Water hydration of polyethylene glycol dimethyl ether

In this work, GHz and THz complex dielectric spectra of a polyethylene glycol dimethyl ether (2000 g/mol) aqueous solution were studied. The reorientation relaxation of water in this kind of macro-amphiphilic molecule solution can be well described by three Debye models, including under-coordinated water, bulk-like water [water molecules in a tetrahedral hydrogen bond network (bulk water) and bulk water molecules affected by hydrophobic groups], and slow hydrating water (water molecules donating hydrogen bonds to hydrophilic ether groups). The reorientation relaxation timescales of bulk-like water and slow hydration water both show increases with concentration from 9.8 to 26.7 ps and from 46.9 to 100.1 ps, respectively. By estimating the ratios of the dipole moment of slow hydration water to the dipole moment of bulk-like water, we calculated the experimental Kirkwood factors of bulk-like and slow hydrating water. The experimental Kirkwood factor of bulk-like water increased from 3.17 to 3.44 with concentrations, while the experimental Kirkwood factor of slow hydrating water roughly remained constant at 4.13 from concentrations of 15%-60%. The estimated water molecule numbers of three water components around monomers also confirm our sorting for water components.

Automated summary

The GHz and THz complex dielectric spectra of a polyethylene glycol dimethyl ether (2000 g/mol) aqueous solution were investigated. The reorientation relaxation of water in this type of macro-amphiphilic molecule solution can be well described by three Debye models: under-coordinated water, bulk-like water, and slow hydrating water. The reorientation relaxation timescales of bulk-like water and slow hydration water both increase with concentration. By calculating the experimental Kirkwood factors of bulk-like and slow hydrating water, the changes in their dipole moments were estimated. The estimated water molecule numbers of three water components around monomers also support the sorting of water components.