Ionic dispersion potentials

Our point here is not to claim a complete description of the phenomena. It is that even at the primitive model level the inclusion of dispersion effects accounts for at least some specific ion effects. Inclusion of potential of mean force (PMF) from simulations improves the understanding of the Hofineister effect significantly. Both ionic dispersion potentials and the PMF depend on the polarisability of the ions. It has been firmly established that the polarisability has an important role for the ion specificity observed in colloidal systems and in biology. 

Recently, Ninham and Parsons presented model calculations of polaris-abilities of many different ions. This opens up for quantitative calculations of ionic dispersion potentials. However, for completeness we give also here some simple estimates of the non-electrostatic (NES) potential acting between an ion and a membrane surface ... 

Bostrom M, Williams DRM, Ninham BW (2002) The influence of ionic dispersion potentials on counterion condensation on polyelectrolytes. J Phys Chem B 106 7908-7912... 

The ionic models discussed in section 1.12 involve some sort of empiricism in the evaluation of repulsive and dispersive potentials. They thus need accurate parameterization based on experimental values. They are useful in predicting interaction energies within a family of isostructural compounds, but cannot safely be adopted for predictive purposes outside the parameterized chemical system or in cases involving structural changes (i.e., phase transition studies). 

We have already introduced the concept of ionic polarizability (section 1.8) and discussed to some extent the nature of dispersive potential as a function of the individual ionic polarizability of interacting ions (section 1.11.3). We will now treat another type of polarization effect that is important in evaluation of defect energies (chapter 4). 

The experiments performed by several research groups showed good agreement of theoretical predictions with the experimental data. This is rather encouraging and a little surprising result, keeping in mind that the experiments with simple electrolytes (where a similar effect called ionic vibration potential exists ) produced data that are often not well explained by the corresponding theory. The CVP technique can be applied to concentrated dispersions. 

Another line of thought has been developed by Ninham and coworkers (Ninham and Yaminsky 1997 Ninham 1999 Bostrom, Williams, and Ninham 2001 Bostrom, Deniz et al. 2006 Bostrom, Tavares et al. 2006 Ninham 2006 Parsons and Ninham 2010), consisting essentially of the introduction of ion-specific interactions through ion dispersion energies. It considers ionic dispersion and image potentials, giving... 

Figure 2 presents the ionic interface profiles of the 1-M NaNOs salt of Fig. 1 Rf 30 A is sufficient). At that concentration, the ion-ion correlations have a non-negligible effect on the shape of the interface distributions. The addition of the air-ion dispersion potentials greatly modifies the bare profiles obtained with pure hard sphere + image force contributions.

Fig. 2. HNC ionic profiles at the air-water interface within the PM description. 1-M NaNOs salt of Fig. 1. Solid lines with air-ion dispersion potentials dotted lines without. Note that, according to the electroneutrality condition, the excess adsorbed quantity is identical for cation and anion. (From Ref 7.)...

Fig. 3. KCl 0.1 M + KI 0.1 M mixture at air-water interface. Top PM interface-ions potentials. Bottom HNC ionic interface distributions. Dotted lines image force + dispersion potentials of Ref 7. Thick lines an extra attraction must be added for the anions in order to reproduce the experimental adsorption. (From Ref. 10.)...

In this chapter, a short history is given about the introduction of non-electrostatic interactions in the Poisson-Boltzmann equation, starting from simple ionic dispersion forces up to very recent approaches, in which both water profile and ion-surfece potentials inferred from molecular dynamics simulations are used. 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

In pseudoplastic substances shear thinning depends mainly on the particle or molecular orientation or alignement in the direction of flow, this orientation is lost or regained at the same speed. Additionally many dispersions show this potential for particle or molecule interactions, this leads to bonds creating a three-dimensional network structure. They are often build-up from relatively weak hydrogen or ionic bonds. When the network is disturbed. 

In SILP carbonylation we have introduced a new methanol carbonylation SILP Monsanto catalyst, which is different from present catalytic alcohol carbonylation technologies, by using an ionic liquid as reaction medium and by offering an efficient use of the dispersed ionic liquid-based rhodium-iodide complex catalyst phase. In perspective the introduced fixed-bed SILP carbonylation process design requires a smaller reactor size than existing technology in order to obtain the same productivity, which makes the SILP carbonylation concept potentially interesting for technical applications. 

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