Aqueous solutions attraction between

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

The internal structure of a liquid at a temperature near its freezing point has been discussed in Sec. 24. Each molecule vibrates in a little cage or cell, whose boundaries are provided by the adjacent molecules, as in Fig. 20, and likewise for each solute particle in solution in a solvent near its freezing point. It is clear that the question of the hydration of ions no longer arises in its original form. In aqueous solution an atomic ion will never be in contact with less than three or four water molecules, which in turn will be in contact with other water molecules, and so on. There is an electrostatic attraction, not only between the ion and the molecular dipoles in immediate contact with it, but also between the ion and molecular dipoles that are not in contact with it. For solvent dipoles that are in contact with a small doubly charged ion, such as Ca++,... 

The crystallographic radius assigned to the ion Fc+++ is comparable with that assigned to the scandium ion Sc+++. The ions K, Ca+t, and Sc+++ have the same number of electrons, and the same closed electronic shells as the argon atom. In aqueous solution there will be electrostatic forces of attraction between Ca++ and Cl, and between 8c+ t+ and Cl- but the quantum-mechanical forces between these ions will be forces of repulsion only. Between Fe+++ and Cl-, on the other hand, there may be quantum-mechanical forces of attraction. In view of the rather intense electrostatic attraction between Fe+++ and a negative ion, a 1 E. Rabinowitch and W. H. Stockmayer, J. Am. Chern. Soc., 64, 341 (1942). 

In general, three basic kinds of sorption mechanisms for trace elements in geologic aqueous systems can be distinguished (56). Due to non-specific forces of attraction between sorbent and the solute, a physical adsorption may occur. This sorption mechanism results in the binding of species from the solution in several consecutive layers on exposed solid surfaces. This would be a rapid non-selec-tive and reversible process, fairly independent of nuclide concentration and only little dependent on ion exchange capacity of the solid. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Water is highly polar, but it is not ionic. How, then, can water act as a solvent for ionic solids A salt dissolves only if the interactions between the ions and the solvent are strong enough to overcome the attractive forces that hold ions in the ciystal lattice. When an ionic solid forms an aqueous solution, the cations and anions are solvated by strong ion-dipole interactions with water molecules. 

Nucleic acids, DNA and RNA, are attractive biopolymers that can be used for biomedical applications [175,176], nanostructure fabrication [177,178], computing [179,180], and materials for electron-conduction [181,182]. Immobilization of DNA and RNA in well-defined nanostructures would be one of the most unique subjects in current nanotechnology. Unfortunately, a silica surface cannot usually adsorb duplex DNA in aqueous solution due to the electrostatic repulsion between the silica surface and polyanionic DNA. However, Fujiwara et al. recently found that duplex DNA in protonated phosphoric acid form can adsorb on mesoporous silicates, even in low-salt aqueous solution [183]. The DNA adsorption behavior depended much on the pore size of the mesoporous silica. Plausible models of DNA accommodation in mesopore silica channels are depicted in Figure 4.20. Inclusion of duplex DNA in mesoporous silicates with larger pores, around 3.8 nm diameter, would be accompanied by the formation of four water monolayers on the silica surface of the mesoporous inner channel (Figure 4.20A), where sufficient quantities of Si—OH groups remained after solvent extraction of the template (not by calcination). 

Adsorption of (bio)polymers occurs ubiquitously, and among the biopolymers, proteins are most surface active. Wherever and whenever a protein-containing (aqueous) solution is exposed to a (solid) surface, it results in the spontaneous accumulation of protein molecules at the solid-water interface, thereby altering the characteristics of the sorbent surface and, in most cases, of the protein molecules as well (Malmsten 2003). Therefore, the interaction between proteins and interfaces attracts attention from a wide variety of disciplines, ranging from environmental sciences to food processing and medical sciences. 

Many anionic dyes (section 1.6) depend on their sulphonic acid groups for their solubility in water. Dye sulphonic acids have pK values within the range of pH 1-2 and are fully ionised under dyeing conditions as either the free acid or the sodium salt. The mutual electrostatic repulsion between dye sulphonate anions ensures their uniform separation and distribution in dilute aqueous solution. At higher concentrations, however, this repulsion is counterbalanced by mutually attractive forces of various kinds operating at shorter range [3] ... 

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