Ionic surface charge density

A qualitative theoretical explanation of this model based on ionic surface charge densities or continuum electrostatics is possible, but quantitative interpretations are difficult, even more so when molecular ions like SCN" are studied, which cannot be approximated by spheres with an isotropic surface charge density. 

Studies of theadsorption of surface-active electrolytes at the oil,water interface provide a convenient method for testing electrical double-layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, a surface ionic area of 100 per ion corresponds to 16 pC per square centimeter. " " ... 

FIG. 10 Distribution of the ions K and A2 in the presence (full lines) and in the absence (dashed lines) of ion pairing. In the former case, the ionic interaction parameter was taken as r = —9kT. The surface-charge density was taken as cr = 2.34/iCcm in phase 1. 

Fig. 10.1. Relationship (Eqn. 10.6) between surface charge density cr and surface potential T for a sorbing surface in contact with solutions of differing ionic strengths I (molal).

Dzombak and Morel, 1990, have illustratively and compactly summarized (Fig. 3.3) the interdependence of the Coulombic interaction energy with pH and surface charge density at various ionic strengths for hydrous ferric oxide suspensions in... 

Relationship between pH, surface potential, xp or Coulombic term, log P, or Coulombic free energy, AGcoui), and surface charge density, a (or surface protonation) for various ionic strengths of a 1 1 electrolyte for a hydrous ferric oxide surface (P = exp(-Fi //RT). 

Less is known about the interaction of the nucleosomes between themselves or with free DNA. The nucleosome-nucleosome interaction has recently been parameterized by using the surface charge density of the known crystal structure [39] in a point-charge model [51]. While in that work only electrostatic interactions were considered and the quantitative influence of the histone tails on the interaction potential still remains obscure, simulations based on this potential allowed to predict an ionic-strength dependent structural transition of a 50-nucleosome chromatin fragment that occurred at a salt concentration compatible with known experimental data (Ref. [65], see below). 

The mobility depends on both the particle properties (e.g., surface charge density and size) and solution properties (e.g., ionic strength, electric permittivity, and pH). For high ionic strengths, an approximate expression for the electrophoretic mobility, pc, is given by the Smoluchowski equation ... 

Table 1. Ionic Radii, hydration numbers, softness parameters a, surface charge densities, polarizabilities, free energies AG° enthalpies AH° and entropies A S° of hydration of metal cations from groups Za o,nd II ...

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

Counterion Binding. The fractional counterion binding on charged mixed micelles is of fundamental interest because it gives an indication of surface charge density which is related to the mechanism of mixing nonidealities in ionic/nonionic micelles. It is also a necessary... 

For simplicity we restrict our discussion to a thin two-dimensional channel of a half width H which is much shorter than its length L, H L, and assume once again equal ionic diffusivities. Let the channel s wall be charged with a surface charge density a. Direct x along the symmetry axis of the capillary, with y directed across the channel and the origin at the middle of its left edge (see Fig. 6.4.1). 

Electroosmotic mobility is proportional to the surface charge density on the silica and inversely proportional to the square root of ionic strength. Electroosmosis decreases at low pH (Si—O —> Si—OH decreases surface charge density) and high ionic strength. At pH 9 in 20 mM borate buffer, electroosmotic flow is 2 mm/s. At pH 3, flow is reduced by an order of magnitude. 

Existing theories of the adsorption of polyelectrolyte allow effects of the polymer charge density, the surface charge density, and the ionic strength on the adsorption behavior to be predicted. The predicted adsorption behavior resembles that of nonionic polymers if the ionic strength is high or the polymer charge density is very low. 

The surface potential 0O, therefore, depends on both the surface charge density cr0 and (through k) on the ionic composition of the medium. If the double layer is compressed (i.e. k increased), then either cr0 must increase, or 0O must decrease, or both. 

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