2011 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.
Hydrophobicity (water repellence) was introduced on the surface of crystals of cellulose I beta, the main component of cotton fibers, by substituting hydroxyl (-OH) groups on individual glucose sugar units (the cellulose monomers). Both methyl (-CH3) and methoxy (-O-CH3) groups were added onto the cellulose molecules that were on the crystal surfaces. Molecular Dynamics simulations were performed on the crystal surface. Water molecules were packed in a cubic box to simulate a hemi-cylindrical drop and initially placed at a distance of 3 Angstroms from the surface. The cellulose chains were held in place to avoid twisting. Constant volume simulations were done at 1 atmosphere of pressure and 300°K. Periodic boundary conditions in only X and Y directions were allowed. Long range corrections were included. The system was further simulated for another 30 nanoseconds. A hemi-cylindrical droplet was chosen to evaluate the line tension effects along the Y axis. The contact angles of the hemicylindrical drops were calculated by fitting a circle to the equimolar (where the density of the drop is half the bulk density) contour of cross section of the droplet. The contact angle of a hemi-cylindrical drop on the methylated surface was evaluated to be around 115°-117° and 80°-85° for the methoxylated surfaces for varying droplet size.
1. The methoxylated (-O-CH3) surface is more hydrophilic than the methylated (-CH3) surface. It could be inferred that the functional group used for substituting the hydroxyl groups on the cellulose chains could play a critical role in controlling the movement of moisture on the crystalline cellulose surface.
2. Within the limits of error, increasing the number of water molecules did not have a significant effect on the contact angle. The number of water molecules is directly proportional to the size of the droplet and hence the radial component. Since the hemi-cylindrical drop is continuous along the X direction due to periodic boundary conditions, the only gradient to surface tension exists in the perpendicular Y direction. However, it could be inferred from current results that the gradient of surface tension along Y direction is very small and could be ignored. This is useful information for studying the different behavior when cylindrical water droplets move forward and backward on patterned crystalline surfaces.
Additional, more realistic computational experiments were carried out involving N- Methylmonpholone N-oxide (NMMO) and NMMO-water mixtures, as well as pure water, as solvents for cellulose. The previous work was carried out at the very high temperature of 523°C. The NMMO molecule’s O2 interacts with the O6 atoms of the cellulohexaose models in preference to the O8 of the NMMO.
The methods used to monitor activities for this agreement were technical visits/e-mails/interactions, presentations at scientific meetings, and publications.