Hydrophobic expulsion

Adsorption may occur in a combination of three possible mechanisms hydrophobic expulsion, electrostatic attraction, and complexation. Most nonpolar compounds, such as various organics, adsorb by this mechanism, and the degree of partitioning is correlated to the octanol/water partitioning coefficient, Kou, Polar substrates such as various metals sorb via electrostatic attraction and complexation. Table 13.1 shows the typical sorption mechanisms and typical examples. 

Hydrophobic expulsion Partitioning Nonpolar organics (e.g., PCBs, PAHs)... 

Hydrophobic expulsion of hydrophobic substances (this includes nonpolar organic solutes), which are usually only sparingly soluble in water and tend to reduce the contact in water and seek relatively nonpolar environments, thus accumulating at solid surfaces and becoming absorbed on organic sorbents. 

Because of its nonpolar and hydrophobic character, the mercury-water may serve as a good model interface for the adsorption study and determination of the organic substances that are adsorbed primarily because of hydrophobic expulsion. There is generally a proportionality of adsorbability (free energy of adsorption) found at the mercury electrode to a number of -CH2 groups in paraffinic hydrocarbon residues in nonpolar surfactants and a similar relation between the octanol water partition coefficient and chain length. This was recently also illustrated in the case of adsorption of aliphatic fatty acids (Ulrich ct al., 1988). 

Protein molecules contain both polar and apolar groups. For proteins dissolved in water, these apolar groups tend to be buried in the interior of the globular structure, as a result of expulsion by the surrounding water. However, other interactions, as well as geometrical constraints, interfere with the hydrophobic effect, so that a minor fraction of the water-accessible surface of the protein molecule may be apolar. Protein molecules that do not spontaneously aggregate in water do not have pronounced apolar patches at their surfaces. 

The aforementioned methodology has been applied to measure the kinetics of a series of monovalent ions by using the oxidation of LBPC [26-29], As the redox probe LBPC is oxidized to the stable hydrophobic cation LBPC+, and the electrode reaction is accompanied by either anion ingress from the aqueous phase (4.12) or cation expulsion from the organic phase (4.13), which depends on the type of ions and their relative affinity for both liquid phases. 

Thus, the possibility of adsorption is of primary importance. Adsorption may originate either from chelating properties of the organic substrate toward surface metal species or, because of the low hydrophobicity of the metal oxide surface, from the expulsion of the organic molecules from the solution for entropy reasons. Because there is depletion of substrate at the catalyst surface when degradation takes place, migration from the solution is assisted by a concentration difference in the two environments. 

The central core is predominantly hydrocarbon. The expulsion of the hydrophobic tails of the surfactant molecules from the polar medium is an important driving force behind micellization. The amphipathic molecules aggregate with their hydrocarbon tails pointing together toward the center of the sphere and their polar heads in the water at its surface. 

In flg. 3. lb the preference of the hydrophobic tails of the (anionic) surfactant molecules for the oil phase gives rise to the double layer. Such double layers are for instance encountered in some emulsions. They may also occur at the air-water interface then the driving force for their formation is the expulsion of the hydrocarbon tails from the aqueous phase. We speeik of ionized monolayers and return to them in Volume III. 

Conveyor of chemicals (metals, pollutants, nutrients) Collector of hydrophobic solutes that accumulate at the surface because of expulsion from the water Organic or inorganic surface ligands (Lewis bases) that interact with protons or metal ions Lewis acids that bind ligands (anions and weak acids) (ligand exchange)... 

M. S. Cheung, A. E. Garcia, J. N. Onuchic (2002) Protein folding mediated by solvation Water expulsion and formation of the hydrophobic core occur after the structural collapse. P. Natl. Acad. Sci. USA 99, pp. 685-690... 

In this type of membrane, as well as in similar products made by other manufacturers, the Teflon-like backbone is responsible of very high chemical resistance (due to the strong bond between carbon and fluorine), high hydrophobic characteristics, and good mechanical properties. The hydrophobic feature is useful to favor the expulsion of product water out of the cell, in order to prevent flooding phenomena, while the mechanical strength permit the production of very thin films (down to 50 pm). 

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