INVESTIGATION OF THE STATE OF WATER ADJACENT TO SOLID SURFACES USING DEUTERIUM NMR

Open Access
Author:
Pokras, Shoshanna Meghan
Area of Honors:
Chemistry
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Alan James Benesi, Thesis Supervisor
  • Karl Todd Mueller, Honors Advisor
Keywords:
  • deuterium NMR
  • solid state NMR
  • starch
  • cellulose
  • water
Abstract:
The state of water adjacent to solid surfaces in freeze-dried starch and cellulose has been investigated using 2H NMR techniques. Quadrupole echo spectra of all samples at room temperature include a sharp central peak that corresponds to 2H2O. Both compounds also showed additional powder patterns that have been attributed to rigid glucose O-2H groups. At lower temperatures (≤ -50°C), the central peak became irresolvable due to transition into the intermediate and slow exchange regime. T1 values of the central peak for a range of temperatures obtained at two magnetic fields closely match previous values for theoretical calculations that assume jumps on a solid-state lattice, and are similar to those observed for 2H2O-synthesized Kanemite and 2H2O-hydrated Zeolite A. The data could not be matched with isotropic rotational diffusion models for liquid state 2H2O. The sharp central peak at room temperature and higher temperatures (up to 85°C) exhibited spin-lattice relaxation times almost two orders of magnitude shorter than those of bulk 2H2O, and exhibited magnetic field dependence, with T1 values at room temperature of 6.31 ms at 7.02 T and 10.6 ms at 11.75 T for cellulose, and 5.59 ms at 7.02 T and 9.67 ms at 11.75 T for starch. These data support the hypothesis that solid-state water exists at low hydration levels within the lattice structures of both starch and cellulose at room temperature and higher.