Fast transport diffusion of bound water in cellulose fiber network

Abstract

A remarkable property of cellulose-based materials is that they can absorb huge amounts of water (25% of the dry mass) from ambient vapor, in the form of bound water confined at a nanoscale in the amorphous regions of the cellulose structure. The control of the dynamics of sorption and desorption of bound water is a major stake for the reduction of energy consumption and material or structure damages, but in the absence of direct observations this process is still poorly known. Here we present measurements of bound water transport thanks to Nuclear Magnetic Resonance relaxometry and Magnetic Resonance Imaging measurements. We show that the bound water is transported along the fibers and throughout the network of fibers in contact. For each material a single transport diffusion coefficient value allows to represent the processes over the whole range of saturation. The dependence of the transport diffusion coefficient on the fiber density and orientation is then analyzed to deduce the (elementary) transport diffusion coefficient of bound water along a cellulose fiber axis. This constitutes fundamental physical data which may be compared with molecular simulations, and opens the way to the prediction and control of sorption dynamics of all cellulosic materials or other hygroscopic materials.

Critical Role of Boundary Conditions in Sorption Kinetics Measurements

“…Following Kelvin’s law, the radius r of the free water–air meniscus is related to the RH according to ln ( n ) = −2σ V m / rRT , with σ = 0.07 N m –1 the water–air surface tension, V m = 18 cm 3 mol –1 the molar volume of water, R = 8.31 J mol –1 K –1 the gas constant, and T = 23 °C the temperature. In our system made of cellulose fibers of typical thickness in the order of a few microns, we expect the minimum size of the menisci to be in the order of one micron, which gives a RH larger than 0.999. In other words, as long as there is a layer of free water covering the fibers, the RH is almost 100%.…”

“…Note that the value of the diffusion coefficient may depend on saturation. Moreover, its physical origin may be complex as one can expect both vapor transport through the porous fiber network and bound water diffusion along the fiber network (inside the fibers), as shown from recent experiments with oil-filled fiber stacks . Also, the exchanges between these two phases may play a significant role.…”

“…To illustrate the above theoretical predictions, we present data obtained with a typical hygroscopic material, a cellulose fiber stack prepared from cellulose fibers having a strip-like shape with an average length of 300 μm, a width of 10 μm, and a thickness of about 1 μm. The sorption curve of this material is shown in Zou et al 65 Some fiber amount previously placed during several weeks in a desiccator at a RH of 97%, is then compressed so as to obtain a sample of controlled porosity of 0.72 (void to total volume ratio, noted ε). Note that under these conditions, ε corresponds to the porosity in the saturated state.…”

“…After the compression, the material is again placed in a desiccator at 97% RH in order to ensure a homogeneously saturated state. Further details on the preparation and characteristics of the material may be found in Zou et al 65 The sample is then subjected to a vertical dry air flux (n ∞ = 0) according to the scheme, as shown in Figure 3. The diameter of the cylindrical sample is 5 cm and its thickness is 2.1 cm.…”

“…Moreover, its physical origin may be complex as one can expect both vapor transport through the porous fiber network and bound water diffusion along the fiber network (inside the fibers), as shown from recent experiments with oilfilled fiber stacks. 65 Also, the exchanges between these two phases may play a significant role. The dynamics of these exchanges are in fact the true sorption dynamics, which these tests initially aim to explore but which are hidden by the global diffusion process and, in a second step, i.e., even when the diffusion coefficient has been determined, this is just one of the potential processes occurring inside the sample, besides vapor and bound water diffusion.…”