Hydrophobic 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. 

To check the efficacy of grease removal, the alkali solution is rinsed away or neutralised by dipping in dilute acid. If, after removal from the acid, the draining metal surface remains wetted evenly all over for 30-60 s (or until it dries by evaporation), hydrophobic soils have been removed. Traces of grease cause the surface to de-wet, and surface tension draws the water into separate droplets. This is the water-break test. Traces of grease which remain when the work is plated do not prevent electrodeposition, but are detrimental to adhesion and corrosion resistance. 

Recent experiments have shown that the non-specific, physical chemical interactions between small hydrophobic, water-insoluble molecules and the hydrocarbon chains of lipid membranes are important determinants of the rate at which these molecules enter cells and are metabolized (3.34). Cholesterol has the capability of modifying these interactions and also increases the affinity of vesicle surfaces for amphiphillic molecules (4) separating the lipid polar groups (35). 

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. 

Hydrophobic forces are long-range attractive forces between macroscopic, hydrophobic surfaces in water. The force is significantly stronger than the Van der Waals attraction and can still be measured at surface separations as large as 70 nm. 

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. 

The effect of NaCl on bubble nucleation in the presence of hydrophobic surfaces has also been examined. Excess nitrogen gas was dissolved in solution by equilibration under 25 atmospheres of pressure. Immediately following decompression the solution was supersaturated with nitrogen gas. In water and 0.02M NaCl, it was found that bubbles nucleated quickly (<25 sec) at a (hydrophobic) teflon surface. However, a 0.20M solution of NaCl was found to inhibit bubble formation. In ref>eat experiments, bubbles were found to form at the same sites on the hydrophobic surface. It would appear that the microstructure of the surface is important for the nucleation of bubbles. Microscopic surface cracks would present hydrophobic surfaces at very close separations, enabling nucleation to occur more readily. 

In general, flotation separation relies on the attachment of properly prepared mineral surfaces to air bubbles in water. Surfaces which are easily wetted by water, such as ordinary ground rock, are not attracted to bubbles and thus sink. Elemental sulfur, graphite (and other forms of elemental carbon, such as the pigment in printers inks), and talc (minerals with layer lattice type structures) have hydrophobic surfaces, which are attracted to the surface of an air bubble and, if small enough, may be lifted to the surface of the water and separated from the water as a froth layer. 

Water molecules are very weakly adsorbed on the surface of hydrophobic activated carbons [356], Nevertheless, when humidity is high water was found to interfere with adsorption of organic compounds. Although, some of them are able to replace preadsorbed water [75], these quasistable conditions affect the kinetics of the process and feasibility of adsorptive separation/removal [357]. The process is even more complex when the surface of carbon is decorated with functional groups. They provide the hydrogen bonding sites, which are the primary adsorption centers [358]. Then water- water interactions lead to the formation of clusters and condensation of water in micropores at much lower humidity than that at which it happens on a fully hydrophobic carbon surface. Due to the importance of this problem in industrial and military applications numerous reviews were published describing the effects of surface chemisby on the adsorption of water [356, 359]. 

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 major factor stabilizing the aggregation of protein subunits is the hydrophobic effect. When separate polypeptide chains fold into compact three-dimensional shapes to expose polar side chains to the aqueous environment and shield nonpolar side chains from water, hydrophobic patches may still appear on the surface, in contact with water. These patches can be shielded from water if two or more monomers assemble so that their hydrophobic patches are in contact. The numbers of subunits of several proteins of knovwi quaternary structure are shown in Table 18.3. 

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