Adsorption Versus Precipitation

Adsorption and Precipitation versus Heterogeneous Nucleation and Surface Precipitation There is not only a continuum between surface complex-ation (adsorption) and precipitation, but there is also obviously a continuum from heterogeneous nucleation to surface precipitation. The two models are... 

The distribution of metals between dissolved and particulate phases in aquatic systems is governed by a competition between precipitation and adsorption (and transport as particles) versus dissolution and formation of soluble complexes (and transport in the solution phase). A great deal is known about the thermodynamics of these reactions, and in many cases it is possible to explain or predict semi-quantita-tively the equilibrium speciation of a metal in an environmental system. Predictions of complete speciation of the metal are often limited by inadequate information on chemical composition, equilibrium constants, and reaction rates. 

The dynamic method permits the purification from better soluble impurities as well as continuous solubility measurements at the same time. An adsorption effect of the stationary phase which is used to precipitate the dyestuff on its surface is not found within the experimental accuracy. The measurements of l,4-bis-(n-alkylamino)-9,10-anthraquinone (with n-alkyl = butyl, octyl) show two intersection points in the plot of pressure versus concentration. 

The behavior of the precipitated silicas with respect to the adsorption of water vapor was even more anomalous (17, 18). Kiselev (11) and others (19) had demonstrated that in the fully hydroxylated form, a wide range of nonporous pyrogenic silicas gave rise to a common reduced water isotherm (i.e., adsorption per unit area versus relative pressure). However, in the... 

As illustrated in Fig. 4. a number of surfactant properties experience discontinuity in slope at a specific concentration called the critical micelle concentration (CMC). Below the CMC, an insignificant concentration of micelles is present, and essentially all the surfactant is present as monomer. Above the CMC. incremental surfactant forms micelles, and the monomer concentration remains nearly constant. Surfactant molecules contribute differently to a given property when present as monomer versus in a micelle, accounting for the discontinuity in slope in Fig. 4. The monomer concentration is often nearly proportional to the thermodynamic activity of the surfactant, and it is this activity that determines such properties as adsorption at interfaces, surface tension reduction, and precipitation. Monomer concentrations become nearly constant above the CMC (for singlecomponent surfactants), resulting in many physical properties plateauing above the CMC. For example, as shown in Fig. 4, surface tension at the air-water interface levels off above the CMC—this is the dominant method of measuring the CMC. E en practical properties, such as... 

The studies of Scheidegger et al. 16,17) have emphasized the importance of combining time-dependent or kinetic studies with spectroscopic and microscopic investigations to better understand sorption processes at the soil mineral/water interface. Such studies can result in a detailed mechanistic understanding (e.g., distinguishing the rate of metal adsorption versus precipitation processes in sorption systems) which would be difficult to determine using a macroscopic approach alone. 

These examples may be related specifically to the information categories in Table 1 under the headings of phase structures, surface structure (from atomic to 5(X) nm scale resolution), surface sites (e.g., structure, chemistry, reactivity and defects) adsorption (e.g., distribution, coverage, monolayer versus multilayer. colloidal, molecular fonii and bonding) and surface reactions (e.g., oxidation. dissolution, precipitation and phase transformation). 

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