Single colloid systems

From 3 e 1 (see below) it will appear that the division "apparent single colloid systems does not not possess a sharp boundary between it and the next one called "gels . 

In the next main division the different kinds of single colloid systems will be treated. Here once more solutions are dealt with as a subdivision, but now the stress is laid on the actual properties of the whole system, including interactions between macromolecular ions among themselves or macromolecular ions and micro ions. 

For the classification of the different kinds of single colloid systems the elimination procedure is used to characteri2 e them. It enables us to divide the liquid single systems into two kinds, which will be called Sols (in the restricted sense) and Coacer-vateSf though both fulfil the definition of sol in the larger sense of 1 b (p. 2). 

In the third main division will be found the apparent single colloid systems and in the fourth gels, containing an interpenetrating liquid system of micro-units. The last main division will deal with solid systems of a nature not further specified, consisting only of macromolecules. 

If, for example, the separated colloid-rich phase remains for some reason or other in a very highly dispersed state of division, then systems can be produced which are macroscopically and microscopically homogeneous. Two types of these systems are known, of which the one has throughout the nature of a liquid, the other that of a solid body. They can be called " apparent single colloid systems, (compare p. 7 Ch. I 3d). This nomenclature indicates that it is appropriate for various reasons to treat them as two-phase systems. In this respect they differ from the true ""single colloid systems (the original sols, the colloid crystals, the coacervates), in which the one-phase conception is the more appropriate. 

The difference between the two kinds of colloid systems resides in the equilibrium character of the true single colloid systems " and the non-equilibrium character of the "apparent single colloid systems . Considered from the two-phase standpoint, the mutual surface of contact between the colloid-rich and the colloid-poor phase is very great in these latter systems, so that they attempt to reduce this surface of contact. They will therefore chaise their properties with time while the true single systems do not change with time. 

From the above it follows that the conditions for the establishment of the liquid type of apparent single colloid systems in general consist of (a) a certain charge of the colloid, (b) a low sol concentration, (c) absence of electrolytic impurities and (d) the added substance which is used to reduce the solubility of the colloid must be a non-electrolyte... 

In still more dilute agar sols (0.02%) flocculation is no longer observed even after one day. We have here again obtained a liquid apparent single colloid system with the behaviour of a lyophobic sol , because if a small concentration of salt is present in this 0.02% sol then it flocculates right enough on cooling. 

D Formation of lyo-phobic sols high dispersion Liquid apparent single colloid system... 

E Gelation high dispersion Solid apparent single colloid system (Gels which are still preferably to be treated as two-phase systems)... 

At other pH values it can still remain in solution but if the pH is brought to the isoelectric point or if some salt is added, flocculation again follows. Under certain circumstances apparent single colloid systems can be formed. Such systems of the solid type (see p. 236, 1 d) are produced for example from the white of an egg on boiling, in which the concentrated sol is transformed into a gel. 

In 1996, Liu et al. reported the selective hydrogenation of cinnamaldehyde, an a,/ -unsaturated aldehyde, to cinnamyl alcohol, an a,/ -unsaturated alcohol, by means of PVP-protected Pt/Co bimetallic colloids prepared by the polyol process [111]. The colloids were obtained as a dark-brown homogeneous dispersion in a mixture of ethylene glycol and diethylene glycol, and characterized by TEM and XRD. These authors prepared different samples of nanoparticles with Pt Co ratios of 3 1 and 1 1, the mean diameters of which measured 1.7 and 2.2 nm, respectively. These colloidal systems were also compared with the single metal-... 

The invention of the electron microscope in the 1930s by Knoll and Ruska cleared the way for scientists to take an even closer look at vesicles and other colloidal structures [5]. Improving the resolution of the optical microscope roughly by the factor that the optical microscope improved that of the unaided eye, the finer structures of colloidal systems became visible. With the electron microscope, single bilayers can be made visible and the distance between lamellae can be determined. Thus, the structure of a given system can be determined to up to 1/10000000 of a millimeter, which is about the distance of six atoms in a molecule. The most impressive results are obtained with the freeze fracture and cryo-TEM methods [6]. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

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