Isoelectric point isotherm

Sodium Poly(4-styrene sulfonate). The sol—gel processing of TMOS in the presence of sodium poly-4-styrene sulfonate (NaPSS) has been used to synthesize inorganic—organic amorphous complexes (61). These sodium siUcate materials were then isotherm ally crystallized. The processing pH, with respect to the isoelectric point of amorphous siUca, was shown to influence the morphology of the initial gel stmctures. Using x-ray diffraction, the crystallization temperatures were monitored and were found to depend on these initial microstmctures. This was explained in terms of the electrostatic interaction between the evolving siUcate stmctures and the NaPSS prior to heat treatment at elevated temperatures. 

Fig. 4. Influence of pH on the plateau-value /T of adsorption isotherms of polyampholytes. At either side of the isoelectric point, i.e.p., the polyampholyte attains a net charge causing intra- and intermolecular electrostatic repulsion. As a result, the mass of adsorbed polyampholyte, that can be accommodated per unit area of the sorbent surface, decreases. Electrostatic interactions are suppressed by increasing ionic strength, yielding /T less sensitive to pH.

Leyva et al. (2001) studied Sb(III) adsorption to hydroxyapatite. They conducted adsorption isotherms in closed vessels at Sb concentrations of 0.05-50 mg/L, constant I (0.01 m), constant solid phase concentrations of 10g/dm3 at pH values between 5 and 8. The hydroxyapatite was characterized by X-ray diffraction (XRD), SF.M-F.DS, X-ray photoelectron spectroscopy (XPS), and infrared (IR) spectroscopy. Langmuir adsorptions models revealed Fmax of 6.7 x 10 xmol/m2 and Xads = 1.5 x 103dm3/mol. As Sb adsorption occurred, the isoelectric point (pHicp) of the hydroxyapatite changed from 4.0 to 12.0. The decline in the pHjep during sorption as well as the absence of... 

While the linear adsorption isotherms of Figure 4 are illustrative only, they are not inconsistent with reality. The simplest theory of the electrical double layer, the Gouy-Chapman approximation, predicts that if the pH is not far from the isoelectric point, the charge represented by counter ions in the diffuse double layer is related to the surface potential as follows (4, 52, 86) ... 

A Ithough the adsorption of polymers onto solid surfaces has been thor-oughly studied (I), relatively few studies can be found in the literature on the adsorption of proteins onto polymer surfaces. In 1905, Landsteiner and Uhliz (2) discussed the interaction of serum proteins with synthetic surfaces. Blitz and Steiner (3) showed that albumin adsorption onto solid surfaces increased with increasing albumin concentration and that adsorption was nearly irreversible. Hitchcock reported (4) that adsorption of egg albumin onto collodion membranes followed a Langmuir isotherm with maximum adsorption occurring near the isoelectric point. Later, Kemp and Rideal (5) reported that protein adsorption onto solids conforms with Langmuir adsorption. 

Determined by titration (the authors refer to it as the isoelectric point). Maximum surface coverage obtained from the Langmuir isotherm. Source Ref 206. 

Let us call this the donor-acceptor complex proposal, similar to that presented recently for adsorption of substituted nitrobenzenes and nitrophenols on mineral surfaces [739]. The experiments on which this proposal is based are (1) isotherms of phenol, nitrobenzene, and m- and / -nitrophenol on one commercial activated carbon at pH = 2-7 and very low solute concentrations ( <1.5% of the solubility limit of these species [6]) and (2) detailed infrared (internal reflection) spectroscopic analysis of the surface after adsorption of / -nitrophenol. Interestingly, neither in this study, nor in any subsequent study that supports this mechanism, has a similar analysis been performed with carbons containing varying concentrations of carbonyl surface groups. Also of interest is that the authors dismiss the electrostatic explanation of the reported pH effects by assuming that the isoelectric point of the carbon (which was dried at 200°C for 12-24 h) was ca. 2.4. 

Thus, the equations of isotherms (30) and (31) can be used to determine the isoelectric points of proteins by the dependence of their sorption on pH. This approach is realized in Refs. [41,42]. However, it should be noted that this determination is only possible for a low affinity of protein molecules to surface sites. As Figs. 12 and 13 show, for a high affinity (a low Ka) a significant sorption is observed also at pH>pI for cation exchangers and at pH[Pg.721]

The support, zirconia (ISA), was supplied by the Norton Company. The oxide was grounded and sieved to a particle size ranged from 0.16 to 0.25 mm, and calcined at 773 K. Its surface properties, 63.3 m g of specific surface and average pore diameter of 8.60 nm, were determined from the nitrogen adsorption isotherms. The catalysts were prepared by adsorption from solution and/or impregnation of precursor(s), ruthenium nitrosyl nitrate (Alfa) and hexachloroplatinic acid (Aldrich), onto the support. Being zirconia isoelectric point 6.5 (determined by electrophoresis [17] using a Malvern Instrument Zetasizer 4) the precursors solution pH value was kept sufficiently low to enable the desired adsorption of complex metal anions. 

The specific adsorption of H, OH, cations, and anions on hydrous oxides and the concomitant establishment of surface charge can be interpreted in terms of the formation of surface complexes at the oxide-water interface. The fixed charge of the solid surface and the pH of its isoelectric point can be measured experimentally by determining the proton balance at the surface (from alkalimetric titration curve) and by the analytical determination of the extent of adsorbate adsorption. Equilibrium constants established for the surface coordination reactions can be used to predict pHiEp, to calculate adsorption isotherms, and to estimate concentration-pH regions for which the hydrous oxide dispersions are stable from a colloid-chemical point of view. 

Because of the relatively rigid structure of adsorbed globular protein molecules, the adsorption isotherms display well-defined plateau values. The adsorption pattern, the effects of pH (i.e., charge of the protein), and ionic strength are in agreement with those of a polyampholyte the adsorbed mass generally is at a maximum around the isoelectric point of the protein/surface complex, that is, at conditions where the charges on the protein and the surface just compensate each other. 

Two globular proteins A and B have their isoelectric points at pH 6. The plateau values Fnijx of the adsorption isotherms are determined as a function of pH. The results are given in the following figure. 

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