Intermediate phases, surfactants

In the early 1970s Li [13] proposed a method that is now called Emulsion (surfactant) Liquid Membrane (ELM) or Double Emulsion Membrane (DEM) (Fig. 3). The name reveals that the three liquid system is stabilized by an emulsifier, the amount of which reaches as much as 5 % or more with respect to the membrane liquid. The receiving phase R, which usually has a smaller volume than the donor solution, F of similar nature, is finally dispersed in the intermediate phase, M. In the next step the donor solution F is contacted with the emulsion. For this purpose, the emulsion is dispersed in the donor solution F by gentle mixing typically in a mixer-settler device. After this step, the emulsion is separated and broken. The enriched acceptor solution is further processed and the membrane liquid M is fed back for reuse. 

The primary effect of temperature on the detergency performance of C12E04 toward C20 is one of acceleration of penetration of the surfactant into the hydrocarbon layer. Increasing the temperature probably also causes a small increase in the CPP of the C12E04 molecules involved in the formation of a relatively small amount of an intermediate phase at the Cw surface. Subsequent penetration of surfactant and water into causes a rapidloss of adhesion of the hydrocarbon to the IRE. 

The phases that exist between hexagonal and lamellar vary from one surfactant to the next. For example, a mesh intermediate phase is formed by the perfluorooctanoate surfactant (and also occurs in the lattice simulation), while cubic phases are produced by the other three surfactants in Fig. 12-19. Also, there are surfactants whose phase behavior in water differs greatly from those of Fig. 12-19. Figure 12-24, for example, shows the phase... 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

An intermediate phase of tetragonal syiiunetry - the T phase - has also been detected in a number of systems. A rod structure related to a square mesh surface was foimd to agree well with X-ray and NMR data on a perfluorinated surfactant-water mixture forming the T phase [22], [34]. These examples demonstrate that surfactant or lipid monolayers lining mesh surfaces as well and bilayers wrapped onto three-periodic minimal surfaces (IPMS) are indeed found in these self-assembled systems. 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

Aqueous solutions containing anionic surfactants and alcohol cosurfactants were contacted with various oils. A microscope which utilized a vertical sample orientation and a video camera was used to observe and record the resulting diffusional processes. As a result, an improved and detailed viewing of intermediate phase growth, interface motion, and spontaneous emulsification was achieved. 

After describing the experimental technique in the next section, we report our observations of intermediate phase formation and spontaneous emulsification in three parts corresponding to three types of equilibrium phase behavior found when equal volumes of oil and the surfactant-alcohol-brine mixtures are equilibrated. The three types are well known (8-9) and, in order of increasing salinity, are a "lower" phase, oil-in-water microemulsion in equilibrium with excess oil, a "surfactant" or "middle" phase, probably of varying structure, in equilibrium with both excess oil and excess brine, and an "upper" phase, water-inoil microemulsion in equilibrium with excess brine. 

The diffusion path method has been used to interpret nonequilibrium phenomena in metallurgical and ceramic systems (10-11) and to explain diffusion-related spontaneous emulsification in simple ternary fluid systems having no surfactants (12). It has recently been applied to surfactant systems such as those studied here including the necessary extension to incorporate initial mixtures which are stable dispersions instead of single thermodynamic phases (13). The details of these calculations will be reported elsewhere. Here we simply present a series of phase diagrams to show that the observed number and type of intermediate phases formed and the occurrence of spontaneous emulsification in these systems can be predicted by the use of diffusion paths. 

The identity of the intermediate phase formed at these conditions can be deduced from the relative movement of the interfaces. Because the phase grew quickly in the direction of the aqueous surfactant solution, it contained predominantly brine. Although small in quantity, some oil did diffuse into it. From this information and from its isotropic appearance, one can conclude that the intermediate phase was an oil-in-water microemulsion. Additional support for this conclusion is that this type of microemulsion is an equilibrium phase at low salinities. 

A comparison between experimental and theoretical results shows that diffusion path analysis can qualitatively predict what is observed when an anionic surfactant solution contacts oil. Experimentally, one or two intermediate phases formed at all salinities. The growth of these phases was easily observed through the use of a vertical-orientation microscope. Except when convection occurred due to an intermediate phase being denser than the phase below it, interface positions varied as the square root of time. As a result, diffusion path theory could generally he used to correctly predict the direction of movement and relative speeds of the interfaces. 

Although the contacting experiments were performed with surfactant systems typical of those used in enhanced oil recovery, application of the results to detergency processes may be possible. For example, the growth of oil-rich intermediate phases is sometimes a means for removing oily soils from fabrics. Diffusion path theory predicts that oil is consumed fastest in the oil-soluble end of the three-phase regime where an oil-rich intermediate microemulsion phase forms. 

In the next section a simple model is described that provides some understanding of the essentials of solubilization. Then some measurement techniques, experimental results, and theories of equilibrium solubilization are presented for micelles and microemulsions. Finally, dynamic phenomena such as solubilization rates and solubilization by intermediate phases formed after the solute contacts a surfactant solution are discussed. 

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. 

Mixtures containing 1 wt% of the pure nonionic surfactant C,2E5 in water were contacted with pure n-hexadecane and n-tetradecane at various temperatures between 25 and 60°C using the vertical cell technique. Similar experiments were performed with C,2E4 and n-hexadecane between about 15 and 40°C. In both cases the temperature ranged from well below to well above the phase inversion temperature (PIT) of the system, i.e., the temperature where hydrophilic and lipophilic properties of the surfactant are balanced and a middle phase microemulsion forms (analogous to the optimal salinity for ionic surfactants mentioned above). The different intermediate phases that were seen at different temperatures and the occurrence of spontaneous emulsification in some but not all of the experiments could be understood in terms of known aspects of the phase behavior, e.g., published phase diagrams for the C12E 5-water-n-tetradecane system, and diffusion path theory. That is, plausible diffusion paths could be found that showed the observed intermediate phases and/or spontaneous emulsification for each temperature. 

Similar experiments were carried out in which drops that were mixtures of /i-decane and various alcohols were injected into dilute solutions of a zwitterionic (amine oxide) surfactant. Here, too, the lamellar phase was the first intermediate phase observed when the system was initially above the PIT. However, with alcohols of intermediate chain length such as /i-heptanol, it formed more rapidly than with oleyl alcohol, and the many, small myelinic figures that developed broke up quickly into tiny droplets in a process resembling an explosion.The high speed of the inversion to hydrophilic conditions was caused by diffusion of n-heptanol into the aqueous phase, which is faster than diffusion of surfactant into the drop. The alcohol also made the lamellar phase more fluid and thereby promoted the rapid breakup of myelinic figures into droplets. Further loss of alcohol caused both the lamellar phase and the remaining microemulsion to become supersaturated in oil, which produced spontaneous emulsification of oil. 

Holmes MC, Leaver MS (2006) Intermediate phases. In Lynch ML, Spicer PT (eds) Bicontinuous liquid crystals. Surfactant science series, vol 127, Taylor and Francis, Boca Raton, pp 15-40... 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

If more surfactant is added above the cmc, the concentration of micelles increases (rather than the concentration of free surfactant) until the micelle concentration becomes so high that they themselves organize to form ordered arrays of lyotropic liquid-crystal phase. There are several well-characterized lyotropic liquid-crystal phases and a host of so-called intermediate phases whose characterization is not unequivocal. While cmc values are typically found in the range lO -lO moldm, formation of lyotropic mesophases typically starts at around 20wt.% of the surfactant in water. 

Ribbon phases have been the most comprehensively studied of the intermediate phases. They occur when the surfactant molecules aggregate to form long flat ribbons with an aspect ratio of about 0.5 located on two dimensional lattices of oblique, rectangular (primitive or centred), or hexagonal symmetry. Ribbon phases were first proposed by Luzzati [68,69] in aqueous sur-... 

Whilst the early reports of intermediate phases concerned systems with reversed curvature [73-76] these were for surfactants where some residual short range order in the polar groups was probably present. There are few definitive reports of fully molten intermediate phases with reversed curvatures. In fact the pattern of how intermediate phases replace the normal bicontinuous cubic phase as alkyl chain size increase only became recognized as systematic studies on homologous series were carried out [37, 66], Here it has required a combination of microscopy, multinuclear NMR and X-ray diffraction to elucidate the structures. Such studies on reversed phases have yet to be carried out, particularly where small variations in alkyl chain structure are made. 

In a second paper [158] Fuller et al. report the phase behavior of further m-n-m OCB surfactants and some straight chain 15-M-15 surfactants (n = 1,2,3,6). Note that these compounds have terminal hydrophobic chains of the same length as the oxycyanobiphenyl compounds. The m-n-m OCB surfactants all give just a lamellar phase from <18°->100°C. Penetration scans on the 15-n-15 surfactants show that they all exhibit HI, V] and phases to > 100 °C. Additionally, a nematic phase is seen for m = 1, 2 and intermediate phases for m = 1, 2, 3. 

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