Location: Cotton Structure and Quality Research2010 Annual Report
1a. Objectives (from AD-416)
Overall project objectives include: Collecting the best available information on the structures within the cotton fiber; Constructing fundamental models of these structures at different size scales; Providing additional fundamental models that have partial surfaces of hydrophobic molecules; and Monitoring moisture movement through the model structure during molecular dynamics simulations. Specific aspects of this work to be carried out at Tulane include development of coarse-grain (or another suitable approach) modeling of the cellulose in cotton fiber and the execution of molecular dynamics studies to show water movement in the model fiber. (Coarse-grain models group several atoms into a “super-atom” to save computer memory and allow greater speed for large systems.)
1b. Approach (from AD-416)
The main feature of this effort will be to construct realistic coarse grain models that will incorporate specific features of cotton fibers that are likely to affect the movement of moisture and to carry out the molecular dynamics simulations that will depict the movement of moisture. Transport of moisture from one side to the other will also be of interest. Suitable computer software for molecular dynamics studies with either atomistic or coarse-grain modeling will be employed, along with specific models of water that are sufficiently realistic. Data will be collected that will permit moving graphics depictions of the water motions
3. Progress Report
A grant of one million hours of computer time on the Louisiana Optic Network Initiative computers was obtained. Models of the different sides of nano-sized cellulose crystals were constructed from the original models and a model droplet of water was placed on each surface. Then, the model atoms were given motion that corresponds to room temperature (a molecular dynamics simulation), and the spreading of the water over the surfaces was studied. Besides the surfaces that would be found for native cellulose, surfaces were also constructed with varying amounts of methyl groups that replaced the hydroxyl groups. Those methylated surfaces were devised as a start to understanding treatments of cotton fabrics for moisture management. The water was not expected to be so attracted to those groups, and that was the case. Although the water droplet did not spread out so much on the heavily methylated surfaces, it did bridge small regions of methylated cellulose. When the cellulose was completely methylated, the water droplet retained much of its original shape, and contact angles could be estimated for the water on the more heavily methylated surfaces. For the native cellulose crystals, however, the water spread out so that a partial monolayer was formed. Not enough water molecules were in the droplet to completely cover the surface. Investigation of the differences between water and an actual solvent molecule for cellulose has begun. This solvent is used to make lyocell that is sold under the commercial name “Tencel”. A model cellulose crystal dissolved in the model of the industrial solvent, n-methylmorpholine n-oxide (NMMO). This finding lets us understand why this very special solvent dissolves cellulose, but water does not, despite both liquids interacting with the hydroxyl groups of the cellulose. This sort of study with molecular dynamics has seldom been reported; the melting of ice is about as close as anything to this work. The methods used to monitor activities for this agreement were annual reports, technical visits/e-mails/interactions, presentations at scientific and industry meetings, and publications.