Clouding liquid crystals

Carbon dioxide gas diluted with nitrogen is passed continuously across the surface of an agitated aqueous lime solution. Clouds of crystals first appear just beneath the gas-liquid interface, although soon disperse into the bulk liquid phase. This indicates that crystallization occurs predominantly at the gas-liquid interface due to the localized high supersaturation produced by the mass transfer limited chemical reaction. The transient mean size of crystals obtained as a function of agitation rate is shown in Figure 8.16. 

The equilibrium in these systems above the cloud point then involves monomer-micelle equilibrium in the dilute phase and monomer in the dilute phase in equilibrium with the coacervate phase. Prediction o-f the distribution of surfactant component between phases involves modeling of both of these equilibrium processes (98). It should be kept in mind that the region under discussion here involves only a small fraction of the total phase space in the nonionic surfactant—water system (105). Other compositions may involve more than two equilibrium phases, liquid crystals, or other structures. As the temperature or surfactant composition or concentration is varied, these regions may be encroached upon, something that the surfactant technologist must be wary of when working with nonionic surfactant systems. 

In spite of these important differences, silicone surfactants share much in common with conventional surfactants. Equilibrium and dynamic surface tension vary with concentration and molecular architecture in similar ways. Silicone surfactants self-associate in solution to form micelles, vesicles and liquid crystal phases. Self-association follows similar patterns as molecular size and shape are varied and silicone surfactants containing polyoxyalkylene groups exhibit a cloud point. HLB values can be calculated for silicone surfactants, although more useful values can be obtained from calculations that take into account the differences between silicone and hydrocarbon species. 

Similar experiments were conducted for other compositions and a cloud point phase diagram was established as shown in Figure 6. As expected the cloud point phase diagram resembles a lower critical solution temperature (LCST) in character. The formation of liquid crystal phase makes it difficult to confirm the reversibility of the phase diagram thus it should be regarded as a virtual one. 

Citrate compounds are salting-out electrolytes — they tie up water molecules in the liquid and as a result help force the formation of liquid crystals or lamellar stmctures. It is sometimes possible to reverse this trend by the addition of salting-in electrolytes, compounds with high lyotropic numbers (>9.5) which can raise the cloud point of a liquid formulation [26], This permits increased concentration without the onset of structuring. 

Liquid-crystalline phase or microemulsion formation between surfactant, water, and oily soil accompanies oily soil removal from hydrophobic fabrics such as polyester (Raney, 1987 Yatagai, 1990). It has been suggested (Miller, 1993) that maximum soil removal occurs not by solubilization into ordinary micelles, but into the liquid-crystal phases or microemulsions that develop above the cloud point of the POE nonionic. 

One of the main features of nonionic water-soluble cellulose derivatives is that they exhibit, like some other polyethers, an inverse solubility-temperature behavior, i.e. there is phase separation on heating above the so-called lower critical solution temperature (LCST). The temperature at which a polymer-rich phase separates is normally referred to as the cloud point (CP). For ideal solutions, this temperature corresponds to the theta-temperature. Actually, for some derivatives, the cloud point may be preceded, if the concentration is not too low, by a sol-gel transformation with an increase in viscosity and possibly formation of liquid crystals (see Sect. 3.5). As it will be seen later, this reversible thermotropic behavior may be detrimental to the performance of the derivatives or can be advantageneously utilized to develop applications. 

Below temperature TB (cloud point for low polymer concentrations) and at a polymer weight fraction lower than wB, isotropic solutions are stable. Above the weight fraction wA and at a temperature below the line AD (clouds points for concentrated polymer concentrations) pure cholesteric phase separates. All other compositions are biphasic, the liquid crystals being in equilibrium with a dilute isotropic solution. At low temperature this domain exists in the small concentration range delimited by wA and wB. Note that TA may be below body or even room temperature. 

The critical concentration wB necessary for the formation of an anisotropic phase at room temperature has been investigated for HPCs varying in their molar mass (Mw = 60000 to 1000000). The values ranged from 39 to 42 wt % and did not change significantly when observations were repeated at 30 and 35 °C. However, at 40 °C (a temperature close to the cloud point) liquid crystal formation took place at a somewhat lower polymer concentration. The nature of the solvent played a greater role. The minimum concentration of a HPC sample with a M of 100000, which was 42% in water, rose to 43 °C in methanol and to 47 °C in the less polar ethanol [103], For another HPC sample, values of 0.21, 0.30,0.38,0.42 and 0.43 were found for dichloroacetic acid, acetic acid, dimethyl acetamide, water and ethanol, respectively [105],... 

AU the water on Earth is connected in a global water cj cle ( FIGURE 18.15). Most of the processes depicted here rely on the phase changes of water. For in.stance, warmed by the Sun, liquid water in the oceans evaporates into the atmosphere as water vapor and condenses into liquid water droplets that we see as clouds. Water droplets in the clouds can crystallize to ice, which can precipitate as hail or snow. Once on the ground, the hail or snow melts to liquid water, which soaks into the ground. If conditions are right, it is also possible for ice on the ground to sublime to water vapor in the atmosphere. 

The counter-ion concentrations around the polyelectrolyte chain may fluctuate, which induces an attractive interaction between the chains sharing the same clouds of counter-ions (Golestanian et al. 1999). Parallel stacking of polyelectrolyte chains favors such kind of attractions, as demonstrated in Fig. 4.11. This effect will result in the spontaneous liquid crystal ordering in polyelectrolyte solutions (Potemkin et al. 2002 Potemkin and Khokhlov 2004). Such a tendency of liquid crystal ordering acmally stabilizes the parallel rolling of DNA long chains, and squeezes them into the very limited space of cell nucleus. 

Fig. 4.11 Illustration of parallel stacking of polyelectrolyte chains dark gray rods with negative charges) favoring the sharing of counter-ion clouds light gray positive charges) and hence forming liquid-crystal-ordering structure...

In the POE(5)/water system, L was observed as liquid crystal. The liquid crystal intrudes into the cloud point... 

In a study by Kahn et al. [9] on a polydisperse octadecyl amide with nine oxyethylene units, the pure surfactant is found to be a viscous liquid at room temperature. Upon solubilization in water, a clear isotropic solution phase is observed for all concentrations between the cloud point and the solidification temperature. No liquid crystal formation is observed. 

An appropriate X-ray source for routine liquid crystal work is illustrated schematically in Fig. 3. Electrons are produced by electrically heating a tungsten wire filament to temperatures in excess of 2000 K. This heating produces a cloud of electrons around the wire. The potential gradient... 

---