Partition of surface-active ions

The concept of the electrochemical instability has been illustrated using a simple model for the coupling of the adsorption and partition of surface-active ions. Three main features of the electrochemical instability, that is, (1) the location of the instability in the values of the phase-boundary potential, (2) the existence of a window-like instability... 

Figure 4 illustrates the dependence of on Aq for the case when r = 1 at several different values of [Fig. 4(a)] and when = 0.5 and at several different values of r [Fig. 4(b)]. From Fig. 4(a), one can see that takes a maximum around y = 0, i.e., Aq The volume ratio affects strongly the value of as shown in Fig. 4(b), which is ascribed to the dependence of the equilibrium concentration on r through Eq. (25). This simple example illustrates the necessity of taking into account the variation of the phase-boundary potential, and hence the adsorption of i, when one tries to measure the adsorption properties of a certain ionic species in the oil-water two-phase systems by changing the concentration of i in one of the phases. A similar situation exists also in voltammetric measurements of the transfer of surface-active ions across the polarized O/W interface. In this case, the time-varying thickness of the diffusion layers plays the role of the fixed volume in the above partition example. The adsorption of surface-active ions is hence expected to reach a maximum around the half-wave potential of the ion transfer. 

Figure 2.10. Schematic of an electrospray droplet showing partitioning of surface-active surfactants to the charged droplet surfaces (where SH = protonated solvent and X = any negatively charged ion). The surfactants represent ideal analytes for ESl-MS because they are both surface active and chargeable. Species that do not have high affinity for droplet surfaces (represented in this case by SH " ) will reside in the electrically neutral droplet interior where they will be paired with coimterions, and they will be lost as neutrals rather than be detected by the mass spectrometer. (Reprinted from Ref. 77, with permission.)...

This area is a development in the usage of NDDO models that emphasizes their utility for large-scale problems. Structure-activity relationships (SARs) are widely used in the pharmaceutical industry to understand how the various features of biologically active molecules contribute to their activity. SARs typically take the form of equations, often linear equations, that quantify activity as a function of variables associated with the molecules. The molecular variables could include, for instance, molecular weight, dipole moment, hydrophobic surface area, octanol-water partition coefficient, vapor pressure, various descriptors associated with molecular geometry, etc. For example, Cramer, Famini, and Lowrey (1993) found a strong correlation (r = 0.958) between various computed properties for 44 alkylammonium ions and their ability to act as acetylcholinesterase inhibitors according to the equation... 

A characteristic of immobilized enzymes that is often ignored is the potential partitioning of ions and substrates and/or products due to electrostatic potentials or hydrophobic moments. This factor could be used to advantage, for example, if the optimal conditions for enzyme activity do not match those of the process stream. To use the example cited earlier, a succinamidopropyl surface was shown by electrostatic partitioning of ions and independent chemical analysis to have 96 ymol charged groups/g dry beads (25). Attachment of 2 ymol trypsin/g did not significantly alter this characteristic. 

Depending on its nature, the layer promotes the separation of molecules by (1) physical sorption of solutes from solution onto the surface-active groups of the layer particles (adsorption) (Scott and Kucera, 1979), (2) dissolving of solutes into a stationary liquid held on the layer (partition), (3) attraction of ions to sites of opposite charge on the layer (ion exchange), or (4) retention or rejection of solutes on the basis of molecular size and/or shape (size-exclusion or gel-permeation TLC). The boundary between adsorption and partition is quite obscure because both can involve the same types of physical forces, that is, permanent and... 

We have shown [19] that, actually, the amphiphilic C ions only are surface active, but their interfacial density depends on the nature of the counter ion. Indeed, the structure of the Pi ion is very similar to the nitrobenzene molecules and the partition coefficient of CiePi Pwater is about 10 times larger than this of C15CI. Thus for the same water total concentration, there are two very different distributions of C (figure 5) [20]. Now, the... 

We have demonstrated similar effects using a zeolite or pillared clay bound to an electrode surface (3). Molecules partition, according to their size, into sites on the external or internal surface of the zeolite or clay, and the externally bound monolayer mediates electron transfer between the electrode and molecules within the particle. An interesting example of such spontaneous partitioning of molecules occurs when zeolite Y is ion-exchanged with small redox-active cations such as FcR" ", and with large metal tris(2,2 -bipyridyl) complexes. The 13 A diameter Os(bpy)32+ ion is substitution inert, and is therefore blocked from entry into the internal... 

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