Hydrophilic water-surface separation

Fig. 6.2 Photographs and schematics showing water-surface separation from a hydrophilic top) and hydrophobic bottom) surface (reproduced with permission from [17], copyright 2014 The American Chemical Society)...

The use of a water-immiscible Hquid to separate coal from impurities is based on the principle that the coal surface is hydrophobic and preferentially wetted by the nonaqueous medium whereas the minerals, being hydrophilic, remain suspended in water. Hence, separation of two phases produces a clean coal containing a small amount of a nonaqueous Hquid, eg, oil, and an aqueous suspension of the refuse. This process is generally referred to as selective agglomeration. 

In water the wetted solid is termed hydrophilic , whereas the non-wetted solid is hydrophobic . Naturally hydrophobic minerals, such as some types of coal, talc and molybdenite are easily separated from the unwanted hydrophilic quartz sand (referred to as gangue ). However, surfactants and oils are usually added as collectors . These compounds adsorb onto the hydrophilic mineral surface and make it hydrophobic. 

The observed values of the layer thickness and the occupied area can indicate that the hydrophobic segments, which lie essentially flat on the water surface at low surface pressures, aggregate at higher pressures, forming a thicker layer with hydrophobic and hydrophilic segments microphase - separated at the air - water interface. This behavior can be illustrated in Figs. 3.26 [129] and 3.27 [130] for different diblock copolymers. 

A surfactant molecule is an amphiphile, which means it has a hydrophilic (water-soluble) moiety and a hydrophobic (water-insoluble) moiety separable by a mathematical surface. The hydrophobic tails of the most common surfactants are hydrocarbons. Fluorocarbon and perfluorocarbon tails are, however, not unusual. Because of the hydrophobic tail, a surfactant resists forming a molecular solution in water. The molecules will tend to migrate to any water-vapor interface available or, at sufficiently high concentration, the surfactant molecules will spontaneously aggregate into association colloids, i.e., into micelles or liquid crystals. Because of the hydrophilic head, a surfactant (with a hydrocarbon tail) will behave similarly when placed in oil or when put in solution with oil and water mixtures. Some common surfactants are sodium or potassium salts of long-chained fatty acids (soaps), sodium ethyl sulfates and sulfonates (detergents), alkyl polyethoxy alcohols, alkyl ammonium halides, and lecithins or phospholipids. 

Fig. 5.1 The behavior of amphiphilic molecules in water, (a) One molecules of water (b) A schematic diagram of a typical amphiphilic molecule, consisting of a hydrophilic head (a ball) and a hydrophobic hydrocarbon tail (c) If there are not too many amphiphilic molecules in contact with water, they prefer to locate on the surface (d) amphiphilic molecules can surround a drop of oil in water (e) in this sense a soap molecule can be said to connect water and oil (f) A liposome contains a small amount of water inside, separate from the bulk of water by an oily shell.

The surface of the zeolite crystal is full of silox-ane groups, and hydrophilic molecules can take advantage of this and also adsorb into the intracrystalline spaces enhancing the permeance of the hydrophilic molecule. This special feature is an advantage in the case of hydrophilic/hydro-phobic separations and makes the organic/water separations with hydrophobic zeolites more difficult. 

In every synthetic reaction where a net amount of water is formed (such as an ester synthesis from an alcohol and a carboxylic acid [38 0]) physicochemical problems arise. Due to the fact that the lipophilic solvent (log F > 1.5) is unable to accommodate the water which is gradually produced during the course of the reaction, it is collected at the hydrophilic enzyme surface. As a consequence, the water gradually forms a discrete aqueous phase which encompasses the enzyme, finally separating substrate and enzyme from each other by a polar interface, which is difficult to penetrate for lipophilic substrate/product molecules. Thus, the rate... 

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