Hydrophobic interactions, colloid stability

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

It is known that interactions between ionic surfactants and polyions with the opposite charge lead to the formation of soluble colloidal complexes. The polyelectrolyte chain binds to surfactant molecules through Coulombic attractions, and the hydrophobic moieties of the surfactant molecules stabilize the complexes due to hydrophobic interactions in the aqueous solution (Morris and Jennings, 1976 Satake and Yang, 1976 Osica etal., 1977 Fendler, 1982 Hayakawa et al., 1983 Jonsson et al, 1998). 

Fig. 20. Photosensitized H2-evolution from an organized assembly composed of a Pt-colloid stabilized by a positively charged polymer matrice. Charge separation is effected by means of complementary electrostatic and hydrophobic interactions...

Kaifer et al. were particularly interested in the catalytic properties of Pd nanoparticles derivatized with surface-attached perthiolated cyclodextrins and their use in various catalytic reactions such as Suzuki reactions [57] or the hydrogenation of alkenes [58] or allylamine [59]. The modified cyclodextrins play the role of a ligand, leading to a steric stabilization. To our knowledge, only one report describes the catalytic hydrogenation of olefins using colloidal Rh dispersions embedded by nahve cyclodextrins [60], generating steric stabilization via hydrophobic interactions. 

Hydrophobic colloids can also be stabilized by hydrophilic groups on their surfaces. Oil drops are hydrophobic, for example, so they do not remain suspended in water. Instead, they abnegate, forming an oil slick on the water surface. Sodium stearate FIGURE 13,32), or any similar substance having one end that is hydrophilic (either polar or charged) and one end that is hydrophobic (nonpolar), will stabilize a suspension of oil in water. Stabilization results from the interaction of the hydrophobic ends of the stearate ions with the oil drops and the hydrophilic ends with the water. 

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