO stearate), a very hpophihc surfactant (Span 20 Sorbitan monolaurate), and an intermediate (Tween 85 Sorbitan 20 EO trioleate or Nonylphenol with an average of 5 EO groups). The two intermediate surfactants correspond exactly to an optimum formulation in the physicochemical conditions, i.e., they exhibit three-phase behavior with the system 1 wt. % NaCl brine-heptane-2-butanol. As the intermediate hy-drophihcity surfactant is replaced by an equivalent mixture of the extreme ... 

It is worth noting that the effect of temperature on ionic and polyethoxy-lated nonionic surfactants is just opposite. As temperature increases, the nonionics become more lipophilic whereas the ionics turn more hydrophilic. By mixing the two types of surfactants in a proper proportion, these effects could cancel each other out, and the mixture is said to be insensitive to temperature. This interesting feature of ionic-nonionic surfactant mixtures may be considered as a synergy, since it could be very important in practice. Analysis of this feature is not included here, because plenty of information may be found in the literature on applications of such mixtures to equihbrated and emulsified systems [10,71-74]. 

The phase behavior of anionic-cationic surfactant mixture/alcohol/oil/ water systems exhibit a similar effect. First of all, it should be mentioned that because of the low solubility of the catanionic compound, it tends to precipitate in absence of co-surfactant, such as a short alcohol. When a small amount of cationic surfactant is added to a SOW system containing an anionic surfactant and alcohol (A), three-phase behavior is exhibited at the proper formulation, and the effect of the added cationic surfactant may be deduced from the variation of the optimum salinity (S ) for three-phase behavior as in Figs. 5-6 plots. Figure 16 (left) shows that when some cationic surfactant is added to a SOWA system containing mostly an anionic surfactant, the value of In S decreases strongly, which is an indication of a reduction in hydrophilicity of the surfactant mixture. The same happens when a small amount of anionic surfactant is added to a SOWA system containing mostly a cationic surfactant. As seen in Fig. 16 (left), the values of In S at which the parent anionic and cationic surfactant systems exhibit three-phase behavior are quite high, which means that both base surfactants, e.g., dodecyl sulfate... 

The hydrophile-lipophile balance (HLB) system is the measure of the surfactant s polarity as well as other physical properties of surfactants and the emulsifying materials. The more lipophilic the surfactant is, the lower the HLB values will be. Table 4.5 empirically classifies and compares surfactants according to their optimum use. Table 4.6 shows the HLB values for a selected group of surfactants. The HLB value of the surfactant or surfactant mixture should be matched with that of the oil or the mixture of oils to ensure a stable emulsion. The required HLB values of a... 

The scope of the chapter will include an introduction to the technique of neutron reflectometry, and how it is applied to the study of surfactant adsorption at the planar solid-solution interface, to obtain adsorbed amounts and details of the structure of the adsorbed layer. The advantages and limitations of the technique will be put in the context of other complementary surface techniques. Recent results on the adsorption of a range of anionic, cationic and nonionic surfactants, and surfactant mixtures onto hydrophilic, hydrophobic surfaces, and surfaces with specifically tailored functionality will be described. Where applicable, direct comparison with the results from complementary techniques will be made and discussed. 

The concentration of black spot formation in microscopic films Cm characterises not only the threshold concentration of the surfactant at which stable foams and emulsions can be obtained but it can also be used as an indirect measure of film stability. The relations between film stability and Cm of the emulsifier depend on the polarity of the organic phase of the emulsion films (aqueous and hydrocarbon) [58], on the hydrophilic-lipophilic balance of the surfactant mixture [59] as well as on other properties. 

At room temperature, the microemulsion is single phase and balanced when the wt. X of the hydrophile is 67 X of the total surfactant mixture. At higher concentrations of the hydrophile, the system splits into two phases a lower mlcroemulslon phase in equilibrium with excess oil. The first manifestation of the phase separation results in a cloudy solution which can be generated by either a change in the H/L properties or by increasing the temperature. It should be realized that the temperature response of anionic surfactants and microemulsions is opposite to that of... 

Water-dispersible lecithins may be produced by adding a hydrophilic surfactant (5-20%) such as polysorbate or ethoxylated monoglycerides. A mixture of lecithin and nonionic surfactants (10-20%) has utility in applications where water dispersibility is needed. Blending of partial glycerides and lecithin, followed by spray cooling, results in flaked or powdered products (33). 

Effect of Demulsifier Mixture. In previous studies (27) Duo-meen C, which was effective in causing flocculation of the water droplets, was not very effective in breaking the interfacial film formed between the water droplets, which inhibits coalescence. (Duomeen C is a mixture of many types of surfactants the general classification is a fatty acid ester nitrogen derivative.) However, Duomeen C in combination with docusate sodium (Aerosol OT), a hydrophilic surfactant, was much more effective in causing water separation compared to the individual chemicals. This effect is shown in Figure 16 for a 6 vol% water-in-oil (Leduc crude) emulsion in which both the UVP signal (20 min after chemical addition) and the volume... 

The poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers were also used to gel the continuous aqueous phase. Poloxamers may be used as the secondary hydrophilic surfactant in the preparation of the w/o/w system, and the finished emulsion is then irradiated. The polymerisation reaction can be monitored by cone-and-plate viscometry. Fig. 9 shows the flow curve obtained for a water/isopropyl myristate/water emulsion as a function of the radiation dose. As the dose of y-irradiation is increased, the viscosity of the w/o/w emulsion increased up to a gel-point1. The gel-point of the emulsion is dependent on the type and concentration of poloxamer. In the example shown, prepared using a mixture of 5% (w/v) Pluronic F87 and 5% (w/v) Pluronic F88 in the external phase, the gel-point was reached at 4.2 (Fig. 9). Fig. 10 shows the changes in the properties of irradiated systems on storage. 

Stable aqueous emulsions of poly(2-ethylhexyl acrylate) (PEHA) were also produced by RESAS from CO2 (68). In this case, a polymer suspension in CO2 was expanded instead of a dissolved solute. A C02-philic surfactant, Monasil PCA (PDMS-g-pyrrolidonecarboxylic acid), was utilized in dispersion polymerization to form a stable polymer suspension at 65°C and 345 bar. A hydrophilic surfactant, (e.g., SAM 185, Pluronic L61, or Pluronic L62), that is soluble in CO2 and CO2/2-EHA monomer mixtures as well as water was added to CO2 to stabilize the suspension after it had been rapidly expanded through a capillary into aqueous solution. The resulting aqueous emulsion with up to 15.6 wt % polymer content was stable for weeks with an average particle size of 2 to 3 pm. Another approach is to introduce the hydrophilic surfactant in the aqueous phase in addition to the surfactant in the CO2 phase. This approach is more general, since many hydrophilic surfactants are not soluble in CO2. During expansion of the suspension into an aqueous solution, the hydrophilic surfactant—for example, triblock Pluronic copolymers—dilfuses to the particle surface to provide stabilization. The resulting aqueous latexes were stable for 100 days for a polymer content reaching 12.7 wt %. 

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. 

Reduction in hydrophilicity with ionic-non-ionic surfactant mixtures... 

It is immediately obvious from the above equations that hydrophilic surfactants will have high HLB values and lipohilic surfactants low values. It is generally recognised that surfactants with HLB values between 4 and 6 are W/O emulsifiers, and those with HLB values between 8 and 18 are O/W emulsifiers. It is also generally recognised, though poorly understood, that mixtures of surfactants create more stable emulsions than the individual surfactants. The overall HLB of a surfactant mixture (HLBM) can be calculated from... 

In a solution containing a single pure surfactant, the monomer concentration and therefore the surfactant adsorption remain constant above the CMC because any additional surfactant is incorporated into micelles. In a surfactant mixture, the distribution of surface-active species in the monomer and micellar phases depends on their relative tendency to form micelles. The more hydrophobic components are incorporated into micelles preferentially, and the more hydrophilic components become en-... 

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

As explained in detail elsewhere [99], it seems that the anionic-cationic surfactant mixtures would contain a certain amount of catanionic equimolecular compound, which is much less hydrophilic than its components, and an excess of either the anionic or cationic surfactant. In other words, a mixture containing 10 mol of cationic surfactant and 90 mol of anionic surfactant would really behave as a mixture of 80 mol of anionic surfactant with 10 mol of the catanionic equimolar component. There is evidence that this anionic-cationic... 

Hydrophilic surfactants (10 < HLB < 20) have generally oiUin-water emulsifying or solubilizing properties. Due to the conditions of their fabrication, these surfactants are usually mixtures of associated substances, so there are sometimes variations in properties between different manufacturers. 

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