Electrostatic interactions, control charge separation

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. 

We have examined two types of organized media that effectively control the charge separation and back reactions of the intermediate photoproducts. These include, (a) charged colloids i.e. SiC>2 and ZrC>2 colloids that introduce electrostatic interactions between the photoproducts and interface (7-10), and (b) water-in-oil microemulsions that provide aqueous-oil two phase systems capable of controlling the reactions by proper design of the hydrophobic-hydrophilic balance of the photoproducts ( 6). 

Ru(bpy) +, is diffusion controlled. In the presence of the SiC>2 colloid this back electron transfer process is substantially retarded and ca. 200-fold slower than in the homogeneous phase. The functions of the Si02 colloid in charge separation and retardation of back reactions are attributed to electrostatic interactions of the photoproducts and the charged colloid interface (Figure 3). 

Fig. 8a, b. Control of photoinduced ET reactions in organized microenvironments a) application of charged interfaces to effect charge separation and retard recombination processes by means of electrostatic interactions b) application of water-oil two phase systems in charge separation and stabilization of photoproducts against back reactions by means of hydrophobic-hydrophilic interactions... 

Nanostructures primarily result from polyelectrolyte or interpolyelectrolyte complexes (PEC). The PEC (also referred to as symplex [23]) is formed by the electrostatic interaction of oppositely charged polyelectrolytes (PE) in solution. The formation of PEC is governed by physical and chemical characteristics of the precursors, the environment where they react, and the technique used to introduce the reactants. Thus, the strength and location of ionic sites, polymer chain rigidity and precursor geometries, pH, temperature, solvent type, ionic strength, mixing intensity and other controllable factors will affect the PEC product. Three different types of PEC have been prepared in water [40] (1) soluble PEC (2) colloidal PEC systems, and (3) two-phase systems of supernatant liquid and phase-separated PEC. These three systems are respectively characterized as ... 

The ordering of liquids can be promoted by attractive interactions between an ion and a polar molecule. For example, electrostatic interactions between a positively charged ion (cation) and a water molecule will occur readily in an aqueous environment. This ion-dipole interaction involves the orientation of the partially localized negative charge of the water molecule toward the positively charged cation, forming a solvation shell (hydration shell in the case of water). At close separations, two solvated particles will be repelled from one another due to the bound water molecules that form the hydration layer around the cation. This hydration force is particularly important for ions or molecules in solution as it minimizes their direct contact due to the van der Waals attraction, controlling the stability of the solution. 

A nice example of double layer effects occurs with the lamellar phases discussed in Section II. We already mentioned the beautiful experiments of Safinya et al.20 where the Helfrich undulation force was clearly demonstrated, using electrostatic interactions between the layers as a control parameter. Let us try to understand how the electrostatic interlayer forces have impact upon the undulation interaction Recall (Eqn. III.3) that the counterion distribution in the neighborhood of a single charged surface falls off as x 2 for x A. Since the counterions may be approximately considered as an ideal gas,the double layer contribution to the disjoining pressure between two lamellae separated by a distance, h, is roughly... 

However, the electrochemical interface is in many respects more complicated than simply a very high electric field. Adsorbed species there, can suffer different degrees of charge transfer, interacting not only electrostatically but also chemically with the metal surface, with neighboring species of their own type, with co-adsorbed ions and sdlvent molecules. These interactions can affect the vibrational energies to an extent similar to that of the electric field, making necessary a separation of effects by an appropriate control of the experimental parameters. 

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