Surfactant phase, birefringent

During the studies of phase behaviour two types of liquid crystalline phases were identified. LC material was viscous and exhibited intense "white" birefingence. material was apparently homogeneous but of low viscosity and exhibited "multi-coloured" birefringence. The liquid crystalline phases observed in the equilibrium studies of surfactant concentrations up to 25 are unlikely to take part in the self-emulsification process due to the presence of two-phase regions between L2 and liquid crystalline phases however, LC material may account for the improved stability of emulsions formed by 25 surfactant systems (Table II). Figure 4c indicates that by increasing the surfactant concentration to 30 the... 

The most common and easily applicable method of characterising liquid crystalline mesophases is polarisation microscopy. In this method, thin samples of the surfactant solution are viewed under a microscope between crossed polarisation filters. Due to optical anisotropy of liquid crystals they are birefringent. Hence, they give rise to a brightness in the microscope and show patterns that are very characteristic for the specific phases examples are shown in Figure 3.17. 

This was not the case earlier, when the interface appeared in the w/0 microemulsion. Fig. 3. Compositions within this interfacial layer were analyzed after 9, 26 and 30 days, (A, B, and C, Fig. 3) as well as within the birefringent layer (D, E, Fig. 3). The compositions A, B, and C are all outside the microemulsion region to the left an obvious consequence of rapid depletion of surfactant due to its faster transport out of the microemulsion. The composition outside the solubility limit explains the interface found in the oil phase. Fig. 3. 

In many of these studies the structure of the middle phase is not established, but it is clearly immiscible in water or oil and its electrical conductivity is closer to water than oil. Phase diagram studies of oil-water-emulsifier systems Ekwall, (5), indicate that surfactant-rich phases immiscible in oil or wa"ter have rodshaped or lamellar micelles with some degree of optical anisotropy or flow birefringence, and these phases have much greater elec-rical conductivity than oil. Figure 1 illustrates that the middle phase composition varies smoothly from a water-rich composition to an oil-rich composition as the emulsifier partition changes from mostly water-soluble to mostly oil-soluble. If lamellar structures are present the relative thickness of oleophilic and hydrophilic layers must vary smoothly from the water-rich compositions to the oil-rich compositions. 

A dispersion of spherulitic liquid crystalline particles in brine exists between 0.8 gm/dl NaCl (Figure 2(a), first sample on the left) and 1.2 gm/dl. As the salinity is increased to about 1.4 gm/dl NaCl, the amount of liquid crystals as well as the birefringence increase, and the texture observed using PLS is intermediate between those of the spherulite (S) and lamellar (L) structures. The aqueous solution is a homogeneous lamellar phase between 1.6 and 1.8 gm/dl NaCl. The surfactant molecules form bilayers with their polar heads toward the brine. Figure 3(a) shows the lamellar structure as observed by polarized microscopy at 1.6 gm/dl salt and without any polymer. The bands represent "oily streaks" in a planar background. 

The contribution of dissolved surfactant, whose concentration was only 0.001M, compared to 7M of decane, to the observed Class I peaks must have been negligible. Class II peaks were not observed in Spectrum 13 of the birefringent phase, and Class I peaks were broadened (linewidth about 30 Hz) compared to the peaks in Spectrum 12 (linewidth less than 5 Hz). Therefore it seems quite possible that the dispersed birefringent phase did give Class I peaks in Spectrum 12, but that these peaks, due to either the surfactant or to absorbed decane or to both, merged with those of the decane in the isotropic phase. 

Surfactant solubility in decane was 0.04 wt% at 25°C and about 9 wt% at 50°C. The surfactant-rich phase in equilibrium with isotropic decane solution was birefringent. About 20 wt% decane was vapor-sorbed by dry surfactant at 25°C. Preliminary polarizing microscopy and nmr results point toward the existence of liquid crystalline states in surfactant-decane and surfactant-decane-water systems. 

Most studies have focused on ternary and quaternary systems (with an electrolyte as the fourth component or an alcohol as cosurfactant in the latter) of both ionic [6,37-41] and nonionic [28,42] surfactants. The shape of the transient birefringence signal and the number, amplitude, and rate of the relaxations typically depend on composition, temperature, and field strength. Since the thermodynamic conditions affect the aggregation number ( , size, and stability of the particles as well as the phase behavior of the system, the distance (Tc — T) from a critical temperature and the distance from a critical composition Cc also have a major influence. 

Figure 8 Schematic representation of the processes leading to birefringence (and turbidity) in a W/O microemulsion, in relation to an applied electric square pulse E. Below a (second) threshold value of the field strength and far from critical conditions, or under any conditions if the pulse is terminated at a time indicated by the dashed line, only birefringence is observed due to the formation of AJ, and Above the threshold of the field strength, close to critical conditions, and with a sufficiently long square pulse, turbidity contributes to the signal due to phase separation or/and percolation. The double wall of the particles symbolizes the water/oil interface. Symbols A, surfactant monomer An, microemulsion droplet (An), cluster LCmp, liquid-crystalline microphase or/and percolation structure. Primed symbols stand for polarized structures oriented parallel to E (- ) reversible step with respect to turning the field on or off (->) irreversible step. (Reprinted with permission from Refs. 6 and 41. Copyright 1989 and 1994 American Chemical Society.)...

Under conditions where no phase separation or percolation can occur, termination of the square pulse is followed by a double exponential decay (t / and T. y) of the birefringence. The forward and reverse relaxations are found to be symmetrical, i.e., t, = t, and An, = Art-i within experimental error [41]. In the faster process the induced dipoles of the droplets (individually and as constituents of clusters) rapidly collapse, the shape of the droplets reverts to spherical, and/or the droplets randomize in their orientation. If the field-free fast relaxation is interpreted as the ellipsoid-to-sphere structural relaxation of the droplets, the bending modulus k of the surfactant monolayer can be estimated from the measured t / [49]. Depending on the polydispersity assumed, the values found in the range k = 0.4-1.0 kT are consistent with those obtained from the static birefringence A o [7,9]. 

In order to improve the understanding of these systems, Kunieda and coworkers examined the thermotropic behavior of poly(oxyethylene) cholesteryl ethers with different chain lengths, ChFOn, mixed with water at a fixed concentration ( 25 wt%) [32]. This study focused on the different fusion mechanisms that were involved in the solid-liquid phase transition. The soUd-Uquid transition temperature for ChFOn as a function of n is shown in Figure 4.3 (for comparison, the transition temperature for polyethylene glycol is also shown). In both cases, the transition temperature decreased when the chain length was diminished. However, for the cholesterol surfactant, when n < 10, a birefringent phase appeared between... 

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