Increasing Polyvinylpyrrolidone Concentration Research Articles

Adsorption of non-ionic and cationic polymers on silica from their mixed aqueous solutions

The adsorption of polyvinylpyrrolidone (PVP) and poly(dimethyldiallylammonium chloride) (PDC) on silica from their mixed aqueous solutions has been investigated as a function of PVP concentration in the presence of PDC. The adsorption of PVP is almost unchanged with the feed concentration of PVP, while the adsorption of PDC decreases with increasing PVP concentration, especially at high concentrations of PDC. The conformation of PVP in the adsorbed layer on silica is relatively flat at low concentrations of PDC, but is extended in solution at high concentrations of PDC. The stability of the silica dispersion is low and the ζ potential is relatively small at low concentrations of PDC, whereas a stable dispersion is obtained at high concentrations of PDC and the ζ potential is large. Thus the stability of the dispersion is well correlated with the electrostatic repulsion and steric repulsion forces operating at the particles.

Nucleation Kinetics of Ice in Undercooled Yeast Cells: Long-term Stability against Freezing

SUMMARY: Undercooling and ice nucleation in yeast cells exposed to increasing hypertonic polyvinyl-pyrrolidone (PVP) concentrations were measured. Ice nucleation rates were analysed in terms of classical nucleation theory. Contrary to earlier reports, nucleation was found to be of the catalysed type, by active catalytic sites within the cell. With increasing PVP concentration, the nucleation temperature tends to a limiting value (approximately 236 K), whereas the homogeneous nucleation temperature of the extracellular PVP solution continues to decrease with increasing PVP concentration. The calculated parameters in the nucleation equation indicate that the nucleation mechanism within the cell is unaffected by hypertonic stress. The experimentally determined freezing kinetics of undercooled water (in oil emulsions) were found to parallel closely previously reported death rates of yeast cells as a function of temperature. The observed kinetics are compatible with a slow crystallization or precipitation of emulsifying agent at the oil/water interface, to yield catalytic sites capable of promoting ice nucleation. Experiments with water- (or yeast)-in-oil dispersions containing no emulsifying agents led to long-term freezing resistance and high recoveries of viable cells.