The "upper phase" is formed by the more hydrophobic polyethylene glycol (PEG), which is of lower density than the "lower phase," consisting of the more hydrophilic and denser dextran solution.
Although PEG is inherently denser than water, it occupies the upper layer. This is believed to be due to its solvent ordering properties, which excludes excess water, creating a low density water environment. The degree of polymerization of PEG also affects the phase separation and the partitioning of molecules during extraction.