The Mechanism of Partition in Aqueous Media

Abstract

From the quantitative determination of the polar surface tension parameters of Dextran 150 (DEX) and polyethylene glycol 6,000 (PEG), it is shown that both polymers are pronounced monopolar Lewis bases. By means of the theory of short-range (SR) (polar or hydrogen bonding) interactions, and also taking into account the influence of long-range Lifshitz-van der Waals (LW) interactions, it is demonstrated that DEX and PEG, immersed in water, repel each other with a sizable repulsion energy. That repulsion energy is highest when the polymers are most strongly dehydrated. Thus it becomes clear why a certain concentration of each polymer must be reached before phase separation can occur. Monopolar repulsion also accounts for the occurrence of the formation of multiple phases, provided the participating polymers all are monopoles of the same sign. The mechanism for the preferential migration of biopolymers or particles to one or the other phase is also elucidated. While in some cases (e.g., affinity partition) the preferential migration is due to a specific attraction to one of the phases, in many other cases the preference for one phase appears to be rather due to the fact that a biopolymer is more strongly repelled by the other phase; this repulsion is enhanced: a) by an increase in M_w of the biopolymer and/or b) by an increase in M_w of the polymer in the repelling phase.