SynopsisThe polyethyleneimine (PE1)-water-silica gel absorption system was used as a model system to investigate the relationship between diffusion into the porous structure, adsorption rate, and molecular weight of the polymer. Three silica gels, Porasil A, B, and and C having a range of characteristic porosity were used as adsorhents. Adsorption of PEI on Porasil C, which has the majority of its pores much larger than the dimensions of the adsorbate molecule, increased initially with increased molecular weight but became nearly constant at higher molecular weight. Little increase in adsorption occurred for this silica gel with increased ionic strength or with increased p H between 9.5 and 10.8. In contrast, adsorption increased sharply with increased ionic strength and for the same p H range on Porasil A.Adsorption decreased with increased molecular weight on Porasil A. In this case, the molecular size of PEI investigated was the same as the majority of pore apertures in the adsorbent. Solution environments (i.e., p H and ionic strength) that decrease the size of the PEI molecule and its affinity for the anionic silica gel surface, thus enabling it to more readily diffuse into the smaller porous regions of the adsorbent, are the apparent causes of the very large adsorption increase. Electrostatic repulsion between PEI molecules do not appear greatly to affect adsorption. Similar adsorption behavior has been reported in the literature for the PEI-cellulosic fiber adsorption system. bIaximum adsorption on Porasil A occurred at p H 10.8, the same maximuni generally reported for adsorption of PEI on cellulosic fibers. In this case, the silica gel (Porasil A ) was found to have a pore size distribution and specific surface area of the same magnitude as cellulosic fibers prepared in the expanded state.Molecular weight dependence was reversed.

The importance of structure in contemporary chemistry is clearly evidenced by the increasing number of papers which deal with this subject in both the solid state and in solution. While the solid crystalline material may be structurally characterized quite accurately by diffraction techniques, the structure of complex molecules or ions in solution must be inferred from their spectral and magnetic properties. Magnetic behavior has long been recognized as a readily obtained physical property which is closely tied to the structure of the compound, as described a number of years ago by Pauling and more recently interpreted by others in terms of the ligand field and molecular orbital theories.-Temperature studies of magnetic properties have been found especially useful in establishing the nature of the ground states for paramagnetic ions which is intimately related to the symmetry of the magnetic species. Recently, a number of transition metal complexes have been reported to undergo structural changes in solution as the temperature is changed. These structural modifications are prominently reflected in the concomitant changes of the magnetic moment of the solution. Several such complexes have been studied and the thermodynamics of the processes causing the changes in magnetic behavior determined. These systems seem to us to be very suitable for undergraduate investigations in as much as they provide experience in the area of synthesis, dynamics, and structure. One of the problems, however, is that doing temperature-dependent magnetic studies is often not possible in many undergraduate laboratories, since they are not equipped with the apparatus for making such measurements. We wish to bring to the attention of those interested in such experiments a very useful and simple way for making solution magnetic measurements.

SUMMARYIn vivo equilibrium dialysis studies were performed to define further the characteristics of calcium binding to bovine albumin. The concentration range for albumin (1 to 9 g/dl) as well as ultrafilterable calcium (0-5 to 2-5 mM) studied encompassed those that might be ordinarily encountered in most clinical situations. Major differences in the regressions of total calcium onultrafilterable calcium occurred at albumin concentrations of 1, 2, and 9 g/dl but only small differences at albumin concentrations between 3, 5 and 7 g/dl. When

The rate of adsorption of polyethylenimine (PEI) onto regenerated cellulose fibers can be described as a mass-transport process. The application of simple mass-transfer theory led to the conclusion that diffusion into the internal porous structure of the fiber represents the predominant barrier to rapid adsorption. Under suitable conditions, mass transfer as a result of electrostatic interactions between polymer and fiber may play a role in determining the sorption rate. Adsorption rate curves were measured as a function of polymer molecular weight, initial polymer concentration, ionic strength, and pH. A rate equation based on diffusion control with Langmuirian adsorption in stirred solution was developed. The equation predicted that the initial rate should depend linearly on initial polymer concentration and on the adsorbate diffusion coefficient to the 0.66 power. This equation served to aid in the design and interpretation of experiments. Fractionated PEI was sorbed onto regenerated cellulose fibers from aqueous solution. This system was chosen to facilitate characterization and control of variables. The polymer was fractionated by means of gel permeation chromatography to limit the effects of polymolecularity. Equilibrium adsorption measurements showed that, over a molecular weight range of from 8,000 to 20,000, the smaller molecules were sorbed to a greater extent than the larger ones. For all molecular weight fractions studied, a maximum in retention was observed at pH 10.9. It was shown that the pH dependence is due primarily to an ion-exchange reaction involving ionized hydroxyls on the fibers. Differences in polymer size greatly affected accessibility to reactive sites. The diffusion coefficient was found to decrease with increasing ionic strength, pH, and molecular weight. The sensitivity to pH and salt concentration can be explained by means of an electrophoretic effect. Adsorption equilibrium was achieved after six to eight hours for most cases. The initial rate was found to increase with initial polymer concentration and to decrease with decreasing pH, ionic strength, and molecular weight. With some exceptions, the magnitude of these effects is in accord with simple mass-transfer theory. The deviations from theory are shown to be a result of electrostatic interactions between the fiber and the polymer. The electrostatic interactions are reflected, in general, by an accelerated initial rate and a retarded approach to equilibrium as compared with simple diffusion alone. An effective diffusional film thickness was defined as the distance over which the polymer must diffuse to produce the observed rate. This quantity was calculated from the rate data and found to be on the order of four centimeters. This physically impossible magnitude was interpreted as an indication of the presence of a physical barrier to mass transfer. It was concluded that this barrier is a result of the necessity of the polymer to penetrate the porous structure of the fiber. Electron micrographs of fiber cross sections treated wit...

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