Pentanol: surfactant mixtures

The results showed distinct and regular changes for the aqueous solubility region in pentanol surfactant mixtures. With increased electrolyte content, the "minimum amount of water for solubility was enhanced, the solubility limit towards the pentanol water axis was shifted to higher soap concentrations, and the "maximum solubility of the aqueous sodium chloride solution was obtained for higher surfactant alcohol ratios (Figure 2). 

Makayssi, A., Bury, R. and Treiner, C., Thermodynamics of micellar solubilization for 1-pentanol in weakly interacting binary cationic surfactant mixtures at 25°C, Langmuir, 10, 1359-1365 (1994). 

Winsor [15] classified the phase equilibria of microemulsions into four types, now called Winsor I-IV microemulsions, illustrated in Fig. 15.5. Types I and II are two-phase systems where a surfactant rich phase, the microemulsion, is in equilibrium with an excess organic or aqueous phase, respectively. Type III is a three-phase system in which a W/O or an O/W microemulsion is in equilibrium with an excess of both the aqueous and the organic phase. Finally, type IV is a single isotropic phase. In many cases, the properties of the system components require the presence of a surfactant and a cosurfactant in the organic phase in order to achieve the formation of reverse micelles one example is the mixture of sodium dodecylsulfate and pentanol. 

The calculated results in the absence of electrolyte will be now compared with the experimental results obtained regarding a lamellar lyotropic liquid crystal SDS (sodium dodecyl sulfate)/pentanol/water/dodecane swollen in a mixture of dodecane and pentanol.24 The weight fraction water/surfactant was 1.552 from the dilution line in the phase diagram, we calculated that the initial concentration of pentanol in the oil-free system was 29 wt % and the concentration of pentanol in the dodecane-based diluant was 8 wt %. The experimental values for the repeat distance were obtained from the X-ray diffraction spectrum (Figure 2 in ref 24) for various dodecane concentrations. 

Microemulsions are thermodynamically stable, transparent (or translucent) dispersions of oil and water that are stabilized by an interfacial film of surfactant molecules. The surfactant may be pure, a mixture, or combined with a co surfactant such as a medium-chain alcohol (e.g., butanol, pentanol). These homogeneous systems, which can be prepared over a wide range of surfactant concentrations and oil to water ratios (20-80%), are all fluids of low viscosity. 

In the presence of 15% pentanol, large amounts of water can be solubilized into heptane or toluene solutions of Ci2-Ci6 alkylpyridinium or alkyltrimethylammo-nium bromides (Venable, 1985). In heptane/pentanol, the longer-chain surfactants appear to be more effective than the shorter ones, while in toluene/pentanol the shorter ones appear to be more effective. In both solvent mixtures, the pyridinium salts are more effective solubilizers than the corresponding trimethylammonium salts. All the quaternaries investigated were more effective than sodium dodecyl sulfate. 

Figure 8.14 Phase behaviour of water/oil/C- 2mE6 mixtures without co-surfactant (a), with 2 wt.% n-pentanol (b), with 4wt.% n-pentanol (c) and with 6wt.% n-pentanol (d). The water oil ratio equals 1 1, with the oil being a mixture of n-alkanes with 95 wt.% undecane. (From Ref. [82], reprinted with permission of Hanser.)...

Nonionic surfactants are also known to exhibit quite high solubilities in CO2. Nonionic polyethers, CgEs (62) and Ci2E4 (63), were extensively studied in CO2 with and without the addition of pentanol as co-solvent. Water solubility up to w = 12 was observed for CgEs with 10 wt % pentanol as a co-solvent. This C12E4 alone dispersed no water, however the addition of pentanol enhanced notably its solubility. Related nonionic surfactants were shown to exhibit solubility in CO2 (64-66). In section 3 of this chapter a new SANS study of this mixture, and other nonionic surfactants in CO2, are reported. 

The phase diagrams of two quaternary mixtures made of sodium dodecylsulfate (SDS)-water-dodecane and hexanol (system A) or pentanol (system B) have been investigated in detail [22,23]. In both cases, sections of the three-dimensional diagram with constant water/surfactant ratio have been examined. These cuts were chosen because they allow a good description of the oil region and also because the water/SDS ratio, termed X in the following, fixes the size of the droplets in the inverse microemulsion phase and the thickness of the bilayers in the oil-rich lamellar phase. In the description of the quaternary mixtures, we emphasize the details of the evolution of the phase equilibria as X is varied. We have focused our attention not only on the characterization and the location of the boundaries of the various phases but also on the equilibria between the phases. 

Figure 1 Phase diagrams of the water-oil (Halpaclean)-surfactants (C12-14E5 Marlipal) system with 3.5 wt% rt-pentanol at different temperatures, (a) r=303 K (b) r=313 K (c) T— >22i K (d) r= 333 K. Symbols < ) = homogeneous mixture, 2

Stability depends upon so many things that it is easy to alter its value. However, in most eases the general phenomenology versus formulation and eomposition is valid. The presenee of aleohol, partieularly an intermediate-solubility alcohol, such as rec-butanol or ter-pentanol, or a mixture of propanol and butanol, tends to reduce the interfacial adsorption of the surfactant, thus reducing all associated effects, in particular the repulsion that eontributes to stabilization. It is worth noting that the use of mixed surfae-tant systems, which is often advised in emulsion making manuals, can be detrimental in some eases in which a selective partitioning of surfactant species takes plaee (191, 192), and little surfactant is left at the interfaee. 

All measurements are performed using the refractive index of CdS. In the case of cadmium sulfide nanoparticles produced in the w/o microemulsion the viscosity rj and the refractive index no of the continuous oil phase, namely the xylene-pentanol (1 1) mixture ( = 1.454 cP, D = 1165) are used. Consequently rj and no for water are used when the CdS nanoparticles are redispersed in the aqueous phase. Morphology and size of the redispersed CdS particles are also determined by transmission electron microscopy. Therefore, a small amount of the aqueous solutions is dropped on copper grids, dried and examined in the EM 902 transmission electron microscope (Zeiss) (acceleration voltage 90 kV). The high amount of surfactant brings also difficulties for the preparation of the samples for TEM measurements and consequently samples have to be washed with water to reduce the amount of surfactant. 

This is not the end of the story, however. A third type of effect can alter the selfassociation structure and is directly related to the surfactant and or alcohol inherent properties. For instance, straight-chain ionic surfactants would produce liquid crystals of the lamellar type unless the temperature is quite elevated. Thus, in most cases of ionic systems, a large amount of alcohol (as much as two or three times the surfactant amount on a mole fraction basis) is required to melt the liquid crystal into a microemulsion, particularly for middle-phase ones [33]. Note, however, that too much alcohol could be detrimental to a high-performance microemulsion because the alcohol molecules which are not playing a cosurfactant role at the interface would dissolve into the bulk of one or both excess phases, making them more compatible [i.e., the alcohol would make the water less polar and the oil more polar (depending on the alcohol, but most particularly, intermediate solubility ones such as secondary butanol or tertiary pentanol)]. This is, of course, a way to narrow the miscibility gap, but this time by favoring the formation of a cosolubilized random mixture of all molecules instead of a microemulsion structure [50,65]. 

Between S/A = 55/45 and S/A = 50/50, the phase behavior changes to mostly lipophilic, and the Winsor Type II region invades the whole diagram. In this case, the surfactant is not hydrophilic enough to compensate for the alcohol effect. It is also probable that such a high amount of alcohol results in a consolute system in which most of the alcohol is miscible in the oil phase at equilibrium with an almost pure water phase. Note that a nonionic surfactant of the alkyl phenol type with even 7.5 EO groups as an average is certainly at least 10 times more soluble in a mixture of heptane and pentanol than in water [49,87]. 

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