Ionic Adsorption from Solution

Complex Ionic Adsorption from Solution. - Much of the metal adsorption that normally is of importance in catalyst preparative chemistry concerns the interaction with complex ions. Obviously, for example, all metal anionic adsorption must of necessity involve the metal as a complex ion. This tendency to form complex ions can cause substantial deviations from what one might normally expect from considerations of simple ion-exchange equilibria. This is true in the case of ferric chloride, for example reactions (12)-(15). 

W Weak Anionic Absorption I Moderate Anionic Absorption S Strong Anionic Absorption M Molarity of Solution 

Similar equations can be written in terms of the hydroxide ion, i.e., pH, giving a series of hydrolysed cations of different valence, even as seen above to the extent of converting a nominal cation into a complex anion. These complex anions tend to be very strongly adsorbed because of their symmetry and high polarizability. 

A very useful review of the subject of metal adsorption has recently appeared, which gives tables of the relative strengths of anionic and cationic adsorption for each of the metals in the periodic table, with separate tables for each of the different anion environments, i.e., F , Q , NO3, CbT, and sol . Table 2 shows some of the results from this reference that pertain to the catalytically important Group VIII and IB metals. There would seem to be ample scope for more detailed work along these lines for systems of more particular catalytic interest, e.g., those containing ammonia as a complexing ligand. Some more qualitative work has appeared in several papers, which will be discussed later. 

It is important to know how strongly a metal is adsorbed since it is this 

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Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

J.S. Clunie, B.T. Ingram, Adsorption of non-ionic surfactants, in Adsorption from Solution at the Solid-Liquid interface (see sec. 2.10b), p. 105. 

Adsorption from Solution at the Solid/Liquid Interface, G.D. Parfitt, C.H. Rochester, Eds., Academic Press (1983). (Contains chapters on adsorption of smEill molecules (G.D. Parfitt and C.H. Rochester), adsorption from mixtures of miscible liquids (J.E. Lane) and adsorption of non-ionic surfactants (J.S. Clunle,... 

Simple Anionic Adsorption from Solution. - Here the relevant equation would be (11), and again the equilibrium would be pH dependent but in this case shifts to the left at a high pH. It is also dependent upon the ionic affinity which increases with anionic polarizability and ionic charge, e.g., I" > Br > Cr > F- and SOf > Q". 

When we introduce an insoluble solid into a solution, a change in composition of the solution usually occurs. This is as a result of preferential adsorption of one of the components on the adsorbent solid. Adsorption from solution is a broad subject including detergent, dye, ion, polymer and biological material adsorption on solids, and a huge amount of literature has been published in this field, since it is important to many industries. In this section, an introduction to the subject will be presented, but excluding ionic adsorption. 

Furthermore, the particle core has a density consistent with amorphous silica. The adsorption of oligomers around the core of the particles has implications that may explain the high charge density and exceptional stability of silica sols (9). Enhanced stability may also arise from solvation forces. Such an interfacial structure has important consequences in both determining the mechanism and enhancing the capacity for sorption of other ionic species from solution. 

The interactions between the sohd support and the IL have a strong influence on the resulting properties of the system (e.g., on ionic mobility) these properties can be also affected by several factors including the way the sample is prepared (adsorption from solution [76, 77], incipient wetness impregnation (IWI) method. 

In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

Althongh van der Waals forces are present in every system, they dominate the disjoining pressnre in only a few simple cases, such as interactions of nonpolar and inert atoms and molecnles. It is common for surfaces to be charged, particularly when exposed to water or a liquid with a high dielectric constant, due to the dissociation of surface ionic groups or adsorption of ions from solution, hi these cases, repulsive double-layer forces originating from electrostatic and entropic interactions may dominate the disjoining pressure. These forces decay exponentially [5,6] ... 

Adsorption is a physicochemical process whereby ionic and nonionic solutes become concentrated from solution at solid-liquid interfaces.3132 Adsorption and desorption are caused by interactions between and among molecules in solution and those in the structure of solid surfaces. Adsorption is a major mechanism affecting the mobility of heavy metals and toxic organic substances and is thus a major consideration when assessing transport. Because adsorption is usually fully or partly reversible (desorption), only rarely can it be considered a detoxification process for fate-assessment purposes. Although adsorption does not directly affect the toxicity of a substance, the substance may be rendered nontoxic by concurrent transformation processes such as hydrolysis and biodegradation. Many chemical and physical properties of both aqueous and solid phases affect adsorption, and the physical chemistry of the process itself is complex. For example, adsorption of one ion may result in desorption of another ion (known as ion exchange). 

In Eq. 30, Uioo and Fi are the activity in solution and the surface excess of the zth component, respectively. The activity is related to the concentration in solution Cioo and the activity coefficient / by Uioo =fCioo. The activity coefficient is a function of the solution ionic strength I [39]. The surface excess Fi includes the adsorption Fi in the Stern layer and the contribution, f lCiix) - Cioo] dx, from the diffuse part of the electrical double layer. The Boltzmann distribution gives Ci(x) = Cioo exp - Zj0(x), where z, is the ion valence and 0(x) is the dimensionless potential (measured from the Stern layer) obtained by dividing the actual potential, fix), by the thermal potential, k Tje = 25.7 mV at 25 °C). Similarly, the ionic activity in solution and at the Stern layer is inter-related as Uioo = af exp(z0s)> where tps is the scaled surface potential. Given that the sum of /jz, is equal to zero due to the electrical... 

Surface Adsorption. From Fig.l and Fig.2 we can calculate the total surface adsorption (17 ) of the RDH-surfac-tant mixture by applying the Gibbs adsorption equation(7). In the case of a mixed aqueous solution with a constant ionic strength, the equation is written as... 

Once (stable) nuclei have formed, there are several ways in which they can increase in size. One is a continuation of the process of embryo growth discussed earlier adsorption of ionic species from the solution onto the nucleus. Crystal... 

ION RETARDATION. A process hused on amphoteric (hifunclionalt ion-exchangc resins containing both anion and cation adsorption sites. These siies will associate with mohile anions and cations in solution and thus remove both kinds of ions from solutions. These ions may be eluted bv rinsing with water. This process can make elean separations of ionic-nonitmic mixtures It has also been suggested fur demineralization of salt solutions. 

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

V. B. Fainerman, S. A. Zholob, M. Leser, M. Michel, and R. Miller, Competitive adsorption from mixed non-ionic surfactant/protein solutions,. /. Colloid Interface Sci. 274, 496-501 (2004). 

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