Organic molecules from aqueous solutions

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

Electrosorption is a replacement reaction. We have already discussed the role of the solvent in the interphase, in the context of its effect on the double-layer capacitance. It is most important for our present discussion to know that the electrode is always solvated and that the solvent molecules are held to the surface both by electrostatic and by chemical bonds. Adsorption of a molecule on such a surface requires the removal of the appropriate number of solvent molecules, to make place for the new occupant, so to speak. This is electrosorption. In this chapter we shall restrict our discussion to the electrosorption of neutral organic molecules from aqueous solutions, without charge transfer. Using the notation RH for an unspecified organic molecule, we can then represent electrosorption in general by the reaction... 

Absorption of Organic Molecules from Aqueous Solutions... 

Groszek, A.J. (1998). Flow adsorption microcalorimetry. Thermochim. Acta, 313,133—43. 70. Moreno-Castilla, C. (2004). Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon, 42, 83—94. 

Adsorption of organic molecules from aqueous solution is equally complicated. As many of the organic adsorptives have functionality (e.g. phenols) then pH of the solution and the chemical nature of the carbon surface are as important as in adsorption of inorganic adsorptives. 

Moreno-Castilla C. Adsorption of organic molecules from aqueous solutions on carbon materials. Carbon 2004 42(l) 83-94. 

Describe the construction and function of a system for the chemical removal of two hazardous organic molecules from aqueous solution, one forming a cation and the other an anion easily. Fashion it after the deionization system of Shimidzu et al. described in Sec. 25.2. Again, what are the properties that would determine the success or failure of such a system (hint- redox potential is one of them) ... 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

Figure 11.1. Schematic views of various ways in which an organic chemical, i, may sorb to natural inorganic solids (a) adsorption from air to surfaces with limited water presence, (b) partitioning from aqueous solutions to the layer of vicinal water adjacent to surfaces that serves as an absorbent liquid, (c) adsorption from aqueous solution to specific surface sites due to electron donor-acceptor interactions, (d) adsorption of charged molecules from aqueous solution to complementarily charged surfaces due to electrostatic attractions, and (e) chemisorption due to surface bonding or inner sphere complex formation.

The dependence of the reactivity on AO°, which is expected from both mechanisms, may help to understand the good correlation with Hammett and Taft s a functions. These may be, therefore, regarded as a measure of the effect of different substituents on the overall electron affinities of organic molecules in aqueous solution. The latter conclusion, if accepted and verified, may be regarded as a major contribution of e q reactions to physical organic chemistry. 

In addition, the polymers of intrinsic microporosity (PIMs), such as phthalocyanine networks and the Co phthalocyanine network-PIM (CoPc20), display high specific surface area, as confirmed by the N2 adsorption isotherm at 77 K, and by the adsorption of small organic probe molecules from aqueous solutions at 298 K [236], This material is basically microporous with an increased concentration of effective nanopores. 

Hydrophobic effects arise from the exclusion of non-polar groups or molecules from aqueous solution. This situation is more energetically favourable because water molecules interact with themselves or with other polar groups or molecules preferentially. This phenomenon can be observed between dichloromethane and water which are immiscible. The organic solvent is forced away as the intersolvent interactions between the water molecules themselves are more favourable than the hole created by the dichloromethane. Hydrophobic interactions play an important role in some supramolecular chemistry, for example, the binding of organic molecules by cyclophanes and cyclodextrins in water (see Chapter 2, Sections 2.7.1 and 2.7.5, respectively). Hydrophobic effects can be split into two energetic components, namely an enthalpic hydrophobic effect and an entropic hydrophobic effect. 

Reviewed here are surface electrochemical studies of organic molecules adsorbed at well-defined Pt(lll) electrode surfaces from aqueous solution. Emphasis is placed upon studies of nicotinic acid (NA), pyridine (PYR), and nine related pyridine carboxylic acids. 

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