Surfactant crystals, formation

There are a number o-f processes which have not been discussed (because o-f space) in which mixture o-f sur-f actants are important. Among these are foaming, emulsion formation, liquid crystal formation, microemulsion formation, adsorption as 1iquid—1iquid interfaces, and phase partitioning of surfactants between immiscible liquid phases. These areas will also see increased interest in the use of surfactant mixtures. 

The formation mechanism of this family of materials is determined by two features [45], The first is the dynamics of surfactant molecules to shape molecular assemblies, which leads to micelle, and, ultimately, liquid crystal formation. The second is the capability of the inorganic oxide to undergo condensation reactions to form extended, thermally stable structures. 

An awareness of crystal packing characteristics and polymorphism helps one to understand incompatibility problems of different fats. Crystal formation has specific demands, and individual crystals in mixed systems each consists of only one species ofTAG. However, surfactants and other molecules can act as impurities and interrupt crystal growth. Different TAG are considered compatible when they co-crystallize as separate crystals under the same conditions without the formation of a eutectic. 

For the 1 M NaCl system the solubility region was further reduced. Fig. 13, and the water solubilization maximum found at even higher surfactant/cosurfactant ratio. The series with the lower ratios of surfactant to cosurfactant showed an uptake of the aqueous solution somewhat similar to the series in the system with 0.5 M NaCl. The series with the surfactant/(cosurfactant + surfactant) ratio equal to 0.4 gave an initial liquid crystal formation lasting for 2-3 days folllowed by a middle phase lasting a longer time. The liquid crystalline and the middle phase layer were both more pronounced for the sample with initial salt concentration equal in the water and in the microemulsion. Fig. 14A, than for the sample with all the salt in the water. Fig. 14B. 

The research on aggregation of surfactants in nonaqueous, polar solvent systems can be motivated, mainly, with two different arguments. First, are the basic considerations of amphiphile aggregation involving a description of the hydrophobic interaction leading to, for example, micelle and liquid crystal formation. What can be learned from comparing water with other polar solvents Much work has been performed to elucidate those properties of the solvent that are essential in order to obtain a hydrophobic (or solvophobic ) interaction. Comparisons of critical micelle concentrations in different solvents with parameters characterizing the solvent are numerous in the literature [1,2],... 

Hydrotropy When there are strong chain-chain and head-head interactions between surfactant molecules (due to long, straight chains and close-packed heads), either insoluble crystal formation (low Krafft point, p. 214) or liquid-crystal formation (Chapter 3, Section IIC) may occur. Since there is much less space available for solubilization in rigid liquid-crystal structures than in the more flexible types of micelles, the onset of crystal formation usually limits the solubilization capacity of the solution. The tendency to form crystalline structures can be reduced by the addition of certain nonsurfactant organic additives called hydrotropes. 

W/o-droplet microemulsions with non-ionic surfactants containing rapeseed oil methyl ester have been successfully used for in situ extraction of polycyclic aromatic hydrocarbons [40]. However, enhancement of oil content and solubilisation capacity failed with these systems. The use of co-surfactants and co-solvents for suppression of liquid crystal formation was considered to be critical for in situ application. 

The formation of surfactant crystals (i.e. liquid crystals) at the oil-aqueous interface can be easily determined by the use of polarized light microscopy (35). 

Due to micelle formation the total surfactant concentration undergoes an abrupt increase. Since true (molecular) solubility of surfactants, determined by the CMC, remains essentially constant, an increased surfactant concentration in solution is caused by an increase in a number of formed micelles. Micellar solubility increases with increase in temperature, and thus a continuous transition from pure solvent and true solution to micellar solution, and further to different liquid crystalline systems and swollen surfactant crystals (see below), may take place in the vicinity of the Krafft point. 

One may also consider a reverse transition from macroscopic heterogeneous system (surfactant crystals in water) to micellar solutions existing above the Krafft point, via gel formation and its spontaneous dispersion stages. In this case, the swelling of soap upon the penetration of water between the lamella formed with polar ( strongly hydrated) groups occurs prior to the formation of colloidal solution. At sufficient dilutions separation of individual particles, e.g. lamella, from crystals occurs due to the... 

Amphoterics can also be used to stabilize emulsions by immobilizing the oil droplets in a network to retard or prevent coalescences (109). Chun (110), in a conference paper presented in 1978, proposed using an amphoteric surfactant (see Figure 15.29) in combination with a fatty amphophile, such as cetyl alcohol, to form a gel network (similar to that of liquid crystal formation) to further improve emulsion stability. A number of commercial skin preparations have used this technique to stabilize treatment lotions. 

The latter are limited to hydrocarbon, perfluorocar-bon and polydimethylsiloxane chains. While the formation of micelles is well known, surfactants also form a wide variety of liquid crystalline phases in water which are much less familiar. Almost all surfactants that form micelles also form liquid crystals, while many do not form micelles but do form liquid crystals. Thus, liquid crystal formation by surfactants is more widespread than micelle formation. Indeed, an understanding and knowledge of liquid crystals can provide a comprehensive guide to the application of surfactants. This is because the size and shape of the surfactant molecules determine the structure of the self-assembled aggregates, which in turn, controls the liquid crystal... 

Table 4.1 Complex crystal formation between aromatic compounds and cationic surfactants, CTAB, MTAB, LTAB and DTAB ...

Equation 6.87 predicts that the time tp until liquid crystal formation begins is proportional to the square of the initial drop radius and inversely proportional to the bulk surfactant concentration. These predictions were in agreement with experiments for systems containing pure nonionic surfactants, n-hexadecane, oleyl alcohol, and water (Lim and Miller, 1991a). Moreover, for a hydrocar-bon alcohol ratio of 3 1 by weight and for solutions of at 30°C, the phase diagram was determined and K calculated as 0.52. When the data were htted to Equation 6.87, D2 was found to be 1.3 x 10" ° m /sec. The Stokes-Einstein equation was then used to estimate micelle radius r. 

Some care is needed to preserve the column performance for long time periods of intensive MLC use, which can be comparable or even longer than in conventional RPLC. First, since most micellar solutions are able to dissolve minute amounts of silica, the mobile phase should be saturated in silica by placing a short precolumn before the injection valve. Second, the micellar solution should never stay motionless in a chromatographic system to avoid the formation of surfactant crystals that can clog the... 

The Krafft point is the temperature at which the solubility of hydrated surfactant crystals increases sharply with increasing temperature and forming micelles. This increase is so sharp that the solid hydrate dissolution temperature is essentially independent of concentration above the critical micelle concentration (cmc) and is therefore often called the Krafft point without specifying the surfactant concentration. The steep increase in solubility above the sharp bend is caused by micelle formation. Micelles exist only at the temperature designated as the Krafft point. This is a triple point at which surfactant mole-... 

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