Solvent composition, flocculation

Figure 4. Critical flocculation solvent composition toluene + n-hexane (v = volume fraction of toluene), versus log (particle volume fraction, (J>) for various SiC -g-PS systems at 24 1°C V, S12/PS13c 0, S12/PS13a ...

Conditions that cause the extended polymer segments to become insoluble in the medium will also, in general, give rise to flocculation and even coalescence of the emulsion. Flory (3) defined theta conditions as conditions of temperature and solvent composition under which the free energy of interaction between polymer segments equals the free energy of polymer-solvent interaction. Under theta conditions, the soluble segments of a steric stabilizer would collapse, the repulsion between droplets would diminish, and flocculation of the system would be expected to ensue. This prediction has been demonstrated experimentally (20, 21). 

Modest solvent composition changes can also affect the state of a colloidal dispersion. A charge-stabilized dispersion of polymer latex particles or gold colloids may flocculate irreversibly upon adding salt, while ion removal through dialysis may turn the dispersion into an ordered structure exhibiting Bragg reflection [23]. Obviously, the physical state of a colloidal dispersion is a function of the interactions between the colloidal particles. 

CE has been used for the analysis of anionic surfactants [946,947] and can be considered as complementary to HPLC for the analysis of cationic surfactants with advantages of minimal solvent consumption, higher efficiency, easy cleaning and inexpensive replacement of columns and the ability of fast method development by changing the electrolyte composition. Also the separation of polystyrene sulfonates with polymeric additives by CE has been reported [948]. Moreover, CE has also been used for the analysis of polymeric water treatment additives, such as acrylic acid copolymer flocculants, phosphonates, low-MW acids and inorganic anions. The technique provides for analyst time-savings and has lower detection limits and improved quantification for determination of anionic polymers, compared to HPLC. 

Numerous studies have been conducted on the nature of the dissolved and particulate organic matter in natural waters. In general, these studies have shown that the composition of the bulk of the organic matter is undefined. Many of the laboratory studies on the nature of the dissolved organic matter in natural waters are of limited value owing to possible alteration of the compounds by the concentration and analytical methods used. The selectivity of the methods used to concentrate the solute to analytically detectable levels presents another problem in many analytical procedures used to study trace compounds in natural waters. Concentration procedures such as freezing, flocculation, sorption columns, and solvent extraction, have been shown to be selective for certain types of compounds (32, 34, 38). Extreme care must be exercised to insure... 

Note that reference to equation (15.60) for the case where the polymer composing the steric layer is different from that in free solution shows that, under some circumstances, AG cannot become negative. Specifically, this requires that be negative in a dispersion medium that is a better than 6-solvent for both of the polymers. A negative value for G corresponds to the free polymer and the stabilizing moieties being compatible (Flory, 1953). If equation (15.60) is correct, it shows that free polymer that differs in chemical composition from the polymer in the steric layers cannot induce flocculation if the two polymers are compatible. No experimental evidence is as yet available to ascertain the validity of this prediction. 

Much of our understanding of entropic stabihzation of pigment dispersions comes from the seminal work of Rehacek (232). A technique has been discussed to determine the thickness and composition of the adsorbed layer on the surface of a pigment dispersed in a resin solution. It has been foimd that if the adsorbed layer thickness of resin plus solvent is less than 9-10 nm, the dispersion is not stable (232-234). With monofimctional surfactants, the adsorbed layer can be thinner and still protect against flocculation. It has been shown that an adsorbed layer thickness of 4.5 nm of surfactant and associated solvent was adequate (235). In contrast to the adsorbed layer of resin, which is nommiform in thickness, the surfactant layer is comparatively uniform, and so it does not have to be as thick to provide stabilization. 

Much effort has been directed at asphaltenes flocculation and precipitation in petroleum crude mixtures that is a significant problem in oil production, transmission and processing facilities. Pressure, temperature, the chemical composition of the oil and the amount of dissolved gases affect this undesired phenomenon. In general, the prediction of asphaltenes precipitation is very difficult and suffers from the definition of an asphaltene. Usually, asphaltenes are defined as the part of the crude oil that is soluble in methylbenzene and benzene but insoluble in pentane or heptane. Asphaltenes consist of many thousands of species, differing in size and chemical structure. The aromatic character of the asphaltenes and their content of heteroatoms influence their solubility in different solvents and the tendency to flocculate. In most of the calculations of the phase equilibria the pseudo-component method has been... 

The effect of PVB additions on the dispersion behavior can be seen from Figure 4.9 to be dependent on the suspension liquid composition. In MEOH suspension, though addition of PVB reduces the viscosity of the suspension, the system is shear-thinning indicating that the suspensions remain flocculated in poorer solvents for the added... 

Pan et al. ° have synthesized a PbS epoxy resin nanocomposite through precipitation of the PbS/resin monomer in an aqueous environment. The precipitated emulsion was cured after isolating die PbS containing resin emulsion from solvents and solvated salts. TEM images of the final material revealed a flocculated composite, with individual particles of an estimated diameter of 7 nm according to XRD measurements. 

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