Cationic surfactant, coagulation

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. 

Coagulation of Polystyrene and PTFE Latices by Cationic Surfactant ... 

The emulgent, phthallylsulfathiazole provides highly stable type of emulsions. Even in the presence of cationic surfactants, these emulsions were found to be reasonable stable. Effect of detergents on the stability was in the order CPB < CPC < CTAB < LPC, which is compatible with their chain length. The rate determining step of coagulation kinetics of the system under investigation is the coalescence as this is slower than flocculation. 

Surfactants may not only stabilize system against coagulation, but may have an opposite effect, i.e. cause destabilization in cases when the surfactant adsorption proceeds against the polarity equalization rule (Chapter III,2), e.g., during chemisorption of surfactants from aqueous medium on a hydrophilic surface. For example, small additives of cationic surfactants cause coagulation of aqueous dispersions of clays and other silicates due to hydrophobization at T< rmax. Further increase in surfactant concentration results in the formation of a second (hydrophilizing) adsorption layer and leads to an increased... 

Gradient coagulation (cf. Van de Ven 1989) of small bubbles with the use of rather high amounts of cationic surfactant under microflotation conditions can be slowed down substantially due to electrostatic repulsion forces. High stresses develop in the zone of bubble... 

If the undialyzed latex is to be coagulated, the addition of 0.5 mg of a cationic surfactant such as hexadecyltrimethylammonium bromide is suggested. 

Based on the principles of precipitate flotation, a rapid and convenient separation technique has been developed for the determination of toxic heavy metals adsorbed on suspended solids in freshwater. Because suspended solids are negatively charged species, they are rendered hydrophobic and coagulate to form bulky floes with a cationic surfactant and sodium chloride (to increase the ionic strength). The floes are easily floated by bubbling and are then treated in nitric acid to determine the desorbed heavy metals (e.g., chromium, manganese, copper, cadmium, and lead) by graphite furnace atomic absorption spectrometry. 

Latex coating. The first objective was to cover the latex with the polycations without coagulation. Cationic surfactants as well as polycations are normally used as titrants for the determination of the charge density of anionically stabilized latexes (4). In an impropriate range of concentration, the point of charge compensation is indicated by clearly pronounced flocculation. 

Anionic surfactants (such as sodium dodecyl sulfate), cationic surfactants (such as cetyltrimethylammonium bromide) (163), and nonionic surfactants (such as the polyoxyethylenated alkylphenols) (136,338) have been used in preparing emulsions. Different types of surfactants can be used in the same recipe (377) to provide additional stability under specific conditions. For example, mixtures of anionic and nonionic surfactants are common. The anionic surfactant controls the particle nucleation stage, and the nonionic surfactant imparts additional electrolyte tolerance, mechanical shear stability (345), and freeze-thaw stability. Mixtures of anionic and cationic surfactants tend to coagulate and are to be avoided. 

Sheets of silica consisting of a single layer of colloidal particles are formed when silica is coagulated under the influence of cationic surfactants (101). The mechanism of formation is discussed in Chapter 4. 

For adsorption of a cationic surfactant on a colloidal particle, the particle must be at least of a certain minimum size, as pointed out by Matijevi6 and Ottewill (297). When there is only sufficient surfactant present to render the surface of the colloidal particles hydrophobic, coagulation occurs but when more is added to form a second layer owing to van der Waals attraction between the hydrocarbon chains, the ionized groups of the second layer are then oriented toward the solution, and the particles are separated and peptized with a reversed surface charge. 

For thicker deposits of colloidal silica it is found that aggregates of limited size (less than 1 micron) can be adsorbed on glass if coagulated and partly redispersed with a slight excess of cationic surfactant or polymer. In this case the aggregates carry a positive charge and are adsorbed to form one layer of aggregates. 

Both non-ionic and cationic surfactants have been used to stabilize the latex. The resulting cyclized NR latex can be coagulated together with untreated NR latex to yield rubber that is a mixture of cyclized rubber intermixed with NR. This modified rubber mix is a hard resinous substance that can be used as a masterbatch for incorporating cyclized rubber into NR compounds. Cyclized NR through the latex route avoids the use of expensive and hazardous solvents in which NR is dissolved before reacting with a Lewis acid such as tin(IV) chloride. 

In the coagulation of PTFE latices by cationic hydrocarbon surfactants> however, a dilTerent behavior is observed in that only very small differences are observed in the magnitude of the ccc with variation of chain length (Richardson, 1979). This effect is illustrated by the data in Table V. Again, it appears to demonstrate the lack of affinity of hydrocarbon chains for fluorocarbon surfaces. 

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