Water diffusion in wood pulp cellulose fibers studied by means of the pulsed gradient spin-echo method

The self-diffusion of water sorbed in wood pulp cellulose fibers was studied by using the pulsed gradient spin-echo (PGSE) method. The observed echo attenuation profiles deviate significantly from those of bulk liquids and can be decomposed into two components: one with a self-diffusion coefficient independent of the diffusion time, and one with an apparent diffusion coefficient that depends on the diffusion time. The relative amplitude of these two components for the different diffusion times used in the PGSE experiment is nearly constant. We attribute the two components to bulk water between cellulose fibers and to water in pores within the fibers, respectively. From the relative amplitude of the components and the known water content in the samples, the amount of water in pores inside the fibers was calculated to 1.4 g/g of cellulose fibers. The self-diffusion coefficient of the bulk water between the fibers is lower than that of neat water. This reduction is mainly caused by the obstruction effect of the cellulose fibers whereas the hydration effect is of minor importance. The apparent self-diffusion coefficient of water trapped in pores inside fibers is approximately one-third of that of the bulk water between fibers when the diffusion time is 12 ms and is reduced further by another factor of 3 when the diffusion time is increased to 80 ms. By using a sheet sample and applying the magnetic field gradient perpendicular or parallel to the sheet it was found that the diffusional motion of water in pores is anisotropic. These results indicate that the pores are elongated along the fiber axis having lengths ranging from a few micrometers up to 20 μm.