Lamellar phase drops

Axial diffusion of several radioactive ions and molecules in capillary tubes containing the lecithin-water lamellar phase was studied by Lange and Gary Bobo (35, 36). As shown in Figure 2, diffusivities increased with increasing water content in the liquid crystal except for a sharp drop... 

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. 

FIGURE 6.20 Video frame showing growth of the intermediate lamellar phase as myelinic figures approximately 21 minutes after contact of 0.05 wt. % CijEj solution with a drop of 5.67 1 n-hexadecane/oleyl alcohol at 50°C. From Miller (1996) with permission. 

The above phases may form during evaporation of a spray drop. In some cases a middle phase is first produced that on further evaporation may afford a cubic phase, which, due to its very high viscosity, may entrap the agrochemical. This could be advantageous for the systemic fungicides that require deposits to act as reservoirs for the chemical. Viscous cubic phases may also enhance the tenacity of the agrochemical particles (particularly with SCs) and, hence, enhance rain fastness. In some other applications, a lamellar phase is preferred as this provides some mobility (due to its lower viscosity). 

The next molecular organisation is more complex. This involves many layers of alternating molecules, intercalated with layers of water—i.e. lamellar phase. It is possible for rafts of these layers to float around in water, but for most real products, they present themselves as fuUy enclosed multi-layered onion-like droplets. The number of layers can nm into hundreds, and these so-called lamellar drops (or vesicles) can be up to several microns in diameter. 

Although many emulsions are stabilized by only one nearly saturated monolayer, this is not always the case. If the surfactant is balanced, it can form a lamellar phase (L ), co-existing with oil and water. If emulsification is performed in such a system, the emulsion droplets are often covered by several monolayers back to back, which drastically improves the emulsion stability. " It should be noted that the original idea that emulsion drops must be stabilized by several monolayers back-to-back in order to be stable belongs to McBain. ... 

With increasing temperature the oil drops become larger and larger because the spontaneous curvature decreases. Accordingly, the volume of the oil phase decreases until at 23°C all oil is incorporated into relatively large oil drops in water and we reach a one-phase region (often denoted Li) of an oil-in-water microemulsion. This point is ideally suited to determine the spontaneous radius of curvature because, here, the radius of the oil drop can be calculated from the added volume fractions with Eq. (12.21). Raising the temperature decreases Co further and from 29°C on lamellar structures are formed (La). At 32°C the PIT is reached and the spontaneous curvature is zero. 

Formation of lamellar liquid crystalline phases at the O/W interface This mechanism, as suggested by Friberg and coworkers [37], proposed that surfactant or mixed surfactant film can produce several bilayers that wrap the droplets. As a result of these multilayer structures, the potential drop is shifted to longer distances, thus reducing the van der Waals attractions. A schematic representation of the role of Hquid crystals is shown in Figure 10.32, which illustrates the difference between having a monomolecular layer and a multilayer, as is the case with hquid crystals. 

Micellar cubic (OD), hexagonal columnar (ID), lamellar (2D), and bicontin-uous cubic (3D) nanostructures are formed by self-assembly of 13. For the complexes with IiC104, the ionic conductivities show discontinuous changes following the phase transitions with change of temperature or molecular structure of the dendritic moiety. For example, the conductivity of the complex of 13 with LiC104 drops from 4.6 x 10 6 to 1.2 x 10 9 S cm, along the phase transition from crystalline lamellar to micellar cubic phases. 

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. 

Fig. 7 a. Time-resolved small-angle diffraction from a multi-lamellar aqueous dispersion (c p 0.2) of dipalmitoyl-phos-phatidylcholine (DPPC) during a temperature-jump and -drop experiment. The respective temperature courses are shown in the inserts, b Contour-line plot of the intensities obtained in the heating-experiment, c Temperature-dependence of the recovery rates of the phase, on the equilibration temperature, upon cooling from 37 °C. (From Ref. 74, with permission)... 

For a given chemical system, phase-type diagrams can be constructed that show the regimes in which each type of microemulsion will exist [136, 137]. These can be used to understand and predict the effects of, for example, increasing salinity or decreasing HLB, which tend to shift the emulsion type directionally from type I to type III to type II. The middle-phase (in type III) microemulsions can be thought of as bicontinuous, as opposed to drop-like or lamellar. It is thought that phase inversion from a W/O microemulsion to an O/W microemulsion takes... 

It is interesting to note that during digestion of triglyceride oils, all the substances discussed above are present as well as the phases mentioned. This was nicely illustrated by Patton and Carey [10] in an in vitro study where the fate of a drop of soybean oil in simulated intestinal fluid was monitored under the microscope. The cubic phase is formed at the oil/water interface, and due to its bicontinuous structure it is capable of both delivering the water molecules necessary for hydrolysis and taking care of the resulting fatty acids. The role of lamellar and micellar phases is to act as carriers of lipids to the intestinal wall [11,12]. 

It is, of course, not necessary to form an emulsion by mechanical dispersion of oil and water phases. One method that has been nsed to form oil-in-water emnlsions with small and uniform drops in a nonionic snrfactant system is to start with the surfactant phase near the PIT and cool it rapidly by perhaps 20°C to 30°C (Friberg and Solans, 1968 Forster et al., 1995 Sagitani, 1992). The capacity for solubilization of oil decreases dramatically npon cooling, and the excess oil nucleates as small drops from the supersaturated microemulsion. Provided that it solubihzes substantial oil, the lamellar liqnid crystalline can also be cooled in this manner to form oil-in-water emulsions (Forster et al., 1995). Spontaneous emulsification can also be prodnced by diffusion, as discussed in Chapter 6. 

Another situation relevant to detergency that was discussed in Chapter 4 is the behavior that occurs when a small amount of a mixture of an alkane and a long-chain alcohol or fatty add is contacted with a surfactant solution that is dilute but above its CMC. Experiments (lim and Miller, 1991a) show that when the aleohol content of the oil is below that of the excess oil phase in equilibrium with a microemulsion and excess water at the balanced condition or phase invasion ternperamre (PIT), no intermediate phase forms and the oil is solubilized into the surfactant solution, albeit at a very slow rate. However, when alcohol contait exceeds the above value, a drop of the oil swells in the surfactant solution. Evai-tually the lamellar liquid crystal forms as an intermediate phase (Figure 6.20). 

In spite of these difficulties, with one notable exception discussed in Section 6.12, simple diffusion is quite common, as discussed in this section and next. A similar analysis, as shown previously for a drop, can be made when the oil is present as a thin layer on a solid surface (Lim and Miller, 1991b), a more interesting situation for detergency. Moreover, the intermediate phase need not be the lamellar liquid crystal. Depending on the phase behavior and the initial compositions of the oil and aqueous phases, it could also be another liquid crystalline phase, a microemulsion, or the Lg (sponge) phase discussed in Chapter 4. Finally, we note that sometimes more than one intermediate phase is seen. An example is given in the next section. 

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