Search

It is known that the addition of phosphate buffer to two polymer aqueous phase systems has a strong effect on the partition behavior of cells and other particles in such mixtures. The addition of sodium phosphate to aqueous poly(ethylene glycol) dextran phase systems causes a concentration-dependent shift in the binodial on the phase diagram, progressively lowering the critical conditions for phase separation as the phosphate concentration is increased. Sodium chloride produces no significant shift in the critical point relative to the salt-free case. Accurate determinations of the phase diagram require measurements of the density of the phases; data are presented which allows this parameter to be calculated from polarimetric measurements of the dextran concentrations of both phases. Increasing polymer concentrations in the phase systems produce increasing preference of the phosphate for the dextran-rich bottom phase. Equilibrium dialysis experiments showed that poly(ethylene glycol) effectively rejected phosphate, and to a lesser extent chloride, but that dextran had little effect on the distribution of either salt. Increasing ionic strength via addition of 0.15 M NaCl to phase systems containing 0.01 M phosphate produces an increased concentration of phosphate ions in the bottom dextran-rich phase.

The use of the rotating drop technique to measure the interfacial tension, γ, of aqueous poly(ethylene glycol) (PEG)/dextran phase systems was studied and compared to the pendant drop method. The dependence of γ on the concentrations of the polymers, sodium phosphate, and sodium chloride was investigated. The difference in polymer concentrations between the phases plotted against γ on a loglog scale yields straight parallel lines for the different salt concentrations. The tension tie line length relationship therefore obeys a power law. The influence of temperature on γ was unusual in that for equal tie line lengths the tension is lower at 6°C than at 22°C in systems containing 0.11 M phosphate.

Two polymer aqueous phase systems have proven useful as partition media for biological cells, the partition coefficients obtained being dependent on cell surface properties. One such property appears to be the surface charge density of cells which prompted this detailed examination of electrostatic potential distributions in dextran/poly(ethylene glycol) two phase systems. The partition coefficients of potassium sulfate and potassium chloride in the phase systems were measured and correlated with the Donnan potential difference between the phases, measured with salt bridges to reversible electrodes. Thermodynamic predictions of the relationship between the two measured quantities were confirmed. The preference of sulfate for the dextran-rich bottom phase, which linearly increases with increasing poly(ethylene glycol) (PEG) concentration, is due to the greater exclusion of sulfate by PEG than by dextran, as indicated by equilibrium dialysis of the polymer against different salt concentrations. Electrokinetic studies of droplets of one phase suspended in the other phase revealed relatively large electrophoretic droplet mobilities which increased linearly with the drop diameter and supralinearly with the sulfate concentration. The sign of the mobility depended on the phase the droplet originated from, but the implied surface charge sign was opposite to that anticipated from the difference in potential between the bulk phases. Assuming the mobility and zeta potential are of the same sign, the simplest potential profile which is consistent with these observations is one in which a dipole potential is present at the phase boundary oriented in such a way as to locally reverse the potential gradient. Levine's partial theory for the electrophoresis of systems of this type was tested and found to be consistent with our results. The correlations obtained suggest that the magnitude of the dipole potential is related to the anion partition coefficient in these systems. © 1984 Academic Press, Inc.

When polymers such as dextran and poly(ethylene glycol) are mixed in aqueous solution biphasic systems often form. On Earth the emulsion formed by mixing the phases rapidly demixes because of phase density differences. Biological materials can be purified by selective partitioning between the phases. In the case of cells and other particulates the efficiency of these separations appears to be somewhat compromised by the demixing process. To modify this process and to evaluate the potential of two-phase partitioning in space, experiments on the effects of gravity on phase emulsion demixing were undertaken. The behavior of phase systems with essentially identical phase densities was studied at one-g and during low-g parabolic aircraft maneuvers. The results indicate the demixing can occur rather rapidly in space, although more slowly than on Earth. The demixing process was examined from a theoretical standpoint by applying the theory of Ostwald ripening. This theory predicts demizing rates many orders of magnitude lower than observed. Other possible demixing mechanisms are considered.

In aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system consisting of a PEG-rich phase floating on top of a dextran-rich phase. Biological particles and macromolecules tend to partition differentially between the phases and the liquid-liquid phase interface in these systems. Bioparticle partitioning has been shown to be related to physiologically important surface properties such as membrane charge or lipid composition.

In aqueous solution at low concentrations, the neutral polymers dextran and poly(ethylene glycol) (PEG) rapidly form a two-phase system consisting of a PEG-rich phase floating on top of a dextran-rich phase. Biological particles and macromolecules tend to partition differentially between the phases and the liquid-liquid phase interface in these systems. Bioparticle partitioning has been shown to be related to physiologically important surface properties such as membrane charge or lipid composition. Affinity partitioning into the PEG-rich phase can be accomplished by coupling PEG to a ligand having affinity for specific cells or macromolecules. Subpopulations can be identified or separated using multi-step countercurrent distribution (CCD). Incomplete understanding of the influence of gravity on the efficiency and quality of the impressive separations achievable by partitioning, and appreciation for the versatility of this efficient technique, have led to its study for low-gravity biomaterials processing. On Earth, two-phase systems rapidly demix because of density differences between the phases. In low-gravity, demixing has been shown to occur primarily by coalescence. Polymer surface coatings, developed to control localization of demixed phases in low-g, have been found to control electroosmosis which adversely affects electrophoretic separation processes on Earth and in space. In addition PEG-derivatized antibodies have been synthesized for use in immunoaffinity cell partitioning.