Adsorbed Organic Cations

Adsorbed polyvalent metal ions with attached organic acid groups (metal soaps). 

Surface silanol groupsi esterified with alcohols to cover the surface with SiOR groups.  

Surface silanol groups reacted with organosilicon intermediates to produce a surface of Si-O-SiRi or similar multiply bonded (SiO)3SiRa groups. 

It is only the last type of surface modification that has proved to be the most widely useful, probably because of the simplicity of application and stability of the resulting surface. However, all approaches are reviewed briefly. 

The adsorption of organic cations, such as long-chain substituted ammonium ions, on the surface of silica is employed in the flotation of silica from ores (496). For example, when dodecylamine hydrochloride is. added to a suspension of finely ground limestone containing silica, the dodecylammonium ion forms an adsorbed layer on the silica, which thus becomes hydrophobic and floats with the froth as air is bubbled through the mixture. 

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Curves for different inorganic cations are nearly coincident. Indicating almost complete absence of specific adsorption of the cations. More strongly adsorbable organic cations, however, depress the cathodic branch of the curye as is demonstrated by Fig. 17 The addition of non-ionic organic substances often results in a more or less symmetric truncation of the electrocapillary curve. See Fig, 18... 

Rytwo et al. (2002) show that diquat and paraquat adsorb on neutral sites of sepiolite the authors speculate that this might be a general pattern for organic cation contaminants interacting with sepiolite. 

It will be noted that with the exception of the organic cations and anions the more mobile hydrogen and hydroxyl ions are most readily adsorbed, whilst in the case of the metallic ions the influence primarily of the valency of the ion and both the position of the metal in the eleotrolytic potential series as well as the ionic mobility is most marked, the higher the valency and the more noble the element the more readily it is adsorbed. 

In this, the adsorbed organic substrate is electrochemically oxidised to form a radical cation, a proton is eliminated in a chemical step to form a radical, which is then further electrochemically oxidised to a carbocation, which reacts with a fluoride ion in the double layer to produce the fluoroorganic compound. 

We conclude that oxidative transformations of organic substrates can be readily understood as emanating from either photogenerated surface adsorbed radical cations or from radicals formed by activated oxygen radicals (surface oxides or adsorbed hydroxy, hydroperoxy or peroxy radicals). Photoelectrochemical methods not only generate the reactive species by interfacial electron transfer, but also control the subsequent activity of the surface adsorbed intermediate. 

At the surface of the solution the cationic soap constitutes a positive layer of organic cations neutralized by Cl" and PoCl62" ions 210Po emits a-rays. The surface density as well as the kinetics of the adsorption of PoCl62" are determined by measuring the radioactivity above the surface of the solution. An analogous technique is used to determine the density of the soap adsorbed at the air-solution interface, when the soap is labelled with 14C. Therefore, these measurements allow a direct analysis of the composition of the adsorbed soap films at equilibrium and during their formation. 

Beside the electron transfer from the semiconductor to adsorbed molecular oxygen also the direct transfer to an organic molecule is possible. This type of photocatalytic reaction, yielding an organic radical anion, has been found to occur with 1,4-benzoquinone [18], tetrachloromethane [19], and several nitroaromatic compounds [20]. But electrons can also be transferred very efficiently to (adsorbed) metal cations [21]. 

After treatment with hydrogen peroxide in 0.3 M hydrochloric acid at 70°C for 30 minutes in order to remove adsorbed organic matter, the silt fraction (4 to 74 ym diameter) was washed repeatedly with distilled deionized water to constant conductivity in order to remove any adsorbed acid. The specific surface area of this material was measured using Lawrie s (S) 1,10-phenanthro-line adsorption method. The cation exchange capacity per gram was determined by Chapman s ( ) method, in which the surface is first saturated with sodium by three successive washings with 1.0 M sodium acetate solution, followed by three successive washings with 1 M ammonium acetate, which is saved and analyzed for the displaced sodium. 

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