Particle surface acid-base equilibria

We shall assume that hydrogen ions are not adsorbed at the electrode and shall denote their concentration directly at the electrode surface by [H. For simplicity we shall only consider singly charged ions A" and BH and assume that adsorption of the components of the acid —base equilibrium obey the Henry isotherm, and we can therefore write for the uncharged particles ... 

The pH at which a protein or particle has an equivalent number of total positive and negative charges as determined by proton exchange is the isoionic point. The pH at which a protein or a particle does not migrate in an electric field is called the isoelectric point. The isoionic point is a whole fiber property of hair and is reflected in the equilibrium acid-base properties of the total fiber the isoelectric point is related to the acid-base properties of the fiber surface. 

Chemical equilibrium models are used to predict the speciation of dissolved solutes in natural systems (e.g., MINTEQA [31]). These models attempt to incorporate all of the various processes that affect the speciation of solutes, including all known solution-phase reactions (e.g., acid-base, precipitation-dissolution, and complexation reactions) and adsorption to solid surfaces. Current models for inorganic chemicals have been successM in predicting speciation in aqueous systems containing well-characterized soUd particles. 

The ability of some components of nucleic acids, especially those with an adenine base, to form complex with 8-cyclodextrin, can also be readily used for chromatographic separations of various nucleotides and nucleosides (59). A substantial problem associated with application of cyclodextrin polymer gels, is that the accessibility of the cyclodextrin cavities on the surface and within the interior of the polymer particle is rather different. The rate of entrapment and release of solutes from the streaming liquid is obviously a diffusion controlled process. Consequently, a longer time is needed to reach an equilibrium within the particle than on its surface. The accessibility of the cyclodextrin rings will be more uniform, if the cyclodextrin is immobilized on the surface of non-complexing polymer particles (polyacrylamide, agarose (60,61) cellulose (62), and silica (63)). Therefore, a better separation (however lower capacity) is expected. 

Finely divided particles in suspension can also interfere with colorimetric mea.surements if one of the indicator forms happens to be preferentially adsorbed. Lanthanum hydroxide is a very striking example of such interference. This compound is a strong base which is very slightly soluble in water. A saturated solution in water at 25° has a pH of 9.0. If the pH of a suspension (turbid solution) of the solid hydroxide is measured with thymol-phthalein, the result obtained is 10.5. The suspension is colored a dark blue, although thymolphthalein is colorless at pH 9.0. The precipitate settles after a time, leaving a colorless supernatant solution although the solid itself is dark blue. Because of the strong basic properties of solid lanthanum hydroxide, it forms on its surface a salt with the indicator acid. In other words, the adsorption of the colored indicator anion predominates, and the presence of the solid phase favors a displacement of the indicator equilibrium towards the alkaline form. Phenol-... 

Seidl created a model based on the state of the surface film (e.g. expanded or condensed), the equilibrium spreading pressure, and the area per film molecule to describe organic film formation from fatty acids, then applied it to rainwater and aerosol particles [245]. He concluded that, in most cases, only dilute films (with concentrations below that necessary to form a complete monolayer) would form on aerosols and raindrops, and such films would not affect their physical or chemical properties. However, dense films were predicted to form on aerosols in the western U.S., mainly attributable to biomass burning. Mazurek and coworkers developed a model to describe structural parameters (elastic properties, etc.) of fatty acid films on rainwater without requiring knowledge of the surfactant concentration or composition by using surface pressure-area and surface pressure-temperature isochors and the rain rate and drop diameter distribution [33]. This model can be used to identify the origin of specific compounds and an approximate chemical composition based on the force-area characteristics of collected rainwater films. 

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