Ions at Interfaces

A film became saturated with respect to aluminium ions when there was one of these in the film for every 12 stearic acid molecules, although photographs of the skim showed that appreciable adsorption could occur if the underlying solution contained only one part of aluminium in 2 X 10 parts of water Under these conditions, even if all the aluminium in the solution were concentrated in the interface, there would be only one atom of metal to every 31 molecules of stearic acid. Copper ions likewise form salts very readily, being effective from bulk concentrations as low as 1 part in 300,000,000. 

The other paper appearing in 1937 dealt with the sensitization of photochemical reactions in the surface by the adsorption of ions of metals. The effect means that the course of a reaction in a surface film may be completely altered by the adsorption of traces of heavy metal ions. Mitchell, Rideal, and Schulman (73) showed that minute quantities of nickel or copper ions may be enough to change or even to initiate a surface reaction. In the decomposition, for example, of films of a-hydroxy-stearic acid on 0.01 N HCl, under the action of radiation, carbon dioxide is apparently split off from the molecule. If, however, the radiation has a 

Evidence was obtained that this catalysis by the nickel ions was due to the adsorption from solution of enough nickel ions for a complex to be formed in the surface. One molecule of a-hydroxystearic acid combines with one nickel ion, so that very small amounts of metal ions are concentrated at the surface through reaction with the molecules in the monolayer. 

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Insulators lack free charges (mobile electrons or ions). At interfaces with electrolyte solutions, steady-state electrochemical reactions involving charge transfer across the interface cannot occur. It would seem, for this reason, that there is no basis at this interface for the development of interfacial potentials. 

To monitor the movement of surfactant ions in the octanol membrane visually, electrical potential oscillation across the octanol membrane was measured with eriochrome black T (EBT) as colored surfactant in phase w2 [20]. Migration of EBT from interface o/w2 toward bulk phase o could be seen during the induction period of oscillation. After EBT reached interface o/wl, the first pulse of oscillation started. Thus, surfactant ions at interface o/wl are indispensable for oscillation. Considerable convection in phase o and... 

Surface and Interfacial Tension. Some properties of liquid surfaces are suggestive of a skin that exercises a contracting force or tension parallel to the surface. Mathematical models based on this effect have been used in explanation of surface phenomena, such as capillary rise. The terms surface tension (gas—liquid or gas—solid interface) and interfacial tension (liquid—liquid or liquid—solid) relate to these models which do not reflect the actual behavior of molecules and ions at interfaces. Surface tension is the force per unit length required to create a new unit area of gas—liquid surface (mN/m (= dyn/cm)). It is numerically equal to the free-surface energy. Similady, interfacial tension is the force per unit length required to create a new unit area of liquid—liquid interface and is numerically equal to the interfacial free energy. 

S. Trasatti. The Electrode Potential, in Comprehensive Treatise of Electrochemistry, Vol. 1, J. O M. Bockris, B.E. Conway and E. Veager. Eds. Plenum (1980), chapter 2 B.E. Conway, The State of Water and Hydrated Ions at Interfaces, Adv. Colloid Interface Sci. 8 (1977) 91 W.R. Fawcett, Molecular Models for Solvent Structure at Polarizable Interfaces. Isr. J. Chem. 18 (1979) 3 M.A. Habib, Solvent Dipoles at the Electrode-Solution Interface. in Modem Aspects of Electrochemistry, Vol. 12, J. O M. Bockris and B.E. Conway. Eds. Plenum (1977) 131 S. Trasatti, Solvent Adsorption and Double Layer Potential Drop at Electrodes, in Modem Aspects of Electrochemistry, B.E. Conway and J. O M. Bockris, Eds. Vol. 13 Plenum (1979) chapter 2 J. O M. Bockris. K-T. Jeng, Water Structure at Interfaces The Present Situation. Adv. Colloid Interface Set 33 (1990) 1. 

For a more ciassicat introduction, see B.E. Conway. The State of Water and Hydrated Ions at Interfaces, Adv. Colloid Interface Set 8 (1977) 91. 

Modem aspects of the structure of the electrochemical double layer at interfaces were recently summarised by Watanabe (1994) in line with lUPAC recommendation for the description of the different parts of an electrical double layer forming at an interface as well as the conditions of electroneutrality and the behaviour of different ions at interfaces. 

As mentioned above, the adsorption of ions at interface can be divided into different cases. The solid can be an extended surface or can consist of disperse particle, such as latices and specific and non-specific adsorption is possible. At the solid surface active sites are distributed. This active sites are the geometrical spots for localised adsorption. Two different planes of adsorption are possible. The first one is the solid/liquid interface itself. We could take the adsorption of negatively charged phosphate ions of amino groups as part of a solid siuface as an example of this. The phosphate groups are a classical case of non-point charges which can... 

Levin, Y. Polarizable ions at interfaces. Phys. Rev. Lett. 102,147803 (2009)... 

Equations (l)-(4) are the foundations of electrical double layer theory and are often used in modeling the adsorption of metal ions at interfaces of charged solid and electrolyte solutions. In a typieal TLM, the outer layer capacitance is often fixed at a lower value (i.e., C2 = 0.2 F/m ), whereas iimer layer capacitance (Ci) can be adjusted to between 1.0 and 1.4 F/m [25]. It should be noted that the three-plane model (TPM) is a variation of the classical triple-layer model, in which the outer layer eapaeitanee is not fixed. Although the physical presentations of the TLM and TPM are identical as shown in Fig. 2, i.e., both involve a surface layer (0), an inner Helmholtz plane (p), and an outer Helmholtz plane d) where the diffuse double layer starts, a one-step protonation process (i.e., 1 piC approach) is, in general, assumed in the TPM, in eontrast to a two-step protonation process (i.e., 2 p/C approach) in the TLM. Another distinct difference is that pair-forming ions are assumed to be on the outer Helmholtz plane in the TPM but on the inner Helmholtz plane in the TLM. In our study, the outer layer capacitance is allowed to vary while the pair-forming ions are placed on the iimer Helmholtz plane with a complete set of surface eomplexation reactions being considered. Therefore, our approach represents a hybrid of the TPM and TLM. 

Further to the bulk behavior the behavior of ions at interfaces must be taken into account to correctly describe most of the systems, for example, ions near phospholipid membranes, the air-water interface, or near electrodes. This evidence was rapidly clear, and consequently, the first models about electrol3rte interface layers came up in the second half of the nineteenth century by Hermann von Helmholtz in 1853... 

However, it is still a matter of debate, whether the potentials used by these authors are reliable. It cannot be excluded that this particular behaviour of the ions at interfaces is simply a consequence of their charge density and only to a minor or negligible extent due to ion polarisabihty. It seems that reliable information about ion specificities at interfaces can also be obtained without taking into account ion polarisabihties. In any case, the influence of the ions on the water structure at the interface is a major reason for the propensity of the ions to the interface, be it caused by polarisation effects or mainly size and charge density. More details are discussed in Chaps. 9-11. 

Netz RIC (2004) Water and ions at interfaces. Curr Opin Coll Interf Sci 9 192-197. 

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