Crystals surfactant gels

Fig. 15.4 Schematic ternary-phase diagram of an oU-water-surfactant microemulsion system consisting of various associated microstructures. A, normal miceUes or O/W microemulsions B, reverse micelles or W/O microemulsions C, concentrated microemulsion domain D, liquid-crystal or gel phase. Shaded areas represent multiphase regions.

Usually the surfactant concentration in ointments and creams is significantly lower than in surfactant gels. Ointments are non-aqueous preparations, whereas creams result from ointments by adding water. The microstructure of both ointments and creams may consist of liquid crystals, as long as a liquid crystalline network or matrix is formed by amphiphilic molecules. In a liquid crystalline matrix, it is easier to deform the system by shear such formulations show plastic and thixotropic flow behavior on shear. In comparison to systems with a crystalline matrix which are usually destroyed irreversibly by shear, those with a liquid crystalline matrix exhibit a short regeneration time of... 

Another consequence of the addition of fatty alcohols to cationic surfactants is the formation, under the right conditions, of liquid crystal and gel networks [41-45] that can greatly increase viscosity and confer stability upon the emulsion. Formation of such liquid crystals has been observed even at low concentrations [44,45] the ready formation of these structures, along with low cost, improved stability, and compatibility with cosmetic ingredients are important reasons why long-chain alcohols are so ubiquitous in conditioning formulations. 

There are various types of molecular orientations possible in the surfactant film. The surfactant molecules may be in the form of crystals, isotropic gels or one or more of several different types of mesophases (33). The techniques which may be used to study the molecular orientations inside these surfactant films have been reviewed by Singer (34). 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Protein recovery via disruption has also been achieved by adsorbing water from the w/o-ME solution, which causes protein to precipitate out of solution. Methods of water removal include adsorption using silica gel [73,151], molecular sieves [152], or salt crystals [58,163], or formation of clanthrate hydrates [154]. In most of the cases reported, the released protein appeared as a solid phase that, importantly, was virtually surfactant-free. In contrast to the dilution technique, it appears that dehydration more successfully released biomolecules that are hydrophilic rather than hydrophobic. 

Figure 1.12 Photochemical titration curves of crystal violet co-entrapped in silica sol-gel matrices with different surfactants, no surfactant ( ) and in solution (x) show the impressive variations in the sensing properties for the same entrapped dye. (Reproduced from ref. 26, with permission.)... 

FIG. 11. Transmission electron micrographs of freeze fractured oily droplets dispersed (a) in a hexagonal and (b) in a cubic liquid crystalline phase, bar 100 nm. From Mueller-Goymann, C., Liquid crystals in emulsions, creams and gels, containing ethoxylated sterols as surfactant, Pharm. Res. 1 154-158 (1984). 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

Mesostructured molecular sieves such as Si-MCM-41 and Al-MCM-41 were synthesized using gels prepared by reacting colloidal silica (Ludox AS) with an Al(OH)3, solution in the presence of a surfactant. Al was incorporated in tetrahedral coordination inside the pristine crystals. However, as expected, dealumination occured upon calcination at 600°C/12h yielding materials having both tetrahedral and octahedral Al-species. 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

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