Dispersion systems aqueous solution medium

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

Above we used the words continuous phase and dispersed phase to refer to the medium and to the particles, respectively, in the colloidal size range. It should be understood that these are solvent and solute in lyophilic systems. In micellar systems, the micelles are dispersed in an aqueous continuous phase. Furthermore, the system as a whole is generally called a dispersion when we wish to emphasize the colloidal nature of the dispersed particles. This terminology is by no means universal. Lyophilic dispersions are true solutions and may be called such, although this term ignores the colloidal size of the solute molecules. 

Emulsions are a class of disperse systems consisting of two immiscible liquids, one constituting the droplets (the disperse phase) and the second the dispersion medium. The most common class of emulsions is those whereby the droplets constitute the oil phase and the medium is an aqueous solution (referred to as O/W emulsions) or where the droplets constitute the disperse phase, with the oil being the continuous phase (W/O emulsions). To disperse a liquid into another immiscible liquid requires a third component, referred to as the emulsifier, which in most cases is a surfactant. Several types of emulsifiers may be used to prepare the system, ranging from anionic, cationic, zwitterionic, and nonioinic surfactants to more specialized emulsifiers of the polymeric type, referred to as polymeric... 

Foamed emulsions are disperse systems with two disperse phases (gas and liquid) in the disperse medium (surfactant solution). Water foamed emulsions are formed when foams or aqueous surfactant solutions are used to clean up oil deteriorated surfaces, in the process of oil flotation of waste waters, in firefighting when the foam contacts various organic liquids and in the processes of chemical defoaming (foam destruction by antifoams). Individual foamed emulsions can have practical importance e.g. a foamed emulsion of bitumen is used in road coating foamed emulsions from liquid fuels are used as explosives. 

In this mode of synthesis of polymeric nanoparticles (e.g., of polybutylcycanoacrylates), monomeric materials (e.g., butylcyanoacrylate) are dispersed in a suitable solvent and emulsified. An initiator is generally added to the system and the monomers condense to form a polymeric matrix, although in the case of the alkyl cyanoacrylates no initiator is required as the aqueous medium acts as the initiator of polymerization. As one example (78) isobutylcyanoacrylate dissolved in ethanol can be mixed with an oil plus an oil-soluble drug to constitute one phase the second phase is an aqueous solution of 0.5% Pluronic F68 (poloxamer 118). The two phases are mixed and the polyisobutylcyanoa-crylate is formed at the interface between the oil and the water. The formation of alkylcyanoacrylate films at oil/water interfaces was investigated over 30 years ago (79). 

The results obtained by Buscall (1981) for the stabilization of polystyrene latices by well-anchored polyacrylate chains, of molecular weight 16000, support the prediction that the steric mechanism is dominant at high ionic strengths. Up to a degree of neutralization of ca 0-5, the polyacrylate stabilized latices when dispersed in an aqueous solution of sodium chloride at a concentration in excess of 1 M, could be flocculated by cooling. A comparison is presented in Fig. 6.7 of the LCFTs and the corresponding 0y-temperature for these systems as a function of the degree of neutralization. The dispersion medium was 1 -6 M NaCl. 

Finally, we stress that the free volume approach is only applicable to nonpolar systems. Aqueous dispersions fall outside its scope. This is vividly illustrated by the data of Evans et al. (1975), who determined experimentally that d(UCFT)/d7 = — 1 x 10 KPa for latex particles sterically stabilized by poly(oxyethylene) in aqueous 0-43 molal magnesium sulphate solutions. Both the sign and magnitude of this quantity is different from that predicted by free volume theory for the UCFT of non aqueous dispersions. Paradoxically, it falls in line with the predictions, both in sign and magnitude, published by Croucher and Hair (1979) for the pressure dependence of the LCFT of poly(a-methylstyrene) in -butyl chloride. This may be merely coincidental, but the very small pressure dependence exhibited by the UCFT of aqueous sterically stabilized dispersions emphasizes the major differences between the origins of flocculation at the UCI T for aqueous and nonaqueous dispersions. The small pressure dependence observed for aqueous systems is scarcely surprising since the UCFT of an aqueous dispersion occurs far from the critical point of water whereas that for nonaqueous dispersions is quite close to the critical point of the dispersion medium. 

Pharmaceutical suspensions consist of solid particles of variable size dispersed in a liquid medium, generally an aqueous solution. Usually the size of the dispersed panicles ranges in the colloidal domain (0.1-10 pm) and this makes pharmaceulical suspensions typical systems where the principles of colloid and surface. science have to be used to deal with their properties. According to Hunter (6). the usual criterion to classify colloidal dispersions concerns mainly the nature of phases forming the system. Table I shows some typical denominations. 

The ability of surface-active agents to adsorb on solid-liquid interfaces and produce a modihcaiion of the interfadal properties depends on the chemical nature of the three mutually interacting components of the system the substrate, solid particles the adsorbate, surfactant molecules and the liquid medium, usually an aqueous. solution. If the nature of the solid substrate is nonpolar, then the adsorption takes place by dispersion force interactions. The orientation of the adsorbed molecules is such that the hydrophobic groups of the chain become associated... 

For treatment of certain diseases (e.g., wound and purulent infections of internal cavities), the preparations based on nanosilica are successfully used (Chuiko 2003). In some of these cases, silica NP can contact blood. Blood as a multicomponent heterogeneous system contains many types of cells and macromolecules, and the aqueous solution of low-molecular organic and inorganic compounds plays a role of the dispersion medium. Therefore, investigations of hydrate shells of blood components, intermolecular interactions between them alone and upon contacts with solid NP are of importance for deeper understanding the mechanisms of actions of medicinal nanocomposites. 

The kinetics and mechanisms of ligand-substitution reactions involving labile metal ions in aqueous solution are now well understood (1). It is therefore of interest to investigate how such reactions are affected when the reaction medium is changed either by addition of charged micelles to an aqueous solution or by dispersing the reactants in an inverse-micellar (water-in-oil microemulsion) system. 

According to Winsor s classification, we have four types of microemulsion systems Winsor I (WI), Winsor II (WII), Winsor III (Will), and Winsor IV (WIV). The ion conductance of WI (o/w system) is reasonably high just like aqueous solution, where water is the dispersion medium whereas the conductance in WII (w/o system) is very low, where the dispersion medium is oil. In Will systems, where both o/w and w/o dispersions are simultaneously present... 

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